Excited States and Reactive Intermediates. Photochemistry, Photophysics, and Electrochemistry 9780841209718, 9780841211421, 0-8412-0971-5

Table of contents :
Title Page......Page 1
Copyright......Page 2
ACS Symposium Series......Page 3
FOREWORD......Page 4
PREFACE......Page 5
Excited States: Characteristics......Page 7
Excited State Quenching Reactions......Page 8
Spectroscopy......Page 9
Electrochemistry......Page 10
PdftkEmptyString......Page 0
1 Excited States of Mononuclear and Dinuclear Chromium(III) Complexes......Page 11
Superexchange in bis(μ-hydroxo)-bridged chromium (III) dimers......Page 12
Nature of the luminescent state in CrCl63-......Page 18
Literature Cited......Page 21
2 Ab Initio Analysis of Charge Transfer Excitations The Cr(CN)63- Complex......Page 22
Ligand Field Considerations......Page 24
Charge Transfer Transitions : General Considerations......Page 26
Energy Pattern of Charge Transfer Transitions......Page 29
Literature Cited......Page 32
3 Excited State Geometries of Coordination Compounds Obtained from Vibronic Spectra and Photon Flux Fluctuation Measured by Time Resolved Spectroscopy......Page 33
Conventional Vibronic Spectra......Page 35
Time Resolved Emission Spectra......Page 41
Acknowledgment......Page 45
Literature Cited......Page 48
4 Excited State Distortions Determined by Electronic and Raman Spectroscopy......Page 49
Excited State Distortions of W(CO)5pyridine and W(CO)5piperidine from Time-Dependent Theory, Pre-resonance Raman Spectroscopy, and Electronic Spectroscopy......Page 50
Calculation of Excited State Distortions and Electronic Spectra from Raman Intensities......Page 54
Correlations between Excited State Distortions and Photochemical Reactivity......Page 58
Excited State Bending Distortions of the MNO Group and Their Photochemical and Spectroscopic Consequences......Page 60
Literature Cited......Page 65
5 Investigation of One-Dimensional Species and of Electrochemically Generated Species Use of Resonance Raman Spectroscopy......Page 67
Wolffram's Red Type Salts......Page 68
Pop Salts of Linear-Chain Complexes......Page 69
Pop Complexes K4[Pt2(pop)4X].nH2O......Page 71
Spectroelectrochemically-generated Species......Page 73
Literature Cited......Page 74
6 Metal-Ligand Charge Transfer Photochemistry Metal-Metal Bonded Complexes......Page 76
Spectroscopic properties......Page 77
Photolysis in 2-Me-THF (133K

Citation preview

ACS SYMPOSIUM SERIES 307

Excited States and Reactive Intermediates Photochemistry, Photophysics, and Electrochemistry A . B . P. Lever, EDITOR York University

Developed from a symposium sponsored by the Divisions of Inorganic Chemistry of both the American Chemical Society and the Chemical Institute of Canada at the 1985 Biennial Inorganic Chemical Symposium, Toronto, Ontario, June 6-9, 1985

American Chemical Society, Washington, DC 1986 In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Library of Congress Cataloging-in-Publication Data Excited states and reactive intermediates. (ACS symposium series, ISSN 0097-6156; 307) "Developed from a symposium sponsored by the American Chemical Society and the Chemical Institute of Canada at the 1985 Biennial Inorganic Chemical Symposium, Toronto, Ontario, June 6-9, 1985." Bibliography: p. Includes indexes. 1. Excited state chemistry—Congresses Physical organic—Congresses I. Lever, A. B. P. (Alfred Beverly Philip) II. American Chemical Society. III. Chemical Institute of Canada. IV. Series. QD461.5.E914 1986 541.3 86-7908 ISBN 0-8412-0971-5

Copyright © 1986 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of the first page of each chapter in this volume indicates the copyright owner's consent that reprographic copies of the chapter may be made for personal or internal use or for the personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc., 27 Congress Street, Salem, MA 01970, for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to copying or transmission by any means—graphic or electronic—for any other purpose, such as for general distribution, for advertising or promotional purposes, for creating a new collective work, for resale, or for information storage and retrieval systems. The copying fee for each chapter is indicated in the code at the bottom of the first page of the chapter. The citation of trade names and/or names of manufacturers in this publication is not to be construed as an endorsement or as approval by ACS of the commercial products or services referenced herein; nor should the mere reference herein to any drawing, specification, chemical process, or other data be regarded as a license or as a conveyance of any right or permission, to the holder, reader, or any other person or corporation, to manufacture, reproduce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. Registered names, trademarks, etc., used in this publication, even without specific indication thereof, are not to be considered unprotected by law. PRINTED IN THE UNITED STATES OF AMERICA

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

ACS

S y m p o s i u m Series

M . Joan Comstock, Series Editor Advisory Board Donald E. Moreland Harvey W. Blanch University of California—Berkele W. H. Norton Alan Elzerman J. T. Baker Chemical Company Clemson University John W. Finley Nabisco Brands, Inc.

James C. Randall Exxon Chemical Company

Marye Anne Fox The University of Texas—Austin

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In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

FOREWORD SYMPOSIUM SERIES

The A C S was founded in 1974 to provide a m e d i u m for p u b l i s h i n g s y m p o s i a q u i c k l y in b o o k f o r m . T h e format o f the Series parallels that o f the c o n t i n u i n g except that, i n order to save time, the papers are not typese by the authors in camera-read the supervision of the E d i t o r s w i t h the assistance o f the Series A d v i s o r y B o a r d and are selected to m a i n t a i n the integrity o f the s y m p o s i a ; however, v e r b a t i m reproductions o f previously p u b lished papers are not accepted. B o t h reviews a n d reports o f research are acceptable, because s y m p o s i a may embrace b o t h types o f presentation.

IN CHEMISTRY SERIES

ADVANCES

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

PREFACE

M ^ O L E C U L E S IN THEIR EXCITED STATES and molecules of transient existence generated by photochemical stimulation or by other processes, such as electrochemistry, are rapidly drawing considerable interest and gaining importance. The excited state of a molecule is, in many ways, a new species different chemically from the ground state molecule and endowed with additional energy; it is often capable of chemical processes that are not possible in the ground state The ability to do "test tube" experiments with such short-lived species is The conference from which this book was developed addressed many of the techniques that may be used to probe these systems and dealt with the new chemistry that is being learned. Approximately 160 participants took part in the Biennial Inorganic Chemical Symposium 1985. The participants came primarily from Canada and the United States; however, some came from as far as Japan, England, Belgium, Italy, East and West Germany, and The Netherlands, thus generating an international atmosphere. They heard the latest ideas in excited state photochemistry and photophysics, species at electrode surfaces, resonance Raman spectroscopy, electrochemiluminescence, photochemistry of organometallic and cluster species, and gas phase organometallic chemistry to name a few topics. Some fifty posters were also presented and the Chemical Abstracts Services displayed the latest in on-line searching. Funding for the conference came from the divisions of inorganic chemistry of the American Chemical Society (ACS) and the Chemical Institute of Canada, the Petroleum Research Fund (ACS), the Natural Sciences and Engineering Research Council, York University, and eight industrial companies: Strem Chemicals Inc.; Merck Frosst Canada Inc.; Union Carbide Canada Limited; EG&G Canada Limited; Xerox Research Centre of Canada; Tasman Scientific Inc.; Guided Wave, Inc.; and Lumonics Inc. I hope this volume will stimulate the readers to consider how they might also contribute to this rapidly growing area. A. B. P. LEVER

York University Toronto Ontario Canada M3J 1P3 January 1986 vii In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

INTRODUCTION

CHEMISTS HAVE BEEN CONCERNED

predominantly with the chemistry o f species that exist i n their m o l e c u l a r g r o u n d state, often stable for an indefinite p e r i o d . Structures c a n , i n p r i n c i p l e , be obtained by X - r a y crystallographic methods, and physical data such as N M R , 1R, and U V / V I S spectra can be obtained with conventional spectrometers. T h e advent o f lasers a n d electronic devices that can record extremely fast events has led to g r o w i n states, usually, though not exclusively, the lowest excited state. T h e detailed study o f excited states is a field that is now g r o w i n g rapidly and promises to deliver a fascinating new view o f chemistry in the future.

Excited States: Characteristics A molecule i n its first excited state is, i n a very real sense, a different molecule from the g r o u n d state o f the species. It possesses a d d i t i o n a l energy and p r o b a b l y has a different structure, at least in respect to small changes in b o n d lengths and angles, and indeed may have a totally different stereochemistry. It has different electronic a n d v i b r a t i o n a l spectra a n d clearly has a different chemistry. S u c h chemistry is referred to as photochemistry because it is accessed by a light a b s o r p t i o n event. These excited state molecules c o m m o n l y exist for time intervals ranging f r o m picoseconds to m i c r o seconds, rarely longer, except when solids at cryogenic temperatures are being studied. Nevertheless, methods o f analysis are available to probe the photophysics and photochemistry o f these species even o n such a short time frame. Light absorption w i l l not generally occur to the lowest excited state, but rather a series o f excited states may become populated. These will generally decay rapidly (in picoseconds) to the lowest excited state. O n e can anticipate the even richer chemistry o f these higher excited states, but their lifetime w i l l usually—though not exclusively—be so short that this chemistry has no time to be expressed. Nevertheless, light emission and photochemistry may sometimes be observed from these higher energy levels. Special techniques such as ultra-short laser pulses (measured i n femtoseconds) are b e c o m i n g available to probe this chemistry (for e x a m p l e , o n the time scale o f b o n d breaking). In general the lowest excited state will decay back to the ground state by IX

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

one or more pathways, i n c l u d i n g radiationless deactivation (loss o f excited state energy as heat to the surroundings), one or more p h o t o c h e m i c a l reactions, or by luminescence (fluorescence or phosphorescence). T h e study of these processes leads to a better understanding o f the electronic structure of the excited state. In the short term, the value o f such studies must lie i n what we can learn about h o w chemistry changes when the q u a n t u m mechanical state of a molecule changes, a n d h o w the a d d i t i o n a l energy, distributed over the m o l e c u l e , modifies its chemistry. In the l o n g t e r m , new i n d u s t r i a l l y important processes may depend u p o n the use o f excited state molecules. W i t h the exception o f a few highly studied states, such as, for example, the redox active lowest metal-to-ligand charge transfer excited state ( M L C T ) in the [ R u ( b i p y r i d i n e ) ] we k n o w relatively little abou knowledge is waiting to be explored. 2

3

Excited State Quenching Reactions A n i m p o r t a n t facet o f the chemistry o f excited states is that a d d i t i o n a l energy confers u p o n the state both greater o x i d i z i n g power a n d greater reducing power, relative to the g r o u n d state. T h e greater reducing power originates i n the higher energy electron that has been excited, while the greater o x i d i z i n g power resides i n the hole created by the excitation of an electron. E l e c t r o n transfer reactions may be observed by reaction o f the excited state with an electron d o n o r or acceptor. Where the excited state luminesces, redox reactions with various species can be m o n i t o r e d by observing the quenching o f excited state luminescence, or reduction i n excited state lifetime, as a function o f the concentration o f quenching species ( S t e r n - V o l m e r plot). In this fashion one can determine the rates o f c h e m i c a l reaction between excited state a n d quencher, a n d , using models such as those developed by M a r c u s or A n g m o n and Levine, determine various parameters such as free energies o f a c t i v a t i o n and reorganization energies. Because o f the m u c h greater d r i v i n g forces potentially available in reactions between substrates and excited state molecules, difficult—but valuable—electron transfer reactions, such as the o x i d a t i o n o f water or c h l o r i d e i o n , may be accessed t h r o u g h excited state photochemistry. T h e question o f h o w to separate hole-electron pairs generated in a quenching reaction, h o w to provide kinetic pathways to lead these two highly reactive species far apart f r o m each another, a n d h o w to couple in some useful chemistry are currently o f interest. O f related interest is the problem o f how far apart, in a fixed sense, an χ

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

excited state a n d a quencher can be, and yet have electron transfer take place. T h u s one can have quenching via a collision process, or by overlap o f d o n o r a n d acceptor orbitals at l o n g distances. If the excited state and quencher are linked by a l o n g conjugated pathway, quenching might be expected to take place. However, what about nonconjugated and throughspace pathways, both o f w h i c h can also lead to quenching? S u c h studies can lead to geometric information about proteins and impurity sites in crystal. S o m e e x t r e m e l y f a s c i n a t i n g o x i d a t i v e a d d i t i o n - t y p e c h e m i s t r y is possible at cryogenic temperatures when metal atoms are irradiated in the presence o f C - H and C - O bonds. M e t a l atoms are inserted, presumably via an excited state. T h i s may well have significance for the activation o f alkanes. S o m e o f the more interestin electron i n nature, suggestin event i n t o a n e x c i t e d state process. T h e study o f the excited state photochemistry a n d photophysics o f binuclear and polynuclear (cluster) molecules is thus b e c o m i n g o f importance, a n d two-electron reactions are being identified. T h e sensitization o f semiconductors is a special example o f electron transfer q u e n c h i n g a n d may prove to be very important. A photoexcited electron may, for example, be injected w i t h high q u a n t u m yield into the semiconductor c o n d u c t i o n b a n d , to produce a p h o t o v o l t a i c device. T h e " h o l e " that is "left b e h i n d " may then perform some useful o x i d a t i o n process. Excited state quenching is not restricted to electron transfer processes, but may also occur by a t o m abstraction (for example, hydrogen a t o m abstraction), or by energy transfer to another species. In a d d i t i o n , the excited state energy may be used a l o n g a reaction coordinate leading ultimately to ligand loss or ligand exchange. N e w molecules may be formed by s h i n i n g light u p o n the o l d . O r g a n o m e t a l l i c photochemistry is particularly rich in p r o v i d i n g unusual molecules after stimulation by light. T h e mechanisms and dynamics o f such reactions are areas o f serious study. Indeed e l u c i d a t i o n a n d understanding o f the m a n y processes that c a n o c c u r u p o n light s t i m u l a t i o n , a n d the c h e m i c a l d y n a m i c s associated therewith, are major goals o f current excited state chemistry. P h o t o e x c i t a t i o n o f biological molecules, proteins, and enzymes also has interest (such as watching a c a r b o n y l group photodissociate from c a r b o n y l heme and studying the chemistry o f the resulting products).

Spectroscopy A s one might expect, various spectroscopies, especially electronic spectroscopy a n d resonance R a m a n spectroscopy, can provide detailed information about the electronic and v i b r a t i o n a l nature o f an excited state. C o n v e n t i o n a l XI

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

electronic spectroscopy, a b s o r p t i o n a n d emission, can provide i n f o r m a t i o n about the geometry a n d b o n d distances i n excited states, while resonance R a m a n reveals the nature o f the c o u p l i n g between a n electronic state a n d v i b r a t i o n a l modes o f the molecule. Transient absorption spectroscopy, wherein one measures the electronic absorption spectrum o f a molecule i n a n excited state, is still in its infancy, but the g r o w i n g a v a i l a b i l i t y o f ultra-high-speed, rapid-scan spectrometers augurs well for this area o f spectroscopy. T h u s one may, i n the future, routinely probe excited state a b s o r p t i o n spectra as well as g r o u n d state a b s o r p t i o n spectra. T h e former can be expected to be as valuable in o b t a i n i n g i n f o r m a t i o n about the excited state as is the latter for the g r o u n d state. Time-resolved spectroscopie p r o b i n g the life o f a n excite t h r o u g h a c h a i n o f species, time-resolved spectroscopy (e.g., luminescence, excitation, resonance R a m a n ) can provide data for these various steps. S u c h studies have led, for example, to the view that the first M L C T excited state in [ R u ( b i p y r i d i n e ) ] , is localized i n one b i p y r i d i n e ring rather than delocalized over a l l three rings. 2+

3

Electrochemistry Electrochemically generated chemiluminescence provides a n unusual method for studying excited state energies. T h u s , for example, a n o x i d a n t a n d a reductant c a n be generated at the same electrode ( w i t h a l t e r n a t i n g p o l a r i z a t i o n ) , o r at t w o closely spaced electrodes. G i v e n a p p r o p r i a t e energetics, the o x i d a n t a n d reductant quench one another to generate a n excited state rather than a g r o u n d state product, and luminescence may be observed.

Xll

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

1 E x c i t e d States of M o n o n u c l e a r a n d D i n u c l e a r Chromium(III) Complexes Hans U. Güdel Department of Chemistry, University of Bern, Freiestrasse 3, CH-3000 Bern 9, Switzerland

3+

Excited states of Cr complexes were explored by single crystal spectroscopy at low temperatures. In the dimeric [a Cr(OH) Cra ] the sharp E single excitations were used to determine orbital exchange parameters. Out-of-plane interactions are dominant. The complex CrCl was studied in two exactly octahedral crystal environments. Broad-band T --> A luminescence with a great deal of fine structure was observed. The equilibrium geometry of the luminescent T state is a distorted octahedron with an equatorial Cr-X elongation of 0.1 Åand a small axial compression. 4+

4

2

2

4

3-

6

4

4

2g

2g

4

2g

3+

Luminescence from Cr complexes, both in the solid state and in solution, is a widespread phenomenon. The great majority belong to type a) in Figure 1, where the luminescent state is E and the optical transitions are sharp. The well-known ruby emission is a prototype for this situation. In a weaker ligand field the situation b) in Figure 1 is approached, the T state becomes competitive with E as the luminescent state. The T emission, corresponding to a spin-allowed d-d transition, is vibronically broadened. Pure T luminescence from Cr^ has been observed in halide and oxide coordinations (J_) . Intermediate situations with both E and ^T emissions ar^ also known. The Ε and A states both derive from the (t ) electron con­ figuration. The two states have approximately the same chemical bond­ ing and thus the same equilibrium geometry. The resulting sharpness of the corresponding optical transitions in absorption and emission at low temperatures provides a great deal of information ^>gut the nature of the excited ^E state. In the case of dinuclear Cr com­ plexes very useful information about the exchange coupling can be ob­ tained from a detailed study of the singly and doubly excited ^E dimer states (2). The reason lies in the intraconfigurational nature of the ^E excitations, which greatly simplifies the theoretical ap2

4

2

2

4

2

4

2

+

2

2

2

2

a

+

0097-6156/86/0307-0001 $06.00/0 © 1986 American Chemical Society In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

2

p r o a c h t o t h e p r o b l e m . I t i s p o s s i b l e t o deduce t h e dominant contributions splittings

i n t h e e x c i t e d s t a t e s which r e s u l t

actions. This

orbital

t o t h e n e t exchange from an a n a l y s i s o f t h e energy from exchange

inter­

i n f o r m a t i o n about t h e mechanisms o f exchange i s n o t

a c c e s s i b l e by s t u d y i n g

the ground-state

g i v e an i l l u s t r a t i o n o f t h i s

type

p r o p e r t i e s a l o n e . We

o f study

i n the f i r s t

will

part of this

paper. The

T

2

s t a t e d e r i v e s from the ( t )

(e^ electron configuration

2

and

i s therefore displaced with

ure

1b. E x p l o r i n g t h e n a t u r e

e q u i l i b r i u m geometry w i t h

respect

to

respect

p h y s i c a l l y r e l e v a n t . In the l o w - f i e l d

4

i s the excited state with

2

photochemical relevance luminescence l i e s

i n the diagram o f F i g ­

2

t o t h e ground s t a t e , i s c h e m i c a l l y

and T

A

4

o f the T ~ s t a t e , i n p a r t i c u l a r i t s s i t u a t i o n s o f F i g u r e 1b

the longest p h y s i c a l l i f e t i m e . I t s

i s thus enhanced. The broad-band ^ T — » ^ A 2

i n the near i n f r a r e d

(NIR)

Solid

2

state materials

w i t h b r o a d NIR l u m i n e s c e n c didates

f o r tunable

laser

In t h e second p a r t o f t h i s paper t h e p o t e n t i a l o f c r y s t a l cence s p e c t r o s c o p y illustrated

lumines­

t o i n v e s t i g a t e e x c i t e d s t a t e p r o p e r t i e s w i l l be

for CrCl^". 6

Superexchange i n bis(μ-hydroxo)-bridged chromium ( I I I ) dimers Optical

spectroscopy

techniques netic Cr^

+

i s a v a l u a b l e complement t o magnetochemical

f o r the study

o f exchange e f f e c t s

complexes. A r e c e n t

r e v i e w was g i v e n

E s

I I |-> I ι ι• >

excited state

|l

Β

7

B,

I

3

0

>

4

3Μςί>

ground state |2M t> s

II M t> s

I 0 0 > F i g u r e 5. H i g h - r e s o l u t i o n a b s o r p t i o n spectrum of [NH^]Br^ · 41^0 i n the r e g i o n of Ε e x c i t a t i o n s at 6K ( a ) Assignment of the components of band Β ( b ) . Reproduced from Ref. 5. C o p y r i g h t 1982, American Chemical S o c i e t y .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

1.

7

3

Mononuclear and Dinuclear Cr * Complexes

GUDEL

l a r i s a t i o n s from t h e p o l a r i z e d c r y s t a l s p e c t r a . The r e s u l t s o b t a i n e d from a f u l l a n a l y s i s o f t h e v e r y e x t e n s i v e e x p e r i m e n t a l d a t a on t h e [en] and [NH^] complexes a r e summarized i n T a b l e I , w h i c h l i s t s t h e a n t i f e r r o m a g n e t i c c o n t r i b u t i o n s t o t h e t o t a l J ^ « The o u t - o f - p l a n e i n t e r a c t i o n i s found t o be dominant. I t s magnitude and, as a r e s u l t , the magnitude o f t h e t o t a l i s c o r r e l a t e d with the p o s i t i o n of a

the Η atom i n t h e b r i d g e . I n [ e n ] B r ^ · 2 H 0 t h e Η atom l i e s more o r l e s s i n t h e CrOCr p l a n e . As a consequence t h e ρ(ττ) o r b i t a l a t t h e oxygen i s f u l l y a v a i l a b l e f o r o u t - o f - p l a n e superexchange. I n t h e [ N H 3 ] s a l t s t h e Η atom l i e s o u t s i d e t h e CrOCr p l a n e , and J ( o u t - o f p l a n e ) i s reduced a c c o r d i n g l y . We have n e g l e c t e d f e r r o m a g n e t i c o r ­ b i t a l c o n t r i b u t i o n s t o J ^ i n t h i s summary d i s c u s s i o n . As shown i n Ref. (5) t h e y c a n a l s o be deduced from t h e o p t i c a l s p e c t r o s c o p i c data. 2

a

T a b l e I . S t r u c t u r a l and Cr-Cr

Cr

H .

J (in plane)

J (out o f plane)

J

Cr deg

(2) [en]Br -2H 0

3.038(4)

[NH ]C1 *4H 0

3.041

4

3

2

4

2

[NH ]Br -4H 0 3

4

1

)

(cm

1

1

(cm

)

6(2)

-5

-145

-16

50(3)

-8

- 65

-0.9

-8

- 55

-0.4

-50

-3.04

2

(cm

)

We c o n c l u d e t h a t t h e h i g h i n f o r m a t i o n c o n t e n t o f t h e i n t r a c o n f i g u r a t i o n a l ^A E t r a n s i t i o n s has e n a b l e d us t o d e r i v e a v e r y a c c u r a t e p i c t u r | o f t h e s i n g l y e x c i t e d s t a t e i n t h e d i m e r i c complexes [a^Cr(0H) Cra ] . By u s i n g s i n g l e c r y s t a l s , h i g h r e s o l u t i o n s p e c t r o ­ scopy i n t h e temperature range 1,5K - 300K and a s t r a i g h t f o r w a r d t h e ­ o r e t i c a l t r e a t m e n t , n u m e r i c a l v a l u e s f o r t h e o r b i t a l exchange p a r a ­ meters were o b t a i n e d . A n t i f e r r o m a g n e t i c o r b i t a l parameters c a n be compared w i t h t h e r e s u l t s o f t h e o r e t i c a l c a l c u l a t i o n s . The parameters 2

2

+

2

J

4

a r e connected i j follows : K

a

with one-electron transfer integrals A

D

a

0

as

K

i

D

j

2

A„ (4) a. b. =

\ b. 2 U where U i s t h e e l e c t r o n t r a n s f e r energy can be r e l a t e d

t o energy

( 7 ) . The t r a n s f e r

d i f f e r e n c e s between m o l e c u l a r

which a r e o b t a i n e d by an approximate method l i k e , 1

Hiickel c a l c u l a t i o n . F o r t h e [a^Cr(OH^Cra^]^" " of

integrals

orbitals,

e.g. an Extended

complexes t h e dominance

o u t - o f - p l a n e superexchange ( F i g u r e 2b) i s n i c e l y r e p r o d u c e d by

such a c a l c u l a t i o n ( 7 ) .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

8

EXCITED STATES AND REACTIVE INTERMEDIATES

A l a r g e number o f d i m e r i c Cr systems, b o t h n a t u r a l d i n u c l e a r complexes and p a i r s o b t a i n e d by d o p i n g C r i n t o a s u i t a b l e host l a t ­ t i c e , have been i n v e s t i g a t e d by o p t i c a l s p e c t r o s c o p y (8) ( 9 ) . Exam­ p l e s o f exchange-coupled 3d complexes o t h e r t h a n C r ^ , whose e x c i t e d s t a t e s were s t u d i e d i n d e t a i l , a r e g i v e n i n R e f s . (10) - ( 1 3 ) . 3 +

+

3Nature o f t h e l u m i n e s c e n t s t a t e i n C r C l ^ 3 +

Cr c a n be doped i n t o t h e c u b i c e l p a s o l i t e l a t t i c e s C s N a I n C l ^ and Cs2NaYCl^. The C r s i t e symmetry i s e x a c t l y o c t a h e d r a l , w h i c h makes t h e s e e l p a s o l i t e systems p a r t i c u l a r l y a t t r a c t i v e . W i t h C r doping l e v e l s o f a p p r o x i m a t e l y 2% broad-band l u m i n e s c e n c e c o r r e s p o n d i n g t o the ^ 2 g — > 2e t r a n s i t i o n i s o b s e r v e d . F i g u r e 6 shows t h e 6K emis­ s i o n s p e c t r a . TRey e x h i b i t a g r e a t d e a l o f f i n e s t r u c t u r e , much more t h a n i n any s p i n - a l l o w e d d-d band e v e r measured i n a b s o r p t i o n We a l s o n o t i c e some d i f f e r e n c e the two l a t t i c e s . They r e s u l 0.1 S i n t h e M - CI d i s t a n c e i n t h e h o s t l a t t i c e s . A v i b r a t i o n a l a n a l y s i s o f t h e s p e c t r a i s s t r a i g h t f o r w a r d . De­ t a i l s f o r t h e C s N a I n C l : C r system were g i v e n i n Ref. ( 1 4 ) . I n b o t h s p e c t r a weak e l e c t r o n i c o r i g i n l i n e s on t h e h i g h - e n e r g y s i d e o f t h e broad band c a n be i d e n t i f i e d as magnetic d i p o l e t r a n s i t i o n s from t h e l o w e s t - e n e r g y s p i n - o r b i t component E " o f **T^ t o t h e ground s t a t e U (^A ) . They a r e f o l l o w e d by s t r o n g e r e l e c t r i c d i p o l e f a l s e o r i g i n s % . . 3i n v o l v m g t ^ and t ^ v i b r a t i o n s o f t h e C r C l ^ u n i t as e n a b l i n g modes. 2

3 +

3 +

T

3 +

2

6

1

u

The

remainder o f t h e f i n e s t r u c t u r e

consists of progressions

in a ^

and e^, based on t h e v i b r o n i c o r i g i n s . I n f o r m a t i o n about t h e n a t u r e of the luminescent ^ T s t a t e i s o b t a i n e d from t h e r i c h f i n e s t r u c 2g t u r e and t h e h i g h l y r e s o l v e d e l e c t r o n i c o r i g i n s . The o b s e r v a t i o n o f an e^ p r o g r e s s i o n i s a c l e a r i n d i c a t i o n o f a J a h n - T e l l e r e f f e c t ( 1 5 ) . An o r b i t a l e l e c t r o n i c T state coupling to an eg v i b r a t i o n i s a c l a s s i c a l J a h n - T e l l e r s i t u a t i o n , w h i c h c a n be t r e a t e d t h e o r e t i c a l l y (14/15) . I t l e a d s t o t h e well-known p i c t u r e o f t h r e e p o t e n t i a l s d i s p l a c e d a l o n g t h e e c o o r d i n a t e s . The ^ 2 s t a t e i n o u r h o s t l a t t i c e s thus shows a t e t r a g o n a l e d i s t o r t i o n i n a d d i ­ t i o n t o t h e normal d i s t o r t i o n a l o n g t h e a- c o o r d i n a t e . I t s e q u i l i b ­ r i u m geometry i s no l o n g e r f u l l y o c t a h e d r a l . T h i s symmetry r e d u c t i o n l e a d s t o a p a r t i a l q u e n c h i n g o f o r b i t a l a n g u l a r momentum, w h i c h i s u s u a l l y c a l l e d a Ham e f f e c t ( 1 6 ) . D e s i g n a t i n g t h e o r b i t a l components of T as = |ξ>, |η> and ~]ζ> we have t h e f o l l o w i n g nonzero m a t r i x elements o f o r b i t a l a n g u l a r momentum: 0

2 g

T

g

g

g

2 g

= < ζ |ΐ,

χ

|η> = = i h

I n t h e b a s i s o f Born-Oppenheimer p r o d u c t s t a t e s

= = ) }

(7)

J/r

where E j

T

All

^ i s the Jahn-Teller s t a b i l i s a t i o n

i n t h e |ξ>, |η>, |ζ> b a s i s a r e thus reduced use

energy.

t h e o f f - d i a g o n a l m a t r i x elements o f t h e s p i n - o r b i t

the e x p e r i m e n t a l l y observed

corresponding

coupling

by t h e f a c t o r γ, and we

quenching t o c a l c u l a t e E j ^

and t h e

geometrica

t i c e the t o t a l spread o 32 cm

, whereas c r y s t a l f i e l d

Teller effect predicts a total

theory without

c o n s i d e r i n g a Jahn-

spread o f approximately

A n o t h e r s o u r c e o f i n f o r m a t i o n about t h e a l o n g a ^ and e , l i e s g

107 cm~^.

distortions,

both

i n the d i s t r i b u t i o n of i n t e n s i t y within the

r e s p e c t i v e p r o g r e s s i o n s i n t h e luminescence by which t h e d i s p l a c e m e n t s

spectrum. The p r o c e d u r e s ,

AQ^ w i t h r e s p e c t t o t h e ground s t a t e e q u i ­

l i b r i u m geometry a l o n g t h e c o o r d i n a t e s i a r e o b t a i n e d from t h e e x p e r ­ i m e n t a l d a t a have been g i v e n i n d e t a i l

i n Refs.

(14) and ( 1 7 ) . The

r e s u l t s f o r o u r systems a r e summarized i n T a b l e I I .

Table I I . D i s t o r t i o n o f the ^ T ground s t a t e ^A host

AQ

2

2

6

6

AQ

e

g

1g

Cs NaYCl

state of C r

J

with respect to the

. The parameters a r e d e f i n e d i n t h e t e x t .

2

lattice

Cs NaInCl

2 g

A(Cr-X)

A(Cr-X) e

g

a x

eq

g

(Ham)

(progr.)

d)

d)

d)

d)

d)

d)

0.150

-0.121

-0.111

-0.116

0.095

-0.006

0.154

-0.132

-0.140

-0.136

0.102

-0.016

We f i n d t h a t t h e e d i s t o r t i o n s d e r i v e d from t h e Ham q u e n c h i n g and t h e i n t e n s i t y d i s t r i b u t i o n i n t h e p r o g r e s s i o n d i f f e r by l e s s t h a n t e n p e r c e n t , thus c o n f i r m i n g t h e soundness o f o u r a n a l y t i c a l p r o c e ­ d u r e . I n o r d e r t o g e t t h e a c t u a l d i s p l a c e m e n t s i n Cr-X bond l e n g t h s , A ( C r - X ) . ^ and A ( C r - X ) . f o r t h e e q u i l i b r i u m geometry o f t h e l u m i n e s cent T s t a t e , t h e AQ^ v a l u e s have t o be l i n e a r l y t r a n s f o r m e d ( 1 7 ) . F o r t h e |ζ> component o f T« t h e v a l u e s i n t h e l a s t two columns o f T a b l e I I a r e o b t a i n e d . The r e s u l t f o r t h e C s N a Y C l l a t t i c e i s v i s u ­ a l i z e d i n F i g u r e 7. g

2 g

2

The

6

h i g h l y r e s o l v e d f i n e s t r u c t u r e o f t h e low-temperature

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

broad-

EXCITED STATES AND REACTIVE INTERMEDIATES

Cs NaYCI 2

6

0.02

0.02

F i g u r e 7. E q u i l i b r i u m Cs NaYCl . 2

geometry o f l u m i n e s c e n t

1

s t a t e o f Cr^" " i n

6

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

3+

1. Güdel Mononuclear and Dinuclear Cr Complexes

Π

+

band luminescence of Cr^ in the Cs NaY(In)Cl host lattices has enZ Ο Λ abled us to derive a very accurate picture of the emitting ^2g * Luminescence spectroscopy, whenever applicable, has the advantage that the low-temperature spectrum consists of only one electronic transition, whereas in absorption the transitions to the four spinorbit components of ^ g superimposed. As a result the absorption spectrum is not as well resolved. In the CrCl^" units the combined distortions along the a-jg and e^ coordinates lead to a compressed octahedron, with equatorial Cr-Cl distances approximately 0.1 8. larger than in the ground state and a much smaller axial compression. Similar distortions were deduced from optical absorption spectroscopy for the T £ *- ( 3)6 ^—^ as well as the Tj and states in Co(NH ) (17) . There are im­ portant photochemical implication f thes distortions It i intui tively clear that the chemica cited state. Bond strengths and force constants in the equatorial plane are reduced and, consequently, ligand substitution is facili­ tated. Such photochemical processes are, of course, much more likely to occur in solution than in the elpasolite systems studied here. But high-resolution crystal spectroscopy is a very powerful technique to study the physical properties of the relevant states. Literature Cited 1. Kenyon, P.T.; Andrews, L.; McCollum, B.; Lempicki, A. IEEE J. Quant. Electronics, 1982, QE-18, 1189. 2. Güdel, H.U. Comments Inorg. Chem., 1984, 3, 189. 3. Hermann, A.M. Sol. Energy, 1982, 29, 323. 4. Güdel, H.U. in "Magneto-Structural Correlations in Exchange Coupled Systems"; Willett, R.D., Ed.; Reidel, 1985; p. 297. 5. Decurtins, S.; Güdel, H.U. Inorg. Chem., 1982, 21, 3598. 6. Decurtins, S.; Güdel, H.U.; Pfeuti, A. Inorg. Chem., 1982, 21, 1101. 7. Leuenberger, B.; Güdel, H.U. Inorg. Chem., submitted. 8. References 7, 8, 10 - 15 quoted in Ref. 4. 9. Riesen, H.;Güdel,H.U.; Chaudhuri, P.; Wieghardt, K. Chem. Phys. Lett., 1984, 110, 552. 10. References 24 - 28 quoted in Ref. 3. 11. McCarthy, P.J.; Güdel, H.U. Inorg. Chem., 1984, 23, 880. 12. Güdel, H.U. Inorg. Chem., 1983, 22, 3812. 13. Riesen, H.;Güdel,H.U. Inorg. Chem., 1984, 23, 1880. 14. Güdel, H.U.; Snellgrove, T.R. Inorg. Chem., 1978, 17, 1617. 15. Sturge, M.D. Solid State Phys., 1967, 20, 91. 16. Ham, F.S. Phys. Rev. A, 1965, 138, 1727. 17. Wilson, R.B.; Solomon, E.I. J. Amer. Chem. Soc., 1980, 102, 4085. 18. Wilson, R.B.; Solomon, E.I. Inorg. Chem., 1978, 17, 1729. 0

£

state

T

a r e

2

s

2g

a t e

n Cr

NH

+

3 6

RECEIVED November 8, 1985

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2 A b Initio Analysis of Charge Transfer Excitations The Cr(CN) Complex 3-

6

L. G. Vanquickenborne, L. Haspeslagh, and M. Hendrickx Department of Chemistry, University of Leuven, Celestijnenlaan 200F, 3030 Leuven, Belgium For the Cr(CN) transitions (both LMCT and MLCT) have been analyzed at the SCF-level of approximation. Apart from the formal electron transfer taking place in the orbitals that change their occupation numbers, significant density shifts take place in the other orbitals as well. The energy spectrum of the charge transfer transitions is found to be considerably simplified by a level clustering, strongly reminiscent of the related d or d ligand field spectrum. In a previous paper (1), ab initio SCF-calculations using a very large basis set have been reported for the ground state and the ligand field excited states of Cr(CN)§". Figure 1 shows a partial molecular orbital diagram, based on the SCF-orbitals for the d^configuration average. All orbitals are either predominantly ligand based, or predominantly metal-based. In fact, this predominancy is very pronounced, since all the orbitals are 85 or 90 per cent metal or ligand. In this respect, the Hartree-Fock calculations are closer to the pure ligand field picture than to certain extended Huckel calculations (2) where one obtained considerable metal-ligand mixture. Another striking difference between SCF and EH calculations is that the metal orbitals in Figure 1 are obviously not the fron­ tier orbitals. Although this fact is quite well known and has been reported for several other transition metal complexes (3-6), it is particularly relevant in the study of charge transfer transitions. The main reason why the t^-configuration of Figure 1 has a lower energy than any other configuration, including t^g coupled to a hole in any one of the topmost orbitals, is connected to the just mentioned dichotomy between ligand orbitals and metal orbitals. Indeed, the orbitals characterized by predominant ligand (L) charac­ ter are highly delocalized, whereas the 2t2 (3d*)-orbital is almost entirely localized on the chromium metal (M) atom. As a consequence, adding a fourth electron into the 3dTT-shell will lead to an increase of the interelectronic repulsion energy, which is much larger than the decrease resulting0097-6156/ from the86/removal of an electron from a ligand 0307-Ό012$06.00/ 0 ε) 1986 American Chemical Society n+1

n-1

g

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

VANQUICKENBORNE ET AL.

Cr(CN% Complex

Cr(CN)^ ENERGY 2t

2 u

—

9t

l u

— 6e

8ti

τι ( C N " )

3dd(Cr

g

u

—ο

Hlg

- » π (CN"

1*2u 1t g 2

7t

-o

l u

5eg

5d ( C N "

2t. 2g

4e

3dn(Cr)

g

6 t 1u Acf

(CN")

7αι

F i g u r e 1. P a r t i a l and q u a l i t a t i v e m o l e c u l a r o r b i t a l diagram o f the C r ( C N ) g ~ - m o l e c u l e . C i r c l e s i n d i c a t e e l e c t r o n o c c u p a t i o n . The r e l a t i v e o r d e r i s based on t h e s o l u t i o n s o f t h e H a r t r e e - F o c k e q u a t i o n s f o r t h e average o f a l l d - s t a t e s ( 1 ) . 3

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

14

EXCITED STATES AND

REACTIVE INTERMEDIATES

o r b i t a l . N u m e r i c a l l y , the dd-Coulomb i n t e g r a l J(2t2 ,2t2g) = 0.790 h a r t r e e , whereas t y p i c a l l i g a n d Coulomb i n t e g r a l s , o r t y p i c a l m e t a l l i g a n d Coulomb i n t e g r a l s are o f the o r d e r o f 0.2 h a r t r e e . T h e r e f o r e t r a n s i t i o n s o f the type 2 t 8 t 5 -»• 2 t ^ 8 t ^ do not l e a d t o a s t a b i l i z a t i o n ; on the c o n t r a r y , they c o r r e s p o n d t o charge t r a n s f e r (CT) e x c i t a t i o n s , c h a r a c t e r i z e d by a r a t h e r h i g h t r a n s i t i o n energy. S C F - c a l c u l a t i o n s are n o t s u f f i c i e n t l y r e l i a b l e t o p r e d i c t q u a n t i t a t i v e v a l u e s f o r these t r a n s i t i o n e n e r g i e s . Rather, they s h o u l d be used t o answer some v e r y g e n e r a l q u e s t i o n s , such as : are the dd bands lower than the CT bands a l s o a t the H a r t r e e - F o c k l e v e l o f a p p r o x i m a t i o n ? Do the SCF r e s u l t s reproduce the q u a l i t a t i v e f e a t u r e s o f l i g a n d f i e l d t h e o r y ? I s t h e r e a p a t t e r n i n the energy o f the CT l e v e l s ? g

2 g

Ligand F i e l d

u

g

u

Considerations

A l t h o u g h the main emphasi u s e f u l to consider f i r s w i t h i n the t | - c o n f i g u r a t i o n , as shown i n F i g u r e 2. In the framework of l i g a n d f i e l d t h e o r y ( p u r e l y atomic d - o r b i t a l s ) , the E g and ^ i g s t a t e s would be degenerate, and the r e m a i n i n g gaps ( E , T i - A2g)/( T - A 2 ) a r e determined by the Racah B, C parameters; they a r e p r e d i c t e d t o be i n the r a t i o 3/5, which i s r e a s o n a b l e i f compared w i t h the e x p e r i m e n t a l r a t i o o f about 0.7. F i g u r e 2 goes beyond l i g a n d f i e l d t h e o r y and i s based on the assumption t h a t the t 2 g - o r b i t a l s are ( p r e d o m i n a n t l y metal-centered) m o l e c u l a r o r b i t a l s . At the l e f t hand s i d e o f F i g u r e 2, the f o u r s t a ­ tes a r e d e s c r i b e d by one s i n g l e s e t o f m o l e c u l a r o r b i t a l s . L e t us suppose t h a t these o r b i t a l s a r e s o l u t i o n s o f the H a r t r e e - F o c k equa­ t i o n s f o r the A2g ground s t a t e . In o r d e r t o d e s c r i b e the i n t e r e l e c t r o n i c r e p u l s i o n w i t h i n these o r b i t a l s , i t i s n o t p o s s i b l e t o work w i t h the two Racah parameters, which a r e based on the s p e c i a l r o t a ­ t i o n a l p r o p e r t i e s o f the atomic d - o r b i t a l s . I n s t e a d , one must r e c u r to the G r i f f i t h parameters ( i n t h i s case a, b and j ) . At the l e f t hand s i d e o f F i g u r e 2 ( f r o z e n o r b i t a l a p p r o x i m a t i o n ) the energy s p l i t t i n g s a r e f u n c t i o n s o f these 3 parameters. C o n c e p t u a l l y and q u a l i t a t i v e l y , the f r o z e n o r b i t a l a p p r o x i m a t i o n i s v e r y s i m i l a r t o the pure l i g a n d f i e l d p i c t u r e . The r i g h t hand s i d e o f F i g u r e 2 shows the r e s u l t o f a complete S C F - c a l c u l a t i o n , where the H a r t r e e - F o c k e q u a t i o n s have been s o l v e d f o r each s t a t e s e p a r a t e l y . T h e r e f o r e , each s t a t e i s now c h a r a c t e r i z e d by i t s own s e t o f o p t i m a l o r b i t a l s . S i n c e a H a r t r e e - F o c k c a l c u l a t i o n i s b a s i c a l l y a v a r i a t i o n a l treatment, the r e l a x a t i o n o f the o r b i t a l s to t h e i r o p t i m a l shape causes the energy t o drop somewhat i n g o i n g from the l e f t t o the r i g h t on the diagram (except f o r A 2 ) . One con­ sequence i s t h a t the t r a n s i t i o n e n e r g i e s can no l o n g e r be d e s c r i b e d by means o f s i m p l e e x p r e s s i o n s as i n the f r o z e n o r b i t a l scheme : each s t a t e i s now c h a r a c t e r i z e d by i t s own s e t o f G r i f f i t h parame­ t e r s . But a l s o the o t h e r o r b i t a l s , more s p e c i f i c a l l y the l i g a n d o r ­ b i t a l s , and even the c o r e o r b i t a l s a r e s l i g h t l y m o d i f i e d , and con­ t r i b u t e t o the t r a n s i t i o n energy : a l l s i m p l i c i t y appears t o be l o s t , at l e a s t f o r m a l l y . The main s i m p l i f i c a t i o n t h a t i s l e f t however, i s t h a t the r e l a x a t i o n energy i s - n u m e r i c a l l y - v e r y s m a l l i n d e e d , b e i n g o f the o r d e r o f a few hundreds o f c m . T h e r e f o r e , the g l o b a l 2

2

2

g

4

2

g

T

4

2 g

g

4

4

g

-1

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Cr(CNf

2. VANQUICKENBORNE ET AL.

15

Complex

6

MOT

(d )

'2g • a - b + 3j

^5

2 a-b-2j [

E

2g

9

Mg

-|(a-b+3j)

~3

^2g

a,b,j ·. G r i f f i t h a =J

parameters

b=J '

f t

t t

j=K ' t t

F i g u r e 2. R e l a x a t i o n o f t s t a t e s . L e f t : e n e r g i e s a r e based on the f r o z e n o r b i t a l s o f t h e ^ 2g g state. Right : r e s u l t s o f f u l l - s c a l e SCF c a l c u l a t i o n s . The e n e r g i e s a r e n o t drawn t o s c a l e . g

A

r

o

u

n

d

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

16

EXCITED STATES AND

REACTIVE INTERMEDIATES

p a t t e r n , and the r e l a t i v e e n e r g i e s a t the r i g h t hand s i d e o f F i g u r e 2 a.re v i r t u a l l y i d e n t i c a l t o the l e f t hand s i d e . Yet, the r e l a x a t i o n p r o c e s s i s i m p o r t a n t from another p o i n t o f view. In the f r o z e n o r b i t a l a p p r o x i m a t i o n , the d o u b l e t s t a t e s have h i g h e r r e p u l s i o n e n e r g i e s than the ground s t a t e . S i n c e the o r b i t a l s a r e o n l y o p t i m a l f o r the ground s t a t e s i t u a t i o n , the o r b i t a l shape w i l l not be i d e a l l y adapted t o t h i s i n c r e a s e d r e p u l s i o n . T h e r e f o r e , the essence o f the r e l a x a t i o n p r o c e s s can be d e s c r i b e d as a r e s h a p i n g o f the o r b i t a l s so as t o r e s t o r e the b a l a n c e . One o f the more impor­ t a n t f a c t o r s i n t h i s r e s h a p i n g p r o c e s s i s an expansion o f the d - o r ­ b i t a l s . S i n c e a H a r t r e e - F o c k c a l c u l a t i o n y i e l d s an energy minimum, a s m a l l e x p a n s i o n means n e x t t o n o t h i n g f o r the t o t a l energy, but i t means v e r y much f o r the energy components V ( p o t e n t i a l energy) and Τ ( k i n e t i c e n e r g y ) . T a b l e I shows the r e s u l t s f o r the i n t e r e l e c t r o n i c r e p u l s i o n energy C (V = C + L, where L i s the n u c l e a r - e l e c t r o n a t ­ t r a c t i o n energy). Table

I : Interelectroni a p p r o x i m a t i o n (C) and i n the SCF a p p r o x i m a t i o n ( C ' ) ( _ l ) . A l l e n e r g i e s are r e l a t i v e t o the ground s t a t e r e p u l s i o n .

State

2

C (cm

T

1

)

C(cm

1

)

33 502

-

91 047

20 111

-

53 187

20 095

-

52 821

2g 2

E g

\

0

0

O b v i o u s l y , the e x p a n s i o n ( w h i l e n o t a f f e c t i n g the t o t a l energy to any s i g n i f i c a n t e x t e n t ) i s so i m p o r t a n t t h a t i t overcompensates the o r i g i n a l change i n i n t e r e l e c t r o n i c r e p u l s i o n : C i s smaller i n the e x c i t e d s t a t e s than i n the ground s t a t e . T h i s r a t h e r thorough m o d i f i c a t i o n o f the c o n c e p t u a l framework o f l i g a n d f i e l d t h e o r y (and a l s o o f atomic m u l t i p l e t t h e o r y ) i s d i s c u s s e d more f u l l y i n o t h e r p u b l i c a t i o n s (1,7)· A p p a r e n t l y , the c o n v e n t i o n a l models o f t r a n s i t i o n metal c h e m i s t r y y i e l d reasonable p r e d i c t i o n s of t r a n s i t i o n energies on the b a s i s o f p h y s i c a l l y unsound assumptions. I f one i s o n l y i n t e ­ r e s t e d i n the energy p a t t e r n , l i g a n d f i e l d t h e o r y remains a r e l i a b l e g u i d e . I f one i s i n t e r e s t e d i n the r e a s o n why a c e r t a i n energy p a t ­ t e r n emerges, l i g a n d f i e l d t h e o r y s h o u l d be abandoned ( 1 , 7 ) . Charge T r a n s f e r T r a n s i t i o n s : G e n e r a l

Considerations

The C T - e x c i t e d s t a t e which was d i s c u s s e d i n the i n t r o d u c t i o n , i s o f the l i g a n d t o metal type (LMCT) : 8 t 2 t . I f i t s energy i s c a l ­ c u l a t e d by u s i n g the ground s t a t e o r b i t a l s , one o b t a i n s a v e r y h i g h v a l u e (more than 110 000 c m " ) ; the e x p e r i m e n t a l CT bands s t a r t a t much lower energy (*\, 40 000 c m ) ( 8 ) . I f a more complete SCF c a l c u ­ l a t i o n i s c a r r i e d out (by s o l v i n g a l s o the SCF e q u a t i o n s f o r the CT l u

2 g

1

-1

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2.

VANQUICKENBORNE ET AL.

Cr(CN%

17

Complex

e x c i t e d s t a t e and u s i n g the same b a s i s s e t as b e f o r e ( 1 ) , the t r a n s i ­ t i o n energy drops t o 68 000 cm~^. The r e l a x a t i o n energy, which was n e g l i g i b l e from one l i g a n d f i e l d s t a t e t o a n o t h e r one, amounts t o more than 40 000 c m from the ground s t a t e t o a CT s t a t e . The main r e a s o n i s t h a t the t r a n s f e r o f an e l e c t r o n from the o u t s i d e o f the m o l e c u l e toward the c e n t e r i n d u c e s i m p o r t a n t s e c o n d a ­ ry s h i f t s i n the o t h e r o r b i t a l s as w e l l . A f i r s t i n d i c a t i o n o f t h e s e s h i f t s can be o b t a i n e d from a s t a n d a r d M u l l i k e n p o p u l a t i o n a n a l y s i s , as shown i n T a b l e I I . In each c a s e , the a c t u a l s h i f t s are much smal-1

T a b l e I I . P o p u l a t i o n s h i f t s upon

L

M

CT-excitation

Cr

6

CN

hypothetical

8

t

X t

*

2 t

2g

2u *

2 t

2g

lu

M + L

2 t

2g

2 t

2g *

*

9

2 t

t

lu 2u

+ 0. ,60

- 0. ,60

+ 0. ,50

- 0. .50

- 1

+ 1

+ 0. .22

- 0..22

-

+ 0..13

0. .13

1er than the f o r m a l t r a n s f e r o f one charge u n i t , i n d i c a t i n g t h a t the r e l a x a t i o n e f f e c t s are c h a r a c t e r i z e d by a s i g n i f i c a n t b a c k f l o w o f e l e c t r o n s . The r e l a x a t i o n i s seen t o be more i m p o r t a n t f o r the MLCT t r a n s i t i o n s , l e a d i n g i n one case ( 2 t + 9 t i ) t o the c o u n t e r i n t u i ­ t i v e r e s u l t shown i n T a b l e I I , where the f o r m a l charge on the m e t a l increases. This suggests that r e l a x a t i o n e f f e c t s i n t h i s p a r t i c u l a r case overcompensate the o r i g i n a l l y i n d u c e d change. S i n c e we use a v e r y l a r g e b a s i s s e t , c o n t a i n i n g a number o f r a t h e r d i f f u s e f u n c ­ t i o n s , t h i s r e s u l t might be an a r t e f a c t o f the M u l l i k e n population analysis. 2 e

u

D e n s i t y d i f f e r e n c e p l o t s are perhaps more i n f o r m a t i v e , as can be seen i n F i g u r e 3 f o r one o f the LMCT s t a t e s . The f i g u r e shows the d i f f e r e n c e between the two o r b i t a l d e n s i t i e s f o r which the population has been changed. The f i g u r e c o r r e s p o n d s t o the s i m p l e p i c t u r e one would g e t from the c l a s s i c a l i d e a s : the l i g a n d π-zone i s d e p o p u l a t e d and the metal d-rr-zone i s p o p u l a t e d . From a c l a s s i c a l p o i n t o f view, one might be i n c l i n e d t o e x p e c t t h a t t h i s f i g u r e d e s c r i b e s the e s s e n ­ ce o f the charge t r a n s f e r phenomenon. Yet, the t o t a l d e n s i t y d i f f e ­ rence i n F i g u r e 4 r e v e a l s a c o m p l e t e l y d i f f e r e n t p i c t u r e : i t shows the g l o b a l d e n s i t y s h i f t s upon e x c i t a t i o n . Δρ i s made up o f two c o n ­ t r i b u t i o n s : f i r s t the f o r m a l o r b i t a l d e n s i t y d i f f e r e n c e ( F i g u r e 3 ) , and next the d e n s i t y s h i f t s , a s s o c i a t e d w i t h the r e l a x a t i o n t a k i n g p l a c e i n the o t h e r o r b i t a l s . O b v i o u s l y , f i g u r e 4 r e t a i n s some o f the

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

CTLM ( L t

2 u

— Mt ] 2g

ρ (Mt )-p(Lt 2g

2u

)

200

Figure dinate dotted equals

10.00

8.00

3. O r b i t a l d e n s i t y d i f f e r e n c e p l o t Δρ i n one o f t h e c o o r ­ p l a n e s . F u l l l i n e s : Δρ > 0; dashed l i n e s : Δρ < 0; l i n e s : Δρ = 0. Δρ a t the d i f f e r e n t i s o d e n s i t y lines ± 0.16, ± 0.08, ± 0.04, ... ± 0.0025.

10.00

CTLM (Lt +Mt ) 2u

p('T ,^ t u

F i g u r e 4. T o t a l i n F i g u r e 3.

2g

density difference

2g

5

2u

)-p( A

plot;

4

2gi

t

3

2g

)

the c o n v e n t i o n s a r e as

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2. VANQUC IKENBORNE ET AL.

Cr(CN%

19

Complex

c h a r a c t e r i s t i c s o f F i g u r e 3, b u t many o t h e r s h i f t s a r e c o m p l i c a t i n g the p i c t u r e . S i m i l a r diagrams can be o b t a i n e d f o r MLCT-states. In t h e 2t^ + 9t c a s e , t h e f o r m a l o r b i t a l d e n s i t y d i f f e r e n c e does c o r r e s ­ pond t o trie c l a s s i c a l p i c t u r e ; as a matter o f f a c t , i t i s q u i t e s i m i ­ l a r t o F i g u r e 3, except f o r t h e r e v e r s a l between d o t t e d l i n e s and f u l l l i n e s , i n d i c a t i n g a d e p l e t i o n o f t h e metal dïï-orbitals and a p o p u l a t i o n i n c r e a s e i n t h e l i g a n d π-region. In t h e t o t a l d e n s i t y d i f ­ f e r e n c e map, remnants o f t h e metal d i T - d e p l e t i o n can s t i l l be o b s e r ­ ved, b u t one a l s o f i n d s a d e p o p u l a t i o n o f t h e o u t e r l i g a n d r e g i o n , which seems t o p o i n t i n t h e same d i r e c t i o n as t h e s u r p r i s i n g r e s u l t s o f t h e p o p u l a t i o n a n a l y s i s . In a l l c a s e s , t h e c l a s s i c a l p i c t u r e i s o n l y c o n f i r m e d , i f we r e s t r i c t o u r s e l v e s t o t h e d e n s i t y d i f f e r e n c e s between t h e o r b i t a l s d i r e c t l y i n v o l v e d i n t h e t r a n s i t i o n . I f we con­ s i d e r the t o t a l d e n s i t y d i f f e r e n c e s , the r e l a x a t i o n e f f e c t s i n the o t h e r o r b i t a l s a r e always v e r y i m p o r t a n t and always c o r r e s p o n d t o density s h i f t s o f opposit not seem t o f o l l o w any s i m p l t r a t e d i n one p a r t i c u l a r s e t o f m o l e c u l a r o r b i t a l s . I t i s as i f each n u c l e u s tends - by whatever c h a n n e l s a v a i l a b l e - t o m a i n t a i n r o u g h l y the same charge i n i t s immediate n e i g h b o r h o o d . Energy P a t t e r n

o f Charge T r a n s f e r

Transitions

As i n d i c a t e d b e f o r e , SCF t h e o r y has no problem i n s i t u a t i n g t h e CT t r a n s i t i o n s w e l l above t h e l i g a n d f i e l d t r a n s i t i o n s , and i n t h i s sense, i t f i t s i n c o m p l e t e l y w i t h t h e t r a d i t i o n a l i d e a s . As f o r t h e r e l a t i v e p o s i t i o n o f d i f f e r e n t CT t r a n s i t i o n s , many problems remain to be s o l v e d . C o n s i d e r f o r example t h e MLCT e x c i t e d c o n f i g u r a t i o n corresponding to 2 t + 9 t . T a b l e I I I shows the 15 s t a t e s r e s u l ­ t i n g from t h e s i n g l e tpg^lu ^ i g u r a t i o n . In an attempt t o d i s c o v e r a p a t t e r n i n t h i s m u l t i t u d e o f s t a t e s , a s i m p l i f y i n g model has been 2 g

l u

c o n

Table I I I . D i f f e r e n t s t a t e s corresponding to the 2 t ^ configuration

24 l

ig x

2g l

2 T„ lu m

24 24 24 lu' 2u' lu' u 2 2 Τ Τ lu» 2u 2 2 2 2 ^Α , Ε , Ί , ^Τ. 2u u lu 2u A

0

2g

Λ

lu

i n t r o d u c e d (9,10), based on t h e d i s t i n c t i o n between weak and s t r o n g i n t e r a c t i o n s . The tt^o - e l e c t r o n s a r e l o c a l i z e d on t h e metal and t h e r e ­ &

f o r e t h e i r r e p u l s i o n s h o u l d be r e l a t i v e l y l a r g e . The t ^ - e l e c t r o n i s s i t u a t e d on t h e l i g a n d s , a t t h e o u t s i d e o f t h e complex and r e l a t i v e l y f a r from t h e m e t a l . T h e r e f o r e , i t was b e l i e v e d t h a t t h i s e l e c t r o n s h o u l d be c o u p l e d o n l y weakly t o t h e metal π-electrons. I f t h i s hypo­ t h e s i s h o l d s t r u e , t h e f i f t e e n s t a t e s can be e x p e c t e d t o e x h i b i t a

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

20

EXCITED STATES AND

REACTIVE INTERMEDIATES

v e r y s i m p l e p a t t e r n , where we have l a r g e energy s e p a r a t i o n s between the f o u r m e t a l - c e n t e r e d s t a t e s , and where the f i n a l d o u b l e t s and q u a r t e t s d e v i a t e o n l y s l i g h t l y from t h e i r p a r e n t m e t a l s t a t e . In o t h e r words, i n the MLCT s t a t e o f a d system, we s h o u l d r e c o g n i z e the s t r u c t u r e o f the l i g a n d f i e l d spectrum o f a d -complex. T h i s mo­ d e l has been a p p l i e d t o a whole s e r i e s o f examples (9,10) and a more r e c e n t and complete r e v i e w has been g i v e n by L e v e r ( 1 1 ) . 3

2

From the p r e s e n t p o i n t o f view, i t would be i n t e r e s t i n g t o v e r i ­ f y i f the model i s c o n f i r m e d by the H a r t r e e - F o c k c a l c u l a t i o n s . In the o r i g i n a l papers (9,10), the a p p l i c a b i l i t y o f the model was l i m i t e d by the o b s e r v a b i l i t y o f the e x p e c t e d energy p a t t e r n ; t h e o b s e r v a b i l i t y had t o be v e r i f i e d i n each p a r t i c u l a r case by assuming an e l e c t r i c d i p o l e t r a n s i t i o n mechanism. In c a r r y i n g out S C F - c a l c u l a t i o n s how­ e v e r , one i s f r e e d from t h e s e l i m i t a t i o n s , and one can v e r i f y the model f o r an a r b i t r a r y c a s e , i n c l u d i n g the case o f T a b l e I I I . F i g u ­ r e 5 shows an SCF energy diagram f o r the CT t r a n s i t i o n s under c o n s i d e r a t i o n . And i n d e e d , e x a c t l y the p a t t e r n p r e d i c t e t h r e e groups o f c l o s e l y spaced l e v e l s , s e p a r a t e d by r e l a t i v e l y l a r g e energy gaps. The c l o s e l y spaced l e v e l s have the same metal p a r e n t a g e . The r e l a t i v e p o s i t i o n o f the p a r e n t a g e a v e r a g e s c o r r e s p o n d s t o the l i g a n d f i e l d s t a t e s o f a t w o - e l e c t r o n system, s a t i s f y i n g the same q u a l i t a t i v e r e l a t i o n s h i p s . The E and *T s t a t e are n e a r l y , a l b e i t not c o m p l e t e l y d e g e n e r a t e . The r a t i o between the two l a r g e r energy gaps i s 0.41 i n s t e a d o f the l i g a n d f i e l d v a l u e o f 0.4. In a p r e v i o u s S e c t i o n , i t has been shown how the l i g a n d f i e l d energy p a t t e r n was c o n f i r m e d a t the SCF l e v e l , but how the under­ l y i n g p h y s i c a l r e a s o n was t h o r o u g h l y m o d i f i e d . The same s i t u a t i o n i s found t o h o l d t r u e f o r the C T - t r a n s i t i o n s : the s t a t e s w i t h T ^ pa­ r e n t a g e have the l o w e s t t o t a l energy, but are c h a r a c t e r i z e d by the l a r g e s t i n t e r e l e c t r o n i c r e p u l s i o n ( T a b l e I V ) . Another r a t h e r s t r i k i n g example o f the r o l e o f i n t e r e l e c t r o n i c r e p u l s i o n energy i s p r o v i d e d by T a b l e V, where two d i f f e r e n t CT c o n f i g u r a t i o n s a r e compared. The t^e (^E ) p a r e n t a g e c o r r e s p o n d s to a h i g h s p i n s t a t e w h i l e the t ( T 7 p a r e n t a g e c o r r e s p o n d s t o a low s p i n s t a t e . The SCF e x c i t a ­ t i o n e n e r g i e s are q u i t e c l o s e t o each o t h e r , but, c o n t r a r y t o the c l a s s i c a l e x p e c t a t i o n s , the low s p i n s t a t e i s c h a r a c t e r i z e d by the l o w e s t r e p u l s i o n energy. 1

3

1

4

l g

T a b l e IV. T o t a l SCF energy (Ε') and r e p u l s i o n energy ( C ) o f the pa­ r e n t a g e averages o f the 2^ 9 t ^ CT s t a t e s . A l l e n e r g i e s are r e l a t i v e t o the T^ _ - v a l u e s u

3

Parentage average

34

L

2g lg

1

Ε'(cm

)

C

λ

(cm

)

903

- 23

268

14 372

- 8

685

14 358

- 8

566

0

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

0

Cr(CNf

VANQUICKENBORNE ET AL.

6

Complex

2

2

ΤM u . Τ 'av

'Ta, : 2

4'

Eg

A

2u

2

Eu

^T

I

U 2

\ 2g 2

l

t 1

\

1

I

U

\ 2

Λ Γ T 1 u ' u ' 'av 2

A

2

L

4

A

l u

4

Eu

V T, 4

Configuration Average

Parentage Average

CT States

F i g u r e 5. SCF Energy l e v e l diagram o f t h e d i f f e r e n t CT s t a t e s c o r r e s p o n d i n g t o t h e t | t ^ - c o n f i g u r a t i o n . The e n e r g i e s a r e n o t drawn t o s c a l e . The symëol T r e f e r s t o t h e average energy o f 2 and ~ 2 u ' Roothaan's open s h e l l f o r m a l i s m does n o t a l l o w t h e s e s t a t e s t o be c a l c u l a t e d s e p a r a t e l y ( 1 ) ; o n l y t h e i r average energy i s a c c e s s i b l e . The same remark h o l d s f o r one o f t h e e n ­ t r i e s i n T a b l e V. The i n t r a c o n f i g u r a t i o n a l energy gaps a r e some 30 t i m e s s m a l l e r than t h e i n t e r c o n f i g u r a t i o n a l energy gaps. u

2

a v

T

T l u

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES 22 Table V. SCF excitation energy Δ Ε and repulsion ΔC for two LMCT states; ^T^ refers to the average of T and T 1

4

4

l u

Configuration and state

2u

λ

ΔΕ'(cm )

Δ C(cm )

75 201

1 036 845

75 994

837 016

1

As a conclusion, Hartree-Fock calculations are seen to be qualitatively compatible with the simple models (ligand field theory for dd-transitions, and the However, the ab initio wor situated in a different conceptual framework. Literature Cited 1. Vanquickenborne, L.G.; Haspeslagh, L.; Hendrickx, M.; Verhulst, J. Inorg. Chem. 1984, 23, 1677-84 2. Alexander, J.J.; Gray, H.B. Coord. Chem. Rev. 1967, 2, 29-43 3. Demuynck, J.; Veillard, Α.; Vinot, G. Chem. Phys. Lett. 1971, 10, 522-5. 4. Wachters, A.H.J.; Nieuwpoort, W.C. Phys. Rev. B, 1972, 5, 4291-301. 5. Sano, M.; Yamatera, H.; Hatano, Y. Chem. Phys. Lett. 1979, 60, 257-60. 6. Sano, M.; Kashiwagi, H.; Yamatera, H. Inorg. Chem. 1982, 21, 3837-41. 7. Vanquickenborne, L.G.; Haspeslagh, L. Inorg. Chem. 1982, 21, 2448-54. 8. Alexander, J.J.; Gray, H.B. J. Am. Chem. Soc. 1968, 90, 4260-71. 9. Vanquickenborne, L.G.; Verdonck, E. Inorg. Chem. 1976, 15, 454-61. 10. Verdonck, E.; Vanquickenborne, L.G. Inorg. Chim. Acta 1977, 23, 67-76. 11. Lever, A.B.P. "Inorganic Electronic Spectroscopy"; Elsevier Science Publishers B.V. : Amsterdam, 1984. RECEIVED November 8,

1985

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

3 Excited State Geometries of Coordination Compounds Obtained from Vibronic Spectra and Photon Flux Fluctuation Measured by Time Resolved Spectroscopy Hans-Herbert Schmidtke Institut für Theorestische Chemie der Universität, D-4000 Düsseldorf 1, Universitätsstraβe 1, Federal Republic of Germany The intensit vibronic spectr emission at low temperature are used to de­ termine the geometric distortions of the electronically excited states of coordina­ tion compounds. In particular for complexes of lower symmetry, band analysis is neces­ sary leading to results with which bond dis­ tance changes can be calculated. For spec­ tra exhibiting no vibrational fine struc­ ture, a new technique is proposed which uses time resolved methods, considering devia­ tions from the Poisson distribution of pho­ tons by recording time intervals between two successively emitted photons. Photochemical reactivity primarily results from electron distributions which are different to those in the ground state. With a change of electron density the geometry of the excited molecule may be distorted from that of the ground state molecule. These excited states,provided with large amounts of excess energy, are short lived species which are difficult to characterize. Since quan­ tum chemistry is not able to calculate molecules of the size we are interested in ( i t also cannot satisfactorily consider cooperative effects resulting from interaction with the environment) one i s restricted to experimental investigation. In particular some spectroscopic methods are fast enough to follow the physical conversions taking place i n the molecule, by detecting the excited state species and by measuring at least some of i t s properties. Vibronic spectra reflect changes in the electronic and vibrational state of a molecule at the same time. It is possible to calculate the geometry of the excited species and the potential hypersurface close to the equi­ librium state. For this, a spectrum i s required with sufficiently well resolved vibronic structure to carry 0097-6156/ 86/ 0307-0023506.00/ 0 © 1986 American Chemical Society In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND

24

REACTIVE INTERMEDIATES

out a b a n d a n a l y s i s . T h i s s h o u l d a l l o w f o r d e c o m p o s i t i o n o f t h e bands i n t o d i f f e r e n t e l e c t r o n i c t r a n s i t i o n s and a f u r t h e r r e s o l u t i o n of the v i b r a t i o n a l f i n e s t r u c t u r e . U s u a l l y , the e l e c t r o n i c s p e c t r a of t r a n s i t i o n metal coor­ d i n a t i o n compounds i n s o l u t i o n , o r i n t h e s o l i d s t a t e , e x h i b i t r e l a t i v e l y b r o a d o v e r l a p p i n g b a n d s , w h i c h do n o t show any s i g n o f v i b r a t i o n a l f i n e s t r u c t u r e . However, t h e r e a r e many c a s e s known where d i s t i n c t v i b r a t i o n a l s t r u c t u r e i s d e t e c t e d although o f t e n , the s t r u c t u r e i s not completely r e s o l v e d . T h i s has l e d some a u t h o r s , on the b a s i s o f such s p e c t r a , to e x p l a i n t h e i r r e s u l t s , i n an i n c o r r e c t f a s h i o n . To a v o i d m i s i n t e r p r e t a t i o n one i s urged to consider only those s p e c t r a w i t h optimal reso­ l u t i o n . T h i s i s , f o r i n s t a n c e , a c h i e v e d when t h e v i b r a ­ t i o n a l quanta measured from a band p r o g r e s s i o n i n a l u m i n e s c e n c e s p e c t r u m a g r e e w i t h t h e f u n d a m e n t a l modes o f a v i b r a t i o n a l spectru (Raman o r I R ) . The MIM i . e . t h e a b s e n c e o f n o r m a l modes i n t h e v i b r a t i o n a l p r o ­ g r e s s i o n i n t e r v a l s (1 « 2 ) , may e x i s t on v a r i o u s occasions where t h e damping i n t h e d i s s i p a t i v e s y s t e m i s t o o l a r g e t o d e t e c t s e p a r a t e modes. However, one c a n n o t e x c l u d e t h e p o s s i b i l i t y t h a t an u n u s u a l p r o g r e s s i o n a l f r e q u e n c y i s s i m u l a t e d by i n c o m p l e t e r e s o l u t i o n i m p o s e d on t h e s y s t e m by i n s u f f i c i e n c i e s i n t h e e x p e r i m e n t . To o b t a i n i n g h i g h q u a l i t y , w e l l r e s o l v e d a b s o r p t i o n o r e m i s s i o n s p e c t r a , v a r i o u s t e c h n i q u e s h a v e b e e n ap­ plied. e.g. ( l ) d e c r e a s i n g t h e t e m p e r a t u r e o f t h e p r o b e , i f n e c e s s a r y b e l o w t h e A p p o i n t o f l i q u i d He (^2 K) investigating single crystals c o n s i d e r i n g d o p e d chromophore m a t e r i a l s w i t h (inert) host c r y s t a l s ( 4 ) d i l u t i n g t h e chromophore t o be i n v e s t i g a t e d w i t h appropriate counter ions or l o o k i n g at double salts e, t.g. [CoiNH U { j r ( C N ) ] Q ) or £Cr(NH^) J(C10 ) Cl.KCl (4) 5) u s i n g p o l a r i z e d l i g K t ' ! 6) i m p r o v i n g t h e a p p a r a t u s t o be u s e d (monochromators, d e t e c t i o n systems, photon c o u n t i n g , micro-optics etc.). However, i n many c a s e s , t h e s e e f f o r t s may n o t l e a d t o any r e s o l u t i o n o f t h e v i b r a t i o n a l f i n e s t r u c t u r e . T h i s o b v i o u s l y has a p h y s i c a l r e a s o n . S i n c e condensed systems a r e i n v e s t i g a t e d , i n t e r a c t i o n w i t h the e n v i r o n ­ ment i s i n v o l v e d i n t h e t r a n s i t i o n . The chromophore i s an open s y s t e m w h i c h d i s s i p a t e s v i b r a t i o n a l e n e r g y i n t o t h e s u r r o u n d i n g medium by i r r e v e r s i b l e p r o c e s s e s . T h i s phenomenon c a n be u s e d f o r d e t e c t i n g f i n e s t r u c t u r e f r o m t h e t i m e r e s o l v e d measurements o f p h o t o n e v e n t s , by m o n i t o r i n g t h e c o r r e l a t i o n s between s u c c e s s i v e l y e m i t t e d p h o t o n s . T h i s new t e c h n i q u e w i l l be r e p o r t e d i n t h e second p a r t of t h i s a r t i c l e .

is)

6

é

4

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

3.

SCHMD ITKE

25 Excited State Geometries of Coordination

Conventional Vibronic

Compounds

Spectra

H e r e we w i l l d i s c u s s t h e v i s i b l e a n d UV a b s o r p t i o n a n d e m i s s i o n s p e c t r a o f some s e l e c t e d t r a n s i t i o n m e t a l a n d m a i n g r o u p c o o r d i n a t i o n compounds* T h e s p e c t r a a r e due to e l e c t r o n i c t r a n s i t i o n s and e x h i b i t e x t e n s i v e v i b r a ­ t i o n a l f i n e s t r u c t u r e i n l o n g band p r o g r e s s i o n s super­ i m p o s e d on e a c h o t h e r . The t r a n s i t i o n metal d-d t r a n s i ­ t i o n s become a l l o w e d b y a c o m p l i c a t e d c o u p l i n g mechanism, which mixes l e v e l s o f d i f f e r e n t p a r i t y by a v i b r o n i c cou­ p l i n g operator, and d i f f e r e n t s p i n s t a t e s by a s p i n - o r b i t coupling operator. These intermixings prepare t h e i n i t i a l and f i n a l s t a t e s o f t h e t r a n s i t i o n f o r a p p r o p r i a t e sym­ metry s e l e c t i o n r u l e s . V i b r a t i o n a l p r o g r e s s i o n s and broad band spectra (in cases where r e s o l u t i o n i s n o t a c h i e v e d ) a r e explained, i n general by s h i f t s o f t h e p o t e n t i a l energy curves along s p e c i f i l a r g e c h a n g e s o f bon of t h e molecule t o a h i g h e r s t a t e (Figure 1). These s t a t e s a r e l i k e l y t o be p h o t o a c t i v e . The t h e o r e t i c a l a n a l y s i s o f t h e s p e c t r u m t o o b t a i n i n g i n f o r m a t i o n a b o u t t h e e x c i t e d s t a t e s t r u c t u r e , must s t a r t f r o m a d e f i n i t e a s s i g n m e n t o f b a n d components a n d a r e l i ­ a b l e band a n a l y s i s c a r r i e d out b y r e s o l v i n g t h e band p r o ­ g r e s s i o n i n t o g a u s s i a n o r l o r e n t z i a n p r o f i l e s . The impor­ t a n c e o f g o o d b a n d a n a l y s e s s h o u l d n o t be u n d e r e s t i m a t e d i f t h e i n t e n s i t y d i s t r i b u t i o n o f t h e measured s p e c t r a i s compared w i t h t h e t h e o r e t i c a l b a n d p r o f i l e f u n c t i o n . A band system which remains u n d e t e c t e d under t h e main p r o ­ g r e s s i o n may l e a d t o d i f f e r e n t r e s u l t s when c a l c u l a t i n g geometry d i s t o r t i o n s . T h e o r e t i c a l band p r o f i l e s a r e u s u a l l y o b t a i n e d from Franck-Condon f a c t o r s which a r e a d j u s t e d t o t h e band p e a k s o f v i b r a t i o n a l components i n t h e p r o g r e s s s i o n . In o u r b a n d a n a l y s e s , we a r e u s i n g d i s t r i b u t i o n f u n c t i o n s I j (ιη,η;Δ, β) w h i c h c o l l e c t F r a n c k - C o n d o n i n t e g r a l s a n d Herzberg-Teller f a c t o r s i n t o a comprehensive f u n c t i o n w h i c h h a s more g e n e r a l a p p l i c a b i l i t y t h a n e a r l i e r b a n d analysis procedures. T h e method c a n b e u s e d f o r j - f o l d d e g e n e r a t e v i b r a t i o n s , where t h e v i b r a t i o n a l q u a n t a may be d i f f e r e n t f r o m t h e g r o u n d s t a t e ^ = *? / φ 1 and f o r v i b r a t i o n a l e x c i t a t i o n i n t h e i n i t i a l s t a t e m>0, b y w h i c h t e m p e r a t u r e dpendence w i l l be i n t r o d u c e d i n t o t h e .band p r o f i l e f u n c t i o n (5-7) · T h e p a r a m e t e r Δ = (/oM/ii) ' ΔΟ. s u p p l i e s t h e s h i f t o f t h e p o t e n t i a l c u r v e minimum o f t h e e x c i t e d s t a t e compared t o t h e g r o u n d s t a t e . We s h a l l a p p l y t h e s e f u n c t i o n s t o some o f t h e s p e c t r a discussed below. e

Emission Spectra. The chance t o o b t a i n v i b r a t i o n a l f i n e s t r u c t u r e i s u s u a l l y h i g h e r f o r luminescence than f o r absorption spectra since i n emission, absolute l i g h t i n t e n s i t i e s w i t h h i g h s e n s i t i v i t i e s a r e measured r a t h e r than i n t e n s i t y d i f f e r e n c e s . However, few compounds e x ­ h i b i t l u m i n e s c e n c e i n t e n s e enough t o measure a r e l i a b l e

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

26

EXCITED STATES AND REACTIVE INTERMEDIATES

F i g u r e 1· P o t e n t i a l e n e r g y c u r v e s i n t h e t o t a l l y sym­ m e t r i c s u b s p a c e Q-j i n t h e c a s e o f o c t a h e d r a l d^ s y s t e m s and r e s u l t i n g band shapes o f o p t i c a l s p e c t r a .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

3.

