Inorganic Chemistry: Toward the 21st Century 9780841207639, 9780841210240, 0-8412-0763-1

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Inorganic Chemistry: Toward the 21st Century Malcolm H. Chisholm,

EDITOR

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.fw001

Indiana University

Based on a symposium cosponsored by the Divisions of Inorganic Chemistry of both the American Chemical Society and the Chemical Institute of Canada, and jointly sponsored by the Dalton Division of the Royal Society of Chemistry, Indiana University, Bloomington, Indiana, May 16-19, 1982

211

ACS SYMPOSIUM SERIES

AMERICAN CHEMICAL SOCIETY WASHINGTON, D.C.

1983

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.fw001

Library of Congress Cataloging in Publication Data Inorganic chemistry. (ACS symposium series, ISSN 0097-6156; 211) "Based on a symposium cosponsored by the Divisions of Inorganic Chemistry of both the American Chemical Society and the Chemical Institute of Canada, and jointly sponsored by the Dalton Division of the Royal Society of Chemistry, [held at] Indiana University, Bloomington, Indiana, May 16-19, 1982." Includes bibliographies and index. 1. Chemistry, Inorganic—Congresses. I. Chisholm, Malcolm H . II. American Chemical Society. Division of Inorganic Chemistry. III Chemical Institute of Canada. Inorganic Chemistry Division. IV. Royal Society of Chemistry (Great Britain). Dalton Division. V . Series. QD146.I55 ISBN 0-8412-0763-1

546 ACSMC8

211

82-24505 1-567 1983

Copyright © 1983 American Chemical Society A l l Rights Reserved. The appearance of the code at the bottom of the first page of each article in this volume indicates the copyright owner's consent that reprographic copies of the article 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. 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 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.

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In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.fw001

ACS Symposium Series M. Joan Comstock, Series Editor

Advisory Board David L. Allara

Robert Ory

Robert Baker

Geoffrey D. Parfitt

Donald D. Dollberg

Theodore Provder

Brian M . Harney

Charles N. Satterfield

W. Jeffrey Howe

Dennis Schuetzle

Herbert D. Kaesz

Davis L. Temple, Jr.

Marvin Margoshes

Charles S. Tuesday

Donald E. Moreland

C. Grant Willson

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.fw001

FOREWORD The ACS SYMPOSIUM SERIES was founded in 1974 to provide

a medium for publishing symposia quickly in book form. The format of the Series parallels that of the continuing ADVANCES IN CHEMISTRY SERIES except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. Papers are reviewed under the supervision of the Editors with the assistance of the Series Advisory Board and are selected to maintain the integrity of the symposia; however, verbatim reproductions of previously published papers are not accepted. Both reviews and reports of research are acceptable since symposia may embrace both types of presentation.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.pr001

PREFACE INORGANIC CHEMISTRY continues to provide the spawning ground for the evolution of vast areas of chemistry. It reaches into solid state chemistry, polymer chemistry, biochemistry, organic synthesis via organometallics, homogeneous and heterogeneous catalysis, energy storage and energyrelated technology, analytical, physical and theoretical chemistry. By definition, inorganic chemistry is concerned with the chemistry of the elements and their compounds, other than those of carbon which fall into the field of organic chemistry. The 1950s were considered the renaissance of inorganic chemistry. Since then, the field has grown rapidly in many new directions. The challenges that the world presents to inorganic chemists have never been greater; nor have the contributions that inorganic chemists make been more than at this time. This symposium brings together a large number of leading chemists to discuss some of the rapidly developing areas. Emphasis was placed on looking toward the future, and some speculation from the speakers was encouraged as to what may arise in the future in their fields of research. I have also encouraged discussion of these ideas by allotting a greater amount of time for questions, answers, and debate after each lecture. This volume is the proceedings of this historic symposium, thefirstsymposium to be jointly sponsored by the inorganic divisions of the American Chemical Society and The Chemical Institute of Canada, and the Dalton Division of the Royal Society of Chemistry. I should like to thank the Office of Naval Research, the Petroleum Research Fund administered by the American Chemical Society, Ε. I. du Pont de Nemours & Company, Exxon Research & Engineering Company, The Chemical Institute of Canada, the ACS Division of Inorganic Chem­ istry, Monsanto Company, Imperial Chemical Industries (London), Union Carbide Corporation, Strem Chemicals, Indiana University's Friends of Chemistry, and the Office of the Vice President of Indiana University for financial support of this symposium. I should also like to thank my wife, the former Cynthia Truax and Symposium Secretary, for all her assistance. MALCOLM H. CHISHOLM

Indiana University October 1982 ix In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1 Solar Electricity: Lessons Gained from Photosynthesis JAMES R. BOLTON

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch001

The University of Western Ontario, Department of Chemistry, Photochemistry Unit, London, Ontario N6A 5B7 Canada

Nature has developed a mechanistically very complex yet conceptually very simple process for the conversion and storage of solar energy. In this lecture I shall first examine the reaction and mechanism of photosynthesis deriving insights into how nature has achieved this remarkable process. I shall then go on to describe various attempts to mimic the primary steps of photosynthesis. Finally, I shall speculate on how these insights into the mechanism of photosynthesis might be used to design a new type of solar cell for the conversion of light to electricity. The Photosynthesis Reaction The reaction of photosynthesis is clearly the most important chemical reaction since life could not exist for long without it. The overall reaction is CO (g) + H O(l) 2

light

2

C H O (s) + O (g) 6

12

6

2

where C H O (s) is D-glucose, the major energy-storage product of photosynthesis from which all plant and animal biomass is derived. The thermodynamic parameters for the photosynthesis reaction at 298K (25°C) are ΔΗ=467 kJmol ; ΔG=496 kJmol and E°=1.24V (1). It is helpful to think of the photosynthesis reaction as the sum of an oxidation half reaction and a reduction half reaction as shown in Figure 1. In fact, nature does separate these half re­ actions, in that the reduction of CO to carbohydrates occurs in the stroma of the chloroplast, the organelle in the leaf where the photosynthesis reaction occurs, - whereas, the light-driven oxidation half reaction takes place on the thylakoid membranes which make up the grana stacks within the chloroplast. Reduced nicotinamide adenine dinucleotide phosphate (NADPH) carries the reducing power and most of the energy to the stroma to drive the fixation of CO with the help of some additional energy provided 6

12

6

-1

1-

2

2

0097-6156/83/0211 -0003$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch001

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C0 (g)

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8 hv

2

/ carbon \f 1 fixation Y V cycle Λ

2NADPH+2H* + 3 ATP ^/light Y driven y 1 electron A JV transportJ\ 2NADP thylakoid +3ADP membrane 3Pi +

Λ stroma

2H 0(I) 2

0 (g) ?

+

l/6C H 0 (s)+ H 0(l) 6

|2

6

2

Figure 1. The separation of the half reaction in the chloroplast of the photosynthetic plant cell. The dark reaction (left) and the light-driven reactions (right) are shown. Key: NADP\ oxidized form of nicotinamide adenine dinucleotide phosphate; ATP, adenosine triphosphate; and P inorganic phosphate. if

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1.

BOLTON

Solar

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Electricity

by adenosine triphosphate (ATP) which i s a l s o generated on the t h y l a k o i d membrane. [Readers i n t e r e s t e d i n the s t r u c t u r e and mechanism of photosynthesis should r e f e r to references (2-4)·] Although the mechanism of the c a r b o n - f i x a t i o n c y c l e i s very i n t e r e s t i n g biochemistry, i t i s the mechanism of the o x i d a t i o n h a l f r e a c t i o n which w i l l concern us most, because i t i s here that the conversion of s u n l i g h t to chemical energy takes p l a c e . We s h a l l now explore some of the d e t a i l s of t h i s remarkable process.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch001

Mechanism of the Primary Photochemical Reaction of Photosynthesis The primary photochemistry of photosynthesis takes place w i t h i n very s p e c i a l i z e d r e a c t i o n - c e n t e r p r o t e i n s s i t u a t e d i n the t h y l a k o i d membrane (see Figure 2) of the c h l o r o p l a s t (2) . Most (>99%) of the c h l o r o p h y l l and other pigments act as an antenna system to gather l i g h t photons and channel them to one of the two r e a c t i o n centers. In essence the antenna c h l o r o p h y l l system acts as a photon concentrator, concentrating the photon f l u x by a f a c t o r of ^300 over what i t would be without an antenna system. In both Photosystems I and I I the photochemically a c t i v e component (P700 or P680) i s thought to be a c h l o r o p h y l l a species. P700 i s l i k e l y a dimer of c h l o r o p h y l l a. The primary photochemical step then i n v o l v e s the t r a n s f e r of an e l e c t r o n from the donor (P700 or P680) to an acceptor species. In Photosystem I I the acceptor Q i s thought to be a molecule of plastoquinone. In Photosystem I 1

0

0 the acceptors are not as w e l l c h a r a c t e r i z e d ; A may be a molecule of c h l o r o p h y l l a i n a s p e c i a l environment and A i s thought to be an i r o n - s u l f u r center. Hence, i t appears that nature has chosen the f a s t e s t and perhaps simplest of a l l photochemical r e a c t i o n s , namely, photochemical e l e c t r o n t r a n s f e r , to trap the energy of the e l u s i v e sunbeam. In e f f e c t , the r e a c t i o n - c e n t e r p r o t e i n s of photosynthesis are s o l a r c e l l s converting l i g h t to e l e c t r i c i t y which i s then used to d r i v e the r e l a t i v e l y slow biochemical r e a c t i o n s which lead u l t i m a t e l y to D-glucose. These r e a c t i o n - c e n t e r s o l a r c e l l s are very e f f e c t i v e - the y i e l d f o r e l e c t r o n t r a n s f e r i s almost u n i t y , and the o v e r a l l s o l a r energy conversion to e l e c t r i c a l energy i s VL6% (5). This i s as good as or g e n e r a l l y much b e t t e r than most commercially a v a i l a b l e s i l i c o n s o l a r c e l l s . Now l e t us examine more c l o s e l y the d e t a i l s of how the x

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch001

NADPH

Figure 2 .

Model of the thylakoid membrane showing the various components involved in electron transport from H 0 to NADP\ 2

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key: Z, the watersplitting enzyme which contains Μη; P680 and Q the primary donor and acceptor species in the reaction-center protein of Photosystem II; Qi and Q , probably plastoquinone molecules; PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside; Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I; P700 and At, the primary donor and acceptor species of the Photosystem I reaction-center protein; At, Fe-SA. and Fe-S , membrane-bound secondary acceptors which are probably Fe-S centers; Fd, soluble ferredoxin Fe-S protein; and fp, is theflavoproteinthat functions as the enzyme that carries out the reduction of NADP+ to NADPH. h

t

B

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1.

BOLTON

Solar

7

Electricity

e l e c t r o n i s t r a n s f e r r e d from one s i d e of the t h y l a k o i d membrane to the other. Figure 3 i l l u s t r a t e s a view of our current knowledge of the composition and s t r u c t u r e of the r e a c t i o n - c e n t e r p r o t e i n from photosynthetic b a c t e r i a which performs a photochemical e l e c t r o n - t r a n s f e r r e a c t i o n analogous to the r e a c t i o n s i n the two photosystems of green-plant photosynthesis (2, 7_ 8). The b a c t e r i a l r e a c t i o n - c e n t e r p r o t e i n contains 4 b a c t e r i o c h l o r o p h y l l molecules, 2 bacteriopheophytins, one nonheme i r o n and a ubiquinone molecule. Two of the b a c t e r i o c h l o r o p h y l l molecules form a s p e c i a l p a i r c a l l e d P870 which acts as the photochemical e l e c t r o n donor. The other two b a c t e r i o c h l o r o p h y l l s absorb at 800 nm and may act as the l a s t l i n k to the antenna system. Within 5...CH4, so although the s h i f t i n i s due mostly to p e r t u r b a t i o n o f o r b i t a l s only populated i n the excited state (12,13), the trend i n X a l s o r e f l e c t s grotmd state i n t e r ­ a c t i o n (23). This i n t e r a c t i o n i s o f very great importance i n understanding the s o l u t i o n behavior o f Cr(C0)5 - i n p a r t i c u l a r , i t suggests that no solvent w i l l be completely innocuous towards "naked Cr(C0)5 and that t r a c e i m p u r i t i e s may be extremely important i n the photochemistry. An important technique f o r probing the photochemical mechanism involves the combination o f matrix i s o l a t i o n and the use o f plane p o l a r i z e d l i g h t f o r both p h o t o l y s i s and IR/UV-vis spectroscopy (24-26). The p o l a r i z e d p h o t o l y s i s can generate p r e f e r e n t i a l l y o r i e n t e d molecules i n the matrix which d i s p l a y d i c h r o i c absorption of l i g h t . In the absence of photochemical s t i m u l a t i o n , the molecules are h e l d r i g i d l y i n the matrix cage and so the photochemically induced dichroism can subsequently be probed by s p e c t r o s c o p i c techniques. [This method i s somewhat akin t o the use of nematic solvents by Gray and colleagues (27) f o r the assignment o f UV s p e c t r a , e.g. Mn2(CO)io-J In t h i s way i t was shown that the v i s i b l e absorption band i n Cr(C0)5...X has a t r a n s i t i o n moment of Ε symmetry and the d e t a i l e d photochemical behavior could be explained by the scheme shown i n Figure 3. I t w i l l be noted that t h i s mechanism i n v o l v e s the i n i t i a l generation from Cr(C0)£ o f C4 Cr(C0)5 i n an e x c i t e d s i n g l e t s t a t e ( Ε ) ; t h i s matches Hay's c a l c u l a t i o n s , (28) although i t has caused some s u r p r i s e (29). The e x c i t e d C4 molecule decays via a s t r u c t u r e t o ground s t a t e C4 0-k]) and subsequent photochemically induced i n t e r c o n v e r s i o n between Cr(C0)5 i n d i f f e r ­ ent o r i e n t a t i o n s proceeds v i a t h i s same U 3 intermediate. Thus the regeneration o f Cr(C0)£ from Cr(C0)5 a r i s e s because the Cr(CO)> fragment i s " s t i r r e d " by the i r r a d i a t i o n u n t i l the empty coordina­ t i o n s i t e encounters the o r i g i n a l l y e j e c t e d CO. This mechanism i s r e l e v a n t to s o l u t i o n chemistry and perhaps f o r t u i t o u s l y p r e d i c t s a quantum y i e l d f o r Cr(CO)£ _ J Û L ^ C r ( C 0 ) 5 ..X o f h which i s i n exact agreement with a recent determination (30) o f the quantum y i e l d f o r the s o l u t i o n r e a c t i o n o f Cr(C0)6 w i t h pyridine. m a x

tf

fl

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch003

11

V

Ί

V

V

n

:

The matrix experiments thus r e v e a l some complex photochemistry of relevance t o s o l u t i o n chemistry but the experiments do not provide information about k i n e t i c s . For t h i s we need a f l u i d medium e.g. gas o r l i q u i d , and we consider such experiments i n the next two s e c t i o n s . F l a s h p h o t o l y s i s suggests i t s e l f as the technique f o r d e t e c t i n g a species as r e a c t i v e as Cr(C0)5 but before d e s c r i b i n g these experiments we show what can be achieved from low-temperature s o l u t i o n s . Low-temperature S o l u t i o n s . The matrix s p e c t r o s c o p i c data f o r Cr(C0)5...X (Figure 2) suggest that the i n t e r a c t i o n between Cr(C0)5 and s a t u r a t e d hydrocarbons may be q u i t e s u b s t a n t i a l ,

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch003

INORGANIC

CO

CO

Figure 3. Scheme of the photochemical behavior of M(CO) in a mixed N /Ar matrix via a trigonal bipyramidal intermediate. Key: O, the matrix cage; and *, *E excited state of Cr(CO) . (Reproduced from Ref. 25. Copyright 1978, American Chemical Society.) 5

5

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

t

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch003

3.

T U R N E R A N D POLiAKOFF

Photochemical

Intermediates

41

perhaps large enough to s t a b i l i z e Cr(C0)5...(hydrocarbon) species at low temperatures i n f l u i d s o l u t i o n s . T y l e r and P e t r y l a k (31) have indeed shown, using IR spectroscopy, that i n e x t r a o r d i n a r i l y w e l l p u r i f i e d methylcyclohexane the species M(C0)5...MCH (M = Cr, Mo, W; MCH = methylcyclohexane) can be photochemically generated w i t h a l i f e t i m e of about an hour at -780C. The degree of p u r i t y of MCH r e q u i r e d i s extremely h i g h s i n c e even t r a c e concentrations of unsaturated hydrocarbons promote the production of TT-olefin complexes. Although there i s l i t t l e doubt that T y l e r and P e t r y l a k produced Cr(C0)5...MCH, a more s e n s i t i v e t e s t of these weak i n t e r a c t i o n s i s U V - v i s i b l e spectroscopy (21) and i t i s hoped such measurements w i l l be made. We have r e c e n t l y been i n v e s t i g a t i n g an a l t e r n a t i v e approach, liquid noble gases. These seem to be i d e a l solvents f o r i n v e s t i g a t i n g organometallic photochemistry; the noble gases are complete l y transparent over a very wide s p e c t r a l range (hence long pathlengths are p o s s i b l e ) . By working i n a s p e c i a l l y designed pressure c e l l (32) i t i s p o s s i b l e to cover the temperature range ^80K to 240K, using Ar, Kr o r Xenon, and these noble gas s o l v e n t s , p a r t i c u l a r l y Ar (Figure 2 ) , are l i k e l y to be weakly i n t e r a c t i n g . S u r p r i s i n g l y , metal carbonyls are s o l u b l e i n these l i q u i d s e.g. Cr(C0)6 i n l i q u i d Xe, and Fe(C0)5 even i n l i q u i d Ar. Our i n i t i a l experiments have i n v o l v e d N2 complexes, s i n c e these are expected to be moderately s t a b l e , and i n the case o f Cr(C0)5N2, allow comparison w i t h both matrix i s o l a t i o n and room temperature f l a s h p h o t o l y s i s experiments. We have generated (33,34) the species C r ( C O ) 5 - ( N 2 ) (x = 1 to 5) by p h o t o l y s i s of Cr(C0)6 i n liquid xenon at -80°C doped w i t h N2. Figure 4 shows both the p h o t o l y t i c production of Cr(CO)5N2 and also photochemical regene r a t i o n of Cr(C0)6, i . e . e x a c t l y mimicking the matrix behavior (22). Prolonged p h o t o l y s i s o f Cr(C0)6 i n l i q u i d xenon w i t h h i g h e r concentration of added N2 generates more h i g h l y ^ - s u b s t i t u t e d s p e c i e s . Figure 5 shows how the s t a b i l i t i e s of the d i f f e r e n t C r ( C O ) 6 - ( N 2 ) species depend on the degree of N2 s u b s t i t u t i o n and suggests that k i n e t i c parameters might be o b t a i n a b l e . For thermally less s t a b l e compounds, s e q u e n t i a l p h o t o l y s i s and s p e c t r a l a n a l y s i s are inadequate because the compounds decompose before they are detected. However, using a s p e c i a l 4-way c e l l we have been able to generate the very unstable species Ni(00)3^, by p h o t o l y s i s o f N i (CO) 4 i n l i q u i d Kr doped with N2 at -170OC, and record the IR spectrum during UV i r r a d i a t i o n . On switching o f f the UV lamp the IR spectrum o f Ni (00)3^ decays and by monitoring the r a t e of t h i s decay we have measured k i n e t i c and a c t i v a t i o n x

x

x

x

energy parameters (35) f o r Ni(C0)3N + CO •Ni(CO)4 + N2. UV p h o t o l y s i s of Cr(CO)£ i n pure l i q u i d xenon ( i . e . i n the absence of N2) produced IR bands of a t r a n s i e n t species ( t i ^ 2 sec at -78°C) which may w e l l be unstable Cr(C0)5...Xe. We b e l i e v e that such experiments w i l l provide an important e x t r a window on s o l u t i o n k i n e t i c s monitored by f l a s h techniques to which we now t u r n . 2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

ΙΑ C σ

5

2

J ι ι 2250 2230

Cr(CO) N

(a)

5

Wavenumber/ cm"

i

g 2

1940

t

6

Figure 4. Absorption spectra of Cr(CO) N and Cr(CO) dissolved in liquid Xe at —79°C. Key: · · ·, before photolysis; , after 3.3 min irradiation with an unfilteredH g arc lamp placed 28 cm from the center of the cryostat; and , after irradiation for about 12 min with the Hg arc lamp and Balzers 367-nm filter about 14 cm from the center of the cryostat. (Where the curves are indistinguishable, only the dotted curve is shown.) For clarity, the noise has been omitted from the left curve. The feature marked with an asterisk (*) is assigned to Cr(CO) (N ) . (Reproduced with permission from Ref. 33. Copyright 1980, Royal Society of Chemistry.)

60
-C5H5)W(CO)3R i n p a r a f f i n matrices. With R = CH3 the intermediate i s b l u e , but when R contains $ hydrogens the i n t e r mediate i s yellow; by analogy w i t h the Cr(C0)5 i n t e r a c t i o n data (21) i t i s concluded that t h i s yellow intermediate adopts a c y c l i c s t r u c t u r e i n which a $-H i s coordinated back to the metal centre. The most potent species f o r a c t i v a t i n g hydrocarbons are l i k e l y to be naked t r a n s i t i o n metal atoms. Margrave (62) and Ozin (63) have shown that cocondensation of Fe atoms o r Cu atoms w i t h methane at ^10K followed by UV i r r a d i a t i o n produces d e f i n i t e s p e c t r o s c o p i c evidence f o r the i n s e r t i o n of the metal atoms i n t o the C-H bond. I t i s l i k e l y that f u r t h e r experiments along these l i n e s w i l l open up new c a t a l y t i c p o s s i b i l i t i e s . Rearrangements. I t w i l l have been obvious from the Cr(C0)5-.. X d i s c u s s i o n above that the matrix technique i s p a r t i c u l a r l y w e l l s u i t e d to the examination o f photochemically induced rearrangements and i s o m e r i z a t i o n s i n c e the matrix holds the p a r t i c i p a t i n g species i n f i x e d p o s i t i o n s and hence allows s p e c t r o s c o p i c study of each, y e t allows ready photochemical i n t e r c o n v e r s i o n . One of the more d e t a i l e d s t u d i e s of t h i s type (£4) has i n v o l ved the u n r a v e l l i n g o f the rearrangement modes o f the c o o r d i n a t i v e l y unsaturated species W(C0)4CS, which was generated by UV p h o t o l y s i s of W(C0)5CS; the experiments i n v o l v e d IR/UV s p e c t r o s copy, wave-selective p h o t o l y s i s and a n a l y s i s based on ^ C 0 subs t i t u t i o n . The r e s u l t s are summarised i n Figure 7. These experiments also c o n f i r m that W(C0)4CS i s i n i t i a l l y formed i n an e x c i t e d s t a t e , as p r e d i c t e d by the scheme o u t l i n e d i n Figure 3. Even more complex experiments have been performed on matrix i s o l a t e d Fe(C0)4, generated by UV p h o t o l y s i s of Fe(C0)5. Isotopic l a b e l l i n g coupled w i t h CW-CO l a s e r pumping (65) o f the CO s t r e t c h i n g v i b r a t i o n s (\>1900 c u r l ) showed that the rearrangement mode o f Fe(C0)4 follows an i n v e r s e Berry pseudo-rotation as shown i n Figure 8. S i m i l a r l y , i n experiments w i t h Fe(fi0) (generated i n a matrix from Fe(C0)4L; L = NMe3) , the C 3 and C forms of the species can be i n t e r c o n v e r t e d w i t h l i g h t o f appropriatE wavelength(66). V

s

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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S

c x

max ~

300 nm

3

0

5

n

m

300 nm 48Sf

S C

432 nm Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch003

489 nm

CS

"max 470 nm

max 445 nm /v

Figure 7. Summary of the photochemistry of W(CO) CS in an Ar matrix. (Reproduced from Ref. 64. Copyright 1976, American Chemical Society.) 5

1919 1898

1902

1894

1894

1881

1901 1894 1881

12

î6

13

18

Figure 8. The observed IR-laser induced isomerizations of Fe( C 0) -j( C 0) species in an Ar matrix. X represents the C 0 group, and the numbers represent the wavenumbers of the CO laser lines that induce the particular isomerizations. h

13

18

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Photochemical Routes. F i n a l l y , two sets of experiments from Rest's laboratory which demonstrate the s u b t l e i m p l i c a t i o n s of q u i t e complex matrix photochemistry experiments. The f i r s t (67) i n v o l v e s (r) -C5H5)Co(C0)2 and can be summarised 5

(n5c H )Co(CO)

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5

5


290

m

/

C

Q

»*

t T i

l

(n3-c H )Co(CO) 5

5

3

nm

[There was no evidence f o r production of (n^-Cs*^)Co(C0) i n Ar matrices, presumably because of very ready recombination w i t h CO i n the matrix cage. Whether a matrix i s o l a t e d carbonyl molecule can e j e c t CO without automatic recombination probably requires the photochemical path to lead to a ground s t a t e fragment i n which the empty coordination s i t e i s o r i e n t e d away from the photoejected CO - see the Cr(C0)5 photochemical scheme (Figure 3) .] In a CO matrix, therefore, the primary product involves an expand­ ed c o o r d i n a t i o n number ("ring-slippage") and i t i s argued that such a species i s c o n s i s t e n t with the a s s o c i a t i v e mechanism prop­ osed f o r room temperature s u b s t i t u t i o n reactions of (r|5-C5H5)Co(C0) . By c o n t r a s t , when (n -C5H5)Fe(C0)2(C0R) i s photolysed (fcâ), even i n CO m a t r i c e s , only CO loss i s observed with (r|5-C5H5)Fe(C0)(C0R) being formed en route to (n5-C5H5)Fe(CO)2R. 2

5

Dinuclear Carbonyls. One o f the c h a r a c t e r i s t i c features i n the UV s p e c t r a of metal-metal bonded d i n u c l e a r carbonyls and s u b s t i t u t e d species i s an intense band i n the 350 nm region (1). For example, f o r Mn2(CO)io t h i s band, at 336 nm, i s assigned t o a t r a n s i t i o n from the f i l l e d σ M-M bonding molecular o r b i t a l to the corresponding antibonding o r b i t a l . Confirmation of t h i s assignment comes from an elegant experiment (27) based on the argument that such a t r a n s i t i o n must be p o l a r i z e d along the Mn-Mh axis ( i . e . of t r a n s i t i o n moment B2) confirmed by examining the d i c h r o i c p r o p e r t i e s of IR and UV bands of MXI2(CO);LO i n a nematic solvent. A great deal of photochemistry of d i n u c l e a r carbonyls i s c o n s i s t e n t with the generation of r a d i c a l s f o l l o w i n g M-M bond schism on i r r a d i a t i o n i n t o the σ — • o*band. However, even f o r the much studied Mri2(CO)io the p i c t u r e i s s t i l l obscure. For i n s t a n c e , pulse r a d i o l y s i s (65) and f l a s h p h o t o l y s i s (70-72) combine to suggest that i n a d d i t i o n to generation of two Mn(C0)5 r a d i c a l s other photochemical routes may i n v o l v e bridged-Mn2(CO)^o> Mn2(CO)9,Mn2(CO)3; however, i n view of the enormous importance o f solvent i n t e r a c t i o n s and i m p u r i t i e s (see Cr(C0)5 above) one wonders what the c o r r e c t explanation i s going to be. Matrix i s o l a t i o n has already provided a v a l u a b l e i n s i g h t i n t o the behavior o f d i n u c l e a r carbonyls. In the f i r s t experiments of t h i s k i n d we were able (ZD to show that, on p h o t o l y s i s i n s o l i d Ar, Fe2(C0)g loses CO to form Fe2(C0)s i n both b r i d g e d and unbridged forms and the behavior of these fragments was s t u d i e d ;

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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i n t e r e s t i n g l y Hoffmann (74) has r e c e n t l y drawn a t t e n t i o n to t h i s Fe2(C0)3 species and to Stone's experiments i n v o l v i n g polynuclear complexes of Fe2(CO>3. More r e c e n t l y , Sweany and Brown (75) have shown that UV p h o t o l y s i s of 002(00)3 i n argon generates unbridged Co2(CO)7, while i n CO matrices there i s evidence f o r Co(CO)4. Thus, there i s evidence that matrix studies could help with Mri2(CO)i0> unfortunately, such experiments have so f a r been rather unrevealing. Presumably the Cage E f f e c t encourages the recombination o f any Mn(C0)5 r a d i c a l s generated by p h o t o l y s i s . Indeed the best way of generating Mn(C0)5 photochemically i s by UV p h o t o l y s i s of H*fci(C0)5 i n a CO matrix (54). The s t r u c t u r e o f Mn(C0)5 as a C4 fragment has now been e s t a b l i s h e d (54) by i s o t o p i c IR and, more r e c e n t l y , confirmed by ESR (£6). I t i s , however, our b e l i e f that subtle wavelength-dependent p h o t o l y s i s experiments, p a r t i c u l a r l y using p o l a r i z e d l i g h t , w i l l e v e n t u a l l y unravel the problem of Mri2(CO)io* Another d i n u c l e a r carbonyl which presents i n t e r e s t i n g problems i s [(n -C5H5)Fe(C0)2] 2· photochemistry proceed e x c l u s i v e l y through homolytic f i s s i o n to produce two (1^-05115) Fe(C0)2 r a d i c a l s or by other p o s s i b l e routes? The d i s c u s s i o n of t h i s r e a c t i o n has involved mechanistic and s y n t h e t i c studies (77), f l a s h p h o t o l y s i s (78) and low-tempe rature p h o t o l y s i s (22.) - the l a t t e r work, i n THF or e t h y l c h l o r i d e at -78°C, invokes an i n t e r mediate i n which the Fe-Fe d i r e c t bond i s broken but the two halves of the molecule are h e l d together by a CO b r i d g e . C l e a r l y such an i n t r i g u i n g problem merits more d e t a i l e d i n v e s t i g a t i o n s .

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch003

V

5

D

o

e

s

t l i e

Polynuclear C l u s t e r s . The photochemistry of polynuclear c l u s t e r s i s a very a c t i v e f i e l d , p a r t l y because of the p o t e n t i a l c a t a l y t i c importance of these compounds. I t i s s t i l l not c l e a r how to p r e d i c t the conditions which w i l l lead t o d e c l u s t e r i f i c a t i o n as opposed to i n t e r n a l rearrangement or s u b s t i t u t i o n (1). There i s an absence of good d e f i n i t i v e evidence f o r s p e c i f i c intermediates and we c l o s e with a h i n t . Some years ago i n rather crude unpublished work (80) we showed that the p h o t o l y s i s of Fe3(CO)i2 i s o l a t e d i n a matrix l e d to production of some CO, but more p a r t i c u l a r l y , complete disappearance of IR bands due to the b r i d g i n g CO groups and the appearance of new terminal CO IR bands. We b e l i e v e that c a r e f u l studies of t h i s k i n d , t a k i n g advantage of s o p h i s t i c a t e d FTIR methods, w i l l provide valuable i n s i g h t i n t o even complex photochemical intermediates. Acknowledgments We thank a l l those who have had discussions with us and are g r a t e f u l to the S.E.R.C. f o r generous support of our work. We wish to acknowledge the help of a l l our colleagues i n Nottingham, p a r t i c u l a r l y Mr P.W. Lemeunier and Dr M.A. Healy.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch003

Literature Cited

1. For an excellent recent review see Geoffroy, G.L.; Wrighton, M.S. "Organometallic Photochemistry", Academic Press: New York, 1979. 2. Kidd, D.R.; Cheng, C.P.; Brown, T.L. J. Am. Chem. Soc. 1978, 100, 4103. 3. McCullen, S.B.; Brown, T.L. J. Am. Chem. Soc. in press 4. Whyman, R. J. Organomet. Chem. 1975, 94, 303, and earlier references quoted therein. 5. Koerner von Gustorf, E.A.; Leenders, L.H.G.; Fischler, I; Perutz, R.N. Adv. Inorg. Rad. Chem. 1976, 19, 65. 6. Chase, D.B.; Weigert, F.J. J. Am. Chem. Soc. 1981, 103, 977. 7. Kazlauskas, R.J.; Wrighton, M.S. J. Am. Chem. Soc. in press. 8. Hermann, H.; Grevels, F.W.; Henne, Α.; Schaffner, Κ., to be published. 9. Brus, L.E.; personal communication. 10. Moskovits, M.; Ozin, G.A. Eds.; "Cryochemistry"; Wiley: New York, 1976 11. Barnes, A.J.; Müller, Α.; Orvilie-Thomas, W.J. Eds. "Matrix Isolation Spectroscopy"; Deidel, 1981. 12. Turner, J.J.; Burdett, J.K.; Perutz, R.N.; Poliakoff, M. Pure and Appl. Chem. 1977, 49, 271. 13. Burdett, J.K. Coord. Chem. Rev. 1978, 27, 1. 14. Stolz, I.W.; Dobson, G.R.; Sheline, R.K. J Am. Chem. Soc. 1962, 84, 3589. 15. Stolz, I.W.; Dobson, G.R.; Sheline, R.K. J Am. Chem. Soc. 1963, 85, 1013. 16. Perutz, R.N.; Turner, J.J. Inorg. Chem. 1975, 14, 262. 17. Burdett, J.K.; Poliakoff, M.; Timney, J.Α.; Turner, J.J. Inorg. Chem. 1978, 17, 948. 18. Burdett, J.K. Inorg. Chem. 1981, 20, 2607. 19. Burdett, J.K. "Molecular Shapes"; Wiley-Interscience: New York, 1980. 20. Ryott, G.J. Ph.D., Thesis, Nottingham University, Nottingham, 1982. 21. Perutz, R.N.; Turner, J.J. J. Am. Chem. Soc. 1975, 97, 4791. 22. Burdett, J.K.; Downs, A.J.; Gaskill, G.P.; Graham, M.A.; Turner, J.J.; Turner, R.F. Inorg. Chem. 1978, 17, 523. 23. Demuynck, J.; Kochanski, E.; Veillard, A. J. Am. Chem. Soc. 1979, 101, 3467. 24. Burdett, J.K.; Perutz, R.N.; Poliakoff, M.; Turner, J.J. J.C.S. Chem. Commun. 1975, 157. 25. Burdett, J.K.; Grzybowski, J.M.; Perutz, R.N.; Poliakoff, M; Turner, J.J.; Turner R.F. Inorg. Chem. 1978, 17, 147. 26. Baird, M.S.; Dunkin, I.R.; Hacker, N.; Poliakoff, M.; Turner, J.J. J. Am. Chem. Soc. 1981, 103, 5190. 27. Levenson, R.A.; Gray, H.B.; Ceasar, G.P. J. Am. Chem. Soc. 1970, 92, 3653. 28. Hay, P.J. J. Am. Chem. Soc. 1978, 100, 2411.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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29. Adamson, A.W.; personal communication. 30. Nasielski, J.; Colas, A.J. Organomet. Chem. 1975, 101, 215. 31. Tyler, D.R.; Petrylak, D.P. J. Organomet. Chem. 1981, 212, 289. 32. Beattie, W.H.; Maier, W.B.; Holland, R.F.; Freund, S.M.; Stewart, B. Proc. SPIE (Laser Spectroscopy) 1978, 158, 113. 33. Maier, W.B.; Poliakoff, M.; Simpson, M.B.; Turner, J.J. J.C.S. Chem. Commun. 1980, 587. 34. Maier, W.B.; Poliakoff, M.; Simpson, M.B.; Turner, J.J. Inorg. Chem., in press. 35. Maier, W.B.; Poliakoff, M.; Simpson, M.B.; Turner, J.J. to be published. 36. Bonneau, R.; Kelly, J.M. J. Am. Chem. Soc. 1980, 102, 1220. 37. Lees, A.J.; Adamson, A.W. Inorg. Chem. 1981, 20, 4381. 38. Breckenridge, W.H.; Sinal, N. J. Phys. Chem. 1981, 85, 3557. 39. Tumes, W.; Gitlin, B.; Rosan, A.M.; Yardley, J.T. J. Am. Chem. Soc. 1982, 104, 55. 40. Nathanson, G.; Gitlin, B.; Rosan, Α.; Yardley, J.T. J. Chem. Phys. 1981, 74, 361, 370. 41. Perutz, R.N.; Turner, J.J. J. Am Chem. Soc. 1975, 97, 4800. 42. Poliakoff, M.; Turner, J.J. J.C.S. Faraday Trans. II, 1974, 70, 93. 43. Mahmoud, Κ.A.; Narayanaswamy, R.; Rest, A.J. J. Chem. Soc., Dalton, 1981, 2199. 44. Kazlauskas, R.J.; Wrighton, M.S. J. Am. Chem. Soc. 1980, 102, 1727. 45. Boxhoorn, G.; Stufkens, D.J.; Oskam, A. Inorg. Chim. Acta, 1979, 33, 215. 46. Boxhoorn, G.; Schoemaker, G.C.; Stufkens, D.J.; Oskam, Α.; Rest, A.J.; Darensbourg, D.J. Inorg. Chem. 1980, 19, 3455. 47. Geoffroy, G.L. Prog. Inorg. Chem. 1980, 27, 123. 48. Green, M.L.H. Pure Appl. Chem. 1978, 50, 27. 49. J.C.S. Grebenik, P.; Downs, A.J.; Green, M.L.H.; Perutz, R.N. Chem. Commun. 1979, 742. 50. Chetwynd-Talbot, J.; Grebenik, P.; Perutz, R.N. J.C.S. Chem. Commun. 1981, 452. 51. Sweany, R.L. X Cong. Organomet. Chem., Toronto, 1981, Abstract IC05, 44. 52. Poliakoff, M; Turner, J.J. J.C.S. Dalton, 1974, 2276. 53. Poliakoff, M.J.C.S. Dalton, 1974, 210. 54. Church, S.P.; Poliakoff, M.; Timney, J.A.; Turner, J.J. J. Am. Chem. Soc. 1981, 103, 7515. 55. Sweany, R.L. Inorg. Chem. 1982, 21, 752. 56. Wermer, P.; Ault, B.S.; Orchin, M. J. Organomet. Chem. 1978, 162, 189. 57. Sweany, R.L. Inorg. Chem. 1980, 19, 3512. 58. Parshall, G.W. "Homogeneous Catalysis"; Wiley Interscience: New York, 1980. 59. Janowicz, A.H.; Bergman, R.G. J. Am. Chem. Soc. 1982, 104, 352.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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60. Poliakoff, M. Chem. Soc. Rev. 1978, 7, 528. 61. Kazlauskas, R.J.; Wrighton, M.S. J. Am. Chem. Soc. in press. 62. Billups, W.E.; Konarski, M.M.; Hauge, R.H.; Margrave, J.L. J. Am. Chem. Soc. 1980, 102, 7394. 63. Ozin, G.A.; McIntosh, D.F.; Mitchell, S.A.; Garcia-Prieto, J. J. Am. Chem. Soc. 1981, 103, 1574. 64. Poliakoff, M. Inorg. Chem. 1976, 15, 2022, 2892. 65. Poliakoff, M.; Turner, J.J. "Chemical and Biochemical Applications of Lasers", Moore, C.B. Ed; Academic Press; New York, 1980; Vol. 5, p.175. 66. Boxhoorn, G.; Cerfoutain, M.B.; Stufkens, D.J.; Oskam, A. J.C.S. Dalton, 1980, 1336. 67. Crichton, O.; Rest, A.J.; Taylor, D.J. J.C.S. Dalton, 1980, 167. 68. Fettes, D.J.; Narayanaswamy, R.; Rest, A.J. J.C.S. Dalton, 1981, 2311. 69. Waltz, W.L.; Hackelberg, O.; Dorfman, L.M.; Wojcicki, A. J. Am. Chem, Soc. 1978, 100, 7259. 70. Hughey, J.L.; Anderson, C.P.; Meyer, T.J. J. Organomet. Chem. 1977, 125, C49. 71. Wegman, R.W.; Olsen, R.J.; Gard, D.R.; Faulkner, L.R.; Brown, T.L. J. Am. Chem. Soc. 1981, 103, 6089. 72. Yasufuku, K.; Yesaka, H.; Kobayashi, T.; Yamazaki, H.; Nagakura, S. Proc. 10th Conf. Organomet. Chem., Toronto, 1981, Abstract IC02, p41. 73. Poliakoff, M.; Turner, J.J. J. Chem. Soc. A 1971, 2403. 74. Hoffmann, R. Nobel Lecture, 1981. 75. Sweany, R.; Brown, T.L. Inorg. Chem. 1977, 16, 421. 76. Symons, M.C.R.; Sweany, R.L. Organometallics, in press. 77. Abrahamson, H.B.; Palazzotto, M.C.; Reichel, C.L.; Wrighton, M.S. J. Am. Chem. Soc. 1979, 101, 4123. 78. Caspar, J.V.; Meyer, T.J. J. Am. Chem. Soc., 1980, 102, 7794. 79. Tyler, D.R.; Schmidt, M.A.; Gray, H.B. Inorg. Chem. 1979, 14, 2753. 80. Poliakoff, M. Ph.D., Thesis, Cambridge,University, Cambridge, England, 1972. 81. Welch, J.Α.; Peters, K.S.; Vaida, V. J. Phys. Chem., 1982, 86, 1941. RECEIVED August 3, 1982

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Discussion

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch003

A.W. Adamson, U n i v e r s i t y o f S o u t h e r n C a l i f o r n i a : You spoke o f an e x c i t e d s t a t e C^ p r o d u c t . We have n e v e r been a b l e t o o b s e r v e s o l u t i o n p h o t o c h e m i s t r y of a c o o r d i n a t i o n compound i n w h i c h l i g a n d d i s s o c i a t i o n p r o d u c e s an e x c i t e d s t a t e p r o d u c t . F u r t h e r , w h i l e c h e m i l u m i n e s c e n t r e a c t i o n s a r e known, t h e ones i n v o l v i n g c o o r d i n a t i o n compounds p r o d u c e l i g h t o n l y i n q u i t e low y i e l d , i n d i c a t i n g t h a t t h e p a t h t h r o u g h an e x c i t e d s t a t e i s n o t a f a v o r e d one. JDo you know o f any d i r e c t e x p e r i m e n t a l e v i d e n c e f o r t h e [C^ ] p r o d u c t ?

J . J . Turner. U n i v e r s i t y of Nottingham: The short answer i s No(!); but the f i r s t point to make i s that we don't claim that C4v Cr(C0>5 i n an excited s t a t e i s a 'product' - undoubtedly the 'product' i s C4 Cr(C0>5 i n the ground s t a t e . What the experiments t e l l us i s t h i s . The p o l a r i z e d p h o t o l y s i s / s p e c t r o s c o p i c experiments ( r e f . 24,25 of above) prove that the Cr(C0>5X species are ' s t i r r e d ' by v i s i b l e l i g h t , f i r s t v i a an e x c i t e d s t a t e of the C4 Cr(C0>5X and then, almost c e r t a i n l y , v i a the D3 intermediate s t a t e (see f i g u r e 9 i n r e f . 25), i . e . a l l the pathways i n f i g u r e 3 (above) are e s t a b l i s h e d except that from Cr(C0)5 to D3 Cr(C0)5 v i a the e x c i t e d C4 fragment. However, i n matrix experiments, my colleague, Martyn P o l i a k o f f ( r e f . 64 above), showed that p h o t o l y s i s of s t e r e o s p e c i f i c a l l y ^ço-labelled trans( C0)W(C0)4CS y i e l d s , as the major product, square-pyramidal cis-(' C0)W(C0)3CS with the CS group i n the a x i a l p o s i t i o n , i . e . the p r i n c i p a l path involves l o s s of e q u a t o r i a l CO and rearrangement of the remaining CS and four CO groups. Whether t h i s i s imagined as CO loss followed by rearrangement ( i . e . analogous to the e x c i t e d s t a t e path i n f i g u r e 3)or as a concerted process does not a f f e c t the argument - we c e r t a i n l y don't consider the e x c i t e d C4 fragment as a ' t h e x i ' s t a t e , we j u s t f i n d i t e a s i e r to p i c t u r e the 6 > 5 process v i a the e x c i t e d fragment. V

V

n

n

V

13

L3

V

G.A. O z i n , U n i v e r s i t y of T o r o n t o ; I n our Cr/CO m a t r i x c o c o n d e n s a t i o n e x p e r i m e n t s (Angew. Chem., I n t . E d . Eng. 1975, 14, 2 9 2 ) , we r e p o r t e d e v i d e n c e f o r t h e f a c i l e f o r m a t i o n of a binuclear chromium c a r b o n y l complex Cr (CO)i or Cr (CO)x which c o u l d be d e s c r i b e d a s s q u a r e p y r a m i d a l C r ( C O ) weakly i n t e r a c t i n g w i t h e i t h e r a Cr(CO) or Cr(CO) moiety i n the v a c a n t ( s i x t h ) s i t e . As a r e s u l t , t h e i n f r a r e d s p e c t r u m of t h i s " w e a k l y - c o u p l e d " b i n u c l e a r s p e c i e s c l o s e l y r e s e m b l e d t h a t of the mononuclear f r a g m e n t C r ( C 0 ) . I w o u l d l i k e t o ask y o u , w h e t h e r o r n o t y o u h a v e any e v i d e n c e f o r t h e e x i s t e n c e of such a binuclear s p e c i e s i n y o u r C r ( C O ) /Xe cryogenic solutions following various photolysis treatments. 2

0

2

5

5

6

5

6

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

x

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch003

3.

TURNER A N D POLIAKOFF

Photochemical

Intermediates

57

J . J . Turner, u n i v e r s i t y o f Nottingham: You are q u i t e correct that species such as Cr(C0>5...X, where X i s a weakly c o o r d i n a t ­ i n g 'ligand* such as Ar, Xe, CH4, Cr(C0>6, show b a r e l y distinguish­ able IR s p e c t r a i n the carbonyl region. However, the v i s i b l e band i s extremely s e n s i t i v e t o X whether measured i n matrices (Ar, 533 nm; Xe, 492 nm; CH4, 489 nm (see r e f . 21 above); Cr(CO)£, ^460 nm ( J . Amer. Chem. S o c , 1975, 97^ 4805)) o r i n s o l u t i o n (cyclohexane, 510 nm; Cr(C0)6, 485 nm (see r e f . 36 above)). The p o s i t i o n of t h i s band r e f l e c t s the strength of Cr(C0)5/X i n t e r ­ a c t i o n which suggests that Cr(C0)£ and Xe w i l l i n t e r a c t s i m i l a r l y with Cr(C0)5; given that i n l i q u i d Xe, Cr(C0)6 i s extremely d i l u t e (^1 ppm) i t seems u n l i k e l y that one would observe Cr(C0)5... Cr(C0)6 i n preference to Cr(C0)5...Xe. Nevertheless, proof of t h i s must await UV data which we are i n the process o f o b t a i n i n g . Two f u r t h e r r e l a t e d comments. F i r s t l y , s i n c e Ν2 i s a "good" l i g a n d ( X , 364 nm) , w i t h N2~doped Xe there i s no trace even of Cr(C0)5...Xe s i n c e Cr/C0/N2 species predominate. Secondly, during experiments w i t h Ni(CO)4/N2/liquid Kr (see above), p h o t o l y s i s i n the complete absence of d i s s o l v e d N2 l e d t o the appearance o f a t r a n s i e n t carbonyl species w i t h IR bands s i m i l a r t o those assigned to NÎ2(C0)7 (J.E. Hulse and M. Moskovits; unpublished data) presumably the i n t e r a c t i o n of Ni(CO)3 with Ni(CO)4 i s considerably stronger than w i t h Kr. m a x

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4 Fuel and Electricity Generation from Illumination of Inorganic Interfaces M A R K S. W R I G H T O N

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

Massachusetts Institute of Technology, Department of Chemistry, Cambridge, MA 02139

Semiconductor-based photoelectrochemical devices represent good systems for the sustained, direct conversion of light to chemical or electrical energy. The interfacial structure, energetics, and redox kinetics control the overall performance of such systems. Examples of improvements in efficiency and durability of photoelectrochemical cells stemming from chemical manipulations at semiconductor/liquid electrolyte interfaces illustrate the critical importance of understanding interface properties. Inorganic chemistry at i n t e r f a c e s i s c r u c i a l to a l a r g e number of processes, systems, and devices that have p r a c t i c a l consequence now and i n t o the 21st century. Heterogeneous c a t a l y s t systems, b a t t e r i e s , fuel c e l l s , f i e l d e f f e c t t r a n s i s t o r s , o p t i c a l and a c o u s t i c a l r e c o r d e r s , sensors, s o l a r c e l l s , and even natural p l a n t photosynthesis are a l l dependent on interfaces. I t i s becoming evident t h a t c h a r a c t e r i z a t i o n , s y n t h e s i s , m a n i p u l a t i o n , and understanding of i n t e r f a c i a l p r o p e r t i e s w i l l comprise a s i g n i f i c a n t f r a c t i o n of the fundamental e f f o r t undergirding many p r a c t i c a l a p p l i c a t i o n s of i n o r g a n i c chemistry. With the advent of an arsenal of new s p e c t r o s c o p i c probes i t i s apparent t h a t chemically complex i n t e r f a c e s can be c h a r a c t e r i z e d with good r e s o l u t i o n . The impact o f the s t r u c t u r a l c h a r a c t e r i z a t i o n of i n t e r f a c e s i s l i k e l y to be as great as from the s t r u c t u r a l c h a r a c t e r i z a t i o n that i s commonplace for r e l a t i v e l y small molecular e n t i t i e s . Elucidation of molecular s t r u c t u r e plays a c e n t r a l r o l e i n the understanding o f r e a c t i o n s and leads to i n s i g h t i n t o d e t a i l s of mechanism and electronic structure. E x p l o i t a t i o n of i n t e r f a c e s i n inorganic-based systems can continue with no new i n s i g h t s . However, as the 21st century approaches i n o r g a n i c chemists have the opportunity to c o n t r i b u t e h e a v i l y to the understanding of 0097-6156/83/021l-0059-$09.25/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

60

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T O W A R D T H E 21ST

CENTURY

i n t e r f a c e s and r a t i o n a l a p p l i c a t i o n of basic knowledge to b e t t e r use i n t e r f a c e s . One current basic research endeavor i n v o l v e s the use of i l l u m i n a t e d i n t e r f a c i a l chemical systems to b r i n g about the sustained conversion of o p t i c a l energy to chemical or e l e c t r i c a l energy. In f a c t , semiconductor-based photoelectrochemical c e l l s l i k e that i n Scheme I represent the best chemical systems for the d i r e c t conversion of o p t i c a l energy to e l e c t r i c a l energy or to chemical energy i n the form of high energy redox p r o d u c t s . ( 1 ) In such devices the l i g h t - a b s o r b i n g semiconductor electrode immersed i n an e l e c t r o l y t e s o l u t i o n comprises a p h o t o s e n s i t i v e i n t e r f a c e where thermodynamically u p h i l l redox processes can be driven with o p t i c a l energy. Depending on the nature of the photoelectrode, e i t h e r a reduction or an o x i d a t i o n h a l f - r e a c t i o n can be l i g h t - d r i v e n with the counterelectrode being the s i t e of the accompanying h a l f - r e a c t i o n . N-type semiconductors are photoanodes, p-type semiconductors are photocathodes, (2^2*1) i n t r i n s i c m a t e r i a l s (5) can be e i t h e r photoanodes or photocathodes depending on the nature of the contact by both the l i q u i d and the wire of the external c i r c u i t . Within the past decade remarkable progress has emerged from conscious e f f o r t s to understand and improve semiconductor-based photoelectrochemical cells. Systems based on η - t y p e GaAs,(6,_7) η - t y p e WSe2, p-type InP (1Ό, 11) have been shown to "Be able to convert s u n l i g h t w i t h greater e f f i c i e n c y than the widely regarded minimum useful e f f i c i e n c y of 10% for large scale energy generation from s u n l i g h t i n the United States.(12^) The aim of t h i s presentation i s both to o u t l i n e recent advances and to i d e n t i f y problems associated with i l l u m i n a t e d semiconductor electrodes for o p t i c a l energy c o n v e r s i o n . Results from t h i s laboratory w i l l be h i g h l i g h t e d . P r a c t i c a l a p p l i c a t i o n s , i f they come at a l l , w i l l be important i n the 21st century and beyond. However, though the prospects for large s c a l e energy generation from s u n l i g h t may appear dim, i t i s c l e a r t h a t e x i s t i n g f o s s i l fuel reserves are f i n i t e , f i s s i o n nuclear power has an uncertain f u t u r e , and fusion does not work now and may not work i n the f u t u r e . S o l a r chemical conversion schemes do work on a l a r g e scale as evidenced by the natural photosynthetic apparatus. E x i s t i n g s o l a r i n s o l a t i o n i s far greater than man's needs. The research e f f o r t required to f u l l y i n v e s t i g a t e i n t e r f a c i a l s o l a r conversion schemes i s worth expending when viewed against the p o s s i b l e return and the prospect t h a t other a l t e r n a t i v e s may prove unacceptable from a t e c h n i c a l , s o c i a l , p o l i t i c a l , economic, or safety standpoint. Semiconductor/Liquid E l e c t r o l y t e Interface

a

n

d

a

n

d

Energetics

Schemes II and I I I represent the e q u i l i b r i u m i n t e r f a c e ener­ g e t i c s for i d e a l n- and p-type semiconductors, r e s p e c t i v e l y , for

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

4.

WRIGHTON

Fuel and Electricity

Generation

Scheme I. Semiconductor-based photoelectrochemical cell. Energy output may be in the form of electricity by putting a load in series in the external circuit, or the output can be in the form of chemical energy as redox products formed at the electrodes. N-type semiconductors effect uphill oxidations upon illumination, and p-type semiconductors effect uphill reductions under illumination. Either or both electrodes in the cell can be a photoelectrode.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

61

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. redox

Representation of the interface energetics for an ideal η-type contact with electrolyte solutions of differing E .

semiconductor in

INCREASING OXIDIZING POWER OF CONTACTING SOLUTION

re

For Eredox more negative than theflatband potential (b) there is little band bending (no barrier) and the elec­ trode is reversible with respect to the redox couple and is in ohmic contact (a). For Ε,··*·»* between theflatband potential and some positive potential represented in (c) the drop in potential occurs across the semiconductor and the behavior is "ideal" because the band bending varies following Equation 1. For more positive E ο

4-> C Ο)

•Γ-

(Ο

(+)

E

0

CB

redox

re

Representation of the interface energetics for an ideal p-type semiconductor contact with electrolyte solutions of differing E dox-

(a)

INCREASING REDUCING POWER OF CONTACTING SOLUTION

in

r

redox

The situation is analogous to the η-type semiconductor (Scheme II) except that the more positive E *io, gives an ohmic contact and the most negative Ε™*** gives inversion. For Ε™*»* between the flat band potential (b) and some negative potential, the behavior is "ideal" because the band bending varies following Equation 1.

Scheme III.

Ε ο

£

s il U ΓΤ

(-)

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

64

INORGANIC CHEMISTRY:

T O W A R D T H E 21ST

CENTURY

contact by e l e c t r o l y t e s o l u t i o n s c o n t a i n i n g d i f f e r e n t redox reagents that vary i n electrochemical p o t e n t i a l , Ε ι . ( 2 , 3 , 4 , 1 3 ) For E j s u f f i c i e n t l y negative for η - , or s u f f i c i e n t l y p o s i t i v e for p-type, semiconductors the electrode behaves as a m e t a l l i c e l e c t r o d e , not b l o c k i n g the flow of e l e c t r o n s i n e i t h e r d i r e c t i o n . This s i t u a t i o n i s analogous to the c r i t e r i a for forming an ohmic contact to an n - or p-type semiconductor.(14) When E j i s between the top of the valence band, Εγβ, ancFthe bottom of the conduction band, EQB» the p-type semiconductor i s b l o c k i n g to reductions and the η - t y p e semiconductor i s b l o c k i n g to o x i d a t i o n s i n the dark. The m i n o r i t y c a r r i e r (e~ or h f o r p- or η - t y p e semiconductors, r e s p e c t i v e l y ) i s only a v a i l a b l e upon p h o t o e x c i t a t i o n with >E« l i g h t and i s driven to the i n t e r f a c e , owing to the f i e l d i n tne semiconductor near the surface. The a v a i l a b i l i t y of the m i n o r i t y c a r r i e r at the i n t e r f a c e upon p h o t o e x c i t a t i o n allows o x i d a t i o n with h or reduction with e", and importantly the o x i d i z i n g power of the h f o r n - and the reducing power for p-type semiconductors can be greater than t h a t a s s o c i a t e d with the electrochemical p o t e n t i a l (Fermi l e v e l ) , E f , i n the bulk of the semiconductor. This means that l i g h t can be used to d r i v e thermodynamically non-spontaneous redox processes. The extent to which a process can be driven i n an u p h i l l sense i s the photovoltage, Εγ, t h a t i s given by equation (1) for i d e a l Γ Ρ (

r e (

Ω Χ

o x

r e (

o x

+

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

+

+

Εγ

= |EpB -

(For

E

r e (

j

0 X

E

r e d o x

|

(1)

between EQB and Εγβ)

semiconductors where Ερβ i s the f l a t - b a n d p o t e n t i a l , i . e . the value of Ef where the bands are not bent. For commonly used c a r r i e r concentrations Ερβ i s w i t h i n 0.1 Y of Εγβ f o r p-type semiconductors, and for η - t y p e semiconductors Ερβ i s w i t h i n 0.1 Y of EÇB» Thus, >Eg i l l u m i n a t i o n of a semiconductor electrode at o p e n - c i r c u i t tends to d r i v e Ef to Ερβ, more p o s i t i v e for p-type e l e c t r o d e s and more negative for η - t y p e electrodes compared to dark e q u i l i b r i u m where Ef = E d . I d e a l l y , the maximum value of Εγ approaches the band gap, Eg, of the semiconductor. However, ~0.3 Y of band bending i s r e q u i r e d to e f f i c i e n t l y separate the e~ - h p a i r s created by l i g h t . The separation of e~ - h p a i r s i s e s s e n t i a l to obtain a high quantum y i e l d for net e l e c t r o n flow, In any energy conversion a p p l i c a t i o n the e f f i c i e n c y , η, for the photoelectrode i s given by equation ( 2 ) , and since i i s p r o p o r t i o n a l to % i t i s r e

0 X

+

+

Εγ χ i

η i

(2) Input Optical Power photocurrent

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4.