SCHMD ITKE

27 Excited State Geometries of Coordination Compounds

s p e c t r u m . A l s o , i n many c a s e s , o n l y e m i s s i o n f r o m t h e l o w e s t e l e c t r o n i c s t a t e can be o b s e r v e d , due t o r a d i a t i o n l e s s d e a c t i v a t i o n from h i g h e r e x c i t e d s t a t e s . t r f R h py^Clg] CI: The powder e m i s s i o n s p e c t r u m o f t h i s compound a t 2 Κ (immersed i n l i q u i d H e l i u m ) has b e e n r e c o r d e d by C r o s b y and c o w o r k e r s (8)· A s i n g l e progres­ s i o n i n 350 cm"" has b e e n f o u n d , w h i c h does n o t c o r r e ­ spond t o any o f t h e v i b r a t i o n a l f r e q u e n c i e s o b t a i n e d f r o m a v i b r a t i o n a l spectrum of the ground s t a t e . FranckCondon a n a l y s i s o f t h e s p e c t r u m , c a r r i e d out by t h e s e a u t h o r s on t h e b a s i s o f an e l a b o r a t e t h e o r y i n c l u d i n g a n h a r m o n i c e f f e c t s , w a s a b o u t t o be r e v i s e d when a b e t t e r r e s o l v e d s p e c t r u m was o b t a i n e d w h i c h e x h i b i t e d t h r e e superimposed p r o g r e s s i o n s w i t h equal v i b r a t i o n a l quanta (2). The s e r i e s o f b a n d peaks ( F i g u r e 2) f o l l o w t h e formula 1

i n which a r e u n g e r a d e p r o m o t i n g modes i n d u c i n g t h e d-d t r a n s i t i o n s w h i c h b y v i b r o n i c c o u p l i n g become e l e c ­ t r i c dipole allowed. The i n t e r v a l s P s 370 cm"" , V> 2 = 251 cm*" , and V ^ s 176 cm" now a g r e e w i t h q u a n t a o b t a i n e d from the f a r i n f r a r e d spectrum. A l s o the p r o g r e s s i o n a l i n t e r v a l V~ = 295 cm" compares w e l l w i t h t h e Raman b a n d \?1 (a-jg) = 2o9 cm"" ( a t room t e m p e r a t u r e ) . With these data, t h e v i b r a t i o n a l s t r u c t u r e o f t h e s p e c t r u m c a n be w e l l understood. ^PtCl^j A t 1.9 Κ t h e e m i s s i o n s p e c t r u m o f c u b i c s i n g l e c r y s t a l s ( F i g u r e 3) e x h i b i t s t h r e e progressions due t o p r o m o t i n g modes ^3("t 1 u) » ^4(*1u) ^o(*2u) w i t h b a n d i n t e r v a l s a g r e e i n g w i t h i n f r a r e d ( t - j ) and t h e o r e t i ­ c a l d a t a (t2u) · P r o g r e s s i o n a l i n t e r v a l s are i n each c a s e i?g s 323 cm"' corresponding to a ^ ( g ) 320 cm" f u n d a m e n t a l v i b r a t i o n r e p o r t e d f r o m a Raman measurement (10). E m i s s i o n o c c u r s from the lowest e x c i t e d e l e c t r o n i c f ^ j s t a t e w h i c h i s one o f t h e ^T-|g(*2g g ) l split by s p i n - o r b i t c o u p l i n g ( j j j . T h i s l e v e l i s degenerate and s u b j e c t t o a J a h n - T e l l e r e f f e c t d i s t o r t i n g t h e m o l e ­ c u l e e i t h e r by e~ o r t2g v i b r a t i o n s . S i n c e a p r o g r e s s i o n i n eg i s o b s e r v e d , we c a n c o n c l u d e t h a t t h i s v i b r a t i o n a l mode i s p r e d o m i n a n t l y J a h n - T e l l e r a c t i v e , A n a l y s i s o f the band p r o f i l e by f i t t i n g the spectrum to the t h e o r e t i ­ c a l f u n c t i o n f o r t h e t r a n s i t i o n r a t e (band p r o f i l e f u n c ­ t i o n ) i n the z e r o - t e m p e r a t u r e l i m i t (maO) (5-7) y i e l d s a p p r o p r i a t e f i t t i n g p a r a m e t e r s A and β f r o m w h i c h a d i s t o r t i o n o f Az = 0.19JÎ and Ax. = Ay = -0.095 & i s c a l c u ­ l a t e d f o r one o f t h e p o s s i b l e Γ3 p e r t u r b a t i o n s ( , i . e . i t s h o u l d n o t e x h i b i t any f i n e s t r u c t u r e i f an ^ I > p l o t i s n o t s t r u c t u r e d . T h i s v a r i a t i o n ( f l u c t u a t i o n ) becomes more e v i d e n t from P o i s s o n d i s t r i b u t i o n i f e l o o k a t photon p a i r s which arrive within relativel define cumulative frequencie i n t e r v a l s equal or l e s s to a c e r t a i n time l i m i t ixt c(At) m

Γ

f(t)dt

t h e f l u c t u a t i o n f r o m t h e mean i n t e n s i t y < I ^ becomes more evident s i n c e f o r small time i n t e r v a l s the e r r o r s i n t r o ­ d u c e d (if is n o t s m a l l enou^ri) by the p a r t i a 1 n e g l e c t o f p h o t o n p a i r s w i t h l a r g e t i m e i n t e r v a l s become l e s s important. The c ( A t ) p l o t ( F i g u r e 7 ) ( 2 0 ) w i t h p h o t o n e n e r g y ex­ h i b i t s a b a n d s t r u c t u r e e v e n a t ^ 7 0 K. I f the time l i m i t At, at a c e r t a i n temperature ( 1 0 K ) , i s i n c r e a s e d to i n f i n i t y , t h e c u r v e w o u l d r e p r e s e n t t h e mean i n t e n s i t y 4.1> c o r r e s p o n d i n g t o a c o n v e n t i o n a l e m i s s i o n measurement s h o w i n g no v i b r a t i o n a l s t r u c t u r e f o r t h i s sample a t t h a t p a r t i c u l a r temperature. From t h i s p r o g r e s s i o n a n e n e r g y i n t e r v a l o f a b o u t 1 5 0 cm" 1 i s c a l c u l a t e d , t h e a v e r a g e a g r e e i n g w i t h the e v i b r a t i o n a l mode o b t a i n e d f r o m Raman measurements (jj)) · The a n a l y s i s i n d i c a t e s a J a h n - T e l l e r e f f e c t i n t h e e x c i t e d Ρ^" s t a t e as a l s o f o u n d f o r c o r r e ­ s p o n d i n g Se compounds ( s . a b o v e ) . Such f l u c t u a t i o n s o f the photon f l u x , e m i t t e d from a m o l e c u l e , h a v e b e e n p r e d i c t e d t o be due t o c o o p e r a t i v e effects (21>22). The t h e o r y i s b a s e d on an i d e a o f P r i g o g i n e and c o w o r k e r s ( s . e . g . ( 2 ^ ) ) who t r e a t e d t h e i r r e v e r s i b l e p a r t o f a p h y s i c a l p r o c e s s by t r a n s f o r m i n g the wavefunctions o f a d i s s i p a t i v e system i n t o another space u s i n g a d y n a m i c a l non-unitary representation Ds exp(-iVx/fi) with a " s t a r - H e r m i t i a n t i m e o p e r a t o r 3~ and V d e s c r i b i n g t h e i n t e r a c t i o n o f a r e l e v a n t l o c a l s y s t e m HQ, e . g . t h e c o m p l e x c h r o m o p h o r e , and t h e t o t a l svjstem H, i . e . o u r c r y s t a l . I n t h e new r e p r e s e n t a t i o n Ύ * = Ό * Ύ · no a d d i t i o n a l t i m e d e p e n d e n c e i s i n t r o d u c e d , d b / d t = 0 , any e x p e c t a t i o n v a l u e o f an o p e r a t o r M=DMD"" s h o u l d be u n c h a n g e d s £M> and t h e t o t a l H a m i l t o n i a n i s t r a n s f o r m e d b y ÎTs DHD*" = Uq t o t h e l o c a l s y s t e m H a m i l ­ tonian ( 2 1 , 2 2 ) . To d e s c r i b e t h e t i m e d e v e l o p m e n t i n t h e new r e p r e s e n t a t i o n , t h e e l e c t r o n d e n s i t y $ = I ^ X ^ t l g

, f

w

M

1

1

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

34

EXCITED STATES AND REACTIVE INTERMEDIATES

c(At)

At = 2 5 0 n s ^70 Κ

1

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+

+

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-50 Κ

0.9H

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0.9-

0,9

0,8-

0.8-

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14.8

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^v[cm" -10" ] 1

3

F i g u r e 7 · C u m u l a t i v e r e l a t i v e f r e q u e n c i e s c ( £ t ) up t o the time l i m i t / \ t f o r the o c c u r r e n c e o f the time i n ­ t e r v a l t between s u c c e s s i v e l y r e c o r d e d photons p l o t t e d a g a i n s t p h o t o n e n e r g i e s f o r a C s 2 T e B r £ powder sample. The mean e r r o r i s a b o u t 0 . 5 % » i« © · t h e e r r o r b a r s h a v e t h e s i z e o f t h e symbols u s e d f o r t h e d a t a p o i n t s .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

3.

SCHMIDTKE

Excited State Geometries of Coordination

Compounds

35

s a t i s f y i n g the von Neumann-Liouville equation i?^/9t = L ^ (L β Γ Η > · · 3 / η ) t r a n s f o r m e d b y D(+L) a n d a l l o t h e r op­ e r a t o r s b y D ( - L ) , l e a d i n g t o t i m e o p e r a t o r s T(+L) a n d 7^(-L) w h i c h , due t o t h e LtO~ symmetry, were shown t o be s u b s t i t u t e d b y T(+L) = where T° i s t h e o d d f u n c t i o n a l p a r t T ° ( - L ) S - ^ ( + L ) o f T. W i t h t h e " i n t r i n s i c t i m e " f ° t h e t o t a l t i m e e v o l u t i o n o f t h e s y s t e m h a s two c o n t r i b u ­ tions i

s

9

*h

w

i

t

h

( ^ ) -

D

i f

D

"

1

One r e s p r e s e n t s e v o l u t i o n w i t h r e s p e c t t o u n i v e r s a l t i m e c o n t r o l l e d b y t h e l o c a l s y s t e m L i o u v i l l i a n LQ, t h e o t h e r part d e s c r i b e s e v o l u t i o n w i t h r e s p e c t t o i n t r i n s i c time b e i n g t h e i r r e v e r s i b l e component o f t h e p r o c e s s d e t e r ­ mined b y t h e i n t e r a c t i o n L i o u v i l l i a n L y The average ( t ) / ? T ^ then correspond u s u a l d e n s i t y change t i o n (decay) o f t h e s t a t e s . F o r a s t a t i o n a r y luminescence experiment, t h i s term t h e r e f o r e d e s c r i b e s the photon f l u c t u a t i o n s from a v i b r o n i c s t a t e o f t h e l o c a l system HQ, c a u s e d b y e n v i r o n m e n t a l interactions. These photon f l u x f l u c t u a t i o n s a r e a l s o observed i n the time r e s o l v e d s p e c t r a o f other systems. Two more examples r e f e r t o Sn^+ d o p e d i n K I ( w h i c h i s a l s o a n s e l e c t r o n i c s y s t e m ) a n d | R u ( b i p y ) 3 U C I 2 i n aqueous s o l u t i o n (Ru2+ t o b i p y r i d i n e charge t r a n s f e r t r a n s i t i o n ) ( 2 0 ) . F i g u r e s 8 a n d 9 show c ( £ t ) p l o t s f o r t h e s e s y s t e m s exhibiting d i s t i n c t band s t r u c t u r e s f o r s m a l l time limits. Assignments t o superimposed p r o g r e s s i o n s and t o several emitting states are p o s s i b l e ; the obtained reso­ l u t i o n d o e s n o t a l l o w , however, a d e t a i l e d a n a l y s i s . W i t h t h e equipment a v a i l a b l e t o u s , t h e method i s l i m i t e d t o compounds w h i c h e x h i b i t a r e l a t i v e l y l a r g e e m i s s i o n i n t e n ­ sity. T h e r e f o r e t h e z e r o phonon r e g i o n s w h i c h e v e n t u a l l y show t h e most p r o n o u n c e d f i n e s t r u c t u r e , c o u l d n o t be investigated. S i n c e t r a n s i t i o n group elements a r e r a t h e r low e m i t t e r s o n l y few measurements ( e . g . f o r ^ P t C l ^ ) were s u c c e s s f u l u n t i l now, s u p p l y i n g b e t t e r r e s o l v e d v i b r a t i o n a l s t r u c t u r e s than those obtained by u s u a l emission or absorption spectroscopy. Acknowledgment I am g r a t e f u l t o D r . A . B. P. L e v e r f o r h i s a s s i s t a n c e t o prepare the manuscript.

editorial

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

36

EXCITED STATES AND REACTIVE INTERMEDIATES

Figure 8. C o r r e s p o n d i n g p l o t s as i n F i g u r e 7 f o r K I : S n ^ s i n g l e c r y s t a l s f o r v a r i o u s g i v e n time l i m i t s A t compared t o t h e e m i s s i o n s p e c t r u m ^ Ι > · +

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

3.

SCHMIDTKE

Excited State Geometries of Coordination Compounds

F i g u r e 9 · E m i s s i o n i n t e n s i t y a n d c ( ^ t ) p l o t s f o r p l u ( b i p y ) ^ J c i 2 i n aqueous s o l u t i o n a t n o r m a l t e m p e r a ­ ture. E x c i t a t i o n wave l e n g t h λ = 358 nm.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

37

38

EXCITED STATES AND REACTIVE INTERMEDIATES

Literature Cited 1. Tutt, L . ; Tanner, D.; Heller, E. J.; Zink, J. I . Inorg. Chem. 1982, 21, 3858. 2. Tutt, L . ; Tanner, D.; Schindler, J.; Heller, E. J.; Zink, J. I. J . Phys. Chem. 1983, 87, 3017. 3· Komi, Y.; Urushiyama, A. B u l l . Chem. Soc. Jpn. 1980, 53, 979. 4. Wilson, R. B . ; Solomon, Ε. I. Inorg. Chem. 1978, 17, 1729. 5. Kupka, H. Mol. Phys. 1978, 36, 685. 6. Kupka, H . ; Enβlin, W.; Wernicke, R.; Schmidtke, H.-H. Mol. Phys. 1979, 37, 1693. 7. Kupka, H . ; Schmidtke, H.-H. Mol. Phys. 1981, 43, 451. 8. Hipps, K. W.; Merrell G Α.; Crosby G A J Phys Chem. 1976, 80 9. Eyring, G.; Schmidtke, Bunsenges Phys Chem. 1981, 85, 597. 10. Woodward, L. Α.; Creighton, J. A. Spectrochim. Acta 1961, 17, 594. 11. Eyring, G.; Schönherr, T.; Schmidtke, H.-H. Theor. Chim. Acta 1983, 64, 83. 12. Wernicke, R.; Kupka, H . ; Enβlin, W.; Schmidtke, H.-H. Chem. Phys. 1980, 47, 235. 13. Fukuda, A. Phys. Rev. Β 1970, 1, 4161. 14. Urushiyama, Α.; Kupka, H . ; Degen, J.; Schmidtke, H.-H. Chem. Phys. 1982, 67, 65. 15. Hakamata, K . ; Urushiyama, Α.; Degen, J.; Kupka, H . ; Schmidtke, H.-H. Inorg. Chem. 1983, 22, 3519. 16. Degen, J.; Schmidtke, H.-H. Theor. Chim. Acta 1985, 67, 33. 17. Degen, J.; Schmidtke, H.-H.; ChatzidimitriouDreismann, C. A. Theor. Chim. Acta 1985, 67, 37. 18. Oliver, C. J. In "Photon Correlations and Light Beating Spectroscopy"; Cummins, Η. Ζ.; Pike, E. R., Ed.; Plenum Press: New York, 1974; p. 151. 19. Stufkens, D. J. Rec. Trav. Chim. Pays Bas 1970, 89, 1185. 20. Degen, J. Ph.D. Thesis, University of Düsseldorf, 1985. 21. Chatzidimitriou-Dreismann, C. A. Int. J . Qu. Chem. Symp. 1982, 16, 195. 22. Chatzidimitriou-Dreismann, C. A. Int. J. Qu. Chem. 1983, 23, 1505. 23. Prigogine, I . "From Being to Becoming-Time and Complexity in Physical Science"; W. H. Freeman: New York, 1980. RECEIVED November 8, 1985

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

4

Excited State Distortions Determined by Electronic and R a m a n Spectroscopy Jeffrey I. Zink, Lee Tutt, and Y. Y. Yang Department of Chemistry, University of California, Los Angeles, CA 90024 The geometry change undergo when excite populate are determined by using a combination of electronic emission and absorption spectroscopy, pre-resonance Raman spectroscopy, excited state Raman spectroscopy, and time-dependent theory of molecular spectroscopy. Bond length changes in the lowest d-d excited states of W(CO)pyridine and W(CO) piperidine are calculated. The results are consistent with the predictions of the ligand field theory and with the observed photochemical reactions. Bond bending distortions are observed in metal-nitrosyl compounds. The photochemical reactions of Co(CO)NO and the photohydrogenation catalyzed by Rh(PPh)3NO provide indirect support for the bending. Excited state Raman spectroscopy of Fe(CN)5NO provides direct support for the bending distortion. 5

5

3

3

2-

The geometry changes which a molecule undergoes when it is electronically excited are important in determining its spectroscopic and photochemical properties. These geometrical distortions can include changes in both the metal-ligand bond lengths and bond angles. Changes in the formal charges may result from these distortions. A simple type of distortion is metal-ligand bond lengthening along one totally symmetric normal coordinate. In a highly symmetrical complex containing a small number of atoms (e.g. PtClij "), there is only one totally symmetric normal mode and the electronic spectrum shows a well defined vibronic progression in that one mode. The distortion can be readily determined by using standard Franck-Condon theory (V). Multiple distortions along more than one totally symmetric normal mode are common in complexes containing many atoms. These distortions are symmetry preserving. The point group of the molecule is the same in both the ground and the distorted excited states. The problem which frequently arises with large molecules in condensed media is that vibronic structure, when 2

0097-6156/ 86/ 0307-0039$06.00/ 0 ε> 1986 American Chemical Society In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

40

EXCITED STATES AND REACTIVE INTERMEDIATES

p r e s e n t a t a l l , i s not w e l l enough r e s o l v e d t o r e v e a l t h e i n d i v i d u a l components o f e a c h o f t h e d i s t o r t e d modes. Typical examples t o be d i s c u s s e d below a r e W ( C 0 ) 5 L , L = p y r i d i n e and piperidine. New t h e o r e t i c a l and e x p e r i m e n t a l methods t o determine t h e magnitudes o f t h e d i s t o r t i o n s a l o n g a l l o f t h e d i s t o r t e d normal modes i n t h e e x c i t e d s t a t e p o t e n t i a l hypersurface a r e the s u b j e c t s o f the f i r s t part o f t h i s paper. D i s t o r t i o n s a l o n g n o n - t o t a l l y symmetric modes may o c c u r i n certain excited states. These d i s t o r t i o n s a r e non-symmetry p r e s e r v i n g ; t h e p o i n t group o f t h e m o l e c u l e changes i n t h e excited state. The s p e c i f i c examples i n t h i s paper a r e t h e l i n e a r t o bent geometry changes o f m e t a l n i t r o s y l s ( e . g . , from Ciiv t o C i n [ F e ( C N ) N 0 ] . ) The a n a l y s e s o f d i s t o r t i o n s and t h e i r s p e c t r o s c o p i c and p h o t o c h e m i c a l consequences a r e t h e s u b j e c t s o f t h i s paper. F i r s t , multi-mode s y m m e t r y - p r e s e r v i n g d i s t o r t i o n s a r i s i n g from d-d e x c i t a t i o n o f m o n o s u b s t i t u t e c a l c u l a t e d by u s i n g a c o m b i n a t i o p r e - r e s o n a n c e Raman s p e c t r o s c o p y , and e l e c t r o n i c s p e c t r o s c o p y . The p o i n t s o f c o n n e c t i o n between t h e l i g a n d f i e l d t h e o r y o f t r a n s i t i o n metal p h o t o c h e m i s t r y , t h e measured p h o t o c h e m i c a l r e a c t i o n s , and t h e e x c i t e d s t a t e d i s t o r t i o n s w i l l be d i s c u s s e d . S e c o n d l y , non-symmetry p r e s e r v i n g d i s t o r t i o n s a r i s i n g from charge t r a n s f e r e x c i t a t i o n o f t h e complexes c o n t a i n i n g t h e MNO group a r e s t u d i e d . Both i n d i r e c t e v i d e n c e f o r t h e d i s t o r t i o n s from p h o t o c h e m i c a l r e a c t i o n s and d i r e c t e v i d e n c e from e x c i t e d s t a t e Raman s p e c t r o s c o p y a r e d i s c u s s e d . 2 +

s

5

E x c i t e d S t a t e D i s t o r t i o n s o f W ( C 0 ) 5 p y r i d i n e and W ( C 0 ) 5 p i p e r i d i n e from Time-Dependent Theory, P r e - r e s o n a n c e Raman S p e c t r o s c o p y , and E l e c t r o n i c S p e c t r o s c o p y The e m i s s i o n spectrum o f W ( C 0 ) p y , shown i n f i g u r e 1, i s t y p i c a l of the s p e c t r a o f l a r g e o r g a n o m e t a l l i c molecules taken i n condensed media a t low temperatures ( 2 ) . T h i s spectrum, t a k e n from a s i n g l e c r y s t a l a t 10 K, shows a l o n g r e g u l a r l y s p a c e d p r o g r e s s i o n w i t h a peak t o peak s e p a r a t i o n o f 550 ± 10 cm" . I n s t r u m e n t a l r e s o l u t i o n i s two o r d e r s o f magnitude h i g h e r . The e m i s s i o n has been a s s i g n e d t o t h e 3E t o A^). I n a l u m i n e s c e n c e spectrum o f a s m a l l m o l e c u l e , a r e g u l a r l y s p a c e d p r o g r e s s i o n i s almost always caused by a d i s t o r t i o n a l o n g a t o t a l l y symmetric normal mode whose f r e q u e n c y i s e q u a l t o t h e v i b r a t i o n a l f r e q u e n c y o f t h a t mode i n the ground e l e c t r o n i c s t a t e . S u r p r i s i n g l y , t h e r e a r e no t o t a l l y symmetric modes w i t h a f r e q u e n c y o f 550 cm" i n W(C0)5py. Thus, a c a l c u l a t i o n o f t h e d i s t o r t i o n a l o n g one 550 cm" mode c o u l d not be c o r r e c t . U n f o r t u n a t e l y , a meaningful c a l c u l a t i o n o f the e x c i t e d s t a t e d i s t o r t i o n s i s i m p o s s i b l e from t h i s spectrum alone. The e m i s s i o n spectrum o f a s i n g l e c r y s t a l o f W ( C 0 ) 5 p i p e r i d i n e i s shown i n f i g u r e 2. The assignment i s t h e same as t h a t f o r t h e p y r i d i n e complex. The major p r o g r e s s i o n i s r e g u l a r l y s p a c e d w i t h a peak s e p a r a t i o n o f 520 ± 15 cm" . The major d i f f e r e n c e between t h i s spectrum and t h a t o f t h e p y r i d i n e complex i s t h a t t h e p r o g r e s s i o n i s s i g n i f i c a n t l y l o n g e r . The 5

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In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Z

Electronic and Raman Spectroscopy: Excited States Distortions

Z I N K E TA L .

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F i g u r e 1. Luminescence spectrum o f a s i n g l e c r y s t a l o f W ( C 0 ^ p y r i d i n e a t 10 K. The s o l i d l i n e i s t h e e x p e r i m e n t a l spectrum. The d o t t e d l i n e i s t h e spectrum c a l c u l a t e d from the p r e - r e s o n a n c e Raman i n t e n s i t i e s as d i s c u s s e d i n t h e text.

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F i g u r e 2. Luminescence spectrum o f a s i n g l e c r y s t a l o f W ( C O ) p i p e r i d i n e a t 10 K. The s o l i d l i n e i s t h e e x p e r i m e n t a l spectrum. The d o t t e d l i n e i s t h e spectrum c a l c u l a t e d as d i s c u s s e d i n t h e t e x t . 5

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

42

EXCITED STATES AND

REACTIVE INTERMEDIATES

peak maximum o c c u r s at the s i x t h v i b r a t i o n a l quantum i n s t e a d o f the t h i r d . A g a i n t h e r e i s no ground s t a t e normal v i b r a t i o n a l mode w i t h an energy near 520 cm"* and a m e a n i n g f u l d e t e r m i n a t i o n of the e x c i t e d s t a t e d i s t o r t i o n s i s not p o s s i b l e from t h i s spectrum a l o n e . The d e t a i l e d c a l c u l a t i o n o f the multi-mode d i s t o r t i o n s i s p o s s i b l e by u s i n g p r e - r e s o n a n c e Raman s p e c t r o s c o p y , the e m i s s i o n s p e c t r a , and time-dependent t h e o r y o f molecular spectroscopy. Time dependent t h e o r y p r o v i d e s b o t h a q u a n t i t a t i v e and an i n t u i t i v e p h y s i c a l p i c t u r e o f the i n t e r - r e l a t i o n s h i p between the e x c i t e d s t a t e d i s t o r t i o n s , p r e - r e s o n a n c e Raman s p e c t r a , and the e m i s s i o n s p e c t r a ( 5 ) . The t h e o r y i s b r i e f l y d i s c u s s e d i n the f o l l o w i n g s e c t i o n and then used t o determine the d e t a i l s o f the e x c i t e d s t a t e d i s t o r t i o n s . 1

E l e c t r o n i c A b s o r p t i o n and E m i s s i o n S p e c t r o s c o p y . The e l e c t r o n i c a b s o r p t i o n p r o c e s s b e g i n s when the ground s t a t e v i b r a t i o n a l wavepacket undergoes a excited state potentia wavepacket, φ, p r o p a g a t e s on the e x c i t e d s t a t e p o t e n t i a l s u r f a c e which i n g e n e r a l i s d i s p l a c e d by r e l a t i v e t o the e x c i t e d s t a t e s u r f a c e . The d i s p l a c e d wavepacket i s not a s t a t i o n a r y s t a t e and e v o l v e s a c c o r d i n g t o the time-dependent Schrodinger equation. The q u a n t i t y o f i n t e r e s t i s the o v e r l a p of the i n i t i a l wavepacket w i t h the time-dependent wavepacket, . The o v e r l a p i s a maximum a t t=0 and d e c r e a s e s as the wavepacket moves away from i t s i n i t i a l p o s i t i o n . A t some l a t e r time t , the wavepacket may r e t u r n t o i t s i n i t i a l p o s i t i o n g i v i n g r i s e t o a r e c u r r e n c e of the o v e r l a p . A p l o t o f the o v e r l a p as a f u n c t i o n o f time i n the time domain shows the i n i t i a l o v e r l a p d e c r e a s i n g and then r e c u r r i n g at a time t when the wavepacket (and thus a l l of the atoms i n the m o l e c u l e ) returns to i t s o r i g i n a l p o s i t i o n . This pattern i s r e p e t i t i v e . In the s i m p l e case o f harmonic p o t e n t i a l s u r f a c e s and no change i n v i b r a t i o n a l f r e q u e n c i e s between the ground and e x c i t e d e l e c t r o n i c s t a t e s , the o v e r l a p i s fc-t k|k(t)> - exp

[-Z(A /2(1-e k k

-iu) t/2)] k

(1)

where E i s the energy d i f f e r e n c e between the minima o f the two s u r f a c e s , Γ i s a phenomenological damping f a c t o r ( v i d e i n f r a ) , and ωμ, and A a r e the f r e q u e n c y and the d i s p l a c e m e n t o f the k normal mode. The e l e c t r o n i c a b s o r p t i o n spectrum i n the f r e q u e n c y domain i s the f o u r i e r t r a n s f o r m o f the o v e r l a p i n the time domain. The spectrum i s g i v e n by 0

t n

k

Κω)

« Cu) \ e œ

At - 1 where Δω i s t h e f r e q u e n c y mismatch. Under s h o r t time c o n d i t i o n s t h e wavepacket moves i n a r e g i o n l o c a l i z e d near t h e e q u i l i b r i u m geometry o f t h e ground e l e c t r o n i c s t a t e , i . e . , t h e Franck-Condon r e g i o n . The p r e - r e s o n a n c e Raman i n t e n s i t y i s dominated by t h e s l o p e o f t h e potential surface i n this region. The g r e a t e r t h e s l o p e , t h e g r e a t e r t h e motion o f t h e wavepacket, t h e g r e a t e r t h e o v e r l a p w i t h t h e f i n a l s t a t e and t h e g r e a t e r t h e i n t e n s i t y . C a l c u l a t i o n of Excited State from Raman I n t e n s i t i e s

D i s t o r t i o n s and E l e c t r o n i c S p e c t r a

The dynamics o f t h e wavepacket on t h e upper p o t e n t i a l s u r f a c e d e t e r m i n e s both t h e a b s o r p t i o n spectrum and t h e Raman s p e c t r u m . The e m i s s i o n spectrum i s determined by t h e dynamics on t h e ground s t a t e p o t e n t i a l s u r f a c e w i t h t h e same d i s p l a c e m e n t s as t h o s e which d e t e r m i n e t h e a b s o r p t i o n and Raman. I n t h e s h o r t time l i m i t , t h e i n t e n s i t i e s i n t h e Raman spectrum a r e r e l a t e d t o t h e d i s p l a c e m e n t s by e q . 7. In t h e s h o r t time l i m i t , t h e a b s o r p t i o n spectrum becomes Κω)

2

2

= Cu> exp [ - ( ω - Ε ) / 2 σ ]

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

(8)

4.

ZINK ET AL.

Electronic and Raman Spectroscopy: Excited States Distortions 2

The q u a n t i t y 2 σ i s the w i d t h o f the e l e c t r o n i c a b s o r p t i o n spectrum a t 1/e o f the h e i g h t . T h i s q u a n t i t y i s a l s o r e l a t e d to the d i s p l a c e m e n t s 2

2σ

2

2

» ZA u> k

k

(9)

2

Thus 2 σ i s e x p e r i m e n t a l l y found from the e l e c t r o n i c spectrum, r a t i o s o f the A's a r e found from the Raman spectrum, and the A s a r e c a l c u l a t e d (except f o r s i g n ) by p a i r w i s e comparison o f the Raman i n t e n s i t i e s . Once the A * s a r e c a l c u l a t e d , the e l e c t r o n i c s p e c t r a a r e c a l c u l a t e d by u s i n g e q . 2 or 3f

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C a l c u l a t i o n o f E x c i t e d S t a t e D i s t o r t i o n s o f W(CO)^L. The e m i s s i o n spectrum d i s c u s s e d e a r l i e r , the t h e o r y d i s c u s s e d above and p r e - r e s o n a n c e Raman d a t a w i l l now be used i n c o n c e r t t o c a l c u l a t e the multi-mod distortions Th relativ intensitie of the peaks i n the p r e - r e s o n a n c by i n t e g r a t i n g the peaks spectrum h a v i n g i n t e n s i t i e s g r e a t e r than t h r e e p e r c e n t of t h a t of the most i n t e n s e peak were measured and used i n the calculations. The e l e c t r o n i c spectrum i s c a l c u l a t e d by u s i n g e q u a t i o n s 3 and 5. The d i s t o r t i o n s used i n t h e s e e q u a t i o n s a r e d e t e r m i n e d from the p r e - r e s o n a n c e Raman i n t e n s i t i e s by u s i n g e q u a t i o n s 7 and 9. Both the v i b r a t i o n a l f r e q u e n c i e s of the normal modes and the d i s p l a c e m e n t s of the e x c i t e d s t a t e p o t e n t i a l s u r f a c e s a l o n g t h e s e normal modes a r e o b t a i n e d from the p r e - r e s o n a n c e Raman s p e c t r u m . The major u n c e r t a i n t y e n c o u n t e r e d i n a p p l y i n g the t h e o r y t o l a r g e m o l e c u l e s i s the unknown e f f e c t of n e a r b y e x c i t e d s t a t e s . The l u m i n e s c e n c e spectrum p r e d o m i n a n t l y o r i g i n a t e s from a s i n g l e s p i n - o r b i t s t a t e , but t h e Raman and a b s o r p t i o n s p e c t r a a r e i n f l u e n c e d by o t h e r s p i n - o r b i t s t a t e s and by nearby charge transfer states. The r e s u l t s r e p o r t e d here a r e based on p r e - r e s o n a n c e Roman d a t a o b t a i n e d as c l o s e t o the e x p e r i m e n t a l l y d e t e r m i n e d o r g i n o f the l o w e s t e x c i t e d s t a t e as p o s s i b l e . Attempts t o o b t a i n f u l l Raman enhancement p r o f i l e s t o probe the p o s s i b l e e f f e c t s o f o t h e r s t a t e s a r e c o n t i n u i n g , but they have been h i n d e r e d by the h i g h p h o t o s e n s i t i v i t y o f the compounds. Once the d i s p l a c e m e n t s o f the e x c i t e d s t a t e s u r f a c e a l o n g the normal modes a r e d e t e r m i n e d , the wavepacket i s p r o p a g a t e d on the m u l t i d i m e n s i o n a l h y p e r s u r f a c e and the o v e r l a p i s c a l c u l a t e d from e q u a t i o n 5. The o v e r l a p i n t h e time domain i s then F o u r i e r t r a n s f o r m e d (eq. 3) t o g i v e the c a l c u l a t e d e l e c t r o n i c spectrum. When good agreement w i t h experiment i s f o u n d , the agreement i n d i c a t e s t h a t the s i m p l i f y i n g assumptions d i s c u s s e d p r e v i o u s l y a r e met and t h a t the d i s t o r t i o n s which a r e c a l c u l a t e d are meaningful. The e m i s s i o n spectrum o f W(C0)5py c a l c u l a t e d as d i s c u s s e d above i s shown i n f i g u r e 1 ( 2 ) . In t h i s c a l c u l a t i o n e x a c t l y the d i s p l a c e m e n t s and f r e q u e n c i e s o b t a i n e d from the e x p e r i m e n t a l

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Raman d a t a were u s e d . E x c e l l e n t agreement between the e x p e r i m e n t a l spectrum and the t h e o r e t i c a l spectrum c a l c u l a t e d from the 18 d i m e n s i o n a l e x c i t e d s t a t e p o t e n t i a l s u r f a c e i s obtained. I n t e r p r e t a t i o n o f t h e s e r e s u l t s w i l l be d i s c u s s e d below. The c a l c u l a t e d e m i s s i o n spectrum o f W(C0)5pip i s shown superimposed on the e x p e r i m e n t a l l y determined spectrum i n f i g u r e 2. The agreement between the c a l c u l a t e d and e x p e r i m e n t a l s p e c t r a i s not as good as t h a t i n f i g u r e 1. Two f a c t o r s a r e p r o b a b l y i n v o l v e d . F i r s t , the e x c i t e d s t a t e i s s i g n i f i c a n t l y more d i s t o r t e d than t h a t i n W(C0)5py. Thus more v i b r a t i o n a l quanta a r e i n v o l v e d and a n h a r m o n i c i t y , which was not i n c l u d e d i n the c a l c u l a t i o n , w i l l p l a y a l a r g e r r o l e . S e c o n d l y , the W-N s t r e t c h i n g mode was not o b s e r v e d i n the Raman spectrum. Even i f a low f r e q u e n c y mode has a s i g n i f i c a n t d i s t o r t i o n , the o b s e r v e d r e l a t i v e i n t e n s i t y w i l l be s m a l l because o f the i n v e r s 7. The major e f f e c t o e m i s s i o n spectrum can be i n c l u d e d i n the damping f a c t o r ( v i d e infra). These c o n s i d e r a t i o n s e x p l a i n why a l a r g e r damping f a c t o r i s r e q u i r e d f o r p i p e r i d i n e than f o r p y r i d i n e . The e x c i t e d s t a t e g e o m e t r i e s of the W i C O ^ p y and W(C0)5pip complexes i n t h e i r l o w e s t e x c i t e d e l e c t r o n i c s t a t e s a r e c a l c u l a t e d by c o n v e r t i n g the r e l a t i v e d i s p l a c e m e n t s from eq. 7 t o bond l e n g t h changes i n A u n i t s . F i r s t , the r e l a t i v e d i s p l a c e m e n t s a r e c o n v e r t e d t o a b s o l u t e d i s p l a c e m e n t s by u s i n g e q s . 7 and 9. The v a l u e o f 2 σ i s o b t a i n e d from the a b s o r p t i o n s p e c t r u m . The r e s u l t i n g d i s p l a c e m e n t s , A , a r e c o n v e r t e d from d i m e n s i o n l e s s normal c o o r d i n a t e s t o l e n g t h s and a n g l e s by t r a n s f o r m i n g t o the d e s i r e d u n i t s . A complete c a l c u l a t i o n r e q u i r e s a complete normal c o o r d i n a t e a n a l y s i s . A good a p p r o x i m a t i o n i s a c h i e v e d by assuming t h a t the normal c o o r d i n a t e s a r e uncoupled and t h a t the masses a p p r o p r i a t e t o a s p e c i f i c normal c o o r d i n a t e can be used. The l a t t e r c a l c u l a t i o n i s r e p o r t e d here because a complete normal c o o r d i n a t e a n a l y s i s i s not a v a i l a b l e . The c a l c u l a t e d changes i n the bond l e n g t h s ( i n A) f o r W(C0)5py show t h a t the most h i g h l y e l o n g a t e d bond i s the W-N bond. The W-C bond t r a n s t o the p y r i d i n e i s a l s o h i g h l y elongated. The l e n g t h s o f the W-C bonds c i s t o the p y r i d i n e a r e o n l y s l i g h t l y changed. The e x c i t e d s t a t e bond l e n g t h changes a r e W-N, 0.18 A, t r a n s W-C, 0.12 A, and c i s W-C, 0.0M A. A l l o f the WCO bond a n g l e s of the c a r b o n y l s c i s t o the p y r i d i n e a r e changed from 180°. A d e t e r m i n a t i o n o f the a n g l e change i n degrees r e q u i r e s a normal c o o r d i n a t e a n a l y s i s . The meaning o f the bond l e n g t h changes i n terms o f the bonding changes i n the e x c i t e d s t a t e and the c o n n e c t i o n s o f these bond l e n g t h e n i n g s t o the p h o t o c h e m i c a l r e a c t i v i t y o f the m o l e c u l e a r e d i s c u s s e d below. The bond l e n g t h changes i n the W(C0)5pip complex a r e l a r g e r than t h o s e i n the p y r i d i n e complex. The W-C bond t r a n s t o the p i p e r i d i n e i s l e n g t h e n e d by 0.25 A and the c i s W-C bonds a r e l e n g t h e n e d by 0.05 A. The bond l e n g t h change o f the W-N bond has an upper l i m i t of 0.3 A. 2

k

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Electronic and Raman Spectroscopy: Excited States Distortions

4. ZINK ET AL.

O r b i t a l C h a r a c t e r i s t i c s o f the Lowest E x c i t e d S t a t e . The l o w e s t energy e x c i t e d s t a t e o f C i | W(C0)5L compounds, (where L i s a l i g a n d lower i n l i g a n d f i e l d s t r e n g t h than CO), has been a s s i g n e d by W r i g h t o n , e t a l . , t o the ( d , d y ) t o d 2 t r a n s i t i o n (3,7). T h i s assignment has been c o n f i r m e d by d e t a i l e d s p e c t r o s c o p i c s t u d i e s i n c l u d i n g MCD s p e c t r o s c o p y ( 8 ) . Both the p y r i d i n e and p i p e r i d i n e complexes have 3E l o w e s t energy e x c i t e d s t a t e s . The o r b i t a l components o f the lowest energy e x c i t e d s t a t e i n Ciiv d ^ complexes a r e g i v e n by e q u a t i o n 10 ( 9 ) . Because o f the p r o x i m i t y o f o t h e r s t a t e s o f the same symmetry, m i x i n g o c c u r s and some d 2 - 2 o r b i t a l c h a r a c t e r i s i n v o l v e d . V

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2 The m i x i n g c o e f f i c i e n t the l i g a n d f i e l d s t r e n g t h unique l i g a n d ( 9 ) . F o r an o c t a h e d r o n (where L=C0), λ - 0 and the sigma i n t e r a c t i o n s between the metal and a l l s i x l i g a n d s a r e e q u a l . When the unique l i g a n d L i s weaker than c a r b o n y l as i s the case f o r p y r i d i n e and p i p e r i d i n e , λ > 0 and the i n c r e a s e i n d 2 c h a r a c t e r over d 2 _ y 2 c h a r a c t e r causes i n c r e a s e d sigma a n t i b o n d i n g i n the ζ d i r e c t i o n compared t o t h a t i n the xy plane. The r e l a t i o n s h i p between λ and the p e r c e n t d 2 c h a r a c t e r has been d i s c u s s e d i n d e t a i l ( £ ) . The most important c o n c l u s i o n s from these o r b i t a l c o n s i d e r a t i o n s a r e 1) t h a t i n the o n e - e l e c t r o n l i g a n d f i e l d p i c t u r e o f W(C0)5py and W(C0)5pip, t h e d 2 o r b i t a l l i e s lower i n energy than the d 2 - 2 o r b i t a l and 2) t h a t the l o w e s t energy e x c i t e d s t a t e w a v e f u n c t i o n i s dominated by a l a r g e d 2 c h a r a c t e r . The e x c i t e d s t a t e d i s t o r t i o n s can be p r e d i c t e d from the o r b i t a l c h a r a c t e r i s t i c s ( U ) ) . The d 2 o r b i t a l and t h e d 2 - y 2 o r b i t a l are both sigma a n t i b o n d i n g m o l e c u l a r o r b i t a l s . P o p u l a t i n g the d 2 o r b i t a l i s thus expected t o weaken sigma bonding p r i m a r i l y i n the ζ d i r e c t i o n . P o p u l a t i n g the d 2 - 2 o r b i t a l i s expected t o weaken sigma bonding i n the xy p l a n e . The l a r g e r t h e v a l u e o f λ i n e q u a t i o n 10, the g r e a t e r the bond weakening i n the ζ d i r e c t i o n . The l a r g e s t d i s t o r t i o n expected f o r W(C0)5py and W(C0)5pip, based on the above c o n s i d e r a t i o n s , i s m e t a l - l i g a n d bond l e n g t h e n i n g a l o n g the ζ a x i s , the a x i s c o n t a i n i n g the unique ligand. S m a l l e r but non-zero d i s t o r t i o n s a r e a l s o t o be e x p e c t e d i n the xy p l a n e . A l t h o u g h m e t a l - l i g a n d bond l e n g t h e n i n g s are p r e d i c t e d t o be the b i g g e s t d i s t o r t i o n s , s m a l l changes i n bond l e n g t h s i n the l i g a n d s themselves a r e a l s o expected. F o r example, l e n g t h e n i n g a W-CO bond s h o u l d reduce back bonding t o the CO thus s t r e n g t h e n i n g the CO bond and d e c r e a s i n g the CO bond l e n g t h . Small changes a r e a l s o expected i n the p y r i d i n e r i n g . Trends i n the magnitudes o f the major d i s t o r t i o n s can a l s o be p r e d i c t e d from e q u a t i o n 10. The p i p e r i d i n e l i g a n d i s a weaker l i g a n d i n the s p e c t r o c h e m i c a l s e r i e s than p y r i d i n e . z

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American Chemical Society Library 1155 16th St., N.W.

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(The energy o f the a b s o r p t i o n maximum i s 22,100 cm"* in W ( C 0 ) p i p and 22,400 cm" i n W(C0) py.) Thus λ i s l a r g e r f o r the p i p e r i d i n e complex than t h a t f o r the p y r i d i n e complex and the d i s t o r t i o n s a l o n g the ζ a x i s s h o u l d be g r e a t e r . The e x p e r i m e n t a l r e s u l t s p r o v i d e the f i r s t s p e c t r o s c o p i c s u b s t a n t i a t i o n o f the l i g a n d f i e l d based bonding p r e d i c t i o n s . The l a r g e s t e x p e r i m e n t a l l y determined d i s t o r t i o n s occur a l o n g the ζ a x i s and s m a l l e r d i s t o r t i o n s o c c u r a l o n g i n the xy p l a n e . The p i p e r i d i n e complex i s more h i g h l y d i s t o r t e d than the p y r i d i n e complex. S m a l l d i s t o r t i o n s o f the bond l e n g t h s w i t h i n the l i g a n d s a r e a l s o o b s e r v e d . The p r e d i c t e d bonding changes which have been used t o p r e d i c t p h o t o c h e m i c a l r e a c t i v i t y a r e v e r i f i e d by the c o m b i n a t i o n o f pre-resonance Raman s p e c t r o s c o p y , e l e c t r o n i c s p e c t r o s c o p y , and time-dependent theory. 1

5

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C o r r e l a t i o n s between E x c i t e Photochemical Reactivit The l i g a n d f i e l d t h e o r y o f t r a n s i t i o n metal p h o t o c h e m i s t r y i s based on the i d e a t h a t the bonding changes i n e x c i t e d e l e c t r o n i c s t a t e s are c o r r e l a t e d with l i g a n d p h o t o l a b i l i z a t i o n (11-13). P o p u l a t i n g the d 2 o r d 2 - 2 o r b i t a l s i n c r e a s e s sigma a n t i b o n d i n g i n the ζ and xy d i r e c t i o n s r e s p e c t i v e l y . D e p o p u l a t i n g d o r b i t a l s which have p i symmetry s i m u l t a n e o u s l y change the p i bond o r d e r w i t h a d i r e c t i o n a l i t y determined by which d o r b i t a l i s d e p o p u l a t e d . Sigma and p i bond weakening a l o n g a g i v e n m e t a l - l i g a n d bond i s c o r r e l a t e d w i t h the p h o t o c h e m i c a l l i g a n d l a b i l i z a t i o n o f t h a t bond. I n cases such as those s t u d i e d here where two d i f f e r e n t l i g a n d s a r e on the same m o l e c u l a r a x i s such as the ζ a x i s , the more c o m p l i c a t e d q u e s t i o n a r i s e s of which o f the two l i g a n d s e x p e r i e n c e s the g r e a t e s t a n t i b o n d i n g . Three approaches t o answering t h i s q u e s t i o n have been u s e d . The f i r s t s u c c e s s f u l approach used m o l e c u l a r o r b i t a l t h e o r y , s p e c i f i c a l l y o v e r l a p p o p u l a t i o n s , t o c a l c u l a t e the d i s t r i b u t i o n o f a n t i b o n d i n g a l o n g a g i v e n a x i s (V\_). T h i s approach i s p r e d i c t i v e , but i t r e q u i r e s a complete c a l c u l a t i o n f o r each compound of i n t e r e s t . A second a p p r o a c h uses l i g a n d f i e l d t h e o r y . When the o r i g i n a l t h e o r y i s r e w r i t t e n i n terms o f a n g u l a r o v e r l a p p a r a m e t e r s , c o n t r i b u t i o n s from each l i g a n d can be a p p o r t i o n e d )· This a p p r o a c h i s a l s o p r e d i c t i v e , but i t i s not u s e f u l f o r many m e t a l s because the r e q u i r e d parameters have not or cannot be determined. The t h i r d a p p r o a c h , used s p e c i f i c a l l y f o r t u n g s t e n c a r b o n y l s , i s an e m p i r i c a l a n a l y s i s based on i n f r a r e d d a t a (^0. I t i s t o some e x t e n t p r e d i c t i v e f o r compounds f a r from the empirical dividing l i n e s . The above t h r e e approaches a r e i n d i r e c t methods o f i n f e r r i n g a n t i b o n d i n g c h a r a c t e r i n a g i v e n excited state. The bond l e n g t h changes determined from p r e - r e s o n a n c e Raman s p e c t r a , e l e c t r o n i c s p e c t r a and time dependent t h e o r y p r o v i d e a d e t a i l e d p i c t u r e o f the r e s u l t s o f bonding changes caused by populating excited electronic states. There i s a d i r e c t but not l i n e a r c o r r e l a t i o n between bond l e n g t h changes and the z

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Electronic and Raman Spectroscopy: Excited States Distortions

p h o t o c h e m i c a l l a b i l i z a t i o n s o f the l i g a n d s . In the p y r i d i n e complex, the most h i g h l y d i s t o r t e d m e t a l - l i g a n d bond, the W-N bond, i s l e n g t h e n e d by 0.18 A. The quantum y i e l d f o r p y r i d i n e l o s s i s 0.22. The much l e s s d i s t o r t e d m e t a l - c a r b o n bond i s much l e s s r e a c t i v e ; the quantum y i e l d f o r CO l o s s i s l e s s than 0.01. I t i s i n t e r e s t i n g t o n o t e t h a t the p i p e r i d i n e complex has a l a r g e r d i s t o r t i o n and a l a r g e r quantum y i e l d f o r unique l i g a n d l o s s , 0.58, than the p y r i d i n e complex. F u r t h e r work i s needed t o d e t e r m i n e whether or not t h i s type o f c o r r e l a t i o n i s g e n e r a l f o r W ( C 0 ) L complexes. 5

The M i s s i n g Mode E f f e c t (MIME) Both o f the compounds W(C0) py and W ( C 0 ) p i p e x h i b i t the " M i s s i n g Mode E f f e c t " (MIME), a r e g u l a r l y s p a c e d v i b r o n i c p r o g r e s s i o n i n the l u m i n e s c e n c e spectrum which does not c o r r e s p o n d t o any ground s t a t e normal mode v i b r a t i o n ( 2 ) . I th luminescenc spectru f W(C0)5py the MIME s p a c i n g i s 55 of W ( C 0 ) p i p the MIME s p a c i n symmetric v i b r a t i o n a l modes o f t h e f r e q u e n c i e s a r e f o u n d i n the v i b r a t i o n a l s p e c t r a of these molecules. 5

5

5

The MIME e f f e c t i s e a s i l y u n d e r s t o o d from the v i e w p o i n t o f time dependent t h e o r y . In the time domain, the most i m p o r t a n t c h a r a c t e r i s t i c s o f t h e o v e r l a p a r e the r a p i d d e c r e a s e n e a r t=0, the p a r t i a l r e c u r r e n c e i n the o v e r l a p near ^ = 2 1 7 / 1 % (where i s t h e f r e q u e n c y s p a c i n g o f the o b s e r v e d p r o g r e s s i o n i n the f r e q u e n c y domain,) and the quenching o f f u r t h e r r e c u r r e n c e s i n the time domain due t o the damping f a c t o r Γ and t o the p r e s e n c e o f d i s p l a c e m e n t s i n s e v e r a l d i f f e r e n t modes. The p a r t i a l r e c u r r e n c e at t - t i s r e s p o n s i b l e f o r the appearance o f the r e g u l a r l y spaced p r o g r e s s i o n a t f r e q u e n c y 0 ^ = 2 ^ / ^ . Two or more d i s p l a c e d modes can c o n s p i r e t o g i v e s u c h a p a r t i a l r e c u r r e n c e which i s not e x p e c t e d o f any mode a l o n e . I n the s i m p l e s t p e d a g o g i c a l example, a two mode c a s e , the t o t a l overlap i s m

2

|

=

exp

(-iE t/fc 0

2

-Γ t

)

(11)

I f and peak a t d i f f e r e n t t i m e s , the p r o d u c t may peak a t some i n t e r m e d i a t e t i m e . The compromise r e c u r r e n c e time t i s not j u s t the average o f t-j and t2. The MIME f r e q u e n c y may be s m a l l e r t h a n any o f the individual frequencies. I t i s u s u a l l y between t h e h i g h e s t and lowest f r e q u e n c i e s . I t cannot be l a r g e r t h a n the h i g h e s t frequency. Each o f the d i s p l a c e d modes i n the m o l e c u l e can c o n t r i b u t e t o t h e MIME f r e q u e n c y . Each o f t h e s e modes k has a time dependence whose magnitude i s g i v e n by e q . 1. The l a r g e r the displacement A i n t h e k t h mode, t h e s h a r p e r the peaks i n . The t o t a l o v e r l a p i s the p r o d u c t ( e q . 10) o f the i n d i v i d u a l modes* o v e r l a p s and t w i l l t e n d t o be c l o s e s t t o tk=2n/u)i< f o r t h a t mode w i t h the l a r g e s t A . m

k

r a

k

1

The 550 cm" MIME f r e q u e n c y o f W(C0) py was c a l c u l a t e d by u s i n g the Raman-determined d i s t o r t i o n s and f r e q u e n c i e s . The normal modes which g i v e a r e c u r r e n c e i n the time domain a t t * t 5

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

m

EXCITED STATES AND REACTIVE INTERMEDIATES

50

1

( i . e . , a MIME f r e q u e n c y w =2-/t =550 cm" ) a r e p r e d o m i n a n t l y the W-C s t r e t c h e s i n the 400-500 cm" r e g i o n , the WCO bend a t 636 cm" and the W-N s t r e t c h a t 195 cm" . A l t h o u g h a l l o f the modes i n c l u d i n g the h i g h f r e q u e n c y CO s t r e t c h i n g modes c o n t r i b u t e t o the MIME f r e q u e n c y , the p r i m a r y e f f e c t on the l u m i n e s c e n c e spectrum o f t h e s e modes w i t h s m a l l d i s p l a c e m e n t s i s t o " f i l l i n " the r e d end o f the spectrum. The 520 cm" MIME f r e q u e n c y o f W(C0)5pip was c a l c u l a t e d by u s i n g the same p r o c e d u r e s as those used f o r W(C0)5py. The major c o n t r i b u t i n g modes t o the MIME f r e q u e n c y a r e the W-C s t r e t c h e s i n the 400-500 cm" r e g i o n and the WCO bending mode a t 596 cm" . The e m i s s i o n s p e c t r a o f W(C0)5py and W(C0)5pip a r e t y p i c a l o f the s p e c t r a o b t a i n e d from p e r t u r b e d p o l y a t o m i c m o l e c u l e s . The s p e c t r a show s t r u c t u r e on a s c a l e o f f i v e hundred wavenumbers a l t h o u g h the i n s t r u m e n t a l r e s o l u t i o n i s two o r d e r s of magnitude h i g h e r . Th r e g u l a r l y spaced p r o g r e s s i o f a r from c o r r e c t . I n s t e a d , e i g h t e e n d i s p l a c e d modes c o n t r i b u t e t o the o b s e r v e d MIME p r o g r e s s i o n . The o r i g i n o f the MIME e f f e c t i s r e a d i l y e x p l a i n e d by u s i n g time-dependent t h e o r y . In a d d i t i o n , d i s p l a c e m e n t s of the modes, (which a r e h i d d e n i n the e m i s s i o n spectrum) a r e d e t e r m i n e d . m

m

1

1

1

1

1

1

E x c i t e d S t a t e Bending D i s t o r t i o n s o f t h e MNO Group and P h o t o c h e m i c a l and S p e c t r o s c o p i c Consequences

Their

Non-symmetry p r e s e r v i n g geometry changes a r e a second i m p o r t a n t type o f e x c i t e d s t a t e d i s t o r t i o n . The s p e c i f i c example t o be d i s c u s s e d h e r e i s the bending of the MNO u n i t a f t e r MLCT e x c i t a t i o n o f a l i n e a r MNO u n i t . Because o f the symmetry change, the bending cannot be as e a s i l y t r e a t e d as a symmetry p r e s e r v i n g d i s t o r t i o n by the time-dependent t h e o r y d i s c u s s e d above. The s p e c t r o s c o p i c g o a l o f the s t u d i e s d e s c r i b e d here i s t o measure the e x c i t e d s t a t e MNO distortion by e x c i t e d s t a t e Raman s p e c t r o s c o p y . F i r s t , the s i m p l e t h e o r e t i c a l i d e a s which m o t i v a t e d the s t u d i e s a r e p r e s e n t e d . Three t y p e s of p h o t o c h e m i c a l r e a c t i o n s which p r o v i d e i n d i r e c t e v i d e n c e f o r MNO bending a r e then d i s c u s s e d . F i n a l l y , the e x c i t e d s t a t e Raman s t u d i e s o f the bending a r e d i s c u s s e d . The t h e o r e t i c a l m o t i v a t i n g i d e a s f o r the p h o t o c h e m i c a l and s p e c t r o s c o p i c s t u d i e s a r e i l l u s t r a t e d by t h e s i m p l i f i e d energy l e v e l diagram drawn below.

l i n e a r M"

- N0

+

}

{ bent M

+

-NO"

The HOMO, which i s m a i n l y m e t a l d i n c h a r a c t e r , i s more s t a b l e i n the l i n e a r geometry than i n the bent geometry. The LUMO, which i s the t o t a l l y a n t i b o n d i n g p i o r b i t a l o f the MNO u n i t , has a component which i s s t a b i l i z e d by the bending. Part of the d r i v i n g f o r c e f o r the s t a b i l i z a t i o n o c c u r s because t h i s

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

4.

ZINK ET AL.

Electronic and Raman Spectroscopy: Excited States Distortions

2

component becomes an s p - l i k e bonding o r b i t a l . Populating the m e t a l t o n i t r o s y l charge t r a n s f e r e x c i t e d s t a t e (which c o r r e s p o n d s t o t h e one e l e c t r o n t r a n s i t i o n shown by t h e arrow) c o u l d cause t h e l i n e a r MNO bond t o bend. In a l i m i t i n g v a l e n c e bond d e s c r i p t i o n , t h e l i n e a r ground s t a t e c o n t a i n s a " N 0 " bonded t o a m e t a l "M" " w h i l e t h e bent e x c i t e d s t a t e c o r r e s p o n d s t o a "NO"" bonded t o a "M ". In t h i s VB d e s c r i p t i o n , t h e bending c a u s e s a two e l e c t r o n o x i d a t i o n o f t h e metal. In t h e remainder o f t h i s d i s c u s s i o n , f o r m a l o x i d a t i o n s t a t e s w i l l be used f o r i l l u s t r a t i v e p u r p o s e s w i t h t h e r e a l i z a t i o n t h a t t h e MO d e s c r i p t i o n o f t h i s h i g h l y c o v a l e n t u n i t i s more a c c u r a t e ( 1 5 ) . 1

+

+1

Photochemistry Three t y p e s o f p h o t o c h e m i c a l r e a c t i o n s s u p p o r t the i d e a t h a t e x c i t e d s t a t e bending o c c u r s . The f i r s t o f t h e s e i s t h e gas phase r e a c t i o n o f C o t C O ^ N O w i t h H C 1 ( 1 6 ) T h i s r e a c t i o n was chose probe t h e geometry an c o r r e c t , t h e H o f H C 1 s h o u l d i n t e r a c t w i t h t h e bent NO" and t h e C I " s h o u l d i n t e r a c t w i t h t h e C o . On t h e o t h e r hand, i f p h o t o l y s i s merely a c t i v a t e s the l i n e a r s p e c i e s , then C l " c o u l d i n t e r a c t w i t h N 0 t o produce N 0 C 1 . The e x p e r i m e n t a l r e s u l t s a r e shown i n eq. 12. The system i s p h o t o a c t i v e (with n e g l i g i b l e t h e r m a l r e a c t i v i t y on t h e time +

+

+

C O - N O " + HC1 +

C 1 " C O - N O - + -> [ C o C l ] + [HNO] +

(12)

+

s c a l e o f the experiment.) H and NO" do i n t e r a c t , u l t i m a t e l y d i s p r o p o r t i o n a t i n g t o produce N 2 O and H 2 O . C o and C I " i n t e r a c t , u l t i m a t e l y u n d e r g o i n g subsequent r e a c t i o n s t o produce non-gaseous C 0 C I 2 which forms a powder. No N 0 C 1 was d e t e c t e d . The second p h o t o c h e m i c a l r e a c t i o n which was s t u d i e d was t h e r e a c t i o n o f C o i C O ^ N O w i t h L e w i s base l i g a n d s L ( 1_6). The o b s e r v e d s o l u t i o n phase p h o t o c h e m i c a l r e a c t i o n i s c a r b o n y l photosubstitution. T h i s r e s u l t i n i t i a l l y d i d n o t appear t o be r e l a t e d t o the proposed e x c i t e d s t a t e bending. Further r e f l e c t i o n l e d t o t h e i d e a t h a t t h e bent m o l e c u l e i n t h e e x c i t e d s t a t e i s f o r m a l l y a 16 e l e c t r o n c o o r d i n a t i v e l y u n s a t u r a t e d s p e c i e s which c o u l d r e a d i l y undergo Lewis base l i g a n d association. Thus, an a s s o c i a t i v e mechanism would s u p p o r t t h e hypothesis. D e t a i l e d m e c h a n i s t i c s t u d i e s were c a r r i e d o u t . The quantum y i e l d o f t h e r e a c t i o n i s dependent on b o t h t h e c o n c e n t r a t i o n o f L and t h e t y p e o f L which was u s e d , s u p p o r t i n g an a s s o c i a t i v e mechanism. Q u a n t i t a t i v e s t u d i e s showed t h a t p l o t s o f 1/φ v s . 1/[L] were l i n e a r s u p p o r t i n g t h e mechanism where a s s o c i a t i v e a t t a c k o f L i s f o l l o w e d by l o s s o f e i t h e r L o r CO t o produce t h e p r o d u c t . These s t u d i e s s u p p o r t t h e h y p o t h e s i s t h a t t h e MNO b e n d i n g c a u s e s a f o r m a l i n c r e a s e i n t h e m e t a l oxidation state. +

The t h i r d type o f p h o t o c h e m i c a l r e a c t i o n , p h o t o c a t a l y t i c hydrogénation o f o l e f i n s , was pursued because o f t h e p o s s i b i l i t y t h a t t h e b e n t , f o u r c o o r d i n a t e , f o r m a l l y 16 e l e c t r o n e x c i t e d s t a t e o f Rh(PPh3)3NO c o u l d a c t i n a manner s i m i l a r t o

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

51

52

EXCITED STATES AND REACTIVE INTERMEDIATES

Wilkinson's c a t a l y s t . I r r a d i a t i o n a t 366 nm o f 0.001 M R h i P P l ^ ^ N O and 1 M c y c l o h e x e n e i n o - d i c h l o r o b e n z e n e was c a r r i e d out under 1 atm H 2 a t room t e m p e r a t u r e . The hydrogen uptake was monitored u s i n g a mercury manometer a t t a c h e d t o t h e r e a c t i o n flask. Hydrogen was added p e r i o d i c a l l y i n o r d e r t o m a i n t a i n 1 atm p r e s s u r e i n t h e system. The s o l v e n t and o l e f i n were d i s t i l l e d t w i c e and degassed by t h r e e freeze-pump-thaw c y c l e s b e f o r e use. A 1000 watt Hg lamp f i l t e r e d w i t h a g l a s s f i l t e r t o i s o l a t e t h e 366 nm Hg l i n e was used f o r a l l p h o t o l y s i s experiments. The l i g h t i n t e n s i t y , measured by f e r r i o x a l a t e a c t i n o m e t r y , was 1.0 χ 10~6 e i n s t e i n s / m i n . F i g u r e 3 shows t h e r e s u l t s o f a t y p i c a l c a t a l y s i s experiment ( V f ) . A f t e r m i x i n g t h e s o l v e n t and c a t a l y s t i n t h e dark, one atmosphere o f H 2 i s i n t r o d u c e d i n t o t h e system. A t h o u s a n d - f o l d e x c e s s o f c y c l o h e x e n e i s then added. Initially, hydrogénation i s n e g l i g i b l e on t h e s c a l e shown When t h e s o l u t i o n i s exposed t o which i s f o l l o w e d by hydroge w i t h t h e number o f photons a b s o r b e d . A f t e r i r r a d i a t i o n ceases the hydrogénation g r a d u a l l y d i m i n i s h e s , a l t h o u g h a f t e r 15 hours i n t h e dark a s i g n i f i c a n t t h e r m a l r e a c t i o n i s s t i l l p r e s e n t . When i r r a d i a t i o n i s c o n t i n u e d , t h e hydrogénation r a t e r i s e s t o a p p r o x i m a t e l y t h e same v a l u e as o b s e r v e d i n t h e p r e v i o u s i r r a d i a t i o n p e r i o d . T h i s r a t e i s l i m i t e d by t h e photon f l u x , and t h e t u r n o v e r s a c h i e v e d t o date a r e l i m i t e d by t h e l e n g t h o f the experiment. I n a t o t a l r e a c t i o n time o f 28 hours (20 t h e r m a l , 8 p h o t o c h e m i c a l ) 15 t u r n o v e r s w i t h r e s p e c t t o moles o f rhodium were o b s e r v e d . One t u r n o v e r per hour was o b s e r v e d w i t h a photon f l u x o f 10^ e i n s t e i n s / m i n u t e . The quantum y i e l d can be d e s c r i b e d i n two ways. The amount o f c y c l o h e x e n e produced (determined by hydrogen u p t a k e ) was measured d u r i n g t h e i n i t i a l i r r a d i a t i o n p e r i o d g i v i n g an average v a l u e o f φ=0.75. The n e t quantum y i e l d i n c l u d e s t h e thermal hydrogénation t h a t o c c u r s i n t h e dark as a r e s u l t o f t h e photogenerated c a t a l y s t . S i n c e t h e number o f moles o f c y c l o h e x a n e produced i s g r e a t e r than t h e number o f e i n s t e i n s o f photons put i n t o t h e system, t h e n e t quantum y i e l d w i l l be g r e a t e r than one. S e v e r a l p o s s i b l e mechanisms f o r t h e c a t a l y s i s a r e b e i n g studied. The p o s s i b i l i t y which m o t i v a t e d t h e s t u d y i s t h e p r o d u c t i o n o f a c o o r d i n a t i v e l y u n s a t u r a t e d s p e c i e s by t h e e x c i t e d s t a t e RhNO bending. T h i s e x c i t e d s t a t e c o u l d then o x i d a t i v e l y add H 2 and f o l l o w a pathway s i m i l a r t o t h a t o f Wilkinson's c a t a l y s t . A l t e r n a t i v e l y , t h e d i h y d r i d e formed a s above c o u l d r e d u c t i v e l y e l i m i n a t e ΗΝ0 y i e l d i n g HRh(PPh3)3 which i t s e l f s h o u l d be a good c a t a l y s t . Another p o s s i b i l i t y i s that h e t e r o l y t i c cleavage o f H 2 occurs with H interacting w i t h NO" as was o b s e r v e d i n t h e r e a c t i o n w i t h H C 1 d i s c u s s e d above. Loss o f HNO would produce an a c t i v e metal h y d r i d e . S t u d i e s a r e i n p r o g r e s s t o d i f f e r e n t i a t e between t h e s e possibilities. +

E x c i t e d S t a t e Raman S p e c t r o s c o p y A d i r e c t method o f s t u d y i n g the geometry o f an e x c i t e d s t a t e i s t o measure t h e v i b r a t i o n a l

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Electronic and Raman Spectroscopy: Excited States Distortions

4. ZINK ET AL.

spectrum o f the m o l e c u l e w h i l e i t i s i n t h e e x c i t e d s t a t e . The d i r e c t v i b r a t i o n a l s p e c t r o s c o p y complements t h e i n d i r e c t p h o t o c h e m i c a l e v i d e n c e f o r MNO b e n d i n g . Raman s p e c t r a o f metal complexes i n e x c i t e d e l e c t r o n i c s t a t e s have been o b t a i n e d by u s i n g e i t h e r p u l s e d o r CW l a s e r s t o produce a near s a t u r a t i o n y i e l d o f e x c i t e d s t a t e s and t o s i m u l t a n e o u s l y p r o v i d e t h e probe beam f o r Raman s c a t t e r i n g from the e x c i t e d m o l e c u l e (18-22). The p i o n e e r i n g s t u d i e s o f Woodruff, e t a l . , showed t h a t the method c o u l d probe e l e c t r o n i c changes i n t h e MLCT e x c i t e d s t a t e of Ru(bipy) (1_8). In t h e e x p e r i m e n t s r e p o r t e d h e r e , e x c i t e d s t a t e Raman s p e c t r a were o b t a i n e d by e x c i t i n g and p r o b i n g w i t h 406 nm, 9 nsec p u l s e s a t a 40 Hz r e p e t i t i o n r a t e from an excimer pumped dye l a s e r (Lambda P h y s i k EMG 102E and FL2001.) The a b s o r p t i o n band a t 400 nm has been a s s i g n e d t o t h e 6 e ( d x z , y z ) t o 7 β ( Π * Ν θ ) MLCT t r a n s i t i o n ( 2 3 ) . The Raman s c a t t e r e d l i g h t was passed t h r o u g h a Spex d o u b l e monochrometer d e t e c t e d by u s i n g a C31034 p h o t o m u l t i p l i e s t r i p chart recorder. A K2[Fe(CN)5N0] was pumped t h r o u g h a n e e d l e t o produce a r o u g h l y 200 um diameter j e t stream a t t h e l a s e r f o c u s . Each l a s e r p u l s e i r r a d i a t e d a f r e s h 1 0 ~ L volume o f s o l u t i o n . 2+

1 1

The Raman s p e c t r a t a k e n a t t h r e e d i f f e r e n t p u l s e e n e r g i e s a r e shown i n f i g u r e 4. The l o w e s t t r a c e was taken a t t h e l o w e s t p u l s e energy and i s t h e s p e c t r u m o f the ground s t a t e m o l e c u l e . The upper two t r a c e s show both t h e ground s t a t e and t h e e x c i t e d s t a t e Raman peaks. Four new peaks a r e o b s e r v e d which grow i n i n t e n s i t y as t h e l a s e r p u l s e energy i n c r e a s e s . The i n t e n s i t i e s o f a l l o f t h e new peaks show a n o n - l i n e a r dependence on t h e l a s e r pulse energy. P l o t s o f the l o g o f t h e i n t e n s i t i e s v e r s u s the l o g o f t h e l a s e r p u l s e energy a r e l i n e a r w i t h s l o p e s o f 1.5 ± 0.2 i n d i c a t i n g t h a t t h e peaks a r i s e from a two photon p r o c e s s w i t h some r e l a x a t i o n o f t h e e x c i t e d s t a t e w i t h i n t h e d u r a t i o n o f the l a s e r p u l s e . The e n e r g i e s o f t h e e x c i t e d s t a t e bands a r e g i v e n i n t a b l e 1. Table

1.

E x c i t e d s t a t e Raman f r e q u e n c i e s i n [ F e ( C N ) N 0 ] c o r r e l a t i o n s w i t h ground s t a t e normal m o d e s . » a

Observed E x c i t e d S t a t e Frequency (cm" ) 1

501 548 716 1835

2 +

5

C o r r e l a t i o n with ground s t a t e stretches 400 Fe-C (eq) 462 Fe-C (ax) 652 Fe-N 1940 NO

and

D

C o r r e l a t i o n with ground s t a t e bends and s t r e t c h e s 462 652 665 1940

ν ν δ ν

Fe-C (ax) FeN FeNO NO

a) The e x c i t e d - s t a t e Raman s p e c t r a were t a k e n by u s i n g t h e 406-nm e x c i t a t i o n from an excimer-pumped dye l a s e r . A l l v a l u e s a r e a c c u r a t e t o 5 cm" . b) The same g r o u n d - s t a t e Raman f r e q u e n c i e s were o b t a i n e d by u s i n g p u l s e d 406 nm and CW 514 nm e x c i t a t i o n w i t h t h e e x c e p t i o n o f t h e 400 cm" mode which was o b s c u r e d by t h e R a y l e i g h s c a t t e r i n g and ASE w i t h 406 nm e x c i t a t i o n . 1

1

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

53

EXCITED STATES AND REACTIVE INTERMEDIATES

Time

F i g u r e 3.

I 400

(Hours)

Photohydrogenation

c a t a l y s i s by

1 1 »—ι 600 700 1750 WAVENUMBER(CM" )

1

500

1

1850

RhCPPl^^NO.

1 1950

1

F i g u r e 4. Raman s p e c t r a o f aqueous s o l u t i o n s o f I ^ L F e C C N ^ N O ] . A l l s p e c t r a a r e n o r m a l i z e d w i t h r e s p e c t t o the 652 cm"' peak. The 1850 cm" r e g i o n i s shown m a g n i f i e d f i v e t i m e s . The magnitude o f the background n o i s e , smoothed d u r i n g d i g i t i z a t i o n , i s shown by the a r r o w s . The broad band w i d t h s a r e caused by t h e l a r g e s l i t widths which were required. The weak ground s t a t e NO s t r e t c h a t 1940 cm" i s o b s e r v e d a t a s c a l e l a r g e r t h a n t h a t shown. The l a s e r p u l s e e n e r g i e s were a) 1.1 mJ/pulse; b) 3.0 mJ/pulse; c ) 4.3 m J / p u l s e . 1

1

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

ZINK ET AL.

Electronic and Raman Spectroscopy: Excited States Disto 1

The new peak which grows in at 1835 cm" is assigned to the NO stretch in the excited state molecule. Its energy is reduced by 105 cm" from that of the NO stretch in the electronic ground State. The lower frequency is expected for the bent nitrosyl where the formal bond order is reduced from three to two. The decrease is similar in magnitude to that observed in RuCl(N0)2(PPh3)2 where both a linear and a bent nitrosyl is observed (24). Two correlations between the observed low frequency excited state modes and the corresponding ground state modes are given in the table (25). The first is a one-to-one correlation with the totally symmetric metal-ligand stretches. In this interpretation, all of the metal-ligand stretching frequencies increase in the excited state. The magnitudes of the increases are larger than those expected for a one-electron oxidation. For example, in a series of Fe(CN)5X" complexes the changes in the M-C stretching frequencie iron is oxidized from observed excited state frequencies are consistent with the large increase in metal formal charge expected in the excited state. The second correlation associates the 716 cm" excited state band with an Fe-NO bending mode and the 548 cm" band with the Fe-N stretching mode as given on the right of the table. This correlation is consistent with the trends observed for these modes in ground state cobalt complexes containing linear and bent NO geometries (27). The 501 cm" mode is correlated with an Fe-C stretch which is increased in frequency by about 40 cm" . These changes, together with the decrease in the NO stretching frequency, are consistent with a linear to bent FeNO geometry change and a concomitant large increase in the positive charge on the metal. 1

n

1

1

1

1

Literature Cited 1. Yersin, H.; Otto, H.; Zink, J.I.; Gliemann, G. J. Am. Chem. Soc. 1980, 102, 951, and references therein. 2. Tutt, L.; Tànnor, D.; Heller, E.J.; Zink, J.I. Inorg. Chem. 1982, 21, 3859. 3. Wrighton, M.S.; Hammond, G.S..; Gray, H.B.; J. Am. Chem. Soc. 1971, 93, 4336. 4. Dahlgren, R.M.; Zink, J.I. Inorg. Chem. 1977, 16, 3 54. 5. Heller, E.J. Acc. Chem. Res. 1981, 14, 368. 6. Heller, E.J.; Sundberg, R.L.; Tannor, D. J. Phys. Chem., 1982, 86, 1822. 7. Wrighton, M.S.; Abrahamson, H.B.; Morse, D.L. J. Am. Chem. Soc. 1976, 98, 4105. 8. Schreiner, A.F.; Amer, S.; Duncan, W.M.; Ober, G.; Dahlgren, R.M.; Zink, J.I. J. Am. Chem. Soc. 1980, 102, 6871. 9. Incorvia, M.J.; Zink, J.I. Inorg. Chem., 1974, 13, 2489. 10. Zink, J.I. Inorg. Chem. 1973, 12, 1018. 11. Zink, J.I. J. Am. Chem. Soc., 1972, 94, 8039. 12. Wrighton, M.; Gray, H.B.; Hammond, G.J. Mol. Photochem. 1973, 5, 165. 1

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

13. Van Quickenbourne, H.G.; Ceulemans, A.J. J. Am. Chem. Soc. 1977, 99, 2208. 14. Zink, J.I. J. Am. Chem. Soc. 1974, 96, 4464. 15. Enemark, J.H.; Feltham, R.D. Coord. Chem. Rev. 1974, 13, 339. 16. Evans, W.; Zink, J.I. J. Am. Chem. Soc. 1981, 103, 2635. 17. Zink, J.I. Laser Chem. 1983, ; Evans, W.E., Ph.D. thesis, UCLA, 1980. 18. Dallinger, R.F.; Woodruff, W.H. J. Am. Chem. Soc. 1979, 101, 4391-3. 19. Dallinger, R.F.; Miskowski, V.M.; Gray, H.B.; Woodruff, W.H. J. Am. Chem. Soc. 1981, 103, 1595-6. 20. Smothers, W.K.; Wrighton, M.S. J. Am. Chem. Soc., 1983, 105, 1067-9. 21. Foster, M.; Hester, R.E. Chem. Phys. Lett., 1981, 81, 42. 22. Schindler, J.W.; Zink, J.I. J. Am. Chem. Soc., 1981, 103, 5968-9. 23. Manoharan, P.T.; 24. Pierpont, C.G.; Va R. J. Am. Chem. Soc., 1970, 92, 4760-2. Linear: 1845 cm , bent: 1687 cm . 25. Work is in progress to differentiate between these correlations by using NO isotopic substitution. 26. Brown, D.B. Inorg. Chimica Acta 1971, 5, 314-6. 27. Quinby-Hunt, M.; Feltham, R.D. Inorg. Chem. 1978, 17, 25152520. -1

-1

15

RECEIVED November 8,

1985

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

5

Investigation o f One-Dimensional Species and of Electrochemically Generated Species Use of Resonance Raman Spectroscopy Robin J. H. Clark Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WC1H0AJ,United Kingdom Resonance Raman spectroscopy, well known to be a sensitive probe of the nature of charge-transfer excited states, is now established to be a sensitive probe of intervalence states. In particular, one-dimensional systems prove to be very amenable to study, and the results on a variety of linear-chain platinum complexes of both the Wolffram's red sort as well as the pop sort (pop = H P O ) are outlined. Brief mention is made of the application of resonance Raman spectroscopy to the study of electrochemically generated species. 2-

2

2

5

In any discussion of one-dimensional materials much mention is, justifiably, placed on linear-chain complexes of the KCP type (KCP = KPt(CN) .Br .30.3H2O) . The striking optical properties and conductivities of this type of complex, which are associated with their very short Pt-Pt distances, have been studied in detail by synthetic chemists, crystallographers, theoretical physicists and materials scientists alike, particularly over the past 15 years (1*2) . However, they are not the only type of one-dimensional system to command attention. More recently it has been realised that halogen-bridged chain-complexes give rise to interesting electronic, Raman and resonance Raman spectra. In particular, mixed-valence chain complexes such as Wolffram's red, [Pt (C2H5NH2)k] [Pt (C2HsNH2) ttCl ] CI4 .4H2O give rise to very intense resonance Raman spectra from which information on the nature of the intervalence state may be obtained; this is a matter of considerable contemporary interest (3). It is the purpose of this article to summarise the key results obtained by way of resonance Raman studies on different chain complexes and to draw attention to the likely implications of these results vis à vis this particular type of excited state, the intervalence state (4). The relevance of these results to the study of electrochemically generated species is also outlined. 2

0

11

IV

2

0097-6156/ 86/ 0307-0057S06.00/ 0 © 1986 American Chemical Society In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

58

EXCITED STATES AND REACTIVE INTERMEDIATES 1

W o l f f r a m s Red Type S a l t s Chain complexes o f t h e Wolffram's r e d s o r t have s t r u c t u r e s o f t h e general type L

, L x

.