WRIGHTON

Fuel and Electricity

65

Generation

d e s i r a b l e to obtain a l a r g e value of Φ . Good Ey at i = 0 ( i l l u m i n a t e d , o p e n - c i r c u i t ) or good i at Εγ = 0 ( i l l u m i n a t e d , s h o r t - c i r c u i t ) are both s i t u a t i o n s that give η = 0. The o b j e c t i v e i s to optimize the product of Εγ and i i n order to achieve the maximum e f f i c i e n c y . Figure 1 shows a steady-state photocurrent-voltage curve (15J for an n-type WS2 (Eg « 1.4 eV) photoanode based c e l l for conversion o f l i g h t to e l e c t r i c i t y employing the Br2/Br" redox couple and the i n s e t shows the f u l l c e l l energetics for operation at the maximum power p o i n t , the value of Ef when Εγ χ i i s maximum. Figure 2 shows a s i m i l a r curve for a c e l l based on p-type WS2 employing Fe(^-C5Me5)2 /° as the redox couple.(16^) In s o l a r energy a p p l i c a t i o n s an Eg i n the range 1.1-1.7 eV i s d e s i r a b l e , since a s i n g l e photoelectrode based device would have optimum s o l a r e f f i c i e n c y , exceeding 20, for such band gaps.(12) The WS2 electrodes represent n - and p-type semiconductors t h a t behave r e l a t i v e l y i d e a l l y (15,16) with respect to i n t e r f a c e energetics i n t h a t the value of ΐ γ does vary with E d according to equation (1) for E w i t h i n ~0.8 V of Ερβ. However, for many electrode materials the v a r i a t i o n i n Εγ does not follow equation ( 1 ) . ( Γ 7 ) For example, n-type CdTe (Eg = 1.4 eV) can give e i t h e r a constant value of Εγ (independent of E j o x ) or a nearly i d e a l v a r i a t i o n i n Εγ, Figure 3, depending on the pretreatment of the surface p r i o r to use.(18) The a b i l i t y to improve the value of Εγ from the constant value o f -0.5 V to ~0.9 V using a reducing surface pretreatment i s c l e a r l y d e s i r a b l e , but at the same time c e r t a i n o x i d a t i o n reactions can be driven i n an u p h i l l sense using the o x i d i z i n g pretreatment that could not be done with the CdTe pretreated with the reducing reagent. For CdTe i t i s evident that the surface pretreatment chemistry i s r e l a t e d to the value of Εγ. In f a c t , Auger and X-ray photoelectron spectroscopy reveal a c o r r e l a t i o n of the Εγ v s . E j ç behavior r e l a t e d to the composition of the surface of CdTe.(18) The o x i d i z i n g surface etch leaves a T e - r i c h o v e r l a y e r t h a t causes the CdTe to behave as i f i t i s coated with a metal having a work function t h a t gives a Schottky b a r r i e r height of -0.6 Y. In the l i q u i d j u n c t i o n system the analogue of Schottky b a r r i e r height i s the E j - EQB or E j - Εγβ separation for p - or n-type semiconductors. I f a Schottky b a r r i e r i s immersed i n t o a l i q u i d e l e c t r o l y t e s o l u t i o n the value of Εγ should be independent of E j , as found from n-type CdTe a f t e r an o x i d i z i n g pretreatment. G e n e r a l l y , when Εγ i s f i x e d for a wide range of E j the semiconductor i s s a i d to be "Fermi l e v e l pinned".(17J Fermi l e v e l pinning simply means t h a t the value of Ef at the surface of the semiconductor i s pinned to some value r e l a t i v e to the band edge p o s i t i o n s , independent of the electrochemical p o t e n t i a l of a c o n t a c t i n g s o l u t i o n or work function of a contacting metal. The pinning of the Ef i s due to surface states of s u f f i c i e n t density θ

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

+

r e

0 X

r e d o x

re(

r e (

X

r e (

r e (

o x

r e (

o x

o x

r e (

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

o x

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

ws„

redox

V

Liquid

•VB

LOAD

E =0.44V

Ihv

•CB

Pt

Pt

2

E

= E

f redox

50

100

Ί50

2

2

I 2

0.4

2

0.6

( η = 12.1%)

Maximum Power P o i n t

n-WS /H 0/Br /Br"

PHOTOVOLTAGE, V

0.2

ι

Figure 1. Cell (lower left), cell energetics at maximum power point (upper left), and output characteristics (right) for an n-type WS -based photoelectrochemical cell. The electrolyte is 12 M LiBr and E = -\-0.64 V vs. SCE; for current density multiply current shown times 32 cm' (IS).

n-WS„

g

E -1.4V

632.8 nm, 16mW/cm2

(+)

(-)

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

3

H

a w

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 2

2

redox

Figure 2. Cell (lower left), cell energetics at maximum power point (upper left), and output characteristics {right) for a p-type WS -based photoelectrochemical cell. The E = —0.38 V vs. Ag*/Ag; for current density multiply current shown by 32 cm' (16).

PHOTOVOLTAGE, V

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

CHEMISTRY:

TOWARD

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Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

INORGANIC

Figure 3. Photovoltage from n-CdTe etched with an oxidizing etch (%) or an oxidizing etch followed by a reducing treatment (NaOH/S O ~) (M) as a function of Ei/g of a contacting couple. Key to redox couples: 1, Ru(bipy) ~; 2, Ru(bipy) * ; 3, Ru(bipy) ^; 4, T&>°; 5, TQ ^; 6, Fe( -C Me ) ^; 7, Fe( -C H )S ; 8, TMPD ^; 9, TMPD+ ; 10, MV ^; and 11, MV . (Reproduced from Ref. 18.) 2

2

k

0/

/0

s

2

2

s

2

/0

2

s

5

v

5

5

s g

v

/0

5

s

+/0

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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and d i s t r i b u t i o n . Thus, the presence of surface s t a t e s can a l t e r the behavior of a semiconductor photoelectrode with respect to output e f f i c i e n c y v s . E j owing to a l i m i t a t i o n on Εγ. The degree of a l t e r a t i o n i n behavior depends on the surface s t a t e d i s t r i b u t i o n . For n-type CdTe the value of Εγ i s f i x e d to -0.5 Y, a r e l a t i v e l y small f r a c t i o n of the band gap. For p-type InP (Eg = 1.3 eV) i t would appear that the surface s t a t e d i s t r i b u t i o n i s such t h a t the value of Εγ i s f i x e d to -0.8 Y, (11) a much l a r g e r f r a c t i o n of the band gap. Understanding a n T c o n t r o l l i n g surface s t a t e s i s thus c r u c i a l to development of e f f i c i e n t semiconductor-based d e v i c e s . Even for i d e a l ( s u r f a c e - s t a t e free) semiconductors the behavior with respect to Εγ v s . E d can be confusing. For the i d e a l p- or n-type semiconductor s u f f i c i e n t l y negative or positive E j , respectively, w i l l result in carrier inversion a t the surface of the semiconductor, Schemes II and III.(14>19) In the region of E j where there i s i n v e r s i o n the semi­ conductor photoelectrode can s t i l l e f f e c t u p h i l l redox processes upon i l l u m i n a t i o n and Εγ can be independent of E j . The Fermi l e v e l , r e l a t i v e to the band edge p o s i t i o n s , simply cannot be d r i v e n s i g n i f i c a n t l y more negative than EQB or more p o s i t i v e than Εγβ. Thus, for a range of E j the band edge p o s i t i o n s vary with E j i n a manner s i m i l a r to when surface s t a t e s between EÇB and Εγβ p i n the Fermi l e v e l . In the i d e a l case i t i s the high density of s t a t e s from the valence οθ conduction band t h a t e v e n t u a l l y gives an Εγ t h a t i s independent of E j | . When the observed value of Εγ i s a l a r g e f r a c t i o n of Eg i t i s not easy to determine whether surface s t a t e s pin Ef or whether a t a i l i n g density of valence or conduction band s t a t e s e f f e c t p i n n i n g . Whenever the Εγ exceeds 1/2E„ there i s a measure of c a r r i e r i n v e r s i o n at the surface at dark e q u i l i b r i u m , but t h i s does not mean t h a t strong i n v e r s i o n i s p o s s i b l e . Strong i n v e r s i o n can only occur when the region between EÇB and Εγβ i s s u f f i c i e n t l y free of s t a t e s that Εγ can approach Eg. G e n e r a l l y , i t i s d i f f i c u l t to e f f e c t strong i n v e r s i o n when the semiconductor i s i n contact with any conductor, i n c l u d i n g l i q u i d e l e c t r o l y t e s o l u t i o n s . The metal chalcogenides, M0S2, MoSe2, WS2, and WSe2, seem to be c l o s e s t to i d e a l of the semiconductors s t u d i e d , though there s t i l l seems to be a r o l e for surface s t a t e s . ( 1 5 » 1 6 * 2 0 ) A m a t e r i a l such as n-type CdTe t h a t gives a f i x e d Εγ at 27) I n t e r e s t i n g l y , the redox decomposition processes o f semiconductors are m u l t i - e l e c t r o n processes that can be s u f f i c i e n t l y slow t h a t k i n e t i c competition with desired redox processes can be successful to b r i n g about sustained generation o f e n e r g y - r i c h products or e l e c t r i c i t y from p h o t o e x c i t a t i o n of a photoanode. Suppression of Photocorrosion of Photoanodes and Manipulation of K i n e t i c s for Anodic Processes In 1976 the f i r s t sustained conversion of v i s i b l e l i g h t to e l e c t r i c i t y using an n-type semiconductor-based c e l l (CdS ( E = 2.4 eV) or CdSe ( E = 1.7 eV)) was reported.(25,27) In the ensuing s i x years remarkable progress has been r e a l i z e d i n t h i s a r e a . The key has been to f i n d reducing reagents, A, t h a t can capture photogenerated o x i d i z i n g e q u i v a l e n t s , h , at a r a t e that precludes decomposition of the semiconductor, equation ( 8 ) . The reverse process, equation ( 9 ) , can then be e f f e c t e d at the q

g

+

A A counterelectrode

+

(8)

+

+

h

+

Θ­

Α

to complete a chemical c y c l e i n v o l v i n g no net

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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chemical change, but y i e l d s i g n i f i c a n t e f f i c i e n c y for e l e c t r i c i t y generation. Now, a v a r i e t y of n-type semi conductor/sol v e n t / e l e c t r o l y t e / A / A / c o u n t e r e l e c t r o d e systems are known to comprise e f f i c i e n t , durable v i s i b l e l i g h t energy to e l e c t r i c a l energy.(£-£,15^,25,27,28) Often, there i s a p o t e n t i a l regime where the process represented by (8) i s completely dominant compared to the anodic decomposition of the semiconductor. In some cases, e . g . C d S / S " , (29) C d T e / T e " , (29) and MoSe2/l3", (30) the redox species i n t e r a c t strongly w i l i ï the electrode material r e s u l t i n g i n changes i n Epg. Such strong i n t e r a c t i o n s can be useful i n p r o t e c t i n g the semiconductor. In the examples c i t e d above, Epg s h i f t s more negative reducing the tendency for anodic decomposition and opening a wider p o t e n t i a l regime where the d e s i r e d o x i d a t i o n process can be effected without competition from anodic decomposition. The unique i n t e r a c t i o n s of a semiconductor with s o l u t i o n s p e c i e s , such as MoSe2 w i t h I3", (30) are very l i k e l y the s o r t s of s i t u a t i o n s that w i l l lead to the f i r s t a p p l i c a t i o n s of semiconductor photoelectrochemical devices for photochemical synthesis of redox products. G e n e r a l l y , the p r a c t i c a l competition w i l l come from c o n v e n t i o n a l , i n c l u d i n g e l e c t r o c h e m i c a l , methods for producing redox reagents. When the semiconductor e l e c t r o d e has unique surface chemistry t h i s can change the product d i s t r i b u t i o n and i n some cases i t may be t h a t the semiconductor may be the only surface at which a desired r e a c t i o n w i l l occur e f f i c i e n t l y . However, even when a semiconductor i s the electrode material of choice i t i s not c l e a r t h a t l i g h t would be used. The d e s i r a b l e i n t e r a c t i o n s that e x i s t for a p-type photocathode, for example, would l i k e l y e x i s t as well for the o p p o s i t e l y doped, n-type, m a t e r i a l . The reductions t h a t require l i g h t at the p-type electrode can be effected i n the dark at the n-type m a t e r i a l . A l s o , degenerately doped semiconductors often behave well i n the dark and are not b l o c k i n g to any redox processes. However, the unique chemistry of semiconductor surfaces needs to be e l u c i d a t e d before a v e r d i c t can be reached regarding the p r a c t i c a l consequences. The strong i n t e r a c t i o n of the I 3 - / I " redox system with the metal dichalcogenide m a t e r i a l s was r e c e n t l y e x p l o i t e d (31) to b r i n g about the v i s i b l e l i g h t - d r i v e n process representecTby equation (10). In 50% by weight H2SO4 the r e a c t i o n as w r i t t e n +

2

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

n

n

S0

2

+ H2O

> H2SO4 + H2 -50% H2SO4, ~10-3 !M

requires -0.3 V or d r i v i n g f o r c e , a good match to the photovoltage at the maximum power point for n-type WS2-based

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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c e l l s for the o x i d a t i o n of I~.(15^) But the o x i d a t i o n of SO2 to SO42- i n H2SO4 has poor k i n e t i c s (32) and does not compete with photoanodic decomposition of the e l e c t r o d e . Further, redox( 4 / 2 ) equation (1) p r e d i c t s a very small photovoltage.(32) The I 3 - / I - , though, i n t e r a c t s to s h i f t Ερβ o f WS2 favorably to give a good Εγ with respect to E e d o x ( ° " / 2 ) and simultaneously provides a mechanism for the o x i d a t i o n of SO2, s i n c e the I3" r a p i d l y o x i d i z e s SO2 to SO42-, Scheme IV. Thus, the I 3 - / I " serves as a redox mediator and favorably a l t e r s the i n t e r f a c e energetics to give a good Εγ and a p o t e n t i a l window of d u r a b i l i t y . The v i s i b l e l i g h t - d r i v e n r e a c t i o n represented by equation (10) i s one of the most e f f i c i e n t o p t i c a l to chemical energy conversions (up to -14% from 632.8 nm l i g h t ) known. The system i l l u s t r a t e s the complex r e l a t i o n s h i p s t h a t must be understood i n order to e f f i c i e n t l y d r i v e m u l t i - e l e c t r o n redox processes. The suppression of photoanodic c o r r o s i o n i s not always difficult. For example, metal dichalcogenides are not durable i n aqueous 0.1 M KC1; equation (11) represents the photoanodic E

S 0

s o

1 5

s

u

c

h

t

h

a

t

s

4

S 0

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

r

+

2

MoS + 18h + 8H 0 —* 16H+ + 2S04 " + M o 2

6 +

(11)

2

decomposition process for MoS2«(33) However, the o x i d a t i o n of low concentrations of C l " o c c u r s T n CH3CN s o l u t i o n with 100% c u r r e n t e f f i c i e n c y . ( 3 4 ) Even i n aqueous s o l u t i o n the o x i d a t i o n of C I " can be effected" at s u f f i c i e n t l y high C I " a c t i v i t y . ( 1 5 , 3 5 ) In aqueous 15 M L i C l the o x i d a t i o n of C I " has 100% current efficiency. Tfie high L i C l concentration y i e l d s very high C I " a c t i v i t y and lower a c t i v i t y of H2O, both c o n t r i b u t i n g to the improved d u r a b i l i t y of i l l u m i n a t e d n-type MoS2.(35) Thus, these experiments show, not s u r p r i s i n g l y , (24) that the medium i n contact with the semiconductor can a l î ê r the o v e r a l l i n t e r f a c i a l c h e m i s t r y , even though C I " i s a v a i l a b l e as a reductant i n each case. The a b i l i t y to e f f e c t the sustained generation of CI2 a t an i l l u m i n a t e d i n t e r f a c e shows that potent oxidants can be made photochemically; CI2 i s thermodynamically more potent, and k i n e t i c a l l y more aggressive, than O2. The a b i l i t y to manipulate the anodic c o r r o s i o n problem using high concentrations of redox a c t i v e e l e c t r o l y t e also makes p o s s i b l e the sustained o x i d a t i o n of Br" at i l l u m i n a t e d metal dichalcogenide-based c e l l s , Figure 1.(15) The use of high concentrations of e l e c t r o l y t e has proven valuable i n s i t u a t i o n s i n v o l v i n g other photoanode m a t e r i a l s , notably n-type S i . ( 3 6 , 3 7 ) Reducing photoanodic c o r r o s i o n with high concentrations of redox a c t i v e m a t e r i a l s l e d to the c o n c l u s i o n that redox reagents c o v a l e n t l y anchored to the photoelectrode might prove u s e f u l . ( 3 8 ) For example, research showed that n-type Si could be a durable photoanode i n Et0H/0.1 M [ n - B u 4 N ] C 1 0 4 / F e ( ^ - C 5 H 5 ) 2 / ° for the generation of e l e c t r i c i " f y . ( 2 6 ) The Fe(ir>-C5H5)2 i s a f a s t , +

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

INORGANIC

Scheme IV. Representation of the I ~/I~ mediated oxidation of S0 at illuminated metal dichalcogenide photoanodes (top) and interface energetics with and without H/V in 6 M H SO /l M S0 for MoS (bottom) (31). In the absence of the mediator system the photovoltage for the S0 oxidation is expected to be negligible. The adsorption of the If IV is unaffected by the S0 so the negative shift of the flat band potential can be exploited to give a photovoltage for the desired process with the Ι 'IV system simultaneously providing an acceleration of the S0 oxidation. s

2

h

2

2

2

2

2

3

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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one-electron reductant t h a t i s durable and gives a durable o x i d a t i o n product. Subsequent study showed that the reagent represented by I_ can be polymerized and attached to the surface

I o f n-type Si to p r o t e c t i t from photoanodic c o r r o s i o n . ( 3 8 , 3 9 , 4 0 ) The important r e s u l t i s t h a t once the electrode i s made^aiirabTê using the surface-confined redox system, [ A / A ] r f . , then the photoelectrode can be used to s u s t a i n many o x i d a t i o n processes, say Β B , where Β i t s e l f i s not successful i n competing with the anodic c o r r o s i o n of the e l e c t r o d e . The a b i l i t y to photooxidize Β would then depend on the thermodynamics and k i n e t i c s for the i n t e r f a c i a l process represented by equation (12) +

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

S U

+

+

+

CA ] urf. + Β +

B + [A]

S

s u r f

(12)

.

+

and not the k i n e t i c s for h capture by B . I t has been shown t h a t n-type Si d e r i v a t i z e d with j[ i s capable of e f f e c t i n g the photoassisted o x i d a t i o n of a v a r i e t y of reagents i n H2O s o l v e n t , where the decomposition (7) i s most severe.(40) In several cases i t has been e s t a b l i s h e d t h a t the process represented by (12) represents the dominant path for production of B . Thus, i n p r i n c i p l e , the molecular p r o p e r t i e s of the surface-confined reagent could be e x p l o i t e d to e f f e c t s p e c i f i c r e a c t i o n s . For the surface species derived from I_ the o x i d i z i n g power i s f i x e d to -+0.45 Y v s . SCE, the formal p o t e n t i a l , Ε ° ' , of the surface reagent.(41) However, the Fe(î) -C5H5)2 /° systems are outer-sphere reagents and do not o f f e r any basis for s e l e c t i v i t y other than the p o t e n t i a l . However, such species may prove useful i n f a c i l i t a t i n g the redox r e a c t i o n s of b i o l o g i c a l reagents, vide infra. Several groups have r e c e n t l y shown ( 3 £ , 4 2 , 4 3 , 4 4 ) t h a t photoanode m a t e r i a l s can be protected from pTiatoanôïïic c o r r o s i o n by an a n o d i c a l l y formed f i l m of " p o l y p y r r o l e " . ( 4 5 ) The work has been extended (46) to photoanode surfaces f i r s t T r e a t e d with reagent U_ that c o v a l e n t l y anchors i n i t i a t i o n s i t e s for the formation of p o l y p y r r o l e . The r e s u l t i s a more adherent p o l y p y r r o l e f i l m t h a t b e t t e r protects n-type Si from p h o t o c o r r o s i o n . Unlike the material derived from p o l y m e r i z a t i o n of I , the a n o d i c a l l y formed p o l y p y r r o l e i s an e l e c t r o n i c conductor.(45) This may prove u l t i m a t e l y important i n that the r a t e of i o n T r a n s p o r t of redox polymers may prove to be too slow +

5

+

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Ο

Ν

TOWARD

i(0Me)

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3

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

II to be useful i n a t t a i n i n g useful photocurrent d e n s i t i e s . For the e l e c t r o n i c a l l y conducting polymer the rate would not be l i m i t e d by ion t r a n s p o r t . At t h i s p o i n t i t i s evident that there are many approaches to the sustained conversion o f v i s i b l e l i g h t using photoanodes. The approaches based on strong i n t e r a c t i o n , e . g . WS2/l3*7I~,(15) or modified surfaces, e . g . d e r i v a t i z e d n-type S i , (38,39,40) seem most i n t e r e s t i n g since the unique p r o p e r t i e s o f the î n ï ê r T a c e can be e x p l o i t e d a t the molecular l e v e l . S i g n i f i c a n t l y , the generation o f potent oxidants such as B r or CI2 can be effected using v i s i b l e l i g h t and with reasonably good e f f i c i e n c y but without electrode d e t e r i o r a t i o n . 2

Improving the K i n e t i c s f o r Hydrogen Generation from P-Type Semiconductors No naked semiconductor photocathode has been demonstrated to have good k i n e t i c s f o r the e v o l u t i o n o f H2, despite the fact t h a t the p o s i t i o n o f EQB i n many cases has been demonstrated to be more negative than E (H20/H2). This means that e l e c t r o n s e x c i t e d to the conduction band have the reducing power to e f f e c t H2 e v o l u t i o n , but the k i n e t i c s are too poor t o compete with e~ - h recombination. The demonstration that N j N ' - d i m e t h y l ^ ^ ' - b i p y r i d i n i u m , M Y , could be e f f i c i e n t l y photoreduced a t i l l u m i n a t e d p-type Si t o form MY i n aqueous s o l u t i o n under c o n d i t i o n s where E ( M V / + ) = E (H20/H2) when no H2 e v o l u t i o n occurs e s t a b l i s h e s d i r e c t l y t h a t the thermodynamics are good, but the k i n e t i c s are poor, f o r H2 e v o l u t i o n . ( 2 3 , 4 7 ) The a b i l i t y t o e f f i c i e n t l y reduce M Y t o MY a t i l l u m i n a t ë ï ï p-type Si l e d t o studies o f the surface d e r i v a t i z i n g reagent I I I o,

+

2 +

+

0 ,

o,

2 +

2+

+

i(0Me) B r

CMe0l !

3

3

III

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2

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f o r use as an e l e c t r o n acceptor on photocathode surfaces.(48) Subsequent d e p o s i t i o n of Pd(0) by electrochemical reduction of low concentrations of aqueous P d C l 4 " leads to the i n t e r f a c e represented i n Scheme V.(49) The Pd(0) i s c r u c i a l to b r i n g about e q u i l i b r a t i o n of the surface-confined viologen reagent, C ( P Q / ) ] s u r f . » with the H 0 / H c o u p l e . The [ ( P Q / ) ] f / P d ( 0 ) c a t a l y s t system can, i n p r i n c i p l e , be used on any photocathode surface to improve H e v o l u t i o n k i n e t i c s . The improvement i n photoelectrochemical H e v o l u t i o n e f f i c i e n c y using the [ ( P Q / ) ] s u r f . / ( ° ) system i s r e f l e c t e d by the data i n Figure 4 . For the naked surface, H e v o l u t i o n b a r e l y onsets at E ( H 0 / H ) . For the photoelectrode bearing [(PQ /+) ] / P d ( 0 ) the onset f o r H e v o l u t i o n i s up to -500 mV more p o s i t i v e than E ° ' ( H 0 / H ) . The extent to which the onset i s more p o s i t i v e than E ( H 0 / H ) i s Εγ. I t i s obvious t h a t the modified photoelectrode gives s u p e r i o r performance. The mechanism of the c a t a l y s i s for the p - S i / [ ( P Q / ) ] f / P d ( 0 ) system i s represented by equations (13) and ( 1 4 ) . The E [ ( P Q / ) ] . = -0.55 ± 0.5 Y v s . SCE, 2

2 +

+

n

2 +

2

2

+

n

s u r

e

2

2

2 +

+

p d

n

2

o ,

2

2

2 +

n

S u r f

2

2

2

o ,

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

2

2 +

2

+

n

s u r

#

0 ,

2 +

+

n

C(PQ ) ]surf.

s u r f

• C(PQ )n3surf.

2 +

+

n

> C(PQ )n] urf.

+

2 +

n H + [(PQ+) ]surf. n

S

+

(13)

l/2nH

2

(14)

o ,

(50) independent of pH, whereas E ( H 0 / H ) v a r i e s with pH. Tïïûs, the c a t a l y z e d process represented by (14) i s only downhill f o r s u f f i c i e n t l y low pH; at the lower pH's the d r i v i n g force i s greater and the rate i s f a s t e r . However, as the pH i s lowered Εγ becomes s m a l l e r . This leads to an optimum i n o v e r a l l energy conversion e f f i c i e n c y at pH = 4 . For monochromatic 632.8 nm l i g h t the e f f i c i e n c y f o r photoassisted H e v o l u t i o n i s up to ~5% whereas naked e l e c t r o d e s have n e g l i g i b l e e f f i c i e n c y . ( 4 9 , 5 0 ) The C ( P Q / ) n ] s u r f . system can a l s o be employed w i t i ï Pt(0) (50) as the c a t a l y s t i n s t e a d of P d ( 0 ) . Both Pt(0) and Pd(0) have e x c e l l e n t H e v o l u t i o n k i n e t i c s . One v i r t u e of Pd(0) i s t h a t i t i s much more e a s i l y detected by Auger e l e c t r o n spectroscopy than i s P t ( 0 ) . ( 4 9 ) Auger e l e c t r o n spectra taken while s p u t t e r i n g the surface w i W A r ions have l e d to the establishment of i n t e r f a c e s t r u c t u r e s l i k e t h a t represented i n Scheme V. T y p i c a l l y , 10~ mol/cm of P Q centers and - 1 0 " mol/cm of Pd(0) are used to give an o v e r l a y e r of -2000 Â i n dimension. A key feature of the i n t e r f a c e represented by Scheme Y i s t h a t there i s no Pd(0) at the p-type S i / S i O s u r f a c e . This means that the only mechanism f o r e q u i l i b r a t i n g H 0/H? w i t h the photogenerated reducing e q u i v a l e n t s i s v i a the L ( P Q / ) n 3 s u r f . system. Studies have a l s o been done with Pt(0) or Pd(0) dispersed throughout the polymer from Π_Ι_ v i a the sequence represented by equations (15) 2

2

2

2 +

+

2

+

8

2

2 +

8

2

x

2

2 +

+

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INORGANIC

CHEMISTRY:

TOWARD

T H E 21 ST

CENTURY

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

78

Sputtering Time , min Scheme V. Representation of the catalytic p-type Si photocathode for H evolu­ tion prepared by derivatizing the surface first with Reagent III followed by deposi­ tion of approximately an equimolar amount of Pd(0) by electrochemical deposition. The Auger/depth profile analysis for Pd, Si, C, and Ο is typical of such interfaces (49) for coverages of approximately 10~ mol PQ */cm . g

8

2

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

WRIGHTON

Fuel and Electricity

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

4.

79

Generation

_ l -0.4

I

l _

-0.2

0.0

POTENTIAL, V v s .

SCE 2

Figure 4. Comparison of photocathodic current (632.8 nm; ~6 mW/cm ) for naked p-type Si ( ), p-type Si bearing [(PQ *) ] rf ( ), and p-type Si bearing [(PQ )n]aurf/Pd(0) (-·-·) at pH — 4. The photocathodic current in the last case is associated with H evolution that occurs more positive than E ' (H 0/H ). For current density multiply values shown by 10 cm' . The current peak for the smooth curve is associated with the uphill reduction [(PQ *)n]surf -> [(PQ )n]surf- (Reproduced from Ref. 49. Copyright 1982, American Chemical Society.) 2

n

8U

2

0

2

2

2

2

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

80

INORGANIC CHEMISTRY:

[(PQ2+.2Br-) ] n

CENTURY

2

s u r f

. + n P d C 1 - — [(PQ2+ · P d C l 2 - ) ] + 2nBr~ (15) 4

and (16) for Pd(0).(49) 0j

4

n

In such cases there i s at l e a s t a small

, _ ο „ [(PQ2+ · P d C l 2 - ) ] r

T O W A R D T H E 21ST

reduce

%

4

n

•

s u r f e

[(PQ2+ · 2C1 . P d ( 0 ) ] n

s u r f

oxidize

>

.

(16)

amount of Pd(0) at the p-type S i / S i O s u r f a c e . The Pd(0) a t the p-type S i / S i O surface should be r e s p o n s i b l e for some H e v o l u t i o n c u r r e n t d e n s i t y , since the d i r e c t d e p o s i t i o n of Pd(0) or Pt(0) (without I I I ) onto p-type S i / S i 0 does improve H e v o l u t i o n d r a m a t i c a l l y . ( 5 0 , 5 1 ) Indeed, the d i r e c t d e p o s i t i o n of c a t a l y t i c metals onto p-^Çype InP has l e d to a demonstration of -12* e f f i c i e n c y for the s o l a r - a s s i s t e d production of H . ( 5 2 ) The a b i l i t y to e f f i c i e n t l y c a t a l y z e the H e v o l u t i o n with a metal deposited onto the p-type semiconductor r a i s e s the l e g i t i m a t e question of why use I I I at a l l , l e t alone attempt to prepare and e x p l o i t the more ordered i n t e r f a c e represented by Scheme V. In t h e o r y , the e f f i c i e n c y of a l l devices based on a given semiconductor would be the same. The d i r e c t d e p o s i t i o n of P t ( 0 ) or the use of C ( P Q ) ] s u r f . / P d ( 0 ) are both ways of c a t a l y z i n g the H e v o l u t i o n . A problem with the redox polymers, as already mentioned, i s t h a t rate i s l i k e l y to be ion t r a n s p o r t l i m i t e d . A problem with Pt(0) and Pd(0) i s t h a t they often give an ohmic c o n t a c t , rather than a Schottky b a r r i e r , with p-type semiconductors. For example, i n attempting to c a t a l y z e H e v o l u t i o n from i l l u m i n a t e d p-type WS by e l e c t r o c h e m i c a l l y d e p o s i t i n g P t ( 0 ) or Pd(0) a l a r g e percentage of the e l e c t r o d e s give an ohmic c o n t a c t . ( 1 6 ) This r e s u l t s i n no photoeffects from the e l e c t r o d e . A uniform c o a t i n g with the redox polymer from I I I gives a r e p r o d u c i b l e , p h o t o s e n s i t i v e surface t h a t can be used to generate H v i a Pd(0) or Pt(0) deposited on the outermost surface. Neither of the approaches has l e d to s u f f i c i e n t l y durable c a t a l y s t s t h a t p r a c t i c a l devices are at hand; the Pt(0) or Pd(0) i s very e a s i l y poisoned. S u r p r i s i n g l y , the C(PQ ) ] s u r f . polymer does not suffer d e t e r i o r a t i o n on the t i m e s c a l e of l o s s of a c t i v i t y of the Pd(0) or Pt(0) c a t a l y s t . The synthesis of c a t a l y t i c photocathodes for H e v o l u t i o n provides evidence that d e l i b e r a t e surface m o d i f i c a t i o n can s i g n i f i c a n t l y improve the o v e r a l l e f f i c i e n c y . However, the s y n t h e s i s of rugged, very a c t i v e c a t a l y t i c surfaces remains a c h a l l e n g e . The r e s u l t s so far e s t a b l i s h that i t i s p o s s i b l e , by r a t i o n a l means, to synthesize a d e s i r e d p h o t o s e n s i t i v e i n t e r f a c e and to prove the gross s t r u c t u r e . Continued improvements i n photoelectrochemical H e v o l u t i o n e f f i c i e n t l y can be expected, w h i l e new surface c a t a l y s t s are needed for N and C 0 r e d u c t i o n processes. x

x

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

X

2

2

2

2 + / +

n

2

2

2

2

z + / +

n

2

2

2

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4.

WRIGHTON

Fuel and Electricity

A Role for B i o l o g i c a l

Generation

81

Redox C a t a l y s t s ?

The enzymes hydrogenase, nitrogenase, and formate dehydrogenase can be used to e q u i l i b r a t e reducing reagents with H2O/H2, N2NH3, and CO2/HCOOH, r e s p e c t i v e l y . ( S 3 ) In no case do the enzymes involve expensive noble metals as c a t a l y s t s . P r a c t i c a l c o n s i d e r a t i o n s a s i d e , the m u l t i - e l e c t r o n t r a n s f e r c a t a l y s i s effected by enzymes provides an existence proof for d e s i r e d photoelectrode c a t a l y s t s . One of the major d i f f i c u l t i e s i s t h a t large b i o l o g i c a l redox reagents are often unresponsive at e l e c t r o d e surfaces. For a v a r i e t y of reasons the heterogeneous e l e c t r o n t r a n s f e r k i n e t i c s for large b i o l o g i c a l reagents are poor. However, small redox reagents d i s s o l v e d i n s o l u t i o n do e q u i l i b r a t e r a p i d l y with the large b i o l o g i c a l reagents.(54) I n t e r e s t i n g l y , M Y , f o r example, w i l l e f f e c t reduction o F T ^ O , N2, or CO2 when the proper enzyme i s present as a c a t a l y s t . ( 5 3 ) The use of s u r f a c e - c o n f i n e d , f a s t , o n e - e l e c t r o n , outer-sphere redox reagents l i k e those derived from I_ or III as redox mediators for b i o l o g i c a l reagents would seem to represent an e x c e l l e n t approach to the e q u i l i b r a t i o n of the e l e c t r o d e with the b i o l o g i c a l reagents. Experiments r e l a t i n g to the o x i d a t i o n and reduction of f e r r o - and ferricytochrome c , c y t c(red) and c y t C(QX) from horse h e a r t , e s t a b l i s h t h a t p h o t o e l e c t r o ï ï e s d e r i v a t i z e d T w i t n molecular reagents can give s i g n i f i c a n t l y improved response to b i o l o g i c a l redox reagents.(55,56) The c y t c provides an example of a r e a d i l y accessible" ïïfomolecule t ï ï a t generally has poor k i n e t i c s a t e l e c t r o d e surfaces.(57) The f i r s t experiments concerned e l e c t r o d e s d e r i v a t i z e d with 111.(55) The f a c t that MY r a p i d l y reduces c y t £ ( o x ) to c y t £ ( e d ) to the use of the [(PQ2+/+) ] f o r t h i s purpose.(58) The E ° ' ( c y t £ ( ) / c y t £(red)) ~ 0 · 0 2 V v s . SCE (59) anëTthe r e a c t i o n represented by equation (17) i s thus downhill by -0.5 V. I n t e r e s t i n g l y , the

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

+

+

r

n

1 S

s u r f #

C( Q )n]surf. p

+

o x

+

+

η cyt £ ( x ) — • C ( Q ) n ] s u r f . p

2 +

0

+

π cyt £ ( d ) r e

(17)

process represented by (17) was shown to account for the r e d u c t i o n of c y t c at i l l u m i n a t e d p-type Si f u n c t i o n a l i z e d with 111.(55) The reduction of c y t £ ( o x ) electrodes d e r i v a t i z e d w i t h Τ Π i s mass t r a n s p o r t l i m i t e d and independent of coverage of P Q centers on the electrode from - 1 0 - 1 ° to 10"" mol / c m . Naked e l e c t r o d e s do not respond to the c y t £ ( o x ) the same p o t e n t i a l range. Inasmuch as adsorption of c y t £ , or i m p u r i t i e s contained i n i t , onto most e l e c t r o d e s leads to o v e r a l l poor k i n e t i c s , i t i s p a r t i c u l a r l y noteworthy t h a t high concentrations of c y t £ ( o x ) can be reduced with good k i n e t i c s v i a the C ( P Q ) n 3 s u r f . · Thus, ihe m o d i f i c a t i o n of e l e c t r o d e surfaces with I I I brings about improvement i n response with respect to reduction of c y t £ ( ο χ ) · a

t

2 +

8

i

2

n

+

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INORGANIC CHEMISTRY:

82

T O W A R D T H E 2 1 ST

CENTURY

While experiments with the c y t £ ( η χ ) a t C ( P Q / ) ] s u r f · e s t a b l i s h a p o i n t , the d i s p a r i t y E°* or the reagent and the mediator precludes the claims t h a t e l e c t r o d e s can i n f a c t e q u i l i b r a t e with the b i o l o g i c a l reagent. The synthesis of j Y has 2 +

+

d

o

n

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

IV l e d to the demonstration t h a t d e r i v a t i z e d e l e c t r o d e s can be e q u i l i b r a t e d with b i o l o g i c a l redox reagents.(56) Representing the surface species from IV by [ P M F c / ° ] f . , the E [ P M F c / ° ] f . = + 0 . 0 4 T v s . SCE, t h a t i s very c l o s e to the E ° ' for c y t c . Conventional Pt e l e c t r o d e s d e r i v a t i z e d with IV can be used To o x i d i z e c y t c ( d ) , or reduce c y t £ ( o x ) near Tfîe E ° ' of c y t £ , v i a the e q u i l i b r i u m process represented by equation ( 1 8 ) . Importantly, n-type Si e l e c t r o d e s d e r i v a t i z e d with IV can +

s u r

0 ,

+

S u r

r e

+

[PMFc ]

+ cyt £ ( d ) ï = ± [ P M F c ° ]

s u r f e

r e

s u r

f . + cyt £ (

o x

)

(18)

be used i n aqueous e l e c t r o l y t e s o l u t i o n at pH = 7 and the process represented by equation (19) can be effected i n an u p h i l l sense, [PMFcO]

s u r f #

+ h

+

•

+

[PMFc ]

s u r f

(19)

. +

Εγ « 300 mV. As on the Pt s u r f a c e , the [ P M F c ] f on n-type Si i s capable of e f f e c t i n g the o x i d a t i o n of c y t c ( j ) ! Thus, the n-type S i / [ P M F c / ° ] f . e l e c t r o d e can be useïï to e f f e c t the u p h i l l o x i d a t i o n of c y t ' c j V p d ) . The rate constant for r e a c t i o n represented by equation T20) i s >7 χ 1 0 M " l s " l . The observed s u r

#

r e (

+

s u r

3

+

[PMFc ]

S u r f

. + cyt £ ( r e d ) — * [ ™ F 0 ] C

s u r f

. + cyt £

( o x

)

4

(20)

heterogeneous e l e c t r o n r a t e constant i s >1 χ 1 0 " cm/s for a v a r i e t y of e l e c t r o d e s independently prepared, representing s u b s t a n t i a l improvement compared to naked e l e c t r o d e s that give n e g l i g i b l e r a t e s under the same c o n d i t i o n s . ( 5 6 ) Significantly, as f o r [ ( P Q ) J s u r f . > the [ P M F c / ° ] f Ts useful at high c o n c e n t r a t i o n s of c y t c . Very pure, low concentration c y t c apparently responds weTl at conventional e l e c t r o d e s , but small amounts of decomposition or i m p u r i t i e s cause severe problems from a d s o r p t i o n . ( 6 £ ) The surface reagents from I I I or IV apparently minimize the adsorption problems, while p r o v i d i n g a mechanism f o r exchanging e l e c t r o n s with the e l e c t r o d e . ( 5 5 , 5 6 ) Preliminary 2 + / +

+

n

s u r

#

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4.

WRIGHTON

Fuel and Electricity

83

Generation

r e s u l t s have e s t a b l i s h e d t h a t hydrogenase can e q u i l i b r a t e with the polymer derived from I I I , suggesting that the one-electron polymers from I I I and jV^ can i n f a c t come i n t o redox e q u i l i b r i u m w i t h m u l t i - e l e c t r o n t r a n s f e r c a t a l y s t s for r e a c t i o n s of p o s s i b l e importance i n energy c o n v e r s i o n . Large Area P h o t o s e n s i t i v e M a t e r i a l s The studies described so far have concerned r e l a t i v e l y s m a l l , -0.1-1 c m , s i n g l e - c r y s t a l photoelectrode m a t e r i a l s . Promising r e s u l t s have been obtained i n t h a t there are a v a r i e t y of durable, e f f i c i e n t paths to generation of high energy chemicals or e l e c t r i c i t y . However, s i n g l e - c r y s t a l photoelectrode m a t e r i a l s are l i k e l y to remain too expensive for s i g n i f i c a n t p r a c t i c a l development. The question i s whether the basic r e s u l t s from s i n g l e - c r y s t a l systems can be a p p l i e d to large area p h o t o s e n s i t i v e m a t e r i a l s not n e c e s s a r i l y f a b r i c a t e d from s i n g l e crystals. In t h i s area as well some promising r e s u l t s have been o b t a i n e d . Thin f i l m and/or p o l y c r y s t a l l i n e GaAS (61) and CdX (62) photoanodes have been shown to have r e l a t i v e l y good e f f i c i e n c y compared to t h e i r s i n g l e - c r y s t a l analogues. R e c e n t l y , r e s u l t s from amorphous hydrogenated s i l i c o n , a - S i : H , Eg = 1.7 eV, obtained i n a glow discharge of SiH4 show s i g n i f i c a n t promise. S o l i d s t a t e devices for s o l a r to e l e c t r i c a l energy conversion based on absorption of l i g h t by a - S i : H have been shown to have almost 10% e f f i c i e n c y (63) and i t i s b e l i e v e d t h a t a - S i : H can be produced i n e x p e n s i v e l y and uniformly i n large areas. In p r i n c i p l e , i n t r i n s i c photoconductors such as a - S i : H can be good photoelectrodes. (5^,64) Scheme VI shows the approximate i n t e r f a c e s i t u a t i o n f o r a r e c e n t l y reported a-Si:H-based c e l l for the generation of e l e c t r i c i t y . ( 6 £ ) C r i t i c a l l y , the i n t r i n s i c t h i n f i l m (1-4 μ ) of a - S i : H was deposited onto a very t h i n (-200 Â ) h e a v i l y η-doped a - S i : H l a y e r on s t a i n l e s s s t e e l i n order to assure that the Fermi l e v e l contacts the bottom of the conduction band. This means t h a t E i p o s i t i o n s of Εςβ w i l l r e s u l t i n a f i e l d across the photoconductor such that photogenerated h s w i l l be driven toward the e l e c t r o l y t e / r e d o x couple s o l u t i o n . For redox c l o s e to Εγβ the Εγ would be expected to approach Eg as for an n-type semiconductor photoanode. As f o r any other photoanode the a - S i : H i s s u s c e p t i b l e to photoanodic decomposition, but the c o r r o s i o n can be completely suppressed by using the 0.1 Μ [ n - B u 4 N ] C 1 0 4 / E t O H / F e ( ^ - C 5 H ) / electrolyte/recTox couple s o l u t i o n , Figure 5. I n t e r e s t i n g l y , the sustained conversion of 632.8 nm l i g h t i s j u s t as e f f i c i e n t for the i n t r i n s i c a - S i : H photoanode as for s i n g l e - c r y s t a l n-type Si e l e c t r o d e s under the same conditions.(5^) The surface of a - S i : H can also be d e r i v a t i z e d with reagent J_ and the Εγ i s -750 mV compared to Εγ « 500 mV on s i n g l e c r y s t a l

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

2

r e (

o x

+ ,

E

+

5

0

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. r e d o a

Scheme VI. Representation of the interface energetics for intrinsic a-Si:H at short circuit, dark equilibrium with ferricenium/ferrocene in EtOH, electrolyte solution (left) and under illumination with 632.8-nm light with a load in series in the external circuit (right). The diagrams are adapted from data in Reference 65 for intrinsic a-Si:H (l-4-μ thick) on stainless steel first coated with heavily η-doped a-Si:H (200-À thick) to ensure an ohmic contact near the bottom of the conduction band. In typical experiments E . = 0.4 V vs. SCE.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

WRIGHTON

Fuel and Electricity

1

0.2

0.4

r

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

τ

85

Generation

0.6

0.8

1.0

PHOTOVOLTAGE, V

Figure 5. Output characteristics and photocurrent density at maximum power point against time (inset) for an intrinsic a-Si:H photoanode-based cell. (Repro­ duced from Ref. 5.)

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

1.2

86

INORGANIC CHEMISTRY:

T O W A R D T H E 21 S T C E N T U R Y

n-type Si.(S) This r e s u l t suggests t h a t a - S i : H could be durable i n aqueous s o l u t i o n s v i a p r o t e c t i o n by the surface reagent. As f o r n-type Si t h i s would allow the use of large area, e f f i c i e n t a - S i : H to e f f e c t a v a r i e t y of l i g h t - d r i v e n o x i d a t i o n processes.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch004

Summary Semiconductor-based photoelectrochemical c e l l s can e f f e c t the s u s t a i n e d , d i r e c t conversion of l i g h t to chemical or e l e c t r i c a l energy with good e f f i c i e n c y . There are several approaches to suppressing the photocorrosion of n-type semiconductor photoanodic m a t e r i a l s , a l l depending on the manipulation of i n t e r f a c e p r o p e r t i e s such as s t r u c t u r e , e n e r g e t i c s , and k i n e t i c s . The v i s i b l e l i g h t - d r i v e n generation of CI2 from photoanodes represents the most potent oxidant generated from non-oxide e l e c t r o d e s . Output parameters depend on surface p r o p e r t i e s as r e f l e c t e d i n experiments with reducing v s . o x i d i z i n g pretreatments for n-type CdTe. I n t e r f a c i a l redox k i n e t i c s can be modified by r a t i o n a l means as i l l u s t r a t e d with r e s u l t s for photocathodes modified to improve H2 e v o l u t i o n k i n e t i c s . However, much more work remains to be done on m u l t i - e l e c t r o n processes to b r i n g about improvements i n k i n e t i c s . C e r t a i n enzymes may prove useful i n N2, CO2, or H2O r e d u c t i o n . Progress i n r e l a t i v e l y e f f i c i e n t , large area, inexpensive photoelectrode m a t e r i a l s has been made, with a - S i : H being one example. At t h i s p o i n t the performance of i n t e r f a c i a l i n o r g a n i c chemistry systems for energy conversion i s s u f f i c i e n t l y good t h a t they cannot be r u l e d out as contenders for large seal6 energy generation. The near-term, pre-2000 c h a r t e r i s to f u l l y e l a b o r a t e the basic science underlying the i n t e r f a c i a l systems w i t h a conscious e f f o r t d i r e c t e d toward e f f i c i e n t (>10%), d u r a b l e , and inexpensive systems for the d i r e c t production of e n e r g y - r i c h redox products from abundant, inexpensive resources such as H2O and CO2. I t i s too e a r l y to focus on fuel v s . e l e c t r i c i t y generation as the u l t i m a t e o b j e c t i v e . Many of the requirements for both outputs are the same but fuel generation poses the g r e a t e s t c h a l l e n g e , since useful fuel generation w i l l r e q u i r e new m u l t i - e l e c t r o n t r a n s f e r c a t a l y s t s . Acknowledgments Research support from the United States Department of Energy, Office of Basic Energy Sciences, D i v i s i o n of Chemical Sciences i s g r a t e f u l l y acknowledged. Support from GTE L a b o r a t o r i e s , Inc. and Dow Chemical Company, U . S . A . for aspects of t h i s work i s a l s o acknowledged. P a r t i a l support from the O f f i c e of Naval Research for work on the surface chemistry of CdTe i s a p p r e c i a t e d .

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Results for platinum silicide/Si photoanodes appear very promising in combination with high concentration electrolytes; Bard, A.J., private communication. Wrighton, M.S.; Austin, R.G.; Bocarsly, A.B.; Bolts, J.M.; Haas, O.; Legg, K.D.; Nadjo, L.; Palazzotto, M.C. J. Am. Chem. Soc., 1978, 100, 1602. Bolts, J.M.; Bocarsly, A.B.; Palazzotto, M.C.; Walton, E.G.; Lewis, N.S.; Wrighton, M.S. J. Am. Chem. Soc., 1977, 101, 1378. Bocarsly, A.B.; Walton, E.G.; Wrighton, M.S. J. Am. Chem. Soc., 1980, 102, 3390. Wrighton, M.S.; Palazzotto, M.C.; Bocarsly, A.B.; Bolts, J.M.; Fischer, A.B.; Nadjo, L. J. Am. Chem. Soc., 1978, 100, 7264. Noufi, R.; Tench, D.; Warren, L.F. J. Electrochem. Soc., 1980, 127, 2310 and 1981, 128, 2596. Skotheim, T.; Lundstrom, I.; Prejza, J. J. Electrochem. Soc., 1981, 128, 1625. Noufi, R.; Frank, A.J.; Nozik, A.J. J. Am. Chem. Soc., 1981, 103, 1849. (a) Kanazawa, K.K.; Diaz, A.F.; Geiss, R.H.; Gill, W.D.; Kwak, J.F.; Logan, J.Α.; Rabolt, J.; Street, G.B. J. Chem. Soc., Chem. Commun., 1979, 854; (b) Diaz, A.F.; Castillo, J.J. J. Chem. Soc., Chem. Commun., 1980, 397; (c) Kanazawa, K.K.; Diaz, A.F.; Gill, W.D.; Grant, P.M.; Street, G.B.; Gardini, G.P.L. Kwak, J.F. Synth. Met., 1979/1980, 2, 329; (d) Diaz, A.F.; Vasquez Yallejo, J.M.; Martinez Duran, A. IBM J. Res. Dev., 1981, 25, 42. Simon, R.A.; Ricco, A.J.; Wrighton, M.S.; J. Am. Chem. Soc., 1982, 104, 2031. Bookbinder, D.C.; Lewis, N.S.; Bradley, M.G.; Bocarsly, A.B.; Wrighton, M.S. J. Am. Chem. Soc., 1979, 101, 7721. Bookbinder, D.C.; Wrighton, M.S., J. Am. Chem. Soc., 1980, 103, 5123. Bruce, J.S.; Murahashi, T.; Wrighton, M.S. J. Phys. Chem., 1982, 86, 1552. Dominey, R.N.; Lewis, N.S.; Bruce, J.Α.; Bookbinder, D.C.; Wrighton, M.S. J. Am. Chem. Soc., 1982, 104, 467. Nakato, Y.; Abe, K.; Tsubomura, H. Ber. Bunsenges. Phys. Chem., 1976, 80, 1002. Heller, Α.; Vadimsky, R.G. Phys. Rev. Lett., 1981, 46, 1153. Summers, L.S., "The Bipyridinium Herbicides", Academic Press: London, 1980, pp. 122-124. (a) Szentrimay, R.; Yeh, P.; Kuwana, T., ACS Symposium Series, 1977, 38, 143;

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RECEIVED

August 9,

1982

Discussion CH. L a n g f o r d , Concordia U n i v e r s i t y : Dr. Wrighton's paper m a i n t a i n s a p p r o p r i a t e d i s c r e t i o n w i t h r e s p e c t t o t h e twentyf i r s t c e n t u r y . As N e i l s B o h r s a i d , " P r e d i c t i o n i s d i f f i c u l t , e s p e c i a l l y a b o u t t h e f u t u r e " . However, he does c a l l a t t e n t i o n t o a number o f p r o b l e m s i n s o l i d s t a t e and i n t e r f a c i a l c h e m i s ­ t r y w h i c h a r e c e r t a i n l y i m p o r t a n t now a n d w o u l d n o t have b e e n l i k e l y t o be p i c k e d o u t f o r e m p h a s i s i n a m e e t i n g o f i n o r g a n i c chemists h e l d t e n y e a r s ago and o r i e n t e d t o t h e theme o f i n o r g a n i c c h e m i s t r y i n 1990. T h i s makes me r e f l e c t on t h e one aspect of t h e a c t i v i t i e s of t h e m a j o r i t y of t h e inorganic c h e m i s t s p r e s e n t w h e r e we a r e r e q u i r e d , l i k e i t o r n o t , t o p r e d i c t t h e f u t u r e . I r e f e r , of course, t o the teaching func­ t i o n . I n t e a c h i n g , we make c h o i c e s d e s i g n e d t o p r e p a r e t h e n e x t g e n e r a t i o n o f c h e m i s t s . T y p i c a l l y , t h e impact of t h e s e c h o i c e s c a n be e x p e c t e d t o e n d u r e o v e r 40 y e a r s . T h i s means t h a t c o n s c i o u s l y o r u n c o n s c i o u s l y we engage i n p r e d i c t i v e a c t i v i t y . After reading the p r e p r i n t of Wrighton's s t i m u l a t i n g paper, I d i d a q u i c k and i n c o m p l e t e survey o f t h e i n o r g a n i c

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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c h e m i s t r y t e x t b o o k s on my s h e l f . Of f i v e t h a t I e x a m i n e d , o n l y one had a s e r i o u s t r e a t m e n t o f s e m i c o n d u c t o r s and one seemed t o me t o d e v e l o p a " M a t e r i a l s " p e r s p e c t i v e a s a subtheme o r s i d e l i g h t . I am t r o u b l e d by t h i s . S u r e l y , t h e m a t e r i a l s t h a t do and w i l l s u p p o r t t h e d e v e l o p m e n t o f e l e c t r o n i c m a t e r i a l s and n o v e l c a t a l y s t s o f f e r a f r u i t f u l f i e l d f o r r e s e a r c h by i n o r g a n i c c h e m i s t s and r e p r e s e n t a f i e l d w h i c h c a n b e n e f i t f r o m t h e participation of inorganic chemists. We should t e l l our s tudents. On a n o t h e r p o i n t , W r i g h t o n r a i s e s t h e q u e s t i o n o f e l e c t r o n i c versus i o n i c c o n d u c t a n c e i n f i l m s and o f f e r s t h e t h o u g h t t h a t o n l y t h e f i r s t w i l l be s u f f i c i e n t l y f a s t f o r e f f e c t i v e d e v i c e development. I t h i n k t h a t there i s a p a r t i c u l a r subspec i e s o f e l e c t r o n i c conductance t h a t i s l i k e l y t o dominate t h e behavior o f chromophore f i l m s i n v o l v i n g m o l e c u l a r u n i t s . I t i s a " h o p p i n g " o r p o l a r o n mechanism w h i c h i s c l o s e l y r e l a t e d t o t h e homogeneous e l e c t r o n t r a n s f e r p r o c e s s w h i c h h a s been e x t e n s i v e l y s t u d i e d by i n o r g a n i c c h e m i s t s . I'd like t o raise the question of t h e l i m i t on t h e r a t e o f t h i s mechanism where d i f f u s i o n t o g e t h e r o f t h e p a r t n e r s i s n o t r e q u i r e d . H e r e , we may f i n d t h e t e s t o f t h e e x i s t a n c e and s i g n i f i c a n c e o f t h e elusive "inverted region".

M.S. Wrighton: I concur with your o b s e r v a t i o n t h a t students i n i n o r g a n i c chemistry r e c e i v e r e l a t i v e l y l i t t l e formal i n s t r u c t i o n i n m a t e r i a l s s c i e n c e . This w i l l l i k e l y change as the i n o r g a n i c chemists begin to e x e r t t h e i r i n f l u e n c e on the p r a c t i c a l aspects of m a t e r i a l s - b a s e d devices. Regarding the question o f the r a t e of e l e c t r o n t r a n s p o r t through polymer f i l m s , i t i s not y e t c l e a r what u l t i m a t e r a t e can be a c h i e v e d . In s o l a r energy a p p l i c a t i o n s the important i s s u e i s whether the r a t e can be high enough so t h a t the net electron transfer rate i s l i g h t i n t e n s i t y l i m i t e d .

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

5 Integrated Chemical Systems: n-Silicon/Silicide/Catalyst Systems ALLEN J. BARD, FU-REN F. FAN, G. A. HOPE, and R. G. KEIL

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch005

The University of Texas, Department of Chemistry, Austin, TX 78712

The paper by Wrighton describes semiconductor systems which incorporate other components, such as polymer layers, to produce useful electrode structures. The development of such multicomponent, multiphase systems, which we call "integrated chemical systems" by analogy to the integrated circuits used in semicon­ ductor devices, clearly represents an important new trend in chemistry. The design of useful semiconductor electrodes and powders will require surface modification to passivate surface states to improve efficiency, to protect the surface from photodecomposition, to catalyze desired reactions and to provide sen­ sitizers. For example the system p-GaAs/viologen polymer/Pt can be used for the photodriven evolution of hydrogen. The develop­ ment of such systems will probably require the application of techniques very different from those normally used in chemical synthesis, e.g., molecular beam epitaxy, sputtering, ion implan­ tation, spin coating, and other methods borrowed from solid state physics and semiconductor technology. These integrated chemical systems will have properties different and, we hope, more useful, than that of the individual components. Such syner­ gistic effects are well-known in biological systems where the overall behavior of the complex structure is usually more than the simple sum of the parts. I would l i k e to describe b r i e f l y an i n t e g r a t e d chemical sys­ tem r e c e n t l y under i n v e s t i g a t i o n i n our l a b o r a t o r y based on ntype s i l i c o n which i l l u s t r a t e s some of the above features (1,2). As Wrighton p o i n t s out i n h i s paper, η-Si e l e c t r o d e s are u s u a l l y unstable i n aqueous s o l u t i o n s , because they tend to form a p a s s i v a t i n g oxide f i l m under i r r a d i a t i o n . We have found that by form­ ing a platinum s u i c i d e l a y e r on the surface of the S i e l e c t r o d e (by f l a s h evaporation of Pt on the p r e t r e a t e d S i surface followed by annealing) the e l e c t r o d e w i l l show s t a b l e operation i n aqueous photoelectrochemical (PEC) c e l l s . The current-voltage curves f o r an η-Si e l e c t r o d e (coated with 40 angstroms of Pt and annealed a t 400 C i n vacuum f o r 10 minutes) i n a s o l u t i o n of 1 M FeCl2, 0.1 M 0097-6156/83/0211-0093$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Photovoltage, V

Figure 1. Photocurrent-photovoltage characteristics of the cell η-Si (Pt suicide coated)/1.0 M FeCl , 0.1 M FeCl , 1 M HCl/Pt at 65 nW/cm illumination. Pt thickness deposited ~ 40 À and annealing temperature 400 °C at ~ 10' torr for 10 min. Key: a, before long-term stability test; and b, after long-term stability test. 2

2

s

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6

Figure 2. Voltammetric curves of Pt (curve a) and n-Si(Ir)/Ru0 (curve b) electrode in 11 M LiCl at pH = 7. Light intensity 65 nW/cm , and scan rate 100 mV/s. (Reproduced with permission from Réf. 1. Copyright 1982, The Electrochemical Society, Inc.) 2

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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z

F e C l ^ , and 1 M HC1 under i l l u m i n a t i o n of 65 mW/cm (tungstenhalogen lamp) are shown i n F i g u r e 1. T h i s c e l l has operated f o r more than 20 days with no a p p r e c i a b l e change i n performance or decomposition at a maximum power conversion e f f i c i e n c y of about 6%. To s t a b i l i z e such an e l e c t r o d e to oxidants stronger than i r o n ( I I I ) a c a t a l y s t such as Ru0£ must be added to the s u r f a c e to allow the r a p i d t r a n s f e r of photogenerated holes to a s o l u t i o n species before S i r e a c t i o n s occur. Under these c o n d i t i o n s even c h l o r i n e and bromine e v o l u t i o n are p o s s i b l e on S i . For example an i r i d i u m s i l i c i d e coated η-Si e l e c t r o d e with Ru02 c a t a l y s t i n 11 M L i C l s o l u t i o n w i l l evolve c h l o r i n e at p o t e n t i a l s about 0.4V l e s s p o s i t i v e than the r e v e r s i b l e c h l o r i d e / c h l o r i n e p o t e n t i a l (Figure 2) and show no a p p r e c i a b l e d e t e r i o r a t i o n a f t e r seven days of continuous i r r a d i a t i o n . In looking towards the 21st Century, I p r e d i c t that i n t e r ­ f a c i a l photochemical and e l e c t r o c h e m i c a l processes at designed and i n t e g r a t e d chemical systems w i l l play an important r o l e i n the development of energy conversion and other devices. LITERATURE CITED

1. Fan, F. R., Hope, G. Α., and Bard, A. J. J. Electrochem. Soc., in press. 2. Fan, F. R., Keil, R. G., and Bard, A. J., J. Am. Chem. Soc., submitted. RECEIVED

December 30,

1982

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

6 Metalloporphyrins Catalysts for Dioxygen Reduction and P-450-Type Hydroxylations D A V I D D O L P H I N and B R I A N R . J A M E S

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

University of British Columbia, Department of Chemistry, Vancouver, British Columbia V 6 T 1Y6 Canada

The functions of the heme proteins, cytochrome oxidase, cytochrome P-450, catalase, and peroxidase, are discussed with special reference to the mechanisms of enzymatic action and the development of protein-free in vitro catalysts. The use of dimeric face-to-face cobalt porphyrins for the electrochemical four-electron reduction of dioxygen to water is limited, probably as a result of destruction of the porphyrin macrocycle by intermediate hydrogen peroxide that is released. The cytochrome oxidase system prevents hydrogen peroxide release, possibly by utilizing it in much the same manner as catalase and peroxidase, as well as P-450 that more commonly functions as a hydroxylating reagent using dioxygen and a two­ -electron reducing source. The mechanisms of each of the enzyme systems can be pictured as involving formation of a ferric peroxide that breaks down, via cleavage of the oxygen-oxygen bond, into an oxo iron(IV) porphyrin cation radical species with release of water. The oxo intermediate, that in the case of P-450 can be generated also via the iron(III) state and iodosylbenzene or peracids, is considered to be the key species governing the reactivity pattern of the enzyme. Figure 1 shows i n bold type the t r a c e elements i n nature e s s e n t i a l f o r the proper f u n c t i o n i n g of b i o l o g i c a l processes. The importance of metals has been long known but i t i s only i n the past three decades that some of t h e i r s p e c i f i c r o l e s have begun to be e l u c i d a t e d . I t i s perhaps not s u r p r i s i n g that i r o n , the most n a t u r a l l y abundant of a l l metals, should play many important r o l e s i n nature. We s h a l l present here one small aspect of t h i s r a p i d l y expanding area of inorganic chemistry, namely that of the f u n c t i o n i n g of i r o n when coordinated to porphyrins ( 1 , 2^, 3 ) . Figure 2 shows the major heme p r o t e i n s 0097-6156/83/0211-0099$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

100

INORGANIC

IA

IIA

III Β IVB

VB

VIB VIIB s ί

CHEMISTRY:

VIII

T O W A R D T H E 21 ST C E N T U R Y

IIB

IB

A

/ \

H

\

Be

/

Κ

Ca

Sc

Ti

V

Cr

Rb

Sr

Y

Zr

Nb

Cs

Ba

La

Hf

Ta

Fr

Ra

Ac

Figure 1.