L Λ

(ι

. . p

1 t

A

(v

L

/

L

L

/ \

L

L

L

/

(v

X

π — χ · · · Pt

/ \

L

L x

. . . χ—pt

/ \ L

,L

· · · X — P t — X

· · ·

/ \

L

L

L

where t h e n e u t r a l e q u a t o r i a l l i g a n d , L, i s an amine such as^NH3, CH3NH2 o r C2H5NH2 and X = C I , Br o r I . B^identate l i g a n d s LL can a l s o form complexes o f t h i s s o r t , where LL = 1,2-diaminoethane, 1,2-diaminopropane, 1,3-diaminopropane, 1,2-diaminobutane, 1,2-diaminocyclopentane 1,2-diaminocyclohexane e t c . as can c e r t a i n t e r d e n t a t e amines LÊL suc N-methyldiethylenetriamin through the f i v e d i f f e r e n t p o s s i b l e charge types f o r t h i s s e r i e s o f ll complexes, v i z . [Pt Lh] [Pt LfX ]Z , ΓΡΐ L Y][Pt LaX Y]Z , [ P t L Y ] [ P t L X Y 2 ] ,A [ P t L Y ] [ P t L X Y ] , A [ P t ^ ] [ P t * Y ] , where X,Y = C l , Br o r I , Ζ = s i n g l y charged a n i o n such as CI , Br , I " , HSOH", C l O i i " , BFit", N0 ~, and A i s a s i n g l y charged c a t i o n such ï v

1 1

2

Ï I

I V

2

2

H

2

2

2

I V

k

3

2

l V

3

2

1

2

3

v

k

6

3

a s

K +

*

TT TV The P t -*· P t i n t e r v a l e n c e t r a n s i t i o n s o f such c h a i n complexes o c c u r i n t h e r e g i o n s 25,000-18,200 cm" , 23,600-14,300 cm" and 20,600-7,500 cm" f o r c h l o r o - , bromo-, and i o d o - b r i d g e d complexes, r e s p e c t i v e l y , t h e t r e n d CI > Br > I b e i n g t h e r e v e r s e o f t h a t o f t h e c o n d u c t i v i t y o f t h e complexes. The t r a n s i t i o n wavenumbers may be d e t e r m i n e d e i t h e r by Kramers-Kronig a n a l y s i s o f s p e c u l a r r e f l e c t a n c e measurements o r from p l o t s o f t h e e x c i t a t i o n p r o f i l e s o f Raman bands enhanced a t o r near resonance w i t h t h e Pt -Pt i n t e r v a l e n c e band. The maxima have been found t o be r e l a t e d t o the P t Pt chain d i s t a n c e , the smaller the l a t t e r the l e s s b e i n g t h e i n t e r v a l e n c e t r a n s i t i o n energy ( 3 ) . The Raman s p e c t r a o f such h a l o g e n - b r i d g e d mixecT-valence complexes o f p l a t i n u m o b t a i n e d a t resonance w i t h P t -*· P t i n t e r v a l e n c e band a r e c h a r a c t e r i s e d by an enormous enhancement t o the Raman band a t t r i b u t e d t o t h e symmetric X - P t - X c h a i n - s t r e t c h i n g mode (Vx), t o g e t h e r w i t h t h e development o f l o n g and i n t e n s e o v e r t o n e p r o g r e s s i o n s v i V i ( v i = v i b r a t i o n a l quantum number o f ν χ ) . V i o c c u r s over t h e ranges 309.1-297.8, 175.7-172.0, and 122.3-114.2 cm f o r c h l o r i n e - , bromine-, and i o d i n e - b r i d g e d complexes, r e s p e c t i v e l y ( 2 ) . P r o g r e s s i o n s r e a c h i n g as f a r as 17Vi have been observed i n t h e resonance Raman s p e c t r a o f some c h a i n complexes, t h e s e l o n g p r o g r e s s i o n s e n a b l i n g v a l u e s f o r t h e harmonic wavenumbers (ωχ) and a n h a r m o n i c i t y c o n s t a n t s ( x n ) t o be o b t a i n e d under t h e usual approximations (6). The c o n s i d e r a b l e l e n g t h o f the V i p r o g r e s s i o n a t resonance w i t h t h e i n t e r v a l e n c e band i m p l i e s a s u b s t a n t i a l (~ 0.1 Â) change i n t h e p o s i t i o n o f t h e b r i d g i n g atom on excitation to this state. Recent s y n t h e t i c and s p e c t r o s c o p i c work has c o n c e n t r a t e d on t h e study o f s t r u c t u r a l l y r e l a t e d l i n e a r - c h a i n p a l l a d i u m complexes ( 7 , 8 ) , on t h e p o s s i b i l i t y o f s y n t h e s i s i n g r e l a t e d n i c k e l complexes, and on the study o f mixed-metal mixed-valence complexes o f the s o r t 1 1

A V

1

1

1

I ; [

I V

1

1

I V

1 1

I V

I V

-1

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

5.

59

Resonance Raman Spectroscopy: Intervalence States

CLARK

[ N i (en) ] [ P t ( e n ) C 1 ] [C10i»] i» and [ P d ( e n ) ] [ P t ( e n ) C 1 ] [ C 1 0 i » ] i » (9). The i n t e r v a l e n c e band maxima a r e found t o v a r y i n t h e o r d e r CI > Br > I and P d / P t > Ni /Pt > Pt /Pt > Pd /Pd , i m p l y i n g t h a t t h e v a l e n c e e l e c t r o n s a r e most d e l o c a l i z e d f o r t h e Pd */Pd complexes; t h i s i m p l i c a t i o n i s c o n s i s t e n t w i t h t h e r e l a t i v e l y h i g h c h a i n c o n d u c t i v i t y o f such complexes ( 1 0 ) . The immense amount o f s p e c t r o s c o p i c work c a r r i e d o u t on t h e s e complexes, p a r t i c u l a r l y on t h e i r e l e c t r o n i c , i n f r a r e d , Raman and resonance Raman s p e c t r a ( 2 ) , has l e d t o Wolffram's r e d b e i n g r e g a r d e d as t h e a r c h e t y p a l c l a s s I I o r l o c a l i z e d v a l e n c e complex i n which t h e two metal atoms d i f f e r i n t h e i r o x i d a t i o n s t a t e s by two, c f . P r u s s i a n b l u e i s so r e g a r d e d f o r mixed-valence complexes i n which t h e two metal atoms d i f f e r i n t h e i r o x i d a t i o n s t a t e s by one. The p r i n c i p a l p o i n t s o f c u r r e n t i n t e r e s t c o n c e r n i n g Wolffram'sr e d type complexes and on which r e s e a r c h i s b e i n g c o n c e n t r a t e d a r e (a) t h e n a t u r e o f t h dependenc f t h X-M X symmetri c h a i n - s t r e t c h i n g mode an band on p r e s s u r e ( 1 1 ) . 1 1

1 V

n

2

2

n

I V

2

2

I V

n

I

V

n

2

I V

n

2

I V

A

IV

(b) t h e u n d e r s t a n d i n g o f t h e o r i g i n o f t h e luminescence e m i t t e d by t h e s e complexes and o f i t s p r e s s u r e dependence. (c) t h e c h l o r i n e i s o t o p i c s t r u c t u r e o f t h e ν χ band o f P t ^ / P t ^ complexes, which d i f f e r s from 9 : 6 : 1 f o r t h e fundamental b u t n o t f o r t h e o v e r t o n e s , and t h e r e l a t i o n s h i p between t h e degree o f v a l e n c e d e r e a l i z a t i o n a l o n g t h e c h a i n , t h e wavenumber o f t h e band gap r e l a t i v e t o t h a t o f t h e e x c i t i n g l i n e , and t h e q u a l i t y o f t h e r e s o l u t i o n o f t h i s s t r u c t u r e t o V i (which i s not i s o t o p i c i n o r i g i n i n t h e case o f P d ^ / P d ^ complexes). (d) t h e d e f i n i t i v e c h a r a c t e r i s a t i o n o f N i ^ / N i ^ l i n e a r - c h a i n complexes ( 1 0 ) . 1

I

1

Pop

S a l t s o f L i n e a r - C h a i n Complexes

As p a r t o f a s e a r c h f o r o t h e r l i g a n d s c a p a b l e o f a d o p t i n g a s q u a r e - p l a n a r c o n f i g u r a t i o n about a metal atom and thus p o t e n t i a l l y a b l e t o form s t a c k e d u n i t s o u r a t t e n t i o n was drawn t o t h e l i g a n d H2P205 " ( d i p h o s p h o n a t e ) , u s u a l l y a b b r e v i a t e d pop. Platinum complexes o f t h i s l i g a n d - i n p a r t i c u l a r [Pt2 (pop) i+] **~ - have a l r e a d y been s u b j e c t t o i n t e r e s t i n g s t u d i e s o f t h e i r luminescence, e l e c t r o n i c , Raman and i n f r a r e d s p e c t r a (12-16) . Our i n i t i a l o b j e c t i v e s were t o t r y t o i n c o r p o r a t e [ P t l ( e n ) 2 X 2 ] (en = 1,2-diaminoethane; X = Br o r I) and [PtI (pop) i f ] * " a l t e r n a t e l y i n t o s t a c k e d c h a i n s . However, t h e r e a c t i o n was found t o generate a d i f f e r e n t type o f c h a i n complex from t h a t e n v i s a g e d , v i z . 2

v

2 +

1

I V

2[Pt (en)2X2]

2 +

+ + I V

1

[Pti (pop) ] h

H

I V

[ P t ( e n ) ] [Pt (en) X2] [ P t * 2

2

1 1

(pop) i>X ] .

2

+

2

1

In e f f e c t , one [ P t ( e n ) 2X2] u n i t o x i d i z e s [ P t I (pop) i»] **" t o [Pti ( p o p ) X ] - , i t s e l f b e i n g reduced t o [ P t (en) ] ; t h e l a s t then c o c r y s t a l l i z e s w i t h the second [ P t ( e n ) 2 X 2 ] u n i t t o form a W o l f f r a m ' s - r e d t y p e c h a i n complex. The s p e c t r o s c o p i c ( e l e c t r o n i c , i n f r a r e d , Raman, resonance Raman) evidence on which t h e s e c o n c l u s i o n s a r e b a s e d i s as f o l l o w s ( 1 7 ) . (a) The a b s o r p t i o n s p e c t r a a r e c h a r a c t e r i s e d by t h r e e bands. l +

t +

1 1

2+

2

2

I V

2 +

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND

60

REACTIVE INTERMEDIATES

The one o f lowest wavenumber o c c u r r i n g a t o r n e a r the wavenumber known t o be c h a r a c t e r i s t i c o f l i n e a r - c h a i n bromine- o r i o d i n e bridged P t / P t complexes (17,000 cm" and 11,000 c m f o r X = Br or I, r e s p e c t i v e l y ) . n

I V

1

-1

(b) The resonance Raman s p e c t r a o f t h e complexes are c h a r a c t e r i s e d by a p r o g r e s s i o n i n the X-Pt^^-X symmetric s t r e t c h i n g mode (νί) o f the c h a i n a t resonance w i t h the c a t i o n - c h a i n i n t e r v a l e n c e band mentioned above, b u t by a p r o g r e s s i o n i n the Pt^-Pt s t r e t c h i n g mode ( v ) o f the d i m e r i c a n i o n [Pt (ρορ)ι*Χ ] ~ a t resonance w i t h second e l e c t r o n i c band i n each case ( a s s i g n e d t o t h e σ -> σ* t r a n s i t i o n o f the a n i o n ) . A summary o f the key r e s u l t s on t h e s e c h a i n complexes i s g i v e n i n T a b l e I . 1

1

1

x

h

2

2

Table

I.

Summary o f S p e c t r o s c o p i c Data on t h e Complexes [ P t ( e n ) ] [PtIV(en) X ] [ P t ^ I ( ) X ] n

a

X = Br copper

Crystals Powder I I

I V

v(Pt ^Pt )/cm*

17000

11000

23300

18000

1

1

xii/cm III III, . - l U)i(Pt -Pt )/cm , -ι xii/cm n

m

v (Pt -X)/cnf , -ι xi2/cm

progression +

v

ViVi

+

2v

v v t t ViVi 2

a

2

2

2

116

±

0.3

-0.39

±

0.03

138.8

±

0.3

121.6

±

0.3

-0.04

±

0.03

-0.04

±

0.03

-0.1

±

-

199.6

228.6

2

viVi

^

175.7

1

viVi

needles

black

a

r n

gold-green

black 1

v(d ->d ycmt TV u)i(X-Pt -X)/cm

needles

X = I

0.05

+0.35 ±

9(12)

V! = 9

vi

=

7(9)

V! = 7

vi

=

5(8)

vi

= 2

v

=

2(2)

v

= 2

vi

vi

=

2

= 10(11)

2

vi

0.05

= 5

Raman d a t a r e l a t e t o measurements taken on samples a t c a . 80 K. Those i n p a r e n t h e s e s r e l a t e t o measurements on s i n g l e c r y s t a l s at c a . 15 K.

The complexes a r e u n i q u e i n t h a t they i n v o l v e (a) p l a t i n u m d i f f e r e n t f o r m a l o x i d a t i o n s t a t e s and (b) b o t h c l a s s I and m i x e d - v a l e n c e i n t e r a c t i o n s w i t h i n a s i n g l e complex.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

i n three c l a s s II

5.

Pop

61

Resonance Raman Spectroscopy: Intervalence States

CLARK

Complexes

K [Pt (pop) X].nH 0 4

2

k

2

Recent s y n t h e t i c work (14-19) on Κι* [ P t (pop) i*] .2H 0 has shown t h a t i t can be p a r t i a l l y o x i d i z e d t o a new type o f s e m i - c o n d u c t o r Kit [ P t (pop) i+X] . η Η 0 , X = C I , Br o r I , i n which t h e average o x i d a t i o n s t a t e o f t h e p l a t i n u m atoms i s 2.5. These complexes form as g o l d e n m e t a l l i c - l o o k i n g c r y s t a l s w i t h c h a i n c o n d u c t i v i t i e s (σ = Ι Ο - ^ - l O - Ω " cm" ) which a r e many o r d e r s o f magnitude g r e a t e r than t h o s e found f o r W o l f f r a m ' s - r e d t y p e complexes ( 1 0 " - 1 0 Ω" c m - ) . X-ray c r y s t a l l o g r a p h i c work shows t h a t b o t h t h e c h l o r i d e (18,19) and t h e bromide (14,16) c o n s i s t o f l i n e a r c h a i n s o f -Χ-Pt-Pt-X-, t h e Pt atoms~beîng l i n k e d , as i s u s u a l f o r t h i s type o f complex, p a i r w i s e by f o u r pop b r i d g e s ; c l e a r l y , t h e c h a i n s p r o v i d e an e f f e c t i v e pathway f o r e l e c t r i c a l c o n d u c t i v i t y . The e l e c t r o n i c s p e c t r a o f t h e s e complexes a r e i n each case dominated by a v e r y i n t e n s e b r o a d band c e n t r e d a t 19,500 15,500 and 11,400 cm" f o r X = c h a r a c t e r i s t i c s (wavenumber l i k e an i n t e r v a l e n c e band o f a h a l o g e n - b r i d g e d s p e c i e s . The Raman spectrum o f K [Pt (pop)4CI].3H 0 a t resonance w i t h t h e i n t e r v a l e n c e band i s dominated by a band a t 291 cm" which i s a t t r i b u t e d t o t h e symmetric P t C l s t r e t c h i n g mode, somewhat lowered (on account o f b r i d g i n g ) from i t s v a l u e (305 cm" ) f o r t h e d i s c r e t e a n i o n [Pt (pop) ifCl ] ~. Six-membered p r o g r e s s i o n s i n t h e 291 cm" band a r e o b s e r v e d i n t h e Raman spectrum under resonance c o n d i t i o n s . The important i m p l i c a t i o n o f t h e s e r e s u l t s i s t h a t t h e c h l o r i n e atom cannot be c e n t r a l l y b r i d g i n g between t h e P t ( p o p ) i * u n i t s f o r , i n t h a t c a s e , t h e symmetric P t C l s t r e t c h i n g mode would be Raman i n a c t i v e . The r e s u l t s a l s o i n d i c a t e t h a t t h e p r i n c i p a l s t r u c t u r a l change on e x c i t a t i o n t o the i n t e r v a l e n c e s t a t e i s along the P t - C l coordinate. There a r e two p o s s i b l e s t r u c t u r e s f o r such a c h a i n , F i g u r e 1. The former s t r u c t u r e c o n s i s t s o f s t a c k e d p o l a r d i m e r s , which would seem t o be u n l i k e l y s i n c e t h e b r i d g i n g pop l i g a n d s a r e themselves symmetric ( p o l a r dimers a r e , however, w e l l e s t a b l i s h e d where t h e b r i d g i n g l i g a n d s a r e unsymmetric). The l a t t e r s t r u c t u r e i s t h e Wolffram's r e d analogue and, i n view o f t h e l a r g e number o f such complexes known ( v i d e s u p r a ) , was c o n s i d e r e d t o be t h e more p r o b a b l e . These p o s s i b i l i t i e s may i n p r i n c i p l e be d i s t i n g u i s h e d by c o n s i d e r a t i o n o f i n f r a r e d and Raman band a c t i v i t i e s , b u t i n p r a c t i c e t h i s p r o v e s t o be v e r y d i f f i c u l t . S t r o n g e v i d e n c e t h a t t h e complex c o n s i s t s o f a s t a c k e d p o l a r dimer has, however, been o b t a i n e d by X-ray c r y s t a l l o g r a p h y , t h e P t - P t , P t ^ - C l and P t - * C l distances b e i n g 2.813(1), 2.367(7) and 2.966(8) ft, r e s p e c t i v e l y (19). Thus K 4 [ P t ( p o p ) C l ] .3H 0 i s a l o c a l i z e d - v a l e n c e ( P t / P t ) complex on account o f t h e f a c t t h a t t h e P t - C l d i s t a n c e s d i f f e r v e r y s u b s t a n t i a l l y (by 0.60 ft). T h i s d i f f e r e n c e i s e n t i r e l y comparable w i t h t h a t found f o r W o l f f r a m ' s - r e d type s a l t s , v i z . t h e c h a i n P t l - C l d i s t a n c e s f o r seven such s t r u c t u r e s c o v e r t h e range 2.30 ± 0.04 ft w h i l e t h e c h a i n P t H C l d i s t a n c e s c o v e r t h e range 3.06 ± 0.23 ft. 2

2

2

2

3

1

1

Μ

12

1

1

1

4

2

2

1

1

h

2

1

2

2

I I e

I I

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lt

Î I Î

2

v

# e e

Both c r y s t a l l o g r a p h i c (14,16,19) as w e l l as s p e c t r o s c o p i c r e s u l t s (18) i n d i c a t e t h a t t h e analogous bromide and i o d i d e complexes are much n e a r e r t o b e i n g d e l o c a l i z e d - v a l e n c e s p e c i e s (as a r e t h e bromide and i o d i d e v e r s i o n s o f W o l f f r a m ' s - r e d type s a l t s ) , c o n s i s t e n t

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

Structure

of

K [Pt (pop) BrJ-3H 0 4

Pt -'^ — Pt -

Br

2±

2

2

4

2

Pt-'-— P t

2

— B r

V ( P t B r ) = 2 2 3 cm" V ( P t P t ) = 133 c m " Tetragonal

(Raman)

1

copper-bronze

Possible

;

Structures

r ( P t - Pt) = 2-793 Â crystals, σ„= Ι Ο " - Ι Ο " 4

for

n

Pt?

Ω Γ cm" 1

1

K [Pt (pop) CI]-3 H 0 4

pt?-— Pt^-CI

-Pt

3

2

-,Pt°

CI

4

2

Pt-—CI---

;Pt^ Pt-—CI---

i M P t CI) = 291 cm"' ( R a m a n ) HO.

^ O

= 2 8 8 cm" (i.r.) 1

v

r (Pt-Cl)-2-367 Â

Figure

1.

structures

= 2-966 Â

Θ

Θ

Structure

o f Kt* [Pt2 (pop) ι+Br] .3H2O and p o s s i b l e

f o r K [Pt (pop) C1].3H 0. 4

2

4

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

5. CLARK

63

Resonance Raman Spectroscopy: Intervalence States

w i t h t h e i r c h a i n c o n d u c t i v i t i e s b e i n g h i g h e r than t h a t o f t h e chloride. A l t h o u g h c r y s t a l l o g r a p h i c work (16) has been i n t e r p r e t e d t o imply e x a c t c e n t r a l b r i d g i n g by bromide f o r Ki+ [ P t ( p o p ) i t B r ] .3H2O, the s p e c t r o s c o p i c r e s u l t s i n d i c a t e t h a t t h e r e must be a d i s t o r t i o n , a l b e i t s l i g h t , from t h i s geometry, c o n s i s t e n t w i t h t h e e x p e c t e d P e i e r l ' s d i s t o r t i o n o f a symmetric l i n e a r c h a i n . The bromide and i o d i d e a r e thus c l o s e l y s i m i l a r i n b o t h s t r u c t u r e s and p r o p e r t i e s t o t h e d i t h i o a c e t a t e complexes o f p l a t i n u m and n i c k e l , M2(CH3CS2)«+I, which a r e l i k e w i s e semiconductors w i t h n e a r l y b u t n o t e x a c t l y e q u a l c h a i n M-I bond l e n g t h s (2.975 and 2.981 Κ f o r P t , 2.928 and 2.940 Κ f o r Ni) (20,21) . 2

Spectroelectrochemically-generated

Species 2

The c h e m i s t r y o f c l u s t e r complexes, e.g. o f t h e s o r t [ F e S (SR)ι+] ~, i s o f p a r t i c u l a r i n t e r e s t s i n c e such complexes a r e known t o be c l o s e representations or syntheti in various iron-sulphu the v a l e n c e e l e c t r o n s a r e l o c a l i z e d o r d e l o c a l i z e d i n such complexes - i n f a c t s e v e r a l s t u d i e s by e . s . r . , n.m.r., and, more r e c e n t l y , resonance Raman s p e c t r o s c o p y have shown t h a t such c l u s t e r s a r e d e l o c a l i z e d r a t h e r than t r a p p e d - v a l e n c e s p e c i e s . This r e s u l t i s l i n k e d w i t h t h e most i m p o r t a n t b i o p h y s i c a l p r o p e r t y o f i r o n - s u l p h u r p r o t e i n s , v i z . t h a t o f e l e c t r o n t r a n s f e r . Rapid e l e c t r o n t r a n s f e r i s p o s s i b l e i f any c o n s e q u e n t i a l g e o m e t r i c rearrangements around t h e m e t a l atom s i t e s a r e s m a l l , as i m p l i e d by many resonance Raman r e s u l t s on such c l u s t e r complexes ( c f . t h e s m a l l - d i s p l a c e m e n t a p p r o x i m a t i o n , which p r o v i d e s a b a s i s f o r enhancement t o fundamental but n o t t o o v e r t o n e bands) (22) . I n i t i a l s t u d i e s o f [MSt|] ~ i o n s (M = Mo o r W) (25,24) have s i n c e been supplemented by s t u d i e s o f d i n u c l e a r species" ëTg. [(PhS)2FeS2MS2] ~ (25) and c l u s t e r s p e c i e s 4

4

2

2

(26) such as t h e c o p p e r ( I ) t e t r a t h i o m o l y b d a t e ( V I ) anions [ M S ^ C u L O J - (M = Mo o r W; L = CN, S C H , SCeH^CHa, C I , o r Br; η = 1-4). The q u e s t i o n o f whether e l e c t r o n s added t o a complex i o n become l o c a l i z e d o r d e l o c a l i z e d i s i m p o r t a n t , n o t o n l y f o r t h e type o f complex mentioned above, b u t a l s o f o r much w i d e r ranges o f complexes. F o r such s t u d i e s t h e use o f an OTTLE ( o p t i c a l l y t r a n s p a r e n t t h i n l a y e r e l e c t r o c h e m i c a l ) c e l l i s most a p p r o p r i a t e (27) . Such c e l l s c a n be adopted n o t o n l y t o e l e c t r o n i c and i n f r a r e d s t u d i e s , b u t a l s o t o Raman and i n p a r t i c u l a r , resonance Raman studies. The time s c a l e o f OTTLE measurements, b e i n g r a p i d by comparison w i t h t h a t o f c o n v e n t i o n a l b u l k e l e c t r o l y s i s , p e r m i t s r a p i d s p e c t r a l sampling o f t h e p r o d u c t . Moreover, by e s t a b l i s h i n g the e l e c t r o n i c band maxima o f t h e e l e c t r o c h e m i c a l l y g e n e r a t e d s p e c i e s , i t i s then p o s s i b l e t o resonance enhance Raman bands o f t h i s s p e c i e s u s i n g an e x c i t i n g l i n e which i s o f f - r e s o n a n c e f o r t h e r e a c t a n t ( s ) ; thus t h e e f f e c t s o f i n t e r f e r i n g r e a c t a n t bands a r e removed from t h e r e q u i r e d spectrum. The o p t i c a l c e l l used c o n s i s t e d o f a p l a t i n u m m i n i g r i d (52 mesh, 0.1 mm d i a m e t e r w i r e ) w i t h a t r a n s p a r e n c y o f 60%; a s i m i l a r c e l l has a l s o been used s u c c e s s f u l l y f o r b o t h Raman (Spex 14018, R6) and i n f r a r e d (Bruker 113 V interferometer) studies. Many i n o r g a n i c systems a r e c u r r e n t l y under s t u d y by t h e s e means, 2

6

5

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

64

EXCITED STATES AND REACTIVE INTERMEDIATES

viz. the tris-dithiolates of Cr, Mo and W and V in all of which the electrons are found to be added to, or removed from, extensively delocalized orbitals on the complex ion (a conclusion again based on the small-displacement approximation), and the confacial bioctahedral complexes [L RuCl L ] and [L - Cl RuCl RuClyY -y] , where L = PEt Ph, As(tol) or PPh (28). Such complexes display at least one and usually two stepwise, reversible one-electron transfer reactions without there being any gross structural change. Such work has already established that the extent of valence-electron derealization depends on the degree of asymmetry (y - x) of the complex and on the basicity of the terminal ligands. Moreover it is established that, where y - χ is zero, e.g. for [(PEt Ph) RuCl Ru(PEt Ph) ] , the intervalence band occurs at low wavenumber (4350 cm" ), implying that the valence electrons are highly delocalized. However, as y - χ increases, the intervalence band loses intensity and moves well into the visible region e.g for [(PEt Ph) RuCl RuCl a localized valence species studies of these systems are currently in progress in order to establish (a) which mode(s) change wavenumber and/or intensity on oxidation or reduction, and (b) by implication, therefore, where in the molecule the odd electron enters on reduction or is removed from on oxidation. The nature of the HOMO and LUMO may then be able to be established. Thin layer electrochemistry thus offers a very convenient way of controlling the oxidation state of a very thin (< 1 mm) layer of an electrochemically generated species. 2+

3

2

3

3

n+

3

3 x

x

3

3

3

2+

2

3

2

3

3

1

2

3

3

Conclusion The combination of several spectroscopic techniques, but particularly with involvement of resonance Raman spectroscopy, offers an effective way of studying the nature of the intervalence state in a wide variety of complexes ranging from linear-chain complexes of the Wolffram's red and Pt(pop)i+ sorts to electrochemically generated species. 2

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

"Extended Linear Chain Compounds", Miller, J.S., Ed., Plenum: New York. Vol.1, 1982. Clark, R.J.H. Chem. Soc. Rev. 1984, 13, 219. Clark, R.J.H. "Advances in Infrared and Raman Spectroscopy", Clark, R.J.H.; Hester, R.E., Eds., Wiley-Heyden: Chichester. Vol.11, 1984, p.95. Allen, S.D.; Clark, R.J.H.; Croud, V.B.; Kurmoo, M. Phil. Trans. Roy. Soc. Lond. A 1985, 314, 131. Fanizzi, F.P.; Natile, G.; Lanfranchi, M.; Tiripicchio, Α.; Clark, R.J.H.; Kurmoo, M. J. Chem. Soc. (Dalton Trans.) in press. Clark, R.J.H.; Stewart, B. Structure and Bonding 1979, 36, 1. Clark, R.J.H.; Croud, V.B.; Kurmoo, M. Inorg. Chem. 1984, 23, 2499. Clark, R.J.H.; Croud, V.B. J. Chem. Soc. (Dalton Trans.) 1985, 815.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

5. CLARK

65 Resonance Raman Spectroscopy: intervalence States

9. Clark, R.J.H.; Croud, V.B. Inorg. Chem. 1985, 24, 588. 10. Yamashita, M.; Ito, T. Inorg. Chim. Acta 1984, 87, L5. 11. Tanino, H.; Koshizuka, N.; Kobayashi, K.; Yamashita, M.; Hoh, K. J. Phys. Soc. Japan 1985, 54, 483. Clark, R.J.H.; Croud, V.B. unpublished results. 12. Sperline, R.P.; Dickson, M.K.; Roundhill, D.M. J. Chem. Soc. (Chem. Comm.) 1977, 62. 13. Filomena Das Remedios Pinto, M.A.; Sadler, P.J.; Neidle, S.; Sanderson, M.R.; Subbiah, A. J. Chem. Soc. (Chem. Comm.) 1980, 13. 14. Che, C.-M.; Schaeffer, W.P.; Gray, H.B.; Dickson, M.K.; Stein, P.; Roundhill, D.M. J. Am. Chem. Soc. 1982, 104, 4253. 15. Stein, P.; Dickson, M.K.; Roundhill, D.M. J. Am. Chem. Soc. 1983, 105, 3489. 16. Che, C.-M.; Herbstein, F.H.; Schaefer, W.M.; Marsh, R.E.; Gray, H.B. J. Am Chem Soc 1983 105 4604 17. Clark, R.J.H.; Kurmoo 1985, 579. 18. Kurmoo, M.; Clark, R.J.H. Inorg. Chem. in press. 19. Clark, R.J.H.; Kurmoo, M.; Dawes, H.M.; Hursthouse, M.B. Inorg. Chem. in press. 20. Bellito, C.; Flamino, Α.; Gastaldi, L.; Scaramuzza, L. Inorg. Chem. 1983, 22, 444. 21. Bellito, C.; Dessy, G.; Fares, V. Mol. Cryst. Liq. Cryst. 1985, 120, 381. 22. Clark, R.J.H.; Dines, T.J. Mol. Phys. 1981, 42, 193. 23. Clark, R.J.H.; Dines, T.J.; Wolf, M.L. J. Chem. Soc. (Faraday Trans.) 1982, 78, 679. 24. Clark, R.J.H.; Dines, T.J.; Proud, G.P. J. Chem. Soc. (Dalton Trans.) 1983, 2019. 25. Clark, R.J.H.; Dines, T.J.; Proud, G.P. J. Chem. Soc. (Dalton Trans.) 1983, 2229. 26. Clark, R.J.H.; Joss, S.; Zvagulis, M.; Garner, C.D.; Nicholson, J.R. J. Chem. Soc. (Dalton Trans.) in press. 27. Heineman, W.R. J. Chem. Ed. 1983, 60, 305. 28. Heath, G.A.; Lindsay, A.J.; Stevenson, T.A.; Vattis, D.K. J. Organometal. Chem. 1982, 233, 353. RECEIVED November 7, 1985

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

6 M e t a l - L i g a n d Charge Transfer Photochemistry Metal-Metal Bonded Complexes D. J. Stufkens, A. Oskam, and M. W. Kokkes Anorganisch Chemisch Laboratorium, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands The complexes (CO) a different photochemistr T 1 3 3 K ) . T h e g r e e n l i n e (λ = 5 1 4 . 5 n m ) o f a n a r g o n - i o n l a s e r w a s u s e d as i r r a d i a t i o n s o u r c e . F i g u r e 6 s h o w s t h e I R bands of the photoproducts in the C O - s t r e t c h i n g region upon photolysis of ( C O ) M n M n ( C O ) ( i - P r - D A B ) at d i f f e r e n t t e m p e r a t u r e s . A t 2 3 0 K M n ( C O ) i o is f o r m e d w i t h C O - s t r e t c h i n g m o d e s at 2 0 4 5 , 2009 a n d 1977 c m " . T h e changes in the e l e c t r o n i c absorption s p e c t r u m a c c o m p a n y i n g the r e a c t i o n at 2 3 0 K a r e s h o w n i n F i g u r e 7. T h e a ->xr* t r a n s i t i o n s h i f t s f r o m 340 t o 350 n m due t o t h e f o r m a t i o n o f M n ( C O ) i o w h i l e t h e M L C T b a n d s h i f t s f r o m 550 t o 7 4 5 n m . T h e f o r m a t i o n o f M n 2 ( C O ) i o is t h e r e s u l t o f a h o m o l y t i c s p l i t t i n g o f t h e m e t a l - m e t a l b o n d ( s c h e m e 1). 5

3

2

1

b

2

A p p a r e n t l y the M n ( C O ) radicals f o r m e d react to M n ( C O ) i o . A t the s a m e t i m e the M n ( C O ) ( i - P r - D A B ) r a d i c a l s react to M n ( C O ) ( i - P r - D A B ) , a b i n u c l e a r m e t a l - m e t a l b o n d e d c o m p l e x w i t h an i - P r - D A B l i g a n d at e a c h m e t a l f r a g m e n t . S u c h c o m p l e x e s h a v e b e e n i d e n t i f i e d b e f o r e as t h e r m a l l y unstable side-products of the r e a c t i o n b e t w e e n [Mn(CO)s]~ and M n ( C O ) X ( R - D A B ) ( X = C 1 , B r o r I) (18). T h e c o r r e s p o n d i n g c o m p l e x M n ( C O ) 6 ( b i p y ' ) 2 ( b i p y = 4 , 4 ' - d i m e t h y l - 2 , 2 ' - b i p y r i d i n e ) has b e e n d e s c r i b e d by M o r s e a n d W r i g h t o n (11). T h e M L C T b a n d at 7 4 5 n m a n d t h e t h r e e C O - v i b r a t i o n s at 1 9 4 6 , 1898 a n d 1888 c m " a r e a s s i g n e d t o t h e b i n u c l e a r c o m p l e x . The shift of the M L C T band to l o w e r energy agrees w i t h the d i f f e r e n c e in e l e c t r o n w i t h d r a w i n g p o w e r b e t w e e n the M n ( C O ) s group of the p a r e n t c o m p o u n d and the M n ( C O ) ( i - P r - D A B ) f r a g m e n t of the p h o t o p r o d u c t M n ( C O ) e ( i - P r - D A B ) . T h i s p h o t o p r o d u c t is p a r t l y s p l i t i n t o i t s r a d i c a l s w h i c h h a v e b e e n i d e n t i f i e d w i t h E S R (1 2). F u r t h e r m o r e , t h i s c o m p l e x is t h e r m a l l y u n s t a b l e . R a i s i n g t h e t e m p e r a t u r e t o 293 Κ c a u s e s the disappearance of the 745 n m band and a shift of the C O - s t r e t c h i n g modes. The same decomposition product, presumably M n ( C O ) e ( i - P r - D A B ) ( 2 - M e - T H F ) , is f o u n d as a p h o t o p r o d u c t w h e n t h e p h o t o l y s i s t a k e s p l a c e at 29 3 K . 5

2

3

2

6

3

,

2

1

3

2

2

2

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2

STUFKENS ET AL.

F i g u r e 5. Copyright

MLCT

Photochemistry

E n e r g y vs d i s t o r t i o n d i a g r a m . ( R e p r o d u c e d 1985, A m e r i c a n C h e m i c a l Society).

from Réf. 1

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

133

Κ

173 Κ

1800

2200

F i g u r e 6. C O s t r e t c h i n g m o d e s ( I R ) o f t h e p r o d u c t s f o r m e d u p o n p h o t o l y s i s of ( C O ) M n M n ( C O ) ( i - P r - D A B ) in 2 - M e - T H F at d i f f e r e n t temperatures. #=Mn (CO)i ; f = M n ( C O ) ( i - P r - D A B ) ; o=Mn ( C O ) ( i - P r - D A B ) ( 2 - M e - T H F ) ; X = M n ( C O ) ( 2 - M e - T H F ) ; ©= [ M n ( C O ) ] ~ @ = [ M n ( C O ) ( i - P r - D A B ) ( 2 - M e - T H F ) ] . ( R e p r o d u c e d f r o m R e f . 12. C o p y r i g h t 1985, A m e r i c a n C h e m i c a l S o c i e t y ) . 5

3

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In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

5

6.

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STUFKENS ET AL.

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T h i s h o m o l y t i c s p l i t t i n g o f t h e m e t a l - m e t a l b o n d is o b s e r v e d f o r a l l c o m p l e x e s ( C O ) M n M ( C O ) ( a - d i i m i n e ) ( M = M n , R e ) at T > 2 0 0 K . When h o w e v e r the photolysis of ( C O ) M n M n ( C O ) ( i - P r - D A B ) i n 2 - M e - T H F takes place below 200K but above the t e m p e r a t u r e at w h i c h the solvent s o l i d i f i e s to a g l a s s (T=i 1 3 0 K ) , n e w b a n d s s h o w up i n t h e C O - s t r e t c h i n g r e g i o n at t h e expense of those belonging to M n ( C O ) i and M n ( C O ) ( i - P r - D A B ) This e f f e c t is s t r o n g e s t w h e n t h b a n d s , t h o s e a t 1 8 8 4 , 186 [ M n ( C O ) ] ~ (19). > ^ s i g n e d to the s o l v a t e d c a t i o n [ M n ( C O ) ( i - P r - D A B ) ( 2 - M e - T H F ) ] and i n d i c a t e d by φ i n t h e I R s p e c t r u m . T h i s a s s i g n m e n t is b a s e d o n t h e c l o s e s i m i l a r i t y b e t w e e n these bands and those of [ M n ( C O ) ( i - P r - D A B ) ( T H F ) ] [ O T F ] " ( O T F = C F S 0 ) , w h i c h c o m p o u n d h a s b e e n p r e p a r e d s e p a r a t e l y . In t h e I R s p e c t r u m o f t h e p h o t o p r o d u c t s o b t a i n e d at 1 7 3 K e x t r a bands ( i n d i c a t e d w i t h X ) are o b s e r v e d , w h i c h do n o t s h o w u p u p o n p h o t o l y s i s at 2 3 0 K a n d w h i c h a r e v e r y weak in the 1 3 3 K s p e c t r u m . These bands apparently belong to a c o m p l e x w h i c h is o n l y f o r m e d w h e n M n ( C O ) i o i s a m a j o r p h o t o p r o d u c t a n d w h i c h i s unstable at higher t e m p e r a t u r e s . Indeed, these bands disappear w h e n the s o l u t i o n , a f t e r p h o t o l y s i s a t 1 7 3 K , is r a i s e d i n t e m p e r a t u r e . T h e s e b a n d s are therefore assigned to the c o m p l e x M n ( C O ) g ( 2 - M e - T H F ) and this assignment is s u p p o r t e d by t h e c l o s e a g r e e m e n t b e t w e e n these f r e q u e n c i e s and those o f o t h e r M n ( C O ) g L c o m p l e x e s (20). A t higher t e m p e r a t u r e s 2 - M e - T H F i s s u b s t i t u t e d by C O f r o m t h e s o l u t i o n . The ions f o r m e d by p h o t o l y s i s at T < 2 0 0 K a r e not stable at higher temperatures. Raising the temperature causes them to react back to the p a r e n t c o m p o u n d w i t h loss o f 2 - M e - T H F . W h e n , h o w e v e r , P ( n - B u ) is added to a s o l u t i o n of t h e ions, [ M n ( C O ) ( i - P r - D A B ) ( P ( n - B u ) ] [ M n ( C O ) ] " is f o r m e d , w h i c h is a stable c o m p o u n d at r o o m t e m p e r a t u r e . F r o m these results one m i g h t c o n c l u d e t h a t photolysis leads to homolytic or heterolytic splitting of the m e t a l - m e t a l bond, depending on t h e t e m p e r a t u r e o f t h e s o l u t i o n . If i n s t e a d o f ( C O ) M n M n ( C O ) ( i - P r - D A B ) , ( C O ) M n M n ( C O ) ( p h e n ) o r ( C O ) M n M n ( C O ) ( b i p y ) a r e p h o t o l y z e d at 1 3 3 K , i t b e c o m e s c l e a r t h a t h e t e r o l y t i c s p l i t t i n g o f t h e m e t a l - m e t a l b o n d is n o t a p r i m a r y p h o t o p r o c e s s . In t h a t c a s e n e i t h e r M n ( C O ) i o n o r [Mn(CO)5] i s f o r m e d b u t i n s t e a d a p h o t o s u b s t i t u t i o n p r o d u c t . F i g u r e 8. shows t h e I R s p e c t r a l changes upon photolysis of ( C O ) s M n M n ( C O ) ( p h e n ) at 1 3 3 K . O n l y a s m a l l a m o u n t o f [ M n ( C O ) ] ~ is f o r m e d . Instead, f r e e C O shows up w i t h V=2132 c m " . F u r t h e r m o r e , a l l C O - v i b r a t i o n s s h i f t t o l o w e r f r e q u e n c i e s , especially those of the metal fragment w i t h the α-diimine ligand. A t the s a m e t i m e the M L C T band at 520 n m dissappears and a b r o a d band shows up b e t w e e n 5 8 0 a n d 7 0 0 n m . T h e s e d a t a p o i n t t o a p h o t o s u b s t i t u t i o n o f C O o f t h e M n ( C O ) ( p h e n ) m o i e t y by 2 - M e - T H F . T h e p h o t o p r o d u c t ( C O ) M n M n ( C O ) ( p h e n ) ( 2 - M e - T H F ) cannot be i s o l a t e d since it d i s p r o p o r t i o n a t e s upon raising the temperature. A similar disproportionation reaction was observed 5

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In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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EXCITED STATES AND REACTIVE INTERMEDIATES

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F i g u r e 7. C h a n g e s i n t h e e l e c t r o n i c a b s o r p t i o n s p e c t r u m u p o n p h o t o l y s i s o f ( C O ) M n M n ( C O ) ( i - P r - D A B ) i n 2 - M e - T H F at 2 3 0 K . ( R e p r o d u c e d f r o m R e f . 12. C o p y r i g h t 1985, A m e r i c a n C h e m i c a l Society). 5

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F i g u r e 8. I R s p e c t r a l c h a n g e s u p o n p h o t o l y s i s o f ( C O ) s M n M n ( C O ) 3 (phen) i n 2 - M e - T H F at 1 3 3 K . ( R e p r o d u c e d f r o m R e f . 1 2 . C o p y r i g h t 1985, A m e r i c a n C h e m i c a l S o c i e t y ) .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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75

for t h e phosphine s u b s t i t u t e d p r o d u c t f o r m e d by t h e r e a c t i o n o f a phosphine ligand w i t h the 2 - M e - T H F substituted c o m p l e x . The corres­ ponding photoproduct ( C O ) R e M n ( C O ) 2 ( i - P r - D A B ) ( P P h ) did however not d i s p r o p o r t i o n a t e a n d c o u l d be i s o l a t e d a n d i d e n t i f i e d . A s m e n t i o n e d a b o v e , the photosubstitution products of ( C O ) M n M n ( C O ) 3 ( a - d i i m i n e ) dispropor­ tionate upon raising the t e m p e r a t u r e . U p to now only the cations [ M n ( C O ) ( a - d i i m i n e ) ( 2 - M e - T H F ) ] c o u l d de i d e n t i f i e d , w h i c h m e a n s t h a t t h e c a t i o n [ M n ( C O ) ( a - d i i m i n e ) ( 2 - M e - T H F ) 2 ] , f o r m e d by h e t e r o l y t i c s p l i t t i n g of the m e t a l - m e t a l bond, readily reacts w i t h C O f r o m the solution. On the basis of these results w e propose the m e c h a n i s m shown i n F i g u r e 9 f o r t h e p h o t o l y s i s o f t h e s e c o m p l e x e s at T < 2 0 0 K . T h i s r e a c t i o n is observed for all complexes ( C O ) M M n ( C O ) ( a - d i i m i n e ) . Irradiation into the M L C T band causes photosubstitution of C O of the M n ( C O ) ( a - d i i m i n e ) m o i e t y by 2 - M e - T H F . T h e d i f f e r e n c e i n e l e c t r o n e g a t i v i t y b e t w e e n b o t h m e t a l f r a g m e n t s i s t h e n so l a r g e t h a t r a i s i n g t h e t e m p e r a t u r e c a u s e s a heterolytic splitting of the m e t a l - m e t a l bond The c a t i o n formed reacts with C O from the solution t e m p e r a t u r e the ions r e c o m b i n is t h e n r e l e a s e d . A d i f f e r e n t p h o t o l y s i s b e h a v i o r a t 1 33 Κ is f o u n d f o r t h e c o m p l e x e s ( C O ) M n R e ( C O ) ( a - d i i m i n e ) . Irradiation of these complexes causes the d i s s a p p e a r a n c e o f t h e M L C T b a n d . W h e n t h e t e m p e r a t u r e is r a i s e d t o 203 Κ t h e M L C T b a n d r e t u r n s . A s i m i l a r r e v e r s i b l e b e h a v i o r is o b s e r v e d i n t h e I R s p e c t r a . T h e s e s p e c t r a do n o t s h o w f r e e C O a n d t h e C O - v i b r a t i o n s o f t h e m a i n p h o t o p r o d u c t do n o t a g r e e w i t h t h o s e o f a n y p h o t o p r o d u c t f o u n d so f a r . A p a r t f r o m t h i s p h o t o p r o d u c t a s m a l l a m o u n t o f i o n s i s o b s e r v e d . T h e d i s a p p e a r a n c e o f t h e M L C T b a n d c a n n o t be t h e r e s u l t o f c o m p l e t e l o s s of t h e α - d i i m i n e l i g a n d s i n c e d i f f e r e n t C O f r e q u e n c i e s a r e found f o r t h e photoproducts of t h e R - D A B and phen c o m p l e x e s . F o r this r e a c t i o n w e p r o p o s e t h e b r e a k i n g o f a m e t a l - n i t r o g e n b o n d by w h i c h a c o m p l e x (CO) 5 M n R e ( C O ) ( a - a - d i i m i n e ) ( 2 - M e - T H F ) is f o r m e d in w h i c h the a - d i i m i n e l i g a n d is σ - m o n o d e n t a t e l y c o o r d i n a t e d t o R e . S u c h σ - m o n o d e n t a t e l y b o n d e d α - d i i m i n e ligands o c c u r e.g. in Cr(CO) (R-DAB)(_5), M ( C O ) ( b i p y ' ) ( M = C r , M o o r W) (21) a n d M C l ( P P h ) ( t - B u - D A B ) ( M = P d o r P t ; t - B u = t e r t i a r y - b u t y l ) ( 2 2 ) . A f t e r b r e a k i n g of t h e m e t a l - n i t r o g e n bond, 2 - M e - T H F c o o r d i n a t e s to R e at t h e o p e n s i t e . T h e s a m e p h o t o p r o d u c t is f o r m e d b y p h o t o l y s i s o f ( C O ) M n R e ( C O ) ( i - P r - D A B ) i n a P V C f i l m at 1 9 8 K . S i n c e t h i s f i l m is c a s t from T H F , the solvent molecules, still present in the f i l m , will stabilize the photoproduct. When these complexes are irradiated w i t h light of higher e n e r g y (X=350nm), d i s p r o p o r t i o n a t i o n t a k e s p l a c e j u s t as f o r t h e ( C O ) s M n M n ( C O ) ( a - d i i m i n e ) complexes. These photolysis reactions are shown s c h e m a t i c a l l y f o r ( C O ) M n R e ( C O ) ( R - D A B ) i n F i g u r e 10. 5

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It h a s b e e n a r g u e d t h a t t h e l o w - e n e r g y a b s o r p t i o n b a n d o f t h e s e ( C O ) s M M ' ( C O ) ( a - d i i m i n e ) c o m p l e x e s has t o b e a s s i g n e d t o a M L C T t r a n s i t i o n f r o m a d ( Μ ' ) o r b i t a l not involved in the m e t a l - m e t a l bond to the lowest π* level of the α - d i i m i n e ligand. The electronic transition w i l l t h e r e f o r e be d i r e c t e d to d ï ï * and from this state i n t e r s y s t e m crossing may o c c u r to both d ττ* and a π*. F r o m both states a reaction may o c c u r . The r e a c t i o n f r o m σ π * leads to s p l i t t i n g of the m e t a l - m e t a l bond a n d o u r r e s u l t s s h o w t h a t t h i s s p l i t t i n g i s h o m o l y t i c a n d t h a t i t is t h e m a i n r e a c t i o n i n 2 - M e - T H F a t T > 2 0 0 K . J u s t as i n t h e c a s e o f t h e c o r r e s p o n d i n g d - c o m p l e x e s M ( C O ) ^ α - d i i m i n e ) ( M = C r , M o o r W) ( 6 , JJD) a n d d - c o m p l e x e s 3

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In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2

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F i g u r e 1 0 . P h o t o l y s i s o f ( C O ) M n R e ( C O ) ( R - D A B ) i n 2 - M e - T H F (S) a t T < 1 7 3 K . V > V i ( R e p r o d u c e d f r o m R e f . 12. C o p y r i g h t 1985, A m e r i c a n C h e m i c a l Society).

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F e ( C O ) 3 ( a - d i i m i n e ) (8), a p h o t o c h e m i c a l r e a c t i o n f r o m d ^ π * w i l l lead to r e l e a s e o f C O . T h i s is i n f a c t o b s e r v e d f o r s e v e r a l o f t h e s e c o m p l e x e s i n 2 - M e - T H F a t 1 3 3 K . It is u n l i k e l y t h a t one o f t h e s e r e a c t i o n s o c c u r s f r o m a ligand field state since the corresponding spin- a l l o w e d transitions are f o u n d b e l o w 4 0 0 n m a n d i r r a d i a t i o n t a k e s p l a c e at 5 1 4 . 5 n m . 3

We t h e r e f o r e c o n c l u d e t h a t the high t e m p e r a t u r e r e a c t i o n o c c u r s f r o m the c r n * s t a t e a n d t h e l o w t e m p e r a t u r e o n e f r o m d 7 r * . It is n o t y e t c l e a r w h e t h e r t h i s c h a n g e o f r e a c t i o n is a m e r e t e m p e r a t u r e e f f e c t o r t h a t t h e i n c r e a s e o f v i s c o s i t y o f t h e s o l v e n t p l a y s an i m p o r t a n t r o l e h e r e . In t h e f i r s t c a s e w e d e a l w i t h a σ ττ* s t a t e h i g h e r i n e n e r g y t h a n d 7r a n d o n l y o c c u p i e d at h i g h e r t e m p e r a t u r e s . T h i s s i t u a t i o n is t h e n s i m i l a r t o that of the c o m p l e x e s [ R u ( N H ) ( 4 R - P y ) ] (23) a n d M ( C O ) L ( L = 4 R - p y r i d i n e , p y r i d a z i n e ; M = C r o r W)(24), i n w h i c h a ^ F s t a t e is c l o s e i n e n e r g y t o a M L C T s t a t e . It is h o w e v e r q u i t e p o s s i b l e t h a t t h e v i s c o s i t y o f 2 - M e - T H F is o f i m p o r t a n c e h e r e . T h i s v i s c o s i t y i n c r e a s e s d r a s t i c a l l y u p o n c o o l i n g . T h e r a d i c a l s f o r m e d by t h e h o m o l y t i c s p l i t t i n g o f t h e m e t a l - m e t a l b o n d c a n t h e n not diffuse t h r o u g h th c o m p o u n d . A s a result the t h e m u c h s l o w e r r e a c t i o n f r o m t h e d 7 r s t a t e ( r e l e a s e o f C O ) c a n be observed. The b r e a k i n g of a m e t a l - n i t r o g e n bond instead of C O release for the c o m p l e x e s ( C O ) M n R e ( C O ) 3 ( a - d i i m i n e ) agrees w i t h the m e c h a n i s m proposed f o r t h e p h o t o s u b s t i t u t i o n o f C O i n F e ( C O ) 3 ( a - d i i m i n e ) (8). T h e p r i m a r y p h o t o p r o c e s s o f t h i s r e a c t i o n w a s p r o p o s e d t o be b r e a k i n g o f a m e t a l - n i t r o g e n bond w i t h f o r m a t i o n of an i n t e r m e d i a t e in w h i c h the α - d i i m i n e l i g a n d is σ - m o n o d e n t a t e l y b o n d e d t o F e . A n u c l e o p h i l i c l i g a n d t h e n a t t a c k s the o p e n s i t e , C O is r e l e a s e d and the σ , σ - c o o r d i n a t i o n of t h e α - d i i m i n e l i g a n d is r e s t o r e d . T h e s a m e m e c h a n i s m is p r o p o s e d f o r t h e p h o t o s u b s t i t u t i o n o f C O by 2 - M e - T H F i n t h e c o m p l e x e s ( C O ) M n M n ( C O ) 3 ( a d i i m i n e ) . A p p a r e n t l y , this r e a c t i o n stops after the p r i m a r y photoprocess in the case of the ( C O ) s M n R e ( C O ) 3 ( a - d i i m i n e ) c o m p l e x e s because the R e - C O b o n d is t o o s t r o n g . 3

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Photochemical reactions with P R 3 P h o t o l y s i s in T H F o r 2 - M e - T H F at 293K i n the absence of P R 3 leads to the f o r m a t i o n of M n 2 ( C O ) i for all c o m p l e x e s ( C O ) s M n M ( C O ) 3 ( a - d i i m i n e ) . This m e a n s t h a t t h e p r i m a r y p h o t o p r o c e s s is a h o m o l y t i c s p l i t t i n g o f t h e m e t a l - m e t a l b o n d . In t h e p r e s e n c e o f P R 3 d i f f e r e n t r e a c t i o n s a r e o b s e r v e d d e p e n d i n g on the c o m p l e x and on the P R 3 l i g a n d . Thus, upon p h o t o l y s i s of ( C O ) M n M n ( C O ) 3 ( a - d i i m i n e ) in the presence of P P h 3 , h o m o l y t i c s p l i t t i n g of t h e m e t a l - m e t a l b o n d o c c u r s a n d M n 2 ( C O ) e ( P P h 3 ) 2 is f o r m e d , p r o v i d e d t h e c o m p l e x is i r r a d i a t e d by v i s i b l e l i g h t w i t h a l o w p h o t o n f l u x ( e . g . m e d i u m p r e s s u r e H g - l a m p , 5 0 0 n m f i l t e r ) . If i n s t e a d a n a r g o n i o n l a s e r is u s e d w i t h a m u c h h i g h e r p h o t o n f l u x ( e . g . λ = 5 1 4 . 5 n m , p=20 m W ) b o t h M n 2 ( C O ) e ( P P h 3 ) 2 a n d M n 2 ( C O ) i o a r e f o r m e d . S u c h a p h o t o n f l u x d e p e n d e n c e has e . g . b e e n o b s e r v e d by S t i e g m a n a n d T y l e r ( 2 5 , 26). U p o n i r r a d i a t i o n w i t h l o w i n t e n s i t y l i g h t t h e c o n c e n t r a t i o n o f M n ( C O ) r a d i c a l s f o r m e d is s m a l l c o m p a r e d w i t h t h a t of P P h . T h e r e a c t i o n w i t h P P h 3 is t h e n f a v o r e d w i t h respect to the f o r m a t i o n of M n 2 ( C O ) i o - A t a higher photonflux, the c o n c e n t r a t i o n o f M n ( C O ) s r a d i c a l s is m u c h l a r g e r a n d t h i s p r o m o t e s t h e f o r m a t i o n of M n ( C O ) i . When the photolysis of ( C O ) M n M n ( C O ) 3 ( a d i i m i n e ) t a k e s place in the presence of P ( n - B u ) 3 , n e i t h e r M n 2 ( C O ) i o nor M n ( C O ) ( P ( n - B u ) ) 2 is f o r m e d . I n s t e a d , t h e i o n s [ M n ( C O ) ( a - d i i m i n e ) ( P ( n Bu)3] a n d [ M n ( C O ) s ] a r e f o r m e d . T h i s c a t i o n has b e e n i d e n t i f i e d w i t h * H 0

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In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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STUFKENS ET AL.

MLCT

Photochemistry

79

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and P - N M R and w i t h IR after performing this p h o t o c h e m i c a l r e a c t i o n on a preparative scale. M o r e o v e r , the C O - f r e q u e n c i e s of this cation closely resemble those of the c a t i o n in [ M n ( C O ) ( b i p y ) ( T H F ) ] [ O T F ] " ( O T F = C F 3 S O 3 ) , w h i c h c o m p o u n d w a s p r e p a r e d by r e a c t i o n of M n ( C O ) 3 ( b i p y ' ) B r with Ag(OTF). A r e m a r k a b l e p r o p e r t y o f t h i s r e a c t i o n is i t s v e r y h i g h q u a n t u m y i e l d (Φ - 1 0 ) . S u c h a h i g h q u a n t u m y i e l d p o i n t s t o a c h a i n r e a c t i o n . In a n a l o g y w i t h the mechanisms of the photodisproportionation of M n 2 ( C O ) i o i n N - d o n o r s o l v e n t s (27) a n d o f ( η - C H C 5 h U ) M o ( C O ) 6 i n t h e p r e s e n c e o f p h o s p h i n e ( 2 9 ) , t h e f o l l o w i n g c h a i n m e c h a n i s m is p r o p o s e d f o r t h e çlisproportionation of ( C O ) M n M n ( C O ) ( a - d i i m i n e ) in the presence of P(n-Bu) : f

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Reaction (CO) MnMn(CO) L + Ρ 5

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Initiation (CO) MnMn(CO) L 5

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Propagation Mn(CO) L + Ρ Mn(CO) LP + (CO) MnMn(CO) L [(CO) s M n M n ( C O ) L ] ~ 5

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Termination Mn(CO) LP + Mn(CO) 3

Mn(CO)

> — » »

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Mn(CO) LP [ M n ( C O ) L P ] +[(CO) 5 M n M n ( C O ) I _ J [Mn(CO) ]"+ M n ( C O ) L 3

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Scheme 2 T h e i n i t a t i o n r e a c t i o n is h o m o l y s i s o f t h e m e t a l - m e t a l b o n d j u s t as i n t h e absence of P ( n - B u ) . The propagation steps start f r o m the r a d i c a l M n ( C O ) ( a - d i i m i n e ) . T h e E S R s p e c t r a s h o w t h a t t h e u n p a i r e d e l e c t r o n is l o c a l i z e d at t h e α - d i i m i n e l i g a n d , w h i c h m a k e s the r a d i c a l a 1 6 - e l e c t r o n species, contrary to the 17-electron radical Mn(CO) . This intermediate t a k e s up P ( n - B u ) f o r m i n g t h e 1 8 - e l e c t r o n s p e c i e s M n ( C O ) ( a - d i i m i n e ) ( P ( n - B u ) ) , w h i c h is t h e k e y f a c t o r i n t h e d i s p r o p o r t i o n a t i o n r e a c t i o n . S u c h i n t e r m e d i a t e s are assumed to play an i m p o r t a n t role in the photo­ d i s p r o p o r t i o n a t i o n o f M n ( C O ) i o (27) a n d ( η - C H - C H ) M o ( C O ) (28,29). Thus, S t i e g m a n and T y l e r proposed that the p h o t o d i s p r o p o r t i o n a t i o n of M n ( C O ) i o i n N - d o n o r s o l v e n t s t a k e s p l a c e v i a e l e c t r o n t r a n s f e r f r o m the 1 9 - e l e c t r o n i n t e r m e d i a t e M n ( C O ) N to M n ( C O ) i o (27). E l e c t r o n transfer f r o m M n ( C O ) ( a - d i i m i n e ) ( P ( n - B u ) ) c a n take place to both M n ( C O ) s a n d t o t h e p a r e n t c o m p o u n d . T h e f i r s t r e a c t i o n is a t e r m i n a t i n g s t e p , t h e l a t t e r one leads to the f o r m a t i o n of the unstable anion [(CO) M n M n ( C O ) ( a diimine]" w h i c h decomposes into [ M n ( C O ) ] " and M n ( C O ) ( a - d i i m i n e ) . The l a t t e r r a d i c a l is r e s p o n s i b l e f o r t h e c h a i n r e a c t i o n . T h e i o n s [ M n ( C O ) ( a d i i m i n e ) ( P ( n - B u ) ) ] and [ M n ( C O ) ] " are the only products observed and t h e r e f o r e no o t h e r t e r m i n a t i n g r e a c t i o n s a r e a s s u m e d t o o c c u r . T h e f a c t o r s influencing the quantum yield of this r e a c t i o n and the c h e m i c a l properties of t h e highly r e d u c i n g r a d i c a l M n ( C O ) ( a - d i i m i n e ) ( P ( n - B u ) ) are subject to further study. 3

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In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

80

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Photolysis in rigid m e d i a If t h e p h o t o l y s i s t a k e s p l a c e i n an i n e r t gas m a t r i x , b o t h t h e h o m o l y t i c s p l i t t i n g of the m e t a l - m e t a l bond and the b r e a k i n g of a m e t a l - n i t r o g e n b o n d w i l l be f o l l o w e d by a f a s t b a c k r e a c t i o n t o t h e p a r e n t c o m p o u n d . T h e r a d i c a l s f o r m e d by h o m o l y s i s o f t h e m e t a l - m e t a l b o n d c a n n o t d i f f u s e f r o m the m a t r i x site and w i l l r e c o m b i n e to the parent c o m p o u n d . M o r e o v e r , t h e p h o t o p r o d u c t o b t a i n e d by b r e a k i n g o f a m e t a l n i t r o g e n b o n d , w i l l n o t be s t a b i l i z e d by a c o o r d i n a t i n g s o l v e n t m o l e c u l e a n d t h e r e f o r e r e a c t b a c k to the parent c o m p o u n d . Because of this the p h o t o c h e m i s t r y of some of t h e s e c o m p l e x e s has a l s o b e e n s t u d i e d i n a C h k - m a t r i x at 1 0 K a n d f o r c o m p a r i s o n i n a P V C f i l m , w h i c h is a l e s s r i g i d m e d i u m t h a n t h e m a t r i x e s p e c i a l l y at r o o m t e m p e r a t u r e . W h e n t h e c o m p l e x ( C O ) M n R e ( C O ) ( i - P r - D A B ) is p h o t o l y z e d i n a C H t t - m a t r i x f i v e n e w C O b a n d s s h o w up w i t h s i m u l t a n e o u s l o s s o f C O . T h e n e w b a n d s do n o t b e l o n g t f th photoproduct observed far Th IR spectral changes, show indicating a well-defined clea n e w b a n d s h o w s up i n t h e v i s i b l e r e g i o n . A n n e a l i n g a C O - m a t r i x a f t e r t h e p h o t o l y s i s d i d n o t c a u s e a r e a c t i o n w h i c h m e a n s t h a t t h e p h o t o p r o d u c t is c o o r d i n a t i v e l y s a t u r e d . D r a s t i c changes also o c c u r in the 1 200-1500 c m r e g i o n o f t h e I R s p e c t r u m . T h e b a n d at 1479 c m " b e l o n g i n g t o v ( C N ) o f t h e c o o r d i n a t e d i - R r - D A B l i g a n d a n d t h e 1 294 c m band belonging to v ( C C ) d i s a p p e a r w h i l e t w o n e w b a n d s s h o w up at 1389 a n d 1304 c m , r e s p e c t i v e l y . T h e s e b a n d s a r e a s s i g n e d to v ( C N ) a n d v ( C C ) , r e s p e c t i v e l y , of the i - P r - D A B ligand in the p h o t o p r o d u c t . This low frequency of v ( C N ) p o i n t s t o a l a r g e i n v o l v e m e n t o f t h e 7r*Tevel o f t h e i - P r - D A B l i g a n d i n t h e b o n d i n g s i n c e t h i s o r b i t a l is a n t i - b o n d i n g b e t w e e n C a n d N . A s i m i l a r l o w f r e q u e n c y f o r V ( C N ) has b e e n f o u n d f o r t h e p h o t o p r o d u c t o f F e ( C O ) ( R - D A B ) , i n w h i c h t h e R - D A B l i g a n d is η , η c o o r d i n a t e d t o F e ( 8 ) O n the basis of these results we propose that the c o m p l e x ( C O ) M n ( i - P r - D A B ) R e ( C O ) (30) is f o r m e d ( F i g u r e 12) i n w h i c h t h e i - P r - D A B l i g a n d is σ , σ - c o o r d i n a t e d t o R e a n d η , η t o M n . T h i s p r o p o s a l is s t r o n g l y s u p p o r t e d by t h e o b s e r v a t i o n s o f A d a m s (31) a n d K e i j s p e r (32). A d a m s a c c i d e n t a l l y synthesized the c o m p l e x ( C O ) M n ( M e - D A B ( C H , C H ) ) M n ( C O ) a n d e s t a b l i s h e d t h e s t r u c t u r e by X - r a y d i f f r a c t i o n . K e i j s p e r s y n t h e s i z e d i n l o w y i e l d an a n a l o g o u s c o m p l e x ( C O ) M n ( t - B u - D A B ) M n ( C O ) by t r e a t i n g M n ( C O ) ( t - B u - D A B ) B r w i t h [ C p F e ( C O ) ] " . The C O - s t r e t c h i n g frequencies of these c o m p l e x e s agree very w e l l w i t h those of our photoproduct. P h o t o l y s i s of the o t h e r ( C O ) M M ' ( C O ) ( i - P r - D A B ) ( M , M ' = M n , R e ) c o m p l e x e s in a C h U - m a t r i x did not lead to the f o r m a t i o n of ( C O ) M ( i - P r - D A B ) M ' ( C O ) . If t h e s e c o m p l e x e s a r e h o w e v e r p h o t o l y z e d i n a P V C f i l m , c a s t f r o m T H F , t h r e e of the four c o m p l e x e s ( C O ) M M ' ( C O ) ( i - P r - D A B ) ( M , M ' = M n , R e e x c e p t M = M ' = R e ) c a n be c o n v e r t e d i n t o ( C O ) M ( i P r - D A B ) M ' ( C O ) . F i g u r e 13 s h o w s t h e I R s p e c t r a l c h a n g e s a c c o m p a n y i n g t h e p h o t o l y s i s o f ( C O ) M n M n ( C O ) ( i - P r - D A B ) i n a P V C f i l m at 2 9 3 K . A t t h i s t e m p e r a t u r e no f r e e C O is o b s e r v e d d u e t o t h e b r o a d n e s s o f t h e I R b a n d . N o w , w h e n t h i s c o m p l e x is p h o t o l y z e d i n t h e f i l m at 1 9 3 K , f i v e e x t r a C O b a n d s s h o w up w h i c h d i s a p p e a r u p o n r a i s i n g t h e t e m p e r a t u r e w i t h f o r m a t i o n of the parent c o m p o u n d . A p p a r e n t l y , a T H F substituted c o m p l e x is f o r m e d . T h i s s u b s t i t u t i o n d o e s n o t t a k e p l a c e at t h e M n ( C O ) ( i - P r - D A B ) m o i e t y s i n c e t h e C O - v i b r a t i o n s do not a g r e e w i t h t h o s e o f t h e 2 - M e - T H F s u b s t i t u t e d c o m p l e x f o r m e d in 2 - M e - T H F at 1 3 3 K . The bands are assigned to ( T H F ) ( C O ^ M n M n C O ^ ( i - P r - D A B ) . R a i s i n g the temperature w i l l lead to b a c k s u b s t i t u t i o n o f T H F by C O , s t i l l p r e s e n t i n t h e f i l m . 5

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In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

STUFKENS ETAL.

MLCT

Photochemistry

0.50τ

F i g u r e 11. I R s p e c t r a l changes in the C O - s t r e t c h i n g region upon photolysis of ( C O ) M n R e ( C O ) ( i - P r - D A B ) i n a C h U - m a t r i x at 1 0 K . 5

3

F i g u r e 12. Structure proposed for the photoproduct of ( C O ) s M n R e ( C O ) ( R - D A B ) i n a C h U - m a t r i x at 1 0 K . 3

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

F i g u r e 13. IR s p e c t r a l changes in the C O - s t r e t c h i n g region upon p h o t o l y s i s o f ( C O ) M n M n ( C O ) ( i - P r - D A B ) i n a P V C f i l m at 2 9 3 K , ( R e p r o d u c e d f r o m R e f . 13. C o p y r i g h t 1985, A m e r i c a n C h e m i c a l Society). 5

3

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

6. STUFKENS ET AL. MLCT Photochemistry

83

The formation of these two photoproducts can be explained with a homolytic splitting of the metal-metal bond. Most radicals will react back to the parent compound. Some of the M(CO)s radicals will however react, presumably associatively (53,34), with the R-DAB ligand of the M'(CO)3 (R-DAB) radical or with THF in the PVC film with formation of these photoproducts and release of CO. In 2-Me-THF we observed, apart from the homolytic splitting of the metal-metal bond, breaking of a metal-nitrogen bond for (CO)sMnRe(CO)3(a-diimine), followed by release of CO in the case of (CO)sMMn(CO)3(a-diimine) (M=Mn,Re), at 1 33K. This reaction is not observed for any of these complexes in the matrix. However, contrary to its behavior in the matrix, (CO) MnRe(CO) (i-Pr-DAB) shows this reaction in the film at 193K upon irradiation into the MLCT band (upon irradiation with u.v. light (CO) Mn(i-Pr-DAB)Re(CO) is formed). The MLCT band disappears and no CO is released. Raising the temperature causes a backreaction to the parent compound just as in 2-Me-THF The different behavior of this complex in the PVC film of the matrix and to the 5

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The intriguing photochemistry of these complexes in relationship to their excited state properties certainly deserves more attention.Subject to further study are also the stability and electron distribution of the radicals M'(CO)3(a-diimine) and the identification and chemical properties of the highly reducing species M'(CO)(a-diimine)(PR3). 3

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

R.W. Balk, D.J. Stufkens and A. Oskam, Inorg. Chim. Acta 1978, 28, 133. R.W. Balk, D.J. Stufkens and A. Oskam, Inorg. Chim. Acta 1979, 34, 267. R.W. Balk, D.J. Stufkens and A. Oskam, J. Chem.Soc.,Dalton Trans. 1982, 275. R.W. Balk, D.J. Stufkens and A. Oskam, J. Chem.Soc.,Dalton Trans. 1981, 1124. L.H. Staal, D.J. Stufkens and A. Oskam, Inorg. Chim. Acta 1978, 26, 255. R.W. Balk, D.J. Stufkens and A. Oskam, Inorg. Chem., 1980, 19, 3015. M.W. Kokkes, D.J. Stufkens and A. Oskam, J. Chem.Soc.,Dalton Trans. 1983, 439. M.W. Kokkes, D.J. Stufkens and A. Oskam, J. Chem.Soc.,Dalton Trans. 1984, 1005. P.C. Servaas, H.K. van Dijk, T.L. Snoeck, D.J. Stufkens and A. Oskam, Inorg. Chem., in press. H.K. van Dijk, P.C. Servaas, D.J. Stufkens and A. Oskam, Inorg. Chim. Acta, in press. D.L. Morse and M.S. Wrighton, J. Am. Chem. Soc. 1976, 98, 3931. M.W. Kokkes, D.J. Stufkens and A. Oskam, Inorg. Chem., in press. M.W. Kokkes, D.J. Stufkens and A. Oskam, Inorg. Chem., in press. M.W. Kokkes, W.G.J. de Lange, D.J. Stufkens and A. Oskam, J. Organomet. Chem., in press. M.W. Kokkes, T.L. Snoeck, D.J. Stufkens, A. Oskam, M. Cristophersen and C.H. Stam, J. Mol Struct., in press.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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16. R.R. Andréa, D.J. Stufkens and A. Oskam, J.Organomet.Chem. 1985,290,63. 17. R.R. Andréa, A. Terpstra, D.J. Stufkens and A. Oskam, Inorg. Chim. Acta 1985, 96, L57. 18. L.H. Staal, G. van Koten and K. Vrieze, J. Organomet. Chem. 1979, 175, 73. 19. N. Flitcroft, D.K. Huggins and H.D. Kaesz, Inorg. Chem. 1964, 3, 1123. 20. M.L. Ziegler, H. Haas and R.K. Sheline, Chem. Ber. 1965, 98, 2454. 21. R.J. Kazlauskas and M.S. Wrighton, J. Am. Chem. Soc. 1982, 104, 5748. 22. H. van der Poel, G. van Koten and G.C. van Stein, J. Chem. Soc., Dalton Trans. 1981, 2164. 23. P.C. Ford in "Progress in Inorganic Chemistry"; Lippard, S.J., Ed.; John Wiley: New York, 1983; vol. 30, pp. 213-271. 24. A.J. Lees and A.W. Adamson J Am Chem Soc 1982 104 3804 25. D.R. Tyler, J. Photochem 26. A.E. Stiegman and D.R 27. A.E. Stiegman and D.R. Tyler, Inorg. Chem. 1984, 23, 527. 28. A.S. Goldmann and D.R. Tyler, J. Am. Chem. Soc. 1984, 106, 4067. 29. A.E. Stiegman, M. Stieglitz and D.R. Tyler, J. Am. Chem. Soc. 1983, 105, 6032. 30. This notation is used to indicate that the i-Pr-DAB ligand is σ-Ν, σ-Ν coordinated to Re an η -CN, η -CN to Mn. 31. R.D. Adams, J. Am. Chem. Soc. 1980, 102, 7476. 32. J. Keijsper, G. van Koten, K. Vrieze, M. Zoutberg and C.H. Stam, Organometallics 1985, 4, 1306. 33. H. Yeasaka, T. Kobayashi, H. Yasufuko and S. Nagakuru, J. Am. Chem. Soc. 1983, 105, 6249. 34. Q.Z. Shi, T. Richmond, W.C. Trogler and F. Basolo, J. Am. Chem. Soc. 1984, 106, 71. 2

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RECEIVED November 8, 1985

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

7 Manipulation of Doublet Excited State Lifetimes in Chromium(III) Complexes John F. Endicott, Ronald B. Lessard, Yabin Lei, Chong Kul Ryu, and R. Tamilarasan Department of Chemistry, Wayne State University, Detroit, MI 48202 The relaxatio energy doublet excited states (2E) of chromi um (III) complexes can be represented,kre=kore + kre(T). The limiting low temperature excited state lifetime, τ=(kore)-1,is a molecular property which is nearly independent of tem­ perature and the condensed phase environment, but τo does decrease with such molecular properties as the number of N-H vibrational modes available to function as acceptors for the electronic excitation energy. The thermally activated decay, kre(T), is a function of the solvent and the coordinated ligands. When kre(T) is fitted to an Arrhenius function, values of ln A vary more than do the apparent activation energies, and varia­ tions in kre(298) are more often determined by the pre-exponential factor than by Ea. The transition between τo and thermally activated relaxation occurs at a temperature, Ttr, which is a strong function of the medium and of the coordinated ligands. The observed room tem­ perature 2E lifetimes are more strongly cor­ related with Ttr than with the apparent Arrhenius activation energy. It is suggested that the thermally activated relaxation channel(s) involves a strongly coupled, but spin forbidden surface crossing to the potential energy surface of a reaction intermediate. However, some preliminary observations suggest that there may be more than one possible decay channel. Vibrationally equilibrated electronic excited states in molecules are unique chemical species. These are metastable species with unusual electronic configurations, and on this basis alone they might be expected to exhibit unusual patterns of reactivity. However, it is probably a more striking feature 0097-6156/ 86/ 0307-0085506.00/ 0 © 1986 American Chemical Society In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

86

EXCITED STATES AND

REACTIVE INTERMEDIATES

o f t h e s e e l e c t r o n i c e x c i t e d s t a t e s t h a t they have s t o r e d a c o n s i d e r a b l e amount o f energy i n a p r e d o m i n a t e l y e l e c t r o n i c form, and t h a t i f t h i s e l e c t r o n i c energy were c o n v e r t e d t o o t h e r energy forms, i t would exceed the energy r e q u i r e m e n t s f o r many s i m p l e c h e m i c a l p r o c e s s e s . The c o n v e r s i o n o f e l e c t r o n i c e x c i t a t i o n energy i n t o a more u s e f u l energy form ( e . g . , v i b r a t i o n a l , e l e c t r i c a l , e t c . ) can be v e r y s e l e c t i v e . The p r i n c i p l e s g o v e r n i n g t h i s s e l e c t i v i t y are not always w e l l understood. The l o w e s t energy e x c i t e d e l e c t r o n i c s t a t e o f the c h r o mium(III) complexes c o n s i d e r e d h e r e i s d e s i g n a t e d the state ( f o r c o n v e n i e n c e , even i n low symmetry c o m p l e x e s ) . This e x c i t e d s t a t e d i f f e r s from the ground s t a t e , **&2g ( i n 0^ symmetry, see F i g u r e 1 ) , i n s p i n m u l t i p l i c i t y , but not i n o r b i t a l population. As a consequence the ^£ and states have n e a r l y i d e n t i c a l m o l e c u l a r g e o m e t r i e s and the d i f f e r e n c e i n energy c o n t e n t o small entropy c o r r e c t i o driving forces). The t y p i c a l e x c i t e d s t a t e - g r o u n d s t a t e energy d i f f e r e n c e i s about 14 χ 10^ cm~l and i t exceeds the energy requirements f o r simple s u b s t i t u t i o n , i s o m e r i z a t i o n , e t c . , r e a c t i o n s o f the ground s t a t e . Thus i t i s not s u r p r i s i n g t h a t ( E ) C r ( I I I ) species are o f t e n very u n s t a b l e with respect t o l i g a n d r e p l a c e m e n t o r i s o m e r i z a t i o n r e a c t i o n s (1-3). However, d e s c r i b i n g such c h e m i c a l p r o c e s s e s i s not s i m p l e s i n c e the and e l e c t r o n i c s t a t e p o t e n t i a l energy s u r f a c e s must be v e r y s i m i l a r i n shape (1-4), a t l e a s t near t h e i r e q u i l i b r i u m n u c l e a r configurations. F u r t h e r m o r e , many o f t h e s e r e a c t i o n s have been found t o be s t e r e o s e l e c t i v e and t o o c c u r i n c o m p e t i t i o n w i t h p h o s p h o r e s c e n c e e m i s s i o n and n o n - r a d i a t i v e r e l a x a t i o n o f the e x c i t e d s t a t e (1-4). In t h i s r e p o r t we d e s c r i b e some o f the s t u d i e s which have been i n i t i a t e d t o i n v e s t i g a t e the f a c t o r s c o n t r i b u t i n g t o the b e h a v i o r o f the lowest d o u b l e t e x c i t e d s t a t e i n c h r o m i u m ( I I I ) . The p r i n c i p l e g o a l o f our work has been t o e x p l o r e t h o s e s t e r i c c o n s t r a i n t s , i n t r o d u c e d by the l i g a n d s , which g r e a t l y a l t e r the p h o t o p h y s i c a l p r o p e r t i e s o f the excited state. In p u r s u i t o f t h i s g o a l , we have r e - i n v e s t i g a t e d some f e a t u r e s o f w e l l known amine and p o l y p y r i d y l complexes i n o r d e r t o o b t a i n i n t e r n a l l y c o n s i s t e n t r e f e r e n c e systems. In p r i n c i p l e one must d e t e r m i n e o r t a k e i n t o a c c o u n t a v a r i e t y of f a c t o r s . Among t h e s e a r e : 2

1. 2.

the energy d i f f e r e n c e s , ΔΕ*, between the l o w e s t energy e l e c t r o n i c a l l y e x c i t e d q u a r t e t and d o u b l e t s t a t e s ; the energy and r o l e , i f any, o f h i g h e r energy d o u b l e t e x c i t e d s t a t e s ( e s p e c i a l l y components o f the T^, state); d i s t o r t i o n s o f the ^E e x c i t e d s t a t e p o t e n t i a l energy surface ; 2

3.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

ENDICOTT ET AL.

3+

Doublet Excited State Lifetimes in Cr

Complexes

b. Lowest Energy Metal Centered Electronic States

Configuration Coordinate F i g u r e 1. S t a t e e n e r g i e s ( a ) and q u a l i t a t i v e p o t e n t i a l energy s u r f a c e s (b) f o r Cr(NH3)| . A l t e r n a t i v e mechanistic proposals f o r ( E ) C r ( I I I ) decay a r e i l l u s t r a t e d i n 1 ( b ) : a, back i n t e r s y s t e m c r o s s i n g ; b, d i r e c t r e a c t i o n t o y i e l d e l e c t r o n i c a l l y c o r r e l a t e d p r o d u c t s ; c, s u r f a c e c r o s s i n g t o some ground s t a t e i n t e r m e d i a t e p o t e n t i a l energy s u r f a c e . +

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

87

88

EXCITED STATES AND REACTIVE INTERMEDIATES 4.

5.

the s i g n i f i c a n c e o f v a r i a t i o n s i n the c h e m i c a l and e l e c ­ t r o n i c p r o p e r t i e s o f C r ( I I I ) i n d u c e d by v a r i a t i o n s i n t h e ligands; the n a t u r e and c h e m i c a l b e h a v i o r o f any p h o t o g e n e r a t e d reaction intermediates.

I n p r a c t i c e , and d e s p i t e much s t u d y o f C r ( I I I ) systems, t h e r e i s o n l y a l i t t l e d e f i n i t i v e i n f o r m a t i o n about any o f t h e s e f a c t o r s . I n t h i s r e p o r t we d i s c u s s some c u r r e n t s t u d i e s o f chromium complexes w i t h s t e r i c a l l y c o n s t r a i n e d l i g a n d s . These s t u d i e s a r e p r o v i d i n g i n f o r m a t i o n m o s t l y about the f o u r t h f a c t o r l i s t e d above. Temperature

2

Dependence o f ( E ) C r ( I I I ) 2

Lifetimes

2

The l i f e t i m e s τ ( Ε ) , o f the E e l e c t r o n i c e x c i t e d s t a t e o f C r ( I I I ) complexes i s d e t a i l e d temperature dependenc and from s o l v e n t t o s o l v e n t . In g e n e r a l , t ( E ) i s s t r o n g l y temperature dependent a t h i g h t e m p e r a t u r e s and temperature independent a t low t e m p e r a t u r e s ( 3 , 5, 6 ) . Thus, 2

2

[x( E)]-l

« k

r e

« k?

e

+ k

r e

(T)

(1)

where k j * ( τ ° ) " ^ i s the l i m i t i n g low temperature r a t e c o n s t a n t f o r e x c i t e d s t a t e r e l a x a t i o n and the r a t e o f r a d i a t i v e r e l a x a t i o n i s assumed t o be s m a l l . In many i n s t a n c e s the tem­ p e r a t u r e dependent term, k ( T ) , can be r e p r e s e n t e d as a s i m p l e A r r h e n i u s t e m p e r a t u r e dependence (6) but more o f t e n a more complex f u n c t i o n i s r e q u i r e d t o f u l l y accommodate the c u r v a t u r e ( 3 , 6, 7, 8). e

r e

2

( E ) C r ( I I I ) L i f e t i m e s i n the Low Temperature Regime. As the temperature d e c r e a s e s , k approaches a l i m i t i n g v a l u e , k j which i s u s u a l l y independent o f the condensed phase medium. The low temperature l i f e t i m e s o f Cr***N£ complexes and cyanoa m i n e - C r ( I I I ) complexes a r e u s u a l l y f o u n d t o be n e a r l y tem­ p e r a t u r e independent o v e r an a p p r e c i a b l e temperature range. V a l u e s o f k°- can t h e r e f o r e be r e g a r d e d as f u n c t i o n s o f s t r u c t u r a l f e a t u r e s o f the chromium complexes. S i n c e the E and p o t e n t i a l energy s u r f a c e s a r e n e s t e d , e x c i t e d s t a t e r e l a x a t i o n i n t h i s regime i s a t t r i b u t e d t o l i m i t i n g weak c o u p l i n g (9) between the e x c i t e d and ground e l e c t r o n i c s t a t e s . In such a l i m i t , the n o n - r a d i a t i v e t r a n s i t i o n between s u r f a c e s depends on n u c l e a r and e l e c t r o n i c t u n n e l i n g . The more im­ p o r t a n t m o l e c u l a r p a r a m e t e r s which a r e e x p e c t e d ( 9 , 10) t o c o n t r i b u t e t o r e l a x a t i o n r a t e s i n t h i s regime a r e : (1) the energy d i f f e r e n c e , Δ Ε ° , between t h e e x c i t e d and ground e l e c ­ t r o n i c s t a t e s ; (2) t h e number o f h i g h f r e q u e n c y v i b r a t i o n a l modes a v a i l a b l e t o d i s s i p a t e t h i s energy; (3) the magnitude o f r e

e

e

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

ENDICOTT ET AL.

Doublet Excited State Lifetimes in Cr

3+

Complexes

the d i f f e r e n c e between t h e e x c i t e d s t a t e and t h e ground s t a t e n u c l e a r c o o r d i n a t e s f o r t h e s e modes; ( 4 ) t h e f r e q u e n c i e s o f t h e n u c l e a r p r o m o t i n g modes; and, ( 5 ) s p i n - o r b i t c o u p l i n g . Of these f a c t o r s , the c o n t r i b u t i o n s of the high frequency acceptor modes have been b e s t documented, (3, 11-13); e.g., i n C r ( N H 3 ) | and C r * * * ( N H 3 ) 5 X complexes, p e r d e u t e r a t i o n i n c r e a s e s τ ° by n e a r l y two o r d e r s o f magnitude, more o r l e s s c o n s i s t e n t w i t h expectation. No s i m p l e dependence on Δ Ε has emerged, b u t t h e compounds i n v e s t i g a t e d span a range o f o n l y , a t most 2000 cm""-*-, i n E ( E ) , and t h e e n e r g i e s o f t h e i r p r o m o t i n g modes p r o b a b l y v a r y o v e r a comparable range. Selected observations are summarized i n T a b l e I . +

0

2

Thermally A c t i v a t e d Relaxation

Rates

Solvent Mediation of E x c i t e d State L i f e t i m e s ; Examples from the B e h a v i o r o f P o l y p y r i d y e x h i b i t a strong solven lifetimes. The most extreme examples o f t h i s b e h a v i o r a r e p r o b a b l y found among t h e p o l y p y r i d y l complexes. F o r example, x ( E ) i s about 50 ns f o r C r ( t p y ) ^ i n water a t 25°C (14), b u t i n c r e a s e s t o 20 μβ i n t h e s o l i d s t a t e ( p e r c h l o r a t e s a l t ; 25°C) (15). T h i s k i n d o f b e h a v i o r has been e x t e n s i v e l y i n v e s t i g a t e d , e s p e c i a l l y by Kemp and co-workers (6), f o r C r ( p h e n ) ^ . The e f f e c t o f s o l v e n t on t ( E ) f o r t h i s complex appears t o be m a n i f e s t e d by compensation between temperature dependent (ΔΗ*) and t e m p e r a t u r e i n d e p e n d e n t components, ( A S * ) : V a l u e s o f τ ( Ε ) have u s u a l l y been found t o be s o l v e n t dependent i n t h e t h e r ­ m a l l y a c t i v a t e d regime, and t o v a r y i n g d e g r e e s t h e s e s o l v e n t d e p e n d e n c i e s a r e m a n i f e s t e d by v a r i a t i o n s i n b o t h t h e A r r h e n i u s a c t i v a t i o n e n e r g i e s and p r e - e x p o n e n t i a l f a c t o r s (3, 6). Thus, i n t h e t h e r m a l l y a c t i v a t e d regime t h e l i f e t i m e o f t h e l o w e s t d o u b l e t e x c i t e d s t a t e cannot be r i g o r o u s l y r e g a r d e d as an i n ­ t r i n s i c molecular property. Rather, values o f i ( E ) a r e de­ t e r m i n e d by some i n t e r a c t i o n between t h e m o l e c u l a r e x c i t e d s t a t e and i t s e n v i r o n m e n t . 2

+

+

2

2

2

P h o t o p h y s i c a l P r o p e r t i e s o f Simple Ammine (Amine) Complexes. Many systems have been s t u d i e d ; we w i l l o n l y c o n s i d e r a r e p r e s e n t a t i v e few. A more e x t e n s i v e r e c e n t r e v i e w o f t h e l i t e r a t u r e c a n be found e l s e w h e r e (3). In many ways t h e p h o t o p h y s i c a l b e h a v i o r o f ( E g ) C r ( N H 3 ) ^ i s p a r a d i g m a t i c o f ammine and amine c h r o m i u m ( I I I ) systems. Near ambient c o n d i t i o n s i n f l u i d s o l u t i o n , t h i s e l e c t r o n i c a l l y e x c i t e d complex has t h e f o l l o w i n g p r o p e r t i e s : ( 1 ) a h i g h l y s t r u c t u r e d e m i s s i o n ( F i g u r e 2) e x h i b i t i n g an i n t e n s e 0-0 l i n e and r e s o l v e d v i b r o n i c components; t h i s i s s t r o n g e v i d e n c e f o r s i m i l a r e q u i l i b r i u m n u c l e a r c o n f i g u r a t i o n s o f t h e e x c i t e d and ground e l e c t r o n i c s t a t e s (3, 16); ( 2 ) a s t r o n g l y t e m p e r a t u r e dependent e x c i t e d s t a t e l i f e t i m e ( F i g u r e 3 ) ; t ( E ) ~ 2 μβ a t 2

2

g

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

+

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

6

3 +

5

3

3 +

5

3

3 +

3

2

3 +

2

3

2

+

2

+

2

1

3+

ir^j?-Cr(L )(CN)

CIS-CT{)en


-Cr(L )(NH )

2

+

+

2

rra/w-CriLjMNH^*

1

+

120 (17)

22.2

52 (17) >80 (17)

18.9 (43) 20.7 (44)

14.2 (17)

14.1 (17)

14.3 (32)

14.8 (11)

23.5 (32)

379 ± 50 (32)

5600 (32)

88 (17)

17.6 (43)

14.4 (17)

f

136 (11)

1580 (18) 116 (18)

21.4 (18)

~15 (18)

21.5 (11)

3690 ± 30 (18)

f

175 ± 5 (18)

370 ± 20 (8, 20)

24.16 (42)

14.0 (32) 22.5 (18)

196 (40)

23.6 (41)

14.2 (40)

~15 (18)

3660 (32)

540 (39)

-21.1 (38)

13.0 (38)

5000 (37)

~23.9 (34, 35)

(36)

13.7 (37)

14.8 (17)

108 ± 12 (15, 23, 24) 2560 (15)

21.88 (34, 35)

9662 (32)

15.0 (33)

100 (32)

22.2

14.7 (30, 31)

(31)

19.25 (27)

14.8 (26)

ira^Cr(L )(CN)

2

41 (28, 29)

e

3500 (15, 25)

(μβ)

2+

5 (15, 23, 24)

75 i

(μβ)

21.64 (11)

3

15.2 (16)

l

C

AN-D) '

2 +

Cr(phen) (NH )

2

Cr(tpy) *

3

Cr(phen)

Crisen) "*"

3

Cr(en)

3

Cr(NH ) CN

3

Cr(NH ) Cl

3

Cr(NH )

Complex

4 4 Ε ( T o r E) ™*1 3 (cm J\0 )

Spectroscopic Parameters o f Soae Chroaiua(III) Complexes

-1 3 (cm V H T )

Table I.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2

2

2

3

3+

3 +

2

3 +

2

+

en phen Lj L2 L3 L4 L5 L5

β

» * * = = = =

e

c

D

(12)

ethylenediaraine; sen * 4,4',4"-Ethylidynetris(3~azabutan-l-amine) 1,10-phenanthraline; tpy » t e r p y r i d i n e ; 1,4,8,1l-Tetraazacyclotetradecane(cyclam); 5,\2-meso-5,7,7,12,14,14-hexamethyl-l,4,8,11-tetraazacyclotetradecane 5,12-rac-5,7,7,12,14,14-hexamethyl-l,4,8,l1-tetraazacyclotetradecane 1,4,7-triazacyclononane; 1,4,7-tris(acetato)-l,4,7-triazacyclononane ( T C T A ) ; 1,2-bi s( 1,4, 7-triazacyclononane)-ethane (BCNE)

50 (40)

20..8 (46)

(40)

13.8

430 (40)

(21)

19.. 5

14.1 (40)

(12)

14.7

91 (40)

20..1 (40) 370 ± 30

(40)

14.7

56 (17)

19..0 (45)

(32)

(17)

94 (17)

208

>108

94 (17)

16..7 (45)

21,.6 (32)

20..0 (45)

17..4 (45)

22..8 (12)

(17)

13.8

(32)

(17)

14.1

13.8

(17)

14.4

Data from (3, 6, 8, 11, 12, 15-18). E l e c t r o n i c o r i g i n of the lowest energy doublet state, except as i n d i c a t e d . Low temperature l i m i t i n g l i f e t i m e ( i n a r i g i d glass matrix), d Coordinated amines (ammines) i n the proteo form. Coordinated amines (ammines) perdeuterated. ^ Unresolved emission spectrum (20).

a

Abbreviations:

HOTES FOR TABLE I:

6

Cr(L )

5

Cr(L )

4

Cr(L )

3

cis-Cv{L >en

A

cy -Cr(L )(NCS)

3

c7*-Cr(L )Cl

3

c^Cr(L )(CN)

+

2

+

) Cl^

tran*-Cr(L )(NCS)

trâns-Cr(

(32)

(teta); (tetb);

3200 (12)

1854

£

^

^

^ δ 2'

£

I

H >

D ο ο

EXCITED STATES AND REACTIVE INTERMEDIATES

77 Κ

15.6

15.4

15.2

15.0

cm7l0

14.8

14.6

14.4

3

F i g u r e 2.

E m i s s i o n s p e c t r a f o r C r ( N H ) ( C 1 0 4 ) 3 a t 250 Κ and 7 7 K . 3

ο

6

ο •

as

4.0

4.5 1A xid\ κ"

·

ο

·

55

5.0

1

2

+

F i g u r e 3. Temperature-dependent l i f e t i m e s of ( E)Cr(NH3>| i n DMF:CHCl3 s o l u t i o n s (upper c u r v e ) and i n Ru(NH3)|+ c r y s t a l s (Ru:Cr - 30:1); from (8).

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

ENDICOTT ET AL.

3+

Doublet Excited State Lifetimes in Cr

3

Complexes

1

25°C and E - 3 . 8 x l 0 cm"- i n DMF (8); ( 3 ) a l a r g e p r o b a b i l i t y for substitution, ~ 0.5, and t h e a p p a r e n t r a t e c o n s t a n t f o r s u b s t i t u t i o n i s a t l e a s t 10* times l a r g e r f o r the E excited s t a t e t h a n f o r t h e ground s t a t e (2); ( 4 ) x ( E ) becomes n e a r l y temperature independent f o r Τ < 250 Κ and approaches a m a t r i x i n d e p e n d e n t low t e m p e r a t u r e l i m i t i n g v a l u e , x ° ( 2 ) - 75 ± 5 ]is\ ( 5 ) t h e e x c i t e d s t a t e l i f e t i m e i s i n c r e a s e d by p e r d e u t e r a t i o n even i n t h e s t r o n g l y t e m p e r a t u r e dependent regime; ( 6 ) x ( E g ) i s dependent on t h e condensed phase environment; t h i s i s most d r a m a t i c a l l y i l l u s t r a t e d by t h e c o n t r a s t between ( E ) C r ( N H ) 3 + i n D M F / C H C I 3 s o l u t i o n and doped i n t o t h e Ru(NH ) Cl s o l i d (Figure 3). a

1

2

g

2

g

E g

2

2

3

3

6

3

2

+

The t h e r m a l l y a c t i v a t e d b e h a v i o r o f ( E ) C r ( N H ) 3 , ( 1 5 ) , ( E)Cr(NH ) CN ( 1 7 ) , and C r ( N H ) C 1 + (17) p r o v i d e some i n ­ structive contrasts. The 77 Κ l i f e t i m e o f t h e hexammine i s b r a c k e t e d by t h o s e o f t h pentammine (75 100 d 42 μ β s p e c t i v e l y ) . The room o r d e r , b u t span many o r d e r CI . The much s h o r t e r ambient s o l u t i o n l i f e t i m e s found f o r t h e c h l o r o complexes r a i s e s t h e p o s s i b i l i t y o f t h e i n ­ t e r v e n t i o n o f second r e l a x a t i o n c h a n n e l ; e.g., such a c h a n n e l c o u l d i n v o l v e d i r e c t p a r t i c i p a t i o n o f t h e l o w e s t energy q u a r t e t e x c i t e d s t a t e s , which probabl reasonabl clos i to the £ s t a t e i n thes r e l a x a t i o n c h a n n e l woul that t

r

2

k

T

re< >

β

k

?e

(

T

)

+

k

r e

(

T

)

(

2

)

I f t h e two c h a n n e l s were t r u l y d i s t i n c t , t h e n d o m i n a t i o n o f k (T) by t h e c h l o r i d e m e d i a t e d r e l a x a t i o n c h a n n e l (designated "b") s h o u l d l e a d t o a r e p r e s s i o n o f ( a n d u l t i m a t e l y e l i m i n a t e ) c h a r a c t e r i s t i c features o f r e l a x a t i o n channel " a " . I f the l o n g e r l i f e t i m e f o r the trans- than f o r t h e c j ^ - g e o m e t r i e s be t a k e n as a c h a r a c t e r i s t i c f e a t u r e o f k ( T ) , t h e n t h i s re f e a t u r e does appear t o be r e p r e s s e d when t h e c o o r d i n a t i o n o f c h l o r i d e i n c r e a s e s k ( T ) by more than t h r e e o r d e r s o f mag­ nitude. T h i s argues t h a t t h e c h l o r i d e m e d i a t e d r e l a x a t i o n pathway i s d i s t i n c t . A p o s s i b l e means f o r a c c o u n t i n g f o r t h e s e observations i s that the E r e l a x a t i o n involves crossing to the p o t e n t i a l energy s u r f a c e s o f r e a c t i o n i n t e r m e d i a t e s (i.e., i n t o some, n o t n e c e s s a r i l y t h e l o w e s t e n e r g y , r e a c t i o n c h a n n e l o r c h a n n e l s o f t h e e l e c t r o n i c ground s t a t e ) which have q u a r t e t spin m u l t i p l i c i t y . Such e l e c t r o n i c a l l y f o r b i d d e n crossings would be f a c i l i t a t e d by s p i n o r b i t c o u p l i n g , and ^ E - ^ - J ^ s p i n o r b i t c o u p l i n g i n c r e a s e s as Δ Ε * E ( ^ T ) - E ( E ) d e c r e a s e s . r

e

a

r e

2

β

2

2

L i g a n d s w i t h a Tendency Towards T r i g o n a l D i s t o r t i o n s The observations summarized i n t h e p r e c e d i n g s e c t i o n seem t o i n d i c a t e t h a t t h e n u c l e a r d i s t o r t i o n s which e f f e c t r e l a x a t i o n o f t h e ( E ) C r ( I I I ) e x c i t e d s t a t e a r e more e a s i l y a c c o m p l i s h e d from a cz'.i?-Cr* -*-(N4)X2 complex t h a n from a trans-Cr^^(H^)X2 complex. T h i s s u g g e s t s t h a t l i g a n d s which f a c i l i t a t e c e r t a i n t y p e s o f n u c l e a r motions s h o u l d reduce t h e e x c i t e d s t a t e lifetimes. W i t h t h i s h y p o t h e s i s i n mind, we have been i n v e s t i ­ gating the p o t e n t i a l f o r h i g h l y s t r a i n e d ( i n the e l e c t r o n i c ground s t a t e ) l i g a n d s t o i n d u c e e x c i t e d s t a t e d i s t o r t i o n s by 2

I

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND

REACTIVE INTERMEDIATES

employing a s e r i e s o f s u b s t i t u t e d 1 , 4 , 7 - t r i a z a c y c l o n o n a n e complexes. These N - s u b s t i t u t e d l i g a n d s t e n d t o promote a t r i g o n a l p r i s m a t i c geometry, a p o t e n t i a l d i s t o r t i o n which might open a r e a c t i o n c h a n n e l not a v a i l a b l e i n the t e t r a g o n a l complexes d i s c u s s e d above. ^ The p a r e n t , Cr( [ Î J j a n e ^ ^ , i s r e l a t i v e l y l o n g l i v e d under ambient c o n d i t i o n s ( 1 2 ) . The t r i - a c e t a t o d e r i v a t i v e , ( E ) C r ( T C T A ) , and ( E ) C r ( [ 9 ] a n e N ) | have comparable v a l u e s o f k° , but ( E ) C r ( T C T A ) has an e x c e p t i o n a l l y s h o r t l i f e t i m e under ambient c o n d i t i o n s . These v a r i a t i o n s i n k ( T ) a r e not m a n i f e s t e d i n E , but i n v e r y d i f f e r e n t v a l u e s o f T (or A). The TCTA l i g a n d has a tendency t o f a v o r a t r i g o n a l p r i s m a t i c geometry, but Cr(TCTA) i s o n l y s l i g h t l y d i s t o r t e d from an a n t i p r i s m a t i c geometry ( 2 1 ) . I f a t r i g o n a l t w i s t i n g mechanism promoted r e l a x a t i o n , then one would e x p e c t a s m a l l e r v a l u e o f E f o r ( E ) C r ( T C T A ) tha fo ( E)Cr([9]aneN )| Thi i t our o b s e r v a t i o n . Onc lowest quartet e x c i t e d e v e l o p e d i n the p r e c e d i n g s e c t i o n , f o r a c h l o r i d e m e d i a t e d pathway, may a l s o be a p p l i c a b l e h e r e . The b e h a v i o r o f the Cr(BCNE) complex i s more i n l i n e w i t h e x p e c t a t i o n based on the tendency o f a s t r a i n e d l i g a n d to f a c i l i t a t e e n t r y i n t o a r e l a x a t i o n channel. The A r r h e n i u s a c t i v a t i o n energy i s e x c e p t i o n a l l y s m a l l and a r e l a t i v e l y b r o a d d o u b l e t e m i s s i o n i s o b s e r v e d a t 77 Κ (fwhh ca 340 cm"-*-). Thus, i t would appear t h a t some o f the ground s t a t e s t r a i n energy i s r e l a x e d t h r o u g h e x c i t e d s t a t e d i s t o r t i o n s , and t h a t t h e s e d i s t o r t i o n s reduce the b a r r i e r f o r the r e l a x a t i o n p r o c e s s . The c o n t r a s t i n b e h a v i o r o f the C r ( e n ) and Cr(sen) complexes might be a t t r i b u t e d t o the e f f e c t s o f a t r i g o n a l l y s t r a i n e d ground s t a t e . Thus, E(^T2> i s l a r g e r , but τ(298 Κ) i s much s m a l l e r i n the sen complex. These complexes d i f f e r o n l y i n the c a p p i n g o f one t r i g o n a l f a c e o f the sen complex. The c a p p i n g C H ^ C i C ^ - ) ^ m o i e t y i s a p p r e c i a b l y s t r a i n e d i n the ground s t a t e . However, the e n t h a l p i c component o f t h i s s t r a i n does not make a c l e a r c o n t r i b u t i o n to the d i f f e r e n c e i n v a l u e of k ( 2 9 8 K ) . Once a g a i n the e f f e c t appears to be "entropie" (manifested i n T or A) and the e m i s s i o n band w i d t h s are comparable. T h i s does not r u l e out the p o s s i b i l i t y t h a t the d i f f e r e n c e i n l i g a n d s t r a i n dominates v a r i a t i o n s i n τ(298 Κ) f o r t h e s e two complexes, but the s t r a i n c o n t r i b u t i o n i s a p p a r e n t l y not m a n i f e s t e d i n an e x c i t e d s t a t e d i s t o r t i o n i n Cr(sen) . +

2

2

+

3

2

r e

a

t r

2

2

+

a

3 +

3 +

3 +

3

r e

t r

3 +

2

An Attempt t o E l u c i d a t e the R o l e o f the T t , S t a t e . We have some p r e l i m i n a r y o b s e r v a t i o n s which may bear on the r o l e o f the T i e x c i t e d s t a t e i n the r e l a x a t i o n p r o c e s s . T h i s s t a t e can o f t e n be found 200 t o 1000 cm" above the ( E ) s t a t e i n s i m p l e amine and ammine complexes ( 2 2 ) . As the symmetry o f the complexes d e c r e a s e s , the Τ χ s t a t e s p l i t s i n t o two o r t h r e e 2

1

2

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

ENDICOTT ET AL.

3+

Doublet Excited State Lifetimes in Cr

Complexes

components, and t h e energy o f t h e lowest o f these can approach E( E). F o r example E ( E ) - E ( T ) - 600 cm" i n C r ( p h e n ) ^ , b u t i n Cr(phen)2(NH3>| t h e lowest component o f T ^ o r i g i n appears a t about 150 cm" h i g h e r energy ( F i g u r e 5 a ) . A t 77 Κ both o f e l e c t r o n i c o r i g i n s are r e s o l v e d , but the v i b r o n i c s t r u c t u r e i s weak enough t h a t o n l y t h e s t r u c t u r e a s s o c i a t e d with the E emission i s e a s i l y detected ( F i g u r e 5a). As t h e temperature i s i n c r e a s e d t h e h i g h e r energy e l e c t r o n i c o r i g i n i s populated s u f f i c i e n t l y that the v i b r o n i c s t r u c t u r e associated w i t h i t i s superimposed on t h e s t r u c t u r e d ( E ) e m i s s i o n (222 Κ spectrum) r e s u l t i n g i n a n e t , broadened e m i s s i o n spectrum. A s i m i l a r e f f e c t appears i n t h e more complex spectrum o f C r ( t e t a ) ( C N ) + ( F i g u r e 5b). I n t h e 222 Κ e m i s s i o n spectrum o f t h i s complex t h e s u p e r p o s i t i o n o f t h e components o f e m i s s i o n s from t h e d i f f e r e n t e l e c t r o n i c o r i g i n s g i v e s a n e t spectrum which appears t o be Stokes s h i f t e d a t ambient t e m p e r a t u r e s 2

2

2

1

+

X

+

2

1

2

2

2

Since t ( E ) i s e s s e n t i a l l Cr( t e t a ) ( C N ) j J , i t i s l y i n g e l e c t r o n i c components o f t h e h i g h e r energy d o u b l e t s t a t e do n o t p r o v i d e an e f f i c i e n t r e l a x a t i o n pathway i n t h i s system. Some I n f e r e n c e s About t h e Mechanism f o r S o l v e n t M e d i a t e d , Thermally A c t i v a t e d , Non-Radiative R e l a x a t i o n o f ( ^ E ) C r ( T l I ) . The ( E ) C r ( I I I ) r e l a x a t i o n r a t e has been f o r m u l a t e d , i n e q u a t i o n 1, as a composite o f t h e c o n t r i b u t i o n s o f mechan­ i s t i c a l l y d i s t i n c t pathways. The temperature independent c o n t r i b u t i o n , k ° seems w e l l behaved, i n a c c o r d w i t h e x re p e c t a t i o n f o r n e s t e d e x c i t e d and ground s t a t e p o t e n t i a l energy s u r f a c e s . The temperature dependent component, k ° ( T ) , e x h i b i t s a number o f i m p o r t a n t g e n e r a l f e a t u r e s : z

1.

k ° ( T ) tends t o v a r y w i t h t h e s o l v e n t medium, and t h e s e v a r i a t i o n s appear i n b o t h E and A f o r t h e t h e r m a l l y activated relaxation rates; a

2.

2

E

l

s

quenched by base f o r many o f t h e amine compounds, and t h i s s e n s i t i v i t y i s most s t r i k i n g l y m a n i f e s t e d i n v a r i a ­ tions i n T ; A r r h e n i u s p r e - e x p o n e n t i a l f a c t o r s (A) v a r y o v e r a con­ s i d e r a b l e range; t h e overwhelming m a j o r i t y o f compounds s t u d i e d i n a s i n g l e s o l v e n t have v e r y s i m i l a r A r r h e n i u s a c t i v a t i o n e n e r g i e s ( E - (3.6 ± 0.5) χ 1 0 cm" i n DMS0-H 0, b u t many o f t h e A r r h e n i u s p l o t s a r e c u r v e d ( 3 , 6, 8); t h e r e i s no c o r r e l a t i o n o f E w i t h ΔΕ*; l i g a n d s i n which t h e n u c l e a r motions a r e e i t h e r s t e r i c a l l y i n h i b i t e d o r promoted have p r o f o u n d e f f e c t s on t h e h i g h temperature l i f e t i m e s , m a n i f e s t e d m o s t l y as v a r i a t i o n s i n T (or A); d e u t e r a t i o n o f c o o r d i n a t e d amines (ammines) r e s u l t s i n a d e c r e a s e i n k ( T ) i n s e v e r a l systems. t r

3. 4.

3

1

a

5. 6.

2

a

t r

7.

r e

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

f

1

χ ίο , κ 3

1

2

F i g u r e 4. A c i d dependence o f the ( E ) C r ( t e t a ) ( C N ) + l i f e t i m e , i n water w i t h a c i d o r base added: 3 χ 10~ M NaOH, t r i a n g l e s ; 3 χ 10~ M HC1, open c i r c l e s ; 2.5 χ 10" M HC1, c l o s e d c i r c l e s . 6

6

6

1.46

1.42 1.38 _ 1

cm /10

1.40 _ 1

cm /10

Cr(phen) (NH ) 2

1.46

4

3

3 + 2

1.34 4

Cr(tet) a ) ( C N )

3 + 2

F i g u r e 5. D o u b l e t e m i s s i o n s p e c t r a o f C r ( P H e u ) ( N H ) 3 + and C r ( t e t a ) ( C N ) + a t 222 Κ and 77 K. 2

3

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

3+

Doublet Excited State Lifetimes in Cr

ENDICOTT ET AL.

Complexes

These o b s e r v a t i o n s f o r c e us t o c o n c l u d e t h a t l a r g e n u c l e a r d i s p l a c e m e n t s make a major c o n t r i b u t i o n t o k ( T ) . That t h e e f f e c t s o f s o l v e n t a r e m a n i f e s t e d i n b o t h t h e temperature de­ pendent and t h e temperature independent components o f k ( T ) i s reminiscent o f the behavior of c l a s s i c a l s o l v o l y s i s r e a c t i o n s or c o n f o r m a t i o n a l rearrangements. Thus, t h e r e l a x a t i o n dynamics might b e t t e r be d i s c u s s e d i n terms o f f r e e energy c o n t r i b u t i o n s than i n terms o f p o t e n t i a l e n e r g i e s . I n such a view t h e v a r i a t i o n i n t h e A r r h e n i u s p r e - e x p o n e n t i a l term t r a n s l a t e s i n t o a ± 24 J K" m o l " (2σ v a l u e ) v a r i a t i o n around a mean AS* « 1 J K" m o l " ; o n l y f o r complexes w i t h c o n s t r a i n e d l i g a n d s a r e t h e AS* v a l u e s f o r e n t r i e s i n T a b l e I I s i g n i f i ­ c a n t l y n e g a t i v e ; i . e . , f o r trans-CriN^)(CN)^ and f o r Cr([9]aneN )| . These a r e a l s o t h e complexes w i t h t h e h i g h e s t energy q u a r t e t e x c i t e d s t a t e s . r e

r e

1

1

1

1

+

3

In summary, o u r t h e r m a l l y a c t i v a t e d r e l a x a t i o n pathways o f ( E ) C r ( I I I ) v e r y l i k e l y i n v o l v e £-to-^(intermediate) surface c r o s s i n g . These ^ ( i n t e r m e d i a t e s ) c a n be a s s o c i a t e d w i t h some, n o t n e c e s s a r i l y the l o w e s t energy, t r a n s i t i o n s t a t e ( o r t r a n s i t i o n s t a t e s ) f o r ground s t a t e s u b s t i t u t i o n . The A r r h e n i u s a c t i v a t i o n b a r r i e r s f o r t h e r m a l l y a c t i v a t e d r e l a x a t i o n a r e r e m a r k a b l y s i m i l a r from complex t o complex, b u t they can be a l t e r e d i n systems w i t h highly strained ligands. Some o f t h i s work i n d i c a t e s t h a t t h e s t e r i c and e l e c t r o n i c p e r t u r b a t i o n s o f t h e l i g a n d s d i c t a t e t h e c h o i c e among p o s s i b l e r e l a x a t i o n c h a n n e l s . 2

ACKNOWLEDGMENTS We a r e g r a t e f u l t o P r o f e s s o r K a r l Wieghardt f o r p r o v i d i n g samples o f t h e s u b s t i t u t e d [ 9 ] a n e N l i g a n d s . P r o f e s s o r N. A. P. Kane-Maguire k i n d l y p r o v i d e d us w i t h a number o f u s e f u l d e t a i l s about t h e c y c l a m complexes p r i o r t o p u b l i c a t i o n . The r e s e a r c h d e s c r i b e d i n t h i s paper has been g e n e r o u s l y s u p p o r t e d by t h e N a t i o n a l S c i e n c e F o u n d a t i o n . 3

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

6

3 +

5

3

2

2

3 +

3

3+

χ

)(CN) *

r

1

2

+

3

2

si

/ Y \

3+

2

+

2

2

^/?5-Cr(L )(CN) "*"

cjjj-Cr( Lj )en

^

trans-Cr(1^)(NCS)

cj ^Cr(L )Cl

1

cj>-Cr(L )(NH )

3 +

3

2

3+ trans-Cr( 1> ) ( NH >

trans-Crih

Cr(phen) (NH )

Cr(tpy)

Cr(phen) ^

6.7

4.3

4.2

2.3

0.7

~4

~5

7.7

7.0

5.8

7.5

5.0

+

5.0

2+

Cr(sen)

2.0

4

4.4

5

3

1

6

δ

C

3

J

A

380

80

2

(lxl0" )

(8xl0" )

1*

136

361

3.6

8

g

g

(1χ10~ )

126

(0.02)

1.2

14

(2χ10~* )

2.2

(μβ)

x(298K)

8

331

243

156

168

~235

~308

314

233

178

235

202

234

287

170

244

(K)

i

3.7

2.1

4.1

3.6

5.6

i

2.9

4.0

3.2

3.5

3.4

4.3

4.5

3.2

a

3

Ε - l (cm V I C T )

C β

2

2

0.4

(2xl0)

5

4xl0

0.03

2x10

4

3

8

6xl0~

90

1.2

88

(2xl0)

0.1

Α

8 h

Photophysical Parase t e r s f o r SomeChroaiua(III) Complexes

(ca/lO »' )

2 +

3

3

Cr(en>

3

Cr(NH ) CN

3

Cr(NH ) Cl

3

Cr(NH )

Complex

Table I I .

h

(32)

(32)

(17)

(17)

(17)

(18)

(18)

(32)

(39)

(39)

(39)

(17)

(15)

(32)

(17)

(8)

Ref

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2

3 +

X

3+

5.2

3 +

2

4

See note a Table I for abbreviations.

7

3

2.8 2.5 1.5

140 134

8

8

3.5

260

196

DMF-CHCI3

Arrhenius a c t i v a t i o n parameters

Temperature dependence seems c o r r e l a t e d with OH" quenching.

1

J 293 Κ

Data c o l l e c t e d i n a r i g i d glass.

n

See f i g u r e 4.

8 Value extrapolated from thermally a c t i v a t e d behavior at very low temperatures

f

e

!

n>

S'

g£

^

6

c

&

m ζ D Ô Ο H H m H >

g

3

(40)

(40)

(12)

(24)

for Cr(NH )

3

(17)

(32)

In DMSO-water (or H 2 O ) except as noted

3

0.025

38

l.lxlO"

150

0.1

2.3

181

E ) obtained r e l a t i v e to 6E set equal to 4 χ 10

(6xl0~ )

3

(5xl0~ )

40

8

4.2

3.8

237

^ Temperature of the t r a n s i t i o n between temperature dependent and temperature independent regimes of τ ( i n DMSO-H2O except as noted)

c

4

3

(2xl0~ )

2.1

assuming band widths do not change.

b Relative values of ΔΕ* » E ( E ) - E ( T or

a

HOTES FOR TABLE I I :

6

Cr(L )

5

2

3.0

4

/τ

Cr(L )

„

+

5.7

•

2

5.4

CIS-CT{L^)en Cr(L )

3

S + ML + Δ η

->-s~ +

->s

+

p> S + ML

exc ip lex format ion

external heavy atom effect

spin-catalysed deact ivat ion

photooxidat ion

photoreduct ion

energy transfer

109

Photocatalytic Systems

8. HENNIG AND REHOREK

zation has been proposed Π , _15 ). The idea behind this concept lies in the linking of a sensitizer S and a metal complex ML , which has to be sensitized, in one closed unit S-ML by an ionic, covalent or coordinate bond interaction between both components. Under such circumstances the formation of the encounter complex is no longer a restriction, and the kinetic scheme of sensitization is reduced considerably, Equation 4. n

1

H

S-ML η

» 5 ^

V

* η

K

sens . ^—\ k r

products

k

(4)

S-ML

η

As essential competitive pçocesses, we have only to consider deactivation processes of S-ML ( kc^i ) ^ transfer (k ). Thus, the efficiency l\ . η primarily depends on the befiavior of the S-ML unit, bBÇ it is independent of the s o l vent properties, provideQ they do not affect the thermodynamic stability of the sensitizer/complex unit: b

a

c

k

e

,

e

c

t

r

o

n

n

k sens k +k + k sens r S-Μ­ η

V prod

(5)

c

The following proposals have been made (J_, 15) for realizing the concept of static sensitization: i. Static sensitization by formation of ion pairs with IPCT be­ havior or with S being an ionic dye ML η

•

S*

χ

»

ML ^ η

9

S*

i i . Static sensitization through the formation of mixed-valence compounds with IT (intervalence charge-transfer) behavior L MX η

+

1

L My η 2

τ

»

LM -X-M L η η 1

2

•

y

i i i . Mixed-ligand complexes with chromophor i c Ii gands ( Chr ) and/or low-lying CT states ML η

•

Chr

χ

»

ML

-Chr

η-1

•

L

Examples for the static sensitization through the formation of mixed-valence cyanometallates are discusses below. Static sensitization by mixed-valence complexes with IT behavior Mixed-valence compounds with IT behavior may be described theo-

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

110

EXCITED STATES AND REACTIVE INTERMEDIATES

retically as proposed by Robin and Day (17) based on the semienpirical model approach of Hush and Allen (18, 19). Especially interesting concerning the static sensitization process are mixed-valence complexes with weakly interacting metal centers (class II mixed-valence compounds according to the classification by Robin and Day (V7) ). The electronic interaction between the two metal centers leads to the appearance of IT bands which can be observed In a region ranging from the ultraviolet to the near infrared part of the spectrum. Optical IT transitions ace due to electron transfer from one metal center M to the other M . The energy of the IT tran­ sition strongly depends on the redox asymmetry of the metal centers as well as the dielectric properties of the solvent. The general behavior of mixed-valence compounds has been reviewed excellently very recent Iy (20). The synthesis of mixed-valence compounds with optical IT transitions in the visibl interesting way to low-energy spectral sensitization. However, ex­ perimental results of photochemical reactions initiated by exci­ tation of IT states are very scarce, see e.g. (21, 22). The IT behavior of mixed-valence cyanometallates (23-26), Aqueous solutions of[Mo(CN)^T , LW(CN)^ , C F e ( C N ) Τ , anà\RKÔÏÏJ" ions undergo remarkable color changes upon addition of F e ( l l i ) , Cu(ll), U0 , and VO ions as well as some cobalt(lll) arrmine complexes, whereas the addition of C r ( l l l ) , Co(ll), N i ( l l ) , Zn(ll), Hg(ll), and Τ 1 ( I ) ions leads to absorption spectra which can be described as the sum of the spectra of the single components with no addi­ tional bands. Figure 3 exhibits some typical electronic spectra of mixedvalence complexes of thefMo(CN)Jr ion with various other metal ions. Mixed-valence compounds formed by interaction of Mo(IV), W(IV), Fe(lI), and Ru(ll) cyanometallates with metal ions such as Fe(111), Cu(lI), U0 , and VO^ belong to the class II of the Robin-Day class if ication: i. In accordance with the theoretical treatment of Hush, the position of the IT band is strongly correlated to the dielectric properties of the solvent. The energy of the optical IT transition, Ε , depends on the inner-sphere reorganization energy, Ε . , the ofrtei—sphere reorganization energy, Ε , on the enthalpy changes, Δ Ε, and on the changes of electrostatic interactions, Δ Ε which are caused by the electron transfer from one metal center to the other, Equation 6 and Figure 6. 9

9

2

z

ρ

Ε

op

=

Ε

. +Ε + ΔΕ r,ι r,o

+ΔΕ . el

(6)

Since the outer-sphere reorganization energy Ε is a function of the solvent term.i1/D - 1/D ), as folTôws from the continuum theory (18,19), / A defends linearly on the dielectric properties of the solvents aiPshown in Figure 4. i î . The energy of the IT t r a n s i t i o n is strongly influenced by ^he redox asynrrretry of the metal centers. Therefore, changing of M

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Photocatalytic Systems

HENNIG AND REHOREK

au.

/

\

V0

2 i

\

" \

\

\

\

\ .

s.

30

«_ 2 *^ÎÔ cm~ 3

F i g u r e 3 . UV/vis s p e c t r a of M compounds.

0.50

/^o(CN)gT

0.55

mixed-valence

(1/Dop-1/D ) s

F i g u r e 4. S o l v e n t dependence o f t h e IT t r a n s i t i o n f o r M^/IMoCCN)^- ( M = V 0 , U 0 , Cu +, Fe3+). Dop: O p t i c a l d i e l e c t r i c c o n s t a n t o f t h e medium ( e q u a l t o t h e square o f t h e r e f r a c t i v e i n d e x n ) ; D : S t a t i c d i e l e c t r i c c o n s t a n t o f t h e medium. n +

2 +

2 +

2

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

112

EXCITED STATES AND REACTIVE INTERMEDIATES

and/or M causes a shift of Λ as illustrated in Figure 5. The change of the redox asymneTry may also be achieved by v a r i ­ ation of the inner coordination sphere of the acceptor site of the mixed-valence cyanometallates. Thus, in the case of CXi(II) it is easily possible to change the redox asyrmetry by substi­ tution of the Cu(l I) aquo species by either d-donor or JTacceptor ligands. The variation of the redox asymTetry by chan­ ging the first coordination sphere leads to a shift of Λ from about 360 nm ( CCufen)^, ; CMo(CN) l "" ) to about 660 mi ([Cu(dmp) l ; CMCNIJ ) as shown in Table I. However, when CCufcVrch)^^ ; CMo(CN) l ist considered, thermal electron transfer due to the inadaquate redox potential , of the Cu(lI) unit prevents the formation of a Cu(ll); [Mo(CN) ] mixed-valence compound. ft

4

4

9

8

8

2**" Table I. The energies o mixed-valence compounds CuL

n

V

2+

Cu(phen)Br Cu(phen)CI^ QjfphenMNDJ, Cu(phen) Br^ Cu(phen)XI, Cu(phen)^(M5^) Cu(ach) TN0j/ Cu(bpy);(ISD^); Cu/5-mpt (|sr3J Cu(dmp) T N O J / Cu(dnrchî,(NO\J Cu(en) (lfc> )2 c

9

9

9

9

9

2

3

( in 10 cm" ) 3

( in V )

1

17.8 17.0 17.0 18.0 18.0 17.5 18.5 18.0 16.9 15.2 c 27.8

9

9

| T

ά

b

+ 0.174 • 0.337 • 0.594 +0.675 - 0.38

Data taken from ref.(26); phen - 1,10-phenanthroline bpy 2,2'-bipyridine, ach - 8-amino-quinoline, 5-mp - 5-methyl1,10-phenanthroline, dmp 2,9-dimethy1-1,10-phenanthroline, dmch - 4,4 -dimethyl-3,3 -dimethylene-2,2 -biquinoline, en ethylenediamine; solvent: methanol; reduct ion potent ia I of CuL at 298 Κ vs. (SHE; thermal electron transfer η l

,

,

c

i i i . From the energy of the IT transition- the bandwidth and the intensity, the de localization parameters may be calculated, Equation 7, (27-29). oC

2

= (4.24-10-*-S^. A-V

1 / 2

)/(P .d ) | T

2

(7)

( d = distance between donor and acceptor site in the mixedvalence compound )

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

8. HENN1G AND REHOREK

113

Photocatalytic Systems

Theoc values obtained for the various mixed-valence cyanorretal lates are ranging between 0.0005 and 0.0121 and, therefore, provide further support for the classification of these compounds as Rob i η-Day c I ass II mi xed-va Ience compounds. iv. The most interesting advantage of mixed-valence compounds with respect to their spectroscopic behavior is related to the fact that simple synthetic variations have a significant influence on the position of the IT absorption as illustrated in the Scheme 2. 1 2 variation of Μ , M

Scheme 2

S

r\ solvent influence L η

M

1

\ X ^

M

L'

2

η

v a r i â t ion of X

The photochemical behavior of mixed-valence cyanometallates. The aim of our photochemical investigations of mixed-valence cyanometallates (23, 26, 30, 31) was to study the spectrally sensitized formation of free cyanide by excitation of the IT states of appropriate mixed-valence compounds. In this way, cyanide ions may be generated by irradiation In the low energy region where the photolysis of the pure cyanometal lates leads no or neglibible cyanide yields. The results obtained with cyanometallates are best i l l u strated by the Scheme 3: Scheme 3 L4T η

...NC4te(CN)^--!3^ / χ—:

tMo(CN) ] " 8

3

•L

H

^

scav

k , k- » scav* 1

k r

!

. . . • CN" (thermal catalyst)

IT excitation of heteronuclear mixed-valence cyanometallates leads to the formation of a vibronically excited valence-isomeric species ( see Figure 6 ) consisting of octacyanomolybdate (V) and the corresponding reduced form of the metal center M. Due to the kinetic lability of[Mo(CN)g] (32-34) fast cyanide aquation can be expected which competes with the back electron transfer (k ).

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

114

EXCITED STATES AND REACTIVE INTERMEDIATES

ι -08

1—τ -0.6 -OA

1 -0.2

τ 0

1— 0.2 V

Figure 5 . IT t r a n s i t i o n s o f d i f f e r e n t m i x e d - v a l e n c e m e t a l l a t e s M ] * / [M ( C N ) ] " ( [ F e ( C N ) ] ~ ; ... 1

4

2

4

[Mo(CN) ] -; asymmetry.

6

4

[W(CN) ]4-;

8

cyano-

4

X

[Ru(CN) ] ~)

8

6

v s . redox

Ε

nuclear coordinates n+ A— F i g u r e 6. P o t e n t i a l energy diagram f o r [ M /Mo(CN)^r m i x e d - v a l e n c e compounds. M /Mo^ : P r e c u r s o r complex; (n-l)/ v. S u c c e s s o r complex; E p : Energy o f t h e o p t i c a l IT t r a n s i t i o n ; E : E n t h a l p y d i f f e r e n c e between p r e c u r s o r and s u c ­ c e s s o r compound; E ^ : A c t i v a t i o n energy o f t h e r m a l e l e c t r o n t r a n s f e r ; E ^ : E n t h a l p y o f t h e r m a l back e l e c t r o n t r a n s f e r ; (3: Resonance e n e r g y . n+

M

M o

v

G

Q

t

T

t

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Photocatalytic Systems

8. HENNIG AND REHOREK

115

Of particular interest are the photochemical investigations of the Cu(11)/£MO(CN)Q1 ~ mixed-valence system. Photochemical investigations have been performed by both monochromatic and polychromatic irradiations at selected energy regions. Low concentrations of octacyanomolybdate(IV) and copper(ll) have been used by reason of the low solubility of polymeric forms which are formed at higher concentrations. The analytical estimation of free cyanide has been used to monitor the photochemical reactions according to Scheme 3. I r r a d i â t i o n i n t o the IT region ( Λ * 500 nm) of the Cu(l l)/[Mo(CN) ] mixed-valence compound leads to an increased formation of free cyanide as compared with free CMo(CN)gl " ions. The efficiency of the spectrally sensitized cyanide formation was monitored by the estimation of the photochemical turnover num­ ber U instead of the quantum yield for practical reasons. ,

4

Γ

Γ

8

u

=

n

cN"

/ (

Ό ·

u

( 8 )

( η ~ . - « moles of cyanide formed; I intensity of incident lignt; t irradiation time ). The results surmBrized in Table II illustrate the increase of the photoinduced formation of cyanide achieved by IT excitation of Cu(ll)/[Mo(CN) 3 " as compared with K [Mo(CN)J . However, despite the increase of cyanide formation the efficiency of the spectral sensitization is rather low. The low efficiency is due to the circumstance that the rate (k-) o4 cyanide aquation in the valence isomeric form Cu( I )/TMo(CN) T " is low compared with the very fast back electron transfer (k ). In order to make the proper choice of a scavenging reaction (k ) which may compete success­ fully with back electron transfer, wl have attempted a rough e s t i ­ mate of the rate constant k of the back electron transfer following the theoretical treatment proposed by Hush (20). s

s

8

4

4

8

S

v

Table II Results of the photochemical studies of the system Cu * /[Mo(CN) r" aq ο 2

^irr/ (

313 436 495 509 546

. I N

χ ™

]

Turnover U I T system K, M O ( C N ) 1.30 0.19 0.048 0.044 0.017

a

ft

1.95 0.12 0.018 0.009 0

U / l L ru tr*i\ π *tMo(CN) l l x

q

,

T

K

8

0.67 1.58 2.67 4.89

Aqueous solutions ( 0.001 M, 30 ml, d = 2 cm, t data taken from ref. (26)

a

, r r

= 0-5 h) ;

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

116

EXCITED STATES AND REACTIVE INTERMEDIATES

Estimation of the back electron transfer rate (k^). A rough e s t i ­ mate of the rate of the back electron transfer can easily be made using Equation 9 or a modified form as proposed by Gratzel (35): k

r

= v

.

e t

exp( - E ' / k T )

(9)

t h

The value of the barrier of the back electron transfer, E ^ is accessible through the experimental data ( Ε , E and oc ), while ν . may be calculated using Equation 10. 1

Q

ν .

= 4 β/η

(10)

The values of k^ ( lated according to Equation Table III

Calculated parameters ( β , EJ^, ν ^) for estimating the rate constant k and . of the mixed-valence systems

β (kJ mole" ) E J 1

a

(10

U

(kJ mole" )

a

1

v

k

et

r

r

b

( s)

16.44 15.26 6.59

22.7 32.4 52.0

1.65 1.53 0.66

1.57 1 0 ™ 6.37 3.60 2.78 10^ 2.45 10 4.09 10

21.10

48.7

2.12

4.95 10

s~ ), Τ * 295 Κ 1

h

4

5

D

2.02 Ι Ο "

6

b , -1 (s ), τ = 295 Κ

The results illustrate cleac^y the short lifetime of the valence isomeric Cu( I )/[Mo(CN)g] ~ mixed-vaIence complex. The increase of E ' ^ cue to the formation of outei—sphere mixedvalence species 3 (25) may account for the higher lifetime of the U0 /[MO(CN)QD " complex. Furthermore, the results summarized in Taole III illustrate that the lower the energy Ε ( which is one of the aims of spectral sensitization ) the shorter is the l i f e ­ time of the va Ience-isomeric form generated from the precursor mixed-valence complex by IT excitation. In order to prove the values of k and 1? experimental ly^we have monitored the back electron transfer of Cu(I)/tMo(CN)gl by low-temperature E^R^spectroscopy (30). k was found to be k (2.74+1.2) 10 s which is in a reasonable agreement w?th the calculated value k = 8.2 10 s § ^ · Extrapo­ lation to room temperature y ï e l d s k^ * 1.3 10 s~ which illustrates 2

p

e

f

r

3

κ

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

8.

HENNIG AND REHOREK

117

Photocatalytic Systems

the good approximation following the theoretical approach for class II mixed-valence compounds proposed by Hush ( 2 0 ) . With respect to the practical application to static spectral sensitization processes, however, the short lifetime of the successor complexes generated by IT excitation of the precursor mixed-valence cyanometaIlates has to be considered seriously. In order to overcome back electron transfer processes we have tried to use scavenging reactions. However, it may be shown by simple kinetic estimates that high concentrations of scavengers are required to quench the back electron transfer completely. Thus,g_ scavenging of Cu(l) from the successor complex Cu( I )/fto(CN)g] may be achieved only at scavenger concentrations higher than about 1 M, even if the scavenging process is diffusion-controlled. Since some of the potential scavengers, e.g. benzonitri le, acetonitri le, triayIphosphines, phenol derivatives etc., undergo thermal redox reactions with Cu(11)/[Mo(CN) l " when they are present at higher concentrations and various diazonium compounds give rise to uncontrollea changes of the association equilibria of the mixed-valence systems. Therefore, it can be assumed that chemical scavenging of back electron transfer processes can be accomplished preferably by extremely fast chemical changes taking place within the va Ience-isomeric species itself, as it has been demonstrated by Vogler ( 2 1 , 2 2 ) . In addition to chemical scavenging, we have tried to use photons as physical scavengers to quencbxjhe back e l e c t r o n transfer in excited mixed-valence complexes M ~ /ÎMO(CN)Q] Photons as scavengers - unusual secondary photolyses. The photochemistry of the lMo(CN)g] ion, which is^regarded as the primary product of IT excitation of MÎ / ^ o ( C N ) J " mixed-valence compounds, is well documented ( 3 2 ) JMO(CN)Q] " is strongly absorbing in the near UV region where it undergoes efficient photoreduction together with the oxidation of water ( 3 7 ) . Hence, it was suggested that polychromatic irradiation of Q J T I I)/fMo(CN)g] " leads to the mixedvalence successor complex Cu( I ) / Γ Μ Ο ( ( 3 Μ ) ^ Γ i n the primarx step which is then converted to Cu( I ) / [ M O ( C N T Q ] " by a second photon as shown in Scheme 4 ( 3 1 , 3 6 ) 1

Scheme 4

hv

Cu(ll)#MCN)J ' 4

k

IT

g

|3-

Cu(l)/fao(CN)J

r

(S = 4-nitroso Ν , Ν - d i m e t h y l a n i I i n e , spin trap )

Cu(l)/Mo(CN)

8

+ 'OH + H

S-OH

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

+

118

EXCITED STATES AND REACTIVE INTERMEDIATES

The formation of hydroxy I radicals was monitored by either scavenging with 4-nitroso Ν , Ν - d i m e t h y I a n i I i n e ( 4-NDMA ) or spin trapping (36). Various experiments confirm the unusual photochemical reaction pathway shown in Scheme 4: Hydroxy I radicals have been detected only during polychromatic irradiation, whereas monochromatic photolysis in the 365 nm or 492 nm region alone did not yield comparable results. Octacyanomolybdate(V) was proved not to form any "OH radicals thermally. Uncontrolled thermal or photochemical reactions of the scavenger 4-NDMA could also be discarded. Further­ more, direct photochemical generation of [Mo(CN) ] ~ by UV light has been excluded by using cutt-off f i l t e r s . Finally, thorough ^ calculations of the stationary concentration of the Cu(I)/[Mo(CN),X mixed-valence intermediate, considering the intensity distribution of the incident light and the absorption by the diverse species present in the solution, do further support the unusual two-photon process given in Schem physical scavengers provides a further possibility to overcome fast back electron transfer processes in mixed-valence compounds. g

Cone lus ion. There is an increasing interest in both photoinduced catalytic and photoassisted reactions, particularly in the field of homogeneous complex catalysis and other organic syntheses, in the search of unconventional information recording materials, in the storage and conversion of solar energy, and in modelling I ightsensit ive metal loenzymes. For a number of applications spec­ tral sensitization of photocatalytic systems is required. It may be achieved by applying the concept of static sensitization. The IT excitation of Rob in-Day class II mixed-valence compounds pro­ vides an interesting route to static sensitization. The advantages in applying mixed-valence compounds are due to the fact that the energy of the IT transition may be varied by convenient synthetic procedures. In addition, the photochemical behavior may be predicted using the theoretical treatment pro­ posed by Hush. However, the most serious restriction for the application of mixed-valence compoun ds in the static spectral sensitization arises from the fast back electron transfer. There­ fore, very efficient scavenging reactions are required in order to suppress back electron transfer.

Literature Cited 1. 2. 3. 4. 5. 6. 7.

Hennig, H.; Rehorek, D.; Archer, R. D. Coord.Chem.Rev. 1985, 61, I. Salomon, R. G. Tetrahedron 1983, 485. Moggi, L.; Juris, Α.; Sandrini, D.; Manfrin, M. F. Rev. Chem. Intermed. 1981, 4, 171. Wrighton, M. S.; Ginley, D. S.; Schroeder, Μ. Α.; Morse, D. L. Pure Appl. Chem. 1975, 4, 671. Carassiti, V. EPA News I. 1984, 21, 12. Mirbach, M. J . EPA News I. 1984, 20, 16. Wubbels, G. G. Acc.Chem.Res. 1983, 16, 285.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

8.

HENNIG A N DREHOREK

8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

Photocatalytic Systems

119

Kisch, H.; Hennig, H. EPA News I. 1983, 19, 23. Hennig, H.; Thomas, Ph.; Wagener, R.; Rehorek, D.; Jurdeczka, K. Z.Chem. 1977, 17, 241. Hennig, H.; Hoyer, E.; Lippmann, E.; Nagorsnik, E.; Thomas, Ph.; Weissenfels, M. J.Signalaufzeichnungsm. 1978, 6, 31. Hennig, H.; Hoyer, E.; Lippmann, E.; Nagorsnik, E.; Thomas, Ph.; Weissenfels, M.; Epperlein, J.; Dowidat, G. DDR Patent 123 024, 1976. Hennig, H.; Rehorek, D.; Mann, G.; Wilde, H.; Salvetter, J.; Weissenfels, M.; Thomas, Ph.; Epperlein, J.; Rehorek, A. DDR Patent 146 351, 1979. Förster, T. Ann.Phys. 1948, 2, 55. Dexter, D. L. J.Chem.Phys. 1953, 21, 836. Hennig, H.; Thomas, Ph.; Wagener, R.; Ackermann, M.; Benedix, R.; Rehorek, D. J.Signalaufzeichnungsm. 1981, 9, 269. Balzani, V.; Moggi Laurence, G. S. Coord.Chem.Rev. 1975, 15, 321. Robin, M. B.; Day, P. Adv.Inorg.Chem.Radiochem. 1967, 10, 247. Allen, G. C.; Hush, N. S. Prog.Inorg.Chem. 1967, 8, 357. Hush, N. S. Prog.Inorg.Chem. 1967, 8, 291. Brown, D., Ed.; "Mixed-Valence Compunds"; Reidel: Dordrecht, The Netherlands, 1980. Vogler, Α . ; Kunkely, H. Ber.Bunsenges.Phys.Chem. 1975, 79, 301. Vogler, Α . ; Kunkely, H. Ber.Bunsenges.Phys.Chem. 1975, 79, 83. Hennig, H.; Rehorek, Α . ; Rehorek, D.; Thomas, Ph.; Graness, G. Z.Chem. 1982, 22, 388. Hennig, H.; Rehorek, Α . ; Rehorek, D.; Thomas, Ph. Z.Chem. 1982, 22, 417. Hennig, H.; Rehorek, Α . ; Ackermann, M.; Rehorek, D.; Thomas, Ph. Z.anorg.allg.Chem. 1983, 496, 186. Hennig, H.; Rehorek, Α . ; Rehorek, D.; Thomas, Ph. Inorg. Chim.Acta 1984, 86, 41. Marcus, R. A. J.Chem.Phys. 1956, 24, 979. Levich, V. G. In "Physical Chemistry: An Advanced Treatise"; Eyring, H.; Henderson, D.; Jost, W.; Eds.; Academie: New York, 1970; Vol. 9B. Dogonadze, R. R. In "Reactions of Molecules at Electrodes"; Hush, N. S., Ed.; Wiley: New York, 1971. Hennig, H.; Rehorek, Α . ; Rehorek, D.; Thomas, Ph.; Bäzold, D. Inorg.Chim.Acta 1983, 77, L 11. Hennig, H.; Rehorek, Α . ; Rehorek, D.; Thomas, Ph. Z.Chem. 1982, 22, 418. Gray, G. W.; Spence, T. J . Inorg.Chem. 1971, 10, 2751. Stasicka, Z.; Bulska, H. Rocz.Chem. 1974, 48, 389. Rehorek, D.; Salvetter, J.; Hantschmann, Α . ; Hennig, H.; Stasicka, Z.; Chodkowska, A. Inorg.Chim.Acta. 1979, 37, L 471. Kiwi, J.; Kalyanasundaram, K.; G r ä t z e l , M. Structure and Bonding 1982, 49, 37. Rehorek, D.; Rehorek, Α . ; Thomas, Ph.; Hennig, H. Inorg. Chim.Acta 1982, 64, L 225. Rehorek, D.; Janzen, E. G.; Stronks, H. J . Z.Chem. 1982, 22, 64.

RECEIVED December 2, 1985

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

9 Electrochemically Generated Transition Metal Complexes Emissive and Reactive Excited States A. Vogler, H. Kunkely, and S. Schäffl Universität Regensburg, Institut fur Anorganische Chemie, D-8400 Regensburg, Federal Republic of Germany A variety of transitio complexe (A) subjec ted to an electrolysis by an alternating current in a simple undivided electrochemical cell. The compounds are reduced and oxidized at the same electrode. If the excitation energy of these compounds is smaller than the potential difference of the reduced (A-) and oxidized (A ) forms, back electron transfer may regenerate the complexes in an electronically excited state (A + A-* -> A* + A). These excited complexes may be emissive (A* -> A + hν) and/or reactive (A* -> B). Chemical transforma­ tions which accompany the ac electrolysis do not only proceed via excited states. As an important alternative the reduced or oxidized compounds can undergo a facile chemical change (A- -> B- or A -> B ). Back electron transfer merely restores the original charges (A +B--> A + Β or A- + B A + B). This mechanism and the ac electrolysis which proceeds via the generation of exci­ ted states are not unrelated processes. Hence the photoreaction and the ac electrolysis can lead to the same product irrespective of the intimate mechanism of the electrolysis. However, i t is also possible that photo­ lysis and electrolysis generate different products. Examples of ac electrolyses proceeding by these diffe­ rent mechanisms are discussed. +

+

+

+

+

+

Bimolecular e x c i t e d s t a t e e l e c t r o n t r a n s f e r r e a c t i o n s have been i n ­ v e s t i g a t e d e x t e n s i v e l y d u r i n g the l a s t decade ( 1 - 3 ) . Electron trans­ f e r i s f a v o r e d t h e r m o d y n a m i c a l l y when the e x c i t a t i o n energy Ε o f an i n i t i a l l y e x c i t e d m o l e c u l e A* exceeds the p o t e n t i a l d i f f e r e n c e o f the redox c o u p l e s i n v o l v e d i n the e l e c t r o n t r a n s f e r p r o c e s s . A + hv -> A* A*

+ Β + A

+

_ B"

Ε (A*)

> E°(A/A )-E(B"/B)

0097-6156/ 86/ 0307-0120$06.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

9.

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121

S t u d i e s o f such systems p r o v i d e d a b e t t e r u n d e r s t a n d i n g o f the mecha­ nism o f e l e c t r o n t r a n s f e r p r o c e s s e s i n g e n e r a l . T h i s r e a c t i o n type i s a l s o the b a s i s o f almost any type o f n a t u r a l o r a r t i f i c i a l p h o t o ­ synthesis. Hence i t i s not s u r p r i s i n g t h a t many i n v e s t i g a t i o n s have been d e v o t e d to e x c i t e d s t a t e e l e c t r o n t r a n s f e r r e a c t i o n s . On the c o n t r a r y , the r e v e r s a l of e x c i t e d s t a t e e l e c t r o n t r a n s f e r has found much l e s s a t t e n t i o n a l t h o u g h i t i s c e r t a i n l y not l e s s i n t e r e s t i n g . In the p r e s e n t paper v a r i o u s a s p e c t s o f t h i s r e a c t i o n type a r e d i s ­ cussed. The p r o d u c t s of a r e d o x r e a c t i o n may be g e n e r a t e d i n an e x c i t e d s t a t e i f to a f i r s t a p p r o x i m a t i o n the e x c i t a t i o n energy i s s m a l l e r than the p o t e n t i a l d i f f e r e n c e of the a s s o c i a t e d r e d o x c o u p l e s . A

+

+ Β " + A*

+ Β

Ε(A*)

+

< E°(A/A )-E°(B~/B)

G e n e r a l l y , t h i s energy r e q u i r e m e n t i s o n l y met when a s t r o n g o x i d a n t reacts with a strong reductant The e x c i t e d s t a t e thus produced does not behave d i f f e r e n t l y fro can be d e a c t i v a t e d by r a d i a t i o t r a n s f e r i n d u c e d e m i s s i o n (chemiluminescence, c l ) i s such a p r o c e s s . While i t i s w e l l known f o r o r g a n i c systems (4) t h e r e are not many o b s e r v a t i o n s o f c l o r i g i n a t i n g from t r a n s i t i o n m e t a l complexes (5-12). The r e a c t a n t s can be p r e p a r e d s e p a r a t e l y . Upon m i x i n g , e l e c t r o n t r a n s f e r t a k e s p l a c e w i t h concommitant e m i s s i o n o f l i g h t . While t h i s type o f experiment i s c o n c e p t i o n a l l y v e r y s i m p l e i t may be d i f f i c u l t to a c c o m p l i s h due to p r a c t i c a l o r t h e o r e t i c a l l i m i t a t i o n s . F o r exam­ p l e , t h i s method cannot be a p p l i e d when the r e d o x p a r t n e r s A and B" a r e not v e r y s t a b l e and have o n l y a s h o r t l i f e t i m e . In t h i s c a s e the redox a g e n t s must be p r e p a r e d i n s i t u . T h i s can be done i n two d i f f e ­ r e n t ways. The redox c a t a l y s i s r e p r e s e n t s one p o s s i b i l i t y . I t may a p p l y to h i g h l y e x o e r g i c r e d o x r e a c t i o n s which do n o t p r o c e e d r a p i d l y due to l a r g e a c t i v a t i o n e n e r g i e s . A s u i t a b l e r e d o x c a t a l y s t may speed up t h i s r e a c t i o n and f i n a l l y take up the energy which i s r e l e a s e d by t h i s redox process. +

Redox c a t a l y s i s l e a d i n g to c l i s i l l u s t r a t e d by two examples. The o x i d a t i o n o f o x a l a t e by P b ( I V ) does n o t p r o c e e d r e a d i l y a l t h o u g h i t i s s t r o n g l y favored thermodynamically. This r e a c t i o n i s catalyzed by R u ( b p y ) 3 ^ w i t h b i p y = 2 , 2 ' b i p y r i d i n e a c c o r d i n g to the f o l l o w i n g mechanism (13): +

2Ru(bipy)^

+

+ Pb0

2

1

Ruibipy)^" " + C 0 ^ " + C0~

+

+ 2Ru(bipy)^

-> R u ( b i p y ) ^

2

Ruibipy)^

+ 4H

+

+

[RuCbipy)**]*

+

[Ru(bipy)^ ]* + Ru(bipy)^"

+

+ C0 +

+

+

2

C0

+ Pb

2 +

+ 2H 0 2

C0~

2

hv

2+ The r e a c t i o n o f R u ( b i p y ) ^ with C0 i s the energy r e l e a s i n g e l e c t r o n t r a n s f e r step l e a d i n g to the f o r m a t i o n o f the e l e c t r o n i c a l l y e x c i t e d (*) complex. I t cannot be c a r r i e d out s e p a r a t e l y . The s t r o n g o x i ­ dant C0~ must be p r e p a r e d i n s i t u s i n c e i t i s a s h o r t - l i v e d r a d i c a l . The c a t a l y z e d d e c o m p o s i t i o n o f e n e r g y - r i c h o r g a n i c p e r o x i d e s i s another t y p i c a l r e a c t i o n of t h i s type. I t was c a l l e d " c h e m i c a l l y i n i t i a t e d e l e c t r o n - e x c h a n g e l u m i n e s c e n c e " (CIEEL) by S c h u s t e r , who used o r g a n i c compounds as r e d o x c a t a l y s t s (14). However, t r a n s i t i o n 2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND

122

REACTIVE INTERMEDIATES

m e t a l complexes work as w e l l . The complex Re (o-phen)(CO)~C1 (R) (o-phen = o - p h e n a n t h r o l i n e ) c a t a l y z e s the d e c o m p o s i t i o n of t e t r a l i n e h y d r o p e r o x i d e (T) to the k e t o n e α-tetralone (K) and water a c c o r d i n g to the mechanism (15): +

R + Τ

-> R

T~

-> K~

+

-> R*

+

R

+

R*

+ K~

R +

+

T" H0 2

Κ hv

The r e a c t i o n o f the k e t y l r a d i c a l a n i o n w i t h the o x i d i z e d rhenium complex i s the e n e r g y - r e l e a s i n g e l e c t r o n t r a n s f e r s t e p . This reaction cannot be c a r r i e d out s e p a r a t e l y While k e t y l r a d i c a l anions are s t a b l e s p e c i e s , the o x i d i z e r a t e d as s h o r t - l i v e d i n t e r m e d i a E l e c t r o l y s i s r e p r e s e n t s a n o t h e r , v e r y e l e g a n t method to p r e p a r e s u i t a b l e r e d o x p a i r s i n s i t u which a r e g e n e r a t e d by c a t h o d i c reduction and a n o d i c o x i d a t i o n . By a p p l i c a t i o n o f an a l t e r n a t i n g c u r r e n t the r e d o x p a i r i s g e n e r a t e d a t the same e l e c t r o d e . Back e l e c t r o n t r a n s f e r takes p l a c e from the e l e c t r o g e n e r a t e d r e d u c t a n t to the o x i d a n t near the e l e c t r o d e s u r f a c e . At an a p p r o p r i a t e p o t e n t i a l d i f f e r e n c e t h i s a n n i h i l a t i o n r e a c t i o n l e a d s to the f o r m a t i o n of e x c i t e d p r o d u c t s . As a r e s u l t an e m i s s i o n ( e l e c t r o g e n e r a t e d c h e m i l u m i n e s c e n c e , e e l ) may be o b s e r v e d (16). Redox p a i r s o f l i m i t e d s t a b i l i t y can be i n v e s t i g a t e d by ac e l e c t r o l y s i s . The f r e q u e n c y o f the ac c u r r e n t must be a d j u s t e d to the l i f e t i m e o f the more l a b i l e redox p a r t n e r . Many o r g a n i c com­ pounds have been shown to undergo e e l (17-19). Much l e s s i s known about t r a n s i t i o n m e t a l complexes. Most o f the o b s e r v a t i o n s involve R u ( b i p y ) ^ and r e l a t e d complexes w h i c h p o s s e s s e m i s s i v e charge t r a n s ­ f e r (CT) m e t a l - t o - l i g a n d (M-*L) e x c i t e d s t a t e s (13,20-31). The organom e t a l l i c compound Re ( o - p h e n ) ( C O ^ C l i s a f u r t h e r example o f t h i s c a t e ­ gory (32). P a l l a d i u m and p l a t i n u m p o r p h y r i n s w i t h e m i t t i n g i n t r a l i g a n d e x c i t e d s t a t e s a r e a l s o e e l a c t i v e (33). Under s u i t a b l e c o n ­ d i t i o n s e e l was a l s o o b s e r v e d f o r Cr ( b i p y ) 3 ~ " ( 2 7 ) . In t h i s case the e m i s s i o n o r i g i n a t e s from a l i g a n d f i e l d (LF) e x c i t e d s t a t e . Almost a l l o f the e e l a c t i v e t r a n s i t i o n m e t a l complexes c o n t a i n b i p y o r r e ­ lated ligands. I t was t h e r e f o r e of i n t e r e s t to see i f e e l c o u l d be extended to o t h e r types o f t r a n s i t i o n m e t a l compounds which have e m i t t i n g s t a t e s of d i f f e r e n t o r i g i n . +

T

F u r t h e r m o r e , e x c i t e d s t a t e s g e n e r a t e d e l e c t r o c h e m i c a l l y may be not o n l y e m i s s i v e but a l s o r e a c t i v e . The p o s s i b i l i t y o f such an "electrophotochemistry" (epc) has been c o n s i d e r e d b e f o r e (34). But r e a l examples were d i s c o v e r e d o n l y q u i t e r e c e n t l y and w i l l be d i s ­ c u s s e d l a t e r (35,36). However, c h e m i c a l t r a n s f o r m a t i o n s i n d u c e d by ac e l e c t r o l y s i s may not o n l y p r o c e e d v i a e x c i t e d s t a t e s . Other me­ chanisms can be a l s o c o n s i s t e n t w i t h t h e s e o b s e r v a t i o n s . While t h i s extends the range of r e a c t i o n types o f ac e l e c t r o l y s i s , i t c o m p l i c a ­ t e s the e l u c i d a t i o n o f the r e a l mechanism. Examples o f the v a r i o u s r e a c t i o n t y p e s are p r e s e n t e d i n the f o l l o w i n g s e c t i o n s .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

9.

VOGLER ET AL.