He

F

Ne

Cl

Ar

C

Ν

Al

Si

Ρ

ο s

Ga Ge

As

Se

Br

Kr

Fe

Co

Ni

Cu

Zn

Mo

Tc

Ru

Rh

Pd

Ag

Cd

In

Sn

Sb

Te

1

Xe

W

Re

OS

Ir

Pt

Au

Hg

TI

Pb

Bi

Ρο

At

Rn

The periodic chart: essential trace elements in nature are shown in bold.

HEMOGLOBIN MYOGLOBIN

Fe

CYTOCHROMES

Fe

2

PEROXIDASE

COgH

HEME

02.

2*

H 0

2

H

Mn

CATALASE

H0 C

\

Β

\

Να M g

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

/

\

Li

VIAVIIAgSSSl

VA

IDA IVA

2

3+V ^

R C

2*

Fe°

H 0 2

H

0

FeV'

r

2

f^HRP

2R*

ROH

+

2Fe * 2Cup^ 3

0^,4e-,4H* Figure 2.

°2

Fe 0 ,2e-,2H

CYTOCHROME OXIDASE

.2°2

2RH

RH CYTOCHROME P-450

' 2*

Fe

H0 2

2Fe * >T2Cu 3

2+

2H 0 2

The major heme proteins and their biochemical functions.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

6.

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AND JAMES

Metalloporphyrins

101

and t h e i r biochemical f u n c t i o n s . In a d d i t i o n t o f u n c t i o n i n g as transport and storage p r o t e i n s f o r e l e c t r o n s and oxygen, hemop r o t e i n s p l a y other important r o l e s . A major theme played by heme enzymes i s the c o n t r o l and u t i l i z a t i o n of oxygen and reduced oxygen d e r i v a t i v e s (4). Figure 3 shows the complex chemistry of oxygen i n terms of the r e d u c t i o n p o t e n t i a l s governing i t s oxidat i o n s t a t e s a t both n e u t r a l (pH 7.0) and a c i d (pH 0.0) c o n d i t i o n s ; shown a l s o are the areas where heme p r o t e i n s exert t h e i r influence. Nature has had a long time to p e r f e c t her i n o r g a n i c chemistry, and understanding the mechanisms of enzymatic a c t i o n can suggest ways f o r the inorganic chemist to develop corresponding i n v i t r o c a t a l y s t s . We s h a l l show how knowledge of cytochrome oxidase and cytochrome P-450 has l e d t o such p o s s i b i l i t i e s . Cytochrome oxidase i s the terminal e l e c t r o n acceptor i n the r e s p i r a t o r y chain of a l l oxygen-breathing organisms (5). Nature uses cytochrome oxidase t o b r i n g about the "concerted" f o u r e l e c t r o n r e d u c t i o n of dioxygen to water without the r e l e a s e o f the t o x i c one- and two-electron r e d u c t i o n products (superoxide and hydrogen peroxide, r e s p e c t i v e l y ) . The study of cytochrome oxidase i s complicated by the f a c t that i t i s a multi-component enzymatic system which f u n c t i o n s w i t h i n and across the b i o l o g i c a l membrane. U n t i l Sanger's group (6) r e c e n t l y sequenced part o f human m i t o c h o n d r i a l genome, even the s i z e of each component of the enzyme, l e t alone the i n d i v i d u a l p r o t e i n sequences, was unknown. The whole enzyme complex, however, i s known t o c o n t a i n two heme porphyrins and two copper c e n t e r s . While these f o u r e l e c t r o n redox centers allow f o r s p e c u l a t i o n on the storage of the four e l e c t r o n s provided v i a cytochrome c_ (another heme prot e i n ) , the mechanism of t h e i r d e l i v e r y to dioxygen to form water i s f a r from c l e a r . The p r a c t i c a l need t o reduce dioxygen to water without the r e l e a s e of peroxide i s important i n f u e l - c e l l technology, where the f a b r i c a t i o n o f a cheap and robust oxygen e l e c t r o d e i s s t i l l a major g o a l . The use of metalloporphyrins f o r t h i s purpose has been widely studied and some c o n s i d e r a b l e success has been achieved r e c e n t l y . By u s i n g c o f a c i a l l y l i n k e d dimeric c o b a l t porphyrins (1), Anson, Collman et a l . (_7, 8) have brought about the " d i r e c t " f o u r - e l e c t r o n r e d u c t i o n of dioxygen to water. However, the c a t a l y t i c l i f e t i m e of the dimeric porphyrins on the carbon e l e c t r o d e s i s short (9). We supposed that h y d r o l y s i s of the amide b r i d g i n g groups, under the s t r o n g l y a c i d i c c o n d i t i o n s employed f o r oxygen r e d u c t i o n , accounted f o r the l o s s of c a t a l y t i c a c t i v i t y . We have prepared an analogous s e r i e s of dimeric c o f a c i a l porphyrins l i n k e d by polymethylene chains (2) (10) which cannot, o f course, undergo h y d r o l y s i s . These complexes p a r a l l e l those o f Anson's and Collman s with respect t o t h e i r a b i l i t y t o promote the f o u r - e l e c t r o n r e d u c t i o n o f water. Unfortunately, t h e i r c a t a l y t i c l i f e t i m e i s a l s o short, suggesting that the l o s s of a c t i v i t y r e s u l t s not from the breaking of the l i n k i n g groups 1

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

but r a t h e r from d e s t r u c t i o n of the porphyrin macrocycle by hydrogen peroxide (11).

How does nature prevent the r e l e a s e of hydrogen peroxide during the cytochrome oxidase-mediated f o u r - e l e c t r o n r e d u c t i o n of dioxygen? I t would appear that cytochrome oxidase behaves i n the same manner as other heme p r o t e i n s which u t i l i z e hydrogen peroxide, such as c a t a l a s e and peroxidase (vide i n f r a ) , i n that once a f e r r i c peroxide complex i s formed the oxygen-oxygen bond i s broken with the r e l e a s e of water and the formation of an oxo iron(IV) complex which i s subsequently reduced to the f e r r o u s aquo s t a t e (12). Indeed, t h i s same sequence of events accounts f o r the means by which oxygen i s a c t i v a t e d by cytochromes P-450. Cytochromes P-450 occur i n both procaryotes and eucaryotes (13) and, while many d i f f e r e n t s p e c i f i c enzymatic r e a c t i o n s are known to be c o n t r o l l e d by these enzymes, they can a l l be g e n e r a l ized i n t o two main c a t e g o r i e s ; namely, the epoxidation of o l e f i n s ( i n c l u d i n g aromatic systems) which gives r i s e to the c a r c i n o g e n i c i t y of fused p o l y c y c l i c aromatic hydrocarbons, and the o x i d a t i o n of unactivated carbon-hydrogen bonds to the corresponding a l c o h o l , such as the conversion of cyclohexane to cyclohexanol (14) . The enzymatic c y c l e f o r cytochrome P-450 i s shown i n Figure 4. Binding of the s u b s t r a t e (RH) to the f e r r i c hemoprotein changes the i r o n from low to high s p i n ; a subsequent one-electron r e d u c t i o n gives the f e r r o u s heme p r o t e i n . L i k e myoglobin (Mb), f e r r o u s P-450 binds both oxygen and carbon monoxide, but the analogy ends there. The m a j o r i t y of heme p r o t e i n s e x h i b i t a strong Soret absorption band at approximately 420 nm. However, the CO complex of P-450 i s a t y p i c a l i n that i t e x h i b i t s a s p l i t Soret band with one t r a n s i t i o n at 450 nm (from which the enzyme

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

6.

DOLPHIN A N D JAMES

103

Metalloporphyrins

OXONIUM ION

HO"*"

Ό (Α) 2

|^H

22.4 K CAL MOLE H eV ) CANCER PHOTOTHERAPY

Ε -0.27 V e*

J 0

2

H

DIOXYGEN β

+

Vi*

0

2

2H

e

-

H

H 0

+

d

Τ pK 118 |\

SUPEROXIDE

H

2

^

2 d

II

+

0 H

2

0

+

HYDROXYL RADICAL

UUATCCS W A I t K

E°+I35V

2

^

E°+ 2 20V e ^ > H -

#

^ ^ •

2

HYDROGEN PEROXIDE

E°+090V e~ ^ > • H 0

ά

E + l.46V

g

+

ρ Κ 45

Ε - 0.4 V e~

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

2 ^

T

2

H

P-450

, , - , >~ HoO + Ο· OXENE

π

E° + I.63V I QI HO* J > - »H 0

+

CYTOCHROME

+

< 2H

CATALASE

- * H

2

0

pK I57JJ

+

OH" HYDROXIDE ION

II K 20 P

E°+0.80V

O2

2H 0 2

PEROXIDE ION

CYTOCHROME OXIDASE

Figure 3. Standard reduction potentials associated with the chemistry of oxygen; values in upper and lower halves of diagram refer to pH 0.0 and pH 7.0 conditions, respectively.

(Product)

ROH Pg

3 +

(

Resting enzyme) RH (substrate)

S

*I0/

Fe - RH 3+

Fe - RH 2+

Fe 0 R H 2

Figure 4.

The enzymatic cycle for cytochrome P-450.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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THE

21ST

CENTURY

was named (15)) and a second higher energy t r a n s i t i o n at 350 nm. This unusual o p t i c a l spectrum has been shown to r e s u l t from the unique a x i a l l i g a t i o n of a t h i o l a t e anion provided by a c y s t e i n e r e s i d u e of the p r o t e i n (16-19). Oxy P-450 a l s o e x h i b i t s such a s p l i t Soret band and t h i s , together with p H - t i t r a t i o n data (19), show that t h i s enzymatically important oxygenated d e r i v a t i v e i s a l s o coordinated by a t h i o l a t e anion. The h i g h e l e c t r o n d e n s i t y on s u l f u r , which can be transmitted to the coordinated oxygen, makes oxy P-450 r e l a t i v e l y unstable at ambient c o n d i t i o n s , and the l o s s of superoxide with the reformation of the f e r r i c com­ plex i s a f a c i l e process (19). At f i r s t s i g h t t h i s i n s t a b i l i t y appears to be a waste of the c e l l s reducing c a p a b i l i t y , but we suspect that i t i s a compromise between the unwanted s i d e - e f f e c t of the t h i o l a t e l i g a n d at t h i s stage of the r e a c t i o n and i t s p o s i t i v e e f f e c t s at l a t e r stages of the enzymatic c y c l e , that we s h a l l d e t a i l below. The next, and r a t e l i m i t i n g step, i n the enzymatic r e a c t i o n i s a f u r t h e r one-electron r e d u c t i o n of oxy P-450. As yet, l i t t l e i s known about the enzymatic r e a c t i o n s beyond t h i s stage. We (20) and others (21, 22), however, have explored t h i s f u r t h e r chemistry u s i n g model systems. Enzymatically, the one-electron r e d u c t i o n of oxy P-450 occurs at approximately -0.20 V (23); t h i s value i s v s . Ag/AgCl, which has a p o t e n t i a l of about +0.23 V w.r.t. the S.H.E. We have found that a one-electron r e d u c t i o n of oxy f e r r o u s o c t a e t h y l p o r p h y r i n i n DMSO/CH3CN at -20° can be brought about e l e c t r o c h e m i c a l l y at a platinum e l e c t r o d e at a very s i m i l a r potential,-0.24 V (vs. Ag/AgCl),to give the η^-peroxy complex (3) (20, 24). However, i n our hands, and those of others (25), t h i s model monoanionic complex e x h i b i t s none of the o x i d i z i n g powers of P-450. This may, of course, mean that a

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

1

complex of the type (3) i s not formed enzymatically. More l i k e l y though i s that w i t h i n the enzyme the t h i o l a t e now plays a c r i t ­ i c a l r o l e . Any s i n g l e bond between two e l e c t r o n e g a t i v e elements i s n e c e s s a r i l y weak. This i s c e r t a i n l y t r u e f o r peroxides and the added e l e c t r o n d e n s i t y supplied by an a x i a l l y coordinate t h i o l a t e anion l i g a n d would a i d i n the cleavage of the peroxy bond i n an enzymatic species corresponding to complex 3 · l a d d i t i o n a l l of the substrates f o r cytochromes P-450 are hydro­ phobic, and the binding s i t e f o r s u b s t r a t e , which i s n e c e s s a r i l y c l o s e to the i r o n porphyrin, must a l s o be hydrophobic. Yet the a d d i t i o n of the l a s t e l e c t r o n i n the enzymatic c y c l e could gen­ erate a species such as 3, that i n f a c t would have a net double n

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

6.

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AND

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Metalloporphyrins

JAMES

negative charge. In such a non-polar environment the peroxo l i g a n d w i l l be s t r o n g l y n u c l e o p h i l i c (basic) and t h i s could ease 0-0 bond cleavage e i t h e r by d i r e c t protonation or a c y l a t i o n (26). By whatever mechanism the bond cleavage occurs, one oxygen atom must leave at the o x i d a t i o n l e v e l of water. Scheme 1 shows that the i r o n oxygen complex (4) remaining, which i s presumed to be the o x i d i z i n g agent, i s f o r m a l l y an F e ^ oxene (oxygen atom) complex. The chemistry of oxenes i s h a r d l y explored, although oxygen atoms can b r i n g about P-450-like hydrocarbon o x i d a t i o n (27). By analogy, however, the chemistry of oxenes can be expected to p a r a l l e l that of n i t r e n e s and carbenes. Here d i r e c t i n s e r t i o n i n t o C-H bonds and the a d d i t i o n across o l e f i n s without l o s s of stereochemistry i s w e l l defined (28). Cytochromes P-450 do not appear to f u n c t i o n v i a d i r e c t i n s e r t i o n , and the oxene analogy i s probably not a reasonable one (29).

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

+

4 Scheme 1 I f the a c t i v e o x i d i z i n g agent i s not formulated as a f e r r i c oxene complex, the questions then remain as to what i s i t s e l e c t r o n i c ground s t a t e and how does hydrocarbon o x i d a t i o n occur? Answers to these questions were i n i t i a t e d over a decade ago during our s t u d i e s on c a t a l a s e (CAT) and horseradish peroxidase (HRP) (30). Both n a t i v e enzymes are f e r r i c hemoproteins and both are o x i d i z e d by hydrogen peroxide. These o x i d a t i o n s cause the l o s s of two e l e c t r o n s and generate a c t i v e enzymatic i n t e r mediates that can be f o r m a l l y considered as F e ^ complexes. U n t i l very r e c e n t l y no complexes of F e ^ were known, and so we examined the e l e c t r o n i c ground s t a t e of these h i g h l y o x i d i z e d hemoproteins, which are known as the compound I d e r i v a t i v e s , CAT I and HRP I. An examination of t h e i r o p t i c a l spectra (Figure 5) suggested that both CAT I and HRP I contained porphyrin 7r-cation r a d i c a l s i n which the porphyrin r i n g had undergone a one-electron oxidat i o n (31). Many porphyrins and metalloporphyrins can be o x i d i z e d to Tt-cation r a d i c a l s and these species can occupy one of two ground states (^A-^ or ^&2\j) * ground s t a t e s have d i f f e r e n t but c h a r a c t e r i s t i c o p t i c a l spectra and can be interchanged as a f u n c t i o n of a x i a l l i g a t i o n of the metalloporphyrin (Figure 5) (30). Catalase i s a x i a l l y coordinated by a phenoxide of t y r o s i n e (32), and HRP by an imidazole of h i s t i d i n e (33), and t h i s may w e l l e x p l a i n t h e i r d i f f e r e n t e l e c t r o n i c ground s t a t e s +

+

T

n

e

s

e

t

w

o

u

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

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INORGANIC

300

400

500

CHEMISTRY:

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nm

Figure 5. Comparison of the optical absorption spectra of cobalt(III) porphyrin π-cation radical species with those of catalase compound I and horseradish peroxi­ dase compound I. The ground states of the bromide and perchlorate species are A and A , respectively. 2

2

l u

2u

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Metalloporphyrins

even though both enzymes contain i r o n protoporphyrin. Oxidation to the ïï-cation r a d i c a l accounts f o r the l o s s of one of the two e l e c t r o n s involved i n the peroxide-mediated o x i d a t i o n . The other e l e c t r o n i s l o s t from the i r o n , as exemplified by o p t i c a l and Mossbauer spectroscopy (34). Hence the e l e c t r o n i c c o n f i g u r a t i o n of CAT I and HRP I i s b e l i e v e d t o be that of an Fe*+ porphyrin π-cation r a d i c a l ; moreover, l a b e l l e d oxygen (35) and ENDOR studies (36) have shown that the complex contains an oxo l i g a n d (5).

0 Fe PorPublication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

4 +

The f e r r i c oxene complex (6) i s , of course, j u s t one reso­ nance formulation of 5 (see scheme 2). Moreover, the a d d i t i o n of dioxygen and two e l e c t r o n s to P-450 i s equivalent to using

:o: 4 s -

:o: P

„

:o:

FL'P—

6

ο

ο

IV-RTP

p= Protoporphyrin Scheme 2

peroxide, and indeed the cytochrome P-450 c a t a l y s i s can be shunted by using a l k y l hydroperoxides (28), Figure 4. I t i s now apparent that the hemoprotein f a m i l i e s of the c a t a l a s e s , peroxidases, and cytochromes P-450 are c l o s e l y r e l a t e d . Scheme 3 i l l u s t r a t e s the suggested s i m i l a r i t i e s f o r each of these sep­ arate enzymes. In a d d i t i o n t o a c t i v a t i n g P-450 by the use of a l k y l hydroperoxides, both iodosylbenzene (7) and peracids (28) can a l s o be used. Groves et a l . have e l e g a n t l y followed up such f i n d i n g s , and have found that f e r r i c porphyrins and iodosylbenzene provide a system, c a t a l y t i c i n porphyrin, f o r the room temperature oxida­ t i o n of hydrocarbons (37), see Figure 4. More r e c e n t l y t h i s group has shown that f e r r i c t e t r a - m e s i t y l p o r p h y r i n (8), when t r e a t e d with a p e r a c i d , y i e l d s an i s o l a b l e oxo intermediate which i s the a c t i v e hydrocarbon o x i d i z i n g agent (38). Depending upon the a x i a l l i g a t i o n , t h i s intermediate e x i s t s as e i t h e r the Fe^~*~ π-cation r a d i c a l or the Fe-* complex (39). These observa­ t i o n s add f u r t h e r support t o the hypothesis concerning the e l e c t r o n i c s t r u c t u r e of the a c t i v e enzymatic o x i d i z i n g agent, +

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Fe * CAT

CENTURY

Fe * CAT**

3

4

H0 2

H,0

2

R0 H

ROH

2

Fe * P-450 ^ >

»

3

0 ,2e-,2H*

H0

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

T O W A R D T H E 21ST

2

Fe P-450* I 4

"S

0 Fe *HRP**

Fe *HRP 3

4

Η>0

2

H0 2

Scheme 3

as o u t l i n e d i n schemes 2 and 3. We can now attempt to d e s c r i b e i n more d e t a i l the i r o n porphyrin complexes that are involved i n the enzymatic c y c l e of cytochrome P-450 (Figure 6, c f . Figure 4 ) .

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

DOLPHIN A N D JAMES

Figure 6.

Metalloporphyrins

The enzymatic cycle for cytochrome P-450, detailing possible electronic structure of intermediates. (Ρ represents protoporphyrin.)

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Studies with both cytochromes P-450 and model systems suggest that hydrocarbon o x i d a t i o n proceeds v i a the intermediacy of a substrate r a d i c a l (28, 29); the hydrophobic nature of the b i n d ing s i t e would a l s o argue f o r an uncharged ( r a d i c a l ) substrate intermediate. The storage of unpaired spins at i r o n , oxygen, and the porphyrin macrocycle, would allow f o r the s t a b i l i z a t i o n of r a d i c a l intermediates during the enzymatic o x i d a t i o n . The a b s t r a c t i o n of a substrate hydrogen atom, and the subsequent t r a n s f e r of a hydroxyl r a d i c a l to substrate from the i r o n porphyrin complex, account f o r the observations concerning hydrocarbon o x i d a t i o n . I t i s i n t e r e s t i n g to note that i n so many areas of oxygen chemistry, nature uses i r o n porphyrins f o r the t r a n s p o r t , s t o r age, and u t i l i z a t i o n of dioxygen and i t s reduced d e r i v a t i v e s . To t h i s end, cytochrome P-450 may be considered as nature's equivalent of Fenton's reagent; i n the enzyme system, h i g h l y r e a c t i v e intermediates such as hydroxyl r a d i c a l s and oxenes appear to be c o n t r o l l e d by t h e i r i n t e r a c t i o n with hemoprotein. What can a l l these s t u d i e s suggest to the inorganic chemist i n t e r e s t e d i n the c o n t r o l l e d and f a c i l e c a t a l y t i c o x i d a t i o n of hydrocarbons? Groves and coworkers have already shown that i r o n porphyrins i n the presence of iodosylbenzene and peracids can be used f o r such c a t a l y t i c r e a c t i o n s (37, 38). However, the cost of the oxidants makes such r e a c t i o n uneconomical at t h i s time. Nature uses dioxygen and two e l e c t r o n s to generate the a c t i v e o x i d i z i n g agent, but we have shown that one such intermediate generated e l e c t r o c h e m i c a l l y with a model system ( l a c k i n g the t h i o l a t e , however) i s not an oxidant. The only d i f f e r e n c e between the a c t i v e o x i d i z i n g and a f e r r i c porphyrin hydroxide complex i s two e l e c t r o n s (scheme 4 ) . Indeed, the e l e c t r o c h e m i c a l o x i d a t i o n of hydroxy f e r r i c t e t r a m e s i t y l p o r p h y r i n shows two r e v e r s i b l e one-electron o x i d a t i o n s (40), and, i n p r i n c i p l e , use of water and an e l e c t r o d e should allow development of a system capable of c a t a l y t i c a l l y o x i d i z i n g hydrocarbons.

-H

+

Scheme 4 Acknowledgments This i s a c o n t r i b u t i o n from the B i o i n o r g a n i c Chemistry Group, which was supported by grants from the Canadian Natural Sciences and Engineering Research Council and the United States N a t i o n a l I n s t i t u t e s of Health (AM 17989).

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

6.

DOLPHIN

AND

JAMES

Metalloporphyrins

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

Literature Cited

111

1. Sigel, H., Ed.; "Metal Ions in Biological Systems"; Vol. 7, Marcel Dekker, New York, 1978. 2. Hughes, M.N. "The Inorganic Chemistry of Biological Proc­ esses"; 2nd Ed., Wiley, New York, 1981, Chapters 5, 7. 3. Ochiai, E. "Bioinorganic Chemistry. An Introduction"; Allyn and Bacon, Boston, 1977, Chapters 5-7, 10. 4. Spiro, T.G., Ed.; "Metal Ion Activation of Dioxygen"; WileyInterscience, New York, 1980. 5. Chance, B. Current Topics in Cellular Regulation, 1981, 18, 343. 6. Anderson, S.; Bankier, A.T.; Barrell, B.G.; de Bruijn, M.H.L.; Coulson, A.R.; Drouin, J.; Eperon, I.C.; Nierlich, D.P.; Roe, B.A.; Sanger, F.; Schreier, P.H.; Smith, A.J.H.; Staden, R.; Young, I.G. Nature, 1981, 290, 457. 7. Collman, J.P.; Marrocco, M.; Denisevich, P.; Koval, C.; Anson, F.C. J. Electroanal. Chem., 1979, 101, 117. 8. Collman, J.P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F.C. J. Am. Chem. Soc., 1980, 102, 6027. 9. Anson, F.C.; Durand, R.R., Jr.; personal communication. 10. Dolphin, D.; Hiom, J.; Paine, J.B., III; unpublished results. 11. Durand, R.R., Jr.; Anson, F.C. J. Electroanal. Chem., 1982, 134, 273. 12. Chan, S.I.; personal communication. 13. Griffin, B.W.; Peterson, J.Α.; Esterbrook, R.W. in "The Porphyrins"; Vol. VII, Dolphin, D., Ed.; Academic Press, New York, 1979, p. 333. 14. Chang, C.K.; Dolphin, D. in "Bioorganic Chemsitry"; Vol. IV, van Tamelen, E.E., Ed.; Academic Press, New York, 1978, p. 37. 15. Omura, T.; Sato, R. J. Biol. Chem., 1962, 237, 1375. 16. Collman, J.P.; Sorrell, T.N. J. Am. Chem. Soc., 1975, 97, 4133. 17. Chang, C.K.; Dolphin, D. J. Am. Chem. Soc., 1975, 97, 5948. 18. Hanson, L.K.; Eaton, W.A.; Sligar, S.G.; Gunsalus, I.C.; Gouterman, M.; Connell, C.R. J. Am. Chem. Soc., 1976, 98, 2672. 19. Dolphin, D.; James, B.R.; Welborn, H.C. J. Mol. Catal., 1980, 7, 201. 20. Welborn, H.C.; Dolphin, D.; James, B.R. J. Am. Chem. Soc., 1981, 103, 2869. 21. Reed, C.A. in Adv. Chem. Series; Vol. 201, Kadish, K.M., Ed.; Am. Chem. Soc., Washington, D.C., 1982, Chapter 15. 22. McCandlish, E.; Miksztal, A.R.; Nappa, M.; Sprenger, A.Q.; Valentine, J.S.; Stong, J.D.; Spiro, T.G. J. Am. Chem. Soc., 1980, 102, 4268. 23. Gunsalus, I.C.; personal communication. 24. Dolphin, D.; James, B.R.; Welborn, H.C. in Adv. Chem. Series; Vol. 201, Kadish, K.M., Ed.; Am. Chem. Soc., Washington, D.C., 1982, Chapter 23. 25. Groves, J.T.; Valentine, J.S.; personal communications.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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112

26. Sligar, S.G.; Kennedy, K.A.; Pearson, D.C. Proc. Natl. Acad. Sci. U.S.A., 1980, 77, 1240. 27. Hori, Α.; Takamuku, S.; Sakurai, H. J. Org. Chem., 1977, 42, 2318. 28. Coon, M.J.; White, R.E. in "Metal Ion Activation of Dioxy­ gen"; Spiro, T.G., Ed.; Wiley-Interscience, New York, 1980, p. 73. 29. Groves, J.T.; ibid., p. 125. 30. Dolphin, D.; Forman, Α.; Borg, D.C.; Fajer, J.; Felton, R.H. Proc. Natl. Acad. Sci. U.S.A., 1971, 68, 614. 31. Dolphin, D.; Felton, R.H. Acc. Chem. Res., 1974, 7, 26. 32. Reid, T.J., III; Murphy, M.R.N.; Sicignano, Α.; Tanaka, N.; Musick, W.D.L.; Rossmann, M.G. Proc. Natl. Acad. Sci. U.S.A., 1981, 78, 4767. 33. Yonetani, T.; Yamamoto, H.; Erman, J.E.; Leigh, J.S., Jr.; Reed, G.H. J. Biol. Chem., 1972, 247, 2447. 34. Dolphin, D. Isr. J. Chem., 1981, 21, 67. 35. Hager, L.P.; Doubek, D.L.; Silverstein, R.M.; Hargis, J.H.; Martin, J.C. J. Am. Chem.Soc.,1972, 94, 4364. 36. Roberts, J.E.; Hoffman, B.M.; Rutter, R.; Hager, L.P. J. Am. Chem.Soc.,1981, 103, 7654. 37. Groves, J.T.; Nemo, T.E.; Myers, R.S. J. Am. Chem. Soc., 1979, 101, 1032. 38. Groves, J.T.; Haushalter, R.C.; Nakamura, M.; Nemo, T.E.; Evans, B.J. J. Am. Chem.Soc.,1981, 103, 2884. 39. Groves, J.T.; personal communication. 40. Dolphin, D.; unpublished observations. RECEIVED August 1 0 ,

1982

Discussion N. S u t i n , Brookhaven N a t i o n a l Laboratory; In the r e d u c t i o n of oxygen by cytochrome c oxidase, i s there any evidence f o r coopérâtivity between heme a n and Cu ? In p a r t i c u l a r , i s there evidence f o r the formation or a peroxo-bridged a^-Cu-Q i n t e r m e d i ate? B

D. Dolphin: C l e a r l y the f a c t that the a3-Cug p a i r can be a n t i f e r r o m a g n e t i c a l l y coupled shows that these two centres can communicate with each other. I t i s not c l e a r , however, that t h i s p a r t i c u l a r "intermediate" has any enzymatic s i g n i f i c a n c e . I know of no d e f i n i t i v e evidence concerning the formation of a μ-peroxo complex between and C u , and i f the analogy between cytochrome oxidase and the other heme p r o t e i n s , I have r e f e r r e d t o , holds then I see no need t o suggest such peroxo-bridging. I suspect that Cug i s there as an e l e c t r o n c a r r i e r and not as a l i g a n d f o r dioxygen i n any of i t s o x i d a t i o n s t a t e s during i t s r e d u c t i o n t o water. B

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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DOLPHIN

AND

113

Metalloporphyrins

JAMES

D.R. M c M i l l i n , Purdue U n i v e r s i t y : L i k e cytochrome oxidase, the blue copper p r o t e i n l a c c a s e c a t a l y z e s the r e d u c t i o n of dioxygen to water, a l b e i t at a somewhat slower r a t e . Given the s t r u c t u r a l analogies that e x i s t between the p r o t e i n s — each contains four metal ions, a s i t e i n v o l v i n g a coupled p a i r of metal ions, e t c . — one might expect there to be analogies i n t h e i r mechanisms of a c t i o n as w e l l . On the other hand, i t seems u n l i k e l y that a copper ion could achieve the valency changes you a s c r i b e to the a 3 heme. Do you have any comment regarding the l a c c a s e system?

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

D. Dolphin: I agree with you that I f i n d i t u n l i k e l y e i t h e r the copper i o n , or the copper ion and i t s attendant l i g a n d s , could achieve the high o x i d a t i o n s t a t e s analogous to those postulated f o r the a3 heme. T.J. Kemp, Warwick: Noting the c r i t i c a l r o l e of higher o x i d a t i o n s t a t e s of Fe i n the f u n c t i o n i n g of cytochrome P-450, i t i s i n t e r e s t i n g to note the recent u . v . - s p e c t r a l c h a r a c t e r ­ i s a t i o n of t r a n s i e n t intermediate i n the o x i d a t i o n of [ F e l l i C N ) ^ - by both H0C1 and X e F i n aqueous solution. » This species i s assigned to an F e species by Purmal et al., » and i n a s s o c i a t i o n with h i s l a b o r a t o r y , Dr. Peter Moore and I have obtained f u l l y time-resolved u . v . - v i s i b l e spectra of these systems i n a stopped-flow apparatus coupled to a r a p i d scanning spectrometer, as exemplified i n the f i g u r e . 1

2

2

I V

1

2

3

Or

20 40 6O

σ ΘΟ IOO

450

400

350

λ/η S p e c t r a l p r o f i l e of the i n i t i a l part of the r e a c t i o n between F e i C N ) ^ - (2.5 χ 10" mol dnT ) and NaOCl (2.5 χ 10" mol dm ) at pH 2.25; t r a c e α represents spectrum on mixing ( i . e . of r e a c t a n t s ) , while b o ... are spectra at successive i n t e r v a l s of 150 ms. g i s the spectrum of the intermediate, a t t r i b u t e d to F e I V ( C N ) - . Kozlov, Yu.N.; M o r a v s k i i , A.P.; Purmal , A.P. Shuvalov, V.F.; Zh. F i z . Khim.1981, 55, 764. Kozlov, Yu.N.; Vorob'eva, T.P.; Purmal ; A.P. Zh. F i z . Khim. 1981, 55, 2279. 3. Kemp, T.J. Kozlov, Yu.N.; Moore, P.; Purmal', A.P. Silver, G.R. to be published. 4

9

3

3

9

2

6

1

1

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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P. Dolphin: A very i n t e r e s t i n g o b s e r v a t i o n . There are, of course, many such examples of the Fe*V o x i d a t i o n s t a t e when the i r o n i s coordinated to a porphyrin. T.J. Kemp, Warwick: Is i t not the case that the product d i s t r i b u t i o n i n h y d r o x y l a t i o n of organic molecules by Fenton's reagent d i f f e r s i n c o n s i d e r a b l e d e t a i l from that r e a l i s e d by cytochrome P-450? 1 2 D. Dolphin: Groves and h i s co-workers ' have shown that f e r r o u s p e r c h l o r a t e and hydrogen peroxide can c a r r y out hydroxyl a t i o n s which bear c l o s e p a r a l l e l s to the P-450 mediated systems.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

1.

Groves, 859. Groves, 5290.

2.

J.T.; McClusky, G.A.

J . Am.

J.T.; Van Der Puy, M.

Chem. Soc. 1976,

J . Am.

98,

Chem. Soc. 1976,

98,

M.S. Wrighton, M.I.T.: Why are the 4e-02 r e d u c t i o n c a t a l y s t s decomposed by H2O2 when i t appears that these c a t a l y s t s operate at p o t e n t i a l s where i t i s thermodynamically impossible to make H 0 ? 2

2

D. Dolphin: Such p o t e n t i a l s r e f e r to f r e e hydrogen peroxide (H2O2). The p o t e n t i a l s at which coordinated peroxide can be formed are lower, but subsequent p r o t o n a t i o n of such coordinated peroxide can r e s u l t i n the l i b e r a t i o n of f r e e peroxide which can then react with the periphery of the porphyrin. E.R. E v i t t , C a t a l y t i c a A s s o c i a t e s , Inc: In t h e i r work with amide-linked c o f a c i a l porphyrins, Collman and Anson have only observed 100% f o u r - e l e c t r o n 0 r e d u c t i o n i n the four-atombridged member of the s e r i e s . Your comments implied f o u r - e l e c tron 0 r e d u c t i o n i s observed with the f i v e and eight as w e l l as four-atom bridged methylene-1inked c o f a c i a l porphyrins you have prepared. Is such a c t i v i t y observed f o r the compounds with longer linkages? 2

2

D. Dolphin: Collman and Anson have found with t h e i r f i v e atom bridged amide systems that the r e d u c t i o n of dioxygen i s p a r t i t i o n e d roughly one t h i r d undergoing a f o u r - e l e c t r o n reduct i o n to water and the r e s t a two-electron r e d u c t i o n to peroxide. We see the same r e s u l t ; indeed the 5-5 systems of ours and that of Collman-Anson are i n d i s t i n g u i s h a b l e . Neither type of c o f a c i a l dimers with chains greater than f i v e show any f o u r - e l e c t r o n reduction.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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DOLPHIN AND JAMES

Metalloporphyrins

115

E.R. E v i t t , C a t a l y t i c a A s s o c i a t e s , Inc.: Have you prepared polymethylene-1inked c o f a c i a l porphyrins that c o n t a i n two d i f f e r e n t metals i n the c o f a c i a l u n i t ? Is i t p o s s i b l e to prepare polymethylene-1inked c o f a c i a l porphyrins that a r e unsymmetrical by v i r t u e of two d i f f e r e n t l e n g t h bridges i n the same molecule? are at present u n c l e a r both of metals w i t h i n the c o f a c i a l metal complexes can be connot looked at the unsymmet-

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch006

P. Dolphin: For reasons that the r a t e s of i n s e r t i o n , or removal, systems are d i f f e r e n t so that mixed v i e n t l y prepared. We have, as y e t , r i c a l l y l i n k e d systems.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

7 Electron Transfer Mechanisms R. J . K L I N G L E R , S. F U K U Z U M I , and J. K . K O C H I

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch007

Indiana University, Department of Chemistry, Bloomington, I N 47405

The finely tunable steric and polar properties inherent to alkyl ligands can be exploited in both the homogeneous and the heterogeneous electron transfers from several classes of organometals. Outer­ -sphere mechanisms pertain to electron transfer with iron(III) oxidants such as Fe(phen)3, since the oxidations of various organometals are singularly unaffected by steric effects and follow the free energy correlation established by Marcus Theory. Likewise the rates of heterogeneous electron transfer at a platinum electrode are shown to be directly related to the homogeneous chemical oxidation with Fe(phen)3 provided they are evaluated at the same potential, i.e., driving force. The free energy relationship for the outer-sphere rates of electron transfer, which can be measured by the electrochemical method over an extended region far from the equilibrium potential, shows the asymptotic behavior at both the endergonic and exergonic limits describable by the empirical Rehm-Weller equation. The contrasting inner-sphere mechanism for electron transfer applies to the oxidation of the same series of organometal donors RM by either hexachloroiridate(IV) or tetracyanoethylene (TCNE), in which steric effects play an important kinetic role. The observation of transient, charge-transfer absorption bands from metastable [RM TCNE] complexes can be used to evaluate the work term for ion-pair formation with the aid of the Mulliken formulation. A single, unified free energy relationship applicable to inner-sphere processes relates the activation free energy to the standard free energy change for electron transfer and the work term. The large variations in the apparent Brønsted slopes (from unity to even negative values) can be attributed to changes in the 3+

3+

0097-6156/83/0211-0117$10.75/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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work term arising from steric effects in ion-pair formation.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch007

T r a d i t i o n a l l y , e l e c t r o n t r a n s f e r processes i n s o l u t i o n and at surfaces have been c l a s s i f i e d i n t o outer-sphere and innersphere mechanisms ( 1 ) . However, the experimental b a s i s f o r the q u a n t i t a t i v e d i s t i n c t i o n between these mechanisms i s not com­ p l e t e l y c l e a r , e s p e c i a l l y when e l e c t r o n t r a n s f e r i s not accom­ panied by e i t h e r atom o r l i g a n d t r a n s f e r ( i . e . , the bridged a c t i ­ vated complex). We wish t o describe how the advantage o f using organometals and a l k y l r a d i c a l s as e l e c t r o n donors accrues from the wide s t r u c t u r a l v a r i a t i o n s i n t h e i r donor a b i l i t i e s and s t e r i c p r o p e r t i e s which can be achieved as a r e s u l t o f branching the a l k y l moiety at e i t h e r the a- or β-carbon centers. S t r u c t u r a l V a r i a t i o n s i n E l e c t r o n Donors We consider the four s t r u c t u r a l l y d i v e r s e c l a s s e s o f organo­ metals I-IV, i n which the c o n f i g u r a t i o n and c o o r d i n a t i o n about the metal centers vary s y s t e m a t i c a l l y from o c t a h e d r a l , square planar, t e t r a h e d r a l t o l i n e a r , r e s p e c t i v e l y .

Me xPMe Ph >t' 2

\>Me Ph 2

II

Et

Et Sn

Et-Hg-Me

I III

IV

These organometals are e s p e c i a l l y d e s i r a b l e f o r k i n e t i c s t u d i e s since they are a l l s u f f i c i e n t l y s u b s t i t u t i o n s t a b l e i n s o l u t i o n to allow meaningful measurements t o be made. Moreover, f o r these n e u t r a l organometal donors, the work term o f the reactants w i s considered to be unimportant. An a d d i t i o n a l b e n e f i t derived from the use o f organometal donors l i e s i n the t r a n s i e n t character o f many o f the o x i d i z e d organometal c a t i o n s . As a r e s u l t , the back e l e c t r o n t r a n s f e r i s g e n e r a l l y minimal, and the e l e c t r o n t r a n s f e r process i s o v e r a l l i r r e v e r s i b l e i n these systems. As r e l a t i v e l y v o l a t i l e and e l e c t r o n - r i c h compounds, the p h o t o e l e c t r o n s p e c t r a of the alkylmetals III and IV are r e a d i l y a c c e s s i b l e , and the v e r t i c a l i o n i z a t i o n p o t e n t i a l s Irj can be a c c u r a t e l y measured. For example, the photoelectron spectra i n Figure 1 i l l u s t r a t e s how the lowest energy band o f a homologous s e r i e s o f d i a l k y l m e r c u r i a l s undergoes l a r g e , systematic v a r i a t i o n s merely by branching at the α-carbon o f the a l k y l l i g a n d (2). In these alkylmetals o f the main group elements, i o n i z a t i o n occurs from the highest occupied molecular o r b i t a l (HOMO) which has σ-bonding character, i . e . , they are σ-donors. Consequently r

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch007

7.

KLiNGLER E T A L .

Electron

8 9 10 l (RHgMe), eV D

Transfer

Mechanisms

119

Figure 1. He (I) photoelectron spectra of the lowest energy bands of Me Hg, EtHgMe, i-PrHgMe, and t-BuHgMe.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2

120

INORGANIC CHEMISTRY: TOWARD THE 21 ST

CENTURY

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch007

a l k y l ligands exert a large dominating i n f l u e n c e on the i o n i z a t i o n p o t e n t i a l s and the s t e r i c p r o p e r t i e s o f a l k y l m e t a l s . Both trends are i l l u s t r a t e d i n Figure 2 f o r the α-branched l i g a n d s : methyl, e t h y l , i s o p r o p y l , and t e r t - b u t y l on the l e f t , as w e l l as the βbranched ligands: e t h y l , η-propyl, i s o b u t y l , and neopentyl on the r i g h t (3). Note that the s t e r i c and p o l a r e f f e c t s g e n e r a l l y i n ­ crease together i n the α-branched a l k y l l i g a n d s , whereas v a r i a ­ t i o n s i n the s t e r i c e f f e c t dominate i n the β-branched a l k y l ligands. A l k y l r a d i c a l s share many o f the d e s i r a b l e p r o p e r t i e s o f or­ ganometals described above, i n s o f a r as 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 are concerned. Thus the s t e r i c p r o p e r t i e s o f a l k y l r a d i c a l s with a- and (3-branches f o l l o w the trends i n Figure 2. Moreover, the d i r e c t p a r a l l e l i n t h e i r donor p r o p e r t i e s i s shown i n Figure 3 by ALKYL RADICALS: d-Β ranched ft-Branched

•CH,

.CH, Me

Me Me I I - C H *C-Me Me

·ΟΗ ΟΗ 3

3

·ΟΗ ΟΗ 2

Me

Me

2

Me I *CH CH 2

Me

Me I -C^Ç-Me Me

a comparison o f the i o n i z a t i o n p o t e n t i a l s o f the s e r i e s o f abranched a l k y l r a d i c a l s (R«) with the I o f the corresponding alkylmethylmercurials (RHgMe) and d i a l k y l d i m e t h y l t i n compounds (R SnMe )(4). The same p a r a l l e l behavior i s a l s o observed with the β-branched a l k y l s e r i e s , although the v a r i a t i o n s i n the absolute values o f I a r e not as large (see Figure 2, r i g h t ) . D

2

2

D

Outer-Sphere E l e c t r o n T r a n s f e r The minimal interpénétration o f the c o o r d i n a t i o n spheres o f the reactants i s inherent i n any mechanistic formulation o f the outer-sphere process f o r e l e c t r o n t r a n s f e r . As such, s t e r i c e f f e c t s provide a b a s i c experimental c r i t e r i o n t o e s t a b l i s h t h i s mechanism. Therefore we wish to employ the s e r i e s o f s t r u c t u r a l l y r e l a t e d donors possessing the f i n e l y graded s t e r i c and p o l a r prop e r t i e s described i n the foregoing s e c t i o n f o r the study o f both homogeneous and heterogeneous processes f o r e l e c t r o n t r a n s f e r . Homogeneous Processes with T r i s - p h e n a n t h r o l i n e Metal(III) Oxidants. The r a t e s o f e l e c t r o n t r a n s f e r f o r the o x i d a t i o n o f these organometal and a l k y l r a d i c a l donors (hereafter designated g e n e r i c a l l y as RM and R*, r e s p e c t i v e l y , f o r convenience) by a s e r i e s o f t r i s - p h e n a n t h r o l i n e complexes ML o f i r o n ( I I I ) , ruthenium(III), and osmium(III) w i l l be considered i n i t i a l l y , s i n c e they have been p r e v i o u s l y e s t a b l i s h e d by Sutin and others as outer-sphere oxidants (5). 3+

3

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Electron

Transfer

Mechanisms

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch007

K L i N G L E R ET AL.

Figure 3.

Direct relationship between the ionization potentials of alkyl radicals (R*) with I of the alkylmetals: RHgMe (O) and R SnMe (Φ). D

2

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

122

INORGANIC CHEMISTRY: TOWARD THE 21 ST CENTURY

3 +

f

The i r o n ( I I I ) complexes F e L , where L = 2 , 2 - b i p y r i d i n e and various s u b s t i t u t e d 1,10-phenanthrolines, cleave a v a r i e t y o f o r ganometals i n a c e t o n i t r i l e according to the general r e a c t i o n mechanism i n Scheme I (6). The a c t i v a t i o n process f o r o x i d a t i v e c l e a -

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch007

3

RM + F e L

3 +

+

RM

3

+

RM

-^Ëi.

R

. +

+ FeL +

2 + 3

, etc.

M

(1) (2)

Scheme I vage i s represented by the e l e c t r o n t r a n s f e r step i n eq 1. The organometal c a t i o n RM i s a t r a n s i t o r y intermediate which subsequently undergoes a r a p i d fragmentation i n eq 2, rendering the electron transfer i r r e v e r s i b l e . The same i r o n ( I I I ) complexes a l s o o x i d i z e a l k y l r a d i c a l s , p a r t i c u l a r l y those with secondary and t e r t i a r y centers, t o the corresponding carbonium ions (7). +

R- + F e L

3 + 3

—*· R

+

+ FeL

2 + 3

(3)

Since these carbonium ions are r e a c t i v e , the subsequent followup steps with solvent are s u f f i c i e n t l y r a p i d to make e l e c t r o n t r a n s f e r i n eq 3 r a t e determining and e s s e n t i a l l y i r r e v e r s i b l e . For a p a r t i c u l a r i r o n ( I I I ) oxidant, the r a t e constant ( l o g kp ) f o r e l e c t r o n t r a n s f e r i s s t r o n g l y c o r r e l a t e d with the i o n i z a t i o n p o t e n t i a l Irj o f the v a r i o u s a l k y l m e t a l donors i n Figure 4 ( l e f t ) ( 6 ) . The same c o r r e l a t i o n extends to the o x i d a t i o n o f a l k y l r a d i c a l s , as shown i n Figure 4 ( r i g h t ) ( 7 ) . [The cause o f the bend (curvature) i n the c o r r e l a t i o n i s described i n a subsequent s e c t i o n . ] S i m i l a r l y , f o r a p a r t i c u l a r a l k y l m e t a l donor, the r a t e constant (log k p ) f o r e l e c t r o n t r a n s f e r i n eq 1 v a r i e s l i n e a r l y with the standard r e d u c t i o n p o t e n t i a l s E° o f the s e r i e s o f i r o n ( I I I ) complexes F e L , w i t h L = s u b s t i t u t e d phenanthroline ligands (6). In order to account f o r the foregoing k i n e t i c behavior, we r e l y on the Marcus theory f o r outer-sphere e l e c t r o n t r a n s f e r t o provide the q u a n t i t a t i v e b a s i s f o r e s t a b l i s h i n g the f r e e energy r e l a t i o n s h i p ( S n A " A ^1 Thus i n a d d i t i o n to the sheer bulk of the a l k y l group i n the t e t r a h e d r a l s t r u c t u r e , one must i n c l u d e the compressional change i n going to the t r i g o n a l bipyramidal s t r u c t u r e . We t r i e d to c a l ­ c u l a t e the l a t t e r with Jeremy Burdett (Chicago), but with no not­ able success, as yet. +

+

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

7.

Electron

KLINGLER ET A L .

Transfer

155

Mechanisms

T.J. Kemp, Warwick: Have you determined the slopes i n the p l o t s of l o g k versus E°? The point i s that s t r i c t adherence to modified Marcus theory* should produce a slope of - 16.9 V " . Your comprehensive study of thermal o x i d a t i o n of metal a l k y l s by t r i s - p h e n a n t h r o l i n e complexes i s complemented by a b r i e f e r one where e l e c t r o n - t r a n s f e r r a t e s from two s e r i e s of m e t a l l o c e n e s and metal c a r b o n y l s to e x c i t e d U 0 2 i o n were determined by l a s e r f l a s h p h o t o l y s i s . Data are combined i n the p l o t of l o g k T versus E ° , which i s l i n e a r , but with a slope of only - 2.07 ± 0.04 V " . E T

1

2

3

2+

E

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch007

1

Key: 1, [Fe( C H ) ]; 2, \Fe( C,H )( C H COPh)]; 3, [Fe( C H )(rrC H COMe)]; 4, [Ru( C H ) ]; 5, [Ru( C H,)( C H CHOHPh)] 6, [Fe( C H COMe) ] 7, [Ru( C H )( C HfiOPh];8, [Ru( C H COPh) ];9, [Os( C H ) ]; 10, NifCOh; 11, Fe(CO) ; 12, W(CO) ; 13, Mo(CO) ; 14, Cr(CO) ; and 15, Mn (CO) . r

5

5 2

T

T

2

;

T

5

5

5

2 2

5

T

T

T

b

5

h

T

T

5

5

h

T

5

&

6

2

3

5

5

;

k

T

2

T

6

5

5

r

5

r

5 2

6

10

1

1

C o r r e l a t i o n of l o g k (/dm mol"" s " ) versus E° f o r the quenching of the e x c i t e d U 0 2 i o n by organometallic molecules i n acetone s o l u t i o n . 2

2+

1.

R.A. Marcus, Ann. Rev. Phys. Chem., V5_ 155 (1964).

2.

D. Rehm and A. Weller, Isr. J. Chem., 8, 259 (1970).

3.

0. Traverso, R. R o s s i , L. Magon, A. C i n q u a n t i n i , and T.J. Kemp, J. Chem. Soc. Dalton Trans. 569 (1978).

4.

S. Sostero, 0. Traverso, P.D. Bernardo, and T . J . Kemp, J. Chem. Soc. Dalton Trans. 658 (1979).

9

9

3

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

156

INORGANIC

CHEMISTRY:

TOWARD T H E 21ST C E N T U R Y

J.K. Kochi: For outer-sphere e l e c t r o n t r a n s f e r , the slope of the p l o t o f the r a t e constant ( l o g k ^ ) versus Ε v a r i e s with the magnitude o f the d r i v i n g f o r c e f o r e l e c t r o n t r a n s f e r . For example, the slope i s expected to be -16.9 V" (which corresponds t o a Br^nsted c o e f f i c i e n t o f α =1) i n the endergonic region o f the driving force. For the o x i d a t i o n s o f organometals by F e L ex­ amined i n our s t u d i e s , the slope i s -8.5 V" o r α =0.5, as pre­ d i c t e d by Marcus theory f o r e l e c t r o n t r a n s f e r i n the r e g i o n about AG = 0. A slope o f only -2.07 V" corresponds t o α =0.12, which l i e s i n the exergonic region according to the Rehm-Weller formula­ t i o n , g r a p h i c a l l y underscored i n Figure 14 with α = β. [Note that the span i n AG over which the slope i s l i n e a r depends on the mag­ nitude o f the i n t r i n s i c b a r r i e r AGo*.] Are the r e l e v a n t standard p o t e n t i a l s and i n t r i n s i c b a r r i e r s f o r the metallocenes, carbonylmetals, and e x c i t e d uranyl i o n known? E

1

1

3 +

3

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch007

3

1

1

(1) (2) (3)

Marcus, R.A. J.Phys.Chem. 1968. 72, 91. Wong, C.L.; Kochi, J.K. J.Am.Chem.Soc. 191g, 101, 5593. K l i n g l e r , R.J.; Kochi, J.K. J.Am.Chem.Soc. 1982, 104, 4186.

T.J. M e y e r , U n i v e r s i t y o f N o r t h C a r o l i n a : One o f t h e most r e m a r k a b l e t h i n g s t o come o u t o f t h i s work i s t h e f a c t t h a t a s t i m e p a s s e s o r g a n i c c h e m i s t s h a v e been f o r c e d t o c o n s i d e r and t h e n have d i s c o v e r e d many c l a s s i c a l r e a c t i o n s w h i c h a p p e a r t o i n v o l v e e l e c t r o n t r a n s f e r . Do y o u h a v e any comments on t h e e x t e n t o f o n e - 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 some o f t h e s e c l a s s i c a l organic transformations?

J.K. K o c h i : I do b e l i e v e t h a t t h e a p p l i c a b i l i t y o f t h e c o n c e p t s i s p e r v a s i v e t o o r g a n i c c h e m i s t r y . However, t h e r e a r e many s k e p t i c s , and we w i l l h a v e t o p r o v i d e i r o n c l a d e x p e r i m e n t s f o r each system.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

8 Excited-State Electron Transfer THOMAS J. M E Y E R

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

The University of North Carolina, Department of Chemistry, Chapel Hill, NC 27514

Many of the features of electron transfer reactions involving excited states can be understood based on electron transfer theory. In other c o n t r i b u t i o n s i n t h i s symposium, a c l e a r case has been made f o r the p o s s i b l e value of molecular e x c i t e d s t a t e s as a b a s i s f o r s o l a r energy conversion processes. Amongst p o s s i b l e approaches i n using molecular e x c i t e d states i s the f o l l o w i n g s e quence of events: 1) O p t i c a l e x c i t a t i o n to give an e x c i t e d s t a t e . 2) E l e c t r o n t r a n s f e r quenching of the e x c i t e d s t a t e to give separated redox products. 3) U t i l i z a t i o n of the separated redox products as a b a s i s e i t h e r f o r a p h o t o v o l t a i c a p p l i c a t i o n where the stored chemical redox energy appears as a p h o t o p o t e n t i a l , or a photochemical a p p l i c a t i o n where the stored energy appears as high energy redox products. In order to i l l u s t r a t e the approach suggested above, i t i s of value to consider a s p e c i f i c case. V i s i b l e or near-UV e x c i t a t i o n of the complex Ru(bpy)3 + r e s u l t s i n e x c i t a t i o n and formation of the w e l l - c h a r a c t e r i z e d metal to l i g a n d charge t r a n s f e r (MLCT) exc i t e d s t a t e Ru(bpy>32+*. The consequences of o p t i c a l e x c i t a t i o n i n the Ru-bpy system i n terms of energetics are w e l l e s t a b l i s h e d , and are summarized i n eq. 1 i n a Latimer type diagram where the p o t e n t i a l s are versus the normal hydrogen e l e c t r o d e (NHE) and are 2

(1)

3

Ru(bpy) + 3

R

u

(b

p

y

)

2 3

+

-

1

·

2

6

Ru(bpy)

+ 3

+2. IV 2+* 3+ -0.84 Ru(bpy) " " Ru(bpy)3 w

3

>

u

+0.84

Ru(bpy)

+ 3

w r i t t e n as r e d u c t i o n p o t e n t i a l s (]L) . In the reduced form of the couple, R u ( b p y ) , the added e l e c t r o n i s i n a ir*(bpy) l e v e l and i n the o x i d i z e d form of the chromophore, Ru(bpy)3^ , the e l e c t r o n has been removed from a dïï l e v e l . +

3

+

0097-6156/83/0211-0157$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

158

INORGANIC C H E M I S T R Y :

TOWARD T H E 21 ST

CENTURY

The redox p o t e n t i a l diagram i n eq. 1 i l l u s t r a t e s that the e f f e c t o f o p t i c a l e x c i t a t i o n i s to create an e x c i t e d s t a t e which has enhanced p r o p e r t i e s both as an oxidant and reductant, compared to the ground s t a t e . The r e s u l t s o f a number of experiments have i l l u s t r a t e d that i t i s p o s s i b l e f o r the e x c i t e d s t a t e to undergo e i t h e r o x i d a t i v e or r e d u c t i v e e l e c t r o n t r a n s f e r quenching (2). An example of o x i d a t i v e e l e c t r o n t r a n s f e r quenching i s shown i n eq. 2 where the oxidant i s the a l k y l pyridinium i o n , paraquat ( 3 ) . (2)

R

u

Ru(bpy)

(

b

2 + 3

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

(PQ

p

)

y

3

2 + ^ 2 +

* + PQ 2 +

R

u

(

b

p

y

)

3

2 + *

— Ru(bpy)

3 +

+ PQ

3

= CH -N^)-

+ + DMA

3

= Me NPh) 2

In the case of e i t h e r eq. 2 or eq. 3, even though e x c i t e d s t a t e energy has been converted i n t o stored chemical redox energy, the storage i s temporary because of back e l e c t r o n t r a n s f e r between the separated redox products, e.g., eq. 4. (4)

Ru(bpy)

3 + 3

+ PQ

+

— * Ru(bpy)

2 + 3

+ PQ

2 +

A major dilemma i n any approach to energy conversion processes based on 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 o f molecular e x c i t e d s t a t e s i s u t i l i z a t i o n o f the stored redox products before back e l e c t r o n t r a n s f e r can occur. The net quenching r e a c t i o n i n eq. 2, which leads to separated redox products capable of o x i d i z i n g and reducing water, r e l i e s on a s e r i e s of e l e c t r o n t r a n s f e r steps. The b a s i c theme of t h i s a c count i s e x c i t e d s t a t e and r e l a t e d e l e c t r o n t r a n s f e r events which occur i n such systems and the b a s i s that we have f o r understanding them both experimentally and t h e o r e t i c a l l y . In order to begin i t i s u s e f u l to consider Scheme 1, i n which the quenching r e a c t i o n i n eq. 2 i s considered i n k i n e t i c d e t a i l . In the scheme, the assumption i s made that the only important quenching event i s e l e c t r o n t r a n s f e r and that energy t r a n s f e r quenching i s n e g l i g i b l e . The s e r i e s o f e l e c t r o n t r a n s f e r events i n the scheme a r e i n i t i a t e d by o p t i c a l e x c i t a t i o n to give the exc i t e d s t a t e and the 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 which occur f o l -

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

8.

Excited-State

MEYER

Electron

159

Transfer

lowing e x c i t a t i o n are: 1) Excited s t a t e decay to the ground s t a t e occuring by a combination of r a d i a t i v e and nonradiative processes (1/τ i n Scheme 1). Nonradiative decay i s by a l i g a n d to metal 0

Scheme 1 RuB

2 + 3

*

+

P Q

2

+

^ RuB

HK RuB

2 3

^PQ %^ R u B ^ P Q ^ â 2

" .. D k

2 +

+ PQ

3

2 +

< (RuB

2 + 3

RuB = Ru(bpy)

2 + 3

2 + 3

,PQ

PQ

+

2 +

) 11

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

+

ν

1 =5

RuB^

2+

11

e l e c t r o n t r a n s f e r r e a c t i o n , ( b p y ) R u ( b p y ) 2 * ^ π 5 π * ) —> ( b p y ) R u (bpy)2 (dïï^). 2) E l e c t r o n t r a n s f e r quenching (k^) . 3) Back e l e c t r o n t r a n s f e r to repopulate the excited s t a t e ( k _ ^ ) . 4) Back e l e c t r o n t r a n s f e r to give the ground s t a t e rather than the excited s t a t e (k2). In any photoredox a p p l i c a t i o n based on e l e c t r o n t r a n s f e r quenching, l i k e the one i n Scheme 1, the c r i t i c a l f a c t o r s determining device performance are the per photon e f f i c i e n c y with which the separated redox products appear, and the u t i l i z a t i o n of the separated redox products before back e l e c t r o n t r a n s f e r between them can occur. The separation e f f i c i e n c y i s determined i n l a r g e part by the s e r i e s of e l e c t r o n t r a n s f e r steps described above: 1) Quenching must occur before e x c i t e d s t a t e decay. 2) Following quenching, the e f f i c i e n c y of separation of the redox products shown by k _n i n Scheme 1 must be r a p i d compared to back e l e c t r o n t r a n s f e r to give the ground s t a t e , k2« Clearly i t i s essential to understand the f a c t o r s which c o n t r o l the r a t e constants f o r such e l e c t r o n t r a n s f e r events. 2+

f

E l e c t r o n Transfer

Theory

The f a c t o r s that determine the r a t e of e l e c t r o n t r a n s f e r between chemical s i t e s are the extent of e l e c t r o n i c coupling between e l e c t r o n donor and acceptor s i t e s and the extent of v i b r a t i o n a l trapping of the exchanging e l e c t r o n by both intramolecular and medium v i b r a t i o n s . V i b r a t i o n a l trapping i s a n a t u r a l consequence of the e f f e c t s of changes i n e l e c t r o n content on molecular s t r u c t u r e . The point i s i l l u s t r a t e d i n Figure 1 where a p l o t i s shown of pot e n t i a l energy versus a normal coordinate f o r the e l e c t r o n t r a n s f e r system, D,A -> D ,A". The p l o t i s f o r a trapping v i b r a t i o n assuming the harmonic o s c i l l a t o r approximation. In order f o r a v i b r a t i o n to be a trapping v i b r a t i o n , the displacement between the bottoms of the p o t e n t i a l w e l l s i n Figure 1, A Q , must be nonzero. In the c l a s s i c a l l i m i t , e l e c t r o n t r a n s f e r can only occur at the i n t e r s e c t i o n between the two p o t e n t i a l curves because i t i s only at that point that energy i s conserved before and a f t e r the e l e c t r o n t r a n s +

eq

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INORGANIC C H E M I S T R Y : TOWARD T H E 21 ST C E N T U R Y

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

160

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

8.

Excited-State

MEYER

Electron

161

Transfer

f e r event. The c l a s s i c a l energy of a c t i v a t i o n w i l l be determined by the thermal energy needed to reach the i n t e r s e c t i o n region, f o r a l l of the normal v i b r a t i o n s which respond to the change i n e l e c ^ t r o n d i s t r i b u t i o n before and a f t e r e l e c t r o n t r a n s f e r occurs. As noted above, there are two c o n t r i b u t i o n s to v i b r a t i o n a l trapping, one from the intramolecular v i b r a t i o n s of the molecule, which are normally i n the frequency range 200-4000 cnT^ and the c o l l e c t i v e v i b r a t i o n s of the surrounding medium, which f o r a solvent are of low frequency and i n the range 1-10 cm"^. In f a c t , e l e c t r o n t r a n s f e r occurs at the microscopic l e v e l where quantum mechanics provides the necessary d e s c r i p t i o n of the phenomenon (5-13). In the quantum mechanical s o l u t i o n , associated w i t h the p o t e n t i a l curves i n Figure 1 are quantized energy l e v e l s ,

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

Ej

=

(VJ

+

1 / 2 ) ^ 0 ) . , where v^

and

ajj

=

2TTVJ

are

the

vibrational

quantum number ancl angular frequency f o r v i b r a t i o n j . The cor­ responding v i b r a t i o n a l wavefunctions are X-:. In the quantum mech­ a n i c a l p i c t u r e the advantage of thermal a c t i v a t i o n to the i n t e r ­ s e c t i o n region i s enhanced v i b r a t i o n a l overlap between the v i b r a ­ t i o n a l wavefunctions f o r the i n i t i a l (D,A) and f i n a l (D ,A~) s t a t e s . The importance of v i b r a t i o n a l overlap i s that where i t occurs the v i b r a t i o n a l c o n f i g u r a t i o n s of both s t a t e s are allowed and e l e c t r o n t r a n s f e r can occur. The presence of the e l e c t r o n acceptor s i t e adjacent to the donor s i t e creates an e l e c t r o n i c p e r t u r b a t i o n . A p p l i c a t i o n of time dependent p e r t u r b a t i o n theory to the system i n Figure 1 gives a general r e s u l t f o r the t r a n s i t i o n r a t e between the s t a t e s D,A and D ,A~. The r a t e constant i s the product of three terms: 1) 2πν /η where V i s the e l e c t r o n i c resonance energy a r i s i n g from the perturbation. 2) The v i b r a t i o n a l overlap term. 3) The density of s t a t e s i n the product v i b r a t i o n a l energy manifold. In the c l a s s i c a l l i m i t where the c o n d i t i o n -ftu) > k 2 , k _ , back e l e c t r o n t r a n s f e r to give the e x c i t e d s t a t e i s r a p i d , and eq. 10 a p p l i e s . D

(10)

RTlnk

1

q

= RTln(k

f

_ + k )K -D I A 0

A

AGI

In the equations, k (0) i s the h y p o t h e t i c a l quenching r a t e con­ stant when ΔΕ (=AG) = 0 and AG^ i s the f r e e energy change on quenching. Eqs. 9 and 10 make c l e a r p r e d i c t i o n s about the dependence of quenching r a t e constants on the f r e e energy change i n the quench­ ing step. One way of t e s t i n g the theory i s to observe the quench­ ing of the e x c i t e d s t a t e by a s e r i e s of r e l a t e d quenchers where the parameters k^(0), K^, and k _Q should remain s e n s i b l y constant and yet where the p o t e n t i a l s of the quenchers as oxidants or r e ductants can be v a r i e d s y s t e m a t i c a l l y . Such experiments have been c a r r i e d out, most notably with the MLCT e x c i t e d s t a t e , Ru(bpy)32+* (1). The experiments have u t i l i z e d both a s e r i e s of o x i d a t i v e nitroaromatic and a l k y l pyridinium quenchers, and a s e r i e s of r e ­ ductive quenchers based on a n i l i n e d e r i v a t i v e s . From the data and known redox p o t e n t i a l s f o r the quenchers, p l o t s of RTlnk q vs. E ° f o r the quencher couple show regions of slope 1/2 and slope 1 as p r e d i c t e d by eq. 9 and 10. In f a c t , the t h e o r e t i c a l equations appear to account f o r the observed v a r i a t i o n of l n k ' ^ with E ° s a t i s f a c t o r i l y . Given the agreement with theory, i t follows that i f an e x c i t e d s t a t e i s thermally e q u i l i b r a t e d , i t can be viewed as a t y p i c a l chemical reagent with i t s own c h a r a c t e r i s t i c p r o p e r t i e s , and that those p r o p e r t i e s can be accounted f o r by using equations and t h e o r e t i c a l developments normally used f o r ground s t a t e reac­ tions. q

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

f

f

1

f

T r a n s i t i o n Between the Normal and Inverted Regions. In the pre­ vious s e c t i o n , e l e c t r o n t r a n s f e r events were considered i n the framework of a v a i l a b l e theory f o r the normal region where -ΔΕ < X. According to eq. 6, as -ΔΕ approaches X i n magnitude, E ·*• 0, and there i s no longer a v i b r a t i o n a l trapping b a r r i e r f o r the exchang­ ing électron. With a f u r t h e r increase i n -ΔΕ, -ΔΕ > X and as shown i n Figure 2, one p o t e n t i a l curve i s "embedded" w i t h i n the other. In t h i s , the i n v e r t e d region, the s i t u a t i o n i s q u i t e d i f ­ ferent from that f o r an e l e c t r o n t r a n s f e r r e a c t i o n i n the normal region. In the normal region, the i n t e r s e c t i o n region occurs out­ side the p o t e n t i a l curves. In the quantum mechanical view, ther­ mal a c t i v a t i o n i s d e s i r e d i n order to reach regions near the i n ­ t e r s e c t i o n region where v i b r a t i o n a l overlap i s maximized. In the i n v e r t e d region, the two p o t e n t i a l curves are w i t h i n each other and v i b r a t i o n a l overlap plays a more important r o l e . In a d d i t i o n , because of the embedded nature of the p o t e n t i a l curves, emission can occur i n the i n v e r t e d region. The r e l a t i o n s h i p between e l e c t r o n t r a n s f e r i n the normal and i n v e r t e d regions i s i l l u s t r a t e d i n Figure 3 f o r the case of quenching of R u i b p y ) ^ * * by a nitroaromatic quencher. E x c i t a t i o n a

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

164

INORGANIC C H E M I S T R Y : TOWARD T H E 21 ST C E N T U R Y

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

Ε

Figure 2. Energy-coordinate diagram for the "inverted region" where — Δ Ε > χ.

1

Q

Figure 3. Energy-coordinate diagram for quenching of Ru(bpy) ** (RuB **) by a nitroaromatic (A rN0 ). 2

3

2

3

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

8.

Excited-State

MEYER

Electron

165

Transfer

of an a s s o c i a t i o n complex between the ruthenium complex and the quencher, Ru(bpy)3 ,ArN0 , would lead to R u ( b p y ) 3 * i n close contact with the quencher. E x c i t e d s t a t e decay of t h i s s t a t e by nonradiative decay ( k ^ i n Figure 3) can occur as an e l e c t r o n t r a n s f e r process i n the i n v e r t e d region. E l e c t r o n t r a n s f e r quen­ ching and e x c i t e d s t a t e repopulation (k^ and are r e a c t i o n s i n the normal region. The quenching step can be viewed as an i n t e r conversion between intramolecular and intermolecular charge trans­ fer excited s t a t e s — ( b p y ) R i ( b p y ) * , A r N 0 — • ( b p y ) R u ( b p y ) , A r N 0 ~ . The f i n a l process to consider i n Figure 3 i s back e l e c t r o n t r a n s f e r to give the e x c i t e d s t a t e ( k ) — R u ( b p y > 3 ^ , ArN0 *~ — * Ru(bpy>3 ,ArN02. This i s a l s o a r e a c t i o n i n the i n v e r ­ ted r e g i o n and can be viewed as nonradiative decay of an "outersphere , charge t r a n s f e r e x c i t e d s t a t e . 2+

2+

2

I I

2 +

2

i n

2

2

2+

2

+

2

2+

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

11

E l e c t r o n Transfer i n the Inverted Region. Decay of an e x c i t e d s t a t e and e l e c t r o n t r a n s f e r i n the i n v e r t e d region are conceptual­ l y the same process (14). As suggested by the p o t e n t i a l curves i n Figure 2, there a r e three l i m i t i n g pathways a v a i l a b l e f o r e x c i t e d s t a t e decay, but a l l three are subject to the l i m i t a t i o n s imposed by energy conservation. Decay can occur by emission, i n which case energy conservation i s met by the l o s s of an emitted photon. Decay could occur n o n r a d i a t i v e l y by thermal a c t i v a t i o n to the i n ­ t e r s e c t i o n r e g i o n between p o t e n t i a l curves, which i s the pathway i n the c l a s s i c a l l i m i t . However, decay can a l s o occur from lowl y i n g v i b r a t i o n a l l e v e l s of the e x c i t e d s t a t e to high energy l e v e l s of the ground s t a t e (Figure 2). Because of energy conser­ v a t i o n , the e l e c t r o n i c energy l o s t i n the t r a n s i t i o n between s t a t e s , e.g., ( b p y ) R u ( b p y ) 2 * —> ( b p y ) R u ( b p y ) , must appear as v i b r a t i o n a l energy i n the product. A c r i t i c a l feature i n determining the r a t e of the t r a n s i t i o n i s the extent of v i b r a ­ t i o n a l overlap between the v i b r a t i o n a l wavefunctions f o r the s t a t e s before and a f t e r e l e c t r o n t r a n s f e r . V i b r a t i o n a l overlap i s obviously of importance i n the i n v e r t e d region because of the n e s t i n g of the p o t e n t i a l curves as shown i n Figure 2, and i t s mag­ nitude i s i n f l u e n c e d by: 1) The v i b r a t i o n a l quantum spacing f o r the acceptor v i b r a t i o n or v i b r a t i o n s , "huty. As the v i b r a t i o n a l quantum number ν increases, the v i b r a t i o n a l amplitude increases near the p o t e n t i a l curve. For a high frequency v i b r a t i o n , the energy conservation c o n d i t i o n i s met f o r v i b r a t i o n s with lower ν values. 2) The extent of d i s t o r t i o n of the acceptor v i b r a t i o n i n the e x c i t e d s t a t e compared to the ground s t a t e , A Q . As A Q increases, v i b r a t i o n a l overlap i n c r e a s e s . 3) The energy gap be­ tween the ground and e x c i t e d s t a t e , ΔΕ. As ΔΕ decreases, energy conservation i s met f o r v i b r a t i o n a l l e v e l s of lower ν number where the amplitudes a r e greater toward the center of the p o t e n t i a l curve f o r the ground s t a t e . In the l i m i t that η ω » kfiT, the r a t e constant f o r nonradia­ t i v e decay i s simply the product of the square of the v i b r a t i o n a l overlap i n t e g r a l < X i i t i a l I X f i n a l * d an e l e c t r o n i c term f o r the Il:L

2+

I I

2 +

2

eq

Μ

a

n

n

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

eq

166

INORGANIC C H E M I S T R Y : TOWARD THE

21 ST

CENTURY

t r a n s i t i o n between s t a t e s . In the l i m i t that "huty >> kgT and assuming a r e l a t i v e l y small e x c i t e d s t a t e d i s t o r t i o n , the r a t e constant f o r nonradiative e x c i t e d s t a t e decay i s given by eq. 11 (11).

k

nr

C

=

1

\ < " Η ί φ Ε 1 - )

/

2

- Ρ - ν

χ

Ρ - ^

ΔΕ;

the i n t e r n a l energy change accompanying e x c i t e d s t a t e decay. ω = 2πν^; the angular frequency of the acceptor v i b r a t i o n or v i b r a t i o n s . S = -^Δ ; a measure of the d i s t o r t i o n of the acceptor v i b r a t i o n i n the e x c i t e d s t a t e . Δ i s the dimensionl e s s , f r a c t i o n a l displacement i n normal v i b r a t i o n M between the thermally e q u i l i b r a t e d e x c i t e d and ground states. I t i s r e l a t e d to A Q by = AQ^C^) / , where M i s the reduced mass f o r the v i b r a t i o n . Ύ = ln(|AE|/S nu) )-l C œ^; note below. Μ

2

M

Μ

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

Μ

1

2

eq

M

M

2

To f i r s t order, the t r a n s i t i o n between e x c i t e d and ground s t a t e i s forbidden because they are both s o l u t i o n s of the same molecular Hamiltonian and t h e i r wavefunctions are orthogonal. However, the t r a n s i t i o n can be induced by e x c i t i n g a promoting v i b r a t i o n or v i b r a t i o n s (ω^) which when a c t i v a t e d r e s u l t i n a change i n e l e c ­ t r o n i c overlap between the e l e c t r o n donor and acceptor s i t e s . The magnitude of the v i b r a t i o n a l l y induced mixing of the states i s given by C.; the angular frequency of the promoting v i b r a t i o n i s ω^. When c o n t r i b u t i o n s from the low frequency, c o l l e c t i v e v i b r a ­ t i o n s of the solvent are included, eq. 11 becomes eq. 12. In eq.

/TON

(12)

ι k

2

η / π l/2 _ = C ω. ( — — - — ) exp-S exp nr k 2hu)..E ' Μ Μ em N

N

γ E Y k Τ ο em_ ο Β , 2, exp + -—fcr—(γ +1) J ηω^. ηω-,τιω., ο M M M Γ

η

v

12 the assumption i s made that the experimentally observed emis­ s i o n energy at X , E , i s r e l a t e d to ΔΕ and X as i n eq. 13. m a x

(13)

e m

Q

Ε

~ ΙΔΕΙ - X em ο The assumption i s reasonable i n the l i m i t that the d i s t o r t i o n i n uty i s not too great i n the e x c i t e d s t a t e . When w r i t t e n i n l o g a r i t h m i c form and noting that the pre-exp o n e n t i a l and Y terms i n eq. 12 are only weakly dependent on E , eq. 12 can be w r i t t e n i n the form of the "energy gap law" (12a, 12b) as shown i n eq. 14. γ E bx (14) Ink = Ink = Ιηβ - ° + zr nr et ο ηω^ ηα^ 1

1

Q

e m

e m

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

8.

Excited-State

MEYER

Electron

C2m(—-!—)l/2 k 2ηω Ε M em

β = o k

b

= 4

167

Transfer

7

>χ

R

T

(

Y

2 +

°

1

)

M One way to t e s t the "energy gap law" and therefore the a p p l i ­ c a t i o n of e l e c t r o n t r a n s f e r to the i n v e r t e d region i s by measuring r a d i a t i o n l e s s decay r a t e constants f o r a s e r i e s of c l o s e l y r e l a t e d excited states. An e s p e c i a l l y u s e f u l s e r i e s has turned out to be the complexes ( b p y ) 0 s L 4 or (phen)OsL4 + (L = py, RCN, 1/2 bpy, 1/2 phen, P R 3 . . . ) , a l l of which contain an MLCT (Os(dTr)-bpy(π*)) based chromophore. The range of ligands that can serve the r o l e of L i s l a r g e , and systematic changes i n L can cause r e l a t i v e l y l a r g e v a r i a t i o n s i n the emission energy. The primary o r i g i n of the v a r i a t i o n s i n E appears to be i n the backbonding a b i l i t i e s of L i n s t a b i l i z i n g the ground s t a t e by metal to l i g a n d backbond­ ing. In any case, f o r a s e r i e s of r e l a t e d chromophores i n a con­ stant solvent, the terms β , f i u ^ , and X i n eq. 14 are expected to remain s e n s i b l y constant and p l o t s of l n k ^ v s . E are expected to be l i n e a r . In a d d i t i o n , i n low temperature (77°K) emission measurements v i b r a t i o n a l s t r u c t u r e can be observed. From the energy spacings between the v i b r a t i o n a l components, *ηω^ ~ 13001400 cm~l f o r the acceptor v i b r a t i o n ( s ) . I t i s a l s o p o s s i b l e to c a l c u l a t e S ( = 1 A ) from the r e l a t i v e i n t e n s i t i e s of the v i b r a ­ t i o n a l components (15).

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

2+

2

e m

0

Q

e m

2

M

m

2

Experimentally, E i s a v a i l a b l e from the emission spectrum and k from a combination of e x c i t e d s t a t e l i f e t i m e (τ ) and emission quantum y i e l d ( φ ) measurements as shown i n eq. 15. An e m

n r

0

Γ

(15)

— τ

= k ο

nr

+ k

r

k r + k r nr extended s e r i e s of osmium complexes i s a v a i l a b l e , and the r e s u l t s of experiments where both emission energies and k allow p l o t s of lnknj- to be made (16) . As the data show, the l i n e a r r e l a t i o n ­ ship p r e d i c t e d by eq. 14 i s observed. I t i s even more s t r i k i n g that the slopes of the l i n e s of the p l o t s are a l s o i n agreement with eq. 14 although a d e t a i l e d a n a l y s i s of the data i s required. The s i m i l a r i t y i n slopes and v i b r a t i o n a l spacings i n low tempera­ ture emission spectra f o r the two s e r i e s show that the acceptor v i b r a t i o n s are r i n g - s t r e t c h i n g i n nature. The d i f f e r e n t i n i n t e r ­ cepts f o r the two s e r i e s i s not s u r p r i s i n g . The i n t e r c e p t i n ­ cludes the term C which i s dependent on the e l e c t r o n i c s t r u c t u r e of the chromophoric l i g a n d . Given the change i n e l e c t r o n i c _ d i s t r i b u t i o n between MLCT e x c i t e d and ground s t a t e s , e.g., ( b p y ) 0 s L 4 — * ( b p y ) 0 s L ^ , a second approach to t e s t i n g the energy gap law should be through k

n r

2

I I I

II

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

168

INORGANIC C H E M I S T R Y : TOWARD T H E

21ST

CENTURY

solvent v a r i a t i o n s . In the c l a s s i c a l d i e l e c t r i c continuum l i m i t , ΔΕ i s p r e d i c t e d to vary with the s t a t i c d i e l e c t r i c constant of the medium (Dg), as i n equation 16, and X with Dg and the o p t i c a l d i Q

e l e c t r i c constant D as i n eq. 17 (17). In eq. 16 and 17 μ and \if are the d i p o l e moments of the metal-ligand e l e c t r o n i c d i s t r i b u ­ t i o n s before and a f t e r e l e c t r o n t r a n s f e r has occurred. In the Qp

±

equations a i s the radius of a sphere enclosing the metal-ligand dipole. According to eq. 14, i f v a r i a t i o n s i n X are r e l a t i v e l y small through a s e r i e s of s o l v e n t s , p l o t s of l n k vs. E should be l i n e a r f o r a s i n g l e e x c i t e d s t a t e . I t has been shown that f o r a s e r i e s of polar organic s o l v e n t s , the p r e d i c t i o n i s borne out f o r s e v e r a l of the (phen)0sL^2+_type MLCT e x c i t e d s t a t e s , i n c l u d i n g O s ( p h e n ) 3 * (18). I t i s e s p e c i a l l y s t r i k i n g that the slopes of the p l o t s are the same w i t h i n experimental e r r o r as i n the e x p e r i ­ ments described above where L was v a r i e d . However, f o r h y d r o x y l i c solvents such as methanol or water, s p e c i f i c solvent e f f e c t s e x i s t , the d i e l e c t r i c continuum r e s u l t i n eq. 17 i s no longer a p p l i c a b l e , and v a r i a t i o n s i n X are appre­ c i a b l e . Even so, eq. 14 s t i l l a p p l i e s i n that f o r a s e r i e s of e x c i t e d s t a t e s l i k e (bpyjOsL^+a, p l o t s of l n k vs. E remain l i n e a r and have the same slope as the p l o t s f o r polar organic s o l ­ vents. The d i f f e r e n c e i s that the l i n e s are p a r a l l e l but o f f s e t , because the term appears i n the i n t e r c e p t and x i s non-negli­ g i b l e f o r the h y d r o x y l i c s o l v e n t s . Studies l i k e those mentioned here on the osmium complexes are more d i f f i c u l t f o r r e l a t e d complexes of ruthenium because of the i n t e r v e n t i o n of a l o w l y i n g , thermally populable d-d e x c i t e d s t a t e . However, i t i s p o s s i b l e to separate the two c o n t r i b u t i o n s to e x c i ­ ted s t a t e decay by temperature dependent measurements. In the case of R u ( b p y ) 3 * , temperature dependent l i f e t i m e s t u d i e s have been c a r r i e d out i n a s e r i e s of s o l v e n t , and the r e s u l t s obtained f o r the v a r i a t i o n of k with E are i n agreement with those ob­ tained f o r the Os complexes (19). A c l o s e l y r e l a t e d t e s t of the energy gap law f o r Ru complexes has come from temperature dependent l i f e t i m e and emission measure­ ments f o r a s e r i e s of complexes of the type R u ( b p y ) 2 L 2 (L = py, substituted pyridines, pyrazine...). From the data, the v a r i a t i o n in l n k with E p r e d i c t e d by the energy gap law has been ob­ served and i t has been p o s s i b l e to observe the e f f e c t of changing the l i g a n d s L on the t r a n s i t i o n between the MLCT and dd states (20). The d i s c u s s i o n i n t h i s s e c t i o n has been o r i e n t e d toward the use of intramolecular r e a c t i o n s and e x c i t e d s t a t e decay to t e s t

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

Q

n r

e m

2+

Q

n r

e m

Q

2+

n r

e m

2+

n r

e m

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

8.

MEYER

Excited-State

Electron

169

Transfer

e l e c t r o n t r a n s f e r theory i n the i n v e r t e d region. However, the same ideas should apply to intermolecular e l e c t r o n t r a n s f e r where e l e c t r o n hopping occurs between d i f f e r e n t chemical s i t e s , as long as the c o n d i t i o n s which d e f i n e the i n v e r t e d region are appropriate. One s t r i k i n g p r e d i c t i o n of the energy gap law and eq. 11 and 14 i s that i n the i n v e r t e d region, the e l e c t r o n t r a n s f e r r a t e constant ( k = k ) should decrease as the r e a c t i o n becomes more favorable ( l n k - Δ Ε ) . Some evidence has been obtained f o r a f a l l - o f f i n r a t e constants with i n c r e a s i n g -ΔΕ (or -AG) f o r i n t e r ­ molecular r e a c t i o n s (21). Perhaps most notable i s the pulse r a d i o l y s i s data of B e i t z and M i l l e r (22). Nonetheless, the a p p l i c a b i l i ­ ty of the energy gap law to intermolecular e l e c t r o n t r a n s f e r i n a d e t a i l e d way has yet to be proven. A p p l i c a t i o n of the energy gap law to the energy conversion mechanism i n Scheme 1 leads to a notable conclusion with regard to the e f f i c i e n c y f o r the appearance of separated redox products f o l ­ lowing e l e c t r o n t r a n s f e r quenching. From the scheme, the separa­ t i o n e f f i c i e n c y , Φ ρ » i s given by eq. 18. D i f f u s i o n apart of the n r

e t

œ

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

n r

3β

k,

(18)

-D 's e p - k2+k'_ D

Y

D

f

quenching products once the quenching step has occurred ( k _ ) i s d i c t a t e d by such features as the charge types on the r e a c t a n t s , and the solvent p o l a r i t y and v i s c o s i t y . However, back e l e c t r o n t r a n s f e r to give the ground s t a t e ^2) i s an e l e c t r o n t r a n s f e r r e ­ a c t i o n . According to the energy gap law, as the quenching step produces redox products where the energy stored becomes more and more favorable, -ΔΕ i n c r e a s i n g , k should decrease. One therefore reaches the s t r i k i n g conclusion that i n a properly designed system both the e f f i c i e n c y of separation of the redox products and the amount of energy stored should increase as the reduction p o t e n t i a l of the quencher approaches that of the e x c i t e d s t a t e (2b). D

2

Directed Electron Transfer. This account was begun by c o n s i d e r i n g the p o s s i b i l i t y of c r e a t i n g energy conversion schemes based on simple e l e c t r o n t r a n s f e r processes i n v o l v i n g e x c i t e d s t a t e s . One obvious extension i s to more complex systems where some of the de­ mands of an o v e r a l l conversion mechanism could be spread amongst d i f f e r e n t chemical s i t e s . In order to b u i l d more complex, coupled systems where l i g h t absorption and chemical redox events are sepa­ rated, i t i s necessary to understand and c o n t r o l the molecular c h a r a c t e r i s t i c s which can lead to d i r e c t e d , intramolecular e l e c ­ t r o n or energy t r a n s f e r . What i s meant by d i r e c t e d e l e c t r o n t r a n s f e r i s that f o l l o w i n g o p t i c a l e x c i t a t i o n of a chromophore, the e x c i t e d e l e c t r o n or e l e c t r o n hole, or perhaps both, d r i f t away from the i n i t i a l chromophoric s i t e by intramolecular e l e c t r o n t r a n s f e r . I f the r a t e of recombination of the e x c i t e d e l e c t r o n e l e c t r o n hole p a i r i s s u f f i c i e n t l y slow, i t i s p o s s i b l e to e x t r a c t them from d i f f e r e n t parts of the molecular at a l a t e r time, before recombination can occur (23). As an example of such a system,

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

170

INORGANIC C H E M I S T R Y :

TOWARD

THE 21ST

CENTURY

+

consider the complex [ R u ( b p y ) ( N C ^ N - M e ) ] ^ + i n which there are both Ru(bpy) chromophoric s i t e s a n d a t t a c h e d , intramolecular e l e c t r o n acceptor pyridinium s i t e s . There i s a c l e a r s i m i l a r i t y between t h i s intramolecular redox complex and the r e a c t i o n i n Scheme 1 based on paraquat. In the complex, the b i m o l e c u l a r , intermolec u l a r quenching r e a c t i o n of Scheme 1 has been converted i n t o an intramolecular quenching r e a c t i o n . Experimentally, o p t i c a l exc i t a t i o n of the Ru-bpy chromophore i n a frozen s o l u t i o n at 77°K leads to e x c i t e d s t a t e decay by an emission which i n terms of emission energy, v i b r a t i o n a l s t r u c t u r e and l i f e t i m e c l o s e l y resemb l e s the emission from R u ( b p y ) 3 . However, at room temperature a s t r o n g l y r e d - s h i f t e d , weaker, s h o r t - l i v e d emission i s observed from the molecule (24). The most s t r a i g h t f o r w a r d i n t e r p r e t a t i o n of the sequence of events that occurs f o l l o w i n g e x c i t a t i o n at room temperature i s summarized i n Scheme 2. 2

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch008

2+

Scheme 2 CT

n

CT

+

(bpy) (bpy) ( L ) R u ^ l ® ^ - M e ]

2

(bpy)

JL

+hv

-hv

(L)Ru

+

*

4+

(NOHgk'-Me] *

1

(bpy) (L) R u

GS

1 1 1

4

2

1 1

+

(Ng> -Me) ]

4

+

At room temperature thermally a c t i v a t e d e l e c t r o n t r a n s f e r occurs from the bpy l i g a n d to the remote pyridinium s i t e followed by decay of the lower, pyridinium-based CT s t a t e . The e l e c t r o n t r a n s f e r step i s the intramolecular analog of the paraquat quenching of R u ( b p y ) * . A feature of i n t e r e s t i n the sequence of events o u t l i n e d i n Scheme 2 i s that f o l l o w i n g o p t i c a l e x c i t a t i o n , d i r e c t e d e l e c t r o n t r a n s f e r does occur away from the chromophoric s i t e to a remote ligand. In that sense, the e x c i t e d s t a t e acts as a molecular photodiode (23). With the proper molecular design i t may be poss i b l e to b u i l d systems where the e x c i t e d e l e c t r o n and/or e l e c t r o n hole cascade through a s e r i e s of redox l e v e l s l e a d i n g to the spat i a l separation of the e l e c t r o n - e l e c t r o n hole p a i r and slow back electron transfer. I t i s important to r e a l i z e that the observat i o n of d i r e c t e d e l e c t r o n t r a n s f e r i s subject to the " r u l e s of e l e c t r o n t r a n s f e r as described above. For the case of the p y r i dinium complex, n o n r a d i a t i v e decay from the i n i t i a l CT s t a t e . CT~, [(bpy)(b^y)(L)RuiII(N^) 2 ^ ) θ

®

0

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch010

Au-Au*,Au-X*

φ

χ

Ο

Ο

»

ο

llr

lb

Q

^ A u # § x O

Û Au-Au*,Au-X

Ο φ

x

O®

A u

ο

%

^

#Au^C

X

®

Ο Au-Au,

Au-X*

Ο l a

g

® * ^ ® Φ

χ

Ο

Au«Au, Au»X

2b

2u

2a

n

lb 2u

la

g

M-M d i s t a n c e Figure 6.

Orbital diagram for dinuclear gold complexes.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

10.

F A C K L E R A N D BASIL

Nonclassical

Coordination

207

Compounds

energy as the d i s t a n c e i s lengthened. At some d i s t a n c e there, indeed, can be a crossover between the 2a^ l e v e l and the l b 2 level. This work suggests t h a t , as a d d i t i o n a l e l e c t r o n d e n s i t y goes i n t o the l i n e a r sigma bonding system through the formation of c o v a l e n t l y bonded l i g a n d s , such as produced by the a d d i t i o n of two methyl groups, the metal-metal bond may be destroyed. Examination of the i n t e r e s t i n g r e a c t i o n ( F i g . 7;2a,b to 4) suggests t h a t , indeed, such a bond r u p t u r i n g transformation can be observed. Schmidbaur and h i s coworkers (10) i n d i c a t e that the methyliodide product and the Au(III) dibromide species r e a c t with m e t h y l l i t h i u m to produce a compound which appears to be the asym­ m e t r i c a l Au (III)-Au (I) d i n u c l e a r species _4. Spectroscopic e v i -

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch010

u

CH -Au-CH 0

f 3 CH -Au-CH 2

CH I 3

R

CH -Au-CH 2

la

1

2 \

/

P R

2

CH -Au-CH 2 ι I

2

0

(R = C H )

0

2a,b

3

b

I

2

(R = C H ) 6

5

CH3I.

LiMe -Lil

I /

2

R P 2

I «

2

PR

CH -Au-CH 0

^CH -Au-CH

2 \ 2

2

PR.

R P 2

\ 0

CH -Au-CH 2

2

I CH

3a,b Figure 7.

CH

3

3

M_ Organometallic reactions of dinuclear gold complexes.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

208

INORGANIC

CHEMISTRY:

TOWARD

T H E 2 1 ST

CENTURY

dence i n our l a b o r a t o r i e s confirms t h i s result,(11), although to date no c r y s t a l l i n e m a t e r i a l has been i s o l a t e d f o r X-ray s t r u c ­ tural analysis. The extreme p h o t o s e n s i t i v i t y i n s o l u t i o n and chemical i n s t a b i l i t y have made i t d i f f i c u l t to achieve s t r u c t u r a l r e s u l t s to date. H o p e f u l l y , before the beginning of the 21st century, t h i s problem w i l l be sorted out, and the compound no longer w i l l be " n o n - c l a s s i c a l . In summary, i t can be concluded that " n o n - c l a s s i c a l coordina­ t i o n compounds" are chemical compounds not explained by the t e x t ­ books. As the papers i n t h i s ACS symposium s e r i e s "Inorganic Chemistry: Towards the 21st Century" are examined, the reader w i l l f i n d much that i s " n o n - c l a s s i c a l " today but w i l l be " c l a s s i c a l " by the 21st century. 11

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch010

Acknowledgment s S t r u c t u r a l work reported here was performed by Dr. H. W. Chen and Dr. C. Paparizos. The N a t i o n a l Science Foundation NSF-CHE 8013141 and the donors of the Petroleum Research Fund as adminis­ tered by the American Chemical Society have supported t h i s work.

Literature Cited

(1) Cotton, F. Α.; Wilkinson, G. "Advanced Inorganic Chemistry", John Wiley and Sons: New York, 1980; 980. (2) Schmidbaur, H. Acc. Chem. Res. 1975, 8, 62. (3) Schmidbaur, H.; Mandel, J. R.; Bassett, I.-M.; Blaschke, G.; Zimmer-Gasser, B. Chem. Ber. 1981, 114, 433; Schmidbaur, H.; Wohlleben, Α.; Schubert, U.; Frank, Α.; Huttner, G. Chem. Ber. 1997, 110, 2751-2757. (4) Jandik, P.; Schubert, U.; Schmidbaur, H. Angew. Chem. Suppl. 1982, 1-12. (5) Balch, A. L.; Hunt, C. T.; Lee, C.-L.; Olmstead, M. M.; Farr, J. P. J. Am. Chem. Soc. 1981, 103, 3764-3772. (6) Brown, M. P.; Puddephatt, R. J.; Rashidi, M.; ManoihovicMuir, L.; Muir, K. W.; Solomon, T.; Seddon, K. R. Inorg. Chim Acta 1977, 23, 633; Cooper, S. J.; Brown, M. P.; Puddephatt, R. J. Inorg. Chem. 1981, 20, 1374-1377. (7) Schmidbaur, H.; Franke, R. Inorg Chim. Acta 1975, 13, 85-89, (8) Fackler, Jr., J. P.; Basil, J. D. Organomet. 1982, 1, 781-873. (9) Hoffman, D. M.; Hoffmann, R. Inorg. Chem. 1981, 20, 35433555. (10) Schmidbaur, H.; Slawisch, A. "Gmelin Handbuch der Anorgani then Chemie. Au Organogold Compounds", 8th ed.; SpringerVerlag: Berlin/Heidelberg, 1980; 264. (11) Basil, J. D. unpublished results. RECEIVED

September

16,

1982

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

11 High- and Low-Valence Metal Cluster Compounds: A Comparison F. A L B E R T COTTON Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch011

Texas A&M University, Department of Chemistry, College Station, T X 77843

The characteristic differences between metal cluster compounds having the metal atoms in low formal oxidation states and those having them in high formal oxidation states are reviewed critically and analytically. There i s now not only a great number but a l s o a great v a r i e t y of metal atom c l u s t e r compounds. In t h i s essay I should l i k e to d i s c u s s the d i f f e r e n c e s between those that have metal atoms i n a r e l a t i v e l y high mean o x i d a t i o n s t a t e (+2 to +4, and even, i n r a r e cases, a b i t higher) and those with metal atoms i n o x i d a t i o n s t a t e s i n the range -1 to +1. To keep the d i s c u s s i o n w i t h i n reasonable l i m i t s I s h a l l r e s t r i c t i t almost e x c l u s i v e l y to c l u s t e r s c o n s i s t i n g of only two or three metal atoms. I eschew the pedantic a s s e r t i o n that two atoms do not a c l u s t e r make. For convenience and b r e v i t y i n the d i s c u s s i o n the abbreviations HVC and LVC f o r high-valent c l u s t e r and low-valent c l u s t e r , r e s p e c t i v e l y , w i l l be adopted. To i l l u s t r a t e the d i s t i n c t i o n between the two types of c l u s t e r compounds the examples shown i n F i g . 1 may be examined. While the HVC/LVC dichotomy has rather r o u t i n e l y been taken f o r granted i n the past, i t has not been the subject of an explanatory d i s c u s s i o n . There are f i v e p o i n t s of comparison and/or contrast to which I should l i k e to draw a t t e n t i o n ; four are e m p i r i c a l and one t h e o r e t i c a l . Ligand Preferences. The LVC* s are e l e c t r o n r i c h and i n order to e x i s t i n s t a b l e compounds they r e q u i r e l i g a n d s , such as CO, that a r e weak donors and good π acceptors. On the other hand the HVC c l u s t e r s are not a t t r a c t i v e to such l i g a n d s nor do they r e q u i r e them f o r s t a b i l i t y . In t h i s respect, there i s a d i r e c t p a r a l l e l to mononuclear compounds where M° p r e f e r s CO, RNC or s i m i l a r l y π-acidic l i g a n d s while the M (n = 2-4) ions do not g e n e r a l l y form CO complexes. n +

0097-6156/83/0211-0209$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

210

INORGANIC C H E M I S T R Y : TOWARD T H E 21 ST C E N T U R Y

HVCs

LVCS

Ο C

CI

CI

Ί2-

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch011

I χι l^ci CI

0 D(Re-Re) = 3 . 0 2 Â

CI

D(Re=Re) = 2 . 2 4 Â

V Mo.

foo.

υ

0 H

iOH

I ,2.80A and the Mo-Mo s i n g l e bond i n ( n - C H ) 2 M o 2 ( C 0 ) has the a s t o n i s h i n g length of 3.22Â. Among m u l t i p l e bonds, s i m i l a r trends are found. For ΜΟΞΜΟ t r i p l e bonds we have a l e n g t h of 2.22A f o r the HVC species [ M o ( H P O i ) ] " " but 2.45Â i n the LVC species ( n - C H ) M o (CO) . I t i s appropriate to s t r e s s that metal-metal bonds show f a r more complex behavior than those between main-group atoms and simple bond-length/bond-order r e l a t i o n s h i p s do not e x i s t except i n s p e c i a l cases. Nevertheless, the general trend of longer bonds i n the LVCs as compared to the HVCs i s unmistakable. Any attempt to e x p l a i n t h i s observation must be s p e c u l a t i v e , but i t i s l i k e l y that two i n t e r r e l a t e d f a c t o r s are l a r g e l y responsible. In the HVCs the bonding i s due to overlap of r e l a t i v e l y compact metal di o r b i t a l s f o r which overlap (and hence bond strength) increases markedly w i t h c l o s e approach of the metal atoms. In the LVCs the M-M bonding i s l a r g e l y due to overlap of JS, £ or s£ h y b r i d o r b i t a l s , which are more d i f f u s e and give best overlap at r e l a t i v e l y long d i s t a n c e s . The 6* e x c i t a t i o n , but has a weaker metal-metal f o r c e constant and a longer metal-metal bond. I t i s 2

1

1

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

LiCHTENBERGER

Metal-Metal

Quadruple

223

Bonds

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch012

12.

2

Figure 1. The Β ionization band ( B positive ion state) of Mo (0 CCH ) equally spaced symmetric vibrational components. 2g

2

2

s h

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

fit with

224

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch012

INORGANIC C H E M I S T R Y : TOWARD T H E 21 ST C E N T U R Y

Figure 2.

The relationship between the δ ionization band and the potential wells of the ground state and the displaced B positive ion state. 2

Xg

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

12.

LiCHTENBERGER

TABLE.

Quadruple

Bonds

225

Related data f o r three e l e c t r o n i c perturbations o f Mo ( 0 ^ Η ) ι * . 2

2

3

formal bond order

state

Mo-Mo distance

Mo-Mo stretch

4.0

406 cm"

2.093 A

δ-δ*

3.0

370(5)

2.20

δ ionization

3.5

360(10)

2.26(1)

neutral

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch012

Metal-Metal

1

important t o note t h a t , i n a d d i t i o n t o the change i n formal bond order, the i o n i z a t i o n process a l s o changes the formal o x i d a t i o n s t a t e o f the metal centers. The i n c r e a s e i n p o s i t i v e charge a t the metal centers w i l l c o n t r a c t the metal d o r b i t a l s and reduce t h e i r overlap. This w i l l decrease the bonding a b i l i t y o f the remaining delta-bonding e l e c t r o n , as w e l l as t h a t o f the p i bonding e l e c t r o n s and q u i t e p o s s i b l y a l s o the sigma-bonding e l e c ­ t r o n s . This e f f e c t o f p o s i t i v e charge i s seldom discussed f o r 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 photoelectron spectroscopy, but has been discussed f o r quadruply bonded metal complexes (3). The important p o i n t t o remember i s that an e l e c t r o n i n the delta-bonding o r b i t a l o f Mo (0 CCH3)i,. has a s u b s t a n t i a l i n f l u e n c e on the strength o f the metal-metal i n t e r a c t i o n . This i n f l u e n c e i s d i r e c t l y evidenced by the metal-metal v i b r a t i o n a l f i n e s t r u c ­ ture observed with i o n i z a t i o n from the d e l t a o r b i t a l , which shows a lowering o f the metal-metal s t r e t c h i n g frequency and a l e n g t h ­ ening o f the e q u i l i b r i u m metal-metal bond d i s t a n c e . 2

2

Acknowl edgment These experiments were conducted by Mr. Charles H. B l e v i n s I I . We wish t o thank the Department o f Energy, c o n t r a c t DE-AC0280ER10746 and the u n i v e r s i t y o f Arizona f o r p a r t i a l support o f t h i s work. D.L.L. i s an A l f r e d P. Sloan Fellow. literature Cited

1. Hubbard, J.L.; Lichtenberger, D.L., J. Am. Chem.Soc.,1982, 104, 2132. 2. Martin, D.S.; Newman, R.A.; Fanwick, P.E., Inorg. Chem., 1979, 18, 2511. 3. Bursten, B.E.; Cotton, F.A., Farad. Disc. Roy. Soc. Chem., 1980, 14, 180. RECEIVED

September 1 6 , 1982

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

13 Higher Nuclearity Carbonyl Clusters BRIAN T. H E A T O N

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch013

University of Kent, Chemical Laboratory, Canterbury CT2 7NH England

Professor Cotton's studies have considerably clarified our understanding of di- and tri-nuclear metal-metal bonded compounds. For higher nuclearity clusters, rationalisation of structures, bonding, reactivity etc., must be much more tenuous because of the increased number of variables (metal-metal, metal-ligand, steric effects) now present. However, I would briefly like to present a few trends which seem to be emerging in this area. Substitution Sites in Tetra- and Penta-Nuclear Clusters Apart from nitrile-substituted derivatives of [Ir (CO) ] (1, 2) which retain the non-carbonyl-bridged structure of [Ir (CO) ], (3) all other derivatives of [Ir (CO) ], both in the solid state and in solution, are based on the C -structure of [M(CO) ][(M = Co, (4-7) Rh (4, 8)](Fig. 1). This structure, with three edge-bridging carbonyls, is better able to dissipate the increased nuclear charge induced on carbonyl substitution. Three possible isomers could result by carbonyl replacement at the apical, radial or axial site but the better back-bonding ability favours carbonyl replacement on the basal metal atom and the substituent is found exclusively in the axial site, eg. [Ir (CO) X] (X = Br, (9) COMe, (10) [Ir(CO) ] (11)), [Ir (CO) L] (L = PR (12)). 4

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Further occugancy of a x i a l s i t e s occurs with small l i g a n d s , eg. [ l r ^ ( C 0 ) ^ Q H ] ~", (13) but minimisation of s t e r i c e f f e c t s becomes important f o r l a r g e r l i g a n d s and the s u b s t i t u t i o n p a t t e r n shown i n F i g . 2 i s p r e f e r r e d . (12, 14) The reason f o r p r e f e r e n t i a l a x i a l s i t e occupancy by monodentate l i g a n d s i s probably r e l a t e d to r e s u l t s of recent CNDO c a l c u l a t i o n s on [ C o ^ i C O ) ^ ] * (15) which show that the a x i a l carbonyls are l e a s t i n v o l v e d i n back-bonding. E x t r a p o l a t i o n to Rh^- and I r ^ - d e r i v a t i v e s seems reasonable and f i n d s some support from n.m.r. measurements, which show that the ^ C n.m.r. chemical s h i f t of the a x i a l carbonyl i s always a t higher f i e l d than the r a d i a l carbonyl i n both rhodium (16) and i r i d i u m d e r i v a t i v e s . (12) 2

0097-6156/83/0211-0227$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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INORGANIC C H E M I S T R Y :

Figure 2. Isomers formed on ligand (L = PR , P(OPh) ) substitution in [M (CO) ], (M = Rh, Ir). Key: ·, terminal CO; and M, bridging CO. S

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In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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HEATON

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Related s u b s t i t u t i o n patterns are observed i n t e t r a n u c l e a r c o b a l t and rhodium c l u s t e r s . Thus, the small l i g a n d , P(OMeK(L), occupies a x i a l s i t e s i n [Co,(CO) L ] (x = 1,2) (17) whereas s t e r i c e f f e c t s become important witS ?(OPhK and the isomers shown i n F i g . 2 are obtained with tetrarhodium d e r i v a t i v e s . U8> υ ) . However, there i s some d e v i a t i o n from these s u b s t i t u t i o n p a t t e r n s , e s p e c i a l l y with cobalt c l u s t e r s . Thus, i o d i d e i n [ C o ^ ( C O ) ^ l ] ~ i s predominantly i n an a p i c a l s i t e , although there i s some s u b s t i t u t i o n (2%) on the b a s a l c o b a l t , (20) and P(CH CH:CH )Me2(L) i n [ C o ( C 0 ) L ] occupies one a x i a l and a trans-apical s i t e . The reasons f o r these d i s c r e p a n c i e s are not c l e a r but are probably a s s o c i a t e d with cobalt being both smaller and l e s s e l e c t r o n e g a t i v e than rhodium or i r i d i u m . _ For the s u b s t i t u t e d penta-nuclear c l u s t e r , [Rh RO-M i n b o t h W „ ( 0 E t ) and Mo„ 0 ( 0 - i - P r )„ (py ) „ > A g W \ 0 > ΟΞΜΟ i n M o 0 ( 0 - i - P r ) ( p y ) . This trend i s r e a d i l y understood i n t e r m s o f t h e r e l a t i v e d e g r e e s o f 0-to-M π-bonding i n t h e series and p l a c e s t e r m i n a l RO l i g a n d s somewhere b e t w e e n t h e i n f i n i t e c h a i n o x y g e n donor l i g a n d s f o u n d i n t h e t e r n a r y m o l y b ­ denum o x i d e and t h e t e r m i n a l oxo l i g a n d s f o u n d i n A g W 0 i ^ w h i c h a p p r o x i m a t e s t o W^O^ , w i t h weak c o o r d i n a t i o n t o Ag ions. 2 . Μ - μ - 0 and Μ - μ - 0 d i s t a n c e s a r e s l i g h t l y s h o r t e r f o r oxo l i g a n d s t h a n f o r a l k o x y l i g a n d s . T h i s g e n e r a l observation may, h o w e v e r , be masked by o t h e r f a c t o r s . F o r e x a m p l e , t h e Μ - μ - 0 d i s t a n c e s i n Ag W,, 0 and W ( 0 E t ) a r e comparable but longer t h a n t h o s e i n M c C a r l e y ' s compounds. These d e v i a t i o n s Χ

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Figure 5. ORTEP view of the centrosymmetric Mo O (O-'i-Pr) molecule. Some pertinent bond distances (λ) and angles (deg) are: Mo(l)-Mo(2) = Mo(l/-Mo(2/ = 2.585(1); Mo(l)-Mo(l/ = 3.353(1); Mo(2)-Mo(3) = Mo(2)'-Mo(3f = 3.285(1); Mo(l)'-Mo(l)-Mo(2) = 146.5(1); Mo(l)-Mo(2)-Mo(3) = 134.3(1); Mo-oxo (terminal) = 1.68 (averaged); Μο-οχο (μ ) = 1.93 (averaged); Mo-OR (terminal) = 1.86 (average); and Mo-OR (μ ) = 2.05 to 2.19. 6

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In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

16.

CHiSHOLM

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T a b l e I I . C o m p a r i s o n o f M e t a l - t o - O x y g e n Bond D i s t a n c e s i n Compounds w h i c h a r e S t r u c t u r a l l y R e l a t e d t o W ^ ( 0 E t ) .

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch016

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10 e l e c t r o n Mo^ c l u s t e r i n Ba . x

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( a ) D i s t a n c e s q u o t e d t o ±0.01 X . ( b ) R e f . 12. ( c ) S k a r s t a d , P.M.; G e l l e r , S. M a t . R e s . B u l l . 1975, 10, 7 9 1 . ( d ) M c C a r l e y , R.E.; L u l y , M.H.; Ryan, T.R.; T o r a r d i , C.C. ACS Symp. S e r i e s 1981, 155, 4 1 .

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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from t h e general r u l e , d > , may be a c c o u n t e d f o r by c o n s i d e r a t i o n s o f t r a n s i n r i u e n c e s ^ ( J 2 ) o p e r a t i n g w i t h i n t h e octahedral u n i t s . The t r i p l y b r i d g i n g oxo g r o u p s i n AgeW^!* are trans to terminal W-0 bonds w h i c h a r e s h o r t (1.79 A a v e r a g e ) b e c a u s e o f m u l t i p l e bond c h a r a c t e r . An e x a m i n a t i o n o f the t h r e e Μο-μ -0 d i s t a n c e s i n M o 0 ( 0 - i - P r ) ( p y ) i sparticu­ l a r l y i n f o r m a t i v e s i n c e t h e bonds a r e t r a n s t o t h r e e d i f f e r e n t l i g a n d s , namely o x o , i s o p r o p o x y and p y r i d i n e , w h i c h have d i f f e r ­ e n t t r a n s i n f l u e n c e s . A s i m i l a r l a r g e asymmetry o c c u r s i n t h e Μ-μ -0 d i s t a n c e s i n Ag W^0 and Moi,0 ( 0 - i - P r ) , , (py)i» and may be t r a c e d t o t h e d i f f e r e n t t r a n s i n f l u e n c e s o f t e r m i n a l oxo and b r i d g i n g oxo l i g a n d s . What i n f l u e n c e w i l l s t r o n g π-donor l i g a n d s have i n o r g a n o metallic chemistry? This question i s one w h i c h c a n n o t be a n s w e r e d r e l i a b l y a t t h i s t i m e . The s t a b i l i z a t i o n o f u n u s u a l c o o r d i n a t i o n numbers and g e o m e t r i e s by π-donor l i g a n d s i n com­ pounds such as M o ( 0 - t - B u ) ( p y ) ( C 0 ) and M o ( C O ) ( S C N R ) has a t t r a c t e d t h e a t t e n t i o n o f Hoffmann ( 3 3 ) and T e m p l e t o n ( 3 4 ) and t h e i r c o w o r k e r s . We have n o t e d t h a t RO π-donors may p r o d u c e anomalous p r o p e r t i e s i n o t h e r l i g a n d s w h i c h a r e c o o r d i n a t e d t o t h e same m e t a l . F o r example, i n M o ( C O ) ( 0 - t - B u ) ( p y ) (35), the c a r b o n y l s t r e t c h i n g f r e q u e n c i e s a r e a n o m a l o u s l y low f o r c a r b o n y l g r o u p s bonded t o Mo(2+), v ( C O ) = 1906 and 1776 cm and, i n Mo(0-i-Pr) (bpy) , t h e 2 , 2 ' - b i p y r i d y l l i g a n d s appear p a r t i a l l y r e d u c e d from X - r a y and Raman s t u d i e s (_36). Q u a l i t a t i v e l y , b o t h o f t h e s e o b s e r v a t i o n s may be r a t i o n a l i z e d i n terms o f R0-to-M π-bonding. S t r o n g π-donor l i g a n d s r a i s e t h e e n e r g y o f t h e "* e l e c t r o n s a n d , t h u s , enhance m e t a l b a c k b o n d i n g t o π*-accept§r l i g a n d s , e v e n t o l i g a n d s such as 2 , 2 ' - b i p y r i d i n e w h i c h do n o t u s u a l l y b e h a v e a s π-acceptor l i g a n d s i n t h e g r o u n d s t a t e i n t r a n s i t i o n m e t a l c o o r d i n a t i o n compounds. M

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o f Molybdenum and

Having established structural and e l e c t r o n i c analogies b e t w e e n m e t a l o x i d e s and a l k o x i d e s o f molybdenum and t u n g s t e n , the key remaining f e a t u r e t o be examined i s t h e r e a c t i v i t y p a t t e r n s o f t h e m e t a l - a l k o x i d e s . M e t a l - m e t a l bonds p r o v i d e b o t h a s o u r c e and a r e t u r n i n g p l a c e f o r e l e c t r o n s i n 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 r e a c t i o n s . S t e p w i s e t r a n s f o r ­ m a t i o n s o f M-M bond o r d e r , f r o m 3 t o 4 ( 3 7 , 3 8 ) , 3 t o 2 and 1 (39) have now been documented. The a l k o x i d e s M ( 0 R ) (M=M) a r e coordinatively unsaturated, as i s e v i d e n t f r o m t h e i r facile r e v e r s i b l e r e a c t i o n s w i t h d o n o r l i g a n d s , e q . 1, and a r e r e a d i l y o x i d i z e d i n a d d i t i o n r e a c t i o n s o f t h e t y p e shown i n e q u a t i o n s 2 ( 3 9 ) and 3 ( 3 9 ) . 2

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Upon o x i d a t i o n , alkoxy bridges seem t o be invariably f o r m e d and t h i s may be v i e w e d as an i n t e r n a l L e w i s b a s e a s s o c i a ­ t i o n r e a c t i o n i n r e s p o n s e t o t h e i n c r e a s e d L e w i s a c i d i t y of t h e m e t a l i o n s w h i c h accompanies the changes ( Μ Ξ Μ ) -*• (M=M) + (M-M) . The s t r u c t u r a l changes w h i c h take the ethane-like Χ3ΜΞΜΧ3 u n i t t o two f u s e d t r i g o n a l b i p y r a m i d s , s h a r i n g a common e q u a t o r i a l - a x i a l edge, t o two face- or edge-shared octahedra are f a s c i n a t i n g . The M ( O R ) compounds p r o v i d e a good s o u r c e of e l e c t r o n s t o l i g a n d s t h a t a r e c a p a b l e of b e i n g r e d u c e d upon c o o r d i n a t i o n . For example, P h C N , r e a c t s w i t h M o ( 0 - i - P r ) i n the presence o f p y r i d i n e t o g i v e t h e a d d u c t Mo ( 0 - i - P r ) ( N C P h ) ( p y ) (4Ç0 i n w h i c h t h e d i p h e n y l d i a z o m e t h a n e l i g a n d may be v i e w e d as a 2e l i g a n d , Mo=N-N=CPh and t h e d i m o l y b d e n u m u n i t may be v i e w e d as ( M o - M o ) . See F i g u r e 6. The four electrons i n v o l v e d i n t h e two Mo-Mo bonds i n Mo 0jο(0-i-Pr)J ( F i g u r e 5) a r e r e a c t i v e t o m o l e c u l a r o x y g e n : Μο 0 0 (0-i-Pr) j + 0 -» 6/n[MoO ( 0 - i - P r ) ] . The s t r u c t u r e of [Mo0 (0-i-Pr) ] i s n o t p r e s e n t l y known, but t h e 2 , 2 ' - b i p y r i dine (bpy) ad9uct M o O ( 0 - i - P r ) ( b p y ) , which i s obtained by a d d i t i o n of bpy t o a h y d r o c a r b o n s o l u t i o n of [Mo0 (0-i-Pr) ] has been structurally characterized and shown t o have the e x p e c t e d o c t a h e d r a l geometry w i t h c i s d i - o x o l i g a n d s ( 4 1 ) . C a r b o n monoxide reacts with M (0R) compounds and the f i r s t s t e p has b e e n shown t o i n v o l v e t h e r e v e r s i b l e f o r m a t i o n o f M ( 0 R ) ( p - C O ) . A s t r u c t u r a l c h a r a c t e r i z a t i o n of M o ( 0 - t - B u ) (μ-CO) r e v e a l s an i n t e r e s t i n g s q u a r e b a s e d p y r a m i d a l g e o m e t r y f o r e a c h molybdenum atom (420. See F i g u r e 7. The two h a l v e s of t h e m o l e c u l e a r e j o i n e d by a common a p i c a l b r i d g i n g CO l i g a n d , a p a i r of b a s a l b r i d g i n g OR l i g a n d s and f o r m a l l y a Mo=Mo bond. Closely r e l a t e d compounds M ( 0 R ) L ^ - C 0 ) have been i s o l a t e d f o r R = i - P r and C H - t - B u where L i s a d o n o r l i g a n d s u c h as an amine (43). The s t r u c t u r e s of t h e M ( 0 - i - P r ) ( p y ) ( μ - C O ) com­ p o u n d s (M = Mo, W) have b e e n d e t e r m i n e d by X - r a y s t u d i e s and 6

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C(30)

Figure 6. An ORTEP view of the Mo (0-\-Pr) (N CPh ) (py) molecule. Some pertinent distances (A) and angles (deg) are: Mo-Mo = 2.661(2); Mo(l)-0(39) = 1.96(1); -0(43) = 1.96(1); -0(47) = 2.24(1); -0(51) = 2.11(1); -0(55) = 2.13(1); -N(3) = 1.76(1); Mo(2)-0(47) = 2.16(1); -0(51) = 2.04(1); -0(55) = 2.04(1); -0(59) = 1.98(1); -N(18) = 1.78(1); -N(33) = 2.225(10); N(3)-N(4) = 1.30(1); N(4)-C(5) = 1.30(2); Ν(18μΝ(19) = 1.30(1); N(19)C(20) = 1.31(2); Μο(ΐμΝ(3μΝ(4) = 164(1)°; N(3)-N(4)-C{5) = 124(1)°; Mo(2hN(18hN(19) = 155(1)°; and N(18hN(19)-C(20) = 122(1)°. 2

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In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch016

CHiSHOLM

Figure 7.