Electrogenerated

123

Electrochemically Generated Transition Metal Complexes Chemiluminescence

F o r o u r e e l s t u d i e s a v e r y s i m p l e t e c h n i q u e was employed. A 1-cm s p e c t r o p h o t o m e t e r c e l l was used as an u n d i v i d e d e l e c t r o c h e m i c a l c e l l . I t was equipped w i t h two p l a t i n u m f o i l e l e c t r o d e s which were d i r e c t l y c o n n e c t e d t o a s i n e wave g e n e r a t o r as an a c v o l t a g e s o u r c e . Much more s o p h i s t i c a t e d methods have been d e s c r i b e d i n t h e l i t e r a t u r e (16) but t h i s simple d e s i g n p e r m i t t e d t h e o b s e r v a t i o n o f e e l which appears at both e l e c t r o d e s . R e c e n t l y we o b s e r v e d e e l o f t h e b i n u c l e a r p l a t i n u m complex t e t r a k i s ( d i p h o s p h o n a t o ) d i p l a t i n a t e ( I I ) (Pt2 (pop)£~) (37). T h i s a n i o n has a t t r a c t e d much a t t e n t i o n due t o i t s i n t e n s e green l u m i n e s c e n c e i n room temperature s o l u t i o n (38-40) (φ = 0.52) (41). The e x c i t e d s t a t e o f t h i s complex undergoes o x i d a t i v e (42) and r e d u c t i v e q u e n c h i n g (41). From t h e q u e n c h i n g experiments t h e redox p o t e n t i a l s were e s t i m a t e d t o be E ° = -1.4 V v s . SCE f o th r e d u c t i o d E° 1 V fo th o x i d a t i o n o f P t ( p o p ) ^ " (41) matches t h e energy o f t h (p°p)^~« C o n s e q u e n t l y , i t s h o u l d be p o s s i b l e to o b s e r v e e e l o f t h i s complex. However, t h e reduced (Pt„(pop)|~) (43) and o x i d i z e d ( P t ~ (pop)|~) (44,45) forms a r e n o t s t a b l e , b u t decay r a p i d l y i n s o l u t i o n . Hence an e e l o f P t ( p o p ) ^ ~ w i l l o n l y take p l a c e i f t h e subsequent g e n e r a t i o n o f b o t h redox p a r t n e r s o c c u r s b e f o r e they undergo a decay. 2

2

2

The e e l experiment was c a r r i e d out i n a s o l u t i o n o f a c e t o n i t r i l e w i t h Bu^NBF^ as s u p p o r t i n g e l e c t r o l y t e ( 3 7 ) . A t an ac v o l t a g e o f 4 V, a f r e q u e n c y o f 280 Hz, and a c u r r e n t o f 13 mA a green e m i s s i o n appea­ red a t the e l e c t r o d e s . I t was i d e n t i c a l w i t h t h e p h o s p h o r e s c e n c e (λ = 5 1 7 nm) o f P t ( p o p ) ^ ~ . This observation i s consistent with \max . s.2 4 the f o l l o w i n g r e a c t i o n sequence: 0

1

Ί

4Pt (pop)^

5+ e -> P t ( p o p ) ^

cathodic

4Pt (pop)^

- e

anodic

Pt (pop)^~

+ Pt (pop)^~

2

2

2

2

3Pt (pop)^ 2

2

cycle

cycle

- [Pt (pop)J~]* + Pt (pop)J" 2

2

[ P t ( p o p ) £ " ] * + P t ( p o p ) £ ~ + hv 2

2

4The r e d u c t i o n and o x i d a t i o n o f P t ( p o p ) ^ takes p l a c e a t t h e same electrode. Back e l e c t r o n t r a n s f e r g e n e r a t e s one o f t h e s t a r t i n g i o n s i n t h e e x c i t e d t r i p l e t s t a t e which undergoes p h o s p h o r e s c e n c e . I n t e ­ r e s t i n g l y , the f l u o r e s c e n c e o f t h e complex which appears on p h o t o excitation at λ = 407 nm, i s n o t o b s e r v e d i n t h e e e l e x p e r i m e n t . T h i s i s n o t s u r p r i s i n g s i n c e the back e l e c t r o n t r a n s f e r does n o t p r o ­ v i d e enough energy (~ 2.4 V) to p o p u l a t e t h e e m i t t i n g s i n g l e t (~ 3.3 V). I t s h o u l d be mentioned h e r e t h a t t h e p r o c e s s e s which a r e i n v o l ­ ved i n t h e appearance o f an e e l o f P t ( p o p ) £ ~ a r e a s s o c i a t e d w i t h changes i n t h e m e t a l - m e t a l b o n d i n g o f t h i s b i n u c l e a r complex (38-40, 42,44,46,47). The P t - P t bond o r d e r which i s z e r o i n t h e ground s t a t e i s i n c r e a s e d t o 0.5 by o x i d a t i o n as w e l l as by r e d u c t i o n . The a n n i ­ h i l a t i o n r e a c t i o n l e a d s t o t h e f o r m a t i o n o f P t ( p o p ) ^ ~ as t h e ground (bond o r d e r = 0) and e x c i t e d s t a t e (bond o r d e r = 1 ) . A r e l a t e d case which was r e p o r t e d q u i t e r e c e n t l y i s t h e e e l o f M o C l ^ ~ . The m e t a l 2

2

2

A

1 9

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

124

EXCITED STATES AND

REACTIVE INTERMEDIATES

m e t a l b o n d i n g o f the c l u s t e r i s i n v o l v e d i n the r e d o x p r o c e s s e s a r e a s s o c i a t e d w i t h the e e l ( 4 8 ) .

which

E l e c t r o g e n e r a t i o n o f E x c i t e d Complexes U n d e r g o i n g E m i s s i o n and

Reaction

The e l e c t r o c h e m i c a l g e n e r a t i o n o f e x c i t e d s t a t e s may n o t o n l y l e a d to an e m i s s i o n . In a d d i t i o n o r as an a l t e r n a t i v e the e x c i t e d s t a t e can undergo a c h e m i c a l r e a c t i o n ( " e l e c t r o p h o t o c h e m i s t r y " , epc) as i t would o c c u r upon l i g h t a b s o r p t i o n ( p h o t o c h e m i s t r y ) . In the e e l e x p e r i ­ ments the o b s e r v a t i o n o f luminescence i s by i t s e l f a p r o o f f o r the generation of e x c i t e d s t a t e s . But the f a c t t h a t e l e c t r o l y s i s and p h o t o l y s i s b o t h l e a d to the f o r m a t i o n o f the same p r o d u c t does n o t prove the e l e c t r o c h e m i c a l g e n e r a t i o n o f an e x c i t e d s t a t e (see b e l o w ) . F o r t h i s r e a s o n i t i s an advantage to s t u d y compounds which a r e s i m u l ­ t a n e o u s l y p h o t o e m i s s i v e and p h o t o r e a c t i v e . A p o s i t i v e c o r r e l a t i o n between e e l and the e l e c t r o c h e m i c a l r e a c t i o i d indicatio that the c h e m i c a l t r a n s f o r m a t i o In t h i s case the e l e c t r o c h e m i c a e l e c t r o l y s i s the complex R u ( b i p y ) ^ undergoes s i m u l t a n e o u s l y e e l and epc ( 4 9 ) . +

J

2

+

The well-known p h o t o l u m i n e s c e n c e o f R u ( b i p y ) ^ o c c u r s from the lowest e x c i t e d s t a t e which i s o f the CT (Ru->bipy) type (50,51). The e m i s s i o n appears i n aqueous as w e l l as i n non-aqueous s o l u t i o n s . While the complex i s h a r d l y l i g h t - s e n s i t i v e i n water (52) i t can under­ go an e f f i c i e n t p h o t o s u b s t i t u t i o n o f a b i p y l i g a n d i n non-aqueous s o l v e n t s (50,51,53-56). The r e a c t i v e e x c i t e d s t a t e seems to be a LF s t a t e which l i e s a t s l i g h t l y h i g h e r e n e r g i e s but can be p o p u l a t e d t h e r m a l l y from the e m i t t i n g CT s t a t e (50-52,55-58). A c c o r d i n g to these o b s e r v a t i o n s the e l e c t r o c h e m i c a l g e n e r a t i o n o f e x c i t e d R u ( b i p y ) ^ i n non-aqueous s o l u t i o n s s h o u l d n o t o n l y be accompanied by the w e l l known e e l but a l s o by an epc. Moreover, the e f f i c i e n c y o f b o t h p r o ­ c e s s e s s h o u l d show a p o s i t i v e c o r r e l a t i o n . P r e l i m i n a r y experiments i n d e e d p r o v i d e e v i d e n c e f o r a s i m u l t a n e o u s o c c u r a n c e o f e e l and epc o f Ru(bipy)2+ ( 4 9 ) . An ac e l e c t r o l y s i s o f [Ru(bipy)^]Cl« was c a r r i e d out i n a s p e c t r o ­ photometer c e l l as an u n d i v i d e d e l e c t r o c h e m i c a l c e l l equipped w i t h platinum f o i l electrodes. A c e t o n i t r i l e was used as s o l v e n t and Bu.NBF, s e r v e d as s u p p o r t i n g e l e c t r o l y t e . The e l e c t r o l y s i s l e d to the t y p i c a l e e l o f R u ( b i p y ) ^ (20,21,23,25). S i m u l t a n e o u s l y , the complex underwent a c h e m i c a l change. The s p e c t r a l v a r i a t i o n s which accompa­ n i e d the e l e c t r o l y s i s ( F i g u r e 1) were v e r y s i m i l a r to those o b s e r v e d d u r i n g the p h o t o l y s i s o f the same s o l u t i o n (λ. > 335 nm). The p r o ­ d u c t o f e l e c t r o l y s i s and p h o t o l y s i s was n o t y e ï i d e n t i f i e d d e f i n i t e l y , but a c c o r d i n g to a p r e l i m i n a r y c h a r a c t e r i z a t i o n i t seems to be [ R u ( b i p y ) (CH CN)C1] . However, i t i s i m p o r t a n t to n o t e t h a t a l l changes o f the e x p e r i m e n t a l c o n d i t i o n s ( e . g . v a r i a t i o n s o f the ac f r e q u e n c y , s t i r r i n g o f the s o l u t i o n ) w h i c h l e a d t o a change o f the e e l i n t e n s i t y a l s o caused a c o r r e s p o n d i n g change o f the e f f i c i e n c y o f the electrochemical reaction. These o b s e r v a t i o n s a r e good i n d i c a t i o n t h a t b o t h p r o c e s s e s p r o c e e d v i a the g e n e r a t i o n o f e x c i t e d R u ( b i p y ) ^ . It i s s u g g e s t e d t h a t the ac e l e c t r o l y s i s can be d e s c r i b e d by the f o l l o ­ wing mechanism: 2

+

r

3

+

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

+

9.

Electrochemically Generated Transition Metal Complexes

VOGLER ET AL.

-4 F i g u r e 1. S p e c t r a l changes d u r i n g ac e l e c t r o l y s i s o f 10 M [Ru(bipy) ]Cl i n a c e t o n i t r i l e / 0 . 1 M Bu.NBF, a t (a) 0 and (e) 120 - min e l e c t r o l y s i s time a t 3 V/20 Hz and 30 mA, 1-cm c e l l . 3

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

125

126

EXCITED STATES AND 2+ Ru(bipy)

Ru(bipy) + e

3

Ru(bipy)

2+ Ru(bipy)

REACTIVE INTERMEDIATES

cathodic

3 3+

anodic

cycle

cycle

- e

3

Ruftipy)* + Ru(bipy)

2+ + +Ru(bipy) [Ru(bipy) ]*

+

3

3+ Ru(bipy)^

+

[Ru(bipy)* ]* +

[Ru(bipy)^ ]*

annihilation

+

3

+

eel

hv

+ C l " + CILCN Ru(bipy) (CH CN)Cl 2

+

3

bipy

epc

The c o n c l u s i o n t h a t the p l a c e v i a an e x c i t e d s t a t A c c o r d i n g t o e l e c t r o c h e m i c a l s t u d i e s the reduced and o x i d i z e d comple­ xes R u ( b i p y ) and R u ( b i p y ) ^ are f a i r l y s t a b l e and n o t e x p e c t e d to undergo r a p i d c h e m i c a l t r a n s f o r m a t i o n s (21,23,25,50). +

+

3

Electrogeneration

of R e a c t i v e E x c i t e d

States

Most compounds which undergo a p h o t o c h e m i c a l r e a c t i o n do not s i m u l ­ t a n e o u s l y show p h o t o l u m i n e s c e n c e . I t i s then more d i f f i c u l t to prove t h a t a r e a c t i o n i n d u c e d by ac e l e c t r o l y s i s p r o c e e d s v i a the i n t e r m e ­ d i a t e f o r m a t i o n of e x c i t e d s t a t e s . A d i f f e r e n t mechanism may be i n operation. In t h i s c a s e the c h e m i c a l t r a n s f o r m a t i o n o c c u r s i n the r e d u c e d and/or o x i d i z e d form. The back e l e c t r o n t r a n s f e r m e r e l y r e g e n e r a t e s the c h a r g e s o f the s t a r t i n g compound: A + e

-> A

A - e

-> A

A

-> Β

chemical

-> A + Β

annihilation

A

+

+ Β

cathodic +

anodic

cycle

cycle reaction

N e v e r t h e l e s s , the r e s u l t o f the e l e c t r o l y s i s may be the same as t h a t o f the p h o t o l y s i s , because the o r i g i n o f the r e a c t i v i t y i s s i m i l a r i n both cases. F o r example, a bond weakening may o c c u r upon r e d u c t i o n o r o x i d a t i o n s i n c e an e l e c t r o n i s added to an a n t i b o n d i n g π* o r b i t a l o r removed f r o m a b o n d i n g π o r b i t a l . The same changes take p l a c e upon ππ* e x c i t a t i o n . A c a s e i n q u e s t i o n i s the ac e l e c t r o l y s i s o f the complex R e ( t r a n s S P ) ( C 0 ) C 1 (SP = 4 - s t y r y l p y r i d i n e ) (59). I t was shown b e f o r e t h a t the c o o r d i n a t e d l i g a n d SP undergoes a p h o t o c h e m i c a l t r a n s / c i s i s o m e r i ­ z a t i o n (60). The r e a c t i v e e x c i t e d s t a t e i s the l o w e s t ππ* i n t r a l i g a n d (IL) s t a t e , which i s not l u m i n e s c e n t . The ac e l e c t r o l y s i s l e a d s a l s o to the t r a n s / c i s i s o m e r i z a t i o n o f the c o o r d i n a t e d l i g a n d ( 5 9 ) . Hence i t i s a r e a s o n a b l e assumption t h a t the e l e c t r o l y s i s p r o c e e d s v i a the g e n e r a t i o n o f the ππ* IL s t a t e : 2

3

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

9.

VOGLER ET AL.

Electrochemically Generated Transition Metal Complexes

Re ( t r a n s - S P )

(CO) C 1

2

3

+ e~ -> Re ( t r a n s - S P )

(CO) Cl""

2

3

Re ( t r a n s - S P ) ( C O ) C 1 - e " + Re ( t r a n s - S P ) ( C 0 ) C 1 2

3

2

Re ( t r a n s - S P ) ( C O ) C 1 2

+

+

3

-> Re ( t r a n s - S P ) Re ( t r a n s - S P ) ( C O ) C 1 * 2

3

2

127

+

3

R e ( t r a n s - S P ) (CO) C l " ( C O ) C 1 * + Re ( t r a n s - S P ) 3

2

(C0) C1 3

-> R e ( c i s - S P ) ( C 0 ) C 1 2

3

However, as an a l t e r n a t i v e the i s o m e r i z a t i o n may take p l a c e r e d u c e d and/or o x i d i z e d form:

i n the

Re(trans-SP) (CO) Cl~ + Re(cis-SP) (C0) C1~ 2

3

Re ( t r a n s - S P ) ( C O ) C 1 2

2

+

3

+ Re(cis-SP) (C0) C1

+

Re(cis-SP) (CO) 2

-> 2 R e ( c i s - S P ) ( C O ) C l 2

3

I n s p e c t i o n o f some a d d i t i o n a l d a t a does n o t l e a d t o a d i s t i n c t i o n between t h e two p o s s i b i l i t i e s . The p o t e n t i a l d i f f e r e n c e o f t h e r e ­ duced and o x i d i z e d complex (2.94 V) exceeds t h e e l e c t r o n i c e x c i t a ­ t i o n energy o f the n e u t r a l complex (~ 2.1 eV) (59). On e n e r g e t i c grounds t h e e l e c t r o c h e m i c a l g e n e r a t i o n o f e x c i t e d s t a t e s i s c e r t a i n l y possible. The r e l a t e d complex R e ( o - p h e n ) ( C O ) C 1 i s n o t l i g h t s e n s i ­ t i v e b u t i s p h o t o l u m i n e s c e n t and a l s o e e l a c t i v e (32). By a n a l o g y one might assume t h a t t h e e l e c t r o l y s i s o f b o t h complexes p r o c e e d s by the same mechanism. On t h e o t h e r s i d e , c y c l i c voltammetry shows t h a t the o x i d i z e d form o f R e ( t r a n s - S P ) (CO)~C1 i s f a i r l y s t a b l e b u t t h e r e d u c e d complex decays i r r e v e r s i b l y (59). Only a t l a r g e s c a n r a t e s (100 Vs~~^) the r e d u c t i o n wave shows b e g i n n i n g r e v e r s i b i l i t y . It i s then n o t u n r e a s o n a b l e to assume t h a t t h e l i g a n d i s o m e r i z a t i o n takes p l a c e i n t h e r e d u c e d complex. The f i n a l back e l e c t r o n t r a n s f e r would m e r e l y r e s t o r e the n e u t r a l complex. Of c o u r s e , i n t h e absence o f e e l any d i r e c t p r o o f o f the e l e c t r o c h e m i c a l g e n e r a t i o n o f e x c i t e d s t a t e s i s d i f f i c u l t to obtain. Nevertheless, i n d i r e c t but conclusive e v i ­ dence showed i n d e e d t h a t an e x c i t e d s t a t e mechanism l e d to t h e e l e c ­ t r o c h e m i c a l i s o m e r i z a t i o n o f the complex. 3

E x p e r i m e n t s were c a r r i e d o u t to determine i f d u r i n g t h e a c e l e c ­ t r o l y s i s the l i g a n d i s o m e r i z a t i o n r e q u i r e s the formation o f the redu­ ced and o x i d i z e d form (59). T h i s would i n d i c a t e an e x c i t e d s t a t e mechanism. I f t h e i n t e r m e d i a t e f o r m a t i o n o f the r e d u c e d 400 nm) i n s o l u t i o n s o f a c e t o n i t r i l e but undergoes an ac e l e c t r o l y s i s which i s accompanied by s p e c t r a l changes as shown i n F i g u r e 3. Accor­ d i n g to a p r e l i m i n a r y a n a l y s i s o f the p r o d u c t s the e l e c t r o l y s i s l e a d s to a l i g a n d exchange: g

11

2 N i ( B A B A ) ( M N T ) -> N i

1 1

(BABA)

+ 2

+ Ni

2(MNT)*

1 1

The e l e c t r o c h e m i s t r y o f Ni(BABA)(MNT) has been i n v e s t i g a t e d r e c e n t l y (64). The f i r s t r e d u c t i o n o c c u r s r e v e r s i b l y a t = -0.7 V v s . SCE. However, the o x i d a t i o n i s i r r e v e r s i b l e ( E / = 0.8 V ) . For the r e l a ­ t e d complex Ni(o-phen)(S^C^?h^) i t was shown t h a t the c a t i o n N i ( o - p h e n ) ( S C P h ) g e n e r a t e d by p h o t o o x i d a t i o n i n h a l o c a r b o n s o l v e n t s undergoes a f a c i l e l i g a n d exchange to y i e l d the symmetric complexes N i ( o - p h e n ) | and N i ( S C P h ) (65). A c c o r d i n g to t h e s e c o n s i d e r a t i o n s the ac e l e c t r o l y s i s can be r a t i o n a l i z e d by the f o l l o w i n g r e a c t i o n scheme : p

2

+

2

2

2

+

2

2

2

2

N i (BABA) (MNT)

+ e~

N i (BABA) (MNT)~

N i (BABA) (MNT)

- e"

+ N i (BABA) (MNT)

2 Ni(BABA)(MNT)

+

2 Ni(BABA)(MNT)" + Ni(MNT)

-> N i ( B A B A )

+ 2

cathodic

+

+ Ni(MNT)

anodic

2

+ 2 Ni(BABA)(MNT) + Ni(MNT)2

cycle cycle

l i g a n d exchange electron transfer

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

130

EXCITED STATES AND REACTIVE INTERMEDIATES

1.2

0.8

0.4

.-4 F i g u r e 2. S p e c t r a l changes d u r i n g ac e l e c t r o l y s i s o f 6.5x10 " M C r ( C 0 ) i n a c e t o n i t r i l e / 0 . 0 5 m Bu^NBF a t (a) 0 and ( f ) 300-min e l e c t r o l y s i s time a t 2.5 V/10 Hz and 5 mA, 1-cm c e l l . 6

1.0

0.8

0,6

0.4

-

0.2

-

400

500

—I 600

1

— X[nm]

F i g u r e 3. S p e c t r a l changes d u r i n g ac e l e c t r o l y s i s o f 1.5x10 M Ni(BABA)(MNT) i n a c e t o n i t r i l e / O . 1 M Bu.NBF, a t (a) 0 and (d) 30min e l e c t r o l y s i s time a t 3 V/20 Hz and 40 mA, 1-cm c e l l .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

9. VOGLER ET AL.

Electrochemically Generated Transition Metal Complexes

131

The l i g a n d exchange produces NiCMNT)^ which i s n o t s t a b l e b u t a s t r o n g o x i d a n t (66). I t o x i d i z e s a p p a r e n t l y t h e r e d u c i n g a n i o n Ni(BABA)(MNT) i n two subsequent e l e c t r o n t r a n s f e r s t e p s . Reactions Related

t o t h e AC E l e c t r o l y s i s

There a r e o t h e r r e a c t i o n s o f t r a n s i t i o n m e t a l complexes w h i c h a r e r e ­ l e v a n t t o o u r o b s e r v a t i o n s on t h e ac e l e c t r o l y s i s . R e c e n t l y , new mechanisms o f l i g a n d s u b s t i t u t i o n r e a c t i o n s have been r e p o r t e d which are c h a r a c t e r i z e d by e l e c t r o n t r a n s f e r r e a c t i o n s as k e y s t e p s a l t h o u g h the o v e r a l l r e a c t i o n s a r e n o t r e d o x p r o c e s s e s , e.g., ML

+ e~

ML"

+ L

1

+ ML " + L

+ L

f

+ ML

ML

overall:

ML

ML~ 1

1

1

+ L

The s u b s t i t u t i o n a l ^ l a b i l e complex may be g e n e r a t e d n o t o n l y by r e ­ d u c t i o n b u t by o x i d a t i o n as w e l l . An immediate r e l a t i o n s h i p o f such a r e a c t i o n to t h e ac e l e c t r o l y s i s p r o c e e d i n g w i t h o u t g e n e r a t i o n o f e x c i t e d s t a t e s c a n be r e c o g n i z e d . The i n i t i a l p r o d u c t i o n o f t h e sub­ s t i t u t i o n a l ^ l a b i l e o x i d a t i o n s t a t e o f ML c a n be a c h i e v e d e l e c t r o ­ c h e m i c a l l y (67-76), c h e m i c a l l y (75-77) o r p h o t o c h e m i c a l l y (78). I n the e l e c t r o c h e m i c a l experiments r e d u c t i o n o r o x i d a t i o n was a c c o m p l i s h e d by a d i r e c t c u r r e n t . I n most c a s e s t h e s e p r o c e s s e s a r e c a t a l y t i c c h a i n r e a c t i o n s w i t h F a r a d a i c e f f i c i e n c i e s much l a r g e r than u n i t y . Electro­ c h e m i c a l s u b s t i t u t i o n o f M(CO). w i t h M = C r , Mo, W was c a r r i e d o u t by c a t h o d i c r e d u c t i o n t o M(CO)~ which d i s s o c i a t e s i m m e d i a t e l y t o y i e l d M(CO)~. Upon a n o d i c r e o x i d a t i o n a t t h e o t h e r e l e c t r o d e c o o r d i n a t i v e l y u n s a t u r a t e d M (CO) i s formed and s t a b i l i z e d by a d d i t i o n o f a l i g a n d L to g i v e M ( C O ) L ( 6 8 ) . P h o t o c h e m i c a l s u b s t i t u t i o n v i a a l a b i l e o x i d a t i o n s t a t e may o c c u r by e x c i t e d - s t a t e e l e c t r o n t r a n s f e r . I f t h e m e t a l complex has a l o n g l i v e d e x c i t e d s t a t e , i t c a n undergo an e l e c t r o n exchange w i t h a r e d u c tant or oxididant i n a bimolecular r e a c t i o n . The l a b i l e r e d u c e d o r o x i d i z e d complex thus produced i s s u s c e p t i b l e to a l i g a n d s u b s t i t u t i o n . A c a t a l y t i c c h a i n r e a c t i o n takes p l a c e when t h e s u b s t i t u t e d complex i n the l a b i l e o x i d a t i o n s t a t e undergoes a f u r t h e r e l e c t r o n exchange w i t h a n o t h e r u n s u b s t i t u t e d complex. The c h a i n t e r m i n a t e s by back e l e c t r o n t r a n s f e r between t h e l a b i l e o x i d a t i o n s t a t e and t h e e x t e r n a l redox p a r t n e r which was g e n e r a t e d i n i t i a l l y . The c a t i o n Re (o-phen)(CO)^~ (CH^CN)" " undergoes t h i s new type o f p h o t o s u b s t i t u t i o n (78). The o c c u r ­ r e n c e o f a c h a i n r e a c t i o n was c o n f i r m e d by t h e quantum y i e l d s which were as l a r g e as φ = 24 depending on t h e e x p e r i m e n t a l c o n d i t i o n s . Of c o u r s e , t h e e f f i c i e n c y o f t h e u s u a l p h o t o s u b s t i t u t i o n s which o r i g i n a t e from L F e x c i t e d s t a t e s o f m e t a l complexes do n o t exceed u n i t y . 5

1

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

132

EXCITED STATES AND REACTIVE INTERMEDIATES

Conclusion The use o f ac e l e c t r o l y s i s i n a l l i t s v a r i a t i o n s i s c e r t a i n l y an i n t e r e s t i n g and v a l u a b l e t e c h n i q u e f o r study o f t h e mechanism o f electron transfer reactions. The g e n e r a t i o n o f a s h o r t - l i v e d redox p a i r as c h e m i c a l i n t e r m e d i a t e s i s an i m p o r t a n t f e a t u r e o f t h e ac electrolysis. I n the f u t u r e i t may even be d e v e l o p e d t o s y n t h e t i c a p p l i c a t i o n s i r r e s p e c t i v e o f the m e c h a n i s t i c d e t a i l s . In some c a s e s i t c o u l d be a c o n v e n i e n t a l t e r n a t i v e t o p h o t o c h e m i c a l r e a c t i o n s . In o t h e r c a s e s i t r e p r e s e n t s a new r e a c t i o n type which has no p r e c e d e n t . Acknowledgments We thank P r o f e s s o r Andreas Merz f o r h e l p f u l d i s c u s s i o n s . Financial s u p p o r t o f t h i s work by the Deutsche F o r s c h u n g s g e m e i n s c h a f t and the Fonds d e r Chemischen I n d u s t r i e i s g r a t e f u l l y acknowledged

Literature Cited 1. Balzani, V.; Bolletta, F.; Gandolfi, M. T.; Maestri, M. Top. Curr. Chem. 1978, 75, 1. 2. Meyer, T. J. Acc. Chem. Res. 1978, 11, 94. 3. Sutin, N.; Creutz, C. Adv. Chem. Ser. 1978, 168, 1. 4. Schuster, G. B.; Schmidt, S. P. Adv. Phys. Org. Chem. 1982, 18, 187. 5. Lyttle, F. E.; Hercules, D. M. Photochem. Photobiol. 1971, 13, 123. 6. Martin, J. E.; Hart, E. J.; Adamson, A. W.; Halpern, J. J. Am. Chem. Soc. 1972, 94, 9238. 7. Gafney, H. D.; Adamson, A. W. J. Chem. Ed. 1975, 52, 480. 8. Jonah, C. D.; Matheson, M. S.; Meisel, D. J. Am. Chem. Soc. 1978, 100, 1449. 9. Bolletta, F.; Rossi, Α.; Balzani, V. Inorg. Chim. Acta 1981, 53, L23. 10. Vogler, Α.; El-Sayed, L.; Jones, R. G.; Namnath, J.; Adamson, A. W. Inorg. Chim. Acta 1981, 53, L35. 11. Balzani, V.; Bolletta, F. J. Photochem. 1981, 17, 479. 12. Bolletta, F.; Balzani, V. J. Am. Chem. Soc. 1982, 104, 4250. 13. Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 512. 14. Schuster, G. B. Acc. Chem. Res. 1979, 12, 336. 15. Vogler, Α.; Kunkely, H. Angew. Chem. Int. Ed. Engl. 1981, 20, 469. 16. Faulkner, L. R.; Bard, A. J. In "Electroanalytical Chemistry"; Bard, A. J., Ed.; Marcel Dekker Inc.: New York, 1977; Vol. 10, p. 1. 17. Faulkner, L. R.; Glass, R. S. In "Chemical and Biological Genera­ tion of Excited States"; Adam, W.; Cilento, G., Eds.; Academic Press, New York, 1982; chapter 6 and references cited therein. 18. Park, S.-M.; Tryk, D. A. Rev. Chem. Intermediates 1981, 4, 43. 19. Pragst, F. Z. Chem. 1978, 18, 41. 20. Tokel, Ν. E.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 2862. 21. Tokel-Takvoryan, Ν. E.; Hemingway, R. E.; Bard, A. J. J. Am. Chem. Soc. 1973, 95, 6582. 22. Chang, M. M.; Saji, T.; Bard, A. J. J. Am. Chem. Soc. 1977, 99, 5399. 23. Wallace, W. L.; Bard, A. J. J. Phys. Chem. 1979, 83, 1350.

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58. Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583. 59. Kunkely, H.; Merz. Α.; Vogler, A. J. Am. Chem. Soc. 1983, 105, 7241. 60. Wrighton, M. S.; Morse, D. L.; Pdungsap, L. J. Am. Chem. Soc. 1975, 97, 2073. 61. Cheim, C. U.; Wang, H. C.; Szwarc, M.; Bard, A. J.; Itaya, K. J. Am. Chem. Soc. 1980, 102, 3100. 62. Picket, C. J.; Pletcher, D. J. J. Chem.Soc.,Dalton Trans. 1976, 749. 63. Schäffl, S.; Vogler, Α., unpublished results. 64. Vogler, Α.; Kunkely, H.; Hlavatsch, J.; Merz, A. Inorg. Chem. 1984, 23, 506. 65. Vogler, Α.; Kunkely, H. Angew. Chem. Int. Ed. Engl. 1981, 20, 386. 66. Davison, Α.; Edelstein, N.; Holm, R. H.; Maki, A. H. Inorg. Chem. 1963, 2, 1227 67. Bezems, G. J.; Rieger Comm. 1981, 265. 68. Grobe, J.; Zimmermann, Η. Z. Naturforsch. 1981, 36b, 301. 69. Tanaka, K.; U-eda, K.; Tanaka, T. J. Inorg. Nucl. Chem. 1981, 43, 2029. 70. Hershberger, J. W.; Kochi, J. K. J. Chem.Soc.,Chem. Comm. 1982, 212. 71. Hershberger, J. W.; Klingler, R. J.; Kochi, J. K. J. Am. Chem. Soc. 1982, 104, 3034. 72. Darchen, Α.; Mahe,C.;Patin, H. J. Chem.Soc.,Chem. Comm. 1982, 243. 73. Miholová, D.; Vlček, A. A. J. Organometal. Chem. 1982, 240, 413. 74. Hershberger, J. W.; Amatore, C.; Kochi, J. K. J. Organometal. Chem. 1983, 250, 345. 75. Hershberger, J. W.; Klingler, R. J.; Kochi, J. K. J. Am. Chem. Soc. 1983, 105, 51. 76. Zizelman, P. M.; Amatore, C.; Kochi, J. K. J. Am. Chem. Soc. 1984, 106, 3771. 77. Harrison, J. J. J. Am. Chem. Soc 1984, 106, 1487. 78. Summers, D. P.; Luong, J. C.; Wrighton, M. S. J. Am. Chem. Soc. 1981, 103, 5238. RECEIVED November 8, 1985

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

10 Surface-Enhanced Raman Spectroscopy Use in the Detection of Adsorbed Reactants and Reaction Intermediates at Electrodes M . J. Weaver, P. Gao, D. Gosztola, M . L. Patterson, and M . A. Tadayyoni Department of Chemistry, Purdue University, West Lafayette, IN 47907

The application o (SERS) for monitorin metal-solution interfaces is illustrated by means of some recent results obtained in our laboratory. The detection of adsorbed species present at outer- as well as inner-sphere reaction sites is noted. The influence of surface interaction effects on the SER spectra of adsorbed redox couples is discussed with a view towards utilizing the frequency-potential dependence of oxidation-state sensitive vibrational modes as a criterion of reactant-surface electronic coupling effects. Illustrative data are presented for Ru(NH3)6 / adsorbed electrostatically to chloridecoated silver, and Fe(CN)63-/4-bound to gold electrodes; the latter couple appears to be valence delocalized under some conditions. The use of coupled SERS-rotating disk voltammetry measurements to examine the kinetics and mechanisms of irreversible and multistep electrochemical reactions is also discussed. Examples given are the outer- and inner-sphere one-electron reductions of Co(III) and Cr(III) complexes at silver, and the oxidation of carbon monoxide and iodide at gold electrodes. 3+

2+

There has r e c e n t l y been much a c t i v i t y i n d e v e l o p i n g molecular s p e c t r o s c o p i c p r o b e s o f e l e c t r o c h e m i c a l i n t e r f a c e s , as f o r o t h e r t y p e s o f h e t e r o g e n e o u s systems. The u l t i m a t e o b j e c t i v e s o f t h e s e e f f o r t s i n c l u d e n o t o n l y the c h a r a c t e r i z a t i o n o f a d s o r b a t e m o l e c u l a r s t r u c t u r e i n t e r a c t i o n s under e q u i l i b r i u m c o n d i t i o n s , b u t a l s o t h e e x t r a c t i o n o f m e c h a n i s t i c and k i n e t i c i n f o r m a t i o n from s p e c t r a l d e t ­ e c t i o n of r e a c t i v e adsorbates. Two problems a r e i n h e r e n t , however, i n a p p l y i n g such t e c h n i q u e s to e l e c t r o c h e m i c a l i n t e r f a c e s . F i r s t l y , t h e e x t r e m e l y s m a l l q u a n t i t y of m a t e r i a l p r e s e n t w i t h i n t h e m o l e c u l a r t h i n i n t e r f a c i a l r e g i o n c a n present severe challenges i n a n a l y t i c a l d e t e c t i o n . Secondly, t h i s d i f f i c u l t y i s e x a c e r b a t e d by t h e u s u a l need f o r t h e i n c o m i n g and o u t g o i n g photons t o t r a v e r s e the b u l k s o l u t i o n , so t h a t under normal 0097-6156/ 86/ 0307-0135506.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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REACTIVE INTERMEDIATES

c i r c u m s t a n c e s the r e s u l t i n g s p e c t r a r e f l e c t s the c o m p o s i t i o n of the s o l u t i o n r a t h e r t h a n t h a t of the i n t e r f a c e . The f i r s t d i f f i c u l t y i s g r a d u a l l y b e i n g overcome by the c o n t i n u i n g development i n h i g h power s o u r c e s ( e . g . , l a s e r s ) and d e t e c t i o n systems. The second d i f f i c u l t y can be m i n i m i z e d by the use of t h i n - l a y e r e l e c t r o c h e m i c a l c e l l s , a l t h o u g h a t the expense of f l e x i b i l i t y i n e l e c t r o d e d e s i g n and the r e s u l t i n g i n a p p l i c a b i l i t y of some e l e c t r o c h e m i c a l techniques. G r e a t i n t e r e s t a r o s e i n the s u r f a c e s c i e n c e community upon the d i s c o v e r y and r e a l i z a t i o n of s u r f a c e - e n h a n c e d Raman s c a t t e r i n g (SERS). (1) Under c e r t a i n c o n d i t i o n s , a d s o r b a t e s a t m e t a l s u r f a c e s e x h i b i t s t r i k i n g l y ( c a . Ι Ο ^ - Ι Ο f o l d ) more i n t e n s e Raman s c a t t e r i n g than i n b u l k media. The p h y s i c a l phenomenon ( o r phenomena) r e s p o n ­ s i b l e f o r SERS i s i n c o m p l e t e l y u n d e r s t o o d as y e t , (2) even though the major r e s e a r c h e f f o r t i n the a r e a has been d e v o t e d t o i t s e l u c i d a t i o n . The v i r t u e s of SERS as an in svtu s u r f a c e m o l e c u l a r probe were r e c o g ­ n i z e d a t the o u t s e t s i n c e the h i g h enhancement f a c t o r s e n a b l e a b s o l u t e Raman s p e c t r a f o r i n t e r f a c i a the p r e s e n c e of h i g h b u l c h e m i c a l c e l l s . (1,2) T h i s t e c h n i q u e t h e r e f o r e surmounts b o t h the d i f f i c u l t i e s of s p e c t r a l d e t e c t i o n n o t e d above. A drawback w h i c h has l i m i t e d somewhat the p r a c t i c a l a p p l i c a t i o n s of SERS to s u r f a c e c h e m i s t r y i s t h a t s a t i s f a c t o r y enhancement can a p p a r e n t l y o n l y be o b t a i n e d a t r e l a t i v e l y few m e t a l s u r f a c e s , most p r o m i n e n t l y s i l v e r , copper, and g o l d , under c o n d i t i o n s where the s u r f a c e i s m i l d l y roughened. These m e t a l s , e s p e c i a l l y g o l d , a r e n e v e r t h e l e s s of importance i n e l e c t r o c h e m i s t r y i n v i e w of t h e i r s t r o n g l y a d s o r p t i v e and e l e c t r o c a t a l y t i c p r o p e r t i e s i n aqueous media. Our i n t e r e s t i n SERS stemmed from our r e s e a r c h a c t i v i t i e s concerned w i t h e s t a b l i s h i n g c o n n e c t i o n s between the m o l e c u l a r s t r u c t ­ u r e of e l e c t r o d e i n t e r f a c e s and e l e c t r o c h e m i c a l r e a c t i v i t y . A c u r r e n t o b j e c t i v e of our group i s to employ SERS as a m o l e c u l a r probe of a d s o r b a t e - s u r f a c e i n t e r a c t i o n s t o systems of r e l e v a n c e to e l e c t r o ­ c h e m i c a l p r o c e s s e s , and t o examine the i n t e r f a c i a l m o l e c u l a r changes b r o u g h t about by e l e c t r o c h e m i c a l r e a c t i o n s . The c o m b i n a t i o n of SERS and c o n v e n t i o n a l e l e c t r o c h e m i c a l t e c h n i q u e s can i n p r i n c i p l e y i e l d a d e t a i l e d p i c t u r e o f i n t e r f a c i a l p r o c e s s e s s i n c e the l a t t e r p r o v i d e s a s e n s i t i v e m o n i t o r of the e l e c t r o n t r a n s f e r and e l e c t r o n i c r e d i s ­ t r i b u t i o n s a s s o c i a t e d w i t h the s u r f a c e m o l e c u l a r changes p r o b e d by the f o r m e r . A l t h o u g h few such a p p l i c a t i o n s of SERS have been r e p o r t e d so f a r the approaches appear to have c o n s i d e r a b l e p r o m i s e . In t h i s c o n f e r e n c e p a p e r , we d i s c u s s some r e c e n t e l e c t r o c h e m i c a l SERS r e s u l t s o b t a i n e d i n our l a b o r a t o r y , b o t h f o r s i m p l e e l e c t r o n t r a n s f e r r e a c t i o n s and more complex m u l t i s t e p p r o c e s s e s , w i t h the o b j e c t i v e of i l l u s t r a t i n g the t y p e s of m o l e c u l a r and d y n a m i c a l i n f o r ­ m a t i o n t h a t can be e x t r a c t e d from t h i s a p p r o a c h . The m a j o r i t y of r e s u l t s d i s c u s s e d h e r e i n v o l v e the gold-aqueous i n t e r f a c e . W h i l e most SERS s t u d i e s to d a t e have u t i l i z e d s i l v e r s u r f a c e s , we have r e c e n t l y formulated a simple pretreatment procedure f o r gold that y i e l d s s u r f a c e s d i s p l a y i n g u n u s u a l l y s t a b l e as w e l l as i n t e n s e SERS w i t h r e d l a s e r e x c i t a t i o n . (3) Gold i s a p a r t i c u l a r l y t r a c t a b l e s u r f a c e f o r e l e c t r o c h e m i s t r y s i n c e i t p r o v i d e s a wide p o l a r i z a b l e window even i n aqueous media and has s t r o n g l y a d s o r p t i v e p r o p e r t i e s , e n a b l i n g a v a r i e t y of c a t a l y t i c e l e c t r o o x i d a t i o n as w e l l as e l e c t r o r e d u c t i o n p r o c e s s e s to be examined. Emphasis w i l l be p l a c e d on a c o n c e p t u a l l y based d i s c u s s i o n of r e p r e s e n t a t i v e r e s u l t s a l o n g w i t h i m p l i c a t i o n s 7

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

10.

WEAVER ET AL.

Surface-Enhanced Raman Spectroscopy

f o r f u t u r e s t u d i e s ; e x p e r i m e n t a l and the v a r i o u s o r i g i n a l p a p e r s c i t e d . Molecular

other

G e n e r a l i t y of SERS; O u t e r - and

d e t a i l s can be

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found i n

Inner-sphere Adsorbates

S i m i l a r l y to e l e c t r o n - t r a n s f e r r e a c t i o n s i n homogeneous s o l u t i o n , i t i s u s e f u l to d i s t i n g u i s h between i n n e r - and o u t e r - s p h e r e pathways, r e f e r r i n g to p r e c u r s o r s t a t e s where the r e a c t a n t does, o r does n o t , p e n e t r a t e the " c o o r d i n a t i o n l a y e r " of s o l v e n t m o l e c u l e s a d j a c e n t to the m e t a l s u r f a c e . ( 4 ) In o r d e r to u t i l i z e SERS t o examine redox p r o ­ c e s s e s a t e l e c t r o d e s , i t would c l e a r l y be d e s i r a b l e to d e t e c t o u t e r as w e l l as i n n e r - s p h e r e a d s o r b a t e s . The d e t e c t i o n of the l a t t e r t y p e , where the a d s o r b a t e i s bound d i r e c t l y to the m e t a l s u r f a c e , i s c l e a r l y f a c i l i t a t e d by the h i g h i n t e r f a c i a l (even monolayer) c o n c e n ­ t r a t i o n s t h a t a r e o f t e n e n c o u n t e r e d . Much lower s u r f a c e c o n c e n t r a t ­ i o n s of o u t e r - s p h e r e a d s o r b a t e s a r e u s u a l l y a n t i c i p a t e d s i n c e t h e i r a d s o r p t i o n i s determine of the l a t t e r a d s o r b a t e s m a l l s u r f a c e Raman s c a t t e r i n g c r o s s s e c t i o n s s i n c e most SERS t h e o r i e s p r e d i c t t h a t the s u r f a c e enhancement f a c t o r s d i m i n i s h s h a r p l y as the a d s o r b a t e - s u r f a c e s e p a r a t i o n i n c r e a s e s and degree of i n t e r a c t i o n decreases.(2) Indeed, under most c i r c u m s t a n c e s SERS appears to p r o b e p r e f e r ­ e n t i a l l y inner-sphere adsorbates. Nevertheless, outer-sphere i o n i c a d s o r b a t e s can a l s o be d e t e c t e d a t s i l v e r and g o l d e l e c t r o d e s by c o a t i n g the m e t a l s u r f a c e w i t h s u i t a b l e charged o r d i p o l a r chemis o r b e d s p e c i e s , thus p r o v i d i n g a s t r o n g e l e c t r o s t a t i c a t t r a c t i o n and g e n e r a t i n g e x t r e m e l y h i g h s u r f a c e c o n c e n t r a t i o n s of o u t e r - s p h e r e i o n s . ( 3 , 5 , 6 ) ( T h i s p r o c e d u r e i s c l o s e l y analogous t o the use of m u l t i c h a r g e d i o n s of o p p o s i t e charge so t o g e n e r a t e d e t e c t a b l e c o n ­ c e n t r a t i o n s of i o n p a i r s t h a t form p r e c u r s o r s t a t e s f o r homogeneous outer-sphere e l e c t r o n t r a n s f e r reactions(7).) Both o u t e r - s p h e r e c a t i o n i c and a n i o n i c s p e c i e s y i e l d d e t e c t a b l e SER s p e c t r a under t h e s e c o n d i t i o n s . M o n o l a y e r s of c h l o r i d e and bromide a n i o n s have been used to g e n e r a t e SER s p e c t r a f o r c a t i o n i c hexaammine and pentaammine t r a n s i t i o n - m e t a l complexes,(5,6) and t h i o u r e a - c o a t e d s u r f a c e s y i e l d s p e c t r a f o r a number of a n i o n i c s p e c i e s , i n c l u d i n g o x y a n i o n s and hexocyano complexes.(3) As might be a n t i c i p a t e d , the SERS f r e q u e n c i e s and bandshapes f o r s u c h o u t e r sphere a d s o r b a t e s , i n c l u d i n g m e t a l - l i g a n d and i n t r a l i g a n d v i b r a t i o n a l modes, a r e e s s e n t i a l l y u n a l t e r e d from the b u l k - p h a s e Raman v a l u e s , i n d i c a t i n g t h a t the a d s o r b a t e - s u r f a c e i n t e r a c t i o n s a r e weak. (5,6) N e v e r t h e l e s s , the s u r f a c e enhancement f a c t o r s f o r s e v e r a l o u t e r sphere m e t a l complexes a r e comparable to ( w i t h i n c a . 2-3 f o l d o f ) those i n v o l v i n g the same v i b r a t i o n a l modes f o r c l o s e l y r e l a t e d i n n e r sphere a d s o r b e d complexes, bound t o the s u r f a c e v i a t h i o c y a n a t e bridging ligands.(5b) This r e s u l t i n d i c a t e s that adsorbate-surface b i n d i n g can have o n l y a r e l a t i v e l y s m a l l i n f l u e n c e upon the degree of s u r f a c e Raman enhancement. S u r f a c e I n t e r a c t i o n E f f e c t s Upon Adsorbed Redox C o u p l e s : Trapped V e r s u s V a l e n c e - D e l o c a l i z e d Cases

Valence-

A c e n t r a l q u e s t i o n i n e l e c t r o c h e m i c a l e l e c t r o n t r a n s f e r i s the manner and e x t e n t to w h i c h the i n t e r f a c e may m o d i f y the r e a c t i o n e n e r g e t i c s , b o t h w i t h r e s p e c t t o the s t a b i l i t y of the p r e c u r s o r and successor

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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s t a t e s and the shape o f the e l e c t r o n - t r a n s f e r b a r r i e r itself.(§) For o u t e r - s p h e r e r e a c t i o n s , the r e a c t a n t - e l e c t r o d e i n t e r a c t i o n s a r e e x p e c t e d to be s u f f i c i e n t l y weak and n o n s p e c i f i c so t h a t the e l e c t r o n t r a n s f e r b a r r i e r i s l a r g e l y u n a l t e r e d by the p r o x i m i t y of the m e t a l surface. F o r i n n e r - s p h e r e p r o c e s s e s , on the o t h e r hand, the degree of r e a c t a n t - s u r f a c e e l e c t r o n i c c o u p l i n g may be s u f f i c i e n t so to d i s t o r t the i n t e r s e c t i n g r e a c t a n t and p r o d u c t p o t e n t i a l - e n e r g y s u r ­ f a c e s , t h e r e b y a l t e r i n g (and u s u a l l y l o w e r i n g ) the b a r r i e r h e i g h t . ( 8 ) In extreme c a s e s , the two p o t e n t i a l - e n e r g y s u r f a c e s may become e n t i r ­ e l y merged, whereupon the Franck-Condon b a r r i e r i s e l i m i n a t e d and the e l e c t r o n w i l l be d e l o c a l i z e d between the donor and a c c e p t o r s i t e s on the redox c e n t e r and the m e t a l s u r f a c e even on the v i b r a t i o n a l time scale. T h i s l a t t e r case may be termed " C l a s s I I I " b e h a v i o r by a n a l o g y w i t h the Robin-Dav c l a s s i f i c a t i o n f o r b u l k - p h a s e mixedv a l e n c e complexes, (9_) w i t h the w e a k - c o u p l i n g l i m i t termed " C l a s s I " and the i n t e r m e d i a t e c a s e o f s t r o n g e l e c t r o n i c c o u p l i n g yet valence trapped, being l a b e l e d "Clas i t s e l f does not a c q u i r e b u l k "mixed v a l e n c e " systems i s a p p r o p r i a t e g i v e n t h a t e l e c t r o n t r a n s f e r o c c u r s from and t o a "donor o r a c c e p t o r s i t e " on the m e t a l s u r f a c e i n the v i c i n i t y o f the a d s o r b e d redox c o u p l e . ) E l e c t r o n d e l o c a l i z a t i o n e f f e c t s f o r a d s o r b e d s p e c i e s , p r i m a r i l y monoatomic a d s o r b e d a n i o n s and c a t i o n s , have p r e v i o u s l y been d i s c u s s e d i n terms of the concept o f p a r t i a l c h a r g e t r a n s f e r , (10) but t h i s i s s u e has r e c e i v e d l i t t l e a t t e n t i o n f o r bona fide adsorbed redox c o u p l e s . We have examined a number o f adsorbed t r a n s i t i o n - m e t a l redox c o u p l e s u s i n g SERS and c o n v e n t i o n a l e l e c t r o c h e m i s t r y i n o r d e r to d e t e c t d i s t i n c t redox s t a t e s u s i n g SERS and to a s c e r t a i n the degree to w h i c h these s t a t e s a r e p e r t u r b e d upon a d s o r p t i o n . Several l i k e l y o u t e r - s p h e r e c o u p l e s have been examined; these i n c l u d e 0β(ΝΗ3) spy *"' (py pyridine) at chloride-coated s i l v e r , ( 6 ) Ru(NH )spy and Ru(NH )6 ' a t c h l o r i d e - c o a t e d s i l v e r and g o l d , ( 5 a , l l ) and F e ( C N ) 6 a t t h i o u r e a - c o a t e d g o l d . (3) C h a r a c t e r i s t i c f e a t u r e s o f these systems a r e the p r e s e n c e of p o t e n t i a l - i n d e p e n d e n t v i b r a t i o n a l modes c h a r a c t ­ e r i s t i c o f the o x i d i z e d and reduced forms t h a t o c c u r a t p o t e n t i a l s p o s i t i v e and n e g a t i v e , r e s p e c t i v e l y , o f the f o r m a l p o t e n t i a l , E f , o f the c o u p l e , w i t h p r o g r e s s i v e d i s p l a c e m e n t o f bands due to one redox form by t h e s e due to the o t h e r as the p o t e n t i a l i s a l t e r e d w i t h i n the v i c i n i t y o f E f . The SERS f r e q u e n c i e s and bandshapes of b o t h o x i d i z e d and reduced a d s o r b a t e forms a r e e s s e n t i a l l y c o i n c i d e n t w i t h the b u l k Raman s p e c t r a l f e a t u r e s . ( 3 , 5 a , 6 ) I t i s of i n t e r e s t to examine q u a n t i t a t i v e l y such p o t e n t i a l dependent redox e q u i l i b r i a as d e t e r m i n e d by SERS i n comparison w i t h t h a t o b t a i n e d by c o n v e n t i o n a l e l e c t r o c h e m i s t r y . F i g u r e 1 shows such data determined f o r R u ( N H 3 ) 6 ^ a t c h l o r i d e - c o a t e d s i l v e r . The s o l i d c u r v e s denote the s u r f a c e c o n c e n t r a t i o n s o f the R u ( I I I ) and R u ( I I ) forms as a f u n c t i o n of e l e c t r o d e p o t e n t i a l , n o r m a l i z e d to v a l u e s a t -100 and -500 mV v s SCE. These a r e d e t e r m i n e d by i n t e g r ­ a t i n g c y c l i c voltammograms f o r t h i s system o b t a i n e d under c o n d i t i o n s [very d i l u t e (50 yM) R u ( N H ) e , r a p i d (50 V s e c ) sweep r a t e ] so t h a t the f a r a d a i c c u r r e n t a r i s e s e n t i r e l y from i n i t i a l l y a d s o r b e d , r a t h e r than from d i f f u s i n g , r e a c t a n t ( c f . réf. 6b). The dashed c u r v e s denote the c o r r e s p o n d i n g p o t e n t i a l - d e p e n d e n t n o r m a l i z e d R u ( I I I ) and R u ( I I ) s u r f a c e c o n c e n t r a t i o n s , o b t a i n e d from the i n t e g r a t e d i n t e n ­ s i t i e s o f the 500 cm" and 460 cm" SERS bands a s s o c i a t e d w i t h the symmetric Ru(III)-NH3 and Ru(II)-NH3 v i b r a t i o n a l modes.(5a) 3

=

3

+

Z +

3

3

+ / 2 +

a V 2

3

3

2+

3+

-1

3

1

1

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

+

WEAVER ET AL.

Surface-Enhanced Raman Spectroscopy

-500 Ε, mV vs S C E F i g u r e 1. P l o t s o f t h e r e l a t i v e s u r f a c e c o n c e n t r a t i o n s ( s o l i d c u r v e s ) and r e l a t i v e peak i n t e n s i t i e s ( p o i n t s , dashed c u r v e ) o f the 500 cm" [ R u ( I I I ) - N H s t r e t c h ] and 460 cm"! [ R u ( I I ) - N H o s t r e t c h J as a f u n c t i o n o f e l e c t r o d e p o t e n t i a l f o r RuiNH^),3+/2+ e l e c t r o s t a t i c a l l y adsorbed a t c h l o r i d e - c o a t e d s i l v e r . Ru^III) and R u ( I I ) s u r f a c e c o n c e n t r a t i o n s d e t e r m i n e d by i n t e g r a t i n g c y c l i c voltammogram o b t a i n e d f o r c o n d i t i o n s [50 π Μ R u C N I ^ ) ^ * i n 0.1 M KC1, 50 V s e c ~ l sweep r a t e ] where f a r a d a i c r e s p o n s e dominated by adsorbed redox c o u p l e . Both s u r f a c e c o n c e n t r a t i o n and SERS i n t e n s i t i e s f o r R u ( I I I ) and R u ( I I ) n o r m a l i z e d t o v a l u e s a t -100 and -500 mV v s SCE. SER s p e c t r a o b t a i n e d u s i n g 647.1 i r r a d i a t i o n . 1

3

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139

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The SERS and e l e c t r o c h e m i c a l d a t a a r e c l e a r l y i n c l o s e agreement, i n d i c a t i n g t h a t the former t e c h n i q u e i s i n d e e d s e n s i n g the p r e p o n d ­ e r a n t a d s o r b a t e d e t e c t e d by e l e c t r o c h e m i c a l means. F u r t h e r , the e f f e c t i v e f o r m a l p o t e n t i a l o f the a d s o r b e d Ru(NH ) 6 * couple, E f , can be e x t r a c t e d from the i n t e r s e c t i o n p o i n t o f the R u ( I I I ) and Ru(II) curves, y i e l d i n g E = -300 ± 10 mV vs SCE. I n t e r e s t i n g l y , t h i s v a l u e i s about 120 mV more n e g a t i v e than t h e c o r r e s p o n d i n g b u l k f o r m a l p o t e n t i a l , E^ =• -180 mV v s SCE. T h i s s h i f t can be u n d e r s t o o d s i m p l y i n terms of the e l e c t r o s t a t i c p o t e n t i a l , φ , a t t h e r e a c t i o n s i t e f o r the adsorbed redox c o u p l e s i n c e ' ( E - E ) = φ = -120 mV. T h i s φ v a l u e i s comparable t o t h a t d e t e r m i n e d a t the o u t e r Helmholtz p l a n e f o r t h e s e e x p e r i m e n t a l c o n d i t i o n s u s i n g the s i m p l e Gouy-Chapman model,(X2)providing f u r t h e r e v i d e n c e t h a t t h e redox c o u p l e i s e x c l u d e d from tEe" i n n e r l a y e r r e g i o n c o n t a i n i n g the adsorbed c h l o r i n e l a y e r . S i m i l a r r e s u l t s a r e a l s o o b t a i n e d from Ru(NH3>6 adsorbed a t h a l i d e - c o a t e d g o l d e l e c t r o d e s . ( % 0 T h e s e d a t a t o g e t h e r w i t h the i d e n t ­ i c a l f r e q u e n c i e s and bandshape s p e c i e s suggest t h a t adsorbe 34

2+

S

3

s

f

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5 i i

s

f

f

Γ

+

S i g n i f i c a n t redox c e n t e r - s u r f a c e i n t e r a c t i o n s a r e a n t i c i p a t e d to be i n d u c e d upon b i n d i n g t r a n s i t i o n - m e t a l redox c e n t e r s t o the metal s u r f a c e v i a s m a l l coordinated l i g a n d s . Under t h e s e c i r c u m s t a n ­ ces t h e v i b r a t i o n a l f r e q u e n c i e s of o x i d a t i o n - s t a t e s e n s i t i v e modes a r e e x p e c t e d t o be a l t e r e d as a r e s u l t o f e l e c t r o n i c c o u p l i n g e f f e c t s . For v e r y s t r o n g c o u p l i n g , such t h a t C l a s s I I I b e h a v i o r i s approached, the d i s c r e t e p a i r s o f v i b r a t i o n a l f r e q u e n c i e s f o r the o x i d i z e d and reduced forms s h o u l d merge i n t o t h o s e r e f l e c t i n g an e f f e c t i v e " h y b r i d " oxidation state. Such b e h a v i o r has been o b s e r v e d i n some b u l k - p h a s e mixed-valence systems.(9b) F o r e l e c t r o c h e m i c a l systems, b e h a v i o r d i a g n o s t i c of such c o u p l i n g e f f e c t s might be o b t a i n e d from the merging o f the v i b r a t i o n a l f r e q u e n c i e s a s s o c i a t e d w i t h the o x i d i z e d and reduced forms o b s e r v e d i n the v i c i n i t y o f the f o r m a l p o t e n t i a l . We have r e c e n t l y been examining v a r i o u s i n n e r - s p h e r e a d s o r b e d redox c o u p l e s w i t h the aim of d e d u c i n g i f such e l e c t r o n d e r e a l i z a t i o n e f f e c t s are indeed encountered. One type of c a n d i d a t e system i s t r a n s i t i o n - m e t a l cyano complexes. Some r e p r e s e n t a t i v e d a t a f o r the F e ( C N ) 6 ^ " c o u p l e adsorbed a t g o l d a r e p r e s e n t e d i n F i g s . 2 and 3. The former i s a p l o t of the peak f r e q u e n c y of t h e SERS C-N s t r e t c h i n g mode, VQq, a g a i n s t e l e c t r o d e p o t e n t i a l f o r g o l d i n 1 mM F e ( C N ) 6 ^ w i t h 0.1 M MCI + 0.01 M HClOi* s u p p o r t i n g e l e c t r o l y t e , where M = Na+, K, and C s . F i g u r e 3 shows a r e p r e s e n t a t i v e p o t e n t i a l - d e p e n d e n t s e t o f SER s p e c t r a o b t a i n e d f o r M = K . I n t e n s e and b r o a d V SERS bands a r e o b t a i n e d , i n d i c a t i v e o f s t r o n g a d s o r p t i o n o f Fe(CN) 6 t o the g o l d s u r f a c e v i a one o r more cyano b r i d g i n g l i g a n d s . A s t r o n g depend­ ence o f the s p e c t r a a t a g i v e n p o t e n t i a l upon the n a t u r e o f the s u p p o r t i n g e l e c t r o l y t e c a t i o n i s o b s e r v e d , not s u r p r i s i n g l y i n v i e w of the m u l t i c h a r g e d a n i o n i c n a t u r e o f t h e Fe(CN) 6 ^ c o u p l e and the e x t e n ­ s i v e i o n p a i r i n g e x p e c t e d w i t h i n the double l a y e r w i t h the c a t i o n i c d i f f u s e - l a y e r charge. 3

+

+

+

C N

More s u r p r i s i n g l y , however, a r e the s t r i k i n g v a r i a t i o n s o b s e r v e d i n the p o t e n t i a l dependence o f V as the c a t i o n i s a l t e r e d . F o r the h e a v i e r a l k a l i c a t i o n s , K , Rb+, and Cs+, one and sometimes two V bands were commonly o b s e r v e d a t each p o t e n t i a l , whose f r e q u e n c i e s s h i f t e d to lower v a l u e s as t h e p o t e n t i a l becomes l e s s p o s i t i v e i n the v i c i n i t y of the F e ( C N ) ' ~ f o r m a l p o t e n t i a l , 200 mV v s SCE. ( T h i s i s C N

+

C N

3

Λ

6

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2160 γ

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I

ι 600

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E, mV vs SCE F i g u r e 2 . F r e q u e n c i e s o f SER C-N s t r e t c h i n g mode, ν ^ , p l o t t e d a g a i n s t e l e c t r o d e p o t e n t i a l f o r FeCCNfc^-M- a d s o r b e d a t g o l d electrode i n supporting e l e c t r o l y t e s containing various a l k a l i metal c a t i o n s . S o l u t i o n s were 1 mM F e ( C N ) f i ~ o r F e ( C N ) with 0.1 M MCI + 0.01 M H C I O 4 , where M = N a , K*, o r C s as indicated. L a s e r e x c i t a t i o n w a v e l e n g t h was 647.1 nm. Ν

3

4 -

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In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

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.

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-100 mV 100 cm ) when r e a c t a n t s a t t a i n the c o r r e c t t r a n s i t i o n s t a t e n u c l e a r c o n f i g u r a t i o n w i l l p r o c e e d to p r o d u c t s , w i t h a p r o b a b i l i t y κ=1. Such r e a c t i o n s a r e called adiabatic. I f the o v e r l a p i s s m a l l , t h e n the appropriate n u c l e a r c o n f i g u r a t i o n can be r e a c h e d many times w i t h o u t net r e a c t i o n , and the r e a c t i o n i s c a l l e d n o n a d i a b a t i c . As d e t a i l e d e l s e w h e r e , the o v e r l a p w i l l g e n e r a l l y d e c r e a s e e x p o n e n t i a l l y w i t h i n c r e a s i n g donoracceptor distances: Α « exp - (aR) where the parameter α may depend on the n a t u r e of the donor and a c c e p t o r , the donor i o n i z a t i o n p o t e n t i a l , as w e l l as the n a t u r e o f the " s t u f f " (eg: p r o t e i n m a t r i x ) i n between the donor and a c c e p t o r s . For,many r e a c t i o n s , α has been e x p e r i m e n t a l l y found t o be α = 1 . 2 ± 0 . 2 ' A We now t u r n t o the. a c t i v a t i o n energy. The most w i d e l y used approach t o r e l a t e AG t o s t r u c t u r a l parameters of the r e a c t a n t s i s due t o Marcus ( f i g 3 ) . In e s s e n c e , Marcus t h e o r y s t a t e s t h a t the a c t i v a t i o n f r e e energy AG , i s d e t e r m i n e d by the b a l a n c e between the r e o r g a n i z a t i o n energy, λ, and the f r e e energy o f r e a c t i o n , AG. The r e o r g a n i z a t i o n energy λ can be u n d e r s t o o d as the energy r e q u i r e d f o r a v e r t i c a l t r a n s i t i o n between the energy minimum of the r e a c t a n t s u r f a c e and the p r o d u c t s u r f a c e a t the same n u c l e a r c o o r d i n a t e . In such a p i c t u r e , λ e s s e n t i a l l y d e f i n e s the F r a n c k Condon f a c t o r f o r v i b r a t i o n a l o v e r l a p o f r e a c t a n t and p r o d u c t s t a t e s . The t o t a l λ i s made up o f i n t e r n a l (λ.) and medium (λ ) d i s p l a c e m e n t s : λ = + \ . An energy λ. i s r e q u i r e d s i n c e , i n g e n e r a l , bond l e n g t h s (and a n g l e s ) w i l l change between an o x i d i z e d and r e d u c e d m o l e c u l e . Thus, Ai can be modeled by a harmonic o s c i l l a t o r t r e a t m e n t . I f ηω >> kT, then r e a c t i o n a l o n g t h i s v i b r a t i o n a l c o o r d i n a t e r e q u i r e s n u c l e a r tunne­ ling. S e m i c l a s s i c a l and quantum m e c h a n i c a l t r e a t m e n t s have been Q

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

Nucleor Configuration

Nucleor Configuration

F i g u r e 2. Top: Note t h a t f o r simple ( S l a t e r ) w a v e f u n c t i o n s , the o v e r l a p between donor (D) and a c c e p t o r (A) d e c r e a s e s e x p o n e n t i a l l y as d i s t a n c e (R) i n c r e a s e s . Bottom: t h i s o v e r l a p c a n e q u i v a l e n t l y be viewed as an i n t e r a c t i o n energy, Η , between r e a c t a n t and p r o d u c t s u r f a c e s , l e a d i n g t o an a v o i d e d c r o s s i n g , ( a ) When is l a r g e (>100 cm ) t h e r e a c t i o n remains on t h e lower s u r f a c e , and t h e r e a c t i o n i s " a d i a b a t i c " . (b) When Η i s s m a l l , some t r a j e c t o r i e s may c r o s s t o t h e upper "R" s u r f a c e and r e t u r n t o the r e a c t a n t w e l l w i t h o u t making p r o d u c t s . Α β

Α

β

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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d e v e l o p e d f o r such c a s e s , and have been a p p l i e d t o low temperature data f o r the photosynthetic r e a c t i o n centers. A second i m p o r t a n t energy, Xs, a r i s e s from r e p o l a r i z a t i o n o f t h e medium around a developing charge. A c l a s s i c a l continuum t r e a t m e n t s u g g e s t s 2 1 1 1 As = Ae ( ~ - - ) (-) ( f o r r >> c o l l i s i o n a l ) op s D

where D and D a r e t h e o p t i c a l and s t a t i c d i e l e c t r i c c o n s t a n t s and r i s t h e d i s t a n c e between donop agd a c c e p t o r . The continuum treatment has been q u e s t i o n e d r e c e n t l y . ' F o r r e a c t i o n s i n polymers ( l i k e p r o t e i n s ) whether t h e s t a t i c d i e l e c t i c c o n s t a n t i s t h e a p p r o p r i a t e parameter i s a l s o q u e s t i o n a b l e . Recent d a t a f o r e l e g t r o n t r a n s f e r i n d r y l e x a n f i l m s ( D ~ 2.4) s u g g e s t λ - 1.0 V, w h i l e c l a s s i c a l c a l c u l a t i o n s r e q u i r e λ < 0.3 V. Thus, i n o r d e r t o u n d e r s t a n t h e o r e t i c a l c o n t e x t , we c o n t r o l t h e p r e f a c t o r , A, t h r o u g h t h e d i s t a n c e dependence (the " a " parameter) and c h a r a c t e r i z e t h e r e o r g a n i z a t i o n energy, λ. p

s

S

3

The Cytochrome c/cytochrome b5 Complex: S t r u c t u r a l F e a t u r e s . One i m p o r t a n t f e a t u r e o f t h e c/b5 system i s t h e d e t a i l e d s t r u c t u r a l i n f o r m a t i o n which i s a v a i l a b l e f o r t h e s e p r o t e i n s . The s t r u c t u r e o f c y t c i s ^ n o w n a t - 1.5Â r e s o l u t i o n i n b o t h t h e o x i d i z e d and reduced states. EXAFS s t u d i e s have a l s o been r e p o r t e d , which show no o b s e r v a b l e changes i ^ m e t a l c o o r d i n a t i o n geometry on oxidation/reduction. The cytochrome b5 s ^ g u c t u r e has a l s o been s o l v e d a t h i g h r e s o l u t i o n by Matthews e t a l . Based on t h e s e s t u d i e s and t h e known c h e m i c a l p r o p e r t i e s o f t h e s e c y t o c h r o m e s , i n 1976 Salemme proposed a n o v e l model f o r t h e s t r o n g n o n c o v a l e n t c/b5 complex. T h i s model i s g r a p h i c a l l y shown i n f i g . 4. Key f e a t u r e s i n c l u d e an e l e c t r o s t a t i c b i n d i n g r e g i o n i n which s e v e r a l l y s i n e r e s i d u e s on c y t c a l i g n w i t h a p p r o p r i a t e a c i d i c amino a c i d s on cytochrome b5 t o form s t r o n g " s a l t - l i n k s " . In t h e p r o p o s e d complex s t r u c t u r e , t h e hemes a r e s e p a r a t e d by a n e a r e s t c o n t a c t d i s t a n c e o f - 8 A, (16 A Fe-Fe d i s t a n c e ) and a r e p r e d i c t e d t o be i n p a r a l l e l p l a n e s . A v a r i e t y o f subsequent s t u d i e s have examined s e v e r a l f e a t u r e s o f t h i s model, and g e n e r a l l y s u p p o r t i t s p r e d i c t i o n s . F o r example, t h e c ( I I I ) / b 5 ( I I I ) binding constant, Κ , i s quite s e n s i t i v e t o . i o n i c s t r e n g t h , d e c r e a s i n g from Κ ~ 3 Χ 1 θ ' M a t μ = 0 M t o K ~10 M at μ = 10 mM, c o n f i r m i n g t h e importance o f i o n i c i n t e r a c t i o n s i n s t a b i l i z i n g t h e complex. Some s p e c i f i c r e s i d u e s i n v o l v e d iûgthis i n t e r a c t i o n have been i d e n t i f i e d by t h e NMR s t u d i e s o f Moore. In agreement w i t h t h e model, r e s o n a n c e s f o r L y s 13, L y s 72, & Phe 82 a r e s e l e c t i v e l y broadened. ^ Energy t r a n s f e r measurements by McLendon e t a l suggest a Zn-Fe d i s t a n c e o f c a . 17 A, i n good agreement w i t h Salemme's p r e d i c t i o n . Thus t h e g e n e r a l s t r u c t u r a l f e a t u r e s o f t h e model appear t o be w e l l founded. Μ

M

E l e c t r o n T r a n s f e r K i n e t i c s i n t h e c y t c / c y t b5 Complex. Pulse r a d i o l y s i s t e c h n i q u e s have been used t o i n v e s t i g a t e t h e r e a c t i o n sequence

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

Nuclear Configuration

F i g u r e 3. Schematic o f Marcus t h e o r y ( z e r o i n t e r a c t i o n ) . Key: (a) a c t i v a t e d p r o c e s s , AG < λ; (b) a c t i v a t i o n l e s s , AG = λ ; ( c ) i n v e r t e d , AG > λ

F i g u r e 4. Tom P o u l o s ,

Computer model o f the c y t c / c y t b^ complex

(courtesy

Genex).

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

11.

Long-Distance Electron Transfer

McLENDON ET AL.

Fe(III) b5/Fe(III)

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155

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A t low i o n i c s t r e n g t h , (μ ύ 10 M) where t h e complex i s f u l l y formed, t h e t r a n s f e r r a t e i s f i r s t _ o r d e r and i s independent o f t h e c o n c e n t r a t i o n s o f [b ] , [ c ] o r [e a q ] . As shown i n f i g 5, t h e decay o f i n i t i a l l y produced F e ( I I ) b5 — > F e ( I I I ) b 5 measured a t ^28 nm i s k i n e t i c a l l V g C O u p l e d t o t h e c o n v e r s i o n F e ( I I I ) c — > F e ( I I ) c measured at 416 nm. A t h i g h i o n i c s t r e n g t h (μ=100 mM P h o s p h a t e ) , where t h e complex i s broken up, we o b s e r v e a second o r d e r r a t e which i n c r e a s e s l i n e a r l y w i t h i n c r e a s i n g [ c ] . The second o r d e r r a t e c o n s t a n t s o obtained i s k = 4X10 M s which a g r e e s w e l l w i t h jjggependent stopped f l o w measurement fei

S p e c i e s Dependence. We rather s e n s i t i v e t o small perturbations. F o r example, when t h e p r i m a r y sequence o f c y t c i s a l t e r e d , t h e r a t e c o n s t a n t c a n change by o v e r a f a c t o r o f 4: k . . , = 4000s" k = 6000s . chicken cow « k

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S i m i l a r examples o f t h e dependence o f i n t r a m o l e c u l a r t r a n s f e r r a t e s on p r o t e i n p r i m a r y ^ g t r u c t u r e a r e found i n r e a c t i o n s i n t h e c y t c / c y t c p e r o x i d a s e system. Me^al S u b s t i t u t i o n : P h o t o i n d u c e d E l e c t r o n T r a n s f e r . The d a t a f o r t h e Fe b5/Fe c r e a c t i o n , by t h e m s e l v e s , a r e i n s u f f i c i e n t t o e s t a b l i s h e i t h e r t h e r e o r g a n i z a t i o n energy λ, o r t h e exchange energy Η , f o r the p r o t e i n c o u p l e . As one approach t o t h i s problem, we p r e p a r e d and c h a r a c t e r i z e d d e r i v a t i v e s o f cytochrome c i n which F e i s r e p l a c e d by m e t a l s w i t h v e r y d i f f e r e n t r e a c t i v i t y (eg: Z n ( I I ) ) . S i n c e Znc, and the a n a l o g o u s f r e e base, Η porphc, have f i l l e d d s h e l l s , they have r e l a t i v e l y l o n g l i v e d e x c i t e d s t a t e s , which can s e r v e as s t r o n g reducing agents: E ° 3 * ' = 0.8 V s 10 msec. Α β

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°( porphc*)/porphc) ' porphc We, and o t h e r s , have shown t h a t t h e s e d e r i v a t i v e s m a i n t a i n t h e same s t r u c t u r e as n a t i v e cytochrome c, as j u d g e d by c i r c u l a r d i c h r o ism, h i g h r e s o l u t i o n magnetic resonance,^gnd by b i n d i n g t o F e ( I I I ) c y t b5, o r o t h e r p h y s i o l o g i c a l p a r t n e r s . Thus, by combining p u l s e - r a d i o l y s i s s t u d i e s o f Fec/Feb5 o r porphc" /Feb5 w i t h p h o t o l y s i s s t u d i e s o f metal s u b s t i t u t e d cytochromes i t i s p o s s i b l e t o vary t h e r e a c t i o n e x o t h e r m i c i t y from AG = 0.2 eV t o AG = 1.1eV. The r e s u l t s of f l a s h p h o t o l y s i s s t u d i e s w i t h porphc/b5 and Znc/b5 a r e shown i n f i g 6. Data from a l l d e r i v a t i v e s a r e summarised i n f i g u r e 7, as a plot of k v s . AG. C o n s i d e r i n g t h e u n c e r t a i n t i e s i n r a t e s which may r e s u l t from s m a l l c o n f o r m a t i o n a l d i f f e r e n c e s t h e d a t a can be a d e q u a t e l y d e s c r i b e d u s i n g s i m p l e Marcus t h e o r y ( s o l i d l i n e ) . This r e s u l t s u g g e s t s a t o t a l r e o r g a n i z a t i o n energy f o r r e a c t i o n o f λ = 0.7 eV. I t remains unknown how such r e o r g a n i z a t i o n energy might be p a r t i t i o n e d between A i and As. P r e l i m i n a r y measurements o f t h e temperature dependence o f t h e r e a c t i o n Fe b5/Fe c suggest Eact - 4 e t

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

156

EXCITED STATES AND REACTIVE INTERMEDIATES .02·

1

3

5

F i g u r e 5. I n t r a m o l e c u l a r e l e c t r o n t r a n s f e r k i n e t i c s i n the c/b complex. Key: t o p , decay of f e ( I I ) b . (428 nm); bottom, growth of F e ( I I ) c (416 nm). t

Time F i g u r e 6. Top: quenching of (porph c y t c ) by F e ( I I I ) c y t b jilO μιη each, pH 7, ImMpi) (460 nm) ; bottom: quenching of (Zn c y t c ) - by F e ( I I I ) c y t b (460 nm).

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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McLENDON ET AL.

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k c a l / M o l ( f i g 8) which i s somewhat h i g h e r than the v a l u e o f 3.6 k c a l / M o l c a l c u l a t e d from Marcus t h e o r y assuming λ = 0.7 eV as i n f i g 7. However, the o b s e r v e d v a l u e might c o n t a i n some c o n t r i b u t i o n from c o n f o r m a t i o n a l changes which a f f e c t o r b i t a l o v e r l a p , and s t u d i e s a r e ongoing. The Marcus type f i t shown i n f i g u r e 7 i m p l i c i t l y assumes t h a t the p r i m a r y e f f e c t o f metal s u b s t i t u t i o n i s t o change the e x o t h e r m i c i t y , w h i l e h o l d i n g c o n s t a n t b o t h the r e o r g a n i z a t i o n energy and the donor acceptor e l e c t r o n i c coupling. We have a l r e a d y n o t e d t h a t e x t e n s i v e c o n f o r m a t i o n a l s t u d i e s s u g g e s t t h a t the metal s u b s t i t u t e d cytochromes c (eg: Z n c y t c , p o r p h c y t c ) a r e e s s e n t i a l l y i s o s t r u c t u r a l w i t h F e c y t c . Thus we expect t h a t f o r the Zn, Fe, and p o r p h y r i n c y t o chromes m e t a l - p o r p h y r i n dependent v a r i a t i o n s i n s t r u c t u r e w i l l not g r e a t l y a f f e c t the g e n e r a l t r e n d o b s e r v e d f o r the dependence o f r a t e on AG. The q u e s t i o n of metal dependent e f f e c t electroni couplin requires d e t a i l e d examination such e f f e c t s ^ F i r s t , as the donor b i n d i n g energy d e c r e a s e s from Fe t o Zn , the e l e c t r o n i c m i x i n g , e x p r e s s e d i n the dependence o f r a t e on d i s t a n c e k « exp-(aR) s h o u l d change. In tljie^simplest b a r r i e r I P

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t u n n e l i n g t h e o r y α = ^ d n o r ~" medium^ > then, α depends s t r o g g l v ^ o n IP 88n8r. I n m o r ^ d e t a i l e d t h e o r i e s , based on superexchange, ' α depends weakly ( r o u g h l y l o g a r i t h m i c a l l y ) on IP donor. The a v a i l a b l e e x p e r i m e n t a l d a t a f o r the dependence o f α on IP s u p p o r t the superexchange model : α depends weakly on IP. Thus, we expect the shape o f the r a t e v s . AG p l o t w i l l be m i n i m a l l y a f f e c t e d by changes i n t h e " a " parameter among the d i f f e r e n t p o r p h y r i n s . A second t y p e of v a r i a t i o n i n e l e c t r o n i c c o u p l i n g c o u l d o c c u r . I t i s p o s s i b l e t h a t i n the Fe system, the donor w a v e f u n c t i o n i s h i g h y l o c a l i z e d a t the i r o n , whereas i n the e x c i t e d s t a t e Zn p o r p h y r i n , the e l e c t r o n i s c l e a r l y w i d e l y d e l o c a l i z e d around the r i n g I f t h i s were t r u e , then the " e f f e c t i v e / d i s t a n c e f o r the Fe r e a c t i o n would be c a : 4 A l o n g e r than f o r the Zn r e a c t i o n , c o r r e s p o n d i n g t o a hundred fold rate difference. However, the a v a i l a b l e e v i d e n c e s t r o n g l y s u g g e s t s t h a t e x t e n s i v e d e r e a l i z a t i o n o c c u r s between the Fe c e n t e r and the p o r p h y r i n π system. F o r example, combined ΝMR s t u d i e s and t h e o r e t i c a l c a l c u l a t i o n s o f F e ( I I I ) p o r p h y r i n s s u g g e s t the s p i n i s e x t e n s i v e l y d e l o c a l i z e d i n t o the π system. (φ = 0.7Φ+ 0.3Φ )· F u r t h e r m o r e , e x c e s s s p i n d e n s i t y , and a s s o c i a t e d e l e B S r B n d e n s i t y , i n cytochrome c i s d i r e c t e d a t the heme edge from which e l e c t r o n t r a n s f e r would o c c u r , r e f l e c t i n g an a n i s o t r o p i c i n t e r a c t i o n o f the d x z , d yz o r b i t a l s w i t h the a x i a l m e t h i o n i n e . ' I f we t a k e the c o n s e r v a t i v e e s t i m a t e s o f 10$ e l e c t r o n d e n s i t y a t the r e a c t i v e edge o f the Fe system, and -25$ e l e c t r o n d e n s i t y a t the edge f o r the more d e l o c a l i z e d Zn system, then a maximum " n o r m a l i z a t i o n f a c t o r " o f about two f o l d i n the e l e c t r o n i c f r e q u e n c y f a c t o r can be e s t i m a t e d t o c o r r e c t f o r d i f f e r e n t i a l w a v e f u n c t i o n o v e r l a p i n the F e c y t v s . Zncyt system. 0

m

1

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η

We c o n c l u d e t h a t the b a s i c t r e n d o f i n c r e a s i n g r a t e w i t h i n c r e a s i n g AG i n the c/b5 system p r i m a r i l y r e f l e c t s a F r a n c k Condon term r a t h e r than an e l e c t r o n i c term. However, s i n c e s m a l l r a t e d i f f e r e n c e s may be p h y s i o l o g i c a l l y s i g n i f i c a n t , " t u n i n g " of the e l e c t r o n i c f a c t o r i s c e r t a i n l y worthy o f f u r t h e r s t u d y .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

158

EXCITED STATES AND REACTIVE INTERMEDIATES

«Η 0

1

1

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05

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15

-AG (eV) F i g u r e 7. P l o t of In ( i n t r a c o m p l e x ) e l e c t r o n t r a n s f e r r a t e (10 μπι each, pH 7, ImM phosphate) vs AG.