Metal

An ORTEP

Alkoxides

view of the central [Mo 0 (CO)] (0-l-Bu) (n-CO) molecule. 2

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r e v e a l a s i m i l a r i t y t o t h e Mo (O-t-Bu) (μ-CO) s t r u c t u r e : t h e p y r i d i n e l i g a n d s complete the c o n f a c i a l b i o c t a h e d r a l geometry by forming bonds trans to t h e M-C bonds. C h a r a c t e r i s t i c f e a t u r e s of t h e s e compounds a r e t h e i r e x c e e d i n g l y low v a l u e s of v ( C 0 ) , c a . 1650 (Mo) and 1550 cm"" (W) and low c a r b o n y l c a r b o n chemical s h i f t s , c a . 325 ppm (Mo) and 315 ppm (W) downfield from Me^Si. These low v a l u e s , w h i c h a r e u n p r e c i d e n t e d for n e u t r a l b r i d g i n g (μ -) c a r b o n y l compounds, i m p l y a g r e a t r e d u c ­ t i o n i n C-0 bond o r d e r . I n a f o r m a l s e n s e , t h e Mo ( O R ) (μ-CO)( p y ) 2 compounds a r e i n o r g a n i c a n a l o g u e s of e y e l o p r o p e n o n e s and i t i s p o s s i b l e t o e n v i s a g e a s i g n i f i c a n t c o n t r i b u t i o n from t h e r e s o n a n c e f o r m I V , shown b e l o w , w h i c h e m p h a s i z e s t h e a n a l o g y with a b r i d g i n g oxy alkylidyne ligand. Bridging alkylidyne c a r b o n r e s o n a n c e s i n [ (Me S i C H ) W^ -CSiMe ) ] are found at 353 ppm ( 4 4 ) . 2

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M o ( 0 R ) compounds i n h y d r o c a r b o n s o l v e n t s r a p i d l y p o l y m e r ­ i z e a c e t y l e n e t o a b l a c k m e t a l l i c - l o o k i n g form of p o l y a c e t y l ene. P r o p y n e i s p o l y m e r i z e d t o a y e l l o w powder, w h i l e b u t - 2 - y n e yields a gelatinous r u b b e r - l i k e m a t e r i a l ( 4 5 ) . The d e t a i l e d n a t u r e of t h e s e p o l y m e r s i s n o t y e t known and t h e o n l y m o l y b d e ­ num c o n t a i n i n g compounds r e c o v e r e d from t h e s e polymerization r e a c t i o n s w e r e t h e M o ( O R ) compounds. When t h e r e a c t i o n s w e r e carried out i n t h e p r e s e n c e of p y r i d i n e / h e x a n e s o l v e n t mix­ t u r e s , s i m p l e a d d u c t s M o ( O R ) ( p y ) ( a c ) were i s o l a t e d f o r R = i - P r and C H - t - B u , and ac = HCCH, MeCCH and MeCCMe ( 4 5 , 4 6 ) . The s t r u c t u r e of M o ( 0 - i - P r ) ( p y ) ( μ - 0 Η ) i s shown i n F i g u r e 8. The c e n t r a l M C u n i t i s t y p i c a l of t h o s e commonly found i n dinuclear organometallie c o m p l e x e s , e.g. Co (C0) (RCCR) ( 4 7 ) , Cp M (C0)„(HCCH) where M = Mo (48) and W (49) and ( C 0 D ) N i ( R C C R ) ( 5 0 ) . The a c e t y l e n i c C-C d i s t a n c e ( 1 . 3 6 8 ( 6 ) X) i n t h e M o ( 0 R ) ( p y ) ( H C C H ) compound i s s l i g h t l y l o n g e r than t h a t i n e t h y l e n e , 1.337(3) Κ ( 5 1 ) . The a c e t y l e n i c d i s t a n c e i n W ( 0 - i - P r ) ( p y ) ( H C C H ) , which i s i s o s t r u c t u r a l w i t h the molybde­ num compound, i s e v e n l o n g e r , 1.413(19) X . A g a i n , we see t h e a b i l i t y of t h e M ( 0 R ) compounds t o d o n a t e e l e c t r o n d e n s i t y t o ττ-acceptor l i g a n d s w i t h t h e o r d e r W > Mo. In a formal sense, t h i s c a n be v i e w e d as an o x i d a t i v e - a d d i t i o n r e a c t i o n accom­ p a n i e d , as u s u a l , by a r e t u r n t o o c t a h e d r a l g e o m e t r i e s t h r o u g h t h e agency of a l k o x y b r i d g e s and, i n t h i s i n s t a n c e , a b r i d g i n g alkyne ligand. The n e o p e n t o x y compound M o ( 0 N e ) ( p y ) ( μ - 0 Η ) , w h i c h can be p r e p a r e d by t h e a d d i t i o n of one e q u i v a l e n t of a c e t y l e n e t o M o ( O N e ) ( p y ) , r e a c t s f u r t h e r w i t h a n o t h e r e q u i v a l e n t of a c e t y 2

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In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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l e n e t o y i e l d Mo ( O N e ) ( μ - C ^ ) (py ) ( 4 5 , 4 6 ) . An ORTEP v i e w o f t h i s i n t e r e s t i n g m o l e c u l e i s shown i n F i g u r e 9. The f o r m a t i o n o f a Μ 2 ( μ - 0 , ^ ) u n i t by t h e c o u p l i n g o f two a c e t y l e n e s a t a dimetal center i s a w e l l recognized f e a t u r e of o r g a n o m e t a l l i c chemistry and was first s e e n i n 1961 w i t h the s t r u c t u r a l c h a r a c t e r i z a t i o n o f (CO) F e ( μ - C M e ( 0 H ) ) , a p r o d u c t obtained f r o m t h e r e a c t i o n between b u t - 2 - y n e and a l k a l i n e s o l u t i o n s of ironhydroxycarbonyl: MeCCMe *H Fe ( CO) (53.)· In Mo (0Ne) (μ-C^H^)(py) , one molybdenum atom i s i n c o r p o r a t e d i n a m e t a l l a cyclopentadiene r i n g , w h i l e t h e o t h e r i s π-bonded t o t h e d i e n e i n a n^-manner. F o r m a l l y , one molybdenum i s i n o x i d a t i o n s t a t e +4^ and t h e o t h e r +3^. I n any e v e n t , t h e d i m o l y b d e n u m c e n t e r has been o x i d i z e d from ( Μ Ο Ξ Μ Ο ) t o (Mo=Mo) and we see a return t o the c o n f a c i a l b i o c t a h e d r a l geometry i n which t h e μ-C^H,, l i g a n d o c c u p i e s two s i t e s o f t h e b r i d g i n g f a c e . 2

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These μ - ^ ^ and μ-^Η,, molybdenum a l k o x i d e s a r e v e r y s o l u b l e i n hydrocarbon solvents which allows t h e i r c h a r a c t e r i z a ­ t i o n i n s o l u t i o n by H and C nmr s p e c t r o s c o p y . I n a l l cases, low t e m p e r a t u r e l i m i t i n g s p e c t r a have been o b t a i n e d w h i c h a r e c o n s i s t e n t w i t h t h o s e e x p e c t e d b a s e d on t h e s t r u c t u r e s f o u n d i n the solid s t a t e . I t i s a l s o p o s s i b l e , by nmr s t u d i e s , t o e l u c i d a t e t h e i r r o l e i n alkyne o l i g o m e r i z a t i o n r e a c t i o n s and a l l a v a i l a b l e evidence i n d i c a t e s t h a t they a r e not a c t i v e i n polymerization, but a r e a c t i v e i n e y e l o t r i m e r i z a t i o n which yields b e n z e n e s . F o r e x a m p l e , when M o ( O N e ) ( μ - 0 ^ Η ) ( p y ) was allowed t o r e a c t w i t h c a . 20 e q u i v . o f C D i n a s e a l e d nmr t u b e , t h e *H i n t e n s i t y o f t h e s i g n a l a r i s i n g f r o m t h e μ-Ο,,Η,, l i g a n d d e c r e a s e d a s a s i g n a l due t o 0 Υ{^Ό grew. S i m i l a r l y , when M o ( 0 - i - P r ) ( p y ) ( μ - 0 Η ) was a l l o w e d t o r e a c t w i t h c a . 20 e q u i v . o f C D , a benzene p r o t o n resonance appeared and, w i t h i n t h e l i m i t s o f *H nmr i n t e g r a t i o n , t h e i n t e n s i t y o f t h i s s i g n a l c o r r e s p o n d e d t o the l o s s of t h e μ - 0 Η s i g n a l . When M o ( 0 - i Pr) (py) (μ-MeCCH) was a l l o w e d t o r e a c t w i t h HCCH, c a . 20 e q u i v . , f o r m a t i o n o f t o l u e n e was o b s e r v e d a l o n g w i t h Mo ( 0 - i P r ) ( p y ) 2 ( μ - C H ) and t h e r a t i o o f t h e \x-C ^2 and t o l u e n e - C H s i g n a l s was 2:3, w h i c h e s t a b l i s h e s t h e s t o i c h i o m e t r y o f t h e r e a c t i o n shown i n e q . 4. 2

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The species r e s p o n s i b l e f o r alkyne p o l y m e r i z a t i o n , which i s k i n e t i c a l l y more f a c i l e t h a n e y e l o t r i m e r i z a t i o n s i n c e o n l y a s m a l l f r a c t i o n o f t h e added a l k y n e i s c o n v e r t e d t o b e n z e n e s , i s n o t y e t known. C a r b e n e - m e t a l c o m p l e x e s , b o t h m o n o n u c l e a r ( 5 4 ) and b i n u c l e a r ^ - C R ) c o m p l e x e s ( 5 5 , 5 6 ) , have been shown t o a c t a s a l k y n e p o l y m e r i z a t i o n i n i t i a t o r s and s e v e r a l y e a r s ago i t was shown t h a t t e r m i n a l a l k y n e s and a l c o h o l s c a n r e a c t t o g i v e a l k o x y c a r b e n e l i g a n d s (5_7). As y e t , we have no e v i d e n c e 2

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In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Figure 9. An ORTEP view of the Mo (ONe) (^-C H )(py) molecule. Some pertinent distances (A) are: Mo(l)-Mo(2) = 2.69(1); Mo-O (terminal) = 1.92 (averaged); Mo-O (μ ) = 2.15; Mo-N = 2.15(1); Mo(l)-C(3) = 2.12(2); Mo(2)C(3) = 2.39(2); Mo(2)-C(4) = 2.34(2); C(3)-C(4) = 1.47(3); and C(4)-C(4/ = 1.44(4). 2

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t h a t carbene l i g a n d s a r e formed i n these r e a c t i o n s , but they r e m a i n h i g h on t h e l i s t o f p o s s i b l y a c t i v e s p e c i e s p r e s e n t i n these r e a c t i o n s . W (u-H) (0-i-Pr) ( 3 0 ) i s one o f t h e v e r y few t r a n s i t i o n m e t a l h y d r i d o a l k o x i d e s t h a t have been c l a i m e d i n t h e l i t e r a ­ ture (58,59). I t i s t h e o n l y one t o be f u l l y structurally c h a r a c t e r i z e d by a n X - r a y s t u d y a n d , t h o u g h i n t h e s o l i d s t a t e each tungsten i s i n a d i s t o r t e d o c t a h e d r a l environment ( F i g u r e 4), i t behaves i n s o l u t i o n as 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 m o l e c u l e . I t i s f l u x i o n a l on t h e *H nmr t i m e - s c a l e i n t o l u e n e de· E v e n a t t h e l o w e s t t e m p e r a t u r e s a c c e s s i b l e i n t h a t s o l v e n t t h e r e i s one t i m e a v e r a g e d t y p e o f 0 - i - P r g r o u p and a h y d r i d e r e s o n a n c e a t δ = 7.87 ppm f l a n k e d by t u n g s t e n s a t e l l i t e s o f r o u g h l y o n e - f i f t h i n t e n s i t y due t o c o u p l i n g t o W w h i c h h a s I = \ and 14.47o n a t u r a l abundance. The a p p e a r a n c e o f o n l y one coupling constant, J = 96 Hz, and t h e i n t e n s i t y o f t h e s a t e l l i t e s i n d i c a t e t h a t t h e h y d r i d e l i g a n d s e e s two e q u i v a l e n t (time-averaged) tungsten atoms. C r y o s c o p i c molecular weight d e t e r m i n a t i o n s i n benzene gave M = 1480 ± 8 0 , w h i c h showed t h a t t h e t e t r a n u c l e a r n a t u r e o f t h e complex i s l a r g e l y o r t o t a l l y m a i n t a i n e d i n n o n - c o o r d i n a t i n g s o l v e n t s . However, i n p - d i o x a n e , t h e m o l e c u l a r w e i g h t was c l o s e t o h a l f t h e v a l u e o b t a i n e d i n benzene, which suggests that t h e t e t r a n u c l e a r complex i s c l e a v e d i n donor s o l v e n t s t o g i v e s o l v a t e d W ( μ - Η ) ( 0 - i - P r ) . I n t h e mass s p e c t r o m e t e r , a v e r y weak m o l e c u l a r i o n was d e t e c t a ­ b l e , b u t t h e m a j o r i o n , by f i e l d d e s o r p t i o n , was νί (μ-Η)(0-i-Pr) . A l l of t h i s i s c o n s i s t e n t w i t h the view t h a t c l e a v a g e o f one o r b o t h o f t h e c e n t r a l a l k o x y b r i d g e s , w h i c h a r e l o n g ( a n d weak) b e i n g t r a n s t o t h e h y d r i d o l i g a n d and span t h e W ( l ) t o W ( l ) ' atoms w h i c h a r e non-bonded ( W ( l ) - W ( l ) ' = 3.407(1) A ) , c r e a t e s a vacant c o o r d i n a t i o n s i t e . T h i s molecule t h u s p r o v i d e s an i n t e r e s t i n g o p p o r t u n i t y f o r t h e s t u d y o f t h e r e a c t i v i t y of t h e b r i d g i n g hydrido l i g a n d . I n a quick survey of i t s r e a c t i o n s w i t h u n s a t u r a t e d h y d r o c a r b o n s c a r r i e d o u t i n nmr tubes i n t o l u e n e - d s o l u t i o n , i t h a s been found ( 3 0 ) t o r e a c t r a p i d l y w i t h e t h y l e n e , aliène and d i p h e n y l a c e t y l e n e . I n each case, t h e W p - h y d r i d e r e s o n a n c e was l o s t . The r e a c t i o n w i t h e t h y l e n e i s e v i d e n t l y r e v e r s i b l e since attempts t o i s o l a t e the presumed e t h y l complex f o r m e d by i n s e r t i o n y i e l d o n l y W«*(y-H) (0-i-Pr) M o r e o v e r , when W (μ-Η) ( 0 - i - P r ) ^ was a l l o w e d t o H

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r e a c t w i t h a l a r g e e x c e s s o f CH =CD i n an nmr t u b e , t h e l a b e l s i n t h e e x c e s s e t h y l e n e w e r e r a p i d l y s c r a m b l e d and when \^^(μ-ϋ) (0-i-Pr) was r e a c t e d w i t h excess CH =CH , t h e ν/^(μ-Η) (O-i-Pr)m compound was recovered. W ( μ-Η) ( O - i - P r ) i n and 1-butene show no a p g a r e n t r e a c t i o n a t room t e m p e r a t u r e i n t o l u e n e - d , b u t a t 60 C, 1-butene i s s e l e c t i v e l y i s o m e r i z e d t o c i s 2-butene ( 6 0 ) . 2

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In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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1. The s t r u c t u r a l a n a l o g i e s b e t w e e n m e t a l o x i d e s , metala l k o x i d e s and m e t a l - a l k o x i d e - o x i d e polymers o r i g i n a l l y noted f o r d° m e t a l systems c a n be e x t e n d e d t o m e t a l - m e t a l bonded systems. 2. M e t a l - m e t a l bonds i n molybdenum and t u n g s t e n a l k o x i d e s provide a ready source of e l e c t r o n s f o r o x i d a t i v e - a d d i t i o n r e a c t i o n s and a d d i t i o n r e a c t i o n s i n v o l v i n g π-acidic l i g a n d s . 3. The s t r u c t u r a l p r o p e r t i e s and c h e m i c a l r e a c t i v i t i e s of t h e M - y - C R , -μ-Ο,,Η,,, -μ-Η g r o u p s have d i r e c t p a r a l l e l s w i t h t h o s e o f " c l a s s i c a l " o r g a n o m e t a l l i c compounds. 4. The c a t a l y t i c c y c l e s t h a t have been documented, namely a l k y n e e y e l o t r i m e r i z a t i o n and o l e f i n i s o m e r i z a t i o n , d e m o n s t r a t e t h a t a d d i t i o n and e l i m i n a t i o n from d i m e t a l c e n t e r s c a n o c c u r readily i n t h e p r e s e n c e o f m e t a l - m e t a l bonds and a l k o x i d e 1igands. 5. A l k o x i d e l i g a n d s , w h i c h a r e s t r o n g π-donors and may r e a d i l y interchange t e r m i n a l and b r i d g i n g (μ o r μ ) b o n d i n g s i t e s , c o u l d p l a y an important r o l e i n t h e f u t u r e development of o r g a n o m e t a l l i c chemistry, particularly t h a t of t h e e a r l y t r a n s i t i o n e l e m e n t s w h i c h may w i s h t o i n c r e a s e t h e i r c o o r d i n a ­ t i o n numbers and v a l e n c e s h e l l e l e c t r o n c o n f i g u r a t i o n s . A l k o x ­ i d e s of t i t a n i u m , s o - c a l l e d organic t i t a n a t e s , i n the presence o f aluminum a l k y l s o r a l k y l a l u m i n u m h a l i d e s , have a l r e a d y b e e n u s e d f o r p o l y m e r i z a t i o n and c o p o l y m e r i z a t i o n r e a c t i o n s o f o l e ­ f i n s and d i e n e s , b u t t h i s has a t t r a c t e d l i t t l e mechanistic attention. S c h r o c k and h i s c o w o r k e r s (6J1) have shown t h a t a l k o x i d e l i g a n d s may be u s e d t o s u p r e s s 3 - h y d r o g e n e l i m i n a t i o n r e a c t i o n s i n o l e f i n m e t a t h e s i s r e a c t i o n s i n v o l v i n g n i o b i u m and tantalum c a t a l y s t s . F i n a l l y , I s h o u l d l i k e t o draw a t t e n t i o n t o t h e work o f o t h e r s who have p r o v i d e d m o d e l s f o r m e t a l o x i d e s and t h e i r interactions with h y d r o c a r b o n and o r g a n o m e t a l l i c fragments. S p e c i f i c a l l y t h e work o f K l e m p e r e r ( 6 2 , 6 3 , 6 4 ) and K n o t h ( 6 5 , 6 6 , 67 ) a n d t h e i r c o w o r k e r s have p r o v i d e d us w i t h d i s c r e t e s a l t s o f the h e t e r o p o l y a n i o n s t o which a v a r i e t y of organic/organometall i c f r a g m e n t s have been a d h e r e d . 2

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3

Acknowledgments I thank t h e Chemical Science D i v i s i o n , O f f i c e of B a s i c R e s e a r c h , U.S. D e p a r t m e n t of Energy, t h e O f f i c e of N a v a l R e s e a r c h , and 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 f o r t h e i r s u p p o r t o f v a r i o u s a s p e c t s o f t h e c h e m i s t r y d e s c r i b e d , and a l s o my many t a l e n t e d c o l l a b o r a t o r s , c i t e d i n t h e r e f e r e n c e s , who made t h e work p o s s i b l e . Literature Cited

1. Bradley, D.C.; Mehrotra, R.C.; Gaur, P.D. "Metal Alkoxides"; Academic Press: London, New York, San Francisco, 1978.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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2. Kochi, J.K. "Organometallic Mechanisms and Catalysis"; Academic Press: New York, 1974. 3. Catalytic Activation of Carbon Monoxide, ACS Sym. Ser. 1981, 152. Ford, P.C. Editor. 4. Nugent, W.A.; Haymore, B.L. Coord. Chem. Rev. 1980, 31, 123. 5. Cotton, F.A.; Wilkinson, G. "Advanced Inorganic Chemistry", John Wiley-Interscience, 4th Edit., 1980. 6. Bradley, D.C. Nature (London) 1958, 182, 1211. 7. Ibers, J.A. Nature (London) 1963, 197, 686. 8. Bradley, D.C. Coord. Chem. Rev. 1967, 2, 299. 9. Watenpaugh, K.; Caughlan, C.N. Chem. Commun. 1967, 76. 10. Bradley, D.C.; Hursthouse, M.B.; Rodesiler, P.F. Chem. Commun. 1968, 1112. 11. Chisholm, M.H.; Huffman, J.C.; Leonelli, J. J. Chem. Soc., Chem. Commun. 1981, 270. 12. Chisholm, M.H.; Huffman, J.C.; Kirkpatrick, C.C.; Leonelli, J.; Folting, K. J. Am. Chem. Soc. 1981, 103, 6093. 13. Skarstad, P.M.; Geller, S.; Mater. Res. Bull. 1975, 10, 791. 14. McCarley, R.E.; Ryan, T.R.; Torardi, C.C. ACS Symp. Ser. 1981, 155, 41. 15. Cotton, F.A.; Fang, A. J. Am. Chem. Soc. 1982, 104, 113. 16. Muller, Α.; Jostes, R.; Cotton, F.A. Angew. Chem., Int. Ed. Engl. 1980, 19, 875. 17. Bino, Α.; Cotton, F.A.; Dori, Z. J. Am. Chem. Soc. 1981, 103, 243. 18. Bino, Α.; Cotton, F.A.; Dori, Z.; Kolthammer, B.W.S. J. Am. Chem. Soc. 1981, 103, 5779. 19. McCarroll, W.H.; Katz, L.; Ward, J. J. Am. Chem. Soc. 1957, 79, 5410. 20. Ansell, G.B.; Katz, L. Acta. Crystallogr. 1966, 21, 482. 21. McCarley, R.E.; Torardi, C.C. J. Solid State Chem. 1981, 7, 393. 22. Bursten, B.E.; Cotton, F.A.; Hall, M.B.; Najjar, R.C. Inorg. Chem. 1982, 21, 302. 23. Chisholm, M.H.; Folting, K.; Huffman, J.C.; Kirkpatrick, C.C. J. Am. Chem. Soc. 1981, 103, 5967. 24. Brandt, B.G.; Skapski, A.G. Acta. Chem. Scand. 1967, 21, 661. 25. Waltersson, K. Acta. Cryst. 1976, B32, 1485. 26. Chisholm, M.H.; Cotton, F.A.; Murillo, C.A.; Reichert, W.W. Inorg. Chem. 1977, 16, 1801. 27. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Reichert, W.W. J. Am. Chem. Soc. 1978, 100, 153. 28. Akiyama, M.; Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Haitko, D.A.; Little, D.; Fanwick, P.E. Inorg. Chem. 1979, 18, 2266. 29. Chisholm, M.H.; et. al. Results to be published. 30. Akiyama, M.; Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Haitko, D.A.; Leonelli, J.; Little, D. J. Am. Chem. Soc. 1981, 103, 779.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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16.

CHiSHOLM

Metal

Alkoxides

31. Chisholm, M.H.; Huffman, J.C.; Kirkpatrick, C.C. J. Chem. Soc., Chem. Commun. 1982, 189. 32. Appleton, T.G.; Clark, H.C.; Manzer, L.M. Coord. Chem. Rev. 1973, 10, 335. 33. Kubacek, P.; Hoffmann, R. J. Am. Chem. Soc. 1981, 103, 4320. 34. Templeton, J.L.; Winston, P.B.; Ward, B.C. J. Am. Chem. Soc. 1981, 103, 7713. 35. Chisholm, M.H.; Huffman, J.C.; Kelly, R.L. J. Am. Chem. Soc. 1979, 101, 7615. 36. Chisholm, M.H.; Huffman, J.C.; Rothwell, I.P.; Bradley, P.G.; Kress, N.; Woodruff, W.H. J. Am. Chem. Soc. 1981, 103, 4945. 37. Chisholm, M.H.; Haitko, D.A. J. Am. Chem. Soc. 1979, 101, 6784. 38. Chisholm, M.H.; Chetcuti, M.J.; Haitko, D.A.; Huffman, J.C. J. Am. Chem. Soc. 1982, 104, 2138. 39. Chisholm, M.H.; Kirkpatrick, C.C.; Huffman, J.C. Inorg. Chem. 1981, 20, 871. 40. Chisholm, M.H.; Folting, K.; Huffman, J.C.; Ratermann, R.L. J. Chem. Soc., Chem. Commun. 1981, 1229. 41. Chisholm, M.H.; Huffman, J.C.; Kirkpatrick, C.C. Results to be published. 42. Chisholm, M.H.; Cotton, F.A; Extine, M.W.; Kelly, R.L. J. Am. Chem. Soc. 1979, 101, 7645. 43. Chisholm, M.H.; Huffman, J.C.; Leonelli, J.; Rothwell, I.P. J. Am. Chem. Soc., submitted. 44. Chisholm, M.H.; Cotton, F.A.; Extine, M.W.; Stults, B.R. Inorg. Chem. 1976, 15, 2252. 45. Chisholm, M.H.; Huffman, J.C.; Rothwell, I.P. J. Am. Chem. Soc. 1982, 104, xxxx. 46. Chisholm, M.H.; Huffman, J.C.; Rothwell, I.P. J. Am. Chem. Soc. 1981, 103, 4245. 47. Cotton, F.A.; Jamerson, J.D.; Stults, B.R. J. Am. Chem. Soc. 1976, 98, 1774. 48. Bailey, W.I. Jr.; Chisholm, M.H.; Cotton, F.A.; Rankel, L.A. J. Am. Chem. Soc. 1978, 100, 5764. 49. Ginley, D.S.; Bock, C.R.; Wrighton, M.S.; Fischer, B.; Tipton, D.L.; Bau, R. J. Organometal. Chem. 1978, 157, 41. 50. Muetterties, E.L.; Day, V.W.; Abdel-Meguid, S.S.; Debastini, S.; Thomas, M.G.; Pretzer, W.R. J. Am. Chem. Soc. 1976, 98, 8289. 51. Kutchitsu, K. J. Chem. Phys. 1966, 44, 906. 52. Chisholm, M.H.; Huffman, J.C.; Leonelli, J. Results to be published. 53. Hock, Α.Α.; Mills, O.S. Acta. Crystallogr. 1961, 14, 139. 54. Katz, T.J.; Lee, S.J. J. Am. Chem. Soc. 1980, 102, 422. 55. Knox, S.A.R.; Dyke, A.F.; Finnimore, S.R.; Naish, P.J.; Orpen, A.G.; Riding, G.H.; Taylor, G.E. ACS Symp. Ser. 1981, 155, 259. 56. Levisalles, J.; Rose-Munch, F.; Rudler, H.; Daran, J.C.; Dromzee, Y.; Jeannin, Y. J. Chem. Soc., Chem. Commun. 1981, 152.

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57. Chisholm, M.H.; Clark, H.C. Acc. Chem. Res. 1973, 6, 202. 58. [Ti (OPh) H]: Flamini, Α.; Cole-Hamilton, D.J.; Wilkinson, G. J. Chem. Soc., Dalton Trans. 1978, 454. 59. [Ti (OEt) H]: Sabo, S.; Ge rvais, D.C.R. Comptes Rend. Acad. Sci., Paris, Ser. C 1980, 291, 207. 60. Chisholm, M.H.; Leonelli, J. Results to be published. 61. Rocklage, S.M.; Fellmann, J.D.; Rupprecht, G.A.; Messerle, L.W.; Schrock, R.R. J. Am. Chem. Soc. 1981, 103, 1440. 62. Besecker, C.J.; Klemperer, W.G.; Day, V.W. J. Am. Chem. Soc. 1980, 102, 7598. 63. Day, V.W.; Fredrich, M.F.; Thompson, M.R.; Klemperer, W.G.; Liu, R.S.; Shum, W. J. Am. Chem. Soc. 1982, 103, 3597. 64. Day, V.W.; Fredrich, M.F.; Klemperer, W.G.; Liu, R.S. J. Am. Chem. Soc. 1979, 101, 491. 65. Knoth, W.H. J. Am. Chem. Soc. 1979, 101, 759. 66. Knoth, W.H. J. Am. Chem. Soc. 1979, 101, 2211. 67. Knoth, W.H.; Harlow, R.L. J. Am. Chem. Soc. 1982, 103, 4265. RECEIVED July 26, 1982 4

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CHEMISTRY:

13

Discussion F. A. Cotton, Texas A&M University: In connection with the very nice results just presented by Dr. Chisholm, I would like to mention work done by Dr. Willi Schwotzer in my laboratory. He has carried out a related but different reaction, namely 1

1

W (OPr ) py + CO • [W (OPr ) (C0)py] 2

6

2

2

6

2

The product is a centrosymmetric "dimer of dimers" each half of which is (py)(Pr 0) W(y-0Pr )(y-CO)W(OPr ) , with W-W = 2.654(1)A. These are linked by bonds from the μ-CO oxygen atom of one to the pyridine-bearing W atom of the other, so that two W—W ÏÏ ο 1

1

2

1

3

units are present. I know of no precedent for this type of n -y -C0 group. 2

3

A.R. Siedle, 3M Central Research Laboratory: How may these novel polynuclear metal alkoxides be studied so as to contri­ bute further to our understanding of chemical reactions which occur on the surfaces of bulk oxides? M.H. Chisholm: No comment.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

17 Dimolybdenum Versus Ditungsten Alkoxides RICHARD A. WALTON

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch017

Purdue University, Department of Chemistry, West Lafayette, IN 47907

In the preceding paper, Malcolm Chisholm (1) has presented a cogent case for the modeling by metal alkoxides of certain aspects of the structural chemistry and reactivities of metal oxides. The focus of this work has been the dinuclear and polynuclear alkoxides of molybdenum and tungsten, an area of research which has also attracted our interest (2-4) and upon which I would now like to take this opportunity to comment. In the case of the triply bonded unbridged alkoxides M(OR), where M = Mo when R = t-Bu, i-Pr or CH-t-Bu, and M = W when R = t-Bu, (1, 5-7), the ditungsten derivative W(O-t-Bu) is much less thermally stable than Mo(OR). Furthermore, attempts to prepare other complexes of the type W(OR), where R represents a bulky alkyl group, through the reactions of W(NMe)6 with the appropriate alcohol leads to facile oxidation to tungsten(IV). This is shown, for example, in the conversion of W (NMe2)6 to W4(O-i2

6

2

2

2

6

6

2

6

2

2

2

Pr)14H ( c o n t a i n i n g W=W bonds) (1,8), a r e a c t i o n which corresponds formally to an o x i d a t i v e a d d i t i o n . Another important example is the formation o f t e t r a n u c l e a r W4(OR)16 upon r e a c t i n g W (NMe )6 with methanol o r ethanol (1,9). In the case of the dimolybdenum Mo (OR)6 complexes, o x i d a t i v e a d d i t i o n s r e q u i r e somewhat more potent reagents, as in the conversions o f Mo (O-i-Pr)6 to doubly bonded M o ( 0 - i - P r ) by i-PrOO-i-Pr and to Mo Xi+(y-0-i -Pr) (0-iPr) i+ by the halogens X (1,10) . Q u a l i t a t i v e l y at l e a s t , the preceding observations h i n t a t an increased s t a b i l i t y of the higher v a l e n t ditungsten alkoxides over r e l a t e d dimolybdenum s p e c i e s , a trend which c o r r e l a t e s with the i n c r e a s i n g s t a b i l i t y o f higher o x i d a t i o n s t a t e s as a t r a n s i t i o n group i s descended. Our own entry i n t o t h i s area stemmed from studies we made i n t o the r e a c t i o n s between a l c o h o l - HCl(g) mixtures and the quadruply bonded complexes M2(mhp)i , where M = Mo or W and mhp i s the anion o f 2-hydroxy-6-methylpyridine. In the presence of CsCl such r e a c t i o n s give Csi Mo2Cls and CS3W2CI9 ( 2 ) , r e f l e c t i n g the greater ease of o x i d i z i n g the W2 " " core. When the t e r t i a r y phosphines P E t 3 or P-n-Pr3 are added i n place o f C s C l , each dimetal system i s o x i d i z e d one step f u r t h e r , Mo2(mhp)i 2

2

2

2

2

2

8

2

L

2

2

+

f

4

1

+

0097-6156/83/0211-0269$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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a f f o r d i n g ( R P H ) M o 2 C l H (11), a member of an already w e l l char­ a c t e r i z e d c l a s s of μ-hydrido bridged dimolybdenum(III) complexes (7), while W (mhp)i produces the dark green ditungsten(IV) alkox­ ides W Cl (y-0R)2(0R) (R0H)2(2-4) · The l a t t e r molecules possess a W=W bond represented by a σ^π " ground s t a t e e l e c t r o n i c c o n f i g u r ­ a t i o n ( 4 ) . X-ray s t r u c t u r e determination on the methoxide and ethoxide show that these complexes possess s t r u c t u r e ^, the short 3

3

2

2

8

+

i+

2

2

c l

\l/°\l/ W = W

V

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch017

c l

W-W d i s t a n c e (2.48A) r e f l e c t i n g the double bond c h a r a c t e r . Oxida­ t i o n of 1 by 0 , H 0 , N0 or Ag(I) s a l t s gives dark red complexes ( s t r u c t u r e ^ ) , which are the ditungsten(V) analogs of ^ (W-W d i s 2

2

2

2

R

ci

o Ι

o .ο. I

R

R

m

^ W ^ - ^ J J ^

R°

2

R

°R Ο tance of 2.72A)(3). In a d d i t i o n to the d i f f e r e n c e i n W-W bond orders, 1 and 2 d i f f e r i n that the former possesses two a l k o x i d e and two a l c o h o l l i g a n d s whereas i n £ there are now four t e r m i n a l a l k o x i d e s . As a r e s u l t , £ does not possess the unsymmetrical hydrogen bond that e x i s t s between the adjacent a l k o x i d e and a l c o ­ h o l l i g a n d s i n ^L, v i z .

In c o n t r a s t to the remarkable thermal and a i r s t a b i l i t y of ^ and the dimolybdenum(V) analogues of Mo Xi (y-0-i^-Pr) ( 0 - i Pr) 1+ (X = CI or Br) are thermally unstable and very moisture sen­ s i t i v e (10). I n t e r e s t i n g l y , molybdenum analogues of ^ are unknown although the c l o s e l y r e l a t e d M o X ( 0 - i - P r ) g may be formed as a very unstable intermediate i n the o x i d a t i o n of M o ( 0 - i - P r ) 5 by halogens ( 1 0 ) , and the dimolybdenum(IV) complex Mo (0-i_-Pr) q i s a very w e l l c h a r a c t e r i z e d species ( 1 , 1 0 ) . Whereas Μ ο Χ ^ ( u - O - i - P r ) 2

2

+

2

2

2

2

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2

17.

WALTON

Dimolybdenum

vs. Ditungsten

271

Alkoxides

(O-i-Pr)4 decompose even at room temperature (to evolve i s o p r o p y l h a l i d e s ) , high temperatures are r e q u i r e d to decompose ditungsten complexes of types ^ and 2, and p r e l i m i n a r y experiments i n d i c a t e that they may decompose by pathways d i f f e r e n t from the isopropoxide complexes of dimolybdenum(V). For example, the ditungsten isopropoxides of types ^ and 2 decompose at 160° i n vacuum to give propene but l i t t l e p r o p y l c h l o r i d e (12). In the case of the n-propoxide of d i t u n g s t e n ( I V ) , W C l i ( y - 0 - n - P r ) ( 0 - n - P r ) ( n - P r O H ) , d i - n - p r o p y l e t h e r i s the p r i n c i p a l v o l a t i l e (12). The doubly bonded complexes V ^ C l i ^ p - O R ) ( 0 R ) ( R 0 H ) are d i f f erent from the f u l l y s u b s t i t u t e d tungsten(IV) a l k o x i d e s W^iOR)^ (1,9), i n that the l a t t e r 8 - e l e c t r o n c l u s t e r e x h i b i t s a c o n s i d e r able degree of delocâlized W-W bonding, the s h o r t e s t W-W d i s t a n c e (2.65Â) being much longer than the W=W bond (2.48A) i n W C l i (y-0R) (0R) (R0H) (4). The p o s s i b i l i t y of c o n v e r t i n g W Cli (y-0R) (0R) (R0H) to t e t r a n u c l e a r Wi^OR)^ by s u b s t i t u t i n g OR" f o r C I " i s being pursued (12). In the course of t h i s work we have studied the a l c o h o l exchange r e a c t i o n s o f the ethoxide W C l i ( p - 0 E t ) ( 0 E t ) ( E t O H ) . Comp l e t e exchange occurs i n r e a c t i o n s with primary a l c o h o l s ROH (R = Me, n-Pr, η-Bu, n-Pent, η-Oct or i-Bu) to give W C l 4 ( y - O R ) ( 0 R ) (R0H) (12,13). On the other hand, the more s t e r i c a l l y demanding secondary a l c o h o l s R'OH (R' = i - P r , s-Bu or £-Pent) a f f o r d the mixed a l k o x i d e s W C l i ( y - 0 E t ) ( 0 R ) ( R O H ) (12). X-ray s t r u c t u r e determinations on the d e r i v a t i v e s where R' = i - P r or s-Pent (14) show that these complexes, while s t i l l possessing the same general type of s t r u c t u r e as depicted i n ^, now contain a symmetrical hydrogen-bond, v i z . 2

+

2

2

2

2

2

2

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch017

2

2

+

2

2

+

2

2

t

2

2

2

2

2

2

2

2

2

f

2

+

R

f

2

2

2

1

R \

.--H»„

f

/

o—

w—

w

Thus the hydrogen-bonding i s apparently dependent, amongst other f a c t o r s , upon the nature of the a l k y l s u b s t i t u e n t s . Perhaps the more h i g h l y branched secondary a l k y l s u b s t i t u e n t s increase the hydrophobic environment about the hydrogen bond, thereby f a v o r i n g a symmetrical i n t e r a c t i o n . In any event, i t i s apparent that t h i s i n t e r a c t i o n i s best described by a very broad, shallow p o t e n t i a l function. The hydrogen-bonding i n t e r a c t i o n s w i t h i n the complexes W Clit( y - 0 R ) ( 0 R ) ( R 0 H ) and W Cli (μ-OR) ( 0 R ) ( R OH) may provide the molecular analogues with which to model the s t r u c t u r e and r e a c t i ­ v i t i e s of t r a n s i t i o n metal oxide c a t a l y s t s that possess surface hydroxyl groups. The thermal treatment which i s o f t e n c a r r i e d out i n the pretreatment of metal oxides ( l e a d i n g to the l o s s of -OH 2

T

2

2

2

2

+

2

f

2

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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T H E 21ST

groups and the e v o l u t i o n of H2O) may have i n t e r e s t i n g i n the thermolysis of the ditungsten a l k o x i d e s .

CENTURY

analogies

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch017

Literature Cited

1. Chisholm, M. H. preceding paper in this symposium. 2. DeMarco, D.; Nimry, T.; Walton, R. A. Inorg. Chem., 1980, 19, 575. 3. Cotton, F. Α.; DeMarco, D.; Kolthammer, B.W.S.; Walton, R. A. Inorg. Chem., 1981, 20, 3048. 4. Anderson, L. B.; Cotton, F. Α.; DeMarco, D.; Fang, Α.; Ilsley, W. H.; Kolthammer, B.W.S.; Walton, R. A. J. Am. Chem. Soc., 1981, 103, 5078. 5. Chisholm, M. H.; Cotton, F. Α.; Murillo, C. Α.; Reichert, W. W. Inorg. Chem., 1977, 16, 1801. 6. Akiyama, Α.; Chisholm, M. H.; Cotton, F. Α.; Extine, M. W.; Haitko, D. Α.; Little, D.; Fanwick, P. E. Inorg. Chem., 1979, 18, 2266. 7. Cotton, F. Α.; Walton, R. A. "Multiple Bonds Between Metal Atoms", J. Wiley and Sons, 1982, and references therein. 8. Akiyama, M.; Chisholm, M. H.; Cotton, F. Α.; Extine, M. W.; Haitko, D. Α.; Leonelli, J.; Little, D. J. Am. Chem. Soc., 1981, 103, 779. 9. Chisholm, M. H.; Huffman, J. C.; Kirkpatrick, C.C.;Leonelli, J.; Folting, K. J. Am. Chem. Soc., 1981, 103, 6093. 10. Chisholm, M. H.; Kirkpatrick, C. C.; Huffman, J. C. Inorg. Chem., 1981, 20, 871. 11. Pierce, J. L.; DeMarco, D.; Walton, R. Α., submitted for publication. 12. DeMarco, D.; Walton, R. A. unpublished results. 13. Clark, P. W.; Wentworth, R.A.D. Inorg. Chem., 1969, 8, 1223. 14. Cotton, F. Α.; DeMarco, D.; Walton, R. A. unpublished results. RECEIVED October 8, 1982

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

18 Metal Clusters and Extended Metal-Metal Bonding in Metal Oxide Systems ROBERT E. McCARLEY

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

Iowa State University, Ames Laboratory and Department of Chemistry, Ames, IA 50011

The results of recent investigations of the synthesis and structures of ternary and quater­ nary molybdenum oxide compounds having metal­ -metal bonded cluster units or infinite chains are presented. In a l l cases the average oxi­ dation state of molybdenum is less than 4+, the lowest state previously known in structurally characterized oxide phases. New compounds con­ taining discrete clusters are LiZn Mo O and Ζn Μo O , with triangular units, and Ba . Mo O16 with two variants of rhomboidal tetranuclear units, one having regular and the other distorted rhomboidal geometry. Infinite metal-metal bonded chains consisting of Μο octahedral cluster units fused on opposite edges are found in the new compounds NaMo O , Ba (Mo O ) , Sc . Zn . Mo O , and Ti . Zn . Mo O . The ef­ fects of the differing metal-based electron counts in the repeat units of the infinite chains among these compounds are discussed with refer­ ence to metal-metal bond order and structural variations within the units. 2

3

3

3

8

8

1 14

8

6

4

0 75

1 25

4

7

6

0 5

4

5

1 5

4

6 8

7

Metal clusters in metal oxide systems have not been wellcharacterized or abundantly investigated up to the present time. Only isolated examples of metal-metal bonded units in oxide lattices have appeared from time to time. It will be the thesis of this presentation to show that highly unusual structures determined by strong metal-metal bonding will be found in ternary and quaternary metal oxide systems, and that oppor­ tunities abound for creative work on the synthesis, theory and structure-property relationships of such compounds. Because of the well-known correlation of d-electron population and d-orbital radial extension with metal-metal bond formation, 0097-6156/83/0211-0273$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

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we may expect that oxide systems c o n t a i n i n g the e a r l y 4d and 5d t r a n s i t i o n elements i n reduced o x i d a t i o n s t a t e s w i l l provide the most f e r t i l e area f o r new i n v e s t i g a t i o n s . We emphasize here our recent work with the reduced ternary oxides of molybdenum, but expect that r e l a t e d chemistry w i l l be observed f o r oxides of Nb, Ta, W, Tc and Re. The most f a m i l i a r examples of metal-metal bonding i n oxide compounds are found i n the d i o x i d e s , VO2, Nb02, M0O2> WO2 and a-Re02 > a l l of which adopt low-symmetry v a r i a n t s of the r u t i l e s t r u c t u r e ÇL-4). In each case the metal atoms occur i n chains of edge-shared ΜΟβ octahedra running p a r a l l e l to the r u t i l e ca x i s . Displacement of each metal atom towards one and away from the other of i t s nearest-neighbor metal atoms along the chain accounts f o r the a l t e r n a t e short and long d i s t a n c e s between metal atoms. The metal-metal bonds thus e s t a b l i s h e d may be r e ­ garded as s i n g l e (VO2> NDO2) or double (M0O2» WO2 and a-Re02) (4). With the edge-shared b i o c t a h e d r a l arrangement of oxide l i g a n d s only 4 e l e c t r o n s may populate M-M bonding o r b i t a l s i n the c o n f i g u r a t i o n σ π (5). A d d i t i o n a l e l e c t r o n s then populate nonbonding (M-M) or antibonding (M-0) o r b i t a l s of 6 and 6 symmetry with respect to the M-M bond a x i s 0 5 , 6 ) . Thus, though 6 e l e c t r o n s r e s i d e i n metal-centered o r b i t a l s , the Re-Re bond i n a-Re02 i s best regarded as a double bond, with two a d d i t i o n a l e l e c t r o n s i n the δ*, e s s e n t i a l l y nonbonding o r b i t a l . The com­ pounds M0O2 and WO2 e x h i b i t m e t a l l i c c o n d u c t i v i t y because of overlap of the f i l l e d M-M π and empty M-0 π* valence bands, which then provide a p a r t i a l l y f i l l e d conduction band (4). More r e c e n t l y metal-metal doublets with m u l t i p l e bond character have been found i n Lai+ReeOig 05), La30s20io (7) and L a R e 0 (8). In the f i r s t two cases the M-M bonds, e s t a b l i s h e d i n an edgeshared b i o c t a h e d r a l arrangement, are a t best considered as double bonds, with e x t r a e l e c t r o n s consigned to M-0 π* del o c a l i z e d o r b i t a l s . The doublets of Re atoms i n La Re 0 o con­ s i s t of Re 0e u n i t s l i k e that of R e C l 8 ~ , i . e . two ReO^ f r a g ­ ments h e l d together i n an e c l i p s e d c o n f i g u r a t i o n by a strong metal-metal bond. Here the Re atoms share 6 e l e c t r o n s i n a t r i p l e bond with the c o n f i g u r a t i o n σ τΛ, and the e c l i p s e d con­ formation of the Re 0s u n i t i s imposed by the arrangement of oxygen atoms i n the l a t t i c e . 2

2

i f

2

lt

2

1 0

2

1

2

2

2

We n o t i c e that i n these examples o f t e r n a r y oxides the oxygen/metal (O/M) r a t i o i s h i g h . T h i s f o s t e r s small metalmetal bonded u n i t s because an octahedral arrangement of 0 atoms about each metal atom can be a t t a i n e d with a minimum of shared atoms, e.g. only two shared 0 atoms i n the O s 0 i o u n i t s o f L a 0 s 0 i o . I t i s reasonable to expect t h a t , as the O/M r a t i o i s decreased, higher degrees of aggregation w i l l ensue i n order f o r the metal atom to a t t a i n higher c o o r d i n a t i o n numbers. For a given electron/metal (e/M) r a t i o l a r g e r c l u s t e r u n i t s with M-M bonds of low bond order then w i l l be favored over the smal­ l e r u n i t s with M-M bonds of higher order. The best known 2

3

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

18.

Metal Oxide

MCCARLEY

Systems

275

examples showing e f f e c t s of t h i s kind are found i n metal h a l i d e systems, e.g. presumed dimers i n WC1J+, trimers i n N b C l ( 9 ) , and hexamers i n Z r C l i 2 (10). 3

8

6

Oxides with T r i n u c l e a r C l u s t e r s The f i r s t compound c o n t a i n i n g a w e l l - c h a r a c t e r i z e d c l u s t e r i n a metal oxide system was Ζ η Μ θ 3 θ , reported i n 1957 by M c C a r r o l l , Katz and Ward (11,12). In t h i s compound the M o 0 c l u s t e r u n i t s are constructed such that three M0O5 octahedra share common edges, and share 0 atoms with adjacent c l u s t e r u n i t s as r e f l e c t e d i n the c o n n e c t i v i t y formula Ζ η Μ ο 0 ^ 0 / Ο3/3· Thus the true c l u s t e r u n i t i s of the type M X as shown i n Figure 1. Although the e/Mo r a t i o i s the same as i n MoO^, a l l e l e c t r o n s are u t i l i z e d i n s i n g l e bonds i n the Mo 0 ~ c l u s t e r as opposed to the double bond i n the Mo dimers of Mo0 . Pos­ s i b l y t h i s d i f f e r e n c e i s d i c t a t e d by the higher 0 / t o t a l metal r a t i o i n Mo0 as opposed to Ζ η Μ ο 0 . ^_ Other compounds c o n t a i n i n g the M o 0 c l u s t e r u n i t s have been prepared as members of two s e r i e s . The f i r s t s e r i e s i s of the type M M o 0 (11) with M = Zn, Cd, Mg, Mn, Fe, Co and N i , and the second s e r i e s i s of the type LiM- ^- Mo 03 (13, 14,15) with M i l l = Ga, In, Se, Y, and r a r e e a r t h ions having atomic numbers greater than 62(Sm). The s t r u c t u r a l arrangement i n c r y s t a l s of the two s e r i e s however i s very s i m i l a r . In each case one-half of the c a t i o n s occupy t e t r a h e d r a l s i t e s and oneh a l f occupy octahedral s i t e s w i t h i n a close-packed oxide l a t t i c e . I n d i c a t i n g these as M° f o r ions i n octahedral s i t e s and M f o r those i n t e t r a h e d r a l s i t e s , we have the formulas Μ Μ ° Μ ο 0 and Li M°Mo 0e f o r the two s e r i e s . Our work was i n i t i a t e d on the reduced ternary molybdenum oxides with the thought that the metal c l u s t e r e l e c t r o n count (MCE) should be v a r i a b l e f o r the M o 0 c l u s t e r u n i t s . Based on Cotton's previous molecular o r b i t a l treatment of such c l u s t e r s (16) i t appeared that MCE s from 6 to 8 could be ac­ commodated, but i t was not c l e a r whether the seventh and e i g h t h e l e c t r o n s would occupy bonding or antibonding o r b i t a l s with respect to the M-M i n t e r a c t i o n s . We thus set about to determine t h i s from s t r u c t u r a l data on s u i t a b l e compounds. The attempted replacement of Z n with S c to secure the compound Zn Sc°Mo 03 conducted v i a the r e a c t i o n shown i n equation 1. 2

8

3

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

3

3

8

2

1 3

3

8

2

2

2

3

I I

2

8

3

2

8

8

1 1

3

8

t

c

3

fc

1:

3

8

t

3

3

8

1

2 +

w

a

3 +

s

3

2Sc 0 2

3

+ 4Zn0 + H M o 0

2

+ Mo

• 4ZnScMo 0 3

8

(1)

1100° X-ray powder patterns showed that the product of t h i s r e a c t i o n i s indeed isomorphous with Z n M o 0 and hence i s the d e s i r e d 7e l e c t r o n c l u s t e r d e r i v a t i v e . Unfortunately s i n g l e c r y s t a l s f o r a complete s t r u c t u r e determination have not been obtained. Sub­ sequent work (17) however showed that a d d i t i o n a l c a t i o n s could 2

3

8

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

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INORGANIC C H E M I S T R Y : TOWARD T H E 2 1 S T C E N T U R Y

Figure 1.

The M X cluster type as found in the oxides Zn Mo 0 , and Zn Mo 0 . (See Table I for atom numbering scheme.) 3

1S

3

2

3

3

8

LiZn Mo 0 ,

8

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2

3

8

18.

MCCARLEY

277

Metal Oxide Systems

be introduced i n t o the l a t t i c e of a modified Z n M o 0 v i a high temperature r e a c t i o n s , equations 2 and 3. 2

L^MoO^ + 4ZnO + 4Mo0

2

+ IMo

3

2LiZn Mo 0 2

3

structure

8

(2)

8

1100° 6ZnO + 5Mo0

+ Mo

2

• 2Zn Mo 0 3

3

(3)

8

1100° Structure determinations of L i Z n M o 0 and Z n M o 0 (18) showed that these compounds are isomorphous but the p a t t e r n of o x i d e - l a y e r s t a c k i n g i s d i f f e r e n t from that of Z n M o 0 . Other­ wise the s t r u c t u r e s are c l o s e l y r e l a t e d and a l l contain the c l u s t e r u n i t s w i t h the same c o n n e c t i v i t y , [Μο 0^0 / 0 / ] ~. Thus we may compare s t r u c t u r a l data f o r Z n M o 0 (6MCE) with those f o r LiZn Mo 0 (7MCE) and Zn Mo 0 (8MCE) to d i s c e r n the e f f e c t s of adding successive e l e c t r o n s to the Mo 0^~ c l u s t e r u n i t . A comparison of Mo-Mo and Mo-O bond d i s t a n c e s i s given i n Table I f o r these compounds. I t i s c l e a r that a d d i t i o n of ο Table I. Comparison of Mo-Mo and Mo-O bond d i s t a n c e s (A) i n Z n M o 0 , L i Z n M o 0 and Z n M o 0 . 2

3

8

3

3

2

3

8

8

n

3

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

2

2

3

8

3

3

3

8

2

3

3

8

8

3

2

3

8

2

3

8

3

3

8

Zn Mo 0

Bond

Zn Mo 0

Mol-Mol

2.524(2)

2.578(1)

2.580(2)

Mol-01

2.058(10)

2.063(6)

2.100(9)

Mol-02

1.928(20)

2.003(8)

2.056(13)

Mol-03

2.128(30)

2.138(5)

2.160(8)

Mol-04

2.002(30)

2.079(7)

2.054(11)

Mo 1-0 (aver.)

2.017

2.058

2.088

2

3

a 8

LiZn Mo 0 2

3

3

8

3

8

Data taken from reference (12). e l e c t r o n s to the M o 0 c l u s t e r causes a net e l o n g a t i o n of both the Mo-Mo and Mo-O bonds. T h i s may be taken as evidence that the seventh and e i g h t h MCE's enter a molecular o r b i t a l which i s antibonding with respect to boçh Mo-Mo and Mo-O i n t e r a c t i o n s . Among the Mo-O bonds, those i n v o l v i n g the three-edge-bridging 0 atoms are the most s t r o n g l y a f f e c t e d . I t t h e r e f o r e appears that the set of three d - o r b i t a l s not involved i n e i t h e r Mo-O or Mo-Mo σ-bonding mix p r i m a r i l y with the set of three ρπ o r b i t a l s l o c a t e d on the edge-bridging 0 atoms to form a set of 3 bonding and 3 antibonding M0 s. The bonding set (a± + e) i s populated by e l e c t r o n s from the 0 atom lone p a i r s , and these probably l i e below the Mo-Mo σ-bonding o r b i t a l s ; the antibonding set ( a + e*) would then c o n s t i t u t e the LUM0 s of Z n M o 0 . Magnetic s u s c e p t i b i l i t y data f o r L i Z n M o 0 showed Curie-Weiss 3

8

f

f

x

2

2

3

3

8

8

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

278

INORGANIC C H E M I S T R Y : TOWARD T H E 21ST

CENTURY

behavior over the temperature range 95-298 Κ with a very l a r g e Weiss constant θ = -545°. At 298 Κ, μ = 1.18 μ_, a low err D value caused by strong a n t i f e r r o m a g n e t i c c o u p l i n g . Magnetic measurements on Z n M o 0 , while l e s s d e t a i l e d , show only a weak paramagnetism, p f f = 0.6 μ , probably i n d i c a t i v e of Van Vleck paramagnetism. We thus i n f e r that the e l e c t r o n spins are p a i r e d i n Z n M o 0 , and that the a* l e v e l l i e s below the e* l e v e l . r

3

3

8

e

3

3

β

8

Oxides with T e t r a n u c l e a r C l u s t e r s In the s t r u c t u r e refinement of L i Z n M o 0 the L i ions were not l o c a t e d because of t h e i r apparent scrambling with Zn ions i n both the t e t r a h e d r a l and o c t a h e d r a l s i t e s . An e f f o r t to overcome t h i s d i f f i c u l t y was made with the attempted s y n t h e s i s of NaZn Mo 0 , where i t was expected that the N a ions would be confined only to the o c t a h e d r a l s i t e s . T h i s attempted s y n t h e s i s l e d i n s t e a d to the formation of the new compound NaMo^Og (19) which grew i n the r e a c t i o n mixture and on the w a l l of the moly­ bdenum container as t h i n needles with s i l v e r y m e t a l l i c l u s t e r . The s e r e n d i p i t o u s d i s c o v e r y of t h i s compound has proven to be extremely important to our v i s i o n s of p o s s i b i l i t i e s f o r new metal-metal bonded s t r u c t u r e s i n reduced oxide phases. In r e t r o s p e c t i t i s amazing that oxide phases c o n t a i n i n g molybdenum i n o x i d a t i o n s t a t e s l e s s than 4 were e s s e n t i a l l y unknown and c e r t a i n l y s t r u c t u r a l l y uncharacterized. The e x i s t e n c e of the p r e v i o u s l y mentioned s e r i e s M J M O 0 and L i M M o 0 should have been a t i p - o f f to an extensive chemistry f o r metal-metal bonded molybdenum oxide systems. Indeed, subsequent work has revealed a p l e t h o r a of new compounds a l l of which (where s t r u c t u r e has been determined) f e a t u r e strong metal-metal bonding i n e i t h e r d i s c r e t e c l u s t e r u n i t s or extended chain a r r a y s . +

2

3

8

+

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

2

3

8

+

I

I I I

3

8

3

8

A chart of these s t r u c t u r a l l y c h a r a c t e r i z e d compounds i s given i n Table I I , showing the c o r r e l a t i o n of average o x i d a t i o n s t a t e of the molybdenum with c l u s t e r formation or extended arrays. There are two notable e f f e c t s which are p e r t i n e n t to the nature of the metal-metal bonding, namely the e l e c t r o n / m e t a l (e/M) r a t i o and the oxygen/metal (O/M) r a t i o . With other f a c t o r s constant an increase i n e/M r a t i o should promote more extensive M-M bonding, and a decrease i n the O/M r a t i o should promote c l u s t e r s of i n c r e a s i n g n u c l e a r i t y , p r o g r e s s i n g from dimers through l a r g e r c l u s t e r s to systems with extended chains, sheets or 3-dimensional arrays (20, 21). The i n t e r e s t i n g compound Bai ii+Mo 0 (17,22) contains t e t r a n u c l e a r rhomboidal c l u s t e r u n i t s , t i e d i n t o i n f i n i t e chains by Mo-O-Mo bonding as represented i n the c o n n e c t i v i t y M o 0 0 / 0 / . The Ba + ions are stacked i n s i t e s along tunnels formed by weaving together the [MO^OQ ] chains, as shown i n Figure 2. In t h i s low-symmetry, metal-metal bonded adaptation of the well-known h o l l a n d i t e s t r u c t u r e the chains forming the four s i d e s of each tunnel are r e l a t e d i n p a i r s by Ρ 1 symmetry. 8

18

2

I+

2

8

2

6

3

qq

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

18.

MCCARLEY

Table I I .

C h a r a c t e r i s t i c s of metal-metal bonded ternary and quaternary molybdenum oxides. O.N.

Compound

a e /

Structural feature

Mo

3.72

2.28

2.0

rhomboidal c l u s t e r s

LiZn2Mo 0e

3.67

2.33

2.67

triangular

clusters

Zn Mo 0

3.33

2.67

2.67

triangular

clusters

2.75

3.25

1.50

i n f i n i t e chains

2.69

3.31

1.50

i n f i n i t e chains

1.75

i n f i n i t e chains

1.75

i n f i n i t e chains

Ba

Mo

l.l*+ 8°16 3

3

3

8

NaMoi^Oe Ba (Moi 0 ) 5

t

6

8

SCo . 7 5Ζηι . 25ΜΟ4Ο7 Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

279

Metal Oxide Systems

2.31

3.69

Tio.sZni.sMoitOy

?

Oxidation number (average) of molybdenum Average e l e c t r o n to metal r a t i o f o r metal-metal bonding Oxygen to molybdenum r a t i o

Figure 2. A three-dimensional perspective of the structure of Ba , Mo 0 as viewed down the unique axis parallel to the direction of chain growth and tunnel formation. 1A t

s

16

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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INORGANIC C H E M I S T R Y : TOWARD T H E 2 1 S T C E N T U R Y

In one p a i r of chains the rhomboidal c l u s t e r u n i t s have f i v e Mo-Mo bonds of n e a r l y equal length ( r e g u l a r u n i t s ) , and i n the other p a i r the ( d i s t o r t e d ) rhomboidal u n i t s have three short and two long Mo-Mo bonds. Figure 3 shows how these rhomboidal u n i t s are bound together w i t h i n the i n f i n i t e chains. A comparison of Mo-Mo and s e l e c t e d Mo-0 bond distances f o r the r e g u l a r and d i s t o r t e d c l u s t e r u n i t s i s given i n Table I I I . Table I I I .

Selected bond d i s t a n c e s (A) w i t h i n the d i s t o r t e d and r e g u l a r c l u s t e r u n i t s of B a i . n*Mo 0i6. 8

Atoms

Distorted Cluster

Mol-Mo2 Mol-Mo2 Mo2-Mo2

2.847(1) 2.546(1) 2.560(1)

2.616(1) 2.578(1) 2.578(1)

Mol-03 ï Mol-08 \ edge Mol-04 J

1.931(6) 1.894(6) 2.079(6)

1.936(6) 2.022(6) 2.143(6)

Mol-01 Ï Mol-02 I capping ΜΟ1-02Ί

2.082(6) 2.046(6) 2.104(6)

2.053(6) 2.055(6) 2.095(6)

1

1

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

Regular C l u s t e r

See Figure 3 f o r atom numbering scheme. Since the Mol-Mo2 distance i n the d i s t o r t e d u n i t corresponds to a P a u l i n g bond order (23) of ca. 0.5 these bonds may be con­ sidered as one-electron bonds. Assuming the Mol-Mo2 and Mo2-Mo2 bonds are each normal two-electron bonds, there are c a . 8 MCE*s f o r the d i s t o r t e d u n i t s . In the r e g u l a r u n i t s the Mo-Mo d i s t a n c e s i n d i c a t e that these bonds should be considered as normal two-electron bonds and an MCE count of 10 i s _ o b t a i n e d . Thus the compound may be formulated as B a i . 14 (Mo^ol MMoi+Oe' ) to represent t h i s unbalanced e l e c t r o n d i s t r i b u t i o n . As noted i n the previous paper by P r o f e s s o r Chisholm the d i s t o r t e d Μοι^Οβ c l u s t e r u n i t has a r e c e n t l y discovered molecular analog i n Wi^OEOie (24,25) • The d i s t o r t i o n i n these 8-electron rhom­ b o i d a l c l u s t e r s has been a t t r i b u t e d by Cotton and Fang (26) to a second order J o h n - T e l l e r e f f e c t . T h i s seems reasonable f o r the molecular Wit(0Et)i6 but i s questionable f o r the d i s t o r t e d u n i t i n Bai.ι^ΜοβΟιβ. Examination of the Mo-0 d i s t a n c e s i n Table I I I r e v e a l s a much s h o r t e r Mol-08 d i s t a n c e i n the d i s ­ t o r t e d c l u s t e r as compared to that i n the r e g u l a r c l u s t e r . Thus i t appears that there i s a push-pull e f f e c t at work, such that e l e c t r o n density removed from Mol through weakening of the Mol-Mo2 bond i s compensated by increased π-bonding i n the Mol-08 bond of the d i s t o r t e d c l u s t e r . f

f

28

I t appears that other compounds having t h i s s t r u c t u r e type should be p o s s i b l e . In p a r t i c u l a r , by appropriate s u b s t i t u t i o n of B a by c a t i o n s of d i f f e r e n t charge the MCE count should be v a r i a b l e over some range, and conceivably even compounds having more than 10 MCE per c l u s t e r u n i t might be prepared, with ad2 +

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

MCCARLEY

Figure 3. A view of one Mo O cluster chain in Βα^Μο^Ο^ which shows intrachain Mo-Mo and Mo-O bonding. (See Table 111 for atom numbering scheme.) k

%

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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282

d i t i o n a l e l e c t r o n s e n t e r i n g a conduction band. Work along t h i s l i n e i s p r e s e n t l y i n progress i n our l a b o r a t o r y . Oxides with Extended Metal-Metal Bonding The remaining compounds l i s t e d i n Table I I a l l adopt s t r u c t u r e s with i n f i n i t e metal-metal bonded chains c o n s i s t i n g of o c t a h e d r a l c l u s t e r u n i t s fused on opposite edges. However, because of the l a r g e d i f f e r e n c e i n e f f e c t i v e i o n i c r a d i u s of the c a t i o n s concerned, very d i f f e r e n t l a t t i c e types are d i c ­ t a t e d . The compounds NaMoi+Oe (19,22) and Β 3 ( Μ θ ι * 0 ) 8 (17) adopt tunnel s t r u c t u r e s with the N a or B a ions l o c a t e d i n s i t e s along the tunnels with 8 - f o l d c o o r d i n a t i o n by oxygen atoms. In the sodium and barium compounds there are 13 and 13.2 MCE*s per Mo^Oe- u n i t of the c h a i n , r e s p e c t i v e l y . Ordering of the c a t i o n vacancies along the tunnels i n Ba5(Moit06)8 r e s u l t s i n a s u p e r - l a t t i c e with a dimension e i g h t times that of N a M o i ^ along the unique chain (or tunnel) a x i s . Views of the N a M o i ^ (tetragonal) and Β35(Μοι»0β)8 (orthorhombic) s t r u c t u r e s as viewed down the c-axes are shown i n F i g u r e 4. C o n s t r u c t i o n of the [MoitOe]^ chains i s shown i n F i g u r e 5. 5

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

+

6

2 +

The compounds Sco . 7 δΖηχ . 5M04O7 and Tio . s Z n i . 5M04O7 are the most recent a d d i t i o n s to the f a m i l y of reduced, ternary or quaternary oxides (27). Once again they were obtained as unexpected products i n experiments designed f o r another purpose. Attempts to o b t a i n s i n g l e c r y s t a l s of ScZnMo 03 produced only m a c r o c r y s t a l l i n e powders i n the range 1100-1400°C. At vL450°C the l a t t e r compound decomposed and b e a u t i f u l gem-like c r y s t a l s of the new compound were found i n the product mixture. Analyses of s e v e r a l c r y s t a l s f o r Sc, Zn and Mo by electron-microprobe x-ray f l u o r e s c e n c e provided c o n s i s t e n t r e s u l t s and the average atomic r a t i o s Sc:Zn:Mo of 0.75:1.25:4. These data, combined w i t h the subsequent x-ray s t r u c t u r e determination, e s t a b l i s h e d the composition S C Q · ι ^ τ ΐ γ . 2 5 ^ 0 ^ 0 7 . The composition of c r y s t a l s of T i . sZni. 5MOI+07, formed i n a r e a c t i o n between T i 0 , ZnO, Mo0 and Mo at 1450°C, was e s t a b l i s h e d i n l i k e manner. Although these new compounds proved to be isomorphous, d i s t i n c t d i f ­ ferences were found i n the metal-metal bonding c h a r a c t e r i s t i c s , e v i d e n t l y caused by d i f f e r i n g MCE counts i n the repeat u n i t s of the i n f i n i t e chains. Because the o x i d a t i o n s t a t e of T i i n T i . s ^ l · 5 ° t + 0 7 has not been e s t a b l i s h e d , the assessment of the MCE count and i n t e r p r e t a t i o n of bonding d i f f e r e n c e s with S C Q . 7 5Zn!. 2 5Moi 0 i s u n c e r t a i n . Fcr t h i s reason only the s t r u c t u r e and bonding f e a t u r e s of the scandium compound w i l l be described here. 2

3

0

2

2

n

M

0

+

7

A view down the c-axis ( p a r a l l e l to the chain axes) of the scandium compound i s shown i n Figure 6. I t can be seen that the [Mo^Oy] chains are bound together through Mo-O-Mo bridge bonding to form l a y e r s which are, i n t u r n , bound together through O-Sc-0 and O-Zn-0 bonding. The s i t e s f o r these metal ions are of two types: t e t r a h e d r a l s i t e s occupied only by Z n i o n s , and o c t a h e d r a l s i t e s occupied by e i t h e r Z n or S c . In 2 +

2 +

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

3 +

Metal Oxide Systems

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

MCCARLEY

Figure 4. A three-dimensional perspective of the structures of NaMofie (top) and Ba (Mo 0 ) (bottom) as viewed down the unique axis of chain growth and tunnel formation. 5

lt

6

8

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

283

284

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INORGANIC C H E M I S T R Y : TOWARD T H E 2 1 S T C E N T U R Y

Figure 5.

Segment of one (Mo Oi)^ chain in NaMo 0 which shows intrachain Mo-Mo and Mo-O bonding. Key: %, Mo; and O, oxygen. k

k

6

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

McCARLEY

Metal

Oxide

Systems

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18.

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285

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t h e o c t a h e d r a l s i t e s t h e m e t a l i o n s a p p e a r t o be s t a t i s t i c a l l y d i s o r d e r e d w i t h 0 . 2 5 Z n + and 0 . 7 5 S c * f r a c t i o n a l o c c u p a t i o n . B a s e d on t h e s e c o n s i d e r a t i o n s and t h e s h a r i n g o f 0-atoms a l o n g and b e t w e e n c h a i n s , t h e compound c a n be f o r m u l a t e d a s 2

Sco.7 5 O - 2 5 l - O u t ( 4 8 / 2 2 / 2 ) ° 2 ] · E l e c t r o n t r a n s f e r f r o m t h e Sc and Zn t o t h e Μοι+Ογ u n i t s o f t h e c h a i n s r e s u l t s i n 1 4 . 7 5 MCE p e r u n i t . The a v e r a g e f o r m a l o x i d a t i o n s t a t e o f 2 . 3 1 + f o r Mo i n t h i s compound i s t h e lowest y e t a t t a i n e d i n a t e r n a r y o x i d e phase. A comparison o f Mo-Mo b o n d i n g a l o n g t h e c h a i n s b e t w e e n NaMoi+Og w i t h 1 3 MCE a n d SCQ.75Zni.25Μοι+θ7 w i t h 1 4 . 7 5 MCE i s i n s t r u c t i v e . Pertinent Mo-Mo bond d i s t a n c e s f o r t h e s e compounds a r e g i v e n i n T a b l e I V . The p r i n c i p a l b o n d i n g d i f f e r e n c e b e t w e e n t h e c h a i n s i n t h e two s t r u c t u r e s i s t h a t i n NaMoi+Og t h e s p a c i n g b e t w e e n Mo atoms a l o n g t h e c h a i n , b o t h a p e x - a p e x and w a i s t - w a i s t , i s p e r f e c t l y r e g u l a r . In c o n t r a s t , w h i l e the w a i s t - w a i s t s p a c i n g i s r e g u l a r , t h e apexapex d i s t a n c e s a l t e r n a t e between s h o r t ( 2 . 6 2 5 ( 2 ) Â ) and l o n g ( 3 . 1 3 9 ( 2 ) X) w i t h i n t h e c h a i n s o f S C Q · 7 5 M . 2 5Μθΐψθ7, a s shown i n F i g u r e 7. The sum o f P a u l i n g bond o r d e r s f o r t h e n e t o f two a p e x - a p e x bonds p e r Μ ο ^ 0 r e p e a t u n i t i n NaMo^Og i s 0 . 7 7 3 . F o r t h e n e t o f one s h o r t a n d one l o n g a p e x - a p e x bond p e r r e p e a t u n i t i n S c . 7 5 Ζ η . 25MO1+07 t h e sum o f P a u l i n g bond o r d e r s i s 1 . 0 9 2 . T h i s i n c r e a s e o f 0 . 3 1 9 f o r t h e a p e x - a p e x bond o r d e r sum i n g o i n g f r o m t h e f o r m e r t o t h e l a t t e r compound d i c t a t e s a n i n c r e a s e o f ca. 0.64 bonding e l e c t r o n s p e r repeat u n i t . Summed o v e r a l l Mo-Mo bonds o f t h e r e p e a t u n i t , t h e t o t a l bond o r d e r f o r NaMoi+Og i s 6 . 3 6 , and f o r S c . 5 n i . 2 5 °i+07 t h e sum i s 6 . 8 4 . T h e s e Z n

Z n

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3

M o

0

0

Z R

6

0

χ

z

0

M

7

ο

Table IV.

A c o m p a r i s o n o f Mo-Mo bond d i s t a n c e s and Sc 7 Zn 25Mo 0 . 0 e

5

l e

4

if

(Mo-Mo) S C Q . 75^1. 5Μοι+0

NaMoi+Og

2

(waist-waist)

a

2.8618(2)

2.8854(5)

(waist-waist)

b

2.753(3)

2.817(2)

apex-waist

6

7

d Bond

(A) i n NaMo 0

2.780(2)

7

2.747(1) 2.782(1)

apex-apex

2.8618(2)

2.625(2) 3.139(2)

Bond d i s t a n c e p a r a l l e l t o c h a i n ^

Bond d i s t a n c e p e r p e n d i c u l a r

direction.

to chain

direction.

f

numbers s t r o n g l y i n d i c a t e t h a t a l l M C E s p a r t i c i p a t e i n b o n d i n g i n t e r a c t i o n s i n NaMo^Og and t h a t t h e a d d i t i o n a l e l e c t r o n s r e ­ q u i r e d t o f o r m t h e c h a i n s i n SCQ . 75^1.2 5 f°7 f enhance t h e Mo-Mo b o n d i n g . Mot

u

r

t

n

e

r

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 0 1!i

U

k

1

Figure 7. A view perpendicular to the direction of chain growth in Sc . Zn 2!>Mo O showing the mode of interchain Mo-O-Mo linking and alternately short and long apex-apex Mo-Mo bonds.

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Several important questions then a r i s e . What range of e l e c t r o n counts per repeat u n i t i s permitted i n chain s t r u c t u r e s of t h i s type? What changes may be expected i n the p a t t e r n of metal-metal bonding w i t h i n the u n i t s as the e l e c t r o n count i s v a r i e d across the permitted range? What i s the d e t a i l e d bond s t r u c t u r e ? How many d i s c r e t e bands separated by forbidden gaps w i l l occur? Answers to these and r e l a t e d questions are very important as guides to f u r t h e r s y n t h e t i c work, i . e . what compounds are p o s s i b l e , and as a base to understand p h y s i c a l p r o p e r t i e s . At the present time s e v e r a l compounds are known having chain s t r u c t u r e s of the same type discussed here. These are found among the rare e a r t h and group I I I t r a n s i t i o n metal subhalides, v i z . T b B r (20) with 6 MCE; and S c C l ( 2 8 ) , G d B r ( 2 9 ) , T b B r (29) and Eri+Is Ç30) with 7 MCE. Thus a wide range of MCE counts per repeat u n i t i s indeed p o s s i b l e . The compounds at the low end of the range are composed of both metal and nonmetal atoms having r e l a t i v e l y l a r g e r a d i i . Because of the chain c o n s t r u c t i o n , which r e q u i r e s that the average M-M separation must equal the nonmetal-nonmetal separation along the chain d i r e c t i o n , the l a r g e nonmetal atoms d i c t a t e long M-M d i s t a n c e s . This c o n d i t i o n i s met by metal atoms having l a r g e r a d i i forming bonds of low net bond order, hence metals with low valence e l e c t r o n counts. At the upper end of the range are the molybdenum oxide compounds with the much smaller nonmetal atoms, comparatively short metal-metal bonds, and higher net bond orders. I t w i l l be i n t e r e s t i n g to see i f the gaps between these two extremes can be f i l l e d - i n by s u i t a b l e synthesis and r e q u i s i t e matching of MCE with metal and nonmetal r a d i i , and e l e c t r o n i c band s t r u c t u r e . 2

5

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CHEMISTRY:

8

5

3

5

8

8

Concluding Remarks As a f i n a l comment we should note that no ternary molybdenum oxide phase has yet been found having d i s c r e t e o c t a h e d r a l c l u s t e r u n i t s l i k e those found i n the famous Chevrel phases M Mo S and M Mo Se (31,32). Only i n the case of Mg3Nb 0 (33) has a d i s c r e t e octahedral c l u s t e r u n i t been found i n an oxide phase. The existence of the l a t t e r however s i g n a l s opportunity f o r f u r t h e r research, and taken together with the r e s u l t s reported here and those known f o r a few other metal oxides, extensive metal-metal bonded, metal oxide chemistry can be a n t i c i p a t e d f o r Nb, Ta, W, Tc and Re. The great s t a b i l i t y and unusual s t r u c t u r e types observed f o r the known metal oxide c l u s t e r compounds should make them i n t e r e s t i n g c a n d i dates f o r the study and development of u s e f u l p h y s i c a l and chemical p r o p e r t i e s . x

8

6

8

x

8

8

11

Acknowledgments The author g r a t e f u l l y acknowledges the s e v e r a l coworkers upon whose work t h i s a r t i c l e i s based and whose c o n t r i b u t i o n s are i n d i c a t e d i n the l i t e r a t u r e c i t a t i o n s . The continuing support of the U.S. Department of Energy, D i v i s i o n of Basic Energy Sciences, f o r t h i s research program i s a l s o g r a t e f u l l y acknowledged.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

18.

MCCARLEY

Metal Oxide Systems

289

Literature Cited

1. 2. 3. 4. 5.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

Marinder, B.; Magneli, A. Acta Chem. Scand. 1957, 11, 1635. Goodenough, J.B. "Magnetism and the Chemical Bond," Interscience Monographs in Chemistry, Inorganic Section, Vol. 1, F. A. Cotton, Ed., Interscience Division, John Wiley and Sons, Inc., New York, Ν. Y. 1963. Goodenough, J.B. Bull. Soc. Chim. France 1965, 4, 1200. Rogers, D.B.; Shannon, R.D.; Sleight, A.W.; Gillson, J.L. Inorg. Chem. 1969, 8, 841. Sleight, T.P.; Hare, C.R.; Sleight, A.W. Mat. Res. Bull. 1968, 3, 437. Skaik, S.; Hoffman, R. J. Am. Chem. Soc. 1980, 102, 1194. Abraham, F.; Trehoux, J.; Thomas, D. J. Solid State Chem. 1979, 29, 73. Waltersson, K. Acta Crystallogr. 1976, B32, 1485. Schäfer, H.; von Schnering, H.G. Angew. Chem. 1964, 76, 833. Imoto, H.; Corbett, J.D.; Cisar, A. Inorg. Chem. 1981, 20, 145. McCarroll, W.H.; Katz, L.; Ward, R. J. Am. Chem. Soc. 1957, 79, 5410. Ansell, G.B.; Katz, L. Acta Crystallogr. 1966, 21 482. McCarroll, W.H. Inorg. Chem. 1977, 16, 3351. Donohue, P.C.; Katz, L. Nature (London) 1964, 201, 180. Kerner-Czeskleba, H.; Tourne, G. Bull. Soc. Chim. Fr. 1976, 729. Cotton, F.A. Inorg. Chem. 1964, 3, 1217. Torardi, C.C. and McCarley, R.E. J. Solid State Chem. 1981, 37, 393. Torardi, C.C. and McCarley, R.E. to be published. Torardi, C.C. and McCarley, R.E. J. Am. Chem. Soc. 1979, 101, 3963. Simon, A. Angew. Chem. Int. Ed. Engl. 1981, 20, 1. Corbett, J.D. Acc. Chem. Res. 1981, 14, 239. McCarley, R.E.; Ryan, T.R.; Torardi, C.C. ACS Symp Ser. 1981, 155, 41. Pauling, L. The Nature of the Chemical Bond, 3rd Ed. Cornell University Press, 1960, p.400. Chisholm, M.H.; Huffman, J.C.; Leonelli, J. J. Chem. Soc., Chem. Commun. 1981, 270. Chisholm, M.H.; Huffman, J.C.; Kirkpatrick, C.C.; Leonelli, J.; Folting, K. J. Am. Chem. Soc. 1981, 103, 6093. Cotton, F.A.; Fang, A. J. Am. Chem. Soc. 1982, 104, 113. Brough, L.F.; Carlin, R.T.; McCarley, R.E. Results to be published. Poeppelmeier, K.R.; Corbett, J.D. J. Am. Chem. Soc. 1978, 100, 5039. Mattausch, H.J.; Simon, Α.; Eger, R. Rev. Chim. Miner. 1980, 17, 516. Berroth, K.; Simon, A. J. Less-Common Met., to be published.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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31 32 33.

INORGANIC

CHEMISTRY:

TOWARD

THE

21ST

CENTURY

Chevrel, R.; Sergent, M.; Prigent, J. J. Solid State Chem. 1971, 3, 515. Yvon, K. Current Topics Mat. Sci. 1978, 3, 55. Marinder, B.-O. Chem. Scripta 1977, 11, 97.

RECEIVED

September 13,

1982

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch018

Discussion J.P. Fackler, J r . , Case Western Reserve University: Bob, I f i n d y o u r s t u d y t o be most i n t e r e s t i n g . Has i t been p o s s i b l e f o r y o u t o i s o l a t e m i x e d 0,S p h a s e s o f t h e C h e v r e l t y p e by o x y g e n s u b s t i t u t i o n ?

R. Ε. McCarley: No, we have done very l i t t l e work along t h i s l i n e , although I t h i n k important r e s u l t s w i l l be obtained from s t u d i e s of mixed 0, S phases. So f a r we have found no d i ­ r e c t s t r u c t u r a l analogies between the reduced ternary molybdenum oxide phases and the v a r i o u s ternary s u l f i d e or s e l e n i d e phases of the Chevrel type.

A.R. S i e d l e , 3M C e n t r a l R e s e a r c h L a b o r a t o r y : I f one o f t h e e x t e n d e d s t r u c t u r e s d e s c r i b e d by P r o f e s s o r M c C a r l e y w e r e t r u n ­ c a t e d t h r o u g h a low M i l l e r i n d e x p l a n e , c a n one, f o l l o w i n g t h e a p p p r o a c h of Solomon, p r e d i c t what m e t a l o r b i t a l s w o u l d p r o ­ t r u d e f r o m t h e s u r f a c e so g e n e r a t e d ? Have u l t r a v i o l e t p h o t o e l e c ­ t r o n s p e c t r a been o b t a i n e d on s i n g l e c r y s t a l s of any o f t h e s e materials?

R. Ε. McCarley: In response to the f i r s t question, i t should be easy to p r e d i c t those metal o r b i t a l s which would pro­ trude from the 001 and 002 faces of N a M o ^ ô , i . e . those faces normal to the d i r e c t i o n of chain growth. However, up to the present time we have not considered such questions because of our g e n e r a l l y poor understanding of bonding d e t a i l s i n these extended systems. In reference to the l a t t e r question, we have obtained UPS data on p o l y c r y s t a l l i n e samples of NaMo406, but not on s i n g l e c r y s t a l s . These measurements i n d i c a t e a r e l a t i v e l y high d e n s i t y of s t a t e s at the Fermi l e v e l , which i s i n agreement with the low r e s i s t i v i t y and m e t a l l i c character of the m a t e r i a l .

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

19 Low-Valent Dimers of Tantalum and Tungsten A. P. SATTELBERGER

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch019

University of Michigan, Department of Chemistry, Ann Arbor, MI 48104

I am not going to discuss early transition metal oxide or alkoxide chemistry here. Instead I would like to use the few minutes allotted me to describe our work with low valent dimers of tantalum and tungsten. Professor McCarley will no doubt see his influence on our work and forgive this digression from the topic of his excellent presentation. The synthesis of the first quadruply bonded tungsten(II) carboxylate, W(OCCF)4 (hereafter referred to as W(TFA)), was reported by us last year (1). The structure of its diglyme adduct, W(TFA).2/3 diglyme is shown in Figure 1. This particular view shows the partial contents of two unit cells and it emphasizes the tridentate nature of the axially coordinated polyether which "stiches" W(TFA) units together in the solid state. We now have in hand several other adducts of W(TFA) and one of these, W(TFA).2PPh (an axial adduct), has been structurally characterized (2). Despite many attempts, X-ray quality crystals of unsolvated W2(TFA)4 have not been obtained. 2

2

2

3

2

4

4

2

4

2

2

4

4

3

The s u c c e s s f u l synthesis of W2(TFA)4 was the culmination of two years of hard work i n our l a b o r a t o r y . In r e t r o s p e c t the synt h e s i s now appears t r i v i a l ( r e a c t i o n 1); a l l of our more grandiose approaches f a i l e d . We have f u r t h e r streamlined r e a c t i o n 1 by THF

W C1 (THF) 2

6

4

+ 2Na/Hg + 4Na0 CCF 2

3

> W (TFA) 2

4

+ 6NaCl

(1)

using polymeric WCI4 as the s t a r t i n g m a t e r i a l and W (TFA)4 i s u s u a l l y obtained i n ^65% y i e l d from a "one-pot" s y n t h e s i s . Our e f f o r t s to extend t h i s method to other carboxylates have only been s u c c e s s f u l i n the case of W2(02CCMeg)4. Sodium p i v a l a t e , l i k e sodium t r i f l u o r o a c e t a t e , i s s l i g h t l y s o l u b l e i n tetrahydrofuran and t h i s f a c t o r appears to be c r u c i a l to the success of r e a c t i o n 1. No quadruply bonded products were i s o l a t e d i n r e a c t i o n s with sodium acetate or sodium benzoate, both v i r t u a l l y i n s o l u b l e i n THF. 2

0097-6156/83/0211-0291$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INORGANIC C H E M I S T R Y ! TOWARD T H E 21ST

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292

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

CENTURY

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch019

19.

SATTELBERGER

Tantalum

and

293

Tungsten

With W2(TFA>4 i n hand and Mo2(TFA>4 already known and w e l l c h a r a c t e r i z e d (3), we began to pursue comparative s p e c t r o s c o p i c s t u d i e s . The r e s u l t s of our IR and Raman experiments have already been presented i n p r e l i m i n a r y form (1) . We have a l s o measured the gas phase photoelectron s p e c t r a i n c o l l a b o r a t i o n with P r o f e s s o r G. M. Bancroft, Department of Chemistry, U n i v e r s i t y of Western Ontario (4). The Hel s p e c t r a are shown i n F i g u r e 2. The spectrum of Mo2(TFA)^ (top) i s i n good agreement with that published pre­ v i o u s l y by Green and coworkers (5). We a l s o f i n d two peaks below the onset of l i g a n d i o n i z a t i o n (ca. 12 eV). The peak at 8.76 has t r a d i t i o n a l l y been assigned as the δ i o n i z a t i o n (5,.6). The assignment of the 10.46 eV band i s another matter. SCF-Xa-SW r e ­ s u l t s favor a s s i g n i n g t h i s band as the π i o n i z a t i o n (7), whereas ab i n i t i o c a l c u l a t i o n s suggest that t h i s band be assigned to over­ lapping π and σ i o n i z a t i o n s (8,9,10). Cotton has documented a l l of the o b j e c t i o n s to t h i s l a t t e r assignment ( 6 ) . Now we t u r n to the W2(TFA)^ spectrum. Here we f i n d three d i s t i n c t bands i n the r e g i o n below 12 eV. These are at considerably lower b i n d i n g energy than i n the molybdenum dimer, a f e a t u r e not i n c o n s i s t e n t with the greater r e a c t i v i t y (e.g., toward 02or HC1) of W2(TFA)4 r e ­ l a t i v e to Mo2(TFA)4 (1). We have assigned the lowest energy band at 7.39 eV as the 6 i o n i z a t i o n . The remaining two bands (9.01 and ca. 9.76 eV) are separated by 0.75 eV and t h i s presents an a s s i g n ­ ment problem. I t might be tempting to a s s i g n these as the s p i n o r b i t components of the E s t a t e of W2(TFA)J. However, 0.75 eV i s o u t s i d e the range expected from the s p i n - o r b i t i n t e r a c t i o n , ca. 0.3 to 0.6 eV (11) and we consider i t u n l i k e l y , even taking 0.75 eV as an acceptable s p i n - o r b i t s p l i t t i n g , that the p h o t o i o n i z a t i o n cross s e c t i o n s , width and s t r u c t u r e of the two l e v e l s would be so d i f f e r e n t . We are t h e r e f o r e i n c l i n e d to a s s i g n the bands at 9.01 and 9.76 eV as the π and σ i o n i z a t i o n s , r e s p e c t i v e l y . The opposite i n t e r p r e t a t i o n has been invoked to e x p l a i n the PES spec­ trum of W2Cl4(PMe3)4 (12). Here the separation between the second and t h i r d bands i n the spectrum i n 0.4 eV, i . e . , w i t h i n the range expected from tungsten s p i n - o r b i t s p l i t t i n g but the band shapes are anomolous even c o n s i d e r i n g mixing with the σ and 6 l e v e l s . 2

Now I would l i k e to leave tungsten chemistry and d e s c r i b e a few of our r e s u l t s i n low v a l e n t tantalum chemistry. Our entry i n t o t h i s area began with the synthesis ( r e a c t i o n 2) and s t r u c ­ t u r a l c h a r a c t e r i z a t i o n (Figure 3) of the tantalum(III) dimer, TaCl

5

+ 2Na/Hg + 2PMe

P h C H 3

3 » Q.5[TaCl (PMe ) j (μ-Cl) 2

3

2

2

2

+ 2NaCl

u

[ T a C l 2 ( P M e 3 ) 2 ] 2 ( - C l ) (13). The short metal-metal bond l e n g t h of 2.710(2)ft has been i n t e r p r e t e d as a formal metal-metal double bond on the b a s i s of molecular o r b i t a l arguments (13,14). An un­ usual f e a t u r e of t h i s complex i s that i t r e a c t s with molecular 2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 2

k

2

k

Figure 2. Hel photoelectron spectra of Mo (TFA) (left) and W (TFA) (right). The spectra have been computer fitted to combination Lorentzian-Gaussian peaks. (Reproduced with permission from Ref. 4. Copyright 1982, The Royal Society of Chemistry.)

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch019

Tantalum

and

Tungsten

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch019

SATTELBERGER

Figure 3.

Molecular structure of [TaCl (PMe ) ] (^-Cl) 2

3 2

2

2

(13, 14).

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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hydrogen under very m i l d c o n d i t i o n s (1 atm H , 25°C) to form the quadruply bridged tantalum(IV) dimer, [TaCl (PMe3) ] ^-Cl) ^-H) (13,15). A view of t h i s dimer l o o k i n g down the Ta-Ta a x i s i s shown i n F i g u r e 4. The b r i d g i n g hydrogens were not l o c a t e d i n the f i n a l d i f f e r e n c e F o u r i e r of t h i s X-ray s t r u c t u r e but a b a t t e r y of spectroscopic techniques 0-H and P NMR and IR) places them i n the c a v i t y below the b r i d g i n g c h l o r i d e s (15)• The ( μ - 0 1 ) ( μ - Η ) dimer o f f e r s a convenient entry back i n t o tantalum(III) dimer chemistry ( r e a c t i o n 3) and the product, [ T a C l ( P M e 3 ) ] ^ - H ) , been c h a r a c t e r i z e d by X-ray c r y s t a l l o g r a p h y (16). This time we were fortunate enough to l o c a t e the b r i d g i n g hydrides (Figure 5 ) . 2

2

2

2

2

2

3 1

2

2

n a s

2

[TaCl (PMe ) ] (y-Cl) (y-H) Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch019

2

3

2

2

2

+ 2Na/Hg

2

g

l

y

m

e

2

2

>

[ T a C l ( P M e ) ] ( y - H ) + 2NaCl 2

3

2

2

2

(3)

2

The geometry of the Ta Cl4(PMe3)4 substructure i s reminiscent of the quadruply bonded dimers Μ ο ^ Ι ^ ί Ρ Μ β β ) ^ and W Cl4(PMe )^ (17) . The room temperature ^H and 31p NMR spectra (Figure 6) of the ( μ - Η ) dimer are i n c o n s i s t e n t with the s o l i d s t a t e s t r u c t u r e . Rapid r o t a t i o n of the end groups and/or b r i d g i n g hydrides i s r e ­ quired to account f o r apparent magnetic e q u i v a l e n c i e s . The mole­ c u l e does, however, have b u i l t i n p s e u d o - c y l i n d r i c a l symmetry, i . e . , one set of metal π-type o r b i t a l s binds the b r i d g i n g hydrides and the other set forms the π-component of the metal-metal double bond. 2

2

3

2

[ T a C l ( P M e 3 ) ] ( y - H ) r e a c t s with many s u b s t r a t e s . Time per­ mits me to mention only one - the r e a c t i o n with molecular hydro­ gen. The l a t t e r provides the quadruply hydrogen bridged tantalum (IV) dimer, [ T a C l ( P M e 3 ) ] ( μ - Η ) ^ whose X-ray s t r u c t u r e i s shown i n F i g u r e 7. This dimer has c r y s t a l l o g r a p h i c a l l y imposed D j symmetry and a very short (2.511(2)Â) tantalum-tantalum bond length (18). Unlike i t s tantalum(III) precursor, the (μ-Η)4 dimer i s s t a t i c on the NMR time s c a l e (Figure 8). This i s i n keeping with the p r e d i c t i o n s of Hoffmann and coworkers on (μ-Η)^ dimers (19). A l l of the metal π-type o r b i t a l s are engaged i n Ta-H-Ta bonding and a s u b s t a n t i a l r o t a t i o n a l b a r r i e r a r i s e s from the over­ lap of o r b i t a l s of δ symmetry. This combination i s shown below. 2

2

2

2

2

2

2

2(

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch019

Figure 5.

Molecular structure of [TaCl (PMe ) ]' (μ-Η) 2

3 2

2

2

(16).

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

NMR

6

6

3 3

3

P H

31

k

-9.9

Π

J

1 4 5 . Θ Μ Η Ζ ^ P l S E l J H l NMR

Jf

=13.4 Hz

31

2

1.52

2

3 2

2

Figure 6. *H and selectively decoupled [from P(CH ) ] P-NMR spectra of [TaCl (PMe ) ] (μ-Η) in C D (*). Proton chemical shifts (B) are in ppm from Me Si. P chemical shifts (S) are in ppm from external H PO

a52

_JLLL___

PH

J =13.4HZ

1

360 MHz H

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch019

H

w

O

H

I s

H

g So H

2

ο ο > g ο ο w

VO 00

SATTELBERGER

Tantalum and

299

Tungsten

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch019

19.

TV 1.47

8.79

Figure 8. 360 MHz Ή-NMR spectrum of lTaCU(FMe ) ] (^-H) in C D (*). Chemical shifts (8) are in ppm from Me Si. The amplitude of the 88.79 signal (4 bridging hydrides) is twice that of the 81.47 resonance (36 PMe protons) (IS). 3 2

2

k

6

k

s

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

6

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CHEMISTRY:

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Acknowledgments I thank the Research Corporation, the Petroleum Research Fund administered by the American Chemical Society and the U.S. Department of Energy (DE-FG02-80ER10125) f o r t h e i r f i n a n c i a l support. I a l s o acknowledge my f r u i t f u l c o l l a b o r a t i o n s with Dr. John C. Huffman, D i r e c t o r of the Molecular Structure Center, Indiana U n i v e r s i t y , as w e l l as my own group whose c o n t r i b u t i o n s are c i t e d i n the r e f e r e n c e s .

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch019

Literature Cited

1. Sattelberger, A.P.; McLaughlin, K.W.; Huffman, J.C. J. Am. Chem. Soc. 1981, 103, 2880. 2. Sattelberger, A.P., Santure, D.J.; McLaughlin, K.W.; Huffman, J.C., manuscript in preparation. 3. Cotton, F.A.; Norman, J.G., Jr. J. Coord. Chem. 1971, 1, 161. 4. Bancroft, G.M.; Pellach, E.; Sattelberger, A.P., McLaughlin, K.W. JCS Chem. Commun. 1982, xxxx. 5. Coleman, A.W.; Green, J.C.; Hayes, A.J.; Seddon, E.A.; Lloyd, D.R*; Niwa, Y. J. Chem. Soc. Dalton 1979, 1057. 6. Cotton, F.A.; Walton, R.A. "Multiple Bonds Between Metal Atoms" John Wiley, New York, 1982, Chapter 8, pp. 415-425. 7. Norman, J.C, Jr.; Kolari, H.J.; Gray, H.B.; Trogler, W.C. Inorg. Chem. 1977, 16, 987. 8. Benard, M. J. Am. Chem. Soc. 1978, 100, 2354. 9. Guest, M.F.; Hillier, I.H.; Garner, C.D. Chem. Phys. Lett. 1977, 48, 587. 10. Guest, M.F.; Garner, C.D.; Hillier, I.H.; Walton, I.B. J. Chem. Soc. Faraday II 1978, 74·, 2092. 11. Bursten, B.E.; Cotton, F.A.; Cowley, A.H.; Hanson, B.E.; Lattman, M.; Stanley, G.G. J. Am. Chem. Soc. 1979, 101, 6244. 12. Cotton, F.A.; Hubbard, J.L.; Lichtenberger, D.L.; Shim, I. J. Am. Chem. Soc. 1982, 104, 679. 13. Sattelberger, A.P., Wilson, R.B., Jr., Huffman, J.C. J. Am. Chem. Soc. 1980, 102, 7911. 14. Sattelberger, A.P.; Wilson, R.B., Jr.; Huffman, J.C. Inorg. Chem. 1982, 21, 2392. 15. Sattelberger, A.P.; Wilson, R.B., Jr.; Huffman, J. C. Inorg. Chem. 1982, 21, xxxx. 16. Wilson, R.B., Jr.; Sattelberger, A.P.; Huffman, J.C. J. Am. Chem. Soc. 1982, 104, 858. 17. Cotton, F.A.; Extine, M.W.; Felthouse, T.R.; Kolthammer, B.W.S.; Lay, D.C J. Am. Chem. Soc. 1981, 103, 4040. 18. Sattelberger, A.P.; Luetkens, M.L., Jr.; Wilson, R.B., Jr.; Huffman, J.C, manuscript in preparation. 19. Dedieu, Α.; Albright, T.A.; Hoffman, R. J. Am. Chem. Soc. 1979, 101, 3141. RECEIVED

October 18, 1982

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

19.

Tantalum

SATTELBERGER

and Tungsten

301

Discussion I would l i k e t o make a few comments r e g a r d i n g the photoe l e c t r o n s p e c t r a presented by Dr. S a t t e l b e r g e r . In r e f e r r i n g t o h i s spectrum o f W (^CCFa)^, he has assigned the i o n i z a t i o n band near 10 eV t o be associated with removal o f an e l e c t r o n from the sigma-bonding o r b i t a l between the two metal centers. This assignment i s made p r i m a r i l y on the b a s i s that the s p l i t t i n g o f t h i s band from the band near 9 eV, and the widely d i f f e r e n t c r o s s - s e c t i o n s f o r these i o n i z a t i o n s , are not c o n s i s t e n t with s p i n - o r b i t e f f e c t s on the predominantly metal p i bond i o n i z a t i o n . The occurrence o f the sigma i o n i z a t i o n i n t h i s r e g i o n would have some important i m p l i c a t i o n s . However, there i s considerable information, both i n the s p e c t r a o f the compounds presented by Dr. S a t t e l b e r g e r , and from other r e l a t e d work, that i n d i c a t e assignment o f t h i s band t o the sigma i o n i z a t i o n i s not warranted. I w i l l r e s t r i c t my comments t o the compounds discussed by Dr. Sattelberger. F i r s t , i n s p e c t i o n o f the low energy i o n i z a t i o n bands presented by Dr. S a t t e l b e r g e r f o r W2 (^CCFa)^ shows that the band near 10 eV i s extremely narrow. I f t h i s band i s assigned t o the sigma bond i o n i z a t i o n , then i t f o l l o w s that the change i n metalmetal bond d i s t a n c e with removal o f an e l e c t r o n from the sigma bond i s very small and i s much l e s s than the change that occurs with removal o f an e l e c t r o n from the d e l t a bond. This point i s r e l a t e d to the comments I made i n my c o n t r i b u t i o n f o l l o w i n g Professor Cotton's t a l k . A narrow band and n e g l i g i b l e change i n bond d i s t a n c e f o r a sigma i o n i z a t i o n i s unprecedented and would be q u i t e remarkable. Second, s p i n - o r b i t e f f e c t s i n a dimetal system must be t r e a t e d r i g o r o u s l y before s t a t i n g what i s o r i s not reasonable f o r the i o n i z a t i o n s t r u c t u r e . One component o f the s p i n - o r b i t s p l i t p i i o n i z a t i o n w i l l mix with the sigma framework o f the molecule, while the other component w i l l mix with the d e l t a framework. These components w i l l have d i f f e r e n t r e l a x a t i o n energies with i o n i z a t i o n , d i f f e r e n t c r o s s - s e c t i o n s f o r i o n i z a t i o n , and d i f f e r e n t displacement o f p o s i t i v e i o n p o t e n t i a l w e l l s above the ground s t a t e o f the molecule. A p r e d i c t i o n o f the magnitude of these d i f f e r e n c e s i s a major c a l c u l a t i o n a l challenge. In view of the present l i m i t o f our knowledge o f the s p i n - o r b i t e f f e c t s , i n t e r p r e t a t i o n o f these i o n i z a t i o n s as o r i g i n a t i n g from the p i bond electrons cannot be considered unreasonable. F i n a l l y , the i m p l i c a t i o n i s made that the sigma i o n i z a t i o n i s a l s o i n the r e g i o n o f the p i i o n i z a t i o n f o r the molybdenum dimers. We have r e c e n t l y c a r r i e d out an examination o f the p i i o n i z a t i o n of M02(02CCH3)i i n which we observe c l e a r v i b r a t i o n a l f i n e s t r u c t u r e across the band. This observation i s not expected i f two d i f f e r e n t i o n i z a t i o n s , the sigma and p i , are overlapped i n the same envelope. Thus, I do not see any evidence a t t h i s stage that the sigma i o n i z a t i o n i s i n the r e g i o n o f the p i i o n i z a t i o n f o r these complexes.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch019

2

t

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

20 Dynamic Processes of Metal Atoms and Small Metal Clusters in Solid Supports G E O F F R E Y A . O Z I N and S. A. M I T C H E L L

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

University of Toronto, Lash Miller Chemistry Laboratories, Toronto, Ontario M5S 1A1 Canada

Major developments in the field of metal vapor synthesis and matrix isolation spectroscopy involving ground state metal atomic and cluster reagents over the past decade are briefly contemplated. A new direction which focuses attention on the chemistry and spectroscopy of these reagents in selected excited electronic states is the subject of this paper. In particular, matrix cage relaxation dynamics, following uv and visible excitation of atomic and small cluster guests of copper and silver in rare gas lattices will be examined. Reactivity patterns of electronically excited metal atomic reagents with methane and dioxygen will also be briefly described. Metal-support effects involving both ground anexcited electronic states of the metal guest-cage unit will feature prominently in these discussions. These interactions play a critical role in determining reactivity patterns and relaxation processes of immobilized metal guests and clearly bear a relationship to metal-support effects known to be important in supported metal catalysts. As the main theme of t h i s meeting i s to assess and c o n s o l i date past achievements i n various key areas of inorganic/organom e t a l l i c chemistry, with the o b j e c t i v e o f gazing deep and hard i n t o the f u t u r i s t i c chemical c r y s t a l b a l l of the 21st century, the purpose of my p r e s e n t a t i o n w i l l be to focus a t t e n t i o n on p i v o t a l developments i n the f i e l d o f t r a n s i t i o n metal atom/metal c l u s t e r chemistry over the past decade and then to attempt to p r o j e c t and f o r e c a s t some of the more promising d i r e c t i o n s that the area i s l i k e l y to f o l l o w i n the years ahead. I f one surveys the e x c i t i n g growth p e r i o d of the e a r l y sevent i e s one cannot help but n o t i c e the n a t u r a l but constrained subd i v i s i o n of the f i e l d of metal vapor (MV) chemistry i n t o a macr o s c a l e s y n t h e t i c s c h o o l , conducting experiments u s u a l l y at 77300K and a matrix s c a l e s p e c t r o s c o p i c school, working i n the lower 0097-6156/83/0211-0303$07.25/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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temperature range of 4.2-300K (1). Recently, the two method o l o g i e s have been s u c c e s s f u l l y merged to the great b e n e f i t of both. C u r r e n t l y , both s o l i d s t a t e cocodensation and s o l u t i o n phase MV experiments of a combined s y n t h e t i c / s p e c t r o s c o p i c type are performed on a routine b a s i s i n our l a b o r a t o r y (2). Moreover, matrix and macroscale MVS equipment i s now commercially a v a i l a b l e (3) . These e a r l y studies were aimed at e x p o l i t i n g the o f t e n unique chemical r e a c t i v i t y of naked metal atoms, molecular metal c l u s t e r s and c o l l o i d a l metal p a r t i c l e s i n t h e i r ground e l e c t r o n i c s t a t e s , with each other to form w e l l defined metal aggregates, as w e l l as with organic ligands to produce n o v e l r e a c t i v e i n t e r mediates and products w i t h v a r y i n g degrees of s t a b i l i t y over the cryogenic temperature to room temperature range. The f i e l d of ground s t a t e MVS enjoyed p r o f u s i v e growth i n the seventies and a t t r a c t e d a m u l t i d i s c i p l i n a r y audience of s c i e n tists. This was because of the wide ranging r a m i f i c a t i o n s of the r e s u l t s i n f i e l d s as diverse as n u c l e a t i o n theory, photographic and xerographic s c i e n c e , c l u s t e r and chemisorption model theory, c a t a l y s i s by supported metal c l u s t e r s , o r g a n o m e t a l l i c s y n t h e s i s , homogeneous c a t a l y s i s , organometal polymers, to name but a few (4) . Along with the elegant chemical achievements and impressive instrumental design, one also witnessed a r a p i d expansion i n s p e c t r o s c o p i c and k i n e t i c techniques f o r c h a r a c t e r i z i n g matrix entrapped a t o m i c / c l u s t e r metal vapor r e a c t i o n products. Noteable amongst these pioneering experiments are SIMS (Michl ( 5 ) ) , MCD ( G r i n t e r ( 8 ) , Schatz ( 7 ) ) , UPS (Jacobi ( 8 ) ) , EXAFS (Montano ( 9 ) ) , NMR (Michl (10)), ESR (Weltner, (11), Kasai (12), Lindsay (13)), Mossbauer ( B a r r e t t , Montano (14)), o p t i c a l absorption and f l u o r escence (Kolb (15), Ozin (16)), Laser f l u o r e s c e n c e , Raman, Resonance Raman (Bondybey (17), Nixon (18) Schulze (19), Gruen (20), Moskovits (21)). Molecular e l e c t r o n i c s t r u c t u r e c a l c u l a t i o n s of naked metal c l u s t e r s , with n u c l e a r i t i e s spanning the range from atom to bulk became the focus of intense s c i e n t i f i c i n t e r e s t i n the 70*s (22). Experimental techniques, on the other hand, f o r f a b r i c a t i n g and s p e c t r o s c o p i c a l l y probing s p e c i f i c c l u s t e r s developed more slowly. P r e s e n t l y , molecular beam and matrix i s o l a t i o n methods have emerged as the premier approaches f o r studying l i g a n d - f r e e metal c l u s t e r s i n the gas and condensed phases r e s p e c t i v e l y . Each method d i s p l a y s advantages and l i m i t a t i o n s . In the i d e a l c o l l i s i o n f r e e environment of a molecular beam, the p r o p e r t i e s of a metal c l u s t e r can be considered to be t r u e l y i s o l a t e d from c l u s t e r - s u b s t r a t e e f f e c t s . Therefore, spectros c o p i c methods that can s e l e c t i v e l y e x t r a c t i n f o r m a t i o n from metal c l u s t e r beams hold great promise f o r i l l u m i n a t i n g diverse s i z e dependent p r o p e r t i e s of aggregates of metal atoms i n t h e i r e q u i l i b r i u m c o n f i g u r a t i o n (23). Condensation of metal atoms and/or metal c l u s t e r s , with or i n rare gases at cryogenic temperatures, from e i t h e r j e t s or beams

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

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represents the other main approach f o r observing n u c l e a t i o n phenomena from i s o l a t e d metal atoms to bulk metal aggregates i n a weakly i n t e r a c t i n g s o l i d support (4,24). In t h i s way, metal d i s p e r s i o n , thermal and p h o t o l y t i c behaviour of p a r t i c u l a r metal c l u s t e r s may be observed. The s p e c t r a obtained are often of low r e s o l u t i o n and observations r e f l e c t not only the p r o p e r t i e s of the guest but a l s o t h e i r i n t e r a c t i o n with the host (25). The matrix method appears to be able to provide i n d i v i d u a l c l u s t e r p r o p e r t i e s up to about s i x atoms (4,24,26). Above t h i s s i z e , s p e c t r a l overlap problems between d i f f e r e n t species i n the matrix u s u a l l y preclude an unambiguous determination of c l u s t e r n u c l e a r i t y . Nevertheless, the important t r a n s i t i o n , atom to molecule to bulk and quantum s i z e e f f e c t s i n small metal aggregates can be studied i n t h i s way (26,27). Because the s i z e regime of n=l-6 atoms i s of great p r a c t i c a l s i g n i f i c a n c e to the s p e c t r o s c o p i c , chemical and c a t a l y t i c prop e r t i e s of supported metal c l u s t e r s i n both weakly and s t r o n g l y i n t e r a c t i n g environments (28), i t i s important to study v e r y small metal c l u s t e r s i n v a r i o u s types of substrate as w e l l as i n the gas phase. In t h i s way, one can hope to develop a s c a l e of metal c l u s t e r - s u p p o r t e f f e c t s (guest-host i n t e r a c t i o n s ) and evaluate the r o l e that they play i n d i v e r s e t e c h n o l o g i c a l phenomena. In the past few years, the f i e l d of metal atom/metal c l u s t e r chemistry has taken an i n t e r e s t i n g turn out of the realm of the ground e l e c t r o n i c s t a t e i n t o the world of the e x c i t e d s t a t e . T h i s promises to be an i n t r i g u i n g new phase and one with considerable p o t e n t i a l f o r new s c i e n t i f i c discovery and t e c h n o l o g i c a l development. The o r i g i n a l swing of emphasis away from ground s t a t e metal atom/metal c l u s t e r chemistry can be traced to i n v e s t i g a t i o n s of the photoprocesses of these reagents embedded i n weakly i n t e r a c t i n g supports, mainly of the rare gas v a r i e t y (24). Studies of t h i s k i n d revealed that the i n t e r a c t i o n s between the e x c i t e d s t a t e s but not the ground s t a t e s of c e r t a i n matrix entrapped metal guests with the surrounding cage, were not q u i t e as innocent as might have been i n i t i a l l y a n t i c i p a t e d . In f a c t , these metal support e f f e c t s were of s u f f i c i e n t magnitude to r e s u l t i n a range of h i t h e r t o f o r e unobserved matrix r e l a x a t i o n processes, i n cluding metal atom p h o t o d i f f u s i o n and aggregation (30), metal c l u s t e r photofragmentation/fluorescence/cage-recombination (31), and metal c l u s t e r photoisomerization (32). These p h o t o e f f e c t s were found to be e x c e p t i o n a l l y s e n s i t i v e to the nature of the support and u n v e i l e d the existence and operation of s u r p r i s i n g l y strong guest-host i n t e r a c t i o n s . I t was as a r e s u l t of i n v e s t i g a t i o n s of the aforementioned kind that a new kind of e x c i t e d s t a t e metal atom/metal c l u s t e r photoprocess was discovered, i n v o l v i n g chemical r e a c t i o n with the support i t s e l f (33). A p r e r e q u i s i t f o r the s u c c e s s f u l e x p l o i t a t i o n of t h i s n o v e l k i n d of chemistry, i s a weakly i n t e r a c t i n g metal atom/metal c l u s t e r - cage ground e l e c t r o n i c s t a t e . Only i n

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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306

t h i s s i t u a t i o n i s i t p o s s i b l e to prepare the metal reagents i n the d e s i r e d e l e c t r o n i c s t a t e f o r i n v e s t i g a t i o n of t h e i r e x c i t e d s t a t e r e a c t i v i t y p a t t e r n s . This phase of development i s very recent and may be considered to have opened up a new era of metal vapor chemistry, c e r t a i n l y worthy of c o n s i d e r a t i o n by chemists of the 21st century. E a r l y i n d i c a t o r s emerging from s t u d i e s of the e x c i t e d s t a t e chemistry of metal atomic and c l u s t e r reagents p o i n t to an i n ­ t e r e s t i n g and b r i g h t future f o r the f i e l d . H i g h l i g h t s over the past year include e x c i t e d s t a t e metal atom agglomeration reac­ t i o n s (34), s e l e c t i v e p h o t o i n s e r t i o n processes of metal atoms i n t o the CH bonds of p a r a f f i n i c hydrocarbons (33,35), the s p l i t ­ t i n g of dihydrogen (36), dioxygen (37), water and ammonia (38) by photoexcited metal atoms, and photoinduced e l e c t r o n - t r a n s f e r processes (37). A taste of the i n t r i g u i n g o p p o r t u n i t i e s that t h i s new f i e l d has to o f f e r , can be appreciated from the r e c e n t l y discovered s e l e c t i v e , room temperature,atmospheric pressure,photoheterogeneous d i m e r i z a t i o n of p a r a f f i n s to higher saturated a l k a nes on a metal z e o l i t e (39) ,an i n t e r e s t i n g new avenue i n the quest f o r transforming n a t u r a l gas u l t i m a t e l y to gasoline hydrocarbons. For the remainder of t h i s p r e s e n t a t i o n , I w i l l concentrate on some of the more i n t e r e s t i n g photophysical and photochemical p r o p e r t i e s r e c e n t l y observed f o r metal atomic and small c l u s t e r reagents entrapped i n non-reactive and r e a c t i v e s o l i d s u b s t r a t e s . A t t e n t i o n w i l l be d i r e c t e d to matrix-cage r e l a x a t i o n e n e r g e t i c s and dynamics f o l l o w i n g uv and v i s i b l e p h o t o e x c i t a t i o n of atomic and small c l u s t e r guests of copper and s i l v e r i n rare gas,methane and dioxygen l a t t i c e s . Metal-support i n t e r a c t i o n s i n v o l v i n g both 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 of the metal guest-cage u n i t w i l l feature prominently throughout t h i s d i s c u s s i o n as they play a c r i t i c a l r o l e i n c o n t r o l l i n g the observed r e a c t i v i t y p a t t e r n s and r e l a x a t i o n processes of the entrapped metal guests. Results and Discussion The major absorption and emission processes of gaseous Cu and Ag atoms i n the u v - v i s i b l e range are r a t h e r s t r a i g h t f o r w a r d l y i n t e r p r e t e d i n terms of e l e c t r o n i c t r a n s i t i o n s between an i s o t r o ­ p i c a d ( n t l - l ) s Si/ ground s t a t e and n d ( n + l ) p , Ρ / ? , / and n d ( n + l ) s , D / , 5 / 2 e x c i t e d s t a t e s (40) .Only the S χ / + P 1/2 > 3/2 t r a n s i t i o n s are both p a r i t y and spin allowed although the D 5 / 2 ~ ^ S i / 2 t r a n s i t i o n i s seen i n the emission spectra of Ag vapor (41). The P i / 2 > 3 / 2 terms are almost c o i n c i d e n t f o r Cu and Ag atoms .Furthermore,the D /2,5/2 term of s i l v e r i s almost e q u i e n e r g e t i c with the P \ l 2 ^ 3 / 2 term, whereas t h i s term f o r Cu l i e s w e l l below that of the P i / » 3/2 term. The spin o r b i t coupling constant f o r Ag020,7cm- ) i s considerably l a r g e r than f o r 1

10

2

y

9

2

1 0

1

2

2

Χ

2

3

2

3

2

2

2

2

2

2

2

3

2

2

2

1

1

Curtail- ) . When these atomic species are embedded i n a s o l i d rare gas, dramatic a l t e r a t i o n s i n t h e i r s p e c t r o s c o p i c and p h o t o l y t i c pro-

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

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p e r t i e s ensue which can be traced to s p e c i f i c guest-host i n t e r a c t i o n s operating i n t h e i r ground and e x c i t e d s t a t e s ( 3 0 , 3 1 , 3 4 ) . These perturbations can be described i n terms of a t t r a c t i v e van der Waals i n t e r a c t i o n s and short range r e p u l s i v e f o r c e s due to overlap between charge clouds on the metal and matrix atoms ( 4 2 ) . It i s evident that the p o t e n t i a l energy of i n t e r a c t i o n between a metal atom and a support w i l l i n general be d i f f e r e n t f o r d i s t i n c t e l e c t r o n i c s t a t e s of the metal atom, since the p o l a r i z a b i l i t y , r a d i a l extent and symmetry of the e l e c t r o n i c charge d i s t r i b u t i o n w i l l g e n e r a l l y vary from one e l c t r o n i c s t a t e to another. One of the o b j e c t i v e s of t h i s paper i s to evaluate the spect r o s c o p i c and photochemical consequences of the occurrence of markedly disparate guest-host i n t e r a c t i o n s i n the ground and o p t i c a l l y e x c i t e d s t a t e s of Cu and Ag atoms, and some of t h e i r low n u c l e a r i t y c l u s t e r s , i n rare gas as w e l l as other supports. O r i g i n a l papers should be consulted f o r d e t a i l s . In a l l d i s c u s s i o n s of metal support e f f e c t s , whether weak or strong, a knowledge of the l o c a l s t r u c t u r e and symmetry of the metal s i t e i s mandatory.For the s o l i d rare gases, the a v a i l a b l e spectroscopic (43) and t h e o r e t i c a l (44) evidence leans h e a v i l y towards s u b s t i t u t i o n a l i n c o r p o r a t i o n i n t o a tetradecahedral s i t e with 0 symmetry, as the most probable l o c a t i o n f o r entrapped Cu, Ag and Au atoms. In p r i n c i p l e ^ m a t r i x EXAFS measurements could place t h i s proposal on a concrete f o o t i n g . Comparison of the estimated van der Waals diameters of Cu, Ag, Au atoms ( 4 . 4 0 , 4 . 7 0 , 4.60Â r e s p e c t i v e l y ) with the known s u b s t i t u t i o n a l s i t e diameters f o r Ar, Kr, and Xe s o l i d s ( 3 . 8 3 , 4.05, 4.41Â respectively) suggest that t h i s would be a reasonable trapping s i t e arrangement e s p e c i a l l y f o r Cu atoms i n Xe, although a rather l a r g e mismatch i n s i z e would occur f o r Ag atoms i n Ar matrices. I t i s expected that these comparisons are u s e f u l only as rough q u a l i t a t i v e guidelines. n

Atomic S i l v e r i n Rare Gas

Supports

Consider the absorption s p e c t r a f o r a t o m i c a l l y dispersed Ag in Ar, Kr, Xe (30) (Figure 1 ) . Instead of d i s p l a y i n g the gas phase s p i n - o r b i t doublet p a t t e r n with i t s c h a r a c t e r i s t i c 1:2 i n t e n s i t y r a t i o f o r the S \ l i l l > $\l$12 t r a n s i t i o n s , one observes three w e l l resolved components (major blue s i t e ) with roughly equal i n t e n s i t i e s , with the s p l i t t i n g between the low energy component and the mean of the two high energy components being close to the gas phase s p i n - o r b i t s p l i t t i n g energy. E x c i t a t i o n of any one of these three l i n e s r e s u l t s i n r a p i d bleaching at equal rates with concurrent growth of Ag c l u s t e r absorptions s t r a d d l i n g the Ag atomic bands ( 2 9 , 3 0 ) . This phenomenon of photoaggregation has been found to be a s e n s i t i v e f u n c t i o n of the metal d i s p e r s i o n , nature and temperature of the support. During the p h o t o l y s i s of these Ag atom-rare gas f i l m s one cannot help but n o t i c e intense v i s i b l e fluorescence (Figure 2 ) . A time 2

2

n

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

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Figure 1. Optical absorption spectra of well-isolated A g atoms in Ar, Kr, and Xe matrices (Ag/inert gas ^l/ΙΟ ) at 10-12 K. (Reproduced from Ref. 30. Copyright 1980, American Chemical Society.) 5

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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Dynamic

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Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

20.