F i g u r e 8.

Arrhenius plot

f o r data

i n F i g u r e 5.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Long-Distance Electron Transfer

11. MCLENDON ET AL.

159

A Second Example: Cytochrome c P e r o x i d a s e / c y t o c h r o m e c . Cytochrome c p e r o x i d a s e ( c e p ) c a t a l y z e s r e d u c t i o n o f H^O^ i n y e a s t , w i t h cytochrome c p r o v i d i n g t h e r e d u c i n g equivalents: Fe ES

III

ccp + H 0 2

IV °+ Fe cep (ES) + H 0

2

2

+ (2) F e ( I I ) c y t c -> (2) F e ( I I I )

cytc + Fe(III)ccp

D e t a i l e d c r y s t a l l o g r a p h i c s t r u c t u r e s a r e a v a i l a b l e f o r c c p , and c , and a d e t a i l e d s t r u c t u r a l p r o p o s a l e x i s t s f o r t h e n o n c o v a l e n t c c p / c y t c complex, as shown i n f i g u r e 9. I n t h i s complex, t h e hemes are r o u g h l y p a r a l l e l w i t h a c l o s e s t approach d i s t a n c e o f c a 16 A, (24 A Fe-Fe). A wide v a r i e t y o f p h y s i c a l e v i d e n c e , i n c l u d i n g c h e m i c a l c r o s s l i n k i n g s t u d i e s , s u p p o r t s t h i s proposed s t r u c t u r e . Thus, t h e i n t e r e s t i n g i n i t i a l q u e s t i o n s f o r r e l a t i n g s t r u c t u r e t o a c t i v i t y a r e : what a r e t h e r a t e s o f r e a c t i o n and how do t h e s e r a t e s depend on r e a c t i o n e x o t h e r m i c i t y structure. The c/ccp syste questions. Fe c c p can be produced i n a v a r i e t y o f r e a c t i v e s t a t e s i n c l u d i n g F e ( I I ) ( h i g h s p i n ) , F e ( I I ) (low s p i n ) , F e ( I I I ) ( h i g h s p i n ) , F e ( I I I ) (low s p i n ) , and F e ( I V ) , and metal s u b s t i t u t i o n t o i n t r o d u c e Zn, Mn and m e t a l s i s f a c i l e . S i m i l a r l y , a v a r i e t y o f m e t a l l o c y t o c h r o m e c d e r i v a t i v e s can be produce^, e g : Ç ç p D c y t c , F e ( I I I ) c y t c , Z n c y t c , porph c y t c . F e

c c

P

/ F e

m

HI

n

The r e a c t i o n F e ccp/Fe cytc Fe ccp/Fe c y t c proceeds w i t h ΔΕ = 0.4V. The r e a c t i o n haç been monj^ored both by p u l s e r a d i o l y s i s , and by s i m p l e m i x i n g o f Fe c c p + Fe c y t c , with equivalent r e s u l t s : k = 0.25 ± 0.07 s ( f i g u r e 10) I t i s i n t e r e s t i n g t h a t a dependence of r a t e on t h e p r i m a r y s t r u c t u r e o f t h e p r o t e i n i s ^ o b s e r v e d : ( a t c o n s t a n t AG) f o r h o r s e c y t c / c c p ( y e a s t ) k = 0.25 s but f o r yeasty cytc/(yeast) ccp k = 4 s and f o r tuna c y t c / y e a s t c c p k = 0.1 s , even though the g e n e r a l t h r e e d i m e n s i o n a l s t r u c t u r e s a r e e s s e n t i a l l y i d e n t i c a l f o r h o r s e , tuna and y e a s t cytochrome^ c . These d e t e r m i n a t i o n s d i s p r o v e an e a r l i e r s u g g e s t i o n based on modulated e x c i t a t i o n s p e c t r o s c o p y , t h a t k ~ 10 s . C l e a r l y t h e r a t e i s slow, but does t h i s slow r a t e r e f l e c t λ o r Η ? ^ The c o m p a r a t i v e ^ r a t e s t u d y o f porpnc /Fe ccp -> porphc/Fe c c p (ΔΕ - 0.9V k=100 s ) s u g g e s t s a h i g h r e o r g a n i z a t i o n energy (X-2V) for t h i s couple. I t i s l i k e l y t h a t much o f t h i s r e o r g a n i z a t i o n energy i s an i n n e r s p h e r e r e o r g a n i z a t i o n , r e f l e c t i n g t h e c o o r d i n a t i o n change between t h e h i g h s p i n 6 c o o r d i n a t e F e ( I I I ) / 5 c o o r d i n a t e Fe(II) couple. As a l r e a d y n o t e d , i t i s p o s s i b l e t o compare t h e r e a c t i v i t y o f another o x i d a t i o n s t a t e of ccp v i a the r e a c t i o n Fe(IV) ccp + cytc(red) Fe(III) ccp + c y t c (ox). For F e ( I I ) c y t c ΔΕ - IV and k Ξ 10^ s while f o r Z n ( I I ) c y t c , ΔΕ . . =°θ]^ k = 2 ± 0.4 s and f o r H porph c y t c reaction ' 2 ΔΕ^ ,. = 0.1V k = 4±0.4x10 s . We a g a i n see a s t r o n g reaction dependence o f r a t e i n d r i v i n g f o r c e , c o n s i s t e n t w i t h λ ~ 1.4V. Γβ

t

0

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

160

F i g u r e 9. Compute ( c o u r t e s y Tom P o u l o s ) .

0.6.

ο 0

0.2 0.4

0.6 0.8

1

t (s) F i g u r e 10. R e a c t i o n of horse c y t c F e ( I I I ) / F e ( I I ) c c p top 436 nm (ccp). R e a c t i o n of horse c y t c F e ( I I I ) / F e ( I I ) c c p m i d d l e 416 nm ( c y t c ) . R e a c t i o n of y e a s t c y t c F e ( I I I ) / F e ( I I ) c c p bottom 436 nm.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

11.

MCLENDON ET AL.

Long-Distance Electron Transfer

161

Thus, the p i c t u r e o b t a i n e d from the c y t c / c c p system i s q u i t e s i m i l a r t o t h a t i n the c y t c / c y t b 5 / b system: λ f o r p r o t e i n - p r o t e i n e l e c t r o n t r a n s f e r appears t o be l a r g e . Comparisons With Other I n t r a m o l e c u l a r P r o t e i n Redox R e a c t i o n s . A l t h o u g h work r e m a i n s l i m i t e d , p r o g r e s s i n t h i s a r e a i s i n d e e d r a p i d , and many new r e s u l t s have been r e p o r t e d w i t h i n the l a s t y e a r . E x p e r i m e n t s can be d i v i d e d i n t o two b a s i c c a t e g o r i e s : t r a n s f e r between p h y s i o l o g i c a l p a r t n e r s (eg: c y t b5/cytC.; c y t c / c y t c p e r o x i d a s e ( c c p ) , c y t b5/Hb), and t r a n s f e r between two groups i n the same g p r o t e i n r a n g i n g from Hoffman's s t u d i e s of α Znporph/gFeporph Hb, to s t u d i e s of Ru s u b s t i t u t e d p r o t e i n s ( c y t c , a z u r i n o r m y o g l o b i n ) by Gray and I s e i d . C u r r e n t l y a v a i l a b l e r e s u l t s a r e summarized i n T a b l e I. S e v e r a l p o i n t s a r i s e from t h i s c o m p i l a t i o n . F i r s t of a l l , f o r the p r o t e i n - p r o t e i n redox c o u p l e s the dependences of r a t e on AG and/or t e m p e r a t u r e , a r e r e o r g a n i z a t i o n e n e r g i e s (0.8 l a r g e v a l u e of λ i n f e r r e d f o r the c/b5 c o u p l e appears l i k e l y t o be a common phenomenon r a t h e r than an anomaly. Recent model r e a c t i o n s (not p r o t e i n ) ^ i n low d i e l e c t r i c media l i k e i s o o c t a n e or polycarbonate show r e l a t i v e l y l a r g e e x p e r i m e n t a l s o l v e n t r e o r g a n i z a t i o n e n e r g i e s , r a n g i n g from λ = 0.6 - 1.0V w h i l e c l a s s i c a l d i e l e c t r i c continuum t h e o r y p r e d i c t s λ = 0 - 0.3V. The r e a s o n s f o r the f a i l u r e of continuum t h e o r y a r e not u n d e r s t o o d , but may r e f l e c t the h i g h l o c a l e l e c t r i c f i e l d s which o c c u r w i t h i n s e v e r a l angstroms of an i n j e c t e d c h a r g e . In t h i s c o n t e x t , v a l u e s o f λ £ 1V f o r p r o t e i n s do not seem anomalous, but a r e q u i t e i n l i n e w i t h o b s e r v a t i o n s o f s i m p l e redox r e a c t i o n s i n low d i e l e c t r i c media. We a l s o note t h a t the r a t e s o b s e r v e d f o r the p r o t e i n complexes a t o p t i m a l e x o t h e r m i c i t y a r e s i m i l a r t o t h o s e ^ o b s e r v e d a t o p t i m a l AG and s i m i l a r d i s t a n c e s i n small molecules. A second p o i n t of i n t e r e s t i s t h a t the λ v a l u e s i n f e r r e d f o r h i g h s p i n F e ( I I I ) complexes (eg: Hb) a r e much l a r g e r t h a n t h o s e seen f o r low s p i n systems (eg: c y t c ) . The slow r e a c t i o n s seen f o r h i g h s p i n Fe heme p r o t e i n s l i k e l y r e f l e c t a l a r g e i n t e r n a l r e o r g a n i z a t i o n energy a s s o c i a t e d w i t h the r e a c t i o n from 5 c o o r d i n a t e F e ( I I ) t o 6 coordinate F e ( I I I ) . A f i n a l p o i n t i s t h a t e l e c t r o n t r a n s f e r r e a c t i o n s of the Ru m o d i f i e d p r o t e i n s a r e g e n e r a l l y slow when compared w i t h the p r o t e i n p r o t e i n r e a c t i o n s or w i t h s i m p l e model systems. The r e a s o n s f o r t h i s d i s c r e p a n c y a r e f a r from c l e a r . L i k e l y e x p l a n a t i o n s i n c l u d e an underestimate of λ (the date of I s e i d , and some d a t a o f W i n k l e r e t al. s u g g e s t λ £ 0.8V), and an u n d e r e s t i m a t e o f the a p p r o p r i a t e d i s t a n c e f o r e l e c t r o n t r a n s f e r . I f t r a n s f e r p r o c e e d s from the heme d i r e c t l y t o the Ru atom, r a t h e r than v i a the i m i d a z o l e l i g a n d , then the l a r g e r d i s t a n c e l i s t e d i n the T a b l e i s a p p r o p r i a t e . Some e v i d e n c e f o r t h i s p o s s i b i l i t y comes from s t u d i e s o f a model r u t h e n i u m - p o r p h y r i n system.25 At h i g h e x o t h e r m i c i t y , a r a t e c o n s t a n t o f k ύ 10 s i s o b s e r v e d f o r e l e c t r o n t r a n s f e r from the p o r p h y r i n t o Ru. W h i l e r a p i d , t h i s r a t e i s f a r s l o w e r than e x p e c t e d f o r an a d i a b a t i c system (k > 10 s ). We i n f e r , t h e r e f o r e , t h a t a d i a b a t i c i t y i n t h i s system i s p r e c l u d e d by the poor o v e r l a p o f the donor and a c c e p t o r o r b i t a l s r e f l e c t i n g i n p a r t the h i g h l y metal

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

162

EXCITED STATES AND REACTIVE INTERMEDIATES

Table I .

Some R a t e s o f I n t r a m o l e c u l a r B i o l o g i c a l E l e c t r o n T r a n s f e r Reactions

System

I I

I I I

Fe cytb /Fe cytc λ * III Z n c y t c /Fe b 3 * III H p o r f c /Fe b III Zncytb_/Fe cytc - I l l H p o r f c /Fe b J

û

0

c

2

F e

i l

R(A)

k(s

0.2

8 (l6)

1.5X10

0.8

8

0.4

8

3X1 0 4

1.1

8

1.1

8

0.05

8(16)

0.01

0 9

8

8X1 0

0.9

20

100

0.9

16(24)

2X1 0

0.4

16

2

a

m

b

/Fe Hb III ZnHb/Fe b_ III ZnbJFe Hb III α Zn β Fe Hb

ref

3

b

5

1X10

5

m

')

4X1 0

3

c

5

3

d

5

5

I V

Fe ccp(ES)/Fe

I ] :

cytc I I

ccp(ES)/Zn cytc ccp(ES)/H porfcytc 2

I I

I

I I

Fe ccp/Fe" ' cytc Fe

I I I

ccp/porfcytc

c c p E S / p r o t e i n r a d i c a l (TRP) 3 * III Znccp /Fe c -.11 . , I l l -

4

9

+

6

p

X 1.,1

+

10

10? 6

8

X

5..8

2..2

8

8

10

10

X

8

8

2. 9

10

X

10

10

10»

X X

10» 10»

X

1010

X X

1010 1010

X

X X

6. 5

6..6

8..1

2. 5

6. 2

5..5

1..5

1..3

- l

X

1 M -1 >

2.,0

q

b

a

( k ' ) are corrected

fordiffusion

effects.

The r a t e c o n s t a n t s

+

-1. .85

k

b.

+

+

3

-1. .67

-1. .57

-1. .52

-1. .52

-1. .49

-1. ,48

-1. .36

-1. .14

-1. .07

-0..93

-0..78

-0..67

a

F o r quenchers 1-3, E ( A / A ) = E ^ ( A / A ) . F o r quenchers 4-13, the r e d u c t i o n s a r e i r r e v e r s i b l e ; t h e r e f o r e , the v a l u e s o f E ( A / A ) a r e the c a t h o d i c peak p o t e n t i a l s , E ( A / A ) , measured a t a c o n s t a n t s c a n r a t e (200 mV/s). Both E ^ ( A / A ) and E ( A / A ) were measured by c y c l i c vo ltammetry (CH3CN, μ - 0.1 M [ ( n - C H ) 4 N P F ] , 22 ± 2 ° C ) .

2

+

E(A /A), V v s . SSCE

quenchers

a.

3

+

2,6-(CH3) -4-OCH

CH3

3

4-C(CH )

3

13.

5

4-CH3

2-OCH3

3

2,4,6-(CH )

CH3

12.

3

CH3

11. 3

2,3,6-(CH )

CH3

10. 2

2,6-(CH )

C H

9. 3

CH3

8.

2

CH3

2

7.

5

2

2

C H

3-CONH

3-CONH

6.

5

C H CH

3

CH3

2

5.

6

2

4-C0NH

4-C0 CH

4.

2

C H

3.

3

CH

CH

4-CN

5

N

R'

3

+

3

t r a n s f e r quenching o f [ I r ( y - p z ) ( C O D ) ] 2 * by a l k y l a t e d

R

-

1.

R

Data f o r e l e c t r o n

2.

p

Table I.

12. MARSHALL ET AL.

8

Dinuclear d -d

8

dimer and reduced methyl v i o l o g e n can be m o n i t o r e d by f l a s h and the r a t e c o n s t a n t i s n e a r t h a t o f t h e d i f f u s i o n l i m i t : [lr(u-pz)(COD)]

+ 2

+ MV+

k

» [Ir(y-pz)(COD)]

b

171

Iridium and Platinum Complexes

photolysis

2

+ MV +

2

(3)

S i m i l a r l y , the back e l e c t r o n t r a n s f e r r e a c t i o n s i n v o l v i n g a l k y l a t e d p y r i d i n i u m a c c e p t o r s a r e v e r y r a p i d and no n e t p h o t o c h e m i s t r y i s observed. To u t i l i z e t h e s t r o n g r e d u c i n g power o f t h e ( d o * p o ) e x c i t e d s t a t e s o f the p l a t i n u m and i r i d i u m dimers, the n o n p r o d u c t i v e back e l e c t r o n t r a n s f e r r e a c t i o n s need t o be i n h i b i t e d . An e f f e c t i v e way to a c c o m p l i s h t h i s i s t o use a c c e p t o r s t h a t a r e t h e r m a l l y unstable a f t e r the i n i t i a l e l e c t r o n t r a n s f e r . R e d u c t i o n o f a l k y l h a l i d e s has been shown t o l e a d t o s h o r t - l i v e d r a d i c a l a n i o n s RX , which r a p i d l y decompose t o g i v e R- and X" ( k = 3 χ 1 0 s " f o r Ο Η β Ο Ι , k 3 χ 1 0 s ' f o r CH3Br ) (22) The t r i p l e t e x c i t e d s t a t e s o f I and 2 are c a p a b l e o f r e d u c i n g a e l e c t r o n r e d u c t i o n step lead r a d i c a l anion fragmentation (23). 3

T

7

1

7

d

8

1

T

C

E x c i t a t i o n o f t h e Ai -*~ Â , 2 U < 4h) ( 1 + 2 > 2 ( 2v^ e l e c t r o n i c t r a n s i t i o n s o f P t ( p o p ) 4 ^ ~ and [ I r ( u - p z ) ( C 0 D ) ] j r e s p e c t i v e l y , i s f o l l o w e d i n each case by a r a p i d r e a c t i o n w i t h 1,2d i c h l o r o e t h a n e (DCE) t o form a d - d d i h a l i d e dimer ( P t ( p o p ) 4 ( C l ) " or [ l r ( y - p z ) ( C 0 D ) ( C l ) ] ) and e t h y l e n e : l

3 a

D

1 a

1 β

3 β

c

2 U

g

2

2

7

7

h

2

2

2

Pt (pop) 2

4 4

-

+ ClCH CH Cl-r^:

[lr(y-pz)(C0D)]

2

2

*C1-Pt

2

+ C1CH CH C1 > 4 2

2

λ

> 5 0

n

m

Pt-Cl + C H 2

C

1

^

I

r

ΐ Γ

—

\

C

l

+

(4)

4

C

H

2 4

5

This binuclear photooxidative addition reaction i s general for a number o f h a l o c a r b o n s ( F i g u r e 3 ) . While DCE and 1,2-dibromoethane r e a c t c l e a n l y t o g i v e the d i h a l i d e m e t a l dimers and e t h y l e n e , s u b s t r a t e s such as bromobenzene o r methylene c h l o r i d e r e a c t through an a l k y l o r a r y l i n t e r m e d i a t e . This intermediate reacts further to y i e l d the d i h a l i d e d - d m e t a l complexes. A mechanism t h a t a c c o u n t s f o r the o x i d a t i v e a d d i t i o n o f h a l o ­ carbons has been proposed f o r the two dimers ( F i g u r e 4) ( 2 3 ) . The mechanism i n v o l v e s t h e o x i d a t i v e quenching o f t h e t r i p l e t e x c i t e d s t a t e o f t h e m e t a l dimer as the p r i m a r y p h o t o p r o c e s s . This gives a r a d i c a l a n i o n s p e c i e s t h a t d i s s o c i a t e s a h a l i d e , t h e r e b y p r o d u c i n g an organic r a d i c a l . The d i s s o c i a t e d h a l i d e adds t o the p a r t i a l l y o x i d i z e d m e t a l dimer t o form a mixed v a l e n c e I r * - I r * * - X o r P t * Pt***-X i n t e r m e d i a t e . T h i s i n t e r m e d i a t e r e a c t s f u r t h e r w i t h the r e m a i n i n g o r g a n i c r a d i c a l (presumably i n a second, t h e r m a l e l e c t r o n t r a n s f e r s t e p ) t o form t h e f i n a l d - d d i h a l i d e dimer. T h i s mechanism i s s u p p o r t e d by t h e p r o d u c t d i s t r i b u t i o n found f o r the p h o t o c h e m i c a l r e a c t i o n s w i t h 1,2-bromochloroethane. A t h i g h c o n c e n t r a t i o n s o f t h i s s u b s t r a t e , the r e s u l t i n g p l a t i n u m and i r i d i u m dimers a r e the d i b r o m i d e s p e c i e s , P t ( p o p ) 4 ( B r ) ^ ~ and [Ir(μ-ρζ)(COD) 7

7

1

7

7

2

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

172

EXCITED STATES AND REACTIVE INTERMEDIATES

[lr(/x-pz)(COD)] Tr

2

+ XCH CH X

2

2

2

— — ^ X - I r — Tr -X + C H 2

4

(X = CI, Br)

Ir

+ CH CI

2

2

^CICH

2

4Pt (pop) 2

Pt

- Ir—Ir-CI

^ C l -Ir-Ir-CI

(non-aqueous solvents)

4

2

2

+ XCH CH X — — 2

2

^X-Pt-Pt-X

+

C H 2

4

(X- CI, Br) Pt

2

+ 2C H X 6

—

5

^C

(X = CI, Br)

hi/ CH CI 2

Pt

2

+

^ C I C H - Pt—Pt-CI

2

2

or CR CI 2

or hl/ 2

5-CI-Pt—Pt-CI

> CICR - Pt—Pt-CI 2

3

3

4

F i g u r e 3. R e a c t i v i t y o f [ I r ( μ - ρ ζ ) ( C O D ) ] * and [ P t ( p o p ) 4 ~ ] * with halocarbons. 2

2

2

F i g u r e 4. Proposed g e n e r a l m e c h a n i s t i c scheme f o r h a l o c a r b o n p h o t o o x i d a t i v e a d d i t i o n t o b i n u c l e a r I r ( l ) and P t ( l l ) .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

12.

8

Dinuclear d -d

MARSHALL ET AL.

H

173

Iridium and Platinum Complexes

(Br)J2» e x c l u s i v e l y . Low c o n c e n t r a t i o n s o f 1,2-bromochloroethane, however, y i e l d the mixed h a l i d e m e t a l dimers P t 2 ( p o p > 4 ( B r ) ( C l ) ~ and Ir2(y-pz)2(C0D)2(Br)(Cl). T h i s r e s u l t i s p r e d i c t e d by the proposed mechanism ( F i g u r e 5 ) . P h o t o l y s i s r e s u l t s i n f o r m a t i o n o f Pt2(pop)4~ ( B r ) ~ or Ir2(u-pz)2(COD>2(Br) as i n t e r m e d i t a e s . The i n t e r m e d i a t e can r e a c t w i t h another bromochloroethane m o l e c u l e , as i t does when the l a t t e r s p e c i e s i s i n h i g h c o n c e n t r a t i o n , to y i e l d the dibromide dimer or i t can r e a c t w i t h the c h l o r o e t h a n e r a d i c a l t o y i e l d the mixed h a l i d e m e t a l s p e c i e s . The l a t t e r pathway becomes c o m p e t i t i v e at low h a l o c a r b o n c o n c e n t r a t i o n s . In g e n e r a l , the o x i d a t i v e a d d i t i o n of h a l o c a r b o n s i s t y p i c a l o f the p h o t o c h e m i s t r y a r i s i n g from e l e c t r o n t r a n s f e r from d - d m e t a l dimers w i t h the f i n a l p r o d u c t b e i n g the s t a b l e d - d m e t a l - m e t a l bonded dimers (24-25). 4

4

8

7

8

7

B i m o l e c u l a r quenching o f the e x c i t e d s t a t e s o f m e t a l complexes g e n e r a l l y i n v o l v e s e l e c t r o n t r a n s f e r or energy t r a n s f e r p r o c e s s e s (_1). R e c e n t l y , however, Pt2(pop>4^~ has been found to undergo a photo­ chemical r e a c t i o n i n v o l v i n cess ( 2 6 ) . The r e a c t i o i s o p r o p a n o l to acetone: Pt (pop) 2

4 4

" +

h v

(CH ) CHOH 3

3

2

> Pt (pop) 2

4 4

" + H

2

+

(CH ) CO 3

2

(6)

4

While [ P t 2 ( p o p ) 4 ~ ] * i s a s t r o n g r e d u c t a n t , i t i s not s u f f i ­ c i e n t l y r e d u c i n g to t r a n s f e r one e l e c t r o n to an a l c o h o l ( 2 7 ) . E x t r a c t i o n o f a hydrogen i n a primary photoprocess would produce an i s o p r o p y l r a d i c a l t h a t c o u l d undergo d i s p r o p o r t i o n a t i o n to y i e l d acetone ( 2 8 ) : 2(CH ) COH 3

»

2

(CH ) CHOH + ( C H ) C O 3

2

3

or f u r t h e r r e a c t w i t h a m e t a l dimer 3

[Pt (pop) 2

4 4

-]* +

(7)

2

(29). 4

(CH ) COH—*Pt (pop) (H) - + 3

2

2

4

(CH ) CO 3

2

(8)

Our k i n e t i c i n v e s t i g a t i o n o f t h i s r e a c t i o n p r o v i d e s c o m p e l l i n g e v i ­ dence f o r an atom a b s t r a c t i o n mechanism. Quenching o f the t r i p l e t e x c i t e d s t a t e o f 1 by v a r i o u s a l c o h o l s o c c u r s o n l y when an α-hydrogen i s p r e s e n t (19). No quenching o c c u r s w i t h _t-butanol o r t r i p h e n y l carbinol. Furthermore, completely deuterated i s o p r o p a n o l y i e l d s a k i n e t i c i s o t o p e e f f e c t o f 1.5 ( T a b l e I I ) . Table

3

II.

4

Data f o r the quenching o f [Pt2(pop)4 ~* ] * by o r g a n i c s t r a t e s i n CH CN s o l u t i o n at 22 ± 2°C.

sub-

3

1

q

(CH ) CHOH

10

3

(C H )CH

10

4

3

6

2

5

3

(C H ) CHOH 6

5

2

k (2-propanol)/k (2-propanol(d-8)) q

1

k (M" s" )

Quencher

q

ΙΟ* =1.5.

Hydrocarbons w i t h r e l a t i v e l y weak C-H bonds a l s o r e a c t w i t h [ P t 2 ( p o p ) 4 " ] * by Η-atom t r a n s f e r . For example, t o l u e n e can be p h o t o c a t a l y t i c a l l y c o n v e r t e d t o b i b e n z y l by a b s t r a c t i o n o f a m e t h y l 3

4

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

hi/

-M + BrCH CH CI 2

3

2

M — M * + BrCH CH CI

M—M , CICH CH Br" +

2

2

M—M-Br, CICH CH • 2

BrCH CH CI 2

2

1

2

CICH CH 2

2

M—M-Br Figure 5 . Proposed pathways f o r B r C I ^ C I ^ C l a d d i t i o n t o b i n u c l e a r I r ( l ) and P t ( l l ) .

Cl-M—M-Br photooxidative

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

8

Dinuclear d -d

12. MARSHALL ET AL.

8

hydrogen and c o u p l i n g o f t h e r a d i c a l Pt (pop) 2

4 4

~

+ C H -CH J ^ P t ( p o p ) 6

5

3

175

Iridium and Platinum Complexes

2

products: 4 4

" + C H -CH -CH -C H 6

5

2

2

6

5

+ H

2

(9)

An i n t e r m e d i a t e has been observed f o r t h e r e a c t i o n i n a l c o h o l s . Narrow band i r r a d i a t i o n (370 nm) o f P t ( p o p ) ~ i n i s o p r o p a n o l r e s u l t s i n the d i s a p p e a r a n c e o f P t ( p o p ) ~ and t h e appearance o f an i n t e r m e d i a t e w i t h a maximum a t 313 nm. T h i s i n t e r m e d i a t e w i l l t h e r m a l l y b a c k - r e a c t ( t i ^ = 85 min) by f i r s t - o r d e r k i n e t i c s t o r e f o r m P t ( p o p ) ~ o r w i l l c o n v e r t back immediately upon 313 nm i r r a d i a t i o n . Long-term p h o t o l y s i s a t low energy (370 nm) does n o t produce acetone, w h i l e e q u a t i o n (6) o c c u r s r e a d i l y w i t h broadband i r r a d i a t i o n . The exact n a t u r e o f t h e i n t e r m e d i a t e has n o t been d e t e r m i n e d ; however, i t s a b s o r p t i o n spectrum s t r o n g l y resembles t h a t o f a (metal-metal-bonded) dimer. One p o s s i b l e s t r u c t u r e would be t h a t d e r i v e d from t h e o x i d a t i v e a d d i t i o n o f i s o p r o p a n o l : ( C H ) C ( H ) - 0 - P t Pt-H. Another p o s s i b i l i t d i a t i o n o f e i t h e r o f thes and r e g e n e r a t e P t ( p o p ) ~ . In summary, t h e t r i p l e t (do*po) e x c i t e d s t a t e s o f t h e d 8 - 8 m e t a l dimers [ I r ( μ - ρ ζ ) ( C O D ) ] and P t ( p o p ) ~ undergo a v a r i e t y o f photo­ c h e m i c a l r e a c t i o n s . E l e c t r o n t r a n s f e r t o o n e - e l e c t r o n quenchers such as p y r i d i n i u m c a t i o n s o r h a l o c a r b o n s r e a d i l y o c c u r s w i t h a c c e p t o r s t h a t have r e d u c t i o n p o t e n t i a l s as n e g a t i v e as -2.0 V. With the l a t t e r r e a g e n t s , n e t t w o - e l e c t r o n , photoinduced e l e c t r o n t r a n s f e r y i e l d s d^-d^ o x i d a t i v e a d d i t i o n p r o d u c t s . A d d i t i o n a l l y , the t r i p l e t (do*po) e x c i t e d s t a t e o f P t ( p o p ) ~ a p p a r e n t l y i s a b l e t o r e a c t by e x t r a c t i n g a hydrogen atom from a C-H bond o f an o r g a n i c s u b s t r a t e . 4

2

4

4

2

4

4

2

4

4

2

4

d

4

2

2

4

4

2

4

Acknowledgment. J . L. M. thanks t h e Sun Co. f o r a g r a d u a t e f e l l o w ­ ship. T h i s r e s e a r c h was supported by N a t i o n a l S c i e n c e F o u n d a t i o n Grant CHE84-19828.

Literature Cited 1. Balzani, V.; Bolletta, F.; Gandolfi, M. T.; Maestri, M. Top. Curr. Chem. 1978, 75, 1-64. 2. Meyer, T. G. Prog. Inorg. Chem. 1938, 30, 389-441 and references therein. 3. Bock, C. R.; Connor, J. Α.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 4815-4824. 4. Bock, C. R.; Whitten, D. G.; Meyer, T. J. J. Am. Chem. Soc. 1975, 97, 2909-2911. 5. Lin, C. T.; Bottcher, W.; Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1976, 98, 6536-6544. 6. Toma, H. E.; Creutz, C. Inorg. Chem. 1977, 16, 545-550. 7. Bock, C. R.; Connor, J. Α.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. Chem. Phys. Lett. 1975, 61, 522-525. 8. Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F.; Balzani, V. J. Am. Chem. Soc. 1978, 100, 7219-7223. 9. Sutin, N. J. Photochem. 1979, 10, 19-40. 10. Sutin, N.; Creutz, C. Pure and Appl. Chem. 1980, 52, 2717-2738.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

176

E X C I T E D STATES A N D R E A C T I V E I N T E R M E D I A T E S

11. Nocera, D. G.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 74397350. 12. The Pt2(pop)44- anion can be made with a variety of counterions. Spectroscopic and aqueous photochemical studies were performed with the barium and potassium salts, respectively. The tetrabutylammonium salt is required for photochemistry in nonaqueous solvents. 13. Che, C.-M.; Butler, L. G.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 7796-7797. 14. Fordyce, W. Α.; Brummer, J. G.; Crosby, G. A. J. Am. Chem. Soc. 1981, 103, 7061-7064. 15. Rice, S. F.; Gray, Η. B. J. Am. Chem. Soc. 1983, 105, 4571-4575. 16. Parker, W. L.; Crosby, G. A. Chem. Phys. Lett. 1984, 105, 544546. 17. Marshall, J. L.; Stobart, S. R.; Gray, Η. B. J. Am. Chem. Soc. 1984, 106, 3027-3029 18. Bock, C. R.; Meyer 96, 4710-4712. 19. Heinrichs, Μ. Α.; Stiegman, A. E.; Gray, Η. B. unpublished results. 20. Miskowski, V.; Stiegman, A. E.; Gray, Η. B. unpublished results. 21. Winkler, J. R.; Marshall, J. L.; Netzel, T. L.; Gray, Η. B. J. Am. Chem. Soc. submitted. 22. Kochi, J. K. "Organometallic Mechanisms and Catalysis"; Academic Press: New York, 1978; Chapter 7. 23. Caspar, J. V.; Gray, Η. B. J. Am. Chem. Soc. 1984, 106, 30293030. 24. Fukuzumi, S.; Nishizawa, N.; Tanaka, T. Bull. Chem. Soc. Jpn. 1983, 56, 709-714. 25. Fukuzumi, S.; Hironaka, K.; Nishizawa, N.; Tanaka, T. Bull. Chem. Soc. Jpn. 1983, 56, 2220-2227. 26. Roundhill, D. M. J. Am. Chem. Soc. 1985, 107, 4354-4356. 27. Bard, A. J.; Lind, H. "Encyclopedia of Electrochemistry of the Elements"; Marcel Dekker: New York, 1973; Vol. XI, p. 181. 28. Hay, J. M. "Reactive Free Radicals"; Academic Press: New York, 1974; pp. 96-102. 29. Nonhebel, D. C.; Walton, J. C. "Free Radical Chemistry"; Cambridge University Press, 1974; p. 19. RECEIVED November 8, 1985

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

13 Photochemical Production of Reactive Organometallics for Synthesis and Catalysis William C. Trogler Department of Chemistry, D-006, University of California at San Diego, La Jolla, CA 92093 Photochemical reaction and of platinum an palladiu complexe chelating oxalate yield intermediates with two reac­ tive sites that are either ligand or metal centered. Unsaturated metallacycles exhibit low lying π* excited states that can also function as photoreceptors to pro­ mote ligand dissociation elsewhere in the molecule. A strong coupling model for excited state reactivity of metal carbonyls is presented. Reactions of photogenerated PtL2 and PdL2 fragments (L = trialkylphosphine) are summarized along with methods of preparing silica attached photocatalysts. S y n t h e t i c a l l y u s e f u l photochemical r e a c t i o n s o f o r g a n o t r a n s i t i o n m e t a l complexes c a n be c l a s s i f i e d a c c o r d i n g t o Scheme I . Scheme I 1)

Ligand

Photodissociation

Cr(C0)

2)

h

V

>»

6

Homolysis o f Metal Ligand

CoMe([l4]aneN )0H2

+

h

4

3)

( 1_) :

V

Cr(C0)

5

+

CO

Bond ( 2 ) :

>

Co([l4]aneN )

2 +

4

+ CH^ +

P h o t o c h e m i c a l H o m o l y s i s o f a M e t a l - M e t a l Bond ( 3 ) : Mn (C0) 2

2Mn(C0)

1 Q

5

or Pd (CNCH )^ 2

3

+

h

V

>

2 Pd(CNCH )^ 3

0097-6156/ 86/ 0307-0177506.00/ 0 © 1986 American Chemical Society

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

178 4)

EXCITED STATES AND Photooxidation

(4):

Fe(n-C H ) 5

5)

REACTIVE INTERMEDIATES

5

2

Photochemical Reductive

g

T

>

[Fe(n-C H ) ] Cl" 5

^

H

2

2

(5):

E l i m i n a t i o n of

hv

5

WW

+

è 18e These r e a c t i o n s g e n e r a t e a s i n g l e r e a c t i v e s i t e and o c c u r v i a 15e, 16e, 17e, o r 18e i n t e r m e d i a t e s ( 6 - 8 ) . One o f our g o a l s was t o ex­ amine t h e p h o t o c h e m i c a l b e h a v i o r o f complexes t h a t c o n t a i n an un­ s a t u r a t e d c h e l a t e chromophore. P h o t o f r a g m e n t a t i o n o f t h e s e systems might l e a d t o two r e a c t i v e c e n t e r s , e i t h e r on t h e l i g a n d o r m e t a l . T h i s c o u l d produce i n t e r m e d i a t e s t h a t e x h i b i t n o v e l c h e m i s t r y . Metallacyclopentadiene, Metalladiazabutadiene, azadiene Photochemistry

and M e t a l l a t e t r a -

C o n s i d e r t h e s e r i e s o f m e t a l l a c y c l e s A-C. These u n s a t u r a t e d r i n g systems were e x p e c t e d t o show low l y i n g e l e c t r o n i c t r a n s i t i o n s

\

/

c—c

1

\

/

Ν —

//

metallacyclo­ pentadiene

metalladiaza­ butadiene

Ν

W

metallatetraazabutadiene

because o f t h e u n s a t u r a t e d m e t a l - l i g a n d π system. The photochemis­ t r y o f CpCo[C4Ph4][PPh3] ( 9 ) , where Cp = n-C^H^ and Ph = C ^ in benzene s o l v e n t i s summarized i n E q u a t i o n 1. I n t h e absence o f 02 phosphine d i s s o c i a t i o n was shown t o y i e l d a 16e i n t e r m e d i a t e t h a t 9

CpCo[C Ph ](PPh ) 4

hv

4

3

CpCo(n-C Ph ) 4

4

CpCo[n -OC(Ph)C(Ph)C(Ph)C(Ph)0]

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

(1)

13.

179

Photochemical Production of Reactive Organometallics

TROGLER

r e a r r a n g e d t o t h e η - t e t r a p h e n y l ( c y c l o b u t a d i e n e ) complex. In the presence o f O 2 , s t e r e o s p e c i f i c o x i d a t i o n o f t h e t e t r a p h e n y l metall a c y c l e occurred t o y i e l d Z-dibenzoylstilbene. Single crystal X-ray d i f f r a c t i o n ( 9 ) showed t h a t t h i s l i g a n d bound t o CO i n an τΑ-eneone f a s h i o n . S e v e r a l t e t r a a z a b u t a d i e n e complexes t h a t c o n t a i n e d t h e CpCo fragment were s y n t h e s i z e d (10,11), E q u a t i o n 2. A l l t h e d e r i v a t i v e s CpCo(C0)

2

+

2N R

R = CH =Me, C H 3

6

y

>

CpCo[N(R)NNN(R)]

C ^ , 2,6-Me^H^

(2)

2,4-F^^

were i n t e n s e l y c o l o r e d and c a l c u l a t i o n s (SCF-Xa-DV) (_12) o f t h e model complex CpCo[N(H)NNN(H)], F i g u r e 1, showed t h e p r e s e n c e o f a l o w - l y i n g 30a m e t a l l a c y c l e π* o r b i t a l S t r o n g π back b o n d i n g to the tetraazabutadien l e n g t h s and a s i n g l e s h o r CpCo[N(C6F )NNN(C5F5)], F i g u r e 2. A f u r t h e r i n d i c a t i o n o f t h e s t r o n g π a c c e p t o r c h a r a c t e r o f t h e N 4 R 2 l i g a n d was t h e f o r m a t i o n (13) o f s t a b l e 19e a n i o n s on e l e c t r o c h e m i c a l o r c h e m i c a l (Na/Hg) r e ­ duction. That a d e l o c a l i z e d m e t a l l a c y c l e π o r b i t a l was t h e a c c e p t o r o r b i t a l was s u g g e s t e d by t h e l a r g e v a r i a t i o n i n r e d u c t i o n p o t e n t i a l s on c h a n g i n g t h e s u b s t i t u e n t a s w e l l a s t h e Co h y p e r f i n e s p l i t t i n g i n t h e EPR s p e c t r a ( T a b l e I , ^60% c o b a l t c h a r a c t e r ) . These r e s u l t s 1

5

Table

I.

R e d u c t i o n P o t e n t i a l s v s . NHE i n CH^CN and EPR S p e c t r a l Data i n THF S o l u t i o n f o r (η-C H )CO(1,4-R N,) Complexes. Q

R CH

E°',V

3

2,6-(CH ) C H 3

C

6

3

A

iso(Co),G

iso 2.055

-1.31

2.061

56.3

-1.01

2.078

50.0

-0.97

2.070

51.6

-O.7O

2.066

51.7

57.9

H

6 5 2,4-F C H 2

C

2

S

-1.53

6

3

F

6 5

c o n t r a s t e d w i t h t h o s e f o r complexes t h a t c o n t a i n r i n g System A where l i t t l e π back-bonding t o t h e l i g a n d i s observed (14). I r r a d i a t i o n o f t h e n e u t r a l R = Me d e r i v a t i v e l e d t o slow decom­ p o s i t i o n ; however, t h e a r y l d e r i v a t i v e s e x t r u d e d N 2 on p h o t o l y s i s (Φ = 10~3-10~ , χ = nm) t o form a s e r i e s o f benzoquinone d i i m i n e complexes (10^11_) i n y i e l d s o f 65-90$, E q u a t i o n s 3-5· Because t h i s r e a c t i o n had no p r e c e d e n t , and because C-F and C-C bond c l e a v a g e was unknown i n t h e o r g a n i c p h o t o c h e m i s t r y o f n i t r e n e s , t h e s t r u c t u r e o f the p e r f l u o r o p h e n y l p h o t o p r o d u c t was v e r i f i e d (V5) by c r y s t a l l o ­ graphy. M e t r i c a l p a r a m e t e r s o f t h e s t r u c t u r e a r e c o n s i s t e n t w i t h s t r o n g Co-N π b o n d i n g i n t h e s e p r o d u c t m e t a l l a c y c l e s . Benzoquinone d i i m i n e s do n o t appear t o be a s good π a c c e p t o r s a s t e t r a a z a b u t a d i e n e s j u d g i n g by t h e i r more n e g a t i v e (13) (by 0.3 t o 0.6 V) r e d u c ­ t i o n p o t e n t i a l s . The mechanism o f f r a g m e n t a t i o n - r e a r r a n g e m e n t t h a t 4

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

180

EXCITED STATES AND REACTIVE INTERMEDIATES

F i g u r e 1. O r b i t a l energy diagram from SCC-Xa-DV c a l c u l a t i o n s o f t h e CpCo and H-N=N-N=N-H fragments a s w e l l a s t h e CpCo(l,4-H N4) molecule. Reproduced from Ref. 12. C o p y r i g h t 1982, American Chemical S o c i e t y . 2

F i g u r e 2. ORTEP (50% e l l i p s o i d s ) o f CpCo[l ^ - ^ F ^ N / J w i t h s e l e c t e d bond d i s t a n c e s and a n g l e s . Reproduced from R e f . 12. C o p y r i g h t 1982, American C h e m i c a l S o c i e t y .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

TROGLER

Photochemical Production of Reactive Organometallics

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

182

EXCITED STATES AND REACTIVE INTERMEDIATES

we f a v o r i s shown i n E q u a t i o n 6.

The f i n a l s t e p o f t h e mechanism,

a t t a c k a t an o r t h o r i n g p o s i t i o n , may o c c u r by a r a d i c a l d i s p l a c e ­ ment mechanism ( 1 6 ) * f o r t h e 2,6-Me C5H d e r i v a t i v e t h e e l i m i n a t e d m e t h y l group l e d t o f o r m a t i o n o f methane ( 1 1 ) . 2

3

P h o t o d i s s o c i a t i o n o f CO From T r i c a r b o n y l i r o n D i a z a b u t a d i e n e s and Tetraazabutadienes The i s o l o b a l b e h a v i o r (17) o f t h e CpCo and F e ( C 0 ) fragments i s known. Both F e ( C 0 ) [ l , 4 - R N ] and F e ( C 0 ) [ N ( R ) C ( R ) C ( R ) N ( R ) ] com­ p l e x e s can be p r e p a r e d and b o t h a r e p h o t o a c t i v e . M o l e c u l a r o r b i t a l c a l c u l a t i o n s (_18) show t h a t s t r o n g back-bonding o c c u r s from t h e F e ( C O ) ^ fragment t o t h e t e t r a a z a b u t a d i e n e l i g a n d π system j u s t as f o r t h e CpCo d e r i v a t i v e . The s i m i l a r i t y between t h e average CO s t r e t c h i n g f r e q u e n c y i n F e ( C 0 ) [ l , 4 - M e N 4 ] and F e i C O ) ^ s u g g e s t s (18) t h a t t h e π a c c e p t o r a b i l i t y o f a t e t r a a z a b u t a d i e n e c h e l a t e compares w i t h t h a t o f two C 0 s . D i a z a b u t a d i e n e complexes, whose c a r b o n y l IR s t r e t c h i n g f r e q u e n c i e s l i e 35-40 cm"'' t o lower energy ( 1 9 ) , a r e weaker π a c c e p t o r s . T h e r e f o r e , t h e r e l a t i v e back-bonding a b i l i t y o f t h e m e t a l l a c y c l e s , C > Β > A, p a r a l l e l s t h e e l e c t r o n e g a t i v i t y o f t h e r i n g atoms. T e t r a a z a b u t a d i e n e (18) and d i a z a b u t a d i e n e (20) complexes c o n ­ t a i n i n g t h e F e ( C O ) ^ moiety e x h i b i t i n t e n s e v i s i b l e a b s o r p t i o n s a t t r i ­ buted t o t r a n s i t i o n s from Fe d o r b i t a l s t o a l o w - l y i n g m e t a l l a c y c l e π* o r b i t a l . A l t h o u g h t h e e x c i t e d s t a t e does n o t d i r e c t l y i n v o l v e Fe-CO bonding o r b i t a l s , e f f i c i e n t CO s u b s t i t u t i o n (21) o c c u r s i n t h e 3

!

3

2

4

!

3

3

2

!

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

13.

183

Photochemical Production of Reactive Organometallics

TROGLER

p r e s e n c e o f n e u t r a l l i g a n d s , E q u a t i o n s 7, 8, and 9· The a b i l i t y t o generate c o o r d i n a t e l y unsaturated i r o n c e n t e r s with v i s i b l e l i g h t i n

Fe(CO) [ l , 4 - M e N ] 2

+

4

L

- ^ - ^

Fe(C0) L[l,4-Me^]

L = NC H , P ( M e ) , PPl^, P i c - C g H ^ ) ^ 5

5

3

P(0Ph)

3

Fe(C0) [N(Ph)C(Me)C(Me)N(Ph)]

^

3

Fe(C0) (PPh,)[N(Ph)C(Me)C(Me)N(Ph] έ 3 o

P(0Me)

3

+

CO

+

2

3 >

CO

C ^

>

+

(8)

CO

Fe(C0)[P(0Me) ] [l,4-Me N ]

+

Fe[P(OMe )]

(7)

(9)

[l,4-Me N ]

3

2

4

t h e p r e s e n c e o f o t h e r UV p h o t o s e n s i t i v e complexes ( e . g . Fe(CO),_) p e r m i t s c o n d e n s a t i o n r e a c t i o n s (21) such as E q u a t i o n 10.

0

II r / \ ^Ν^Γ j ^ ^ χ ^ \

CH v

Fe(C0Ul,4-(CH ),NJ 3 3 2 4 +

Fe(C0)

i

i

b

e

. ^ ^ > irradiation

J

3

CH 3

(0C) Fe

Fe(C0) [P(CH ) ] 3

3

3

2

hv P(CH ) 3

(11)

3

Fe(C0) [P(CH ) ][N(t-C H )C(CH )C(CH )N(t-C H )] 2

3

3

4

9

3

3

4

9

S t e r i c crowding i n t h e t h e r m a l S t r a n s i t i o n s t a t e f o r E q u a t i o n 11 f a v o r s l o s s o f t h e b u l k y d i a z a b u t a d i e n e l i g a n d , w h i l e under p h o t o ­ chemical c o n d i t i o n s simple s u b s t i t u t i o n occurs. N 2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

184

EXCITED STATES AND

REACTIVE INTERMEDIATES

R e c e n t l y Kokkes, S t u f k e n s , and Oskam (23) have q u e s t i o n e d whe­ t h e r CO d i s s o c i a t i o n o c c u r s i n t h e m e t a l l a c y c l e systems we s t u d i e d . T h e i r e v i d e n c e a g a i n s t d i s s o c i a t i o n was t h a t s t e r i c a l l y h i n d e r e d de­ r i v a t i v e s t h e y examined, Fe(CO)-3[NRCHCHNR], where R = 2 , 6 - i - ( C 3 H ) 6 3 > £ - 6 1 1 > 4-Me(C H4), t-Bu = t-C^Hg, and C H [ C H ( C H 3 ) ] , d i d n o t photodecompose i n s o l u t i o n " ( i n t h e absence o f l i g a n d s ) . They f a i l e d t o n o t i c e our o b s e r v a t i o n (21) ( t h e f i r s t s e n t e n c e under t h e h e a d i n g Photochemical Reactions) t h a t " v i s i b l e l i g h t p h o t o l y s i s of F e ( C O ) y [1,4-Me N4] ( i n hexanes, c y c l o h e x a n e , benzene, C H C 1 , THF, o r CH3CN) r e s u l t s i n t h e l o s s o f CO t o y i e l d an u n s t a b l e s p e c i e s " . Even i f t h e i r statement were t r u e f o r F e ( C 0 ) [ l ^ - ( C H ^ ) ^ ] i t i s d o u b t f u l whether a p h o t o d e c o m p o s i t i o n c r i t e r i o n f o r p h o t o d i s s o c i a t i o n i s meaningful. F o r example, C r ( C O ) ^ does n o t decompose e f f i c i e n t l y when i r r a d i a t e d i n pure h y d r o c a r b o n s o l v e n t s ( i n t h e absence o f l i ­ gands) because o f r a p i d r e v e r s e b i n d i n g (24) o f d i s s o c i a t e d CO. I r o n p e n t a c a r b o n y l e x h i b i t s e f f i c i e n t p h o t o d e c o m p o s i t i o n ( i n t h e ab­ sence o f l i g a n d s ) becaus and p h o t o g e n e r a t e d F e ( C 0 ) unhindered Fe(C0) [l,4-Me N4] mimics t h e b e h a v i o r o f F e ( C O ) ^ i n f o r ­ ming a c l u s t e r on i r r a d i a t i o n i n t h e absence o f l i g a n d s . Furthermore s e l e c t i v e p h o t o d i s s o c i a t i o n o f CO from t h e t e t r a a z a b u t a d i e n e complex produces a c o o r d i n a t i v e l y u n s a t u r a t e d s p e c i e s t h a t r e a c t s ( l i k e p h o t o g e n e r a t e d F e ( C 0 ) 4 ) w i t h Fe(CO)^ t o form a dimer, E q u a t i o n 10. 7

C

H

c

H

2

6

2

2

2

2

3

2

3

T h e r e f o r e , we a t t r i b u t e t h e p h o t o s t a b i l i t y o f t h e complexes s t u d i e d by Kokkes e t a l . , t o t h e r e v e r s i b l e p r o c e s s o f E q u a t i o n 12.

Fe

(C0) (DAB) 3

DAB

^

Fe(C0) (DAB) 2

+

CO

(12)

= diazabutadiene chelate

The F e ( C 0 ) ( D A B ) s p e c i e s s h o u l d r e s i s t b i n u c l e a r d e c o m p o s i t i o n p a t h ­ ways because o f s t e r i c h i n d r a n c e from t h e b u l k y s u b s t i t u e n t s on t h e DAB l i g a n d . I n a d d i t i o n c o o r d i n a t e l y u n s a t u r a t e d s p e c i e s may be f u r t h e r s t a b i l i z e d by weak c o o r d i n a t i o n t o benzene s o l v e n t employed i n the photochemical s t u d i e s . The a l t e r n a t i v e mechanism t o CO d i s s o c i a t i o n , proposed by S t u f k e n s (23) f o r t h e DAB complexes, i s n o t c o n s i s t e n t w i t h t h e d i f f e r e n c e between t h e r m a l and p h o t o c h e m i c a l r e a c t i o n p r o d u c t s , E q u a t i o n 11. I n s o l u t i o n Kokkes e t a l . propose t h a t one end o f t h e DAB c h e l a t e d i s s o c i a t e s on p h o t o l y s i s . I f t h i s were t h e case i t would be d i f f i c u l t t o u n d e r s t a n d why t h e p h o t o c h e m i c a l r e a c t i o n (where t h e DAB l i g a n d i s h a l f a t t a c h e d ) l e a d s o n l y t o CO d i s p l a c e ­ ment, w h i l e t h e a s s o c i a t i v e t h e r m a l r e a c t i o n l e a d s o n l y t o DAB d i s ­ placement. C o n s i d e r t h e mechanism, E q u a t i o n 13, e s t a b l i s h e d (19) f o r t h e r m a l l o s s o f DAB. The key t o DAB l o s s i s f o r m a t i o n o f t h e monod e n t a t e s p e c i e s D o f E q u a t i o n 13· This intermediate i s i d e n t i c a l to t h a t proposed by Kokkes e t a l . (23) f o r p h o t o c h e m i c a l CO r e p l a c e m e n t . A c c o r d i n g t o t h e i r mechanism, E q u a t i o n 14, the same s p e c i e s D, forms i n a two s t e p p r o c e s s and would t h e r e f o r e be t h e r m a l l y e q u i l i b r a t e d . Thus t h e a l t e r n a t i v e mechanism i s n o t c o n s i s t e n t w i t h t h e r m a l chem­ i s t r y o f t h e s e systems. 2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Photochemical Production of Reactive Organometallics

13. TROGLER

-CH 0=C—Fe

hv

185

H r

0=C—Fe. sCH I R

R

(14)

L»

R I / ^CH N

^C

L'

I

0r=C~Fe 0

U

I R

0*»

J

„

C a r r y i n g t h e a n a l o g y between t h e p h o t o c h e m i s t r y o f F e ( C O ) ^ and Fe(C0)3[l,4-Me2N4] one s t e p f u r t h e r we n o t e t h a t b o t h compounds (25,26) behave a s p h o t o a s s i s t e d o l e f i n h y d r o s i l a t i o n and i s o m e r i z a ­ tion catalysts. One d i s t i n c t i o n between t h e two c a t a l y s t systems i s t h e l a t t e r (26) o p e r a t e s e f f e c t i v e l y w i t h l o n g wavelength r a d i a t i o n , Table I I . H y d r o s i l a t i o n a c t i v i t y r e q u i r e s continuous p h o t o l y s i s ;

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

186

EXCITED STATES AND

REACTIVE INTERMEDIATES

o l e f i n i s o m e r i z a t i o n a c t i v i t y remains d u r i n g dark r e a c t i o n s a f t e r c a t a l y s t g e n e r a t i o n . F u r t h e r study o f t h e s e c a t a l y t i c r e a c t i o n s i s needed. Table I I .

P h o t o c a t a l y t i c R e a c t i o n s o f Fe(CO) [1 ^ ( C H ^ ) ^ ] O l e f i n s and T r i a l k y l s i l a n e s

with

a

O l e f i n (M) Ethylene (0.12)

S i l a n e (M) HSiEt (0.42)

Fe, M 0.01

3

Irrad. i (min) Conv. 80 65

Products (%) S i E t 4 (98) E t S i C H = C H (2) 3

Ethylene (0.12)

HSiMe (0.36)

0.01

1-Pentene (2)

HSiMe

0.005

3

80

75

2

EtSiife

(90) J

3

90

b

>

95

(2)

Pentene i s o m e r s and Pentane

Pentenylsilanes (-15) cis-2-Pentene

HSiMe

0.02

3

58°

>

(0.8)

(8)

95

Pentene i s o m e r s (no h y d r o s i l a t i o n products)

a R e a c t i o n s a t 25°C i n benzene ( o r n e a t ) u s i n g a t o t a l s o l u t i o n v o l ­ ume o f 0.3 mL. The r e a c t i o n s were monitored by p r o t o n NMR and f o r t h e f i r s t t h r e e e n t r i e s , t h e p r o d u c t s were a n a l y z e d by GC-mass spectrometry. A 200W mercury-xenon a r c lamp was used f o r t h e i r ­ r a d i a t i o n s t o g e t h e r w i t h C o r n i n g 3-74 (λ > 400 nm, f i r s t two en­ t r i e s ) o r 0-52 (λ > 340 nm, l a s t e n t r y ) f i l t e r s . No t h e r m a l r e a c ­ t i o n s were o b s e r v e d p r i o r t o p h o t o l y s i s . b The l a s t 40$ o f t h e r e a c t i o n took p l a c e d u r i n g 10 h i n t h e C o n t i n u e d p h o t o l y s i s f o r 265 min gave no change i n t h e NMR

dark. spectrum.

c P a r t o f t h e r e a c t i o n took p l a c e , a f t e r i r r a d i a t i o n , d u r i n g 20 h i n the dark. d Mostly trans-2-pentene

and < 5%

1-pentene.

S t r o n g C o u p l i n g Model F o r O r g a n o m e t a l l i c

Photoreactions

We n o t e d (21_) t h a t t h e quantum y i e l d f o r p h o t o s u b s t i t u t i o n o f CO i n Fe(C0) [l,4-Me N4], F e ( C 0 ) ( P P h ) [ 1 , 4 - M e N 4 J , and F e ( C 0 ) [ P h N C ( M e ) C (Me)NPh] ( e . g . F i g u r e 3) i n c r e a s e d i n an e x p o n e n t i a l f a s h i o n w i t h i n c r e a s i n g e x c i t a t i o n energy. There was no c o r r e l a t i o n w i t h a b s o r p ­ t i o n s p e c t r a l f e a t u r e s . The h i g h quantum e f f i c i e n c y f o r CO s u b s t i t u ­ ât l o n g wavelengths was unexpected because Χα c a l c u l a t i o n s f o r i r o n t r i c a r b o n y l t e t r a a z a b u t a d i e n e complexes (18) and M0 c a l c u l a t i o n s f o r d i a z a b u t a d i e n e analogues (20) s u g g e s t t h a t t h e l o w e s t e x c i t e d s t a t e s do n o t a l t e r metal-C0 b o n d i n g . Resonance Raman s p e c t r a o f t h e d i a z a ­ b u t a d i e n e complexes (27) s u p p o r t t h i s c o n c l u s i o n . To r a t i o n a l i z e t h e o b s e r v a t i o n s we s u g g e s t e d (21) a s t r o n g c o u p l i n g model f o r ex­ cited state r e a c t i v i t y . 3

2

2

3

2

3

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

13.

Photochemical Production of Reactive Organometallics

TROGLER

187

Most d i s c u s s i o n s (28-32) o f i n o r g a n i c p h o t o c h e m i c a l r e a c t i o n s have f o c u s e d on t h e s p e c i f i c n a t u r e o f e x c i t e d s t a t e s and c o r r e l a ­ tions with photoreactivity. T h i s a s s u m e s t h e weak c o u p l i n g m o d e l (33) f o r e x c i t e d s t a t e r e a c t i v i t y . T h i s a p p r o a c h w i l l be s u c c e s s f u l when t h e e x c i t e d s t a t e t h a t p r e c e d e s t h e p h o t o r e a c t i o n h a s a l o n g l i f e t i m e o r i s l o c a l i z e d ( e . g . , M L C T , a + o* ...). F r e q u e n t l y one f i n d s f l a t Φ vs λ p r o f i l e s and r e a c t i v i t y from the l o w e s t e x c i t e d s t a t e i n t h e weak c o u p l i n g l i m i t . There i s another l i m i t t h a t s h o u l d be c o n s i d e r e d . I f the e x c i t e d state i s short l i v e d , not w e l l l o c a l ­ i z e d , and i f p h o t o c h e m i s t r y competes w i t h v i b r a t i o n a l d e a c t i v a t i o n of an e x c i t e d s t a t e then a s t r o n g c o u p l i n g (33) ( i . e . , s t r o n g c o u p ­ l i n g between t h e i n i t i a l l y p r e p a r e d v i b r o n i c s t a t e and t h e d i s s o c i a ­ t i o n c o n t i n u u m ) m o d e l may b e m o r e a p p r o p r i a t e . We i n t r o d u c e d t h e p r e m i s e t h a t a c o n s t a n t f r a c t i o n o f t h e e x c i ­ t a t i o n e n e r g y i s a v a i l a b l e f o r M-CO d i s s o c i a t i v e p r o c e s s e s . A quasis t a t i s t i c a l ( 3 4 ) p a r t i t i o n i n g o f e x c i t a t i o n e n e r g y w o u l d be f a v o r e d by 1 ) d e n s e m a n i f o l d s o t i o n r u l e s on n o n r a d i a t i v e l e c t r o n i c s t a t e s t h a t do n o t c o u p l e s t r o n g l y w i t h a n y s i n g l e v i b r a ­ t i o n m o d e ; we q u a l i f y t h e l a s t c o n d i t i o n b y n o t i n g t h a t e v e n l o c a l ­ i z e d e x c i t a t i o n s can l e a d (35) t o " s t a t i s t i c a l " b e h a v i o r . The w o r d " s t a t i s t i c a l " i s not used i n a s t r i c t thermal sense, because p a r t i ­ t i o n i n g o f e x c i t a t i o n energy depends on t h e s p e c i f i c s o f i n t r a m o l e ­ c u l a r v i b r o n i c coupling of the i n i t i a l l y prepared s t a t e . Experimental manifestations of strong coupling that areexpected i n c l u d e the f o l l o w i n g : 1) quantum y i e l d s f o r p h o t o d i s s o c i a t i v e p a t h ­ ways t h a t depend on t h e amount b y w h i c h t h e e x c i t a t i o n e n e r g y e x c e e d s the thermodynamic t h r e s h o l d f o r bond b r e a k i n g ; 2) m u l t i p l e r e a c t i o n p a t h w a y s t h a t become a v a i l a b l e a t h i g h e r e x c i t a t i o n e n e r g i e s ; 3) s t r u c t u r e s e n s i t i v i t y t o r e a c t i o n quantum y i e l d s b e c a u s e e n e r g y f l o w r e l i e s o n t h e v i b r a t i o n a l modes t h a t i n i t i a l l y r e c e i v e t h e e n e r g y a n d how t h e y c o u p l e t o o t h e r m o d e s ; 4 ) b o n d i n g c h a r a c t e r o f t h e e x c i t e d s t a t e becomes i r r e l e v a n t . P h o t o r e a c t i o n s o f t h e m e t a l l a c y c l e s d i s c u s s e d ( 2 1 ) show l i n e a r p l o t s o f 1 η Φ ς v s e x c i t a t i o n e n e r g y b e f o r e l i m i t i n g quantum y i e l d s are reached. T h e r e was a c o r r e l a t i o n b e t w e e n t h e d o n o r a t o m s e t about Fe and quantum y i e l d s . Thus F e ( C 0 ) ( P P h 3 ) [ 1 , 4 - M e N 4 ] and F e ( C 0 ) 3 [ l , 4 - M e N 4 ] have s i m i l a r a b s o r p t i o n s p e c t r a , but q u i t e d i f ­ f e r e n t q u a n t u m y i e l d s f o r CO s u b s t i t u t i o n . Absorption spectra of F e ( C 0 ) [ l , 4 - M e N 4 ] and Fe(C0)3[PhNC(Me)C(Me)NPh] are d i f f e r e n t ; how­ e v e r , b o t h c o m p o u n d s p o s s e s s t h e same d o n o r a t o m s e t a n d e x h i b i t s i m i l a r q u a n t u m y i e l d s f o r CO l o s s . I t i s a l s o noteworthy t h a t i s o e l e c t r o n i c C p C o [ l , 4 - R N 4 ] c o m p l e x e s t h a t do n o t c o n t a i n a n e a s i l y d i s s o c i a b l e group photofragment by N l o s s ( 1 0 , 1 1 ) . T h e r e was a c o r ­ r e l a t i o n b e t w e e n t h e mode o f p h o t o c h e m i c a l d e c o m p o s i t i o n o f t h e F e ( C 0 ) 3 a n d CpCo t e t r a a z a b u t a d i e n e c o m p l e x e s a n d t h e l o w e s t e n e r g y f r a g m e n t i n t h e i r e l e c t r o n i m p a c t mass s p e c t r a (11 ) . For these rea­ s o n s we f a v o r a s t r o n g c o u p l i n g d e s c r i p t i o n o f t h e p h o t o r e a c t i o n s o f t h e s e compounds where l i g h t e x c i t a t i o n i s r a p i d l y c o n v e r t e d i n t o v i ­ b r a t i o n a l energy t h a t t h e n r e s u l t s i n bond b r e a k i n g as governed by e n e r g e t i c and s t a t i s t i c a l c o n s i d e r a t i o n s . I t s h o u l d be n o t e d t h a t t h e r e are o t h e r e x p l a n a t i o n s (36) f o r wavelength dependencies o f quantum y i e l d s . f

0

2

2

2

3

2

2

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND

188

REACTIVE INTERMEDIATES

Recent s t u d i e s (37-39) o f t h e gas phase p h o t o c h e m i s t r y o f FeiCO)^ * C r ( C 0 ) 5 by time r e s o l v e d IR methods shows t h a t e j e c t i o n of two CO s p e r i n c i d e n t photon o c c u r s as t h e e x c i t a t i o n energy i n ­ c r e a s e s . That i s c o n s i s t e n t w i t h t h e s t r o n g c o u p l i n g model p r o p o s e d . I t a l s o appears (40) t h a t few o f t h e e j e c t e d CO groups a r e v i b r a tionally excited. We s p e c u l a t e t h a t t h e energy gap between h i g h f r e q u e n c y CO v i b r a t i o n s and low f r e q u e n c y M-C o r M-M v i b r a t i o n s in simple metal c a r b o n y l s (e.g., Cr(CO)^ or M ^ Î C O ) ^ ) t r a p s e x c i t e d s t a t e energy i n t h e M-C and M-M v i b r a t i o n a l modes. T h i s c o u l d ex­ p l a i n t h e h i g h quantum e f f i c i e n c i e s f o r CO d i s s o c i a t i o n o r M-M bond h o m o l y s i s i n such compounds. T h i s may a l s o be why quantum e f f i c i e n ­ c i e s f o r CO d i s s o c i a t i o n i n s u b s t i t u t e d c a r b o n y l s (41) d e c r e a s e markedly. I n t r o d u c t i o n o f l i g a n d s w i t h low f r e q u e n c y v i b r a t i o n a l modes p r o v i d e s a s i n k f o r v i b r a t i o n a l e x c i t a t i o n energy and perhaps a b e t t e r p a t h f o r energy m i g r a t i o n t o t h e s u r r o u n d i n g s i n condensed phases. a n c

1

Photochemistry of Oxalat P a l l a d i u m , and P l a t i n u m P h o t o o x i d a t i o n o f c o o r d i n a t e d o x a l a t e has been known s i n c e t h e e a r ­ l i e s t s t u d i e s of t r a n s i t i o n metal photochemistry (42). In these r e a c t i o n s o x a l a t e l i g a n d i s p h o t o o x i d i z e d t o C O 2 , and up t o two m e t a l c e n t e r s a r e r e d u c e d by one e l e c t r o n ( e . g . f e r r i o x a l a t e ) . We wondered whether t h e o x a l a t e l i g a n d c o u l d be a t w o - e l e c t r o n p h o t o r e d u c t a n t , by s i m u l t a n e o u s o r r a p i d s e q u e n t i a l e l e c t r o n t r a n s f e r , w i t h m e t a l s prone t o 2e redox p r o c e s s e s . A p p l i c a t i o n o f t h i s c o n c e p t t o 16e square p l a n a r d® complexes, E q u a t i o n 15, was a t t r a c t i v e because i t s h o u l d produce s o l v a t e d 14e m e t a l complexes t h a t a r e i n o r g a n i c analogues o f

M(C 0 )L £

16e

4

-*^->-

2

2C0

2

+

ML

M = N i , Pd, P t

(15)

2

14e

carbenes. The r e p o r t s (43,44) t h a t p l a t i n u m ( O ) complexes c o u l d be i s o l a t e d by i r r a d i a t i n g P t X c ^ Z f ) ( P P h - z ) 2 and t h a t r h o d i u m ( l ) s p e c i e s were o b t a i n e d by i r r a d i a t i n g R h ( C 2 0 ) C l ( p y ) s u g g e s t e d t h a t t h i s p r o ­ c e s s might work. Because 14e P t L fragments can be made t h e r m a l l y (45,46) when L i s a b u l k y phosphine [ e . g . , PCy^ o r P ( t - B u ) ] , we examined (47,48) t h e p h o t o c h e m i s t r y o f s t e r i c a l l y u n h i n d e r e d com­ plexes. The p h o t o r e a c t i v i t y o f PtiC^O^)(PEt )2, E t = C 2 H 5 , i s sum­ m a r i z e d i n Scheme I I . P h o t o c h e m i c a l c o n v e r s i o n s a r e h i g h and few s i d e p r o d u c t s ( e . g . , F i g u r e 4) form. A l l the r e a c t i o n s suggest f o r ­ mation o f a r e a c t i v e P t ( P E t ) 2 fragment t h a t can be t r a p p e d as a p l a t i n u m ( O ) s p e c i e s o r combined w i t h o x i d a t i v e a d d i t i o n s u b s t r a t e s t o y i e l d p l a t i n u m ( l l ) compounds. 4

3

2

3

3

3

T h i s c h e m i s t r y has been extended t o produce p a l l a d i u m ( O ) i n t e r ­ mediates ( 4 8 ) . Much o f t h e c h e m i s t r y i s s i m i l a r t o t h a t o f t h e P t analogues e x c e p t t h a t t h e p a l l a d i u m ( O ) complexes a r e more u n s t a b l e and d i f f i c u l t t o i s o l a t e . A reaction c h a r a c t e r i s t i c of palladium i s t h e a d d i t i o n o f a l l y l compounds t o form c a t i o n i c a l l y l complexes, E q u a t i o n 16. T h i s has been p o s t u l a t e d (49) as a key s t e p i n t h e mechanism f o r P d ( d i p h o s ) p c a t a l y z e d r e a c t i o n s o f a l l y l compounds.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

13.

TROGLER

Photochemical Production of Reactive Organometallics

F i g u r e 3· E l e c t r o n i c and quantum y i e l d s f o r p h o t o s u b s t i t u t i o n o f CO by PPh3. Repro­ duced from Ref. 21. C o p y r i g h t 1981, American C h e m i c a l S o c i e t y .

Pt < P E t > ( C H > 3

6 19.6 ppM,

'j

2

2

_p

p t

4

* 3486 Hz

I

Pt(PEt ) 6 4 . 3 ppM,

3

2

P t

_p

, J

2

4

*

3

5

0

3

H

z

F i g u r e 4. S u c c e s s i v e P{ H}NMR s p e c t r a (109 MHz) showing t h e p h o t o c h e m i c a l c o n v e r s i o n under 1 atm e t h y l e n e o f P t ( P E t ^ ) 2 ( 0 2 0 4 ) t o P t ( P E t ^ ) 2 ( C H 4 ) . I r r a d i a t i o n t i m e s a r e shown a t r i g h t . The s y m m e t r i c a l l y d i s p o s e d s a t e l l i t e peaks r e s u l t from t h o s e mole­ c u l e s t h a t c o n t a i n ^95pt (33·8$ abundance, I = § ) . S i g n a l s marked by an a s t e r i s k i n t h e f i n a l spectrum a r e u n i d e n t i f i e d s i d e p r o d u c t s , which form a t l o n g i r r a d i a t i o n t i m e s . Reproduced from Ref. 48. C o p y r i g h t 1985, American C h e m i c a l S o c i e t y . 2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

189

EXCITED STATES AND REACTIVE INTERMEDIATES

190

Scheme I I Reactions o f P t ( C 0 ^ ) ( P E t ) 2

3

2

On U l t r a v i o l e t I r r a d i a t i o n

t r a n s - P t ( C H 0)H(PEt 3

2Pt(PEt ) 3

P

t

3

(

P

E

) 3

1

* *·>

trans-Pt(R)X(PEt ) 5 £

•» CH 0H

n

3

r

PtCl (PEt ) 2

3

2

V 4

2

RX = C>-H_C1, 6 5 CH C l , C H I 3

hv >

Pt

[Pt(H 0)H(PEt ) ]0H^ 2

3

cis-Pt(R,Si)H(PËt )

RjSiH

olefin =

2

C H 2

Pt(C0) (PEt ) 2

3

4

2

and t r a n s - P t H ( P E t ) 2

C0

3

2

-*-|— Pt(olefin)(PEt ) ?

o

PEt^ Η—PtL

-Pt H

[0 CH] 2

PEt^

X = OAc, OPh, OH OEt, C l L = diphos or [ P ( n - B u ) ] 2

3

2

Presumably P d ( d i p h o s ) i s g e n e r a t e d i n t h e c a t a l y t i c c y c l e by decom­ p o s i t i o n o r l i g a n d d i s s o c i a t i o n from t h e b i s ( d i p h o s ) complex. The r e a c t i v i t y o f p h o t o g e n e r a t e d P d L d i f f e r s from P t L s i n c e t h e l a t t e r s p e c i e s does n o t add a l l y l s u b s t r a t e s c l e a n l y . P h o t o c h e m i c a l r o u t e s 2

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2

13.

Photochemical Production of Reactive Organometallics

TROGLER

191

t o PtL2 and P d L s p e c i e s , t h a t c o n t a i n l e s s s t e r i c a l l y h i n d e r e d p h o s p h i n e s , complements t h e r m a l c h e m i s t r y known (45,46,50) f o r b u l k y ML2 s p e c i e s (M = Pd and P t , L = PCV3 o r P ( t - B u ) 3 ) . H i g h r e a c t i v i t y o f the p h o t o g e n e r a t e d s p e c i e s i s i l l u s t r a t e d by the f o l l o w i n g com­ p a r i s o n (49,51,52). 2

Ph-Cl

+

Pt(C 0 )(PEt ) 2

4

3

Ph-Cl

+

Pt(PEt )

Ph-Cl

+

Pt(PCy )

3

>

2

3

1

1

Q

0

C

trans-PtClPh(PEt ) 3

>

trans-PtClPh(PEt ) 3

14 days 20° C ^

3

2

2

trans-PtClPh(PCy )

Our attempts t o p r e p a r e Ni(0204)!^ complexes i n v a r i a b l y l e d t o f o r m a t i o n o f i n s o l u b l e Ni(C204) a b i l i t y o f the weak f i e l Ni(il). We t h o u g h t t h a t a s t r o n g f i e l d o r s o f t v e r s i o n o f the oxa­ l a t e l i g a n d might be u s e f u l . I t seemed t h a t the d i t h i o o x a l a t e (S2C20 ~) l i g a n d would e x h i b i t p h o t o c h e m i s t r y analogous t o c h e l a t i n g oxalate. T h e r e f o r e the s e r i e s o f d i t h i o o x a l a t e complexes M(S2C2~ 02)L.2 have been p r e p a r e d (53) where L = PMe3 o r , L2 = d i p h o s = Ph2~ PCH -CH PPh2 and depe = Et PCH -CH PEt2»and M = N i , Pd, and P t . The IR s t r e t c h o f the C=0 group (1680-1750 cm-1 ) p r o v e s s u l f u r c o o r d i n a ­ t i o n f o r the S2C2O2" l i g a n d . I r r a d i a t i o n o f the d i p h o s d e r i v a t i v e s i n CH2CI2 produced f r e e SCO and MCI2(diphos). T h e r e f o r e i t appears t h a t t h e d i t h i o o x a l a t e l i g a n d can a l s o be r e d u c t i v e l y e l i m i n a t e d by photolysis. The c h e m i s t r y o f t h e s e systems i s c o m p l i c a t e d by s e c o n ­ dary r e a c t i o n s w i t h SCO and i s under i n v e s t i g a t i o n . 2

2

2

2

2

S y n t h e s i s and P h o t o r e a c t i v i t y

2

o f Surface-Bound P l a t i n u m O x a l a t e s

The p h o t o c h e m i c a l l y produced Pt(PEt3)2 fragment, s t a b i l i z e d a s t h e c i s - and t r a n s - P t H p ( P E t ^ ) ? complexes, has proved (54,55,56) t o be an e f f i c i e n t and l o n g l i v e d homogeneous c a t a l y s t f o r H2/D2 exchange ( E q u a t i o n 17), d e u t e r a t i o n o f acetone o r a c e t o n t r i l e ( E q u a t i o n s 18 and 19), d e c o m p o s i t i o n o f f o r m i c a c i d ( E q u a t i o n 20), and h y d r o l y s i s o f a c e t o n i t r i l e ( E q u a t i o n 21). Because o f the c a t a l y t i c promise Η,

3D.