300

400

500

600

700 nm

Figure 2. Comparison of the major emission bands of A g atoms isolated in Ar, Kr, and Xe matrices. The ordinate represents emission intensity in arbitrary units, and the corresponding excitations are illustrated on the absorption spectra at the left. (Reproduced from Ref. 30. Copyright 1980, American Chemical Society.)

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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resolved study of the two main emissions f o r Ag/Xe using a pulsed Ν 2 l a s e r , y i e l d e d corrected r a d i a t i v e l i f e t i m e s (^12ns) w i t h i n a f a c t o r of two of that of the ^3/2 l e v e l of f r e e s i l v e r atoms (6.5±0.6ns at 328nm)(45). T h i s suggests that the f l u o r e s c e n t states i n question decay e x c l u s i v e l y by the observed r a d i a t i v e pathways since l i f e t i m e s s i g n i f i c a n t l y shorter than the f r e e atom values would be expected i f concurrent n o n - r a d i a t i v e decay pro­ cesses were important. The measurements a l s o demonstrate that both f l u o r e s c e n t s t a t e s were formed w e l l w i t h i n nano seconds f o l l o w i n g e x c i t a t i o n of the s i l v e r atoms, c o n s i s t e n t with the p i c t u r e of a f a s t v i b r a t i o n a l r e l a x a t i o n process and a r e l a t i v e l y long r a d i a t i v e l i f e t i m e such that emission o c c u r s a f t e r the matrix cage has a t t a i n e d i t s f u l l y relaxed c o n f i g u r a t i o n . T h i s r a i s e s the issue of the occurrence of s h i f t s between the matrix and the gas phase e l e c t r o n i c t r a n s i t i o n energies. These can be understood i n terms of d i f f e r e n c e s between the interatomic p o t e n t i a l s which apply f o r ground and e l e c t r o n i c a l l y e x c i t e d states of the metal atom i n i t s matrix cage. I f the net i n t e r ­ a c t i o n energy between the metal atom and the matrix i s not iden­ t i c a l i n the two s t a t e s involved i n an e l e c t r o n i c t r a n s i t i o n then a matrix s h i f t i n the t r a n s i t i o n energy r e s u l t s as seen f o r Ag in Ar, Kr, and Xe. The i n t e r a c t i o n energy f o r the e x c i t e d s t a t e of the metal atom r e f e r s i n t h i s context to the i n t e r a c t i o n where the c o n f i g u r a t i o n of the rare gas atoms about the metal atom i s unchanged between the ground and e x c i t e d s t a t e s , that i s the l a t t i c e i s " f r o z e n " during the e l e c t r o n i c t r a n s i t i o n . Since the interatomic p o t e n t i a l s change as a r e s u l t of the e l e c t r o n i c t r a n s i t i o n , there w i l l be a tendency f o r the rare gas atoms to r e l a x to a new e q u i l i b r i u m c o n f i g u r a t i o n about the e x c i t e d metal atom. I f the guest-host i n t e r a c t i o n s are appreciably d i f f e r e n t between the ground and e x c i t e d s t a t e s , then the s i t u a t i o n imme­ d i a t e l y f o l l o w i n g absorption i s one i n which the matrix cage i s i n a s t a t e of high v i b r a t i o n a l p o t e n t i a l energy. In the absence of very f a s t n o n - r a d i a t i v e e l e c t r o n i c r e l a x a t i o n processes the system would tend to v i b r a t i o n a l l y r e l a x and thus the matrix cage would a t t a i n the e q u i l i b r i u m c o n f i g u r a t i o n appropriate f o r the e x c i t e d s t a t e of the metal atom. In t h i s way p a r t of the e l e c ­ t r o n i c energy i s channelled d i r e c t l y i n t o v i b r a t i o n a l e x c i t a t i o n of the host l a t t i c e . I t i s u s e f u l to view o p t i c a l absorption and emission processes i n such a system i n terms of t r a n s i t i o n s between d i s t i n c t v i b r a ­ t i o n a l l e v e l s of 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 of a metal atom-rare gas complex or quasi-molecule. Since the v i b r a ­ t i o n a l motions o f the complex are coupled with the bulk l a t t i c e v i b r a t i o n s , a complicated p a t t e r n of c l o s e l y spaced v i b r a t i o n a l l e v e l s i s involved and t h i s r e s u l t s i n the appearance of a smooth, s t r u c t u r e l e s s absorption p r o f i l e (25). Thus the homogeneous width of the absorption band a r i s e s from a coupling between the e l e c t r o n i c s t a t e s of the metal atom and the host l a t t i c e v i b r a ­ t i o n s , which i s induced by the d i f f e r e n c e s between the guest-host

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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i n t e r a c t i o n s i n the ground and e x c i t e d s t a t e s of the metal atom. Inhomogeneous c o n t r i b u t i o n s to the band width most l i k e l y a r i s e from the occurrence of a d i s t r i b u t i o n of s l i g h t l y d i f f e r e n t matrix trapping s i t e s f o r the metal atom. The fluorescence p r o f i l e s of Ag i n say Kr, b a s i c a l l y shows two major emission bands around 415 and 510 nm which can be traced through t h e i r e x c i t a t i o n dependence to o r i g i n a t e from the three S+ P atomic components. S p e c i f i c a l l y , the lowest energy absroption band gives r i s e to only the lowest energy emission system, whereas the two higher energy absorption bands give r i s e to both emission band systems. The observation of l a r g e Stokes s h i f t s f o r the P - S t r a n ­ s i t i o n of entrapped Ag atoms i n d i c a t e s that the guest-host i n t e r ­ a c t i o n s are markedly d i f f e r e n t f o r the S and P states of t h i s system and can be explained i n terms of matrix cage r e l a x a t i o n effects. An i n t e r e s t i n g way to v i s u a l i z e the o r i g i n of these s h i f t s i s to consider the ways i n which twelve rare gas atoms i n the symmetry of a s u b s t i t u t i o n a l s i t e can approach a s i l v e r atom i n i t s ground and e x c i t e d s t a t e s and to evaluate the van der Waals a t t r a c t i v e forces and r e p u l s i v e i n t e r a c t i o n s that c o n t r i ­ bute to the b i n d i n g energy of the cage-complex (46). It i s w e l l known that one e l e c t r o n wave functions f o r the |j,m. > s p i n - o r b i t l e v e l s a r i s i n g from a np c o n f i g u r a t i o n can be w r i t t e n i n the f o l l o w i n g form (47): 2

2

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

2

2

2

1

I 3/2,

± 3/2

1 > = (1/2)

I 3/2,

± 1/2

> = (2/3)^

11/2,

± 1/2

/ 2

(ρ

± ip )|±l/2>

χ

p J ± l / 2 > + ( l / 6 ) ( p + i p ) | ; l/2> / 2

a

1/2

> = -(l/3) p |±l/2> +(l/3)

1/2

z

y

(p ±ip ) | ; l/2> x

where Ρ , Py and p r e f e r to the r e a l p - o r b i t a l s and |± l/2> i s the spin f u n c t i o n corresponding to m = ± 1/2. Here 13/2, ± 3/2> and |3/2, ± l/2> are the degenerate components of P3/2 I 1/2, ± l/2> corresponds to P i / 2 > where each s t a t e i s a Kramers doublet (48). M u l t i p l i c a t i o n of these wave functions by t h e i r complex conjugates and i n t e g r a t i n g over the spin v a r i ­ able y i e l d s the f o l l o w i n g expressions f o r the associated charge distributions : χ

z

g

a n d

P

P

P

2

2

3/2,

± 3/2

=

1/2(P

X

3/2,

± 1/2

=

1/6(Ρ

χ

l / 2 , ± 1/2

=

1/3(Ρ

χ

2

+

2

P ) y

2

+ P )

+ 2/3

y

2

+ P

2 y

+

p

2 z

2

P ) z

Thus the van der Waals s t a b i l i z a t i o n should be maximized i n the P3/2 312 s t a t e by allowing the rare gas atoms above anà below the xy plane to approach the metal atom more c l o s e l y than those l y i n g i n the xy plane. In terms of the s u b s t i t u t i o n a l s i t e , t h i s corresponds to an a x i a l c o n t r a c t i o n along the z-axis, which 2

±

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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21 ST

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should l e a d t o a d e s t a b i l i z a t i o n o f t h e P 3 / 2 > l / 2 s t a t e because o f t h e o v e r l a p b e t w e e n t h e p c h a r g e d e n s i t y on t h e m e t a l atom and t h e c h a r g e c l o u d s on t h e a x i a l r a r e gas atom. An e x p a n s i o n a l o n g t h e z - a x i s w o u l d r e v e r s e t h i s s i t u a t i o n and c a u s e t h e P 3 / , ± 1/2 s t a t e t o be s t a b i l i z e d . I t c a n b e s e e n i n t h i s way t h a t an a x i a l d i s t o r t i o n f r o m 0^ symmetry g i v e s r i s e t o a s p l i t ­ t i n g of the P / 2 l e v e l i n t o P / 2 > ± 3/2 * 3 / 2 > l / 2 com­ p o n e n t s s u c h t h a t one o f t h e s e components i s s t a b i l i z e d b y t h e distortion. T h e s e i d e a s c a n be p u t on a more s o l i d f o o t i n g b y d i s c u s s i n g t h e m a t r i x cage r e l a x a t i o n p r o c e s s i n t h e P e x c i t e d s t a t e o f Ag i n terms o f a J a h n - T e l l e r e f f e c t . I t c a n be shown t h a t an a x i a l d i s t o r t i o n i s r e q u i r e d b y symmetry c o n s i d e r a t i o n . The above s i m p l y p r o v i d e s a p h y s i c a l p i c t u r e o f t h i s r e q u i r e m e n t i n terms o f v a n d e r Waals s t a b i l i z a t i o n o f t h e s i l v e r a t o m - r a r e gas c o m p l e x . The a s p e c t o f t h e J a h n - T e l l e r p r o b l e m (49) w h i c h i s o f i n ­ t e r e s t i n the present context i s the form o f the n u c l e a r poten­ t i a l e n e r g y s u r f a c e f o r a P e l e c t r o n i c s t a t e i n t h e AgXj2 quasi-molecule. This s t a t e transforms as T ± i n 0 ^ symmetry. O r b i t a l a n g u l a r momentum i s n o t quenched i n a T state. I f s p i n - o r b i t c o u p l i n g i s v e r y s t r o n g , as i n t h e case o f Ag, then the J a h n - T e l l e r e f f e c t i s o f s e c o n d a r y i m p o r t a n c e a n d t h e T state s p l i t s into G /2 d Ei/2 s p i n - o r b i t components ( c o r r e l a t i n g d i r e c t l y w i t h the P3/2 and Ρχ/2 states of the f r e e atom) where t h e symmetry l a b e l s d e n o t e i r r e d u c i b l e r e p r e ­ s e n t a t i o n s o f t h e d o u b l e g r o u p O^ appropriate f o r handling h a l f a n g u l a r momenta s t a t e s . I n t h e l i m i t o f s t r o n g s p i n - o r b i t coupling (50), i t i s reasonable t o consider J a h n - T e l l e r i n t e r ­ a c t i o n s only i n the G / 2 u s t a t e r a t h e r than i n the T term as a w h o l e , w h i c h seems more a p p r o p r i a t e f o r Cu ( s e e l a t e r ) . The v i b r a t i o n a l modes t h a t c a n c o u p l e i n f i r s t o r d e r w i t h a ^3/2u s t a t e a r e contained i n the antisymmetric square i 3/2 } l g + g + T . C o u p l i n g w i t h e modes s t a b i l i z e s t e t r a g o n a l d i s t o r t i o n s f r o m Oh symmetry w h e r e a s c o u p l i n g w i t h t2g modes s t a b i l i z e s t r i g o n a l d i s t o r t i o n s . These a x i a l d i s ­ t o r t i o n s remove t h e d e g e n e r a c y o f t h e two K r a m e r s d o u b l e t s c o r ­ r e s p o n d i n g t o t h e G / 2 u s t a t e . The s o l u t i o n o f t h e s t a t i c p a r t of the J a h n - T e l l e r problem i n v o l v i n g a G /2u e l e c t r o n i c s t a t e i n t e r a c t i n g w i t h e g o r t 2 g v i b r a t i o n a l modes i s w e l l known ( 5 1 ) . On t h e b a s i s o f t h e d e s c r i p t i o n g i v e n e a r l i e r o f t h e symmetry p r o p e r t i e s o f the charge d i s t r i b u t i o n a s s o c i a t e d w i t h the de­ generate sublevels of a P /2 s t a t e i ti s suggested that t h e s t r o n g e s t v i b r o n i c c o u p l i n g s h o u l d i n v o l v e e,g s t r e t c h i n g v i ­ brations. The f o r m o f t h e t e t r a g o n a l d i s t o r t i o n w h i c h i s e x p e c ­ t e d t o be s t a b l i l i z e d b y c o u p l i n g w i t h e g s t r e t c h i n g modes ( 5 2 ) i s i l l u s t r a t e d i n F i g u r e 3. T h e f o r m o f t h e p o t e n t i a l e n e r g y s u r f a c e (50,51) f o r t h i s c a s e i n t h e s i m p l e s t t h e o r y i s a l s o illustrated i n F i g u r e 3. The u p p e r a n d l o w e r b r a n c h e s o f t h e p o t e n t i a l e n e r g y s u r f a c e l a b e l l e d U i a n d U2 r e p r e s e n t t h e two e l e c t r o n i c s t a t e s which replace the o r i g i n a l degenerate s t a t e . ±

z

2

2

2

2

3

a n c

3

±

2p

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

2

2

2

l u

2

l u

2

a

3

n

2

U

u

2

2

1

2

2

3

l u

2

G

u

2

=

A

E

2 g

g

2

3

3

2

3

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

20.

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MITCHELL

Dynamic

Processes of

Metals

313

Figure 3. A: Tetragonal distortion of an MX tetradecahedral complex. B: Schematic potential energy surface for a doubly degenerate electronic state in 0 symmetry interacting with a doubly degenerate vibrational mode, neglecting anharmonicity effects. V represents the nuclear potential energy and Q and Q represents the degenerate components of an e vibrational mode. C: Scheme of the ground and excited state potential energy surfaces for AgX cage complexes (X — Ar, Kr, or Xe). The absorption and emission processes are illustrated on this configuration coordinate diagram f49—51Λ 12

h

2

3

g

12

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

314

INORGANIC C H E M I S T R Y : TOWARD THE

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The separation of the Ui and U2 branches at any p o i n t i n the (Q2> Q3) coordinate space represents the v i b r o n i c s p l i t t i n g of the G 3 / 2 U s t a t e . I f anharmonic terms are included i n the nu­ c l e a r p o t e n t i a l then the energy surface looses i t s c y l i n d r i c a l character and r e t a i n s only the three f o l d symmetry a r i s i n g from the cubic symmetry of the Hamiltonian (53). In t h i s case, the p o t e n t i a l surface has three e q u i v a l e n t , t r i g o n a l l y disposed minima separated by saddle p o i n t s , the minima corresponding to e q u i v a l e n t t e t r a g o n a l d i s t o r t i o n s along the x,y,z d i r e c t i o n s of the molecule f i x e d c a r t e s i a n coordinate system. The curve l a b e l l e d U3 represents the p o t e n t i a l energy surface f o r t h e P x / 2 s t a t e i n which account has been taken of the e f f e c t s of P 3 / 2 ~ l / 2 mixing by a second order p e r t u r b a t i o n treatment (53). The c o n f i g u r a t i o n coordinate diagram described above can be used to i n t e r p r e t the major s p l i t t i n g s i n the absorption and emission s p e c t r a of entrapped Ag atoms, i n c l u d i n g the o r i g i n of the Stokes s h i f t s , the d i r e c t i o n of the s h i f t s Xe>Kr>Ar, the temperature dependence of the absorption spectrum, fluorescence l i f e t i m e s and provides a b a s i s f o r s p e c u l a t i o n concerning the o r i g i n of photoinduced d i f f u s i o n and agglomeration e f f e c t s described e a r l i e r (30). Thus, o p t i c a l e x c i t a t i o n to the ϋχ, U2 or U l e v e l s should be followed by v i b r a t i o n a l r e l a x a t i o n to the minimum point of the r e s p e c t i v e p o t e n t i a l energy surfaces and subsequent r a d i a t i v e decay to the ground s t a t e surface as i l l u s t r a t e d i n Figure 3. The large Stokes s h i f t s of the emission bands are seen to be the consequence of the tendency f o r d i s t o r t i o n of the e x c i t e d s t a t e complexes. The s h i f t s a r i s e both from the s t a b i l i z a t i o n of the e x c i t e d s t a t e and accompanying d e s t a b i l i z a t i o n of the ground s t a t e and i s expected to follow the order Xe>Kr>Ar as observed(30). I t i s l i k e l y that the d e s t a b i l i z a t i o n caused by producing ground s t a t e complexes i n the relaxed e x c i t e d s t a t e c o n f i g u r a t i o n , provides the d r i v i n g force f o r photoinduced d i f f u s i o n of the Ag atoms (29). Perhaps the a n i s o t r o p i c f o r c e s a r i s i n g from the a x i a l d i s t o r t i o n of the A g X x 2 complex, causes the Ag atom i n i t s ground e l e c t r o n i c s t a t e to be e j e c t e d from i t s trapping s i t e . The e j e c t e d s i l v e r atom could d i s p l a c e one of the surrounding rare gas atoms i n such a way that an exchange of the o r i g i n a l s i t e p o s i t i o n s occur. T h i s would have the e f f e c t of moving the s i l v e r atom one i n t e r s i t e distance and r e s t o r i n g the o r i g i n a l s t r u c t u r a l arrangement of the trapping s i t e . As the Ag atom absorption p r o f i l e s g e n e r a l l y tend to r e t a i n t h e i r w e l l d e f i n e d s t r u c t u r e during photoinduced aggregation experiments t h i s suggests that the Ag atoms migrate between s i t e s which are not very d i f f e r e n t in structure. 2

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

2 p

3

Atomic Copper i n Rare Gas

Supports 2

2

At f i r s t glance, the three f o l d s p l i t t i n g of the P - S absorp­ t i o n band of Cu i n Ar, Kr and Xe matrices might lead one to

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

20.

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AND

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Processes of

315

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expect s i m i l a r s p e c t r o s c o p i c and p h o t o l y t i c p r o p e r t i e s to those described f o r Ag atoms (Figure 4). This i s c e r t a i n l y not the case and d i f f e r e n c e s can be traced to diminished importance of s p i n - o r b i t coupling e f f e c t s and to the a c c e s s i b i l i t y of the low l y i n g Ι>3/2>5/2 term f o r Cu compared to Ag atoms (34). These changes t r a n s l a t e experimentally i n t o d i s t i n c t r e l a x a t i o n mecha­ nisms f o r the e x c i t e d s t a t e cage complexes of Cu and Ag atoms, which manifest themselves f o r example, i n d i f f e r e n t photoaggregation k i n e t i c behaviour (54). This can be seen i n Figure 5 where the M/Kr r a t i o i s lower by a f a c t o r of two f o r Ag compared to Cu. I f s i m i l a r considerations described f o r Ag atoms are a p p l i e d to Cu atoms i n rare gas s o l i d s taking i n t o account that the JahnT e l l e r e f f e c t i n the T e x c i t e d e l e c t r o n i c s t a t e dominates over that of s p i n - o r b i t coupling, then one can proceed with the a n a l y s i s using the o r b i t a l wave functions representing the T s t a t e i n 0^ symmetry (49). By n e g l e c t i n g s p i n - o r b i t coupling and anharmonicity e f f e c t s , the s o l u t i o n to the problem of l u ® e ^ v i b r o n i c coupling, i s an e x c i t e d s t a t e p o t e n t i a l energy surface i n (Q , Q3) normal coordinate space which c o n s i s t s of three d i s j o i n t (mutually orthogonal) paraboloids (49,51). In t h i s case, the T +• A^g absorption band i s expected to be a s i n g l e Gaussian with a s i n g l e relaxed emission band from the minimum of the upper s t a t e paraboloids. This scheme i s not c o n s i s t e n t with the s p e c t r a l observations of a t h r e e f o l d s p l i t ­ t i n g of the P +• S absorption p r o f i l e (34,55). On the other hand,for dominant T ^ t v i b r o n i c coupling, the upper s t a t e p o t e n t i a l surface has been shown to c o n s i s t of three sheets with four equivalent minima i n (Qt+j Q5, Qg) space the l a t t e r being the normal coordinates spanning the t r e p r e s e n t a t i o n (49,51).Such a p o t e n t i a l energy hypersurface can be seen to y i e l d a t h r e e f o l d s t r u c t u r e i n absorption with the expectancy of a complex emis­ sion p r o f i l e . Thus the T j © t g coupling scheme can a c c u r a t e l y describe the absorption p r o f i l e of C u X i , whereas a strong s p i n o r b i t and weak v i b r o n i c coupling model would appear to be more appropriate f o r A g X (30,34,55). P h o t o e x c i t a t i o n of Cu rare-gas f i l m s i n the region of the P S absorption band produces intense narrow emissions bands showing large s p e c t r a l red s h i f t s as seen i n Figure 4,(34). In contrast to Ag, these emission p r o f i l e s are i n s e n s i t i v e to v a r i a t i o n s of the e x c i t a t i o n wavelength w i t h i n the t h r e e f o l d s t r u c t u r e of the P «- S absorption band. Simultaneous with the p h o t o l y s i s of any of the three P +• S components, one observes gradual bleaching of a l l l i n e s with concurrent formation of C i ^ where η = 2 - 5 (34,56). A f u r t h e r i n t r i g u i n g observation con­ cerns the appearance of a weak s t r u c t u r e d emission near 420 nm f o r e x c i t a t i o n s centered on the secondary atomic s i t e band of Cu i n a l l three rare gas f i l m s (Figure 4), which has been found from independent s t u d i e s of the absorption and emission s p e c t r a of matrix entrapped Cu to a r i s e from the Α-state of Cu (34). 2

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

l u

2

l u

T

2

2

2

l u

2

2

l u

2

g

2

u

g

2

2

12

2

2

2

2

2

2

2

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INORGANIC C H E M I S T R Y : TOWARD THE

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

316

350

400

450

700

800

750 l\(nm)

850

21ST C E N T U R Y

900

Figure 4. Fluorescence spectra of Cu atoms isolated in solid Ar at 12 Κ (Cu/Ar ~ 1/10 ) uncorrected for instrumental factors. The corresponding excitation wave­ lengths are indicated on the absorption spectrum shown at the upper left. (Repro­ duced from Ref. 34. Copyright 1982, American Chemical Society.) 4

Figure 5. Agglomeration of Cu and A g atoms in solid Kr at 10-12 Κ induced by P C u ( 2 p )

1

Cu (A n ) ^ C u ( X ^ ^ + h v (415nm) 2

2

2

-» (3) Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

u

Cu( S)

where the wavelengths i n parenthesis apply to Ar. Here (1) and (3) represent n o n - r a d i a t i v e t r a n s i t i o n s which d i s s i p a t e p a r t , or a l l , r e s p e c t i v e l y of the P +• S e x c i t a t i o n energy i n t o the rare gas l a t t i c e . The occurrence of process (1) can be recognized d i r e c t l y from the fluorescence spectra since P S photoexcit a t i o n i s followed by D S emission with no s i g n of P •> D fluorescence. S i m i l a r l y the absence of P S fluorescence suggests that process (3) i s important, since the quantum y i e l d f o r D •> S fluorescence i s s i g n i f i c a n t l y l e s s than u n i t y as seen from comparing the fluorescence i n t e n s i t i e s f o r Cu atoms trapped i n two d i f f e r e n t matrix s i t e s , where the m a j o r i t y s i t e (>90%) c l e a r l y shows a smaller quantrum y i e l d f o r D -> S f l u o r e scence f o l l o w i n g P «- S p h o t o e x c i t a t i o n . Although the d e t a i l s of these n o n - r a d i a t i v e t r a n s i t i o n s are not p r e s e n t l y c l e a r , i t seems reasonable to suggest that the energy released i n t o the matrix by processes (1) and/or (3) could account f o r the observed photo-induced d i f f u s i o n and aggregation processes of Cu atoms i n rare gas matrices (34,56). Related to t h i s photoinduced d i f f u s i o n e f f e c t i s process (2) of the above scheme, which d e p i c t s an e x c i t e d s t a t e d i m e r i z a t i o n r e a c t i o n i n v o l v i n g Cu( D)+Cu( S) separated atoms, forming Cu i n an e l e c t r o n i c a l l y e x c i t e d s t a t e as seen by the observation of v i b r a t i o n a l ^ relaxed A •> X Cu fluorescence f o l l o w i n g P S Cu atomic e x c i t a t i o n . T h e f o l l o w i n g points can be c i t e d i n support of the e x c i t e d s t a t e r e a c t i o n p r o p o s a l . F i r s t the D Cu atom e x c i t e d s t a t e i s l i k e l y to be q u i t e long l i v e d as a consequence of the forbidden nature of the o p t i c a l t r a n s i t i o n s to the ground s t a t e . Second, the formation of these s t a t e s should be accompanied by a release of considerable energy i n t o the matrix as a r e s u l t of the P •> D n o n - r a d i a t i v e t r a n s i t i o n . This could r e s u l t i n " l o c a l " melting or matrix s o f t e n i n g which would promote d i f f u s i o n of the D e x c i t e d Cu atoms and allow f o r r e a c t i v e encounters with nearby ground s t a t e S Cu atoms. These considerations suggest that photoinduced d i f f u s i o n and aggregation of Cu atoms i n rare gas s o l i d s i s promoted by P ->• S and P •> D n o n - r a d i a t i v e t r a n s i 2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INORGANIC C H E M I S T R Y :

318

TOWARD T H E

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CENTURY

tions (34), i n c o n t r a d i s t i n c t i o n to Ag atoms, which i s b e l i e v e d to i n v o l v e P e x c i t e d Ag atom-cage r e l a x a t i o n processes r e s u l t i n g i n strong d e s t a b i l i z a t i o n of ground s t a t e Ag atoms (30). 2

Dicopper and P i s i l v e r i n Rare Gas Supports Studies of the energetics and dynamics of Cu2 and Ag2 i n rare gas s o l i d s have a l s o been completed (31,34). The absorption and fluorescence s p e c t r a are s i m i l a r l y i n d i c a t i v e of strong guesthost i n t e r a c t i o n s i n the low l y i n g s t a t e s of Cu and Ag2» Rather than presenting the s p e c t r o s c o p i c and photolyt ftic details, a summary of the observed r a d i a t i v e r e l a x a t i o n processes of v i s i b l e and uv e x c i t e d Cu2 and Ag2 i n rare gas s o l i d s i s shown below: 2

Cu( D / )+Cu( S /2)^Cu( Si 2)+Cu( S 2

Cu (B) Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

2

hv(400nm)

2

3

2

2

1

2

/

' ^

1 / 2

)

+hv( 756nm)

Cu ocr

(2)

2

hv(280nm)" Cu (C) 2

(3) >Cu (A) + Cu (X) + hv(415nm) 2

2

It i s proposed that the B-state o f C u (bound i n the gas phase) (57) i s s u f f i c i e n t l y s t r o n g l y d e s t a b i l i z e d i n the matrix to the extent that i t i s unstable with respect to d i s s o c i a t i o n to Cu( D3/2) + C u ( S i / 2 ) fragments f o l l o w i n g p h o t o e x c i t a t i o n of Cu2 from the ground s t a t e , process (1) i n above scheme. The extent to which the d i s s o c i a t i o n a c t u a l l y occurs depends on the l o c a l dynamics f o l l o w i n g p h o t o e x c i t a t i o n and the d e t a i l s of the Cu2~rare gas p o t e n t i a l s f o r the s p e c i f i c trapping s i t e involved. The absorption and fluorescence s p e c t r a o f A g i n rare gas s o l i d s are a l s o c l e a r l y i n d i c a t i v e of strong guest-host i n t e r actions i n v o l v i n g the A ! * and C Instates of Ag2 as summarized below: 2

2

2

2

1

(1) hv (260nn0 J 2 < \ > A

1

1

1

A g ( n ' ) _ ^ A g ( Z + ) + hv(285nm) ( 2 )

2

u

2

l

A g ( 2 S ) z

1

Ag( P)+Ag ( S+) 2

\ hv(390nm)

l ( 3 )

2

1

Ag( S)+Ag ( E+)+hv(326,365,420,460,480nm) 2

1

Ag ( Z+) 2

2

2

Ag( P) + Ag( S) 2

2Ag( S) + hv(460,480nm)

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

20.

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Processes of

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The emission spectrum produced by A

1

+

C u ( A n )+Cu (X E )+hv(415) 2

2

U

12K

9

i n d i c a t e d that a p r e r e q u i s i t f o r performing condensed phase e x c i t e d s t a t e metal atom chemistry was a weak i n t e r a c t i o n between the ground s t a t e metal atomic species and the matrix. This would allow f o r the preparation of the desired e x c i t e d state metal atomic s p e c i e s , as i l l u s t r a t e d i n the r e c e n t l y discovered a c t i v a t i o n of CH^ by photoexcited metal atoms (33): 2

Cu( S

1 / 2

){CHi } +

h

v

(

3

2

Q

n

1 2

m

)

2

>

Cu( P

l / 2

,

3 / 2

){CHi } +

1 2

+ CH CuH 3

12K weak ground s t a t e cage-complex

strong e x c i t e d state cage-complex

photoinsertion

On the other hand to perform e x c i t e d s t a t e metal atom chemistry i n the M/CH Br system i s not quite so s t r a i g h t f o r w a r d (58): 3

Fe Co Ni

CH Br 3

M{BrCH } hv h i g h l y coloured strong ground state charge t r a n s f e r complex 3

12K

n

^

no

reaction

because the metal atomic reagents tend to form strong ground s t a t e charge t r a n s f e r complexes,thereby precluding the a b i l i t y of achieving the desired metal atomic e x c i t e d s t a t e s . One way to surmount t h i s d i f f i c u l t y i s to work i n d i l u t e matrices as can be seen i n the ground and e x c i t e d s t a t e r e a c t i v i t y patterns of Cu atoms towards dioxygen summarized i n the scheme below (37):

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

320

INORGANIC C H E M I S T R Y : TOWARD THE

Ρ P-Cu-0

21ST C E N T U R Y

hv(290) ^O-Cu-0

/

0

0 345nm(LMCT)

290nm(LMCT)

1

1090cm" (vOO)

1

1100cm~ (v00)

(photoisomerization) z

Cu( S!/ ) 2

0 /Xe

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

2

\

(1/10-1/100) 12K ^ O-Cu-0

/

+

2

Cu-0

290nm _ 1100cm"

+ Cu( S / ){Xe}i2 1

2

hv(310) 12K

224nm _ 1000cm" 2

(Cu( P){Xe}

12

I 0··-Cu = 0 "0 2

Λ Cu

4

2

Cu( D){Xe}

(perturbed copper oxide monomer)

12

(mobile)

-»0

0 The above r e a c t i o n scheme was e s t a b l i s h e d by a combination of u v - v i s i b l e absorption and fluorescence, i r i s o t o p i c s u b s t i t u t i o n , esr and k i n e t i c measurements (37). The important point to note here i s that i n 0 r i c h Xe matrices, ground s t a t e C u ( S i / ) can­ not avoid r e a c t i v e encounters with 0 to form C u ( 0 ) and Cu(0 ) dioxygen complexes,whereas i t i s proposed that the formation of CuO, Cu(03) and 0 i n d i l u t e 0 /Xe matrices a r i s e s from the r e ­ a c t i o n of a long l i v e d mobile e x c i t e d s t a t e Cu( D) with 0 . On the other hand the r e a c t i o n s of photoexcited Ag( P) with 0 are d i f f e r e n t (37), e l e c t r o n t r a n s f e r being favoured to form Ag*0 ~". I t i s thought that t h i s d i f f e r e n c e between the GS/ES r e a c t i v i t y of Cu and Ag atoms with 0 o r i g i n a t e s i n the a c c e s s i b i l i t y of the r e a c t i v e Cu( D) s t a t e from the p h o t o l y t i c a l l y prepared Cu( P) s t a t e which i s not p o s s i b l e f o r Ag( P) as i l l u s t r a t e d i n Figure 6. R e a c t i v i t y d i f f e r e n c e s between P · D n o n - r a d i a t i v e r e l a x a t i o n i s favoured f o r apex p o s i t i o n s , then t h i s could lead to H-atom a b s t r a c t i o n , whereas the edge or face o r i e n t a t i o n could lead to d i r e c t i n s e r t i o n of C u ( P ) . In e i t h e r case, photoexcited Cu( P) i s e f f i c i e n t l y chemically quenched by CH^. On the other hand,the D s t a t e of Ag appears not to be a c c e s s i b l e from the P state,so cage r e l a x a t i o n of apex o r i e n t e d CH^ molecules around the Ag( P) s t a t e could lead to ground s t a t e Ag i n a relaxed CH^ cagf, This would cause photoinduced d i f f u s i o n of A g ( S ) atoms f o r reasons s i m i l a r to those proposed f o r the rare gases, r e s u l t i n g i n Ag atom photoagglomeration as a competing pathway to CH^ a c t i v a t i o n . L i k e Cu, a Ag( P) atom l y i n g on an edge or face of CH^ i s proposed to be r e a c t i v e g i v i n g d i r e c t i n s e r t i o n to CH AgH. 2

+

2

2

2

2

2

1

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

2

2

2

2

2

2

2

2

2

2

2

2

2

2

3

Conclusion From the above d i s c u s s i o n , i t should be p o s s i b l e to apprec i a t e how extremely subtle d i f f e r e n c e s i n guest-host i n t e r a c t i o n s i n the ground and e x c i t e d s t a t e s of Cu and Ag atoms and dimers i n both non-reactive and r e a c t i v e supports can lead to d r a m a t i c a l l y d i s t i n c t chemical r e a c t i v i t y patterns and dynamical processes. Photochemical and photophysical phenomena of t h i s kind should provide chemists of the 21st century with a r i c h f i e l d f o r fundamental and a p p l i e d research, o f f e r i n g considerable scope f o r experimental challenges and i n t e l l e c t u a l s t i m u l a t i o n .

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

20.

OZIN A N D M I T C H E L L

Dynamic

323

Processes of Metals

A g / C H ( ^ = ' 3 3 2 n m ; A = 3PO nm)

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

4

ex

Ο t(min) 2

2

Figure 7. First-order kinetic plots for the quenching of P

2

^2,3 (photoaggregation)

( non-radiative 2_ retaxation,mobile AgCS)) CI-^AgH (insertion)

Figure 8.

2

2

Proposed relaxation schemes for P CH CuH (insertion) {CH CuH -> C H C u H (secondary p h o t o l y s i s ) Thermal annealing of the CH ,,,CuH i n t e r a c t i n g p a i r at 30-35K leads t o e f f i c i e n t generation of the i n s e r t i o n product CH CuH, which suggests that t h i s r e a c t i o n i s to some degree exothermic. 3

3

3 t t f t

3

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch020

3

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

21 Synthetic Inorganic Chemistry in Low Temperature Matrices

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch021

J. L . M A R G R A V E , R . H . H A U G E , Ζ. K . I S M A I L , L . F R E D I N , and W. E . B I L L U P S Rice University, Department of Chemistry, Houston, TX 77001

Simplicity and spontaneity are two of the attractive features of many metal atom reactions which justify the prediction that synthetic inorganic chemistry at low temperatures will be an important technique in the next two decades. Pimentel, Skell, Timms, Green, Klabunde, Andrews, Ozin, and others have made significant contributions to this area. 1

3

4

5

6

2

7

For over 15 years we have conducted research utilizing metal atoms in low temperature spectroscopic and synthetic studies at Rice University. Our synthetic work was started in the late 1960s with the work of Krishnan, on lithium atom reactions with carbon monoxide, extended by Meier in his studies of lithium atom reactions with water and ammonia and expanded over the next several years to include metal atom interactions with HF, HO, HN, HC, and their hundreds of organic analogs--RF, RO, ROH, RN, . . . HN, RC, RCH, etc. A most exciting aspect of metal atom chemistry seems to lie in the photoassisted insertion processes by which one can produce molecules like HMF, HMOH, HMNH, HMCH, HNCH, etc. simply by co-depositing metal atoms with the potential reactant material and either through spon­ taneous reaction or through photochemically induced reactions achieving the formation of new molecular species. This approach has already been taken to a multigram preparative scale and there seems to be no reason why one cannot move on to preparation of industrially significant quantities of organometallic materials by this technique. Table I summarizes the studies of Fe and 8

9

10

2

3

4

3

11-15

3

2

3

4

2

3

2

5

2

0097-6156/83/0211-0329$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INORGANIC C H E M I S T R Y : TOWARD T H E 2 1 S T C E N T U R Y

330

TABLE I

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch021

A.

Reactions of Metal Atoms with H 0 and I s o e l e c t r o n i c Species ?

(a)

M + HF

(b)

M + H 0

->

(c)

M + NH

->

(d)

M + CH. 4

(e)

M + ROR

[M:FH]

HMF

+

—» MO HOB

[M:FH ] -*

2

2

3

[M:NH ]

->

1 0 1 0 , 1

[M...CH ] -

HMCH^™^

4

[M:0R ]

RMOR

+

2

MNH

Τ Τ Λ Λ Τ Τ Τ

+

3

•+ MF

Reactions of Metal Atoms with CO and C 0 (a)

M + CO

+

[M ,C0"]

->-

+

[^.(CO)^

o

*

C.

+

[Li ;"0C Ξ C0";Li ]

M + C0

+

+

2

[M ,CO~]

+

+

[M , C 0^] + 2

M

C

2 2°4

Reactions of Metal Atoms with A l i p h a t i c & C y c l i c Hydrocarbons (a)

M + C H — î i U * HMC H

5

(b)

M + C H

?

(c)

Fe + c y c l o - C H , — 3 6

2

6

3

2

HMC H

g

3

Q

^

H C 0

2 |

Fe D.

2^2 +

e.g. (b)

A

CH ,2 CH

0

2

Reactions of Elemental F l u o r i n e i n Low-Temperature Matrices Al (a)

F

2

+ UF

> UF + UF

4

5

6

(complete r e a c t i o n )

CO

» HF^ + F (b)

F

+ CH. *

Z

(c)

F

9

+ C H, ^ 9

2

(no r e a c t i o n )

rΝ

° ?.) CH (no r e a c t i o n ) 10K * D a r k

> 10K

(no r e a c t i o n ) — — — * 10K

CF., CF H, e t c . * C H,F , C H F, e t c . ?

z

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

J

21.

MARGRAVE

E T AL.

Low Temperature

Matrices

331

metal atom chemistry already performed a t Rice U n i v e r s i t y and some of the v a r i o u s molecules which can be prepared. New formulas, new s t o i c h i o m e t r i e s , new s t r u c t u r e s , new r e a c t a n t s , new reducing agents, new o x i d i z i n g agents, and new areas of chemical research w i l l r e s u l t from the s t u d i e s of chemical r e a c t i o n s i n low-temperature matrices. ACKNOWLEDGMENTS Studies o f chemical r e a c t i o n s i n low-temperature matrices have been supported by funding from the N a t i o n a l Science Foundat i o n , the U.S. Army Research O f f i c e , Durham, and from the Robert A. Welch Foundation. Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch021

Literature Cited

1. G.C. Pimentel, Pure Appl. Chem. 4, 61 (1962). 2. P.S. Skell, J.J. Havel and M.J. McGlinchey, Acc. Chem. Res., 6, 97 (1973). 3. P.L. Timms and T.W. Turney, Adv. Organomet. Chem., 15, 53 (1977). 4. M.L.H. Green, Journal of Organomet. Chem., 200, 119 (1980). 5. Chemistry of Free Atoms and Particles, K.J. Klabunde, Academic Press (1980). 6. L. Andrews, Ann. Rev. Phys. Chem. 22, 109 (1971). 7. Cryo Chemistry, M. Moskovits and G.A. Ozin, eds. (Wiley, N.Y., 1973). 8. J.W. Hastie, R.H. Hauge and J.L. Margrave, Spectroscopy in Inorganic Chemistry, Vol. 1, C.N.R. Rao and J.R. Ferro, eds. (Academic Press, 1970), pp. 57-106. 9. C.H. Krishnan, Ph.D. Thesis, Rice University (1975). 10. J.L. Margrave, P.F. Meier and R.H. Hauge, J. Am. Chem. Soc., 100, 2108 (1978). 11. (a) J.L. Margrave, W.E. Billups, M.M. Konarski and R.H. Hauge, Tetranedron Letters, 21, 3861-3864 (1980). (b) J.L. Margrave, W.E. Billups, M.M. Konarski and R.H. Hauge, J. Am. Chem.Soc.,102, 7393 (1980). 12. (a) J.L. Margrave, R.H. Hauge and J.W. Kauffman, J. Am. Chem. Soc., 102, 19, 6005-6011 (1980). (b) J.L. Margrave, R.H. Hauge, J.W. Kauffman and L. Fredin, Proc., Symp. on High Temperature Chemistry, ACS Advances in Chemistry Series, Ed. J.L. Gole (1981) pp. 363-376. 13. J.L. Margrave, M.A. Douglas and R.H. Hauge, Symp. on High Tempeature Chemistry, ACS Advances in Chemistry Series, Ed. J.L. Gole (1981) pp. 347-354. 14. Z.K. Ismail, R.H. Hauge, L. Fredin, J.W. Kauffman and J.L. Margrave, J. Chem. Phys., Aug. 15 (1982). 15. J.L. Margrave, R.H. Hauge, J.W. Kauffman, N.A. Rao, M.M. Konarski, J.P. Bell and W.E. Billups, J. Chem.Soc.,Chem. Comm., 24, 1258-1260 (1981). RECEIVED September 16, 1982

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

22 Heteroatom Cluster Compounds Incorporating Polyhedral Boranes as Ligands NORMAN N. GREENWOOD

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch022

University of Leeds, Department of Inorganic and Structural Chemistry, Leeds LS2 9JT England

Synthetic routes to metalloboranes are briefly summarized. Two specific aspects of the field are selected for more detailed comment: (i) the occurrence of exopolyhedral cycloboronation of P-phenyl groups; (ii) oxidative cluster closure reactions. These topics are illustrated by reference to reactions of closo-B H- 2-, nido­ -B H - and arachno-B H - with a variety of Ir complexes, and several novel metalloborane cluster geometries (as determined by X-ray diffraction analysis) are described, notably nido-[Ir(B H )Η (PPH )2], iso-nido-[{IrC(OH)B H (OMe)}(C H PPh ) (PPh )], closo-[Ir B H )(C6H PPh2) (CO)3(PPh )], iso-closo-[(HIrB H )(C H PPh )(PPh )], arachno[(HIrB H Cl)(CO)(PMe )2], nido-[(IrB H )(CO) (PMe ) ], and closo-[(HIrB H Cl)(PMe )2]. The significance of these results is discussed. 10

10

I

9

12

9

14

9

3

8

3

2

9

8

3

11

4

2

8

4

6

4

6

6

2

3

3

8

8

7

2

2

3

2

13

4

11

3

Boron hydrides are generally regarded as electron deficient compounds in which the lack of electrons is compensated by forming multicentre bonds which lead to polyhedral cluster geometries of the molecules. Three principal series of boranes are now recognized: closo-B H ^", nido-B H +l| and arachno-B H . In addition, there are the hypho-B H +fl and precloso-B H series (so far known only as derivatives) and also the general class of conjuncto-boranes B H formed by joining together various polyhedral fragments from the parent series. Most metals share with boron the property of having fewer valency electrons than orbitals available for bonding; accordingly, in the early 1960 s, we initiated a research programme to investigate the consequences of considering metal atoms as honorary boron atoms, so that they could be incorporated as vertices within the polyhedral borane clusters. This idea has proved enormously fruitful and many groups on both sides of the Atlantic have made notable contributions to the burgeoning field of metalloborane cluster chemistry. As a n

n

n

n

n

n

n

n

n

n

n

m

f

0097-6156/83/0211-0333$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

334

INORGANIC

CHEMISTRY:

TOWARD

THE

21ST

CENTURY

r e s u l t , i t i s now known t h a t most metals (except the most e l e c t r o ­ p o s i t i v e metals of Groups I , I I , and I I I of the P e r i o d i c Table) can be so i n c o r p o r a t e d and t o date some 3 0 metals, having e l e c t r o n e g a t i v i t i e s i n the range 1 . 5 - 2 . 2 , have been shown t o form p o l y h e d r a l metalloborane c l u s t e r s ( c f . e l e c t r o n e g a t i v i t i e s of Β 2 . 1 , C 2 . 5 ) . E q u a l l y s i g n i f i c a n t has been the growing r e c o g n i t i o n that boranes and borane anions can act as e x c e l l e n t polyhapto l i g a n d s t o appropriate metal centres (2). These new perceptions have g r e a t l y expanded the f i e l d o f boron-hydride chemistry and have l e d t o a deeper understanding of the f a c t o r s i n f l u e n c i n g the s t r u c t u r e , thermal s t a b i l i t y , and chemical r e a c t i v i t y of these compounds.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch022

Synthesis of Metalloboranes Numerous routes have been devised f o r the s y n t h e s i s of metalloboranes. Among the most u s e f u l of these are: 1. Coordination o f a metal by a (polyhapto) borane l i g a n d ; examples of a l l modes from t o r r are known. 2. Oxidative i n s e r t i o n o f a metal i n t o a borane c l u s t e r . 3. P a r t i a l degradation o f a borane c l u s t e r i n the presence of a metal complex. k. Expansion, degradation, or m o d i f i c a t i o n of a preformed metalloborane c l u s t e r . These routes are not mutually e x c l u s i v e and many syntheses i n v o l v e aspects o f more than one category. For example r e a c t i o n of arachno-BoHiU" with Vaska's compound t r a n s - [ I r ( C O ) C l ( P P I 1 3 ) 2 ] g i v e s , amongst other products, a small y i e l d o f the n i d o - i r i d a decaborane c l u s t e r [Ir(Bc;Hi3)H(PPh3)2], Figure 1 , whereas i t s r e a c t i o n with the i s o e l e c t r o n i c platinum complex cis-[PtCl2PMe2 Ph)2] gives a high y i e l d of the monodeboronated product arachno[Pt(n3-B8H-|2)(PMe2Ph)2], Figure 2 . T h i s l a t t e r product can be deprotonated and r e a c t e d with a f u r t h e r mole of c i s-[Pt CI2(PMepPh) ^ to y i e l d the 1 0 - v e r t e x diplatinadecaborane c l u s t e r arachno-L£t2 (n ,n -B8H-|o)(PMe2Ph)l|], Figure 3 . Whilst 6-subrogated m e t a l l o decaboranes were p r e v i o u s l y known [see r e f e r e n c e s c i t e d i n (_1_) ] , dimetalladecaboranes had not p r e v i o u s l y been r e p o r t e d . Mixedmetal c l u s t e r s e.g. PtPd, and P t l r can a l s o be prepared by t h i s and r e l a t e d r o u t e s . Rather than attempt a general review o f metallaborane chemistry and the use o f boranes as l i g a n d s (which would n e c e s s a r i l y be s u p e r f i c i a l i n the space a v a i l a b l e ) two s p e c i f i c t o p i c s w i l l be s e l e c t e d i n order t o i l l u s t r a t e the advances t h a t are at present being made and which p o i n t t o some of the ways i n which the f i e l d i s l i k e l y t o develop. Both t o p i c s i n v o l v e borane c l u s t e r complexes with i r i d i u m i n v a r i o u s o x i d a t i o n states. 3

3

Exo-polyhedral Cycloboronation Recent work by Janet Crook on the r e a c t i o n of t r a n s - [ I r ( C O ) C I -

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

GREENWOOD

Polyhedral

335

Boranes as Ligands

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch022

22.

Figure 2.

3

Structure of 2LT2ichno-[Ptfa -B H )(PMe Ph) ]; are not determined (4). 8

12

2

2

positions of H atoms

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch022

336

INORGANIC C H E M I S T R Y : TOWARD T H E 21ST C E N T U R Y

Figure 3.

Structure of arachno-/Ft^ -tf-BJi )(FMe ?h) ]; only the borane H atoms have been shown for clarity (4). >

10

2

k

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

22.

GREENWOOD

Polyhedral

Boranes as

337

Ligands

( P P h ^ o ] i n b o i l i n g methanol with the normally u n r e a c t i v e c l o s o d i a n i o n l e d t o the i s o l a t i o n o f ten new m e t a l l a - and dimetalla-boranes, and the s t r u c t u r e s o f e i g h t o f these have so f a r been unambiguously e s t a b l i s h e d , i n c l u d i n g the novel ten-vertex i s o - n i d o - c l u s t e r [ {IrC(OH)EQE^(OMe )} ( C6HUPFI12 ) (PPh3 ) ] shown i n Figure h and t h e e q u a l l y unexpected e x o - b i c y c l i c c l o s o s i x - v e r t e x dimetallahexaborane [ ( i ^ B ^ ) ( CgHl+PPhg ) 2 ( ° ) 3 ( 3 ) 1 shown i n Figure 5 . Of t h e numerous s i g n i f i c a n t aspects o f these two compounds (5_, 6) a p a r t i c u l a r l y i n t e r e s t i n g f e a t u r e common t o both i s t h e occurrence o f ortho-cycloboronation o f one o r more P-phenyl groups on t h e t e r t i a r y phosphine l i g a n d s . The mechanistic o r i g i n o f t h i s feature (which i s unprecedented i n metalloborane chemistry though w e l l e s t a b l i s h e d i n organometallic chemistry) was u n c l e a r . Further work suggested that i t d i d not occur by s t r a i g h t f o r w a r d e l i m i n a t i o n o f H2 from the a c y c l i c analogues. Jonathan Bould has now shown t h a t ortho-cycloboronation occurs e s s e n t i a l l y q u a n t i t a t i v e l y i n c e r t a i n reactions using [lrCl(PPh3)3] as the source o f i r i d i u m (T.). Thus, as shown i n Scheme 1 r e a c t i o n with nido-B9Hi2~ i n CH2CI2 s o l u t i o n at room temperature y i e l d s two isomeric forms o f the yellow nido i r i d i u m ( I I I ) compound ^ , [ ( ΗΙΓΒΟ,Η-Ι 2 ) ( C6Hl|PPh2 ) ( ΡΡηβ ) ] by the unusually c l e a n r e a c t i o n :

B10H10

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch022

c

[lrCl(PPh ) ] + Β Η 3

3

"

p P n

> [(HIrB H ^ ) ( Û g H ^ P P h g ) ( P P h ) ] + C l " + PPh 3

3

The p r e c i s e stoichiometry and high y i e l d o f t h i s r e a c t i o n suggest that a d r i v i n g f o r c e o f t h e ortho-cycloboronation i n t h i s case i s the p r o v i s i o n o f t h e two a d d i t i o n a l c l u s t e r e l e c t r o n s r e q u i r e d f o r t h e nido ten-vertex s t r u c t u r e . These can be thought o f as being provided by two H atoms (the ortho-H atom on the c y c l o boronating phenyl group and t h e t e r m i n a l H atom on B ( 5 ) ) ; the nett r e s u l t i s t h e formation o f an Ir-H-^ and an a d d i t i o n a l I r - H - B ( 5 ) bond. In f a c t , two isomers o f the ortho-cycloboronated nidoiridadecaborane are formed which d i f f e r i n t h e arrangement o f the l i g a n d s about t h e pseudo-octahedral I r ( l l l ) c e n t r e ; the more labile, can be q u a n t i t a t i v e l y converted i n t o the more s t a b l e , , by m i l d thermolysis at 65°C. At s l i g h t l y higher temperatures 5°C) q u a n t i t a t i v e l o s s o f 2H2 r e s u l t s i n t h e smooth production o f t h e novel b r i g h t orange-yellow i s o - c l o s o - i r i d i u m ( V ) compound, 2 (Scheme 1). I t w i l l be noted t h a t the {irBp} c l u s t e r (Figure 6) has the p r e v i o u s l y unobserved C symmetry { 1 ^ 3 6 3 6 3 } r a t h e r than t h e normal Dl|^ bicapped square a n t i p r i s m a t i c arrangement o f v e r t i c e s . A formal e l e c t r o n count r e q u i r e s 22 e l e c t r o n s (2n + 2) f o r the c l o s o - c l u s t e r bonding; o f these 18 are s u p p l i e d by the nine boron atoms, l e a v i n g k t o be c o n t r i b u t e d by t h e I r atom. T h i s , together with the Ir-H t e r m i n a l bond i n d i c a t e s that the compound can be regarded as a f u r t h e r example o f the growing number o f compounds i n which the high o x i d a t i o n s t a t e I r ( v ) i s s t a b i l i z e d by c o o r d i n a t i o n t o a s o f t p o l y h e d r a l borane l i g a n d u

3 v

1

1

(5,7,8).

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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338

Figure 5. Structure of close--/"(lr B H )(C H PPh ) (CO) (PPh )] with the P-phenyl groups omitted (apart from their ipso C atoms), but with the two ortho-cycloboronated P-phenylene groups retained (6). 2

i

2

6

!t

2

2

3

s

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

GREENWOOD

Polyhedral

Boranes as Ligands

339

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22.

(2)

Scheme 1. Reactions of mdo-B H ~ and arachno-B H " with [IrCl(PPh ) ] at room temperature, and the quantitative thermolvtic dehydrogenation of the products at 85 °C to give iso-closo-KHlrÈgHsXCeH^PPhzXPPhs)]. (Reproduced with permission from Ref. 7. Copyright 1982, Royal Society of Chemistry.) 9

12

9

n

s s

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INORGANIC C H E M I S T R Y : TOWARD T H E 21 ST C E N T U R Y

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COD

Figure 6. Structure of iso-c\oso-f(HIrn H )(è H PPh )(PPhs)] with P-phenyl groups omitted for clarity (except for their ipso C atoms). All H atoms were located Oh 9

8

6

If

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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GREENWOOD

Polyhedral

Boranes as

Ligands

341

Scheme 1 a l s o shows t h a t t h i s ortho-cycloboronated i s o - c l o s o i r i d i u m ( v ) compound, ξ, can be formed d i r e c t l y and q u a n t i t a t i v e l y by the mild thermolysis of compound ^ which i s the a c y c l i c analogue of the cycloboronated isomers ^ and Compound ^, whose molecular s t r u c t u r e has already been given i n Figure 1, was formerly obtained (3.) i n Pt-CD CHDCHMe (X)

2

Pt-CH < CHDCDMe (XI)

Pt—I π CDMeo ^

2

Scheme III

? CHD L-Fft-« 2

C l

CI

M^VtHDMe (ΧΠ)

MÎ3 C H Me (XIII) N

4

2

5

(4)

5

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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CENTURY

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24.

PUDDEPHATT

Platinacyclobutane

359

Chemistry

( i i ) . The f i r s t hydride s h i f t occurs as a 1,3-hydride s h i f t with t r a n s f e r of hydride from a CH2 group t o the most s u b s t i t u t e d carbon atom, to give a platinum-carbene complex. Some examples to i l l u s t r a t e t h i s s e l e c t i v i t y are given i n equations (2) and (3) and i n Scheme I I . ( i i i ) . In some cases the carbene complex i s trapped as an y l i d e complex by a t t a c k of the l i g a n d L. T h i s occurs i f the β-carbon i s primary (e.g. f o r Pt=CHCH2CHMe2 but not f o r Pt=CHCHMeC H 2 M e ) , i f the l i g a n d i s compact (e.g. p y r i d i n e or 2-methylp y r i d i n e but not u s u a l l y 2,6-dimethylpyridine) and i f the l i g a n d i s a strong base (e.g. p y r i d i n e but not a c e t o n i t r i l e ) . The f i r s t two c o n d i t i o n s a r e s t e r i c i n o r i g i n while the t h i r d i s e l e c t r o n i c . (iv) . I f any of the above c o n d i t i o n s i s not met, then a subsequent 1,2-hydride s h i f t y i e l d s a platinum-alkene complex. The observed 1 , 3 - s h i f t together with the high s e l e c t i v i t y suggest a p o s s i b l e mechanism f o r the Z i e g l e r - N a t t a p o l y m e r i s a t i o n of alkenes to add to the mechanisms already put forward (17, 18, 19), as i l l u s t r a t e d i n equation (5), Ρ = growing polymer.

Ρ

(5) Amongst other p r e d i c t i o n s that can be made i s that p o l y m e r i z a t i o n of CH2=CD2 should give -CHD- u n i t s as w e l l as CH2 and CD u n i t s i n the l i n e a r polymer, but we know of no experimental evidence on this point. 2

S k e l e t a l I s o m e r i z a t i o n and Ring

Expansion

Since β-elimination r e a c t i o n s are o f t e n r a p i d and r e v e r s i b l e , i t i s s u r p r i s i n g that no examples o f metallacyclobutane t o metallacyclopentane r i n g expansion r e a c t i o n s according to equation (6) have been found.