+ 2

+

>

2 HD

>

3H

CH CN 3

=>•

li

HC00H -

>

H

H0

>

0 II •NH H_C-C3

D

2

-

CH C(0)CH 3 3

1| D 2

Η C-C=N

+

+

2

-

2

(17)

+

2

H

2

+

CD C(0)CD 3 3

+

CD CN 3

co

(18)

(19)

2

(20)

2

(21)

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

192

E X C I T E D STATES A N D R E A C T I V E I N T E R M E D I A T E S

o f t h e s e systems we d e c i d e d t o s y n t h e s i z e s u r f a c e a t t a c h e d o x a l a t e derivatives· F o r c e r t a i n c a t a l y s t a p p l i c a t i o n s ( e . g . , ease o f s e p a r a t i o n ) i t i s d e s i r a b l e t o have h e t e r o g e n e o u s r a t h e r t h a n homogeneous c a t a l y s t s . From a f u n d a m e n t a l view t h e r e i s i n t e r e s t i n comparing t h e s u r f a c e e f f e c t on c a t a l y t i c r a t e s and mechanisms o f s u r f a c e - a t t a c h e d homo­ geneous c a t a l y s t s . Of t h e two commonly used s u p p o r t s ( 5 7 ) , o r g a n i c polymers o r s i l i c a , we chose s i l i c a o f h i g h pore d i a m e t e r (140A) be­ cause o f i t s r i g i d i t y and p e r m e a b i l i t y i n p o l a r media. Most p r e v i o u s (57) s t u d i e s o f phosphine s u p p o r t e d t r a n s i t i o n m e t a l complexes have employed a r y l p h o s p h i n e l i g a n d s . T h i s p r e s e n t s problems s i n c e a r y l p h o s p h i n e s o f t e n d i s s o c i a t e and c a t a l y s t l e a c h i n g poses a p r o b l e m . S m a l l t r i a l k y l p h o s p h i n e s , by c o n t r a s t , a r e among t h e most d i f f i c u l t l i g a n d s t o d i s p l a c e from a m e t a l c e n t e r . Aryl­ p h o s p h i n e s a r e b u l k y and h i n d e r s u b s t r a t e a c c e s s t o t h e m e t a l c e n t e r . C l e a v a g e o f t h e P-C bond ( i . e . d e g r a d a t i o n ) as w e l l as o r t h o m e t a l l a t i o n o c c u r s more r e a d i l s y n t h e t i c p r o c e d u r e s wer o x a l a t e complexes. The b e s t p r o c e d u r e (59) i s o u t l i n e d i n Scheme I I I . I n t h e s y n t h e s i s o f Scheme I I I we used D a v i s o n S i l i c a (Grade 62, 1402 p o r e d i a m e t e r . 340 m^/g) and a c h i e v e d a maximum s u r f a c e c o v e r a g e o f 1 molecule/113% which amounts t o 70% f u n c t i o n a l i z a t i o n o f t h e s u r f a c e ( w i t h t h e assumption t h a t 6 s u r f a c e h y d r o x y l s a n c h o r one platinum complex). Key p o i n t s o f t h e s y n t h e s i s i n c l u d e : 1) t h e v o l ­ a t i l i t y o f r e a c t a n t s i n s t e p 1 and t h e h i g h y i e l d (97%) of the photo­ c h e m i c a l a d d i t i o n make i t p o s s i b l e t o p r e p a r e t h e L - P E t l i g a n d i n g r e a t e r t h a n 99% p u r i t y ; 2) t h e v o l a t i l e SMe l i g a n d can be removed i n s t e p 2 and t h e d e r i v a t i z e d p l a t i n u m complex, which i s an o i l , can be i s o l a t e d i n h i g h p u r i t y ; 3) c a p p i n g r e m a i n i n g s u r f a c e h y d r o x y l groups w i t h h e x a m e t h y l d i s i l a z a n e i n s t e p 4 p r e v e n t s r e a c t i o n s o f p h o t o g e n e r a t e d P t ( 0 ) w i t h t h e s u p p o r t ; 4) p u t t i n g t h e complex on t h e s u p p o r t as a s t a b l e P t ( l l ) s p e c i e s p r o t e c t s t h e b a s i c p h o s p h i n e l i ­ gand from o x i d a t i o n ( 6 0 ) . B e s i d e s t h e a n a l y t i c a l d a t a ( P t / P r a t i o = 1/2) t h a t c h a r a c t e r i z e t h e s u p p o r t e d complex t h e IR spectrum e x h i b i t s s t r e t c h e s t h a t a r e i d e n t i c a l t o t h o s e i n t h e homogeneous analogue (49) P t ( C 0 4 ) ( P E t 3 ) . I f t h e sample i s i r r a d i a t e d (as a n u j o l m u l l ) t h e o x a l a t e s t r e t c h e s d i s a p p e a r and a new peak a p p e a r s a t 2330 c m , a t t r i b u t e d to CO2· Thus, E q u a t i o n 22 o c c u r s on t h e s u r f a c e . R e c a l l (Scheme I I ) t h a t 9

2

2

2

2

-1

Et? "CHhv 2C0 {

< y

CH.

-Ρ EΡ t

A

2

(22)

. υ'

p h o t o g e n e r a t e d P t ( P E t 3 ) c o u l d be t r a p p e d w i t h CO t o form P t ( P E t 3 ) (C0) . S i n c e t h e c a r b o n y l s t r e t c h e s (1930 and 1973 cm~1) a r e c h a r a c ­ t e r i s t i c (47) o f t h i s complex we i r r a d i a t e d t h e s u r f a c e s u p p o r t e d complex under CO. O b s e r v a t i o n o f t h e peaks a t 1929 and 1965 cm~1 2

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2

193

Photochemical Production of Reactive Organometallics

13. TROGLER

Scheme I I I MeO^ (1)

MeO—Si—CH==CH

+ HPEt^

2

MeO „ \ MeO-Si-(CH ) -PEt =

hv .

Λ

2

MeO^

(2)

2

2

L-PEt

MeO^

2L—PEt

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2

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+ +

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4

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(3) 4 Silica —0' —0'

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3 (4) HN(SiMe ) 3

2

caps o f f f r e e s u r f a c e OH w i t h i n e r t SlMe., group, and wash w e l l .

suggests t h a t t h e surface generated species o f Equation t r a p p e d , E q u a t i o n 23·

22 c a n be

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

2

2

194

EXCITED STATES AND

>

Et

Et.

-CFT

1

Pt

-CH„

REACTIVE INTERMEDIATES

9

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2

In c o n t r a s t t o homogeneous analogues (48,61) t h e s i l i c a bound P t L fragment does n o t c a t a l y z e a c e t o n i t r i l e h y d r o l y s i s . Initial experiments showed h y d r o l y s i s a c t i v i t y < 1/1000 t h a t o f t h e homo­ geneous system. T h i s p u z z l e d us u n t i l we found t h a t homogeneous c a t a l y s t systems where t h e phosphine l i g a n d s a r e c o n s t r a i n e d t o be c i s [ e . g . , P t ( C 0 4 ) ( d i p h o s ) ] show s i m i l a r low a c t i v i t y . Molecular modeling s t u d i e s (CPK models) o f the s u r f a c e a t t a c h e d r e a g e n t o f Scheme I I I s u g g e s t t h a t f i g u r a t i o n necessary f o P r e v i o u s work (48) w i t h homogeneous analogues showed t h a t S i - H oxidative additions y i e l d c i s products. A c i s geometry o f h y d r i d e and s i l y l may be a l l o w e d i n c a t a l y t i c h y d r o s i l a t i o n . Because t h e i n d u s t r a l homogeneous h y d r o s i l a t i o n c a t a l y s t (62) i s H P t C l £ we t e s t e d t h e a c t i v i t y o f our s u r f a c e g e n e r a t e d r e a g e n t f o r t h e r e a c t i o n of E q u a t i o n 24. A s u s p e n s i o n o f the c a t a l y s t was i r r a d i a t e d i n 12

2

2

heptene and a v i o l e n t r e a c t i o n ensured (400 t u r n o v e r s / P t ) on a d d i t i o n of d i c h l o r o m e t h y l s i l a n e . The h y d r o s i l a t i o n p r o d u c t formed i n over 97$ y i e l d and was pure by gc and % i NMR a f t e r f i l t r a t i o n from t h e catalyst. On a p e r p l a t i n u m b a s i s the c a t a l y s t has c a 1/100 t h e a c t i v i t y of H P t C l ^ . P r e s e n t work f o c u s e s on c a t a l y t i c mechanisms o f photo and t h e r m a l g e n e r a t e d c a t a l y s t s . 2

2

Conclusions P h o t o c h e m i c a l r e a c t i o n s o f t r a n s i t i o n metal complexes t h a t c o n t a i n u n s a t u r a t e d c h e l a t e s f a l l i n t o t h r e e c a t e g o r i e s : 1) f r a g m e n t a t i o n o f t h e l i g a n d t o y i e l d two r e a c t i v e f u n c t i o n a l i t i e s ; 2) e l i m i n a t i o n o f t h e l i g a n d t o g e n e r a t e two r e a c t i v e s i t e s a t t h e m e t a l ; 3) c h e l a t e l o c a l i z e d e x c i t e d s t a t e s can f u n c t i o n as p h o t o r e c e p t o r s t o promote p h o t o d i s s o c i a t i o n o f o t h e r m e t a l - l i g a n d bonds i n t h e complex. These p r o c e s s e s can be used as an e n t r y t o new r e a c t i v e i n t e r m e d i a t e s and catalysts. Acknowledgments I thank the s t u d e n t s (C.E. Johnson, M.E. G r o s s , R.S. Paonessa, A.L. P r i g n a n o , D. P o u r r e a u , R.L. Cowan), and p o s t d o c t o r a l s (C.E. J e n s e n , M.J. Maroney) who c o n t r i b u t e d t o t h e r e s e a r c h program d e s c r i b e d . F i n a n c i a l s u p p o r t o f our r e s e a r c h by the A i r F o r c e O f f i c e o f

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

13. TROGLER

Photochemical Production of Reactive Organometallics

S c i e n t i f i c R e s e a r c h , Army R e s e a r c h Foundation i s a p p r e c i a t e d .

195

O f f i c e , and N a t i o n a l S c i e n c e

Literature Cited 1. 2. 3.

Strohmeier, W. Angew. Chem. 1964, 76, 873. Mok, C.Y.; Endicott, J.F. J. Am. Chem. Soc. 1977, 99, 1276. Wrighton, M.S.; Ginley, D.S. J. Am. Chem. Soc. 1975, 97, 2065; Reinking, M.K.; Kullberg, M.L.; Cutler, A.R.; Kubiak, C.P. J. Am. Chem. Soc. 1985, 107, 3517. 4. Brand, J.C.; Snedder, W. Trans. Faraday Soc. 1957, 53, 894. 5. Green, M.L.H. Pure Appl. Chem. 1978, 50, 27. 6. Geoffroy, G.L. Prog. Inorg. Chem. 1980, 27, 123. 7. Bock, C.R.; von Gustorf, E.A.K. Adv. Photochem. 1977, 10, 222. 8. Geoffroy, G.L.; Wrighton, M.S. "Organometallic Photochemistry"; Academic Press: New York 1979 9. Trogler, W.C.; Ibers 10. Gross, M.E.; Trogler Organomet 407. 11. Gross, M.E.; Johnson, C.E.; Maroney, M.J.; Trogler, W.C. Inorg. Chem. 1984, 23, 2968. 12. Gross, M.E.; Trogler, W.C.; Ibers, J.A. J. Am. Chem. Soc. 1981, 103, 192; Gross, M.E.; Trogler, W.C.; Ibers, J.A. Organometal­ lics 1982, 1, 732. 13. Maroney, M.J.; Trogler, W.C. J. Am. Chem. Soc. 1984, 106, 4144. 14. Thorn, D.L.; Hoffmann, R. Nouv. J. Chim. 1979, 3, 39. 15. Gross, M.E.; Ibers, J.A.; Trogler, W.C. Organometallics 1982, 1, 530. 16. March, J. "Advanced Organic Chemistry", 2nd ed.; McGraw-Hill: New York, 1977. 17. Elian, M.; Chen, M.M.L.; Mingos, D.M.P.; Hoffmann, R. Inorg. Chem. 1976, 5, 1148. 18. Trogler, W.C.; Johnson, C.E.; Ellis, D.E. Inorg. Chem. 1981, 20, 980. 19. Shi, Q.-Z.; Richmond, T.G.; Trogler, W.C.; Basolo, F. Organo­ metallics 1982, 1, 1033. 20. Kokkes, M.W.; Stufkens, D.J.; Oskam, A. J. Chem. Soc., Dalton Trans. 1983, 439. 21. Johnson, C.E.; Trogler, W.C. J. Am. Chem. Soc. 1981, 103, 6352. 22. Chang, C.-Y.; Johnson, C.E.; Richmond, T.G.; Chen, Y.-T.; Trogler, W.C.; Basolo, F. Inorg. Chem. 1981, 20, 3167. 23. Kokkes, M.W.; Stufkens, D.J.; Oskam, A. J. Chem.Soc.,Dalton Trans. 1984, 1005. 24. Church, S.P.; Grevels, F.-W.; Hermann, H.; Schaffner, K. Inorg. Chem. 1985, 24, 418-422. 25. Wrighton, M.S.; Graff, J.L.; Reichel, C.L.; Sanner, R.D. Ann. N.Y. Acad. Sci. 1980, 333, 188. 26. Johnson, C.E., Ph.D. Thesis, Northwestern University, 1981. 27. Balk, R.W.; Stufkens, D.J.; Oskam, A. J. Chem.Soc.,Dalton Trans. 1982, 275. 28. Wrighton, M.; Gray, H.B.; Hammond, G.S. Mol. Photochem. 1973, 5, 164. 29. Zink, J.I. Mol. Photochem. 1973, 5, 151.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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196

30. Vanquickenborne, L.G.; Ceulemans, A. Coord. Chem. Rev. 1983, 48, 157. 31. Ford, P.C. Coord. Chem. Rev. 1982, 44, 61. 32. Adamson, A.W. Coord. Chem. Rev. 1968, 3, 169· 33. Robinson, G.W.; Frosch, R.P. J. Chem. Phys. 1962, 37, 1962; 1963, 38, 1187. 34. Chock, D.P.; Jortner, J.; Rice, S.A. J. Chem. Phys. 1968, 49, 610. 35. Jortner, J.; Rice, S.A.; Hochstrasser, R.M. Adv. Photochem. 1969, 7, 149. 36. Langford, C.H. Acc. Chem. Res. 1984, 17, 96. 37. Ouderkirk, A.J.; Wermer, P.; Schultz, N.L.; Weitz, E. J. Am. Chem. Soc. 1983, 105, 3354. 38. Seder, T.A.; Church, S.P.; Ouderkirk, A.J.; Weitz, E. J. Am. Chem. Soc. 1985, 107, 1432. 39. Tumas, W.; Gitlin, B.; Rosan, A.M.; Yardley, J.T J Am Chem Soc. 1982, 104, 55. 40. Poliakoff, M.; Weitz 41. von Gustorf, E.A.K.; Leenders, L.H.G.; Fischler, I.; Perutz, R. Adv. Inorg. Chem. Radiochem. 1976, 19, 65. 42. Balzani, V.; Carassiti, V. "Photochemistry of Coordination Com­ pounds"; Academic Press: New York, 1970. 43. Blake, D.M.; Nyman, C.J. J. Am. Chem. Soc. 1970, 92, 5359. 44. Addison, A.W.; Gillard, R.S.; Sheridan, P.S.; Tipping, L.R.H. J. Chem.Soc.,Dalton Trans. 1974, 709. 45. Otsuka, S. J. Organomet. Chem. 1980, 200, 191. 46. Shaw, B.L. ACS Symp. Ser. 1982, 196, 101. 47. Paonessa, R.S.; Trogler, W.C. Organometallics 1982, 1, 768. 48. Paonessa, R.S.; Prignano, A.L.; Trogler, W.C. Organometallics 1985, 4, 647. 49. Trost, B.M. Acc. Chem. Res. 1980, 13, 385. 50. Stone, F.G.A. Angew. Chem., Intl. Ed. Engl. 1984, 23, 89. 51. Gerlach, D.H.; Kane, A.R.; Parshall, G.W.; Jesson, J.P.; Muetterties, E.L. J. Am. Chem. Soc. 1971, 93, 3543. Fornies, J.; Green, M.; Spencer, J.L.; Stone, F.G.A. J. Chem. Soc., Dalton Trans. 1977, 1006. 53. Cowan, R.L.; Pourreau, D.; Trogler, W.C., to be published. 54. Paonessa, R.S.; Trogler, W.C. J. Am. Chem. Soc. 1982, 104, 1138. 55. Paonessa, R.S.; Trogler, W.C. J. Am. Chem. Soc. 1982, 104, 3529. 56. Paonessa, R.S.; Trogler, W.C. Inorg. Chem. 1983, 22, 1038. 57. Bailey, D.C.; Langer, S.H. Chem. Rev. 1981, 81, 109. 58. Parshall, G.W. Acc. Chem. Res. 1970, 3, 139. 59. Prignano, A.L.; Trogler, W.C., to be published. 60. Bemi, L.; Clark, H.C.; Davies, J.A.; Fyfe, C.A.; Wasylishen, R.E. J. Am. Chem. Soc 1982, 104, 438. 61. Jensen, C.M.; Trogler, W.C., submitted. 62. Parshall, G.W. "Homogeneous Catalysis"; Wiley: New York, 1980. RECEIVED November 8, 1985

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

14 Chemistry of Rhodium and Iridium Phosphine Complexes Flash Photolysis Investigations of Reactive Intermediates David Wink and Peter C. Ford Department of Chemistry, University of California, Santa Barbara, CA 93106

Flash photolysis complexes MCl(CO)(PPh3) of the unsaturated species MCl(PPh3)2, the reaction kinetics of which have been investigated. Reactions with CO to reform MCl(CO)(PPh )2 occur with second order rate constants of 7 x 10 and 2.7 x 108 M-1s-1 for M = Rh and Ir, respectively. The RhCl(PPh )2 species also undergoes fast reactions with PPh (k = 2.6 x 10 M-l-s-1) and with ethylene (>2 x 10 M- s- ) to form RhCl(PPh ) and RhCl(H C=CH )(PPh ) respectively; however, reaction with H2 to form the dihydride is much slower, (l x 10 M- s- ). Also described are flash photolysis studies of the dinitrogen species IrCl(N2)(PPh )2 and the dihydride H2IrCl(CO)(PPh )2. In both cases, the transient IrCl(PPh )2 is formed. These results indicate that CO labilization from the Ir(III) dihydride is a facile photochemical pathway and the photo-reductive elimination of H2 is a more complicated mechanism than previously inferred. 3

7

3

3

6

1

7

1

3

2

2

5

1

2

3

2,

1

3

3

3

Phosphine complexes of low v a l e n t m e t a l complexes have a l o n g h i s t o r y i n the c h e m i s t r y o f homogeneous c a t a l y t i c a c t i v a t i o n of s m a l l molecules(l,2). For example, the c a t a l y s i s c h e m i s t r y of r h o d i u m ( I ) phosphine complexes c o n t i n u e s to h o l d much i n t e r e s t s e v e r a l decades s i n c e the d e s c r i p t i o n of such r e a c t i o n s by W i l k i n s o n ( 3 ) . However, d e s p i t e c o n s i d e r a b l e q u a n t i t a t i v e s c r u t i n y ( 3 - 1 0 ) , the m e c h a n i s t i c d e t a i l s of key c a t a l y t i c s t e p s f o r even the o r i g i n a l W i l k i n s o n ' s c a t a l y s t R h C l ( P P h ) a r e not f u l l y r e s o l v e d ( 7 ^ 1 0 ) . The r e a s o n l i e s w i t h i n the v e r y n a t u r e of c a t a l y t i c p r o c e s s e s , namely t h a t the a c t i v a t i o n of s u b s t r a t e s o f t e n i n v o l v e s r e a c t i o n s of u n s t a b l e t r a n s i e n t s p e c i e s , the p r o p e r t i e s of which can o n l y be i n f e r r e d from k i n e t i c r a t e laws or from s p e c t r a l s t u d i e s under c o n d i t i o n s c o n s i d e r ­ a b l y d i f f e r e n t from t h o s e of an o p e r a t i n g c a t a l y s t . I n some c a s e s i t may be p o s s i b l e to use f l a s h p h o t o l y s i s t o g e n e r a t e s i g n i f i c a n t c o n c e n t r a t i o n s of such a t r a n s i e n t and to i n v e s t i g a t e the r e a c t i o n s of t h a t s p e c i e s more d i r e c t l y . Here we d e s c r i b e some i n v e s t i g a t i o n s 3

3

0097-6156/ 86/ 0307-0197506.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

198

EXCITED STATES AND

REACTIVE INTERMEDIATES

u s i n g f l a s h p h o t o l y s i s t e c h n i q u e s t o probe the r e a c t i o n dynamics of r e a c t i v e i n t e r m e d i a t e s i n the c h e m i s t r y of r h o d i u m ( I ) and i r i d i u m ( I ) p h o s p h i n e complexes. Rhodium(I) Complexes A key i n t e r m e d i a t e i n proposed mechanisms f o r W i l k i n s o n ' s c a t a l y s t i s the t r i c o o r d i n a t e s p e c i e s R h C l ( P P t i 3 ) 2 o r i t s s o l v a t e d a n a l o g (7-10) « Thus, i t would be p a r t i c u l a r l y d e s i r a b l e to prepare t h i s s p e c i e s and to i n t e r r o g a t e i t s r e a c t i v i t y under c a t a l y t i c a l l y r e l e v a n t conditions. G i v e n the commonly o b s e r v e d p h o t o l a b i l i t y o f c a r b o n monoxide complexes(11), a l o g i c a l p h o t o c h e m i c a l p r e c u r s o r t o RhCl(PPh ) would be the c a r b o n y l R h C l ( C O ) ( P P h 3 ) 2 . Although e a r l i e r i n v e s t i g a t o r s found t h a t under c o n t i n u o u s p h o t o l y s i s the l a t t e r d i d not d i s p l a y n e t p h o t o c h e m i s t r y ( 1 2 ) , i t appeared l i k e l y t h a t r e v e r s i ­ b l e l i g a n d l a b i l i z a t i o n would be the r e s u l t of f l a s h e x c i t a t i o n . When t r a n s - R h C l ( C O flash photolysis (λ^ > t r a n s i e n t a b s o r p t i o n was o b s e r v e d w i t h the s p e c t r a l c h a r a c t e r i s t i c s i l l u s t r a t e d i n F i g u r e 1. T h i s t r a n s i e n t (when m o n i t o r e d a t X 410 nm) decayed v i a second o r d e r k i n e t i c s over a p e r i o d o f s e v e r a l ms ( F i g u r e 2 ) . When the s o l u t i o n was f l a s h e d under CO (1.0 atm, 0.006 M i n b e n z e n e , ( 1 3 ) ) , no t r a n s i e n t h a v i n g a l i f e t i m e l o n g e r than the f l a s h was d e t e c t e d ; however, a l o n g - l i v e d t r a n s i e n t w i t h the same spectrum as R h C l ( P P h 3 ) 3 was seen when the f l a s h p h o t o l y s i s was c a r r i e d out i n the p r e s e n c e of e x c e s s ΡΡΙΊ3 (0.05 M, see b e l o w ) . Thus CO, not phosphine, p h o t o l a b i l i z a t i o n appears to be the major primary p h o t o r e a c t i o n (Equation 1). 3

2

Γ Γ

m o n

hv RhCl(CO) ( P P h ) 3

R h C l ( P P h ) o + CO

2

(1)

3

When the f l a s h p h o t o l y s i s of RhCl(CO) ( P P l v ^ ^ was c a r r i e d out i n the absence o f o t h e r r e a c t a n t s but w i t h X 450 nm, i t was noted t h a t the t r a n s i e n t a b s o r p t i o n decayed i n two s t a g e s ( F i g u r e 3 ) . The r e l a t i v e l y r a p i d second o r d e r decay noted a t 410 nm was f o l l o w e d by a s l o w e r f i r s t o r d e r decay back t o RhCl (CO) (PPI13) 2 w i t h a k ^ of 1.8 s"" (298 K ) . The spectrum of the l o n g e r l i v e d t r a n s i e n t ( F i g u r e 1) i s v e r y c l o s e t o t h a t of t h e dimer [ R h C l ( P P h 3 ^ ] 2 » d e s c r i b e d p r e v i o u s l y ( 1 4 ) and d i s c u s s e d i n the m e c h a n i s t i c schemes f o r W i l k i n s o n ' s c a t a l y s t ( 7 - 1 0 ) . Presumably, [ R h C l i P P t v ^ ^ formed v i a the d i m e r i z a t i o n of R h C K P P l ^ ^ : m o n

Q

s

1

i

k

2 RhCl(PPh ) 3

2

s

2 [RhCl(PPh ) ]2 3

( ) 2

2

and decays by a u n i m o l e c u l a r r a t e - l i m i t i n g s t e p , presumably d i s s o c i a ­ t i o n t o monomers, i . e . , the k_2 s t e p . (The s t o p p e d - f l o w k i n e t i c s of the r e a c t i o n between [ R h C l i P P l v ^ ^ * p r e p a r e d t h e r m a l l y , (14) and CO (P 0.1 - 1.0 atm) i n benzene t o g i v e R h C l ( C O ) ( P P h ) a l s o gave f i r s t o r d e r r a t e s w i t h t h e near l y i d e n t i c a l k , 1.7 s " a t 298 K ) . C a l c u l a t i o n of k_-^ and k 2 from the f l a s h p h o t o l y s i s d a t a r e q u i r e s the e x t i n c t i o n c o e f f i c i e n t o f R h C l ( P P h 3 ) 2 a t >^ i n order c o

3

2

1

o b s

mon

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

14.

WINK A N D FORD

Chemistry of Rhodium and Iridium Phosphine Complexes

_i

r

400

450

199

ι 500

λ ( in nm )

F i g u r e 1. T r a n s i e n t s p e c t r a r e s u l t i n g from t h e f l a s h p h o t o l y s i s o f RhCl(CO) (PPI13) 2 i n benzene s o l u t i o n . A) Spectrum o b s e r v e d 100 ys a f t e r f l a s h (λ± > 315 nm). P o i n t s i n d i c a t e d r e p r e s e n t a c t u a l e x p e r i m e n t a l o b s e r v a t i o n s ; c u r v e i s drawn f o r i l l u s t r a t i v e purposes. B) Spectrum o b s e r v e d 20 ms a f t e r f l a s h . C) Spectrum o f RhCl(CO)(PPh ) . ντ

3

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

200

EXCITED STATES AND REACTIVE INTERMEDIATES

5

10

Time ( i n ms)

Time ( i n ms)

F i g u r e 2. L e f t : Absorbance changes r e s u l t i n g from the f l a s h p h o t o l y s i s (λ. > 415 nm) of R h C l ( C O ) ( P P h ^ i n benzene solution under Ar at The m o n i t o r i n g wavelength was 410 nm. Right: A l i n e a r second-order p l o t [(A ^ * data i above c u r v e . v

s

o r

t

i e

n

t

n

e

5 Time ( i n ms) Figure 3 . Decay c u r v e f o r t h e f l a s h p h o t o l y s i s o f RhCl(CO) (PPI13)2 i n 25° benzene under Ar showing the f o r m a t i o n o f a n o t h e r i n t e r ­ m e d i a t e s p e c i e s (B) as a p r o d u c t o f t h e s e c o n d - o r d e r decay o f t h e i n i t i a l transient (A).

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

14.

Chemistry of Rhodium and Iridium Phosphine Complexes

WINK AND FORD

201

to d e t e r m i n e t h e c o n c e n t r a t i o n s o f t h i s s p e c i e s . T h i s was e s t i m a t e d by assuming t h a t t h e r e a c t i o n w i t h e x c e s s P P h (see above) t r a p p e d a l l RhCl(PPh ) as R h C l ( P P h ) . From t h e known spectrum o f t h e l a t t e r s p e c i e s ( 1 4 ) , t h e i n i t i a l c o n c e n t r a t i o n , thus t h e e x t i n c t i o n c o e f f i c i e n t (8 χ 1 0 M" cm" a t 410 nm, t h e i s o s b e s t i c p o i n t f o r the dimer i n t e r m e d i a t e and t h e s t a r t i n g m a t e r i a l ) , o f R h C l ( P P h ) 2 c o u l d be c a l c u l a t e d . W i t h t h i s e x t i n c t i o n c o e f f i c i e n t , t h e second o r d e r r a t e c o n s t a n t f o r t h e d i s a p p e a r a n c e o f R h C l ( P P h ) 2 was d e t e r ­ mined t o be 1 χ 1 0 M " s " . The amount o f dimer produced by t h e f l a s h p h o t o l y s i s i n t h e absence o f added r e a c t a n t s ( c a l c u l a t e d from the spectrum o f t h e l o n g - l i v e d i n t e r m e d i a t e ) i n d i c a t e d t h a t under t h e s e c o n d i t i o n s about 40% o f R h C l ( P P h ) 2 d i m e r i z e d i n c o m p e t i t i o n w i t h t h e back r e a c t i o n w i t h t h e p h o t o l i b e r a t e d CO t o g i v e RhCl(CO) (PPh )2Thus k _ i and k were e s t i m a t e d a s 6 χ 1 0 M ' ^ " and 4 χ 1 0 M" s~-'-, r e s p e c t i v e l y . 3

3

2

3

2

1

3

1

3

3

8

1

1

3

7

3

1

2

7

1

The above k _ i v a l u confirmed b c a r r y i n th f l a s h p h o t o l y s i s experiments i 10" M. Under t h e s e c o n d i t i o n s t r a n s i e n t decay was f i r s t - o r d e r w i t h k ^ v a l u e s l i n e a r l y dependent on [CO]. The second o r d e r r a t e c o n s t a n t (k_^) o b t a i n e d from t h e plot of k v s [CO] was (6.9 ± 0.2) χ 1 0 M" s" . The r e a c t i o n o f t h e t r a n s i e n t R h C l ( P P h ) 2 w i t h excess t r i p h e n y l p h o s p h i n e t c g i v e R h C l ( P P h ) 2 d i s p l a y e d f i r s t o r d e r r a t e s dependent on t h e c o n c e n t r a t i o n o f P P h . From t h e s e d a t a , t h e second o r d e r r a t e c o n s t a n t f o r t h e r e a c t i o n d e p i c t e d i n E q u a t i o n 3 was c a l c u l a t e d as 2.8 χ 1θ6 M"* s~* . G i v e n t h e r a t e c o n s t a n t o f 0.71 s~^ d e t e r m i n e d (4) f o r t h e d i s s o c i a t i o n o f P P h from R h C l ( P P h ) i n benzene ( k _ ) , the e q u i l i b r i u m c o n s t a n t f o r d i s s o c i a t i o n ( k _ / k ) i s c a l c u l a t e d t o be 0.25 χ 10~6 M , c o n s i s t e n t w i t h t h e p r e v i o u s e s t i m a t e o f < 10"""* M(4). 0

s

7

1

1

o b s

3

3

3

1

1

3

3

3

3

3

3

k RhCl(PPh ) 3

+ PPh

2

-

3

RhCl(PPh ) 3

(3)

3

k

-3 When t h e f l a s h p h o t o l y s i s o f R h C l ( C O ) ( P P h ) 2 was c a r r i e d o u t as above b u t under d i h y d r o g e n (1.0 atm, 0.0028 M) (5) , t h e i n t e r m e d i a t e RhCl(PPh ) underwent f i r s t o r d e r r e a c t i o n ( k = 2.8 χ 1 0 s " ) t o g i v e a new t r a n s i e n t spectrum h a v i n g an even s m a l l e r absorbance t h a n t h a t o f t h e c a r b o n y l complex over t h e s p e c t r a l range 360-450 nm (Figure 4). T h i s new t r a n s i e n t decayed over a p e r i o d o f seconds t o give RhCl(CO)(PPh ) again. We i n t e r p r e t t h e s e o b s e r v a t i o n s i n terms of t h e r e a c t i o n o f R h C l ( P P h ) with H to give the dihydride ( E q u a t i o n 4) f o l l o w e d by r e a c t i o n o f t h e l a t t e r w i t h CO t o r e g e n e r a t e RhCl(CO)(PPh )2I f a second o r d e r r a t e law f o r E q u a t i o n 4 i s assumed, then t h e c a l c u l a t e d v a l u e o f k^ i s 1.0 χ 10^ M'^-s"!, v e r y much i n agreement w i t h H a l p e r n ' s e s t i m a t e o f k^ (> 7 χ 10^ M~ s~^) drawn from k i n e t i c s a n a l y s i s o f t h e W i l k i n s o n ' s c a t a l y s i s ( 4 ) . F l a s h p h o t o l y s i s under D 2 (1.0 atm) gave i d e n t i c a l s p e c t r a l changes and a c a l c u l a t e d k o f 0.7 χ 10^ M^^s" , i . e . , a k i n e t i c i s o t o p e e f f e c t k£/k^ o f about 1.4. 3

2

3

2

1

Q b s

3

2

3

2

2

3

1

1

3

k

RhCl(PPh ) 3

2

+ H

2

4 >

H RhCl(PPh ) 2

3

(4) 2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

202

EXCITED STATES AND REACTIVE INTERMEDIATES

RhCl(PPh )

·-

I

+ H

>

H RhCl ( P P h )

IT

In CA - A ^) H

Abe.

A

C

/

^ •I

0

5

— . — ι1

.L -A

w

d

10

Time ( i n ms)

Figure 4. F l a s h p h o t o l y s i s o f R h C l ( C O ) ( P P h ) 2 i n 25° benzene under H2 (1.0 atm) showing decay o f t h e t r a n s i e n t R h C l ( P P h ) 2 by r e a c t i o n w i t h e x c e s s hydrogen to form a new i n t e r m e d i a t e s p e c i e s (presumably H 2 R h C l ( P P h ) 2 ) h a v i n g an absorbance (Ay) l e s s t h a n t h a t o f t h e s t a r t i n g m a t e r i a l A Q . The c u r v e r e p r e s e n t s t h e t e m p o r a l a b s o r b a n c e changes ( s c a l e t o t h e l e f t ) ; t h e l i n e r e p r e s e n t s a p s e u d o - f i r s t - o r d e r p l o t o f t h i s decay ( s c a l e t o t h e right) . 3

3

3

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

14.

Chemistry of Rhodium and Iridium Phosphine Complexes 203

WINK AND FORD

F l a s h p h o t o l y s i s o f R h C l ( C 0 ( P P h 3 ) 2 under e t h y l e n e ( 0 . 0 1 atm, 0 . 0 0 1 1 M) l e d t o immediate s p e c t r a l changes c o n s i s t e n t w i t h t h e f o r m ­ a t i o n o f t h e e t h y l e n e complex(_3) R h C l ( H 2 C = C H 2 ) ( P P h ) η w i t h i n t h e duration of the f l a s h . T h i s o b s e r v a t i o n p r o v i d e s a lower l i m i t o f 2 χ 1 0 ^ M ~ l s ~ l f o r t h e second o r d e r r a t e c o n s t a n t f o r t h e r e a c t i o n o f RhCl(PPh )2 with ethylene. The back r e a c t i o n o f t h e e t h y l e n e adduct w i t h CO t o r e f o r m R h C l ( C O ) ( P P h ) 2 was a l s o r a t h e r r a p i d and o c c u r r e d w i t h i n a p e r i o d o f a few m i l l i s e c o n d s . 3

3

3

In summary, t h e f l a s h p h o t o l y s i s o f R h C l ( C O ) ( P P h ) 2 i n benzene leads p r i n c i p a l l y to formation of the c o o r d i n a t i v e l y unsaturated Wilkinson's c a t a l y s t intermediate R h C l ( P P h ) 2 (or i t s solvated analog). The r e a c t i o n s o f t h i s s p e c i e s a r e summarized i n Scheme I . In t h e absence o f o t h e r r e a c t a n t s , t h i s i n t e r m e d i a t e recombines w i t h CO o r d i m e r i z e s v i a v e r y r a p i d r e a c t i o n s , b u t i n t h e p r e s e n c e of added P P h , e t h y l e n e o r d i h y d r o g e n , a d d u c t s o f R h C l ( P P h ) a r e formed. Rate c o n s t a n t s f o r t h e second o r d e r r e a c t i o n s o f R h C l ( P P h ) 2 with various substrates a d d i t i o n o f t h e two e l e c t r o c a n t l y h i g h e r than f o r t h e o x i d a t i v e a d d i t i o n o f d i h y d r o g e n . In a l l c a s e s , however, t h e i n i t i a l a d d u c t s r e a c t e v e n t u a l l y w i t h t h e p h o t o l a b i l i z e d CO t o r e f o r m t h e more s t a b l e s t a r t i n g complex RhCl(CO) (PPh )2« These o b s e r v a t i o n s a r e c o n s i s t e n t w i t h t h e c o n t i n u o u s p h o t o l y s i s s t u d i e s w h i c h r e p o r t e d no n e t p h o t o c h e m i s t r y o f t h i s s p e c i e s i n t h e absence o f oxygen ( 1 2 ) . 3

3

3

3

2

3

Table I .

Second Order Rate C o n s t a n t s Substrates with RhCl(PPh ) 3

Substrate

2

> 2 χ 10

7

(4 ± 1) χ 1 0

7

(2.8 ± 0.4) χ 1 0

6

(9.8

± 0.5) χ 1 0

4

(7.0

± 0.5) χ 1 0

4

2

3

2

Do

Iridium

7

4

3

H

± 0.2) χ 1 0

(6.9

RhCl(PPh ) PPh

f o r the Reactions of Various i n 25° Benzene S o l u t i o n .

k ( i n M "^"s ^)

CO C H

2

Complexes

Vaska's complex t r a n s - I r C l ( C O ) ( P P h ) 9 has s e r v e d as an i m p o r t a n t model f o r m e c h a n i s t i c i n v e s t i g a t i o n o f c a t a l y t i c a l l y r e l e v a n t r e a c t i o n s such as t h e o x i d a t i v e a d d i t i o n and r e d u c t i v e e l i m i n a t i o n of s m a l l m o l e c u l e s ( 1 5 ) . The l a t t e r p r o c e s s e s have a l s o been t h e s u b j e c t o f some p h o t o c h e m i c a l i n v e s t i g a t i o n . F o r example, t h e r e d u c t i v e e l i m i n a t i o n o f H 2 d e p i c t e d i n E q u a t i o n 5, w h i c h i s a r e l a t i v e l y s l o w t h e r m a l l y a c t i v a t e d p r o c e s s ( k ^ = 3.8 χ 10"^ s ~ l i n 25° benzene s o l u t i o n ( 1 5 ) ) , has been shown t o o c c u r r e a d i l y when t h e d i h y d r i d e complex was s u b j e c t e d t o c o n t i n u o u s p h o t o l y s i s w i t h 366 nm light(16). However, Vaska's compound i t s e l f was r e p o r t e d t o be 3

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

204

EXCITED STATES AND REACTIVE INTERMEDIATES

RhCl(CO)(PPh ) 3

2

Scheme I . A summary o f t h e r e a c t i o n s o f RhCl(PPH3)2 as s t u d i e d by the k i n e t i c f l a s h p h o t o l y s i s o f RhCl(CO)(PPh3>2.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

14.

Chemistry of Rhodium and Iridium Phosphine Complexes 205

WINK AND FORD

p h o t o i n e r t under c o n t i n u o u s p h o t o l y s i s ( _ 1 6 ) . I n t h e s e c o n t e x t s , we were i n t e r e s t e d i n e s t a b l i s h i n g whether V a s k a s compound would show the same type o f t r a n s i e n t b e h a v i o r upon f l a s h p h o t o l y s i s as d i d t h e rhodium a n a l o g (above) and whether t r a n s i e n t s c o u l d be d e t e c t e d i n the p h o t o r e a c t i o n s o f t h e r e l a t e d I r ( I I I ) o x i d a t i v e a d d u c t s . T

H IrCl(C0)(PPh ) -* trans-IrCl(CO)(PPh ) + H 2

3

2

3

2

(5)

2

Flash p h o t o l y s i s of t r a n s - I r C l ( C O ) ( P P h ) i n s t r i n g e n t l y deaerated benzene s o l u t i o n under an argon atmosphere ( λ ^ > 254 nm) r e s u l t e d i n the formation o f a t r a n s i e n t with strong absorption i n the s p e c t r a l r e g i o n 390-550 nm ( * - 430 nm) which decayed t o t h e i n i t i a l b a s e l i n e v i a c l e a n l y second o r d e r k i n e t i c s ( F i g u r e 5 ) . The r e t u r n t o t h e i n i t i a l spectrum i s c o n s i s t e n t w i t h t h e e a r l i e r r e p o r t (16) t h a t c o n t i n u o u s p h o t o l y s i s o f t r a n s - I r C l ( C O ) ( P P h 3 ) 2 gave no n e t photoreaction. When s i m i l a r f l a s h experiments were c a r r i e d o u t under v a r i o u s p r e s s u r e s o f CO w i t h t h e observed r a t e c o n s t a n t Thus, we c o n c l u d e t h a t t h e i n t e r m e d i a t e s p e c i e s o b s e r v e d i s t h e p r o ­ duct o f CO p h o t o d i s s o c i a t i o n ( E q u a t i o n 6) and t h a t t h e decay p r o c e s s i s t h e r e c o m b i n a t i o n t o t h e s t a r t i n g complex ( E q u a t i o n 7 ) . The second o r d e r r a t e c o n s t a n t k-j = (2.7 ± 0.7) χ 10^ M ~ l s ~ l was d e t e r ­ mined from t h e p l o t o f k v s [CO]. These r e s u l t s a r e v e r y s i m i l a r to t h e c h e m i s t r y i n d u c e d by t h e f l a s h p h o t o l y s i s o f t h e r h o d i u m ( I ) analogue above, a l t h o u g h ky i s about a f a c t o r o f f o u r f a s t e r f o r t h e Ir(I) transient. 3

2

Γ Γ

m a x

o b s

hv trans-IrCl(CO)(PPh ) 3

k

IrCl(PPh ) 3

+ CO

2

>

2

IrCl(PPh ) 3

2

+ CO

(6)

7 >

trans-IrCl(CO)(PPh ) 3

(7)

2

F l a s h p h o t o l y s i s o f t h e analogous d i n i t r o g e n complex t r a n s - I r C l (N )(PPh ) (17) demonstrates t h a t f l a s h p h o t o l y s i s l e a d s i n b o t h c a s e s t o immediate appearance o f a t r a n s i e n t spectrum t h e same, w i t h i n e x p e r i m e n t a l u n c e r t a i n t y , as t h a t a t t r i b u t e d above t o IrCl(PPh ) . Continuous p h o t o l y s i s o f I r C l ( N ) ( P P h ) i n C^D, under o t h e r w i s e analogous c o n d i t i o n s r e s u l t s i n t h e d i s a p p e a r a n c e or t h e i n f r a r e d band a t t r i b u t e d t o t h e c o o r d i n a t e d N ( F i g u r e 6 ) , and t h e p r o d u c t s o l u t i o n d i s p l a y s a p r o t o n nmr resonance a t -22.5 ppm i n d i ­ c a t i n g t h e f o r m a t i o n o f an i r i d i u m h y d r i d e ( 1 8 ) . Thus, p h o t o l a b i l i zation of N to give I r C l ( P P h ) i s i r r e v e r s i b l e ( E q u a t i o n 8 ) , and, i n t h e absence o f t h e o t h e r r e a c t a n t s , t h i s r e a c t i v e i n t e r m e d i a t e a p p a r e n t l y undergoes i n t e r n a l o r t h o m e t a l l a t i o n o f a t r i p h e n y l phos­ p h i n e t o g i v e an i r i d i u m ( I I I ) h y d r i d e p r o d u c t ( s ) . I n t h e f l a s h p h o t o l y s i s experiment, t h e l a t t e r p r o c e s s i s e v i d e n c e d by slow absorbance changes i n t h e 340 t o 550 nm range w i t h i s o s b e s t i c p o i n t s at 460 and 334 nm c o n s i s t e n t w i t h f o r m a t i o n o f I r ( I I I ) p r o d u c t s . 2

3

3

2

2

2

3

2

2

2

3

2

hv IrCl(N )(PPh ) 2

3

IrCl(PPh ) 3

2

2

a* I r C l ( P P h ) 3

^lr(III)

2

+ N

2

hydride

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

(8) (9)

206

EXCITED STATES AND REACTIVE INTERMEDIATES

irCl (C0)L

2

F i g u r e 5. Top: Absorbance changes r e s u l t i n g from t h e f l a s h p h o t o l y s i s o f I r C l ( C O ) ( P P h ) i n 25° benzene under A r . The m o n i t o r i n g w a v e l e n g t h was 420 nm. Bottom: A l i n e a r s e c o n d - o r d e r p l o t f o r t h e d a t a i n the above c u r v e . 3

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

14.

WINK AND FORD

Chemistry of Rhodium and Iridium Phosphine Complexes 207

F i g u r e 6. Top: IR absorbance changes r e s u l t i n g from t h e CW photolysis ( X = 405 nm) o r I r C l ( N ) (ΡΡΐΐβ^ i n 25 benzene under Ar. Bottom: E l e c t r o n i c s p e c t r a l changes r e s u l t i n g from a s i m i l a r p h o t o l y s i s o f I r C l ( N ) (PPI13) . The top c u r v e i n each c a s e r e p r e ­ s e n t s t h e spectrum o f t h e s t a r t i n g m a t e r i a l . i r r

2

2

2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND

208

REACTIVE INTERMEDIATES

In agreement w i t h the e a r l i e r r e p o r t ( 1 6 ) , we found t h a t H 2 l r C l ( C 0 ) ( P P h ) undergoes f a c i l e p h o t o e l i m i n a t i o n of H (Equation 5) when i r r a d i a t e d . The quantum y i e l d was 0.56 m o l e s / e i n s t e i n f o r c o n t i n u o u s p h o t o l y s i s a t 313 nm, a v a l u e c l o s e to t h a t r e p o r t e d f o r the s i m i l a r complex H I r C l ( P P h ) (0.55). However, a p a r t i c u l a r l y s t r i k i n g o b s e r v a t i o n was t h a t f l a s h p h o t o l y s i s of H I r C l ( C 0 ) ( P P h ) i n benzene under 1 atm. H (λ^ > 254 nm) r e s u l t e d i n t r a n s i e n t absorbance i n the s p e c t r a l r e g i o n 400-550 nm e x p e r i m e n t a l l y i n d i s t i n ­ g u i s h a b l e from t h a t seen f o r the f l a s h p h o t o l y s e s of t r a n s IrCl(CO)(PPh ) and of t r a n s - I r C l ( N ) ( P P h ) . T h i s t r a n s i e n t decayed v i a second o r d e r k i n e t i c s t o a p r o d u c t h a v i n g the spectrum of V a s k a s compound. Over a p e r i o d of 10 m i n u t e s , the l a t t e r underwent s u b s e ­ quent r e a c t i o n w i t h H t o r e f o r m the s t a r t i n g complex a c c o r d i n g t o E q u a t i o n 10 f o r which the r a t e c o n s t a n t 1.2 Μ" ^"" has p r e v i o u s l y been r e p o r t e d ( 1 5 ) . 3

2

2

2

3

3

2

2

3

3

2

Γ Γ

2

2

3

2

T

2

-

1

trans-IrCl(CO)(PPh The f l a s h p h o t o l y s i s of D I r C l ( C 0 ) ( P P h ) under 1 atm D demonstrated no i s o t o p e e f f e c t on the r e l a x a t i o n of the f i r s t t r a n s i e n t to g i v e t r a n s - I r C l ( C O ) ( P P h ) . However, f l a s h p h o t o l y s i s of H I r C l ( C 0 ) ( P P h ) under H /C0 m i x t u r e s gave decay r a t e s l i n e a r l y dependent on P . P l o t s of k v v s [CO] as above gave the second o r d e r r a t e c o n s t a n t (2.6 ± 0.7) χ 10° M"~ls~"^ w i t h i n e x p e r i m e n t a l u n c e r t a i n t y o f the ky v a l u e r e p o r t e d above. These r e s u l t s i n d i c a t e t h a t H I r C l ( C O ) ( P P h ) f i r s t undergoes p h o t o d i s s o c i a t i o n o f CO ( E q u a t i o n 11) f o l l o w e d by e l i m i n a t i o n of H from the r e s u l t i n g p e n t a c o o r d i n a t e d i n t e r m e d i a t e ( E q u a t i o n 12) to g i v e the I r C l ( P P h ) t r a n s i e n t formed d i r e c t l y v i a f l a s h p h o t o l y s i s of t r a n s - I r C l ( C O ) ( P P h ) . hv H IrCl(C0)(PPh ) > H IrCl(PPh ) + CO (11) 2

3

3

2

3

2

2

2

2

2

C Q

Q

s

2

3

2

2

3

2

3

2

3

2

2

2

k-, H IrCl(PPh ) 2

3

3

2

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T h i s v i e w c o n t r a s t s t o the p r o p o s a l t h a t the d i h y d r i d e p h o t o e l i m i n ­ a t i o n o c c u r s by a s i n g l e c o n c e r t e d s t e p but i s c o n s i s t e n t w i t h t h e o r e t i c a l arguments(19) and e x p e r i m e n t a l o b s e r v a t i o n s ( 2 0 , 2 1 ) t h a t r e d u c t i v e e l i m i n a t i o n from complexes o f t e n o c c u r s much more r e a d i l y a f t e r l i g a n d d i s s o c i a t i o n from the o r i g i n a l hexacoordinate species to give a pentacoordinate intermediate. Given that f o r m a t i o n of I r C l ( P P h ) was complete w i t h i n the l i f e t i m e of the f l a s h (20 u s ) , a lower l i m i t f o r k ^ can be e s t i m a t e d as 5 χ 10^ s . Thus, we c o n c l u d e t h a t d i s s o c i a t i o n of CO a c c e l e r a t e s d i h y d r o g e n e l m i n a t i o n by n i n e or more o r d e r s of magnitude. Notably, t h i s r a t e a c c e l e r a t i o n o c c u r s d e p s i t e the d i s s o c i a t i o n of the π-acid CO w h i c h would be e x p e c t e d t o f a v o r the lower o x i d a t i o n s t a t e o f the m e t a l center. (*The f o l l o w i n g o b s e r v a t i o n argues a g a i n s t the p o s s i b i l i t y o f a s e q u e n t i a l two-photon p r o c e s s i n v o l v i n g i n i t i a l H photolabiliz a t i o n to generate I r C l ( C O ) ( P P h ) f o l l o w e d by s e c o n d a r y p h o t o l y s i s of t h i s p r o d u c t to g i v e " I r C l ( P P h ) " . The r e l a t i v e p u l s e i n t e n s i t y r e q u i r e d to g e n e r a t e the same c o n c e n t r a t i o n of the l a t t e r t r a n s i e n t was f i v e times l a r g e r when the i n i t i a l s u b s t r a t e was trans-IrCl(CO) PPh ) (under argon) than when H I r C l ( C O ) ( P P h ) (under H ) was the 3

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Chemistry of Rhodium and Iridium Phosphine Complexes 209

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i n i t i a l substrate. However, an a l t e r n a t i v e mechanism s h o u l d a l s o be c o n s i d e r e d , namely, t h e p o s s i b i l i t y t h a t b o t h CO and H l o s s o c c u r s i m u l t a n e o u s l y o r s e q u e n t i a l l y from t h e e x c i t e d s t a t e o f H 2 l r C l ( C 0 ) (PPh3)2A t p r e s e n t such a mechanism, a l t h o u g h u n p r e c e d e n t e d , c a n not be d i f f e r e n t i a t e d from t h e s t e p w i s e pathway p r o p o s e d i n E q u a t i o n s 11 and 12.) 2

Preliminary i n v e s t i g a t i o n s of the I r ( I I I ) species HlrCl(CO) (PPI12C6H4) ( P P h 3 ) 2 , t h e o r t h o m e t a l l a t e d isomer o f Vaska's compound(22), are c l o s e l y analogous. Continous p h o t o l y s i s i n t h i s case leads to the f o r m a t i o n o f I r C l ( C O ) (ΡΡΙΊ3) 2However, f l a s h p h o t o l y s i s l e a d s to a t r a n s i e n t spectrum q u a l i t a t i v e l y t h e same a s t h a t a t t r i b u t e d t o I r C l ( P P h 3 ) 2 and t h i s t r a n s i e n t decays by a second o r d e r pathway ([CO] dependent, i . e . E q u a t i o n 7) t o form t r a n s - I r C l ( C O ) ( P P t v j ) 2 a s t h e f i n a l product. Thus, a g a i n i t appears t h a t t h e s t a r t i n g complex has undergone CO p h o t o d i s s o c i a t i o n f o l l o w e d by H / a r y l e l i m i n a t i o n t o form I r C l ( P P h 3 ) 9 w i t h i n the 20 ys l i f e t i m e of the f l a s h This leads to a 5 χ 1 0 s " l lower l i m i t f o from t h e p e n t a c o o r d i n a t o f magnitude f a s t e r t h a n t h e r a t e o f about 3 χ 1 0 ~ s ~ l we have measured f o r t h e t h e r m a l r e a c t i o n o f H l r C l ( C O ) (PC^H^Ph) ( P P t v j ^ t o give trans-IrCl(CO)(PPh ) i n 70° benzene. 4

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In summary, we have o b s e r v e d t h a t a common t r a n s i e n t i s produced i n t h e f l a s h p h o t o l y s i s o f f o u r s p e c i e s , t r a n s - I r C l (CO) (PPI13) 2 , trans-IrCl(N )(PPh )2, H I r C l ( C O ) ( P P h C H ) ( P P h ) and H I r C l ( C 0 ) ( P P h ) . These r e s u t l s s u g g e s t t h a t t h e p h o t o - i n d u c e d v e r s i o n o f E q u a t i o n 5 i s a s t e p w i s e mechanism i n v o l v i n g i n i t i a l CO d i s s o c i a t i o n as t h e primary photoreaction of H 2 l r C l ( C 0 ) (PPti3)2. The r e s u l t i n g p e n t a c o o r d i n a t e d I r ( I I I ) i n t e r m e d i a t e a p p e a r s t o be d r a m a t i c a l l y a c t i v a t e d toward H2 r e d u c t i v e e l i m i n a t i o n as p r e d i c t e d i n t h e o r e t i c a l t r e a t ­ ments. These t r a n s f o r m a t i o n s a r e i l l u s t r a t e d i n Scheme I I . 3

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The f l a s h p h o t o l y s i s a p p a r a t u s was t h a t d e s c r i b e d previously(23) m o d i f i e d by t h e u s e o f B i o m a t i o n 805 t r a n s i e n t d i g i t i z e r i n t e r f a c e d t o a H e w l e t t P a c k a r d 86 computer f o r d a t a c o l l e c t i o n , a n a l y s i s and plotting. Wavelength s e l e c t i o n was a c c o m p l i s h e d by u s e o f an aqueous NaNÛ3 s o l u t i o n a s a UV and IR f i l t e r . The benzene used i n t h e s e s t u d i e s was s c r u p u l o u s l y d e a e r a t e d by freeze/pump/thaw c y c l e s and d r i e d by d i s t i l l a t i o n from a Na/K amalgam. S o l u t i o n s were p r e p a r e d by vacuum m a n i f o l d t e c h n i q u e s . S t o p p e d - f l o w k i n e t i c s were c a r r i e d out u s i n g a Gibson-Durram D110 s p e c t r o p h o t o m e t e r e q u i p p e d f o r t h e h a n d l i n g o f s o l u t i o n s under d e a e r a t e d c o n d i t i o n s ( 2 4 ) . The d a t a s t a t i o n d e s c r i b e d above was used f o r c o l l e c t i o n , a n a l y s i s and p l o t t i n g of d i g i t a l data. Acknowledgments T h i s r e s e a r c h was s p o n s o r e d by t h e N a t i o n a l S c i e n c e F o u n d a t i o n . The rhodium and i r i d i u m used i n t h e s e s t u d i e s was p r o v i d e d on l o a n by Johnson Matthey, I n c .

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Scheme I I . A summary o f t h e p h o t o r e a c t i o n s intermediate ΐΓ01(ΡΡη ) . 3

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Literature Cited 1. Pignolet, L.H., Ed. "Homogeneous Catalysis with Metal Phosphine Complexes"; Plenum Press: New York, N.Y., 1983. 2. Halpern, J. Acc. Chem. Res. 1970, 3, 386-392. 3. Osborn, J.A.; Jardine, F.H.; Young, J.F.; Wilkinson, G. J. Chem. Soc.(A) 1966, 1711-1732. 4. Halpern, J.; Wong, C.S. J.C.S. Chem. Commun. 1973, 629-630. 5. Tolman, C.A.; Meakin, P.Z.; Lindner, D.L.; Jesson, J.P. J. Amer. Chem. Soc. 1974, 96, 2762-2774. 6. Halpern, J.; Okamoto, T.; Zakhariev, A. J. Mol. Catal. 1976, 2, 65-69. 7. Tolman, C.A.; Faller, J.W. Chapt. 2 in ref. 1. 8. Halpern, J. Trans. Amer. Cryst. Assoc. 1978, 14, 59-70. 9. Jardine, F.H. Prog. Inorg. Chem. 1982, 28, 63-201. 10. James, B.R. "Homogeneous Hydrogenation"; Wiley-Interscience: New York, N.Y. 1973 11. Geoffroy, G.L.; Wrighton Academic Press, New York, N.Y. 1979. 12. Geoffroy, G.L.; Denton, D.A.; Keeney, M.E.; Bucks, R.R. Inorg. Chem. 1976, 15, 238202385. 13. Braker, W.; Mossman, A.L. "The Matheson Unabridged Gas Databook: A Compilation of Physical and Thermodynamic Properties of Gases"; Matheson Gas Products: East Rutherford, NJ, 1974, Vol. I, p. 11. 14. Geoffroy, G.L.; Keeney, M.E. Inorg. Chem. 1977, 16, 205-207. 15. Vaska, L. Acc. Chem. Res. 1968, 1, 335. 16. Geoffroy, G.L.; Hammond, G.S.; Gray, H.B. J. Amer. Chem. Soc. 1975, 97 3933-3936. 17. Collman, J.P.; Kubota, M.; Vastine, F.D.; Sun, J.Y.; Kang, J.W. J. Amer. Chem. Soc. 1968, 90, 5430-5437. 18. Bennett, M.A.; Milner, D.L. J. Amer. Chem. Soc. 1969. 91, 69836994. 19. Tatsumi, K.; Hoffman, R.; Yamamoto, Α.; Stille, J.K. Bull. Chem. Soc. Japan 1981, 51, 1857-1867. 20. Basato, M.; Morandini, F.; Longato, B.; Bresadola, S. Inorg. Chem. 1984, 23, 649-653. 21. Milstein, D.; Calabrese, J.C. J. Amer. Chem. Soc. 1982, 104, 3773-3774; 5227-5228. 22. Valentine, J. J.C.S. Chem. Commun. 1973, 857-858. This paper reports that the orthometallated product is a mixture of two geometric isomers. 23. Durante, V.A.; Ford, P.C. Inorg. Chem. 1979, 18, 588-593. 24. Trautman, R.J.; Gross, D.C.; Ford, P.C. J. Amer. Chem. Soc. 1985, 107, 2355. RECEIVED November 8, 1985

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

15 Intrazeolite Organometallics Chemical and Spectroscopic Probes of Internal Versus External Confinement of Metal Guests Geoffrey A. Ozin and John Godber Lash Miller Chemical Laboratories, University of Toronto, Toronto, ON M5S 1A1 Canada The process of loadin complexes always brings to the forefront the question of internal versus external confinement of the metal guest. In this paper we present some experiments based on size exclusion, metal loading and intrazeolite chemistry which in conjunction with FT-FAR-IR, EPR and UV-visible reflectance spectroscopy, critically probe the question of internal versus external location for the case of five representative organometallics, (η -C H ) V, (η -C H )2Cr, (η -C H ) Fe, (C H CH ) Co, and Co (CO) . 6

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I t i s now w i d e l y a c c e p t e d t h a t c e r t a i n o r g a n o m e t a l l i c s can be an­ chored t o t h e s u r f a c e s of s u p p o r t s such a s , o x i d e s , c a r b o n s , p o l y ­ mers ( 1 ) . However*the placement of m e t a l g u e s t s i n t o the i n t e r n a l v o i d s of z e o l i t e h o s t s u s i n g o r g a n o m e t a l l i c p r e c u r s o r s , i s by com­ p a r i s o n a r a t h e r new f i e l d of i n v e s t i g a t i o n ( 2 - 4 ) . These i n t r a z e o l i t i c g u e s t s may be the o r i g i n a l o r g a n o m e t a l l i c i t s e l f , or some ag­ g l o m e r a t e d and/or o x i d i z e d form, the l a t t e r b e i n g c o n t i n g e n t upon the c h o i c e of z e o l i t e and p r e - and p o s t - t r e a t m e n t c o n d i t i o n s . What­ ever the c h e m i c a l f a t e of the m e t a l , an i n t i m a t e knowledge of the p h y s i c a l l o c a t i o n , geometry, d i s t o r t i o n s and s u p p o r t - i n t e r a c t i o n s of the m e t a l guest i n the z e o l i t e i s a p r e r e q u i s i t e of any subsequent a p p l i c a t i o n of the m e t a l - z e o l i t e c o m p o s i t i o n . In the one-to-one replacement of i n t r a z e o l i t e m e t a l c a t i o n s by ion-exchange l o a d i n g p r o c e d u r e s , the d i s t r i b u t i o n of c a t i o n i c g u e s t s a t i n t e r n a l l a t t i c e s i t e s i s w e l l e s t a b l i s h e d (_5) . However, the i n t r u s i o n of uncharged, o r g a n o m e t a l l i c m o l e c u l e s to the i n t e r n a l l a t t i c e of a z e o l i t e i s not so c l e a r c u t and s p e c i a l p r o c e d u r e s a r e r e q u i r e d to probe the phenomenom. An e l e g a n t r e c e n t example of t h i s i s Schwartz's R h ( n - C H ) / HY system, i n which the p r e s e n c e of supercage Z O - R h ( n - C H ) H s p e c i e s was demonstrated by s i z e s e l e c t i v e o l e f i n hydrogénation and phosphine poisoning experiments; o n l y those o l e f i n s t h a t were s m a l l enough t o pass through the 8A twelve r i n g window and e n t e r the supercage were h y d r o g e n a t e d (6,_7) ; moreover, o n l y those phosphines t h a t were s t e r i c a l l y a c c e s s i b l e to the supercage were a b l e to p o i s o n the a c t i v e 3

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rhodium s i t e (j6,_7) . Some o t h e r approaches which a r e e q u a l l y as e f ­ f e c t i v e f o r e s t a b l i s h i n g t h e outcome o f a z e o l i t i c organometallic i m p r e g n a t i o n a r e d e s c r i b e d i n t h i s paper. A. M e t a l L o a d i n g S t u d i e s :

Bis(toluene)cobalt(0)

B i s ( t o l u e n e ) c o b a l t ( 0 ) , ( d e c o m p o s i t i o n temperature -50°C) was s y n t h e ­ s i z e d by d e p o s i t i n g c o b a l t vapour i n t o l i q u i d t o l u e n e h e l d a t around -90°C i n a m e t a l vapour r o t a r y r e a c t o r ( 8 ) . The s o l u t i o n c o n t a i n i n g (C H CH3)2Co was t r a n s f e r r e d a t -80°C to~"a S c h l e n k tube h e l d a t t h e same temperature. E q u a l volume a l i q u o t s o f ( C e H s C H s ^ C o were then t r a n s f e r r e d t o e q u a l w e i g h t s o f d e h y d r a t e d sodium and a c i d forms o f f o u r d i f f e r e n t f a u j a s i t e samples h a v i n g S i / A l r a t i o s o f 1.25, 2.5, 3.8, 6.3 ( a c i d o n l y ) and 18.0 (sodium o n l y ) . F o l l o w i n g a 36 hour i m p r e g n a t i o n , each sample was a n a e r o b i c a l l y f i l t e r e d a t -80°C, washed w i t h -80°C t o l u e n e brought t o room temperature s l o w l y under a dynamic vacuum, and f l u s h e t o l u e n e (GC a n a l y s i s ) . Th mined by NAA ( n e u t r o n a c t i v a t i o n a n a l y s i s ) and t h e r e s u l t s a r e d e ­ p i c t e d g r a p h i c a l l y i n F i g u r e 1. The key f e a t u r e s o f t h e s e d a t a a r e t h a t ( i ) t h e Co l o a d i n g o f the sodium f a u j a s i t e NàFAU i s e s s e n t i a l l y i n v a r i a n t t o t h e S i / A l r a t i o , ( i i ) t h e Co l o a d i n g o f t h e a c i d f a u j a s i t e HFAU analogues a r e c o n s i s t e n t l y h i g h e r than t h e i r sodium c o u n t e r p a r t s and m o n o t o n i c a l l y increase with the S i / A l r a t i o . An e x p l a n a t i o n o f these o b s e r v a t i o n s takes t h e f o l l o w i n g çack. The m o l e c u l a r dimensions e s t i m a t e d f o r ( 0 Η 0 Η ) Co, 5.85x7.08A ( 9 ) a r e such t h a t t h e r e i s no s t e r i c b a r r i e r t o passage through t h e 1 2 - r i n g window o f f a u j a s i t e , h a v i n g a k i n e t i c d i a m e t e r o f 8.1 A ( 1 0 ) . The r e s u l t s f o r sodium and a c i d f a u j a s i t e impregnated w i t h (C6HsCH3)2Co a r e f a s c i n a t i n g . Both show t h e p r e s e n c e o f c o b a l t b u t t h e t r e n d s are q u i t e d i s t i n c t . L e t us f i r s t c o n s i d e r t h e sodium forms. Here we know from C nmr, and n e u t r o n d i f f r a c t i o n (11-12) s t u d i e s t h a t a r o m a t i c s r e s i d e o v e r N a s i t e I I c a t i o n s i n t h e supercage (C3V axial interactions) of Y z e o l i t e . We a r e a l s o i n f o r m e d from c r y s t a l ­ l o g r a p h i c and FT-FAR-IR s t u d i e s (13) , t h a t t h e number o f a c c e s s i b l e Na s i t e I I c a t i o n s d i m i n i s h e s a t t h e expense o f i n a c c e s s i b l e N a s i t e I c a t i o n s , as t h e f a u j a s i t e s become more s i l i c i o u s . Concomi­ t a n t l y , t h e b a s i c i t y o f t h e l a t t i c e d e c r e a s e s w h i l e t h e hydrophob i c i t y increases. Thus one c a n argue t h a t d i m i n i s h i n g i n t e r a c t i o n s of t h e N a s i t e I I c a t i o n s w i t h t h e e l e c t r o n d e n s i t y o f t h e t o l u e n e l i g a n d s o f ( C R H S C H S ) 2 C 0 , ( o r d e c r e a s i n g Van d e r Waal i n t e r a c t i o n s w i t h t h e l a t t i c e oxygens) a r e a p p r o x i m a t e l y c o u n t e r b a l a n c e d by t h e i n c r e a s i n g hydrophobicity of the f a u j a s i t e s with i n c r e a s i n g S i / A l r a t i o s ; hence a r a t i o n a l e f o r t h e e s s e n t i a l l y i n v a r i a n t c o b a l t l o a d ­ i n g w i t h i n c r e a s i n g s i l i c o n c o n t e n t i n sodium f a u j a s i t e s impregnated with b i s ( t o l u e n e ) c o b a l t ( 0 ) . Thus t h e B r o n s t e d a c i d i t y and h y d r o ­ p h o b i c i t y o f t h e Η-faujasites b o t h i n c r e a s e w i t h t h e S i / A l r a t i o , p r o v i d i n g s o l v e n t c o m p a t i b i l i t y , f a v o u r a b l e i n t e r a c t i o n s between t h e supercage a c i d i c p r o t o n s and t h e charge d e n s i t y o f t h e c o o r d i n a t e d t o l u e n e , as w e l l as o x i d a t i o n o f t h e (C6H5CH3) Co(0) t o s t r o n g l y bound supercage l o c a t e d s i t e I I and I I I C o cations (optical ref­ l e c t a n c e and FT-FAR-IR s p e c t r o s c o p y , see l a t e r ) . These c h e m i c a l p o t e n t i a l s work t o g e t h e r t o enhance t h e c o b a l t l o a d i n g o f t h e a c i d f a u j a s i t e s with i n c r e a s i n g s i l i c o n content. 6

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M e t a l l o a d i n g s t u d i e s of t h i s type s e r v e to e n l i g h t e n the f a c ­ t o r s which c o n t r o l the i m p r e g n a t i o n of o r g a n o m e t a l l i c s i n t o z e o l i t e s , as w e l l as p r o v i d i n g an i n d i r e c t probe of i n t e r n a l v e r s u s e x t e r n a l l o c a t i o n o f the metal g u e s t s . I n the f o l l o w i n g , a d i r e c t s p e c t r o ­ s c o p i c probe o f t h i s k i n d o f p r o c e s s i s p r e s e n t e d . B. D i r e c t Probe FT-FAR-IR S p e c t r o s c o p y : D i c o b a l t O c t a c a r b o n y l I n a number o f r e c e n t p u b l i c a t i o n s (14-19) we demonstrated the g r e a t u t i l i t y o f the 30-350 cm"! r e g i o n o f the f a r - I R spectrum of f a u j a s i t e and A - t y p e z e o l i t e s f o r i d e n t i f y i n g m e t a l c a t i o n s and e s t a b l i s h i n g t h e i r s i t e d i s t r i b u t i o n s i n the l a t t i c e . The i m p r e s s i v e power o f the method stems from the r e a l i z a t i o n t h a t a b s o r p t i o n s between 30250 cm~l a r e e s s e n t i a l l y m e t a l l o c a l i z e d modes, whose number, f r e ­ q u e n c i e s and i n t e n s i t i e s a r e s t r a i g h t f o r w a r d s i g n a t u r e s of m e t a l c a t i o n t y p e , s i t e symmetry s i t e l o c a t i o n and p o p u l a t i o n ( 2 0 ) For d e t a i l s of t h i s work th Thus i n F i g u r e s 2A and 2B l o c a t e d Na+ i o n s i t e s of NaY ( I I , I , I , I I I , ) and NaA (Α, Ε , H) a r e e a s i l y i d e n t i f i e d , as i n d i c a t e d by the r e s p e c t i v e l e t t e r d e s i g n a ­ t i o n s i n the F i g u r e s . A b s o r p t i o n s to h i g h e r energy than 250 airf­ a r e a s s i g n e d to pore opening modes (21) and w i l l not be f u r t h e r d i s ­ cussed here. Bands marked w i t h an a s t e r i s k denote the IR a c t i v e symmetric c o u n t e r p a r t N a i o n modes t h a t accompany the more i n t e n s e asymmetric modes i n d i c a t e d i n F i g u r e s 2A, 2B. B e f o r e d e s c r i b i n g the Co2(CO) i m p r e g n a t i o n experiments some p r e l i m i n a r y remarks a r e ne­ c e s s a r y c o n c e r n i n g the assignment of r e s i d u a l N a i o n modes i n de­ h y d r a t e d sodium and a c i d f a u j a s i t e s and Α-zeolites w i t h i n c r e a s i n g Si/Al ratio. A summary of the f a r - I R d a t a f o r such samples i s shown i n F i g u r e s 3 and 4. From these t r a c e s , i t i s p o s s i b l e t o a s ­ sess c a t i o n s i t e o c c u p a n c i e s and e s t i m a t e r e l a t i v e s i t e p o p u l a t i o n s as the sodium, B r o n s t e d a c i d and s i l i c o n c o n t e n t i s a l t e r e d w i t h i n a f a u j a s i t e and an Α-zeolite s e r i e s . The f a r - I R s p e c t r a of the C o + ion-exchanged v e r s i o n s of the above samples a r e e q u a l l y i n f o r m a t i v e concerning C o s i t e l o c a t i o n s . Some of t h i s d a t a i s summarized i n F i g u r e 5 and 6 which show the e f f e c t of i n c r e a s i n g Co l o a d i n g and S i / A l r a t i o , on the d i s t r i b u t i o n s of C o i o n s i t e s i n a s e r i e s of sodium f a u j a s i t e s . An a c c u m u l a t i o n of and e v a l u a t i o n o f d a t a of t h i s k i n d a l l o w s one to ( i ) p i n p o i n t the c h a r a c t e r i s t i c f a r - I R s i g ­ natures of Co + ions i n s i t e s I I , I, I I I and I , ( i i ) i d e n t i f y residual Na i o n s i t e s , ( i i i ) e s t a b l i s h any o v e r l a p s between the a b s o r p t i o n s o f C o + and N a s i t e II cations (iv) evaluate s i t e pre­ f e r e n c e s and d i s t r i b u t i o n s f o r Co + and N a i o n s and (v) probe c r y s ­ t a l - f i e l d e f f e c t s and m e t a l - s u p p o r t i n t e r a c t i o n s f o r C o sites. The r e a d e r i s r e f e r r e d to the o r i g i n a l papers f o r d e t a i l s of the above (13-20). 1

+

8

+

2

2 +

2 +

2 +

2

1 1

1

+

2

+

2

+

2 +

With a l l o f these p r e l i m i n a r i e s a t hand, l e t us now move to the q u e s t i o n of probing i n t e r n a l versus e x t e r n a l metal guests, f o l l o w i n g f o r example, s u b l i m a t i o n o f Co2(C0)e i n t o f a u j a s i t e and A - z e o l i t e s u s i n g FT-FAR-IR s p e c t r o s c o p y . The m o l e c u l a r dimensions of C o ( C 0 ) a r e e s t i m a t e d to be 9.76 χ 6.29 χ 5.45 A f o r the n o n - b r i d g e d form and 8.72 χ 5.95 χ 5.92 A f o r the b r i d g e d isomer (31) ; hence t h i s m o l e c u l e s h o u l d be e x c l u d e d from e n t e r i n g A z e o l i t e s but s h o u l d be a b l e to e n t e r the pores o f f a u j a s i t e z e o l i t e s . C o n s i d e r f i r s t , the f a r - I R spectrum of dehydrated N a H Y d e p i c t e d i n F i g u r e 7. Because 2

9

4 7

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

8

216

EXCITED STATES AND REACTIVE INTERMEDIATES

350

300

250

200

150

Ï5Ô

50

Wavenumber

F i g u r e 2A. I n s i t u FT-FAR-IR spectrum o f vacuum t h e r m a l l y dehy­ d r a t e d Na 6Y. I n s e r t shows t h e f a u j a s i t e u n i t c e l l , framework oxygen numbering and c a t i o n s i t e d e s i g n a t i o n s . (Asterisks refer to symmetric m e t a l c a t i o n modes - see t e x t ) . 5

350

300

250 200 150 Wavenumber

100

50

F i g u r e 2B. I n s i t u FT-FAR-IR spectrum o f vacuum t h e r m a l l y dehy­ drated Na A. I n s e r t shows the Α-zeolite pseudo u n i t c e l l and cation s i t e designations. 1 2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

217

Intrazeolite Organometallics

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IT

2§Q

2CÔ

Ï3Ô T5c ic" '*AvENUM9EP

F i g u r e 3. I n s i t u FT-FAR-IR spectrum o f vacuum t h e r m a l l y dehy­ d r a t e d NaFAU f o r S i / A l = 1.25, 2.5 and 3.8. Sodium c a t i o n s i t e s a r e d e s i g n a t e d , I I , I , I I I and I . 1

350

250 '50 Wavenumbor

300

200 Wavonumbor

T

50

100

350

250 '50 Wavonumber

3C0

200

50

Ό"

Wavonumber

F i g u r e 4. I n s i t u FT-FAR-IR s p e c t r a o f NHi+NaZ ( l o w e r ) and HNaZ (upper) f o r (A) z e o l i t e A (B-B) f a u j a s i t e , S i / A l = 1 . 2 5 , 2.5, 3.8. (F r e p r e s e n t s a framework mode, A i s a supercage NH^ c a t i o n mode and S a r e r e s i d u a l sodium c a t i o n modes). +

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

218

EXCITED STATES AND REACTIVE INTERMEDIATES

300

25

F i g u r e 5. I n s i t u FT-FAR-IR s p e c t r a o f Co + ion-exchanged N a Y c o n t a i n i n g 6, 14 and 17 Co + c a t i o n s p e r u n i t c e l l . (S r e p r e s e n t s a r e s i d u a l N a s i t e I I c a t i o n mode, I , I I I and I ' d e s i g n a t e t h e r e s p e c t i v e C o + c a t i o n s i t e modes. H i g h e r l o a d i n g s t u d i e s show t h a t s i t e I I C o + l i e s i n t h e same r e g i o n as s i t e I I N a ) . 5 6

2

+

1 1

2

2

+

RED SHIFTING Co

zee

:sc

(D

iqç

2

F i g u r e 6. I n s i t u FT-FAR-IR s p e c t r a o f C o + ion-exchanged NaFAU ( S i / A l = 1 . 2 5 , 2.5, 3 . 8 ) . Note the r e d - s h i f t i n g o f t h e C o site I and I modes w i t h i n c r e a s i n g s i l i c o n c o n t e n t ( s e e t e x t ) . 2 +

T

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Intrazeolite Organometallics