(6)

Schrock has proposed that the reverse r e a c t i o n occurs during some c a t a l y t i c alkene d i m e r i s a t i o n r e a c t i o n s (20) but, i n s t u d i e s of decomposition of a l k y l s u b s t i t u t e d p l a t i n a c y c l o b u t a n e s , no

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p l a t i n a c y c l o p e n t a n e s have ever been observed even though they would be thermally s t a b l e under the r e a c t i o n c o n d i t i o n s used. A d i f f e r e n t approach was t h e r e f o r e used, based on the r i n g expansion commonly observed during s o l v o l y s i s of cyclopropylmethyl or cyclobutylmethyl e s t e r s (equation 7) (21). CH OTs 2

H

0

A

Q _

M

O

A

C

^ - j O A c

+

100°C (7) 99%

1%

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The r e q u i r e d p l a t i n a c y c l o b u t a n e s were prepared according t o equation (8) (OMs = mesylate).

[{PtCl (C H-)} ] 2

2

Cl

L

2

+

[XCH 0MS 2

"

C

2

H

S

t

{ p t C 1

2CH2CR(CH 0Ms)CH } ] 2

R

I \ Cl

and s o l v o l y s e d

2

Cl

V

L

XVII, R=H, L=py XVIII, R=H, L=h bipy XIX, R=Me, L=py

I Cl

n

(8)

CH OMs 2

XIV, R=H, L=py XV, R=H, L=h b i p y XVI, R=Me, L=py

The s o l v o l y s e s were conducted i n 60% aqueous acetone a t 36°C, and products were i s o l a t e d i n good y i e l d and shown t o be e s s e n t i a l l y pure p l a t i n a c y c l o p e n t a n e d e r i v a t i v e s (22). C h a r a c t e r i ­ s a t i o n o f products i n c l u d e d elemental a n a l y s i s and *H and C NMR s p e c t r a . P a r t i c u l a r l y s t r i k i n g are the d i f f e r e n c e s i n the coupling constants *J(PtC) which are V350 Hz i n p l a t i n a c y c l o b u ­ tanes but 490-550 Hz i n p l a t i n a c y c l o p e n t a n e s , no doubt due to opening up of CPtC angles i n the l a r g e r r i n g . In order to determine the mechanism of t h i s novel rearrange­ ment, some k i n e t i c and l a b e l l i n g s t u d i e s were c a r r i e d out. Some r e s u l t s are given i n Table I . 1 3

TABLE I F i r s t Order Rate Constants and D i s t r i b u t i o n of I s o t o p i c L a b e l f o r the Reaction of Equation (9)

L py py h bipy

[py] 0 0.32 M 0

k (36°C)/s" 2.4 χ Ι Ο " 6.0 χ ΙΟ" 5.9 χ 10" o b s

5

6

6

1

% product 1-D 3-D 86 14 32 68 27 73 2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

2

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The s o l v o l y s e s followed simple f i r s t order k i n e t i c s but the observation that XIV, with p y r i d i n e l i g a n d s , was s o l v o l y s e d four times f a s t e r than XV, with 2 , 2 ' - b i p y r i d i n e l i g a n d s , suggested that l i g a n d d i s s o c i a t i o n might be i n v o l v e d at some stage. In confirmation the s o l v o l y s i s of XIV was found to be retarded i n the presence of p y r i d i n e , the observed f i r s t order r a t e constants being given by the two term r a t e expression k b s 5.90 χ 10"" + 1.81 χ 10~ /{1 + 275 [py]}. The component which i s independent of [py] i s equal to the r a t e constant f o r s o l v o l y s i s of (XV). Next, l a b e l l i n g experiments were c a r r i e d out using the complexes [ P t C l L C H 2 C H ( * C H 2 0 M s ) C H 2 ] or [ P t C l L 2 C H C H ( C D 2 0 M s ) C H 2 ] , where *C = carbon enriched to 7.5% with C . In e i t h e r case, a n a l y s i s by C { H } NMR spectroscopy of the s o l v o l y s i s product when L = py showed that the l a b e l l e d carbon l a r g e l y occupied the 1 - p o s i t i o n but with some i n the 3 - p o s i t i o n (equation 9, Figure 2). =

6

0

5

2

2

2

2

13

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13

1

(9)

(XX) 1 3

However, the C NMR method i s not q u a n t i t a t i v e and the products were t h e r e f o r e a l s o analysed by the q u a n t i t a t i v e H { H } NMR s p e c t r a . T y p i c a l s p e c t r a are shown i n Figure 3 and r e s u l t s are summarised i n Table I. I t can be seen that s o l v o l y s i s of (XX), L = p y r i d i n e , i n the absence of added p y r i d i n e gives l a r g e l y the I - D 2 isomer but t h a t , when s u f f i c i e n t p y r i d i n e i s added, the s e l e c t i v i t y i s reversed and the product i s l a r g e l y the 3-D2 isomer. In no case i s there evidence f o r deuterium at e i t h e r the 1- or 4 - p o s i t i o n . S o l v o l y s i s of (XX), L = 2 , 2 ' - b i p y r i d y l gives l a r g e l y the 3-D2 isomer. These r e s u l t s are i n t e r p r e t e d i n terms of the mechanism shown i n Scheme IV. For (XX), L = py, i t i s l i k e l y that the major r e a c t i o n path i n v o l v e s i n i t i a l s k e l e t a l i s o m e r i z a t i o n to give (XXI) followed by r a p i d s o l v o l y s i s of t h i s isomer. The s o l v o l y s i s of t h i s isomer i s s t r o n g l y m e t a l - a s s i s t e d s i n c e the intermediate carbonium i o n i s s t a b i l i s e d by the metal-alkene resonance form as shown i n the Scheme. The product i s the I - D 2 isomer. Now, the s k e l e t a l i s o m e r i z a t i o n of (XX) i s expected to be retarded by f r e e p y r i d i n e and cannot occur when L 2 = 2 , 2 - b i p y r i d y l (7). Hence under these c o n d i t i o n s the r e a c t i o n must occur by s o l v o l y s i s of (XX) g i v i n g l a r g e l y the 3-D2 isomer. However, the product formed under these c o n d i t i o n s i s s t i l l about 30% of the 1-D isomer (Table I ) . E i t h e r there i s a route to s k e l e t a l i s o m e r i z a t i o n which can occur without l i g a n d d i s s o c i a t i o n or e l s e s o l v o l y s i s of isomer (XX), Scheme IV, can give 30% of the I - D 2 isomer. These s o l v o l y s i s r e a c t i o n s e s s e n t i a l l y transform the i s o b u t y l to the η-butyl skeleton and the above r e s u l t s c l e a r l y show that 2

2

f

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

X

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363

Chemistry

Figure 3. V^HyNMR spectra of products of hydrolysis of {PtCl L [CH CH(CD OMs)CH ]}. Key: a, without added pyridine, largely 1-D isomer; and b, with added pyridine, largely 3-D isomer. 2

2

g

i

2

2

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t h i s can be accomplished e i t h e r by s k e l e t a l i s o m e r i z a t i o n of the p l a t i n a c y c l o b u t a n e or by a normal carbonium i o n rearrangement. Taken together with the e a r l i e r rearrangements to y l i d e or alkene complexes, the r e s u l t s e s t a b l i s h c l e a r l y that the s k e l e t a l rearrangement i s of fundamental importance i n the chemistry of p l a t i n a c y c l o b u t a n e s and lend credence to the proposed mechanism of alkane i s o m e r i z a t i o n given i n Scheme I .

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch024

Acknowledgments The new r e s u l t s described are from the work of graduate students S.M. L i n g (α-elimination) and J.T. Burton ( r i n g expansion). We thank NSERC (Canada) f o r f i n a n c i a l support. Acknowledgment i s made t o the donors of the Petroleum Research Fund, administered by the American Chemical S o c i e t y , f o r p a r t i a l support. We thank Dr. J.B. Stothers f o r the C and H NMR spectra. 1 3

2

Literature Cited

1. Amir-Ebrahami, V.; Garin, F.; Weinsang, F; Gault, F.G. Nouv. J. Chim. 1979, 3, 529. Gault, F.G. Gazz. Chim. Ital. 1979, 109, 255. 2. Parshall, G.W.; Herskovitz, T.; Tebbe, F.N.; English, A.D.; Zeile, J.V. Fundam. Res. Homogen. Catal. 1979,3,95· Parshall, G.W., "Homogeneous Catalysis", Wiley, N.Y., 1980. 3. Al-Essa, R.J.; Puddephatt, R.J.; Thompson, P.J.; Tipper, C. F.H. J. Am. Chem. Soc. 1980, 102, 7546. Puddephatt, R.J. Co-ord. Chem. Rev. 1980,33,149. 4. Janowicz, A.H.; Bergman, R.G. J. Am. Chem. Soc. 1982, 104, 352. 5. Tulip, T.H.; Thorn, D.L. J. Am. Chem. Soc. 1981, 103, 2448. 6. Foley, P.; Whitesides, G.M. J. Am. Chem. Soc. 1979, 101, 2732. 7. Puddephatt, R.J.; Quyser, M.A.; Tipper, C.F.H. J. Chem. Soc. Chem. Comm. 1976, 626. Al-Essa, R.J.; Puddephatt, R.J.; Quyser, M.A.; Tipper, C.F.H. J. Am. Chem. Soc. 1979, 101, 364. 8. Casey, CP.; Scheck, D.M.; Shusterman, A.J. J. Am. Chem. Soc. 1979, 101, 4233. 9. Al-Essa, R.J.; Puddephatt, R.J. J. Chem. Soc. Chem. Comm. 1980, 45. 10. Whitesides, G.M.; Gaasch, J.F.; Stedronsky, E.R. J. Am. Chem. Soc. 1972, 94, 5258. 11. Tebbe, F.N.; Parshall, G.W.; Reddy, G.S. J. Am. Chem. Soc. 1978, 100, 3611. 12. Schrock, R.R. Accounts Chem. Res. 1979, 12, 98. 13. Johnson, T.H.; Cheng, S.-S. J. Am. Chem. Soc. 1979, 101, 5277. 14. Cushman, B.M.; Brown, D.B. Inorg. Chem. 1981, 20, 2490.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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15. Ling, S.M.; Puddephatt, R.J. J. Chem. Soc. Chem. Comm. 1982, 412. 16. Al-Essa, R.J.; Puddephatt, R.J.; Perkins, D.C.L.; Rendle, M.C.; Tipper, C.F.H. J. Chem. Soc. Dalton Trans. 1981, 1738. 17. Watson, P.L. J. Am. Chem. Soc. 1982, 104, 337 and refs. therein. 18. Turner, H.W.; Schrock, R.R. J. Am. Chem. Soc. 1982, 104, 2331 and refs. therein. 19. McKinney, R.J. J. Chem. Soc. Chem. Comm. 1980, 490. 20. McLain, S.J.; Sancho, J.; Schrock, R.R. J. Am. Chem. Soc. 1979, 101, 5451. 21. Richey, H.G. Jr.; Wiberg, K.B.; Hess, B.A. Jr.; Ashe, A.J. III, in "Carbonium Ions", vol. III, Olah, G.A.; Schleyer, P. von R., eds., Wiley, N.Y., 1972, pp. 1201 and 1295. 22. Burton, J.T.; Puddephatt, R.J. J. Am. Chem. Soc., in the press. RECEIVED August 19, 1982

Discussion W.L. G l a d f e l t e r , U n i v e r s i t y o f M i n n e s o t a : A t t h i s t i m e , can y o u make any g e n e r a l i z a t i o n s w h i c h w o u l d a l l o w y o u t o p r e d i c t when y o u w i l l s e e α-hydrogen a b s t r a c t i o n v e r s u s 3 - h y d r o gen abstraction?

R. J . Puddephatt: Not r e a l l y . I t h i n k almost a l l organo­ m e t a l l i c chemists would have p r e d i c t e d ^ - e l i m i n a t i o n from p l a t i n a ­ cyclobutanes and t h a t , i f p l a t i n a c y c l o b u t a n e s d i d α-eliminate, then most other metallacyclobutanes should a l s o . Both p r e d i c t i o n s would have been wrong and the reasons are s t i l l obscure. In a more general o b s e r v a t i o n , although we now know the mechanisms of many of the fundamental organometallic r e a c t i o n s , we know very l i t t l e about f a c t o r s which i n f l u e n c e s e l e c t i v i t y . In the present case, one could argue i n terms o f the r e l a t i v e s t a b i l i t i e s o f carbene-hydride v s . η-alkyl-hydride intermediates but, s i n c e nothing i s known about e i t h e r c l a s s of compound i n platinum (IV) chemistry, the arguments are not convincing. More experimental work i s needed to e s t a b l i s h general patterns of behavior f o r other metals.

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367

A.J. C a r t y , G u e l p h - W a t e r l o o C e n t r e ; I t m i g h t be e x p e c t e d that the facility with which metallocyclobutanes undergo a - o r β-eliminations w o u l d be r a t h e r d e p e n d e n t on t h e p r o x i m i t y of h y d r o g e n atoms on t h e a- o r 3 - c a r b o n atoms t o t h e m e t a l . In o t h e r words, t h e r e m i g h t be a s t r u c t u r a l dependence. I s t h e r e any e v i d e n c e t h a t such s t r u c t u r a l features contribute s u b s t a n t i a l l y t o t h e p r e f e r r e d r e a c t i o n pathway?

R. J . Puddephatt: The ^ - e l i m i n a t i o n mechanism r e q u i r e s considerable puckering of the platinacyclobutane r i n g to b r i n g the 3-hydrogen atom c l o s e to platinum. We, i n c o l l a b o r a t i o n with J.A.Ibers group, have shown that some platinacyclobutanes have almost planar PtC3 r i n g s i n the s o l i d s t a t e but that the r i n g s are puckered by about 25° i n s o l u t i o n , i n d i c a t i n g that the a c t i ­ v a t i o n energy f o r puckering i s s m a l l ( l ) . However, i t i s p o s s i b l e i n these Pt((IV) complexes that the more extreme puckering which must precede 3-elimination i s prevented by s t e r i c hindrance of the a x i a l halogen s u b s t i t u e n t s . I t would be i n t e r e s t i n g to f i n d i f platinum(II) d e r i v a t i v e s , f o r which t h i s hindrance should be much l e s s , undergo a- or 3-elimination. (1) J . T. Burton, R. J . Puddephatt, N.L. Jones and J . A. Ibers, J . Am. Chem. Soc., submitted f o r p u b l i c a t i o n .

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch024

1

R.R. S c h r o c k , M.I.T.: Have y o u o r anyone e l s e p r e p a r e d p l a t i n u m ( I V ) m e t a l l a c y c l o b u t a n e complexes w i t h a l k o x i d e l i g a n d s i n p l a c e o f c h l o r i d e s ? One m i g h t e x p e c t t h e a l k o x i d e c o m p l e x e s t o behave c o n s i d e r a b l y d i f f e r e n t l y t h a n t h e c h l o r o c o m p l e x e s , perhaps l i k e e a r l y t r a n s i t i o n metal complexes.

R. J . Puddephatt: No. Nobody has prepared such complexes and the synthesis i s not t r i v i a l . S u b s t i t u t i o n of h a l i d e ligands i n octahedral platinum(IV) d e r i v a t i v e s i s t y p i c a l l y very slow, and a b e t t e r route (suggested by J . K. Kochi) might i n v o l v e o x i d a t i o n of platinum(II) metallacyclobutanes with peroxides. I t would c e r t a i n l y be worthwhile to attempt t h i s synthesis i n view of the promise of enhanced r e a c t i v i t y .

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

25 Multiple Metal-Carbon Bonds in Catalysis RICHARD R. SCHROCK

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

Massachusetts Institute of Technology, Department of Chemistry, Cambridge, M A 02139

The presence of alkoxide ligands slows down the rate of rearrangement of tantalacyclobutane rings and probably also speeds up the rate of reforming an alkylidene complex and an olefin. However, tantalum and niobium alkylidene complexes are not good olefin metathesis catalysts because either intermediate methylene complexes decompose rapidly, or because intermediate alkylidene ligands rearrange to olefins. Tungsten(VI) oxo and imido alkylidene complexes will metathesize olefins, probably because rearrangement processes involving a β-hydride are even slower as a result of the π-electron donor abilities of the oxo or imido ligand. Disubstituted acetylenes are metathesized by tungsten(VI) alkylidyne complexes containing t-butoxide ligands. When chloride ligands are present instead of t-butoxides, a tungstenacyclobutadiene complex can be isolated. It reacts with additional acetylene to give a cyclopentadienyl complex. Tanta­ lum neopentylidene hydride complexes react with ethylene to form new alkylidene hydride complexes in which many ethylenes have been incorporated into the alkyl chain. The polymer that slowly forms in the presence of excess ethylene is approximately a 1:1 mixture of even and odd carbon olefins in the range C50-C100. E.O. Fischer's discovery of (CO)5W[C(Ph)(OMe)] in 1964 marks the beginning of the development of the chemistry of metal-carbon double bonds (1). At about this same time the olefin metathesis reaction was discovered (2) but i t was not until about five years later that Chauvin proposed (3) that the catalyst contained an alkylidene ligand and that the mechanism consisted of the random reversible formation of all possible metallacyclobutane rings. Yet low oxidation state Fischer-type carbene complexes were found not to be catalysts for the metathesis of simple olefins. It is now 9

0097-6156/83/0211-0369$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

INORGANIC C H E M I S T R Y : TOWARD T H E

370

21 ST

CENTURY

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

v i r t u a l l y c e r t a i n t h a t the a l k y l i d e n e chain t r a n s f e r mechanism i s c o r r e c t and t h a t the most a c t i v e c a t a l y s t s are d° complexes (count­ i n g the CHR l i g a n d as a d i a n i o n ) . In t h i s a r t i c l e I ' d l i k e f i r s t to t r a c e the events and f i n d i n g s which l e d to our concluding t h a t d ° a l k y l i d e n e complexes were r e s p o n s i b l e for the r a p i d c a t a l y t i c metathesis of o l e f i n s . Then I want to present some recent r e s u l t s concerning the p o l y m e r i z a t i o n of ethylene by an a l k y l i d e n e hydride c a t a l y s t , and f i n a l l y some r e s u l t s concerning the metathesis of acetylenes by tungsten(VI) a l k y l i d y n e complexes. We w i l l see t h a t an important feature of much of the chemistry of m u l t i p l e metal carbon bonds i s the r o l e played by a l k o x i d e , oxo, or other π-bonding l i g a n d s . Such observations are congruent with some recent ideas and r e s u l t s Chisholm discusses elsewhere i n t h i s volume concerning a l k o x i d e l i g a n d s i n organometallic chemistry. Tantalum and Niobium Neopentylidene Complexes (4) The f i r s t neopentylidene complex was prepared by the r e a c t i o n shown i n equation 1 (5). Although the exact d e t a i l s of t h i s r e a c Dentane Ta(CH CMe3) Cl2 + 2LiCH CMe3 • Ta(CHCMe3)(CH CMe3) -CMe4 2

3

2

2

(1)

3

t i o n are s t i l l unclear ( £ ) , i t i s almost c e r t a i n l y a v e r s i o n of what has come to be c a l l e d an α-hydrogen atom a b s t r a c t i o n r e a c t i o n . The best s t u d i e d example of α-hydrogen atom a b s t r a c t i o n i s the i n t r a m o l e c u l a r decomposition of Ta(T) -C5H5)(CH2CMe )2Cl2 to Ta(îi -C5H5)(CHCMe )Cl2 (7_). But the s i m p l e s t and most general i s the α-hydrogen atom a b s t r a c t i o n i n M(CH2CMe )2X3 M = Nb or Ta, X = Cl or Br) promoted by oxygen, n i t r o g e n , or phosphorus donor l i g a n d s ( 8 ) . The r e s u l t i n g octahedral molecules of the type M(CHCMe )L2X oTfered an i d e a l opportunity to study how a neopentylidene complex o f Nb or Ta r e a c t s with a simple o l e f i n . A complex such as Ta(CHCMe )(PMe )2Cl r e a c t s r e a d i l y with e t h y l e n e , propylene, or styrene to give a l l of the p o s s i b l e p r o ­ ducts (up to four) which can be formed by rearrangement of i n t e r ­ mediate metallacyclobutane complexes (two for s u b s t i t u t e d o l e f i n s ) by a β-hydride e l i m i n a t i o n process ( e . g . , equation 2) ( £ ) . We saw 5

3

5

3

3

3

3

3

3

3

+

(2)

a b s o l u t e l y no evidence for a m e t a t h e s i s - l i k e r e a c t i o n u n t i l we s t u d i e d the complexes M(CHCMe )(THF)2Cl (M = Nb or T a ) . In t h i s case we found low but r e p r o d u c i b l e y i e l d s (5-15%) of 3 , 3 - d i m e t h y l 1-butene upon r e a c t i n g M(CHCMe )(THF)2Cl with e t h y l e n e , and i n the case of c i s - 2 - p e n t e n e , -6 turnovers to 3-hexenes and 2-butenes. We reasoned~"tiïat the r a t e of metathesis of the MC r i n g was f a s t e r 3

3

3

3

3

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

25.

SCHROCK

Multiple

Metal-Carbon

Bonds in

371

Catalysis

r e l a t i v e to the rate of rearrangement of the MC3 r i n g when an oxygen donor l i g a n d was present i n place of a phosphine l i g a n d . Therefore we prepared t-butoxy complexes such as Ta(CHCMe3)(0CMe3)2(PMe3)Cl i n order to see i f s e l e c t i v e metathesis of an i n c i p i e n t MC3 r i n g would r e s u l t . Ta(CHCMe3)(0CMe3)2(PMe3)Cl r e a c t s with e t h y l e n e , s t y r e n e , or 1-butene to give l a r g e l y t - b u t y l e t h y l e n e (equation 3 ) .

Ta=CHR •

Bu*CH=CH

M

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

(R = H,Ph,or Et) When styrene i s the o l e f i n the r e s u l t i n g benzylidene complex can be trapped i n the presence of a d d i t i o n a l PMe3 to give Ta(CHPh)(0CMe3)2(PMe3)2Cl. Neither the methylene nor the propylidene com­ p l e x could be observed, but i n the case of 1-butene we could t r a c e the fate of intermediate metallacyclobutane and a l k y l i d e n e com­ p l e x e s . Metathesis of 1-butene was not successful for two reasons. F i r s t , an intermediate β-ethylmetallacyclobutane complex rearranges t o 2-methyl-1-butene. Second, intermediate methylene complexes decompose by a b i m o l e c u l a r r e a c t i o n to give e t h y l e n e . In c o n t r a s t to the f a i l u r e to metathesize terminal o l e f i n s , i n t e r n a l o l e f i n s such as cis-2-pentene can be metathesized to the e x t e n t of ~50 t u r n o v e r s . The chain t e r m i n a t i n g r e a c t i o n i n t h i s case i s rearrangement of intermediate e t h y l i d e n e and propylidene complexes (equation 4). Both rearrangement of intermediate t r i s u b -

M=C'" CH R

•

CH =CHR 2

(R = H or Me)

(4)

N

2

s t i t u t e d metallacyclobutane complexes and b i m o l e c u l a r decomposition o f monosubstituted a l k y l i d e n e complexes must be slow enough to a l l o w a s i g n i f i c a n t number of steps i n the metathesis r e a c t i o n to proceed. Rearrangement of intermediate a l k y l i d e n e complexes then becomes the major t e r m i n a t i o n s t e p . There had been some evidence t h a t a l k o x i d e l i g a n d s slow down r e a c t i o n s which i n v o l v e e l i m i n a t i o n of a β-hydride from an a l k y l l i g a n d . α - O l e f i n s are dimerized to a mixture of h e a d - t o - t a i l and t a i l - t o - t a i l dimers by o l e f i n complexes of the type Ta(Ti -C5Me5)(CH2=CHR)Cl2 ( 1 0 K The β,β'- and a , β ' - d i s u b s t i t u t e d t a n t a l a c y c l o pentane complexes are intermediates i n t h i s r e a c t i o n . T h e i r decomposition i n v o l v e s the sequence shown i n equation 5. When one 5

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

372

INORGANIC

CHEMISTRY:

TOWARD

T H E 21ST

CENTURY

c h l o r i d e l i g a n d i n the c a t a l y s t i s replaced by a methoxide l i g a n d the rate of o l e f i n d i m e r i z a t i o n decreases by a f a c t o r of a p p r o x i ­ mately 1 0 as a r e s u l t of the greater s t a b i l i t y of the t a n t a l a cyclopentane complexes. Although i t could not be proven, i t was f e l t t h a t the f i r s t step, β-hydride e l i m i n a t i o n , had been slowed down s i g n i f i c a n t l y . Therefore i t was f e l t t h a t the β - e l i m i n a t i o n process by which tantalacyclobutane complexes rearranged to o l e f i n s a t l e a s t would be slowed down by r e p l a c i n g two c h l o r i d e l i g a n d s i n Ta(CHCMe3)(PMe )2Cl w i t h t - b u t o x i d e l i g a n d s . The question as to whether the r a t e of metathesis of the TaC r i n g increases upon r e p l a c i n g c h l o r i d e by t - b u t o x i d e l i g a n d s i s s t i l l open. However, on the basis of some r e s u l t s we present l a t e r concerning acetylene metathesis i t seems l i k e l y t h a t t - b u t o x i d e l i g a n d s encourage reformation of the metal-carbon double bond. 2

3

3

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

3

O l e f i n Metathesis by Tungsten Oxo and Imido Complexes In r e t r o s p e c t i t i s not s u r p r i s i n g t h a t the niobium and t a n t a ­ lum a l k y l i d e n e complexes we prepared are not good metathesis c a t a ­ l y s t s since these metals are not found i n the " c l a s s i c a l " o l e f i n metathesis systems [2). Therefore, we set out to prepare some tungsten a l k y l i d e n e complexes. The f i r s t successful r e a c t i o n i s t h a t shown i n equation 6 (L = PMe3 or P E t ) These oxo 3

Ta(CHCMe )L Cl 3

2

3

+ W(0)(0CMe3)4

•

(6)

Ta(0CMe ) Cl + W(0)(CHCMe )L Cl 3

4

3

2

2

a l k y l i d e n e complexes are octahedral species i n which the oxo and a l k y l i d e n e l i g a n d s are c i s to one another and the W(0)(CHCp) atoms a l l l i e i n the same plane ( 1 2 ) . This type of s t r u c t u r e cah be r a t i o n a l i z e d e a s i l y on the B a s i s of the f a c t t h a t the oxo l i g a n d i s an e x c e l l e n t π - e l e c t r o n donor ( 1 3 ) . Therefore, the oxo l i g a n d uses two of the a v a i l a b l e three d o r B T t a l s of π-type symmetry to bond to W, l e a v i n g only one to form the π-bond between W and the a l k y l i d e n e l i g a n d . Several d i f f e r e n t types of oxo neopentylidene complexes have been prepared i n c l u d i n g W(0)(CHCMe )(L)Cl , [W(0)(CHCMe3)L Cl] , and [W(0)(CHCMe3)L ] . C h a r a c t e r i z e a b l e oxo neopentylidene complexes have not y e t been prepared d i r e c t l y from oxo neopentyl complexes by α-hydrogen a b s t r a c t i o n r e a c t i o n s , although Osborn has presented some evidence t h a t they could be ( 1 4 ) . Another p o t e n t i a l l y important method of preparing oxo aTFylidene complexes i s by adding water or hydroxide to a l k y l i d y n e complexes (see l a t e r ) as shown i n equation 7 ( 1 5 ) . 3

+

2

[W(CCMe )Cl ]- + 2PEt 3

2

2+

2

4

3

+ Et N + H 0 3

2

—^

W(0)(CHCMe )(PEt ) Cl 3

3

2

(7) 2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

25.

SCHROCK

Multiple

Metal-Carbon

Bonds in

Catalysis

Imido a l k y l i d e n e complexes were f i r s t prepared by a r e a c t i o n analogous to t h a t shown i n equation 6. Recently they have been prepared from imido a l k y l complexes by well-behaved α-hydrogen a b s t r a c t i o n r e a c t i o n s (16). Imido neopentylidene complexes seem to be more stable than oxo neopentylidene complexes, p o s s i b l y because the oxo l i g a n d i s s t e r i c a l l y more a c c e s s i b l e to Lewis a c i d s , i n c l u d i n g another tungsten c e n t e r . Oxo a l k y l i d e n e complexes react with o l e f i n s i n the presence of a t r a c e of A1C13 to give new a l k y l i d e n e complexes ( e . g . , b e n z y l i dene, methylene, e t h y l i d e n e ) (11a). Both terminal and i n t e r n a l o l e f i n s can be metathesized slowly i n the presence of aluminum c h l o r i d e . Probably the best c a t a l y s t s are the i o n i c s p e c i e s , [W(0)(CHCMe3)(PEt3)2Cl] AlCl4- and [W(0)(CHCMe3)(PEt3)2] (AlCl -)2 i n dichloromethane or chlorobenzene ( 1 7 ) . Of the order of 10-20 turnovers per hour for a day or more are p o s s i b l e with these c a t i o n i c c a t a l y s t s . These studies demonstrate that t r a n s a l k y l i d e n a t i o n i s p o s s i b l e with a d° tungsten a l k y l i d e n e complex that w i l l metathesize o l e f i n s s l o w l y , but c o n v i n c i n g l y . There i s s t i l l con­ s i d e r a b l e doubt concerning the r o l e of the Lewis a c i d . However, the f a c t that W(0)(CHCMe3)(PEt3)Cl2 (18) metathesizes o l e f i n s more r a p i d l y i n i t i a l l y than the s i x - c o o r d i n a t e complexes ( i n the presence of AICI3) e s t a b l i s h e s that a Lewis a c i d i s not r e q u i r e d . On the basis of these studies and some c a l c u l a t i o n s by Rappe and Goddard (19) i t would seem i n c o n t r o v e r t i b l e that the oxo l i g a n d prevents reduction of the metal and perhaps also enhances the rate of reforming an a l k y l i d e n e complex from a metallacyclobutane com­ p l e x . The next question was whether other strong π-donor l i g a n d s such as a l k o x i d e s could take over the oxo's function ( l i b ) . Osborn's discovery (14) that aluminum h a l i d e s bincTto oxo l i g a n d s i n tungsten oxo neopentyl complexes, and t h a t these com­ plexes decompose to give systems which w i l l e f f i c i e n t l y metathesize o l e f i n s , r a i s e d more questions concerning the r o l e of the Lewis a c i d . A subsequent communication (20) answered some of the ques­ t i o n s ; the aluminum h a l i d e removes Îiïe oxo l i g a n d and replaces i t w i t h two h a l i d e s to y i e l d neopentylidene complexes (equation 8 ) . +

2+

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

4

+ AIBr

Br 3

A d d i t i o n a l aluminum h a l i d e coordinates to an a x i a l h a l i d e to give a species which w i l l metathesize o l e f i n s extremely e f f i c i e n t l y . These studies demonstrate t h a t two a l k o x i d e ligands can take the place of an oxo l i g a n d and t h a t aluminum h a l i d e s , by c o o r d i n a t i n g t o a h a l i d e l i g a n d , can generate an e f f i c i e n t and l o n g - l i v e d c a t a lyst. I t i s p o s s i b l e that [W(CHR)(0R)2(Br)] AlBr4" i s responsible for the c a t a l y t i c a c t i v i t y , but at low c o n c e n t r a t i o n s , and i n the +

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

374

INORGANIC C H E M I S T R Y : TOWARD T H E 2 1 S T C E N T U R Y

presence of excess o l e f i n , bimolecular decomposition of the c a t i o n i c species should be slow. The Reaction of Tantalum Neopentylidene Hydride Complexes with Ethylene

—

A few years ago I v i n , Rooney, and Green made a provocative suggestion for which there was only tenuous experimental support ( 2 1 ) . They suggested t h a t s t e r e o s p e c i f i c propylene p o l y m e r i z a t i o n b y ~ 7 i e g l e r - N a t t a c a t a l y s t s could be explained by a mechanism i n v o l v i n g r e a c t i o n of the o l e f i n with an a l k y l i d e n e l i g a n d i n an a l k y l i d e n e hydride c a t a l y s t . We set out to t e s t t h i s proposal by preparing and studying tantalum a l k y l i d e n e hydride complexes (22). One of these, Ta(CHCMe )(H)(PMe )3Cl2, reacted r e a d i l y with e t h y l ­ ene to give n e i t h e r products of rearrangement nor metathesis o f an intermediate tantalacyclobutane complex. High b o i l i n g products were formed but we could not obtain c o n s i s t e n t r e s u l t s . However, r e s u l t s using Ta(CHCMe )(H)(PMe ) l2 were c o n s i s t e n t and r e p r o ­ ducible (23). Ta(CflCMe3)(H)(PMe ) l2 i s probably a pentagonal bipyramidal complex c o n t a i n i n g an a x i a l neopentylidene l i g a n d with the phosp h i n e s , one i o d i d e , and the hydride i n the pentagonal plane ( c f . Ta(CCMe )(H)(dmpe)2(ClAlMe ) ( 2 4 ) ) . The H NMR s i g n a l for the hydride l i g a n d i s a c h a r a c t e r i s t i c o c t e t at δ 8.29 w h i l e the broad a l k y l i d e n e α-proton s i g n a l i s found at δ - 1 . 7 6 . On a d d i t i o n of a l i m i t e d quantity of ethylene v i r t u a l l y i d e n t i c a l H NMR patterns appear at δ 7.74 and δ -0.73 c o n s i s t e n t with formation of a new complex, T a ( C H R ) ( H ) ( P M e ) l 2 ; -50% of the o r i g i n a l Ta(CHCMe )(H)(PMe )3l2 remains. The v o l a t i l e s formed on treatment of t h i s mixture with CF3CO2H c o n s i s t of neopentane (-50%), and the alkanes Me3C(CH2CH2) CH3 where η = 1, 2, 3, and 4 (-50% t o t a l y i e l d ) , c o n s i s t e n t with h y d r o l y s i s of Ta[CH(CH2CH2) CMe ](H)(PMe )3l2. When CD2CD2 i s used the new product i s Ta(CDR)(D)(PMe3)3l2. When excess ethylene i s added to Ta(CHCMe3)(H)(PMe3)3l2 a pale green polymer slowly forms which weighs approximately four times the o r i g i n a l weight of Ta(CHCMe3)(H)(PMe3)3l2 a f t e r two days; a t t h i s p o i n t any f u r t h e r increase of the weight of the polymer i s n e g l i g i b l e . H y d r o l y s i s of the pale green polymer y i e l d e d a white organic polymer which was shown by f i e l d desorption mass s p e c t r a l s t u d i e s to c o n s i s t of approximately a 1:1 mixture of even and odd carbon o l e f i n s i n the range C S Q - C I O O Therefore, the pale green polymer i s l a r g e l y o r g a n i c . By s i m i l a r l y studying the polymer prepared from Ta(CDCMe3)(D)(PMe3)3l2 we showed t h a t only the odd carbon polymers increased by two mass u n i t s . Therefore, most of the even carbon polymers must be p o l y e t h y l e n e . The mechanism o f chain t r a n s f e r i s at present unknown, but the preceding r e s u l t suggests t h a t i t i s not metathesis of metallacyclobutane r i n g s . There are two ways of viewing the r e a c t i o n between Ta(CHCMe3)(H)(PMe3)3l2 and e t h y l e n e . The one shown i n equation 9 (nonessen-

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

3

3

3

3

3

3

3

l

3

3

l

3

3

3

3

n

n

3

3

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

25.

Metal-Carbon

Multiple

SCHROCK

375

Catalysis

Bonds in

H

I

C9H4

Ta=CHCMe

3

— • TaCH CMe3

TaCH CH CH CMe3

2

2

2

•

2

Ta=CHCH CH CMe3 2

2

t i a l ligands omitted) contains i n p a r t the c l a s s i c a l Cossee-type step (25) where ethylene " i n s e r t s " i n t o the tantalum(III)-neopentyl and subsequent t a n t a l u m ( I I I ) - a l k y l bonds. I t cannot y e t be r u l e d out since magnetization t r a n s f e r experiments show t h a t the a l k y l i ­ dene α-proton and the hydride l i g a n d i n Ta(CHR)(H)(PMe3)3l exchange r e a d i l y , most l i k e l y by forming Ta(CH R)(PMe3)3l . The a l t e r n a t i v e shown i n equation 10 i s analogous to t h a t proposed by I v i n , Rooney and Green. We do not think i t w i l l be easy to d i s ­ t i n g u i s h between these two p o s s i b i l i t i e s , i f i t i s p o s s i b l e at a l l . But since a l k y l i d e n e ligands i n other tantalum(V) complexes r e a c t r a p i d l y with o l e f i n s , and since there are few examples of i s o l able t r a n s i t i o n metal a l k y l complexes t h a t r e a c t r e a d i l y with ethylene ( 2 6 ) , we feel t h a t the second a l t e r n a t i v e i s more p l a u s i b l e . 2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

2

H

2

H

Ta=CHCH CH CMe3 2

2

(10)

One of the most i n t e r e s t i n g aspects of the mechanism shown i n equation 10 i s the l a s t s t e p , an α - e l i m i n a t i o n r e a c t i o n to give the new a l k y l i d e n e hydride complex. Our r e s u l t s do not imply t h a t β - e l i m i nation to give an o l e f i n hydride intermediate i s r e l a t i v e l y slow. I t i s p o s s i b l e t h a t although K > K j , k i > k (equation 11), i . e . , β-elimination i s s t i l l faster. I f t h i s i s t r u e , i t must also 2

2

H ^

TaCH CH R 2

CHR

"k"

-1

2

k

Ta=CHCH R 2

(11)

-2

be true t h a t the o l e f i n hydride complex i s r e l a t i v e l y s t a b l e toward displacement of CH =CHR by ethylene under the r e a c t i o n c o n d i t i o n s which we employ. 2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

376

INORGANIC C H E M I S T R Y : TOWARD T H E 2 1 S T C E N T U R Y

These r e s u l t s at l e a s t demonstrate t h a t ethylene can be p o l y merized by an a l k y l i d e n e hydride c a t a l y s t , probably by forming a metallacyclobutane hydride i n t e r m e d i a t e . The extent to which t h i s i s r e l e v a n t to the more c l a s s i c a l Z i e g l e r - N a t t a p o l y m e r i z a t i o n systems (27) i s unknown. Recent r e s u l t s i n l u t e t i u m chemistry ( 2 8 ) , where a l k y l i d e n e hydride complexes are thought to be u n l i k e l y , provide strong evidence for the c l a s s i c a l mechanism.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

Tungsten(VI) A l k y l i d y n e Complexes and Acetylene Metathesis On the basis of the f a c t t h a t tungsten(VI) a l k y l i d e n e complexes w i l l metathesize o l e f i n s one might p r e d i c t t h a t acetylenes should be metathesized by tungsten(VI) a l k y l i d y n e complexes ( 2 9 ) . Acetylene metathesis i s not unknown, but the c a t a l y s t s are i n F F f i c i e n t and poorly understood (30, 31). The f i r s t tungsten(VI) aTFylTcTyne complex was prepared i n low y i e l d (-20%) by r e a c t i n g WCle * h s i x equivalents of neopentyl l i t h i u m ( 3 2 ) . Three e q u i v a l e n t s of the l i t h i u m reagent are used simply to reduce W(VI) to W ( I I I ) . Therefore the y i e l d i s l i m i t e d and the mechanism by which W(CCMe )(CH?CMe ) forms obscure. A higher y i e l d route to W(CCMe )(CH2CMe ) c o n s i s t s of the r e a c t i o n shown i n equation 12 ( 3 3 ) . Reproducible y i e l d s of 50-70% can be obtained on a r e l a t i v e l y l a r g e scale (30 g ) . The mechanism by which W(CCMe3)(CH2CMe3)3 forms v i a t h i s route i s only s l i g h t l y w i

3

3

3

3

3

3

ether W ( 0 M e ) C l + 6NpMgCH CMe 3

3

2

3

•

W(CCMe )(CH CMe ) 3

2

3

(12)

3

b e t t e r understood; the methoxide l i g a n d s are b e l i e v e d to prevent r e d u c t i o n of tungsten(VI) and so allow a tungsten(VI) n e o p e n t y l i dene complex to form. I t i s f e l t t h a t once a neopentylidene complex forms, formation of a neopentylidyne complex would be f a s t . Since both W(0Me ) (CH2CMe ) and W(0Me)2Np4 can be prepared, and shown not to be converted i n t o W(CCMe )(CH2CMe ) under the r e a c t i o n c o n d i t i o n s , the c r u c i a l intermediate most l i k e l y s t i l l contains some h a l i d e ( s ) . W(0Me)2(CH2CMe )2Cl2 i s an i n t e r e s t i n g p o s s i b i l i t y since W(0CH2CMe )2(CH2CMe )2Br2 i s a p l a u s i b l e p r e c u r s o r to W(CHCMe )(0CH2CMe )2Br2 (equation 8 ) . W(CCMe )Np r e a c t s with three equivalents of HC1 i n ether or dichloromethane i n the presence of NEt4Cl to y i e l d blue [NEt4][W(CCMe )Cl4] q u a n t i t a t i v e l y ( 3 4 ) . I f 1,2-dimethoxyethane i s present instead of NEt4Cl, the product i s purple W(CCMe )(dme)Cl . E i t h e r reacts smoothly with three equivalents of L i X to give W(CCMe )X (X = 0CMe , SCMe , NMe2)· A l l are thermally s t a b l e , s u b l i m a b l e , monomeric pale y e l l o w to w h i t e , c r y s t a l l i n e s p e c i e s . W(CCMe )(0CMe ) r e a c t s r a p i d l y with symmetric acetylenes to give the new a l k y l i d y n e complexes shown i n equation 13. W(CPh)(0CMe ) i s orange and W(CCH2CH CH )(0CMe ) i s w h i t e . Both can be sublimed. The l a t t e r i s an important species since i t 3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

3

2

3

3

3

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

3

25.

Multiple

SCHROCK

Metal-Carbon

W(CCMe )(0CMe3)3 + RCECR

•

3

Bonds in Catalysis

W(CR)(0CMe3)3 + RC=CCMe3

(13)

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

R = Ph or Pr (excess RC=CR required) proves t h a t β-hydrogen atoms are t o l e r a t e d i n the a l k y l i d y n e l i g a n d , and t h a t the bulk o f the s u b s t i t u e n t i n W(CR)(00^3)3 i s probably not a f a c t o r i n determining whether W(CR)(0CMe3)3 decomposes to W2(0CMe3)ç and RC=CR, or not. (So f a r we have not observed t h i s r e a c t i o n . ) We have shown t h a t the s t o i c h i o m e t r i c r e a c t i o n i s f i r s t order i n tungsten and f i r s t order i n acetylene over a wide range of concentrations ( 3 5 ) . W(CCMe3)(0CMe3)3 r e a c t s r a p i d l y wvth unsymmetric acetylenes to give the i n i t i a l metathesis products, RC=CCMe3 and/or R'CECCMes, and the symmetric acetylenes c a t a l y t i c a l l y . The most impressive i s the metathesis of 3-heptyne where the value for k ( M " s e c " ) i s between 1 and 10. Therefore, i n neat 3-heptyne (-1 M) at 25° the number of turnovers i s of the order of several per second. I f we assume t h a t W(YI) or Mo(YI) a l k y l i d y n e s i t e s or complexes are r e s p o n s i b l e for the r e l a t i v e l y slow metathesis i n the known heterogeneous (30) and homogeneous (31) systems, then i t becomes c l e a r t h a t the c o n c e n t r a t i o n of a c t i v e species on the surface or i n s o l u t i o n must be extremely s m a l l . W(CCMe3)(0CMe3)3 i s not the only a l k y l i d y n e complex which w i l l metathesize a c e t y l e n e s . W(CCMe3)(NMe2)3 w i l l a l s o , although the data so f a r have not been q u a n t i t a t e d . A l l others (W(CCMe3)Np3, [W(CCMe3)Cl4]", W(CCMe )(dme)Cl3, and W(CCMe3)(SCMe3)3) w i l l not. Of these, we have s t u d i e d the r e a c t i o n between the h a l i d e complexes and a l k y l acetylenes most c l o s e l y . A d d i t i o n of excess 2-butyne to W(CCMe3)(dme)Cl3 y i e l d s r e d , paramagnetic, s o l u b l e W d ^ - C s l ^ B u * ) (MeC=CMe)Cl2> and orange, paramagnetic, [W(* -C5Me4But)Cl4]2, each i n -50% y i e l d ( 3 6 ) . The i d e n t i t y of W(n -C5Me4But)(MeC=CMe)Cl2 was proven by an x-ray s t r u c t u r a l study which showed i t to be s i m i l a r to Ta(n -C5Me5)(PhC=CPh)Cl2 (37) and r e l a t e d s p e c i e s . The W(Y) dimer and W(III) acetylene complex probably form by d i s p r o p o r t i o n a t i o n of some intermediate W(IY) complex. What we were most i n t e r e s t e d i n was whether any i n t e r mediates not c o n t a i n i n g a T ^ - C S I ^ B U * l i g a n d could be i s o l a t e d . W(CCMe3)(dme)Cl3 r e a c t s with one e q u i v a l e n t of 2-butyne to give a v i o l e t complex whose C NMR data are c o n s i s t e n t with i t being a tungstenacyclobutadiene complex (equation 14) ( 3 6 ) . In p a r t i c u l a r , two s i g n a l s are found at 268 and 263 ppm (cT7 335 f o r the neopentylidyne α-carbon atom i n W(CCMe3)(dme)Cl3) and a t h i r d 1

1

3

5

5

5

1 3

W(CBu )(dme)CI + MeC=CMe f

(14)

3

Me

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

378

INORGANIC

CHEMISTRY:

TOWARD T H E 2 1 S T C E N T U R Y

a t 151 ppm. An x-ray s t r u c t u r a l study showed t h a t i t i s indeed a tungstenacyclobutadiene complex, a t r i g o n a l bipyramidal monomer w i t h a x i a l c h l o r i d e l i g a n d s and a planar WC3 r i n g . Perhaps the most s u r p r i s i n g feature of t h i s molecule i s the f a c t t h a t VI·"Co (2.12Â) i s l e s s than a t y p i c a l W i V D - C ^ y i bond d i s t a n c e . This i s probably the reason why the C - C o - C angle i s so large ( 1 1 9 ° ) , and c o u l d be the reason why the α - s u b s t i t u e n t s are bent away from the m e t a l . There i s c l e a r l y plenty of room f o r a d d i t i o n a l 2-butyne to coordinate to tungsten, probably between C l q and C ( / C l q - W - C » 1 4 0 ° ) , to produce a WC5 r i n g which then c o l l a p s e s to a c y c l o p e n t a d i e n y l system (equation 15). As a r e s u l t of t h i s process the metal a

a

e

a

e

a

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

lit

(15)

(not observed)

(disproportionates)

i s reduced from W(YI) to W(IV). Perhaps at l e a s t one r o l e of a t - b u t o x i d e l i g a n d i s to make the metal more d i f f i c u l t to reduce. I t probably a l s o d e s t a b l i z e s the tungstencyclobutadiene complex r e l a t i v e to the a l k y l i d y n e complex so t h a t the actual c o n c e n t r a t i o n of a tungstenacyclobutadiene t r i - t - b u t o x i d e complex i s s m a l l . I n t e r e s t i n g l y , one t - b u t o x i d e l i g a n d can be added i n the e q u a t o r i a l p o s i t i o n but i f a d d i t i o n of a second i s attempted, only t r i - t b u t o x y a l k y l i d y n e complexes r e s u l t (equation 16).

Cl Bu* Bu*0-W^-Me C

'

M e

,

Λ

Ο

• •

WtCRKOBu^

(16)

(R = Bu or Me) f

In view of the s e n s i t i v i t y of o l e f i n metathesis c a t a l y s t s to f u n c t i o n a l groups we were somewhat s u r p r i s e d to f i n d t h a t acetylene metathesis c a t a l y s t s are apparently much more t o l e r a n t of func­ t i o n a l groups than o l e f i n metathesis c a t a l y s t s . For example, W(CCMe3)(0CMe3)3 w i l l metathesize 3-heptyne i n the presence of many e q u i v a l e n t s of a c e t o n i t r i l e , e t h y l a c e t a t e , phenol, t r i e t h y l ami ne, or i n t e r n a l o l e f i n s ( 3 5 ) . Consequently, W(CCMe3)(0CMe3)3 w i l l metathesize some f u n c t i o n a l i z e d a c e t y l e n e s . P r e l i m i n a r y s t u d i e s show t h a t i t w i l l metathesize EtC=CCH2NMe2, and t h a t i t w i l l cross metathesize Me3SiOCH2C=CCH20SiMe3 with 3-hexyne. However, f i r s t attempts to metathesize EtC=CCH2Cl, H0CH2C=CCH20H with 3-hexyne, or EtC=CC02Me f a i l e d . There are several p o s s i b l e reasons why and we are i n the process of attempting to f i n d out i n d e t a i l what they are.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

25.

SCHROCK

Multiple

Metal-Carbon

Bonds in

Catalysis

379

Literature Cited

1.

Fischer, E.O.; Maasböl, A. Angew Chem. Int. Ed. Eng. 1964, 3, 580.

2.

(a) Grubbs, R.H. Prog. Inorg. Chem. 1978, 24, 1-50; (b) Katz, T.J. Adv. Organomet. Chem. 1977, 16, 283-317; (c) Calderon, N.; Lawrence, J. P.; Ofstead, E.A". Adv. Organomet. Chem. 1979, 17, 449-492; (d) Rooney, J.J.; Stewart, A. Spec. Period. Rep.: Catal. 1977, 1, 277. Hérrison, J.L.; Chauvin, Y. Makromol. Chem. 1970, 141, 161.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

3. 4.

Schrock, R.R. in "Reactions of Coordinated Ligands"; Braterman, P.S., Ed.; Plenum: New York, in press.

5.

Schrock, R.R. J. Am. Chem. Soc. 1974, 96, 6794.

6.

Schrock, R.R.; Fellmann, J.D. J. Am. Chem. Soc. 1978, 100, 3359.

7.

Wood, CD.; McLain, S.J.; Schrock, R.R. J. Am. Chem. Soc. 1979, 101, 3210. Rupprecht, G.A.; Messerle, L.W.; Fellmann, J.D.; Schrock, R.R. J. Am. Chem. Soc. 1980, 102, 6236.

8. 9.

Rocklage, S.M.; Fellmann, J.D.; Rupprecht, G.A.; Messerle, L.W.; Schrock, R.R. J. Am. Chem. Soc. 1981, 103, 1440.

10

McLain, S.J.; Sancho, J.; Schrock, R.R. J. Am. Chem. Soc. 1980, 102, 5610. 11. (a) Schrock, R.R.; Rocklage, S.; Wengrovius, J.; Rupprecht, G.; Fellmann, J. J. Molec. Catal. 1980, 8, 73; (b) Wengrovius, J.H.; Schrock, R.R. Organometallics 1982, 1, 148. 12. Churchill, M.R.; Rheingold, A.L. Inorg. Chem. 1982, 21, 1357. 13. Griffith, W.P. Coord. Chem. Rev. 1970, 5, 459. 14. Kress, J.; Wesolek, M.; Le Ny, J.; Osborn, J.A. J. Chem. Soc. Chem. Commun. 1981, 1039. 15. Rocklage, S.M.; Schrock, R.R.; Churchill, M.; Wasserman, H.J. Organometallics, in press.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

380

INORGANIC C H E M I S T R Y : TOWARD T H E 2 1 S T C E N T U R Y

16. Pedersen, S.F.; Schrock, R.R. J. Am. Chem. Soc., in press. 17. Wengrovius, J.H.; Ph.D., Thesis, Massachusetts Institute of Technology, 1981. 18. Wengrovius, J.; Schrock, R.R.; Churchill, M.R.; Missert, J.R.; Youngs, W.J. J. Am. Chem. Soc. 1980, 102, 4515. 19. Rappé, A.K.; Goddard, W.A. Ill J. Am. Chem. Soc. 1982, 104, 448; 1980, 102, 5114.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

20. Kress, J.; Wesolek, M.; Osborn, J.A. J. Chem. Soc. Chem. Commun. 1982, 514. 21. Ivin, K.J.; Rooney, J.J.; Stewart, C.D.; Green, M.L.H.; Mahtab, R. J. Chem. Soc. Chem. Commun. 1978, 604. 22. Fellmann, J.D.; Turner, H.W.; Schrock, R.R. J. Am. Chem. Soc. 1980, 102, 6608. 23. Turner, H.W.; Schrock, R.R. J. Am. Chem. Soc. 1982, 104, 2331. 24. Churchill, M.R.; Wasserman, H.J.; Turner, H.W.; Schrock, R.R. J. Am. Chem. Soc. 1982, 104, 1710. 25. Cossee, P. J. Catal. 1964, 3, 80. 26. An example of a complex that reacts slowly with ethylene in a manner consistent with insertion of ethylene into the metal alkyl bond is Co(n5-C5H5)(PPh)Me2: Evitt, E.R.; Bergman, R.G. J. Am. Chem. Soc. 1979, 101, 3973. 3

27. Boor, J. Jr. "Ziegler-Natta Catalysts and Polymerizations"; Academic: New York, 1979. 28. Watson, P.L. J. Am. Chem. Soc. 1982, 104, 337-339. 29. Katz, T.J. J. Am. Chem. Soc. 1975, 97, 1592-1594. 30. (a) Panella, F.; Banks, R.L.; Bailey, G.C. J. Chem. Soc. Chem. Commun. 1968, 1548-1549; (b) Moulijn, J.Α.; Reitsma, H.J.; Boelhouwer, C. J. Catal. 1972, 25, 434-436. 31. (a) Mortreux, Α.; Delgrange, J.C.; Blanchard, M.; Labochinsky, Β. J. Molec. Catal. 1977, 2, 73; (b) Devarajan, S.; Walton, O.R.M.; Leigh, G.J. J. Organomet. Chem. 1979, 181, 99-104.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

25. SCHROCK Multiple Metal-Carbon Bonds in Catalysis

381

32. Clark, D.N.; Schrock, R.R. J. Am. Chem. Soc. 1978, 100, 6774. 33.

Schrock, R.R.; Clark, D.N.; Sancho, J.; Wengrovius, J.H.; Rocklage, S.M.; Pedersen, S.F. Organometallics, in press.

34. Wengrovius, J.H.; Sancho, J.; Schrock, R.R. J. Am. Chem. Soc. 1981, 103, 3932. 35.

Sancho, J.; Schrock, R.R. J. Molec. Catal. 1982, 15, 75-79.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

36. Schrock, R.R.; Pedersen, S.F.; Churchill, M.R.; Wasserman, H.J. J. Am. Chem. Soc., in press. 37. Smith, G.; Schrock, R.R.; Churchill, M.R.; Youngs, W.J. Inorg. Chem. 1981, 20, 387. RECEIVED August 1 1 ,

1982

Discussion W.L. G l a d f e l t e r , U n i v e r s i t y o f M i n n e s o t a : Does t h e s t a b i l i t y of t h e t r i c h l o r o t u n g s t e n a c y c l o b u t a d i e n e complex i m p l y t h a t t h e " s p e c i a l e f f e c t " o f t h e OR l i g a n d i s t o enhance t h e r a t e o f decomposition of t h i s intermediate?

R.R. S c h r o c k : We b e l i e v e s o , b u t t h e t - b u t o x i d e ligand seems t o be a u n i q u e a l k o x i d e . We have p r e p a r e d tungstenacyclob u t a d i e n e complexes c o n t a i n i n g o t h e r a l k o x i d e l i g a n d s which a r e s t a b l e t o w a r d d e c o m p o s i t i o n t o an a l k y l i d y n e c o m p l e x .

M.H. Chisholm, Indiana University: I should like to make a p r e d i c t i o n . I n v i e w o f t h e a b i l i t y of tungsten t o form m u l t i p l e bonds w i t h c a r b o n , a s i s w e l l i l l u s t r a t e d by y o u r work w i t h a l k y l i d e n e and a l k y l i d y n e l i g a n d s , I b e l i e v e we s h a l l soon see t h e emergence o f a new c l a s s o f t u n g s t e n compounds i n c o r p o r a t i n g c a r b i d e (C * ) a s a l i g a n d . The p r o b a b l e modes o f b o n d i n g f o r carbide should compliment t h a t found f o r oxo and n i t r i d o c o m p l e x e s . 1

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

382

INORGANIC C H E M I S T R Y :

TOWARD THE

W.L. G l a d f e l t e r , U n i v e r s i t y of M i n n e s o t a : i z e p r o p y l e n e w i t h your t a n t a l u m c a t a l y s t ?

21ST

Can you

CENTURY

polymer-

R.R. S c h r o c k : No. Propylene reacts to g i v e primarily p r o p a n e and a complex of t h e t y p e T a ( C H C M e ) ( C H P M e ) ( P M e ) I . 3

2

2

3

2

2

J.M. B u r l i t c h , C o r n e l l U n i v e r s i t y : I s t h e r e any e v i d e n c e f o r an exchange of a l k y l i d y n e l i g a n d o f t h e s o r t shown below? L 'Ci, WECr

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch025

2

3

+

R.R. Schrock: n o t b e e n done.

L CJt WECR' 2

No.

3

The

*

L 'Cfc WECR' 2

required

3

+

labelling

L C£ W=CR 2

3

experiment

has

A.J. C a r t y , G u e l p h - W a t e r l o o C e n t r e : What a r e t h e f r e q u e n cies of the metal-carbon (V(MEC)) stretching frequencies i n t h e t u n g s t e n a l k y l i d y n e compounds? One m i g h t e x p e c t t h e s e t o be q u i t e h i g h i n v i e w o f t h e x - r a y d a t a showing s h o r t M E C bonds and e v i d e n c e of m u l t i p l e bond r e a c t i v i t y .

R.R. S c h r o c k : T h e r e a r e bands a t c a . 1 3 0 0 cm i n t h e IR s p e c t r a of s e v e r a l a l k y l i d y n e c o m p l e x e s w h i c h m i g h t be a s s i g n e d t o WEC s t r e t c h i n g modes a n a l o g o u s t o t h o s e o b s e r v e d by F i s c h e r i n c o m p l e x e s o f t h e t y p e ( X ) ( C 0 ) W E C R ( X = h a l i d e ) , but we have n o t done t h e a p p r o p r i a t e l a b e l l i n g o r Raman s t u d i e s t o c o n f i r m the assignments. 5

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

26 The Alkylidyne Compound [(η-C H ) (OC) W ≡CC H Me-4] 5

5

2

6

4

A Reagent for Synthesizing Complexes with Bonds Between Tungsten and Other Metals F. G O R D O N A . S T O N E

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

The University, Bristol, Department of Inorganic Chemistry, Bristol BS8 1TS England

In 1979 we reported (1) that the compound [(η-CH)(OC)W = CR] (R = C H Me-4), discovered by Fischer et al. (2), would displace ethylene from the complexes [Pt(C H )(PR ) ] to give species (1), with platinum-tungsten bonds and bridging tolylidyne ligands. Because of the isolobal relationship existing between the groups CR and W(CO)(η-CH)[W(d ), WL ], the compound (1) may be compared with the long known alkyne­ -platinum complexes (2). It thus became apparent that the molecule [(η-CH)(OC)W = CR] would show a reactivity pattern towards other transition metal complexes similar to that of an alkyne, thus affording numerous complexes with metal - metal bonds involving tungsten. This area of study has grown sufficiently to merit a review. A survey is particularly timely for two reasons. Firstly, considerable interest has developed recently in the chemistry of transition metal complexes contain­ ing heteronuclear metal - metal bonds. Secondly, in