OZIN AND GODBER

300

250

200

'SO

00

50

Wavenumbof

Figure 7 . I n s i t u FT-FAR-IR s p e c t r a o f (A) vacuum t h e r m a l l y deamminated/dehydrated NagHi+yY showing r e s i d u a l sodium s i t e s , (B-C) t h e outcome o f exposure t o C o ( C 0 ) vapour f o r 5 and 1 5 minutes r e s p e c t i v e l y , showing t h e f o r m a t i o n o f a c c e s s i b l e Co ' s i t e I I and I I I cations (see t e x t ) . 2

8

2

1

1

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

220

of the low Na+ i o n c o n t e n t o f t h i s sample, the spectrum i s d i s p l a y e d on a x5 o r d i n a t e s c a l e e x p a n s i o n . The p r e s e n c e of r e s i d u a l Na+ i o n s i n s i t e I I and I* i s q u i t e a p p a r e n t , a l t h o u g h the band w i d t h s a r e such t h a t they do not u n e q u i v o c a l l y e x c l u d e the c o n t r i b u t i o n of some Na i o n s i n s i t e s I and I I I . F o l l o w i n g a c o n t r o l l e d i n s i t u s u b l i m a t i o n of C o ( C 0 ) i n t o the N a H Y z e o l i t e w a f e r , the f a r - I R spectrum f o r i n c r e a s i n g l o a d i n g s of C o ( C 0 ) c l e a r l y d e p i c t the growth of two new a b s o r p t i o n s a t 180 and 145 cm"-'-, f l a g g i n g the g e n e r a t i o n of C o + c a t i o n s o c c u p y i n g s i t e I I and I I I ' i n the supercage of the f a u j a s i t e . (Note t h a t i n sepa­ r a t e experiments, C o ( C O ) s u b l i m e d onto p o l y e t h y l e n e and ALPO-5 shows no a b s o r p t i o n s of any i n t e n s i t y i n the 350-30 cm~l r e g i o n of the f a r - I R ) . The sample changes from w h i t e t o b l u e f o l l o w i n g the i m p r e g n a t i o n of the C o ( C 0 ) i n t o NagHi+yY i n d i c a t i n g the p r e s e n c e o: C o + i o n s w i t h i n the z e o l i t e . The o b s e r v a t i o n of C o i C O K " i n the c o r r e s p o n d i n g mid-IR experiment s u g g e s t s the f o l l o w i n g i n t r a z e o l i t i * redox c h e m i s t r y : C o ( C 0 ) S i m i l a r experiments performe C o ( C 0 ) a d s o r p t i o n - i n d u c e d c a t i o n v i b r a t i o n a l s h i f t s ( F i g u r e s 8A,B' comparable t o t h o s e r e p o r t e d f o r the a d s o r p t i o n of p y r i d i n e on the same f a u j a s i t e samples (13-22). I n c o n t r a s t , the s u b l i m a t i o n of Co (CO) onto sodium and a c i d Α-zeolites ( t h e former h a v i n g a k i n e ­ t i c d i a m e t e r o f 3.9A (10)) has e s s e n t i a l l y no e f f e c t on the f a r - I R spectra. The above o b s e r v a t i o n s a r e s i g n i f i c a n t f o r a number of r e a s o n s . F i r s t l y , they u n e q u i v o c a l l y demonstrate t h a t C o ( C 0 ) e n t e r s the supercages of NagHi+yY, f a c i l i t a t i n g i n t r a z e o l i t i c o x i d a t i o n by the a c i d s i t e s , to Co + i o n s l o c a t e d " e x c l u s i v e l y " i n a c c e s s i b l e s i t e s I I and I I I . T h i s i s t o be s h a r p l y c o n t r a s t e d w i t h C o ionexchanged and [ C o ( N H ) ] ion-exchanged/deamminated/autoreduced f a u j a s i t e samples ( t h e l a t t e r shown i n F i g u r e 9) which c l e a r l y i l ­ l u s t r a t e the p r e f e r e n c e of Co + i o n s f o r b o t h a c c e s s i b l e ( I I , I I I ) and i n a c c e s s i b l e ( I , I ) s i t e s , l o c a t e d throughout the z e o l i t e l a t ­ tice. Thus the C o ( C 0 ) e/NagHi+yY p r e p a r a t i o n y i e l d s a NaCoHY z e o l i t e i n which a l l o f the C o s i t e s a r e a c c e s s i b l e t o r e a g e n t s , whereas the [ C o ( N H ) ] + / N a Y r o u t e y i e l d s a NaCoHY z e o l i t e w i t h some of the C o + s i t e s l o c a t e d i n l a t t i c e r e g i o n s i n a c c e s s i b l e t o most c h e m i c a l r e a g e n t s , (namely s i t e s I , I ) . I n c a t a l y t i c a p p l i c a t i o n s f o r example, such d i f f e r e n c e s can o f t e n be e x p l o i t e d t o advantage. A second p o i n t t h a t emerges from t h i s s t u d y , i s t h a t i n t r a z e o l i t i c o x i d a t i o n of C o ( C 0 ) i s e n j o y e d o n l y by NagH^yY and not by N a Y , C o i N a Y , NaA o r HA. F o r the N a Y sample i n t r u s i o n o f the C o ( C O ) guest i n t o the supercage o f the f a u j a s i t e i s f l a g g e d by the o b s e r ­ ved f a r - I R a d s o r p t i o n i n d u c e d v i b r a t i o n a l s h i f t s of the N a ions. By c o n t r a s t , the e x c l u s i o n of C o ( C 0 ) from the i n t r a z e o l i t i c v o i d s of NaA and HA i s seen by i n v a r i a n c e of the f a r - I R c a t i o n s p e c t r a of t h e s e samples, when exposed t o the vapour o f C o ( C 0 ) . +

f

2

9

8

1+7

2

8

2

1

2

8

2

8

2

2

2

8

2

8

2

8

2

1 1

2 +

3 +

3

6

2

1 1

1

2

2 +

3

3

6

2

1

2

8

5 6

7

5 6

2

8

+

2

8

2

Thus by u s i n g C o ( C 0 ) as a r e p r e s e n t a t i v e and a c o m b i n a t i o n of s i z e e x c l u s i o n experiments d a t i o n c h e m i s t r y , as probed through the f a r - I R sodium and a c i d f a u j a s i t e s and Α-zeolites, one u i s h between i n t e r n a l and e x t e r n a l confinement i n the z e o l i t e . 2

8

8

organometallic guest, and i n t r a z e o l i t e o x i ­ c a t i o n s p e c t r a of i s a b l e to d i s t i n g ­ of the m e t a l g u e s t ( s )

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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Β

A

350

300

250

200

150

100

50

Wavenumber F i g u r e 8A. I n s i t u FT-FAR-IR s p e c t r a o f A) vacuum t h e r m a l l y d e ­ h y d r a t e d N a Y and B) t h e outcome o f exposure t o t h e vapour o f C o ( C 0 ) f o r 15 minutes ( s e e t e x t ) . 5 6

2

8

F i g u r e 8B. I n s i t u FT-FAR-IR s p e c t r a o f A) vacuum t h e r m a l l y dehy­ d r a t e d C o N a Y and B) t h e outcome o f exposure t o t h e vapour o f C o ( C 0 ) f o r 15 minutes ( s e e t e x t ) . 1 7

2

2 2

8

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C. O r g a n o m e t a l l i c EPR S p i n Probes I. B i s ( b e n z e n e ) v a n a d i u m ( O ) : S i z e e x c l u s i o n and d i f f u s i o n s t u d i e s . The m o l e c u l a r dimensions o f (n -C6He)2V a r e e s t i m a t e d t o be 3.88A χ 5.24Â, ( 2 3 ) , hence i t s h o u l d be e s s e n t i a l l y e x c l u d e d from e n t e r i n g the supercages o f Α-zeolite b u t n o t Y - z e o l i t e . The c h o i c e o f (n -CeH6)2V f o r these z e o l i t e i m p r e g n a t i o n s t u d i e s was a l s o p r e d i ­ c a t e d on i t s i d e a l e p r p r o p e r t i e s , t h a t i s , a n o n - d e g e n e r a t e , t o t a l l y symmetrical A g e l e c t r o n i c ground s t a t e and a h i g h l y d i a g n o s t i c p a t t e r n o f V ( I = 7 / 2 ) h y p e r f i n e and H ( I = 1/2) s u p e r h y p e r f i n e l i n e s , whose c o u p l i n g c o n s t a n t s and l i n e shapes a r e q u i t e r e v e a l i n g as t o i t s i n t e r a c t i o n s w i t h i t s immediate s u r r o u n d i n g s , as w e l l as i t s r i n g and whole-molecule dynamical m o t i o n s . F o r d e t a i l s o f t h i s work t h e r e a d e r i s r e f e r r e d t o t h e o r i g i n a l .papers (24) . As a b r i e f preamble t o the e p r z e o l i t e work t h a t f o l l o w s , one f i n d s t h a t (n -C(>H6) 2V i n weakly i n t e r a c t i n g s o l v e n t s l i k e pentane, shows a t room temperature an i s o t r o p i t i a l l y r e s o l v e d superimpose 10). I n t h e r i g i d l i m i t below 123K t h i s e p r spectrum d r a m a t i c a l l y t r a n s f o r m s t o one d i s p l a y i n g a x i a l symmetry, w i t h f u l l r e s o l u t i o n of a 1 3 - l i n e i s o t r o p i c p r o t o n s u p e r h y p e r f i n e p a t t e r n on each o f t h e 8 p e r p e n d i c u l a r V - h y p e r f i n e l i n e s ( F i g u r e 10). The p a r a l l e l V-hyperf i n e components i n t h i s case have been proven t o have a much s m a l l e r s p l i t t i n g and g i v e r i s e t o t h e s m a l l c e n t r a l f e a t u r e i n t h e e p r spectrum ( 2 4 ) . The m a g n e t o g y r i c g, h y p e r f i n e a , and s u p e r h y p e r f i n e aH parameters which emerge from a l i n e shape s i m u l a t i o n o f t h e r i g i d l i m i t e p r spectrum a r e l i s t e d below: b

6

2

2

5 1

1

6

v

g

xx

g

1.9810G

yy

1.9857G

ν a

x

ν

91.4955G 6.733G

Η a

1 Η

4.264G 4.264G

Thus t h e room temperature spectrum i n pentane c h a r a c t e r i z e s a f r e e l y tumbling ( n - C H ) V m o l e c u l e , w i t h c o o r d i n a t e d benzene r i n g r o t a ­ t i o n , b u t e x p e r i e n c i n g some e l e c t r o n s p i n exchange l i n e b r o a d e n i n g due t o f r e q u e n t m o l e c u l a r c o l l i s i o n s . I n t h e r i g i d l i m i t below 123K the a x i a l n a t u r e o f (η -0βΗ6)2ν i s r e v e a l e d i n t h e powder spectrum of t h e f r o z e n s o l i d ; however, t h e m o l e c u l e i s t r a p p e d i n a v o i d o f s o l i d pentane o f s u f f i c i e n t l y s p a c i o u s dimensions t h a t r i n g r o t a t i o n i s s t i l l p e r m i t t e d , as seen by t h e magnetic e q u i v a l e n c y o f t h e 12 p r o t o n s o f t h e c o o r d i n a t e d benzenes. The i n t e r m e d i a t e temperature regime 300-123K has been i n v e s t i g a t e d i n d e t a i l i n pentane ( F i g u r e 10) as w e l l as more s t i c k y s o l v e n t s l i k e a r o m a t i c s and e t h e r s ( 2 4 ) . I n pentane, t h e e f f e c t o f l o w e r i n g t h e temperature (synonymous w i t h i n c r e a s i n g v i s c o s i t y ) i s seen f i r s t as an enhancement o f t h e r e s o l u ­ t i o n o f t h e p r o t o n s u p e r h y p e r f i n e ( m o l e c u l a r tumbling w i t h d i m i n i s h ­ ed e l e c t r o n s p i n exchange b r o a d e n i n g ) and then as t h e appearance o f 6

6

6

2

6

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F i g u r e 10. Temperature dependent e p r s p e c t r a o f (n -C H6)2V i n pentane over t h e range 300-123K (see r e f e r e n c e 2A_ f o r d e t a i l s ) . 6

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a n i s o t r o p y ( d i f f e r e n t l i n e w i d t h s ) on t h e V - h y p e r f i n e l i n e s ( v i s c o u s drag on t h e t u m b l i n g dynamics). A t around 138K t h e c o a l e s c e n c e regime i s met, below which t h e a x i a l r i g i d l i m i t spectrum p e r s i s t s ( F i g u r e 10). A g g r e g a t i o n o f (n -C6H6)2V i n s o l i d pentane i s d i a g ­ nosed by t o t a l c o l l a p s e o f t h e e i g h t vanadium h y p e r f i n e components ( d i p o l e - d i p o l e b r o a d e n i n g ) t o a s i n g l e l i n e c e n t r e d around g = 2 (more pronounced on slow c o o l i n g than r a p i d c o o l i n g - see F i g u r e 10). The e f f e c t o f more s t r o n g l y i n t e r a c t i n g s o l v e n t s i s m a n i f e s t as p a r t i a l o r complete l o s s o f p r o t o n s u p e r h y p e r f i n e s p l i t t i n g , t h e o b s e r v a t i o n o f unequal l i n e widths f o r t h e V - h y p e r f i n e l i n e s and t h e a t t a i n m e n t o f t h e r i g i d l i m i t a x i a l spectrum a t h i g h e r temperatures than those observed f o r pentane under i d e n t i c a l c o n c e n t r a t i o n con­ ditions. These e f f e c t s a r e t r a c e a b l e t o g r e a t e r s o l u t e - s o l v e n t i n ­ t e r a c t i o n s i n a r o m a t i c and e t h e r media, and t h e concomitant p e r t u r ­ b a t i o n s o f t h e r i n g and m o l e c u l e dynamics. With t h i s background a t hand we a r e now i n a p o s i t i o n t o a s s e s s the outcome o f i m p r e g n a t i n F o r these m a t e r i a l s we w i l a d i a g n o s t i c of the extent of impregnation of (n -C H )2V i n t o the supercages and t h e l i n e shapes as an i n d i c a t o r o f t h e dynamical p r o ­ p e r t i e s (and p o s s i b l e l o c a t i o n ) o f supercage e n c a p s u l a t e d 6

6

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(n -c H ) v. 6

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C o n s i d e r f i r s t t h e room temperature pentane i m p r e g n a t i o n o f ( n - C H ) 2 V i n t o NaY f o l l o w e d by a thorough pentane wash. The e p r spectrum was r e c o r d e d i n t h e range 300-123K and showed l i t t l e change i n l i n e shape. F e a t u r e s o f i n t e r e s t t o t h e p r e s e n t study i n c l u d e ( i ) t h e o b s e r v a t i o n o f a r i g i d l i m i t spectrum a t room temperature ( F i g u r e 11A) which i s u n a f f e c t e d on c o o l i n g t o 123K ( F i g u r e 11B), ( i i ) the loss o f proton superhyperfine coupling, ( i i i ) adsorption of oxygen o r water (38) i r r e v e r s i b l y changes t h e spectrum t o one c h a ­ r a c t e r i s t i c o f t h e v a n a d y l i o n V 0 , ( F i g u r e 11C), ( t h e low i n t e n ­ s i t y and bowed b a s e l i n e i n t h i s f i g u r e i s an i n h e r e n t p r o p e r t y o f the 0 / H 0 o x i d a t i o n p r o d u c t ( 3 8 ) ) , ( i v ) a d s o r p t i o n o f weakly i n ­ t e r a c t i n g s o l v e n t s l i k e t o l u e n e o r pentane t o t h e sample i n F i g u r e 11A have no e f f e c t on t h e spectrum, ( F i g u r e 11D). Taken t o g e t h e r w i t h t h e o b s e r v a t i o n t h a t a s i m i l a r room temperature treatment o f NaA w i t h (η -06Ηβ)2V/pentane y i e l d s a v e r y much l e s s i n t e n s e spec­ trum (y2% o f NaY) s u g g e s t i v e o f r e s i d u a l e x t e r n a l s u r f a c e c o n f i n e ­ ment o f ( n - C H ) V , we c a n draw t h e f o l l o w i n g c o n c l u s i o n s : (a) ( n - C H ) V e n t e r s t h e supercages o f NaY b u t n o t NaA; (b) ( n - C H ) V i s r i g i d l y l o c k e d i n t h e supercage o f NaY e i t h e r by way of N a s i t e I I c a t i o n s o r l a t t i c e oxygen i n t e r a c t i o n s w i t h t h e c o ­ o r d i n a t e d benzenes o r t h e vanadium atom i t s e l f . I n t e r e s t i n g l y , the Na+ c a t i o n - a r e n e r i n g i n t e r a c t i o n model i s r o u g h l y i n l i n e w i t h ( i ) the d i s t a n c e between the c e n t r e s o f benzene adsorbed onto s i t e I I Na c a t i o n s i n Y z e o l i t e (25) and t h e vanadium-benzene r i n g d i s t a n c e s ( 1 . 6 6 Â ) t h e r e b y a b l e t o f i x ( η - 0 Η ) ν i n the supercage (ii) t h e l o s s o f p r o t o n s u p e r h y p e r f i n e s p l i t t i n g , through t h e N a -benzene(or l a t t i c e oxygen) i n t e r a c t i o n s r e s t r i c t i n c j t h e f r e e r o t a t i o n o f t h e c o ­ o r d i n a t e d benzenes, thereby c r e a t i n g m a g n e t i c a l l y i n e q u i v a l e n t r i n g p r o t o n s (3 s e t s o f 4 e q u i v a l e n t p r o t o n s ; ENDOR experiments a r e planned t o t e s t t h i s i d e a ) w i t h concomitant l i n e b r o a d e n i n g and l o s s of r e s o l u t i o n and ( i i i ) t h e o b s e r v a t i o n o f an a x i a l l i m i t e p r spec6

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trum w i t h a s l i g h t l y i n c r e a s e d aj h y p e r f i n e c o u p l i n g compared t o ( n - C H ) V i n pentane. The o b s e r v a t i o n ( F i g u r e 11D) t h a t weakly i n t e r a c t i n g s o l v e n t s l i k e t o l u e n e o r pentane do n o t p e r t u r b t h e spectrum shown i n F i g u r e 11A i s i n l i n e w i t h t h e h y p o t h e s i s o f a s t r o n g i n t e r a c t i o n between the z e o l i t e and ( n - C H ) V . I t might be e x p e c t e d t h a t a d s o r p t i o n of t o l u e n e o r pentane would s e r v e t o s o l v a t e t h e ( n - C H ) V and r e l e a s e i t from i n t e r a c t i o n w i t h t h e w a l l s o f t h e z e o l i t e . 5

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In a s e r i e s o f s e p a r a t e experiments t h e e f f e c t o f i m p r e g n a t i n g e q u a l w e i g h t s o f d e h y d r a t e d NaY w i t h ( n - C H 6 ) V / p e n t a n e solution h a v i n g i n c r e a s i n g c o n c e n t r a t i o n s o f ( n - C H ) V was i n v e s t i g a t e d u s i n g t h e above type of s p i n - p r o b e e p r method; one o f t h e most w e l l d e f i n e d V - h y p e r f i n e component l i n e s (unchanging band width) was used as a measure o f t h e s p i n c o n c e n t r a t i o n o f supercage i m m o b i l i z e d (r| -C6H6) V. The r e s u l t 12 f o r t h e a n a e r o b i c p r o c e d u r a 2 hour i m p r e g n a t i o n time From t h e s e d a t a one c a n deduce t h a t t h e r e e x i s t s a p r o p o r t i o n a t e r e l a t i o n s h i p between t h e c o n c e n t r a t i o n o f ( n - C H ) V i n t h e pentane phase and t h e amount o f i n t r a z e o l i t i c ( n - C H ) V . C l e a r l y under t h e s e i m p r e g n a t i o n c o n d i t i o n s t h e r e appears t o e x i s t no a p p r e c i a b l e d i f f u s i o n a l b a r r i e r t o t h e e n t r y o f ( n - C 6 H e ) V i n t o t h e supercages of NaY. Thus by u s i n g o r g a n o m e t a l l i c e p r s p i n p r o b e s , s i z e e x c l u s i o n and d i f f u s i o n e x p e r i m e n t s , one i s a b l e t o d i s t i n g u i s h i n a f a i r l y c o n ­ v i n c i n g way between i n t e r n a l and e x t e r n a l confinement o f t h e m e t a l guest i n t h e z e o l i t e . 6

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I I . B i s ( c y c l o p e n t a d i e n y l ) c h r o m i u m ( I I ) ; S i z e E x c l u s i o n and I n t r a z e o ­ l i t e Chemistry The c h o i c e o f ( n . - C H ) C r as an e p r s p i n probe i n t h e s e a p p l i c a t i o n s , o f f e r s numerous o p p o r t u n i t i e s f o r i n v e s t i g a t i n g t h e u l t i m a t e form and l o c a t i o n o f t h g m e t a l g u e s t . I t s m o l e c u l a r dimensions a r e e s ­ t i m a t e d t o be 4.34A χ 4.44A (26) and so i t s h o u l d be a u s e f u l c a n ­ d i d a t e f o r s i z e e x c l u s i o n e x p e r i m e n t s i n f a u j a s i t e and A - z e o l i t e s . The chromium i s f o r m a l l y i n o x i d a t i o n s t a t e +2 w i t h an e l e c t r o n i c ground s t a t e term o f E 2 g ( 2 7 ) . I n t h i s form t h e m o l e c u l e has a s c a r l e t hue and has a c h a r a c t e r i s t i c e l e c t r o n i c spectrum showing U V - v i s i b l e l i g a n d - f i e l d and UV c h a r g e - t r a n s f e r e x c i t a t i o n s and a D j c e n t r o s y m m e t r i c sandwich s t r u c t u r e a s shown i n Scheme I . In f r o z e n pentane g l a s s , down t o 123K, ( n - C H ) C r d i s p l a y s a v e r y b r o a d almost i s o t r o p i c e p r l i n e (g = 3.89,Δ Η = 3400 G) (28) which a t room temperature broadens even f u r t h e r , almost t o d i s a p ­ pearance. This t y p i f i e s the epr behaviour of a f i r s t t r a n s i t i o n s e r i e s complex h a v i n g a h i g h s p i n , o r b i t a l l y d e g e n e r a t e e l e c t r o n i c ground s t a t e , a n d a r e s u l t i n g s h o r t Τ s p i n l a t t i c e r e l a x a t i o n t i m e . Under t h e s e c o n d i t i o n s t h e J a h n - T e l l e r i n s t a b i l i t y o f ( n - C H ) C r c o u l d be a dynamic t y p e . However, when e n t r a p p e d i n t h e supercage o f a z e o l i t e one c a n a n t i c i p a t e a q u i t e d i f f e r e n t s t a t e o f a f f a i r s f o r the e p r p r o p e r t i e s o f chromocene. Consider f i r s t the impregnation of r i g o r o u s l y d e h y d r a t e d / c a l c i 5

5

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5(

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χ

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6

F i g u r e 11. EPR s p e c t r a of NaY e n c a p s u l a t e d (n -C H ) V.(pentane i m p r e g n a t i o n ) , r e c o r d e d (A) a t room t e m p e r a t u r e , S c a l e : 2X, (B) a t 123K, S c a l e : IX, (C) a f t e r a d d i t i o n of H 0 vapour a t room temperature and r e c o r d e d at 123K, S c a l e : 10X, (D) sample i n (A) wet w i t h pentane, r e c o r d e d a t room t e m p e r a t u r e , S c a l e : 2X. 6

6

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F i g u r e 12. EPR s p i n probe of the c o n c e n t r a t i o n dependence of the d i f f u s i o n of ( n - C H ) V / p e n t a n e i n t o N a Y (see t e x t ) . 6

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n e d / d e f e c t removed NaY w i t h a pentane s o l u t i o n o f C p C r . After a thorough pentane wash an e p r spectrum a t t r i b u t a b l e t o C p C r c a n be o b s e r v e d ( F i g u r e 13C,D). A t room temperature t h e spectrum c o n s i s t s of a s i n g l e a b s o r p t i o n a t g = 2.084 which i s almost i s o t r o p i c ( F i g u r e 13D). On c o o l i n g t o -150°C, t h e sample e x h i b i t s growth o f a resonance a t g = 4.079, and t h e h i g h f i e l d s i g n a l shows an a x i a l d i s t o t i o n , t h e g v a l u e c h a n g i n g s l i g h t l y t o g = 2.065 ( F i g u r e 13C). When t h e same experiment i s r e p e a t e d b u t w i t h an e q u a l weight o f a c i d z e o l i t e NagH^yY, e p r s i g n a l s c o r r e s p o n d i n g t o o r t h o r h o m b i c Cp Cr a r e o b s e r v e d ( F i g u r e 13B, a l l s p e c t r a a r e on t h e same s c a l e ) (30). A t -150°C t h e e p r spectrum has g = 4.13, g = 3.86 and g = 1.98; when warmed t o room temperature t h e spectrum remains e s s e n t i a l l y t h e same except f o r a d e c r e a s e i n i n t e n s i t y , w i t h r e s o ­ nances = 4.12, g = 3.89, g = 1.98. The m a g n e t o g y r i c (g) t e n s o r and temperature b e h a v i o u r o f t h i s sample i s s i m i l a r t o t h a t observed f o r Cp Cr+ i n other diamagnetic h o s t s except that z e o l i t e encapsulated Cp Cr+ e x h i b i t sharp c o n t r a s t , when C p o n l y v e r y low i n t e n s i t y h i g h and low f i e l d r e s o n a n c e s a r e o b s e r v e d , ( F i g u r e 13A, r u n a t - 1 5 0 ° C ) . T h i s we a t t r i b u t e t o r e s i d u a l , s u r ­ f a c e c o n f i n e d chromocene o r chromicenium. 2

2

+

2

x

x

v

v

z z

g x x

y

y

z

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F i n a l l y we w i s h t o p o i n t o u t t h a t when C p C r i s r e a c t e d w i t h S i 0 , vacuum t h e r m a l l y p r e t r e a t e d a t 400°C, r o u g h l y one c y c l o p e n t a d i e n e l i g a n d i s r e l e a s e d w i t h f o r m a t i o n o f t h e proposed s u r f a c e a n ­ chored o r g a n o m e t a l l i c CpCr-OSIL ( 3 7 ) . While i t i s not i m p o s s i b l e t h a t such a h a l f sandwich C r s p e c i e s c o u l d be r e s p o n s i b l e f o r t h e e s r spectrum o b s e r v e d from i m p r e g n a t i n g C p C r i n t o HY, we f i n d t h a t the o p t i c a l r e f l e c t a n c e spectrum o f t h e l a t t e r s u p p o r t t h e proposed Cp Cr+ZY f o r m u l a t i o n (see a l s o C p F e Z Y i n t h e n e x t s e c t i o n ) . Never­ t h e l e s s , s t u d i e s a r e underway t o f u r t h e r c l a r i f y t h i s i n t r i g u i n g detail. From t h e s e o b s e r v a t i o n s we c a n draw t h e f o l l o w i n g c o n c l u ­ sions : 2

2

1

1

2

+

2

2

5

( i ) ( η - 0 Η ) 0 Γ c a n e n t e r t h e supercages o f NaY and HY b u t i s ex­ c l u d e d from e n t e r i n g NaA. ( i i ) The o b s e r v a t i o n o f a temperature dependent e p r spectrum f o r Cp Cr/NaY, and i t s d i s t i n c t d i f f e r e n c e t o Cp Cr/HY s u g g e s t s t h a t C p C r has p e n e t r a t e d i n t o t h e p o r e s o f NaY and i s l o c k e d i n p l a c e inthe supercage i n a d i s t o r t e d c o n f i g u r a t i o n n o t p r e v i o u s l y a t t a i n e d f o r C p C r , (see Scheme I I ) ; t h e l a t t e r e f f e c t c o u l d be r e s p o n s i b l e f o r i t s o b s e r v a b l e e p r spectrum. F o r such a low symmetry t r i p l e t s t a t e C p C r s p e c i e s one c a n e x p e c t b e s i d e s s h o r t T i r e l a x a t i o n t i m e s , t h a t z e r o f i e l d s p l i t t i n g s , dynamic J a h n - T e l l e r e f f e c t s and AMg=±2 t r a n s i t i o n s can provide a d d i t i o n a l f a c t o r s r e s p o n s i b l e f o r the un­ u s u a l e p r spectrum and i t s temperature s e n s i t i v i t y . The a n a l y s i s of t h e s e f a s c i n a t i n g C p C r e p r s p e c t r a a r e under c o n t i n u i n g study i n our l a b o r a t o r y . ( i i i ) When C p C r i s impregnated i n t o a c i d f a u j a s i t e , i t i s o x i d i z e d to e p r d e t e c t a b l e C p C r which has undergone an o r t h o r h o m b i c d i s ­ t o r t i o n i n i t s supercage l o c a t i o n . (Note t h a t C p C r C p C r + e~ has = 0.55 V v s SCE ( 2 7 ) ) . 5

5

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D. O p t i c a l R e f l e c t a n c e P r o b e : B i s ( c y c l o p e n t a d i e n y l ) i r o n ( I I ) ; E x c l u s i o n and I n t r a z e o l i t e C h e m i s t r y

Size

The o p t i c a l p r o p e r t i e s o f t h e a r c h i t y p i c a l m e t a l l o c e n e C p F e , t h e 2

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

F i g u r e 13. EPR s p e c t r a o f C p C r / p e n t a n e impregnated i n t o ( A ) NaA, r e c o r d e d a t -150°C, (B) NagH^yY, r e c o r d e d a t -150°C, (C-D) N a Y r e c o r d e d a t -150°C and RT r e s p e c t i v e l y (see t e x t ) . 2

5 6

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15.

OZIN AND GODBER

Intrazeolite Organometallics

Scheme I I

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c a t i o n C p F e and complexes c o n t a i n i n g s u b s t i t u t e d r i n g s have been e x t e n s i v e l y s t u d i e d (29). The m o l e c u l a r dimensions of f e r r o c e n e , 4.13 χ 4.20A (26) a r e s u f f i c i e n t f o r u n h i n d e r e d d i f f u s i o n i n t o f a u ­ j a s i t e z e o l i t e s , but are too l a r g e t o p e n e t r a t e A z e o l i t e s . Fer­ rocene has an e l e c t r o n i c ground s t a t e term of &ig and i n pentane has c h a r a c t e r i s t i c a b s o r p t i o n s a t 440 and 325 nm due to d-d t r a n s i ­ t i o n s , and charge t r a n s f e r a b s o r p t i o n s i n the UV a t 265, 240 and 200 nm ( F i g u r e 14A). The f e r r i c e n i u m c a t i o n has an o r b i t a l l y de­ g e n e r a t e ground s t a t e term which has been shown by e s r and magnetic s u s c e p t i b i l i t y measurements t o be E g (32,33). In aqueous s o l u t i o n the c a t i o n e x h i b i t s d-d t r a n s i t i o n s a t 565, 524, 467 and 380 nm, and charge t r a n s f e r a b s o r p t i o n s a t 617, 283 and 251 nm ( F i g u r e 14G). The i n t e n s e charge t r a n s f e r band a t 617 nm i s e s p e ­ c i a l l y d i a g n o s t i c f o r the p r e s e n c e of C p F e + as C p F e has no a b s o r p ­ tions i n this region. The o p t i c a l r e f l e c t a n c e s p e c t r a of the p r o d u c t s o b t a i n e d from a room temperature i m p r e g n a t i o c i n e d / d e f e c t removed NaY 14 B-F, H. F e r r o c e n e i s e x c l u d e d from NaA as e v i d e n c e d by the l a c k of any bands i n the spectrum which c o u l d be a s s i g n e d to e i t h e r C p F e or C p F e . The UV bands t h a t a r e o b s e r v e d a r e due to z e o l i t e 0 -*· Τ (where Τ = A l , S i ) c h a r g e - t r a n s f e r t r a n s i t i o n s . This result also s t r e n g t h e n s our c o n t e n t i o n t h a t C p F e i s c o n t a i n e d w i t h i n the p o r e s of NaY r a t h e r than c o n f i n e d t o the e x t e r n a l s u r f a c e . D i f f u s i o n of C p F e i n t o r i g o r o u s l y p r e t r e a t e d NaY p r o c e e d s w i t h s l i g h t p a r t i a l o x i d a t i o n to the f e r r i c e n i u m c a t i o n , p o s s i b l y v i a Lewis a c i d s i t e s ( t h e samples were c a r e f u l l y d e f e c t removed by washing with NaCl s o l u t i o n and m e t i c u l o u s l y d e h y d r a t e d and c a l c i n e d ) . T h i s i n ­ teresting observation i s c u r r e n t l y under s t u d y . Even w i t h the s m a l l amount of o x i d a t i o n as e v i d e n c e d by the weak C p F e CT band a t 617 nm, the i m p o r t a n t f e a t u r e to be n o t e d from the r e f l e c t a n c e spectrum i s the e x i s t e n c e of an e s s e n t i a l l y u n p e r t u r b e d f e r r o c e n e w i t h i n the c o n f i n e s of the z e o l i t e ; no d i s t o r t i o n of the m o l e c u l e i s a p p a r e n t . I f f e r r o c e n e had been bent one might have e x p e c t e d a l o w e r i n g i n energy of the Α - > t r a n s i t i o n as o b s e r v e d i n (CpCH CH Cp)Fe, i n which the m o l e c u l e i s ' t e t h e r e d i n t o a bent c o n f i g u r a t i o n ( 3 4 ) . Of p a r t i c u l a r i n t e r e s t i s the e f f e c t t h a t a i r exposure has on the Cp Fe/NaY sample, as i l l u s t r a t e d i n F i g u r e s 14C, D; the f e r r o c e n e i s s l o w l y o x i d i z e d (whether i t i s oxygen o r water or b o t h i s s t i l l un­ der i n v e s t i g a t i o n ) t o the f e r r i c e n i u m c a t i o n as r e f l e c t e d by the drop i n i n t e n s i t y of the 440 nm f e r r o c e n e band and the c o n c o m i t a n t growth of the c h a r a c t e r i s t i c 617 nm charge t r a n s f e r a b s o r p t i o n of Cp Fe+. The f a t e of f e r r o c e n e a f t e r i m p r e g n a t i o n i n t o NagHuyY i s e a s i l y d i s c e r n i b l e from the r e f l e c t a n c e spectrum ( F i g u r e 14E) as i t shows e s s e n t i a l l y complete absence o f f e r r o c e n e a b s o r p t i o n s and o n l y f e r r i c e n i u m bands. To c o n f i r m the e x i s t e n c e of [Cp Fe]NaHY we i n ­ d e p e n d e n t l y p r e p a r e d C p 2 F e B F 4 ~ (33) and ion-exchanged i t i n t o Na Y i n aqueous 0.01M HClOi*. The r e s u l t i n g b l u e - g r e e n z e o l i t e , w i t h a c o m p o s i t i o n of [Cp,Fe]NaHY, was washed u n t i l f r e e of p e r c h l o r a t e and d r i e d a t 100 C. The r e f l e c t a n c e spectrum of t h i s sample ( F i g u r e 14F) i n d i c a t e s t h a t C p F e has ion-exchanged i n t o the z e o l i t e l a t t i c e presumably t o occupy a supercage c a t i o n s i t e . R e g a r d i n g the r e a c t i o n of C p F e w i t h an a c i d f a u j a s i t e , i t i s p o s s i b l e to e x c l u d e a s i m p l e a c i d - b a s e r e a c t i o n , y i e l d i n g p r o t o n a t e d 2

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200 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 Wave length (nm) F i g u r e 14. O p t i c a l s p e c t r a o f (A) C p F e / p e n t a n e (ABS), (B) Cp Fe/NaY (REFL), (C-D) t h e same as (B) b u t a f t e r 6 and 24 h o u r s exposure t o a i r (REFL), (E) Cp Fe/H* Na Y (REFL), (F) [ C p F e ] BF4/O.OIM HClOi» ion-exchanged i n t o N a Y (REFL), (G) [Cp Fe]BFif/ 0.01M HCIO^ (ABS), (H) Cp Fe/NaA (REFL). 2

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f e r r o c e n e [ C p F e H ] as a c a n d i d a t e f o r t h e o b s e r v e d p r o d u c t based on the r e f l e c t a n c e spectrum. Thus [Cp FeH]+ e x h i b i t s an o p t i c a l s p e c ­ trum q u i t e s i m i l a r t o f e r r o c e n e , except t h a t t h e A g-> E i g d-d t r a n s i t i o n a t 325 nm i s broadened ( 3 5 ) . Note a l s o t h a t w h i l e " t e t h e r e d " (CpCH CH Cp)Fe, with the r i n g s f i x e d i n a t i l t e d p o s i t i o n can be p r o t o n a t e d by l e s s than 0.1% H2SO4 i n e t h a n o l , C p F e by con­ t r a s t i s u n a f f e c t e d by t h e p r e s e n c e o f f o r example 10% H2SO4 i n ethanol (36). I n t h e l i g h t o f t h i s i n f o r m a t i o n we p o s t u l a t e t h a t the i n t r a z e o l i t i c o x i d a t i o n o f f e r r o c e n e i n a c i d f a u j a s i t e i n v o l v e s an i n i t i a l b e n d i n g back o f t h e r i n g s f o l l o w e d by p r o t o n a t i o n and o x i ­ dation. From a c o m b i n a t i o n o f i n t r a z e o l i t e o x i d a t i o n c h e m i s t r y o f Cp Fe/HY, C p F e ion-exchanged NaY and pentane i m p r e g n a t i o n o f C p F e i n t o NaY and NaA, one c a n deduce t h a t ( i ) C p F e e n t e r s t h e α-cage o f f a u j a s i t e b u t i s e x c l u d e d from t h e i n t e r n a l v o i d s o f NaA; ( i i ) t h e a c i d s t r e n g t h o f NagHi+yY i s s u f f i c i e n t t o o x i d i z e C p F e t o C p F e ( C p F e + + e " ^ ± Cp Fe,E-J = 0.41 V v s SCE ( 2 7 ) ) confirming that i t e n t e r s t h e α-cage; (iii i n t o NaY; ( i v ) C p F e impregnate when exposed t o a i r g r a d u a l l y o x i d i z e s t o C p F e . 2

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O v e r a l l i t i s q u i t e c l e a r , that a combination o f epr spin-probes, U V - v i s r e f l e c t a n c e s p e c t r o s c o p y , s i z e e x c l u s i o n and i n t r a z e o l i t e o x i d a t i o n experiments a r e able t o e f f e c t i v e l y d i f f e r e n t i a t e those o r g a n o m e t a l l i c - z e o l i t e i m p r e g n a t i o n s which p l a c e m e t a l g u e s t s w i t h i n the i n t e r n a l v o i d s o f t h e z e o l i t e compared t o t h o s e on t h e e x t e r n a l surface o f the z e o l i t e l a t t i c e . Conclusions T h i s paper d e s c r i b e s some new z e o l i t e o r g a n o m e t a l l i c i m p r e g n a t i o n e x p e r i m e n t s , i n which a c o m b i n a t i o n o f m e t a l l o a d i n g , s i z e e x c l u s i o n , i n t r a z e o l i t e c h e m i s t r y and d i f f u s i o n c o n s i d e r a t i o n s i n c o n j u n c t i o n w i t h e p r , f a r - I R and U V - v i s i b l e r e f l e c t a n c e s p e c t r o s c o p i c p r o b e s , serve t o d i s t i n g u i s h metal guests l o c a t e d i n the i n t r a c r y s t a l l i n e v o i d s o f t h e z e o l i t e from t h o s e l o c a t e d on t h e e x t e r n a l s u r f a c e . R e p r e s e n t a t i v e o r g a n o m e t a l l i c s (C6H5CH3)2C0, Co2(C0)e, ( η - 0 6 Η ) 2 V , ( n - C 5 H ) 2 C r and ( n - C 5 H s ) 2 F e impregnated i n t o sodium and a c i d f a u j a s i t e s and Α-zeolites were s e l e c t e d f o r t h i s i n i t i a l investigation. The key p o i n t s t o emerge from t h i s study a r e l i s t e d below: a) A l l o f t h e above o r g a n o m e t a l l i c s pass f r e e l y i n t o t h e s u p e r c a g e s o f sodium and a c i d f a u j a s i t e s , whereas they a r e a l l e f f e c t i v e l y s i z e - e x c l u d e d from sodium A - z e o l i t e s . b) The l o a d i n g o f ( C H C H ) 2 C o i n t o sodium f a u j a s i t e s i s e s s e n t i a l l y i n s e n s i t i v e t o t h e S i / A l r a t i o whereas i n t h e c o r r e s p o n d i n g a c i d forms i t f o l l o w s t h e S i / A l r a t i o . Supercage Na+ c a t i o n s (and/or l a t t i c e oxygen charge d e n s i t y ) and h y d r o p h o b i c i t y work i n oppo­ s i t i o n i n t h e sodium f a u j a s i t e s , w h i l e t h e B r o n s t e d a c i d i t y , c o b a l t o x i d a t i o n and h y d r o p h o b i c i t y r e i n f o r c e t h e i m p r e g n a t i o n / d i f f u s i o n f o r the a c i d f a u j a s i t e s . c) C o ( C 0 ) a d s o r b s i n t h e m o l e c u l a r form on a c c e s s i b l e supercage Na c a t i o n s o f NaY b u t o x i d i z e s t o supercage l o c a t e d C o + s i t e II, I I I c a t i o n s i n t h e HY form. d) ( n - C H ) V f r e e l y e n t e r s t h e supercages o f NaY and i s l o c k e d i n p l a c e by s p e c i f i c i n t e r a c t i o n s w i t h e i t h e r Na+ supercage c a t i o n o r l a t t i c e oxygens. 6

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( n - C 5 H ) C r f r e e l y e n t e r s t h e supercages o f NaY and i s l o c k e d i n p l a c e , p r o b a b l y i n a J a h n - T e l l e r d i s t o r t e d form, whereas on e n ­ t e r i n g t h e supercages o f HY i t i s o x i d i z e d t o an o r t h o r h o m b i c d i s t o r t e d form o f i n t r a z e o l i t e C p C r . (r^-CsHs) F e f r e e l y p a s s e s i n t o t h e supercage o f NaY whereas on i m p r e g n a t i o n i n t o HY i t s u f f e r s i n t r a z e o l i t e o x i d a t i o n t o t h e ferricenium cation. 5

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Acknowledgments The f i n a n c i a l a s s i s t a n c e o f t h e N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h C o u n c i l o f Canada's Major Equipment, O p e r a t i n g and S t r a t e ­ g i c G r a n t s programmes and t h e Connaught F o u n d a t i o n o f t h e U n i v e r s i t y o f T o r o n t o i s g r a t e f u l l y acknowledged. We w i s h a l s o t o acknowledge the d o n a t i o n o f h i g h p u r i t y and h i g h c r y s t a l l i n i t y z e o l i t e samples from D r s . E d i t h F l a n i g e n (Union C a r b i d e ) N i c o l a s Spencer (W.R Grace) and P a u l K a s a i (IBM sions. The a s s i s t a n c e o s p e c t r a , and Mr. T e d Huber i n t h e s y n t h e s i s o f some o f t h e m a t e r i a l s is also greatly appreciated.

Literature Cited 1. Yermakov, Yu. I.; Kuznetsov, B.N.; Zakharov, Y.A., "Studies in Surface Science and Catalysis Vol. 8, Catalysis by Supported Complexes"; Elsevier; New York, 1981. 2. Bein, T.; Jacobs, P.Α., J. Chem. Soc. Faraday Trans. I. 1983, 79, 1819. 3. Bein, T.; Jacobs, P.Α., J. Chem. Soc. Faraday Trans. I., 1984, 80, 1391. 4. Schneider, R.L.; Howe, R.F.; Watters, K.L., Inorg. Chem., 1984, 23, 4600-4607. 5. Mortier, W.J., "Compilation of Extra Framework Sites in Zeolites"; Butterworth, 1982. 6. Schwartz, J.; Huana, T., J. Am. Chem.Soc.,1982, 104, 5244. 7. Corbin, D.R.; Seidel, W.C.; Abrams, L. Herron, N.; Stucky, G.D.; Tolman, C.A., Inorg. Chem., 1985, 24, 1800-1803. 8. Nazar, L.F.; Ozin, G.A.; Hugues, F.; Godber, J.; Rancourt, D., J. Mol. Cat., 1983, 21, 313. 9. These dimensions are based on the assumption that (i) the Co­ -ring distance will not be significantly different than that found in [η -C (CH3)6] Co+PF . This is true for the crystallographically defined (η -C H ) Cr, (Cr-ring = 1.61Å) and (η -C H )2Cr I- (Cr-ring = 1.60Å); Muetterties, E.L.; Blecke, J. R.; Wucherer, E.J.; Albright, T.A.; Chem. Rev., 1982, 82,499; and (ii) that the C-C and C-H bond distances and angles are not significantly different from those of [η -C (CH ) ] Co+PF -, Thompson, M.R.; Day, V.W.; Minks, R.F.; Muetterties, E.L.; J. Am. Chem.Soc.,1980, 102, 2979. 10. The cristallographic diameter is 7.4Å but the kinetic diameter of 8.1Å is the more applicable in this situation, Breck, D.W. "Zeolite Molecular Sieves"; Wiley Interscience; New York, 1974. 11. Fitch, A.N.; Jobic, H.; Renouprez, Α., J. Chem. Soc. Chem. Comm. 1985, p.284. 6

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12. Lechert, H.; Wittern, K.P.; Ber. Bunsenges Phys. Chem.,1978, 82, 1054. 13. Ozin, G.A.; Baker, M.D.; Godber, J. in "Heterogeneous Catalysis"; Shapiro, B. Ed.; Texas A and M University Press: College Station, 1984. See also reference 5. 14. Ozin, G.A.; Baker, M.D.; Godber, J., J. Phys. Chem., 1984, 88, 4902. 15. Ozin, G.A.; Baker, M.D.; Godber, J., J. Phys. Chem., 1985, 89, 305. 16. Ozin, G.A.; Baker, M.D.; Godber, J., J. Phys. Chem., 1985, 89, 2299. 17. Ozin, G.A.; Baker, M.D.; Godber, J.; Shiuhua, W., J. Am. Chem. Soc., 1985, 107, 1995. 18. Ozin, G.A.; Baker, M.D.; Helwig, K.; Godber, J., J. Phys. Chem., 1985, 89, 1846. 19. Ozin, G.A.; Baker, M.D.; Godber, J., Catal Rev -Sci Eng., 1985, 27, 591. 20. Ozin, G.A.; Baker, 107, 3033. 21. Flanigen, E.M., in "Zeolite Chemistry and Catalysis"; Rabo, J.A., Ed.; ACS Monograph No.171, American Chemical Society: Washington, D.C., 1976. 22. Butler, W.M.; Angell, C.L.; McAllister, W.; Risen, W.M., J. Phys. Chem., 1977, 81, 2061. 23. Fischer, E.O.; Fritz, Η.Ρ.; Manchot, J.; Driebe, E.; Schneider, R., Chem. Ber. , 1963, 96, 1418. 24. Andrews, M.P.; Mattar, S.; Ozin, G.A., J. Phys. Chem., (in press). 25. Lechert, Η.; Wittern, K.P.; Schweitzer, W. Acta. Phys. Chem., 1978, 24, 201. 26. Haaland, Α., Topics Curr. Chem., 1975, 53, 1. 27. Robbins, J.L.; Edelstein, N.; Spencer, B.; Smart, J.C., J. Am. Chem.Soc.,1982, 104, 1882. 28. Warren, K.D., Struct and Bonding 1976, 27, 45. 29. Sohn, V.S.; Hendrickson, D.N.; Gray, Η.Β., J. Am. Chem. Soc., 1971, 93, 3603. 30. Ammeter, J.H., J. Magn. Res., 1978, 30, 299. 31. Somner, G.G.; Klug, H.P.; Alexander, L.E. Acta. Cryst., 1964, 17, 732. 32. Prins, R. Mol. Phys.,1970, 19, 603. 33. Hendrickson, D.N.; Sohn, V.S.; Gray, H.B., Inorg. Chem., 1971, 10, 1559. 34. Barr, T.H.; Watts, W.E., J. Organometal. Chem., 1968, 15, 177. 35. Rosenblum, M.; Santer, J.O.; Howells, W.G., J. Am. Chem. Soc., 1963, 85, 1450. 36. Lentzner, H.L.; Watts, W.E., J. Chem. Soc. Chem. Comm., 1970, p.26. 37. Karol, F.G.; Kavapinka, G.L.; Wu,C.;Dow, H.W.; Johnson, R.N.; Carrick, W.L., J. Polym. Sci. A-l, 1972, 10, 2621. 38. We find that low concentration pentane impregnation of (η C6H )2V into a wide range of rigourously pretreated zeolites (sodium and acid faujasities, Si/A1=1.25/1 to 3.8/1, and ALPO-5) yielded samples which displayed well defined epr spectra (level baselines, of Figure 11C) of vanadyl, VO+ the temperature and solvent dependence of which indicate the VO moiety to be 6-

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strongly bound to the zeolite. However, the vanadyl species generated in this way, although similar to that produced by intrazeolite O or H2O oxidation of (η -C H )2V, is nevertheless sufficiently different to suggest that the oxidant in the former involves framework oxygen or defect sites, rather than trace O2/H2O contaminants. 39.Using the SKM theory for the quantitative analysis of reflectance spectra as detailed by Klier (ref. J. Opt. Soc. Am., 62, 882 (1972), our preliminary estimates of the metallocene loadings in the samples of the present study, fall in the range of 0.5-2.5 molecules per unit cell. 6

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In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

16 Spectroscopic Studies of Active Sites Blue Copper and Electronic Structural Analogs Edward I. Solomon, Andrew A. Gewirth, and Susan L. Cohen Department of Chemistry, Stanford University, Stanford, CA 94305

An understanding o active sites is reactivity. An important example of the contributions of spectroscopic, crystallographic and theoretical studies in elucidating electronic structure is in investigations of the blue copper active site in plastocyanin. These studies underscore the large role of covalent delocalization in determining the electronic and spectro­ scopic properties of the site. Further confirmation of the role of covalent delocalization comes from photoemission studies of small molecule spectral analogs. I n o r g a n i c s p e c t r o s c o p y has e v o l v e d t o t h e p o i n t where a g r e a t d e a l o f i n s i g h t i n t o e l e c t r o n i c s t r u c t u r e and i t s c o n t r i b u t i o n t o r e a c t i v i t y can be o b t a i n e d from d e t a i l e d s t u d i e s on h i g h symmetry t r a n s i t i o n m e t a l complexes. A t t e n t i o n c a n now be d i r e c t e d toward some r a t h e r u n u s u a l i n o r g a n i c complexes which a r e a c t i v e s i t e s i n v o l v e d i n c a t a ­ lysis. These i n c l u d e m e t a l l o p r o t e i n s i n v o l v e d i n enzymatic c a t a l y s i s and m e t a l i o n s on s u r f a c e s i n v o l v e d i n heterogenous c a t a l y s i s . These a c t i v e s i t e s o f t e n e x h i b i t unique s p e c t r a l f e a t u r e s compared t o h i g h symmetry i n o r g a n i c complexes. These unique f e a t u r e s g e n e r a l l y d e r i v e from u n u s u a l g e o m e t r i c and e l e c t r o n i c s t r u c t u r e s imposed on the m e t a l by t h e b i o p o l y m e r o r the s u r f a c e . An u n d e r s t a n d i n g o f t h e s e geomet­ r i c and e l e c t r o n i c s t r u c t u r e s s h o u l d p r o v i d e s i g n i f i c a n t i n s i g h t i n t o the h i g h l y s p e c i f i c r e a c t i v i t y o f t h e s e a c t i v e s i t e s . A c l e a r example o f t h e c o n t r i b u t i o n o f i n o r g a n i c s p e c t r o s c o p y i n u n d e r s t a n d i n g the unique p r o p e r t i e s a s s o c i a t e d w i t h an a c t i v e s i t e i s the b l u e copper c e n t e r (1-3) i n p l a s t o c y a n i n . The b l u e copper s i t e e x h i b i t s unique s p e c t r a l p r o p e r t i e s when compared w i t h those o f normal copper complexes. These s p e c t r a l f e a t u r e s i n c l u d e an u n u s u a l l y s m a l l copper h y p e r f i n e s p l i t t i n g o f t h e EPR s i g n a l in^thej|„ r e g i o n (A„ < 70x10 cm as compared t o A_L = 150x10 cm f o r normal t e t r a g o n a l copper) [ F i g u r e 1] and an e x t r e m e l y i n t e n s e low e n e r g y ^ a b s o r p t i o n band ( V = 600 nm, ε * 4000 M cm compared t o ε - 100 M cm f o r normal copper complexes). T h e o r i g i n a l g o a l o f s p e c t r o s c o p y on the b l u e copper s i t e was t o under0097-6156/ 86/ 0307-0236508.75/ 0 © 1986 A m e r i c a n C h e m i c a l Society

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stand these f e a t u r e s and use them to g e n e r a t e a " s p e c t r o s c o p i c a l l y e f f e c t i v e " working model o f the a c t i v e s i t e . In p a r t i c u l a r , i n f r a r e d c i r c u l a r d i c h r o i s m (IRCD) s t u d i e s (4-5) [ F i g u r e 2] demonstrated t h a t a t l e a s t t h r e e d-d t r a n s i t i o n s e x i s t e d i n b l u e copper p r o t e i n s to e n e r g i e s below 5000 cm . A l i g a n d f i e l d a n a l y s i s (4-5) o f t h e s e t r a n s i t i o n s then i n d i c a t e d t h a t the s i t e s h o u l d have a geometry c l o s e to t e t r a h e d r a l and t h a t a l l d-d t r a n s i t i o n s o c c u r a t e n e r g i e s below 800 nm. T h e r e f o r e , the i n t e n s e 600 nm a b s o r p t i o n band must i n v o l v e a charge t r a n s f e r (CT) t r a n s i t i o n which, based on o t h e r c h e m i c a l and s p e c t r o s c o p i c s t u d i e s ( 6 - 8 ) , p r o b a b l y d e r i v e d from c y s t e i n e l i g a t i o n a t the s i t e . In 1978, h i g h r e s o l u t i o n s t r u c t u r e s (9-10) appeared which c o n f i r m e d the g e n e r a l t e t r a h e d r a l geometry and c y s t e i n e l i g a ­ t i o n and demonstrated t h a t the r e m a i n i n g l i g a n d s a r e two imidazoles o f h i s t i d i n e and a t h i o e t h e r from m e t h i o n i n e . While the length of the i m i d a z o l e to copper bond i s f a i r l y normal when compared to model complexes, the Cu-S ( t h i o l a t e ) bond i s found to be q u i t e s h o r t (2.1 Â ) and the Cu-S ( t h i o e t h e r With establishmen features concerning the e l e c t r o n i c s t r u c t u r e o f the b l u e copper s i t e can be a d d r e s s e d . These f e a t u r e s are 1) the n a t u r e o f the t h i o l a t e and t h i o e t h e r bonds, 2) the n a t u r e o f the ground s t a t e w a v e f u n c t i o n and 3) t h e e x t e n t o f c o v a l e n c y . We h a v e a l s o become s t r o n g l y i n v o l v e d i n u s i n g p h o t o e l e c t r o n s p e c t r o s c o p y as a p o w e r f u l approach toward d e t e r m i n i n g c o v a l e n c y i n t r a n s i t i o n m e t a l complexes. These w i l l be d i s c u s s e d i n t u r n . T h i o l a t e and

Thioether

Bonds

The f i r s t e x p e r i m e n t s to be d i s c u s s e d i n v o l v e d p o l a r i z e d s i n g l e c r y s ­ t a l o p t i c a l ( 11) s t u d i e s on the charge t r a n s f e r r e g i o n o f p l a s t o c y anin. The p l a s t o c y a n i n c r y s t a l has f o u r symmetry r e l a t e d m o l e c u l e s i n the Ρ 2.2^2. ( o r t h o r h o m b i c ) u n i t c e l l . The c r y s t a l morphology combined w i t h the o p t i c a l p r o p e r t i e s o f c r y s t a l s a l l o w e d p o l a r i z e d s p e c t r a t o be o b t a i n e d p a r a l l e l and p e r p e n d i c u l a r to the a, a x i s o f the (011) f a c e . The s p e c t r a i n F i g u r e 4A a r e o b s e r v e d to be s t r o n g ­ e s t i n the p a r a l l e l ( t o "a") polarization. As the Cu-S thioether bond i s o r i e n t e d approximately along the ς, a x i s , CT t r a n s i t i o n s a s s o c i a t e d w i t h t h i s l i g a n d s h o u l d appear d o m i n a t e l y i n the perpen­ d i c u l a r ( t o "a") p o l a r i z a t i o n . Thus, Cu-S t h i o e t h e r CT t r a n s i t i o n s c o n t r i b u t e a t most weakly to the a b s o r p t i o n s p e c t r a , a f e a t u r e which r a i s e d s i g n i f i c a n t c o n c e r n w i t h r e s p e c t to the n a t u r e o f a copper t h i o e t h e r bond. The absence o f a l o n g bond between the copper and the t h i o e t h e r a t the b l u e s i t e has, i n f a c t , been c o n s i d e r e d as a p o s s i b l i t y based on resonance Raman (12) and EXAFS (13) s t u d i e s . A c o m b i n a t i o n o f v a r i a b l e temperature a b s o r p t i o n , CD and MCD spectroscopies (4-5) i n d i c a t e d t h a t a t l e a s t f i v e t r a n s i t i o n s are present i n the CT r e g i o n o f p l a s t o c y a n i n . A c o r r e l a t i o n o f these w i t h the p o l a r i z e d s i n g l e c r y s t a l a b s o r p t i o n s p e c t r a gave the pos­ s i b l e band assignments shown i n F i g u r e 4B. C l e a r l y , the imidazole contributes in this region. In a d d i t i o n , t h e s e r e s u l t s i n d i c a t e t h a t t h r e e t h i o l a t e to Cu CT t r a n s i t i o n s may be p r e s e n t . O r i g i n a l l y , two were c o n s i d e r e d : a low energy d o u b l y d e g e n e r a t e p i s e t and a h i g h e r energy, more i n t e n s e sigma t r a n s i t i o n (bands 5 and 4, r e s p e c t i v e l y ) . T h i s p a t t e r n however c o n s i d e r s o n l y the bonding o f a s u l f u r ^ to a copper. I f the C-S-Cu a n g l e i s s i g n i f i c a n t l y l e s s than 180 , the

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

238

EXCITED STATES AND REACTIVE INTERMEDIATES

"Blue" Copper ( "Normal" Copper 3000

1000

2A000

16000 Energy (cm ) -1

8000

2500

3300

2900 Field (gauss)

F i g u r e 1. O p t i c a l ( l e f t ) and EPR ( r i g h t ) s p e c t r a o f a b l u e copper p r o t e i n ( s o l i d l i n e ) and a t e t r a g o n a l copper s i t e (dashed l i n e ) . Reproduced from R e f . 1. C o p y r i g h t W i l e y .

5.0

0.50

0.25

w

0.0

-2.5

-0.50

14000

4000

F i g u r e 2. N e a r - i n f r a r e d c i r c u l a r d i c r h o i s m spectrum o f p l a s t o ­ c y a n i n i n D 2 0 a t 290 K. Spectrum A c o r r e s p o n d s t o s c a l e on left. Spectrum Β c o r r e s p o n d s t o s c a l e on r i g h t . Reproduced from Ref. 5. C o p y r i g h t 1980, American C h e m i c a l S o c i e t y .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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F i g u r e 3· The b l u e copper s i t e i n p l a s t o c y a n i n as determined by X-ray c r y s t a l l o g r a p h y . L i g a n d s (and c o p p e r - l i g a n d bond l e n g t h s ) are h i s t i d i n e 37 (2.04 A ) , c y s t e i n e 84 (2.13 A ) , h i s t i d i n e 87 (2.10 A) and m e t h i o n i n e 92 ( 2.90 A ) . Reproduced w i t h p e r m i s s i o n from Ref. 9. C o p y r i g h t 1983, J o u r n a l o f M o l e c u l a r B i o l o g y .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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EXCITED STATES AND

REACTIVE INTERMEDIATES

F i g u r e 4. A) Room-temperature o p t i c a l spectrum o f a s i n g l e c r y s t a l o f p l a s t o c y a n i n o b t a i n e d w i t h l i g h t i n c i d e n t on the ( 0 , 1 , 1 ) f a c e and p o l a r i z e d p a r a l l e l ( s o l i d l i n e ) and perpendi­ c u l a r (dashed l i n e ) to a. ( f r o m Ref, 1 1 ) . B) G a u s s i a n r e s o l u t i o n of the 35 Κ v i s i b l e a b s o r p t i o n spectrum of a p l a s t o c y a n i n f i l m w i t h suggested assignments; the symbols ( · ) r e p r e s e n t the e x p e r i m e n t a l a b s o r p t i o n spectrum. Right: plastocyanin unit c e l l p r o j e c t e d on the (0,1,1) p l a n e , showing the p o s i t i o n s o f the f o u r s y m m e t r y - r e l a t e d Cu atoms a t t h e i r f i r s t c o o r d i n a t i o n shells.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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strong i n t e r a c t i o n o f the s u l f u r with the carbon o f the c y s t e i n e r e s i d u e would r e s u l t i n t h r e e t r a n s i t i o n s . The R-S-Cu a n g l e i n t h e p l a s t o c y a n i n s t r u c t u r e i s i n f a c t 107 . These r e s u l t s have l e d us t o a more q u a n t i t a t i v e e v a l u t a t i o n o f the bonding i n the b l u e copper s i t e through a many e l e c t r o n SCF-Xa-SW c a l c u l a t i o n (14). The s t r u c t u r e s c a l c u l a t e d i n c l u d e the f r e e l i g a n d s and a p p r o x i m a t i o n s t o the s i t e shown i n F i g u r e 5 w i t h t h e Xa-SW parameters as i n d i c a t e d . F i r s t c o n s i d e r i n g the t h i o l a t e bond, t h e v a l e n c e o r b i t a l e o f t h e f r e e t h i o l a t e l i g a n d i n c l u d e the h i g h e s t energy o c c u p i e d d o u b l y de­ g e n e r a t e 2e l e v e l c o n s i s t i n g o f s u l f u r ρ o r b i t a l e o r i e n t e d perp­ e n d i c u l a r t o t h e S-C bond, and t o a p p r o x i m a t e l y 2 eV d e e p e r b i n d i n g energy, t h e 3a^ l e v e l which i s t h e s u l f u r ρ o r b i t a l involved i n sigma bonding w i t h t h e c a r b o n . C o o r d i n a t i o n o f the t h i o l a t e l i g a n d t o the copper s p l i t s t h e s e v a l e n c e o r b i t a l s i n t o t h r e e r o u g h l y e q u a l ­ l y spaced l e v e l s , [ F i g u r e 6] each w i t h s i g n i f i c a n t bonding interac t i o n s with the o r b i t a l T h i s s p l i t t i n g r e s u l t s fro f r e e t h i o l a t e b e i n g s i g n i f i c a n t l y s t a b i l i z e d due éof^onding and mixed w i t h t h e C-Sp o r b i t a l , which has the same symmetry. The c o n t o u r diagrams a s s o c i a t e d w i t h these t h r e e bonding levels are given i n F i g u r e 7. The h i g h e s t energy o c c u p i e d o r b i t a l (7a") i s the s u l f u r ρ which i s p e r p e n d i c u l a r t o the C-S-Cu p l a n e and i n v o l v e d i n a s t r o n g p i bond w i t h t h e d 2_ 2 o r b i t a l on t h e copper. The m i d d l e level i n v o l v e s t h e s u l f u r p J which i s i n p l a n e and mixes w i t h t h e s u l f u r ρ , forming a pseudo-Jigma bond w i t h t h e copper. Here, t h e e l e c t r o n d e n s i t y i s no l o n g e r maximized a l o n g t h e S-Cu bond. The l e v e l to d e e p e s t b i n d i n g energy i n v o l v e s a m o l e c u l a r o r b i t a l which i s sigma bonding w i t h copper b u t a l s o s i g n i f i c a n t l y d e l o c a l i z e d i n t o t h e S-C bond. χ

X

The l o n g (2.9 A) C u - S ( t h i o e t h e r ) bond i s next c o n s i d e r e d . The v a l e n c e o r b i t a l s o f t h e f r e e l i g a n d a r e 2b^ which i s a ρ o r b i t a l o f the s u l f u r , p e r p e n d i c u l a r t o the C-S-C p l a n e , and, t o 2.^1 eV deeper b i n d i n g energy, t h e 4a^ l e v e l which i s t h e p o r b i t a l o f t h e s u l f u r i n v o l v e d i n a sigma bonding i n t e r a c t i o n w i t h t h e symmetric c o m b i n a t i o n o f methyl valence o r b i t a l s . C o o r d i n a t i o n o f the t h i o e t h e r to the copper a t the same d i s t a n c e and a n g l e as i n t h e p r o t e i n l e a d s t o a s t a b i l i z a t i o n o f t h e Ua^ o r b i t a l by 0.4 eV r e l a t i v e t o o t h e r v a l e n c e o r b i t a l s due t o bonding w i t h about 36% d e r e a l i z a t i o n o f t h e wavef u n c t i o n onto t h e copper [ F i g u r e 8 ] , The n a t u r e o f t h e bonding i n ­ t e r a c t i o n i s shown i n F i g u r e 9. Here the s u l f u r ρ o r b i t a l i s i n ­ v o l v e d i n a pseudo sigma type bond i n t o t h e d 2 o r b i t a l o f t h e cop­ per. As mentioned p r e v i o u s l y , a number o f p h y s i c a l methods have r a i s e d the q u e s t i o n as t o whether t h e r e i s a Cu-S ( m e t h i o n i n e ) bond. The l a c k o f CT i n t e n s i t y i s now seen t o be a consequence o f t h e o r ­ i e n t a t i o n o f t h e S ( t h i o e t h e r ) ρ donor o r b i t a l which i s o r t h o g o n a l t o the h a l f - o c c u p i e d d 2_ 2 a c c e p t o r ( v i d a i n f r a ) . The c a l c u l a t i o n does, however, indicate s p e c t r a l f e a t u r e s which a r e s e n s i t i v e to t h i o e t h e r i n t e r a c t i o n with the copper. The e f f e c t o f a x i a l t h i o e t h e r c o o r d i n a t i o n i n the Χ α c a l c u l a t i o n s i s p r e s e n t e d i n F i g u r e 10. Here, the r e l a t i v e c a l c u l a t e d energy o f t h e copper d o r b i t a l s a r e g i v e n f o r the copper s i t e i n C symmetry w i t h and w i t h o u t t h e t h i o e t h e r . Upon a d d i t i o n o f t h e a x i a l l i g a n d , t h r e e o f t h e f o u r l e v e l s go down s l i g h t l y i n energy r e l a t i v e t o t h e h a l f - o c c u p i e d l e v e l . The d^2 l e v e l , however, goes up i n energy due t o a n t i b o n d i n g i n t e r a c t i o n s z

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND REACTIVE INTERMEDIATES

242

Χα PARAMETERS FOR Cu(S(CH ) )(SCH3)(NH ) 3

L

2

3

2

OVERLAP

MAX

OUTER SPHERE

3

Cu

-

S(THIOLATE)

2%

Cu

2

Cu

-

S(THIOETHER)

0%

S

1

Cu - N

Ν

1

C

1

H

0

0%

CS SYMMETRY CONVERGED AFTER 29

ITERATIONS

(EPS=0.009; -2T/V=0.9998) Figure tion.

5. Top: A p p r o x i m a t i o n s c o n s i d e r e d by S C F - Χ α -SW c a l c u l a ­ Bottom: Χ α parameters f o r Cu(S(CH ) )(SCH ) ( N H ) . q

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

SOLOMON ET AL.

Spectroscopic Studies of Active Sites

Cu(SCH )(NH ) 3

2 e

3

2

l

(PxV

2.163 eV

i 7a'

ChL 2e

9

1

Λ Λ

(Cu-S-C)

CH

Cu 7a"

35°ki, 7°ip

9a

43°ki, 2°ip

1

7a'

\3%ά,

9°is

3%

32%P , V

l o ° i p , , 21f p , , 3%s z

23°ip

3

0

1 l°ip . w

10% 31%

F i g u r e 6. R e p r e s e n t a t i o n o f t h e i n t e r a c t i o n o f t h r e e h i g h e s t energy o r b i t a l s o f m e t h y l t h i o l a t e w i t h a c o p p e r ( I I ) i o n . Top: s h i f t s i n e n e r g i e s r e l a t i v e t o t h e s u l f u r 2s o r b i t a l . Bottom: c h a r a c t e r o f t h e o r b i t a l s i n terms o f a t o m i c o r b i t a l s . Primed c o o r d i n a t e s o f t h e copper-bound s u l f u r ρ o r b i t a l s i n d i c a t e a l i g a n d - b a s e d c o o r d i n a t e system. Reproduced from Ref. 14. C o p y r i g h t 1985, American C h e m i c a l S o c i e t y .

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EXCITED STATES AND REACTIVE INTERMEDIATES

F i g u r e 7. C o n t o u r s o f t h e t h r e e bonding o r b i t a l s w i t h s u b s t a n ­ t i a l t h i o l a t e s u l f u r 3p c h a r a c t e r and t h e geometry o f Cu(SCH^)(NH >2 with copper and t h i o l a t e based c o o r d i n a t e systems indicated. A l lnuclei indicated are i n the planes of the f i g u r e s e x c e p t f o r t h e c o n t o u r o f t h e 7a" o r b i t a l i n t h e xy plane. O n l y t h e copper n u c l e u s i s i n t h e p l a n e o f t h i s f i g u r e . V a l u e s o f t h e c o n t o u r s a r e +0.003, +0.009, +0.027 and +0.081 Reproduced from Ref. 14. C o p y r i g h t 1985, American Chemical Society.

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

SOLOMON ET AL.

S(CH ) 3

2b

2

Spectroscopic Studies of Active Sites

245

Cu(S(CH ) )(SCH )(NH ) 3

2

2

3

3

2

V 18a'

(p ) yl

2.125 eV

2.506 eV 4a (S-C) 1

Î 1.140 eV

14a' (Cu-S-C)

t

i

0.673 eV 9a" (S-C)

i

CH

0

92.ρ

Q\ y

64 p , 3 s z

3b,

47"'ρ

31.. 48

18a' 14a' 9a"

6 d 2, 2 d 28'd 2, 5'',d z

lod

, 2%s

80%p , y

6%

39%p ,, l%s

18%

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44%

z

v

F i g u r e 8. I n t e r a c t i o n o f the three h i g h e s t energy occupied orbitals of dimethylsulfide with copper. Top: s h i f t s i n e n e r g i e s r e l a t i v e t o t h e s u l f u r 2s o r b i t a l . Bottom: c h a r a c t e r of t h e o r b i t a l s i n terms o f atomic o r b i t a l s . Primed c o o r d i n a t e s o f t h e copper-bound s u l f u r ρ o r b i t a l s i n d i c a t e a l i g a n d - b a s e d c o o r d i n a t e system. Reproduced from Ref. 14. C o p y r i g h t 1985, American Chemical S o c i e t y .

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

246

EXCITED STATES AND REACTIVE INTERMEDIATES

CU(S(CH3)2)(SCH3)(NH3)2

1-

14A'

f

F i g u r e 9. Contour o j the 14a l e v e l and t h e g e o m e t r y o f Cu(S(CH ) )(SCH )(NH ) w i t h copper and t h i o e t h e r based c o o r d i ­ n a t e systems i n d i c a t e d . I n the c o n t o u r , a l l n u c l e i i n d i c a t e d a r e i n t h e p l a n e o f the diagram e x c e p t f o r t h o s e o f the amine n i t r o g e n s and t h i o e t h e r c a r b o n s . V a l u e s of the c o n t o u r s a r e the same as i n F i g u r e 7. Reproduced from R e f . 14. C o p y r i g h t 1985, American C h e m i c a l S o c i e t y . 3

2

3

In Excited States and Reactive Intermediates; Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

16.

SOLOMON ET AL.

Spectroscopic Studies of Active Sites

247

WITH THIOETHER

WITHOUT THIOETHER

γ

hUG.V

Y

-OK:

d

ο

χ -y2

xy

10000

20000

J

F i g u r e 10. C a l c u l a t e d e f f e c t s o f a x i a l t h i o e t h e r l i g a t i o n upon copper d o r b i t a l s . The r e l a t i v e e n e r g i e s o f t h e a n t i b o n d i n g o r b i t a l s and t h e i r predominant copper d c h a r a c t e r a r e i n d i c a t e d f o r t h e two s i t e s shown. The e n e r g i e s o f t h e h a l f - o c c u p i e d l e v e l have been s e t t o z e r o . Reproduced from R e f . 14. C o p y r i g h t 1985, American C h e m i c a l S o c i e t y .

American Chemical Society Library 1155 16th St., N.W. In Excited States and Reactive Intermediates; Washington, DC. 20036Lever, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

EXCITED STATES AND

248

REACTIVE INTERMEDIATES

w i t h the s u l f u r ρ o r b i t a l o f the t h i o e t h e r . Thus, the d-d t r a n s i ­ t i o n a s s o c i a t e d w f t h the d 2 o r b i t a l s h o u l d go down i n energy. W h i l e removing the t h i o e t h e r from the b l u e copper s i t e v i a s i t e d i r e c t e d m u t a g e n e s i s and a t the same time r e t a i n i n g the r e m a i n i n g geometric f e a t u r e s i s u n r e a l i s t i c , copper c h l o r i d e s p e c t r a l analog s t u d i e s do c l e a r l y demonstrate d e s t a b i l i z a t i o n o f the d 2 o r b i t a l due to a n t i bonding i n t e r a c t i o n s w i t h c h l o r i d e l i g a n d s a t a p p r o x i m a t e l y 3 % d i s ­ tance as shown i n F i g u r e 11. A t the top o f the f i g u r e i s the l i g a n d f i e l d spectrum o f bis(N-methylphenethylammonium) C u C l ^ (15) , a square p l a n a r complex which c o n t a i n s no a x i a l l i g a n d . At the bottom i s the spectrum o f b i s ( e t h y l ammonium) C u C l ^ (16) which does c o n t a i n a x i a l chlorides. The a s s i g n e d t r a n s i t i o n s , based on p o l a r i z e d s p e c t r a f o r each complex are i n d i c a t e d a t the top o f the a b s o r p t i o n bands i n F i g u r e 12. Upon a d d i t i o n o f a p i c a l c h l o r i d e s , ( F i g u r e 11 top to b o t ­ tom), t h e ^ t r a n s i t i o n frqm the d 2 o r b i t a l d e c r e a s e s i n energy from 16000 cm t o 11000 cm , w h i l e the o t h e r t r a n s i t i o n s o c c u r a t ap­ p r o x i m a t e l y the same e n e r g i e a p i c a l l i g a n d has s i g n i f i c a n i t a l o f the complex. Z

2

Ground S t a t e W a v e f u n c t i o n and

Covalency

S i n g l e c r y s t a l EPR s t u d i e s (_11) o f p l a s t o c y a n i n i n c o n j u n c t i o n w i t h a l i g a n d ^ . e l d c a l c u l a t i o n enabled a d e t e r m i n a t i o n o f the o r i e n t a t i o n o f the g t e n s o r r e l a t i v e to the copper s i t e . F i g u r e 12 p r e s e n t s the EPR s p e c t r a o f a s i n g l e c r y s t a l o f p l a s t o c y a n i n , o r i e n t e d w i t h the magnetic f i e l d (H) p e r p e n d i c u l a r to a, and r o t a t e d about the a. a x i s . An a p p r o x i m a t e l y g„ spectrum i s observed when H i s a l o n g c. i n d i c a t i n g t h a t g„ i s o r i e n t a t e d i n the g e n e r a l d i r e c t i o n o f the t h i o e t h e r - C u bond. S i m u l a t i o n o f f o u r d i f f e r e n t r o t a t i o n s f o r the f o u r m o l e c u l e s i n the u n i t c e l l demonstrate t h a t g and A a r e c o l i n e a r and t h a t g ο ο ζ ζ ζ is 8 o f f £, and 5 out o f t h i s p l a n e . T h u s , the d 2_ 2 o r b i t a l , which i s p e r p e n d i c u l a r to g and c o n t a i n s the unpaired* eïectron, i s l e s s than 15 d e g r e e s above tfie plane formed by the S ( c y s ) and the two Ν ( h i s ) l i g a n d s . T h i s o r i e n t a t i o n o f the d 2_ 2 o r b i t a l i s reproduced i n F i g u r e 13, a l o n g w i t h the energy l e v e l 5ia§ram a s s o c i a t e d w i t h the ligand f i e l d of p l a s t o c y a n i n . W h i l e t h i s energy l e v e l diagram r e ­ f l e c t s a low symmetry, r h o m b i c a l l y d i s t o r t e d s i t e , i t i s o f import­ ance to c o n s i d e r the a x i a l l i m i t s so as t o e v a l u a t e the c l o s e to a x i a l n a t u r e o f the e x p e r i m e n t a l X-band EPR spectrum, shown i n F i g u r e 1. Two a x i a l subgroups o f a t e t r a h e d r o n are p o s s i b l e , D^j and C^ « I f the rhombic s p l i t t i n g o f the d o r b i t a l s i s removed from t h a t g i v e n i n F i g u r e 13 o n l y the energy l e v e l o r d e r i n g i s r e a s o n a b l e . This energy o r d e r i n g i s c o n s i s t e n t w i t h an e l o n g a t e d C ^ s t r u c t u r e , the l o n g a x i s b e i n g a l o n g the Cu t h i o e t h e r bond, as shown i n F i g u r e 13. This elongated C^ e f f e c t i v e symmetry o f the b l u e copper s i t e r a i s e s a s i g n i f i c a n t problem w i t h r e s p e c t to the p r e s e n t i n t e r p r e ­ t a t i o n s (17-18) o f the s m a l l copper h y p e r f i n e s p l i t t i n g observed i n the EPR spectrum shown i n F i g u r e 1. The s m a l l s p l i t t i n g had been a t t r i b u t e d to a D m i x i n g o f Cu 4 p i n t o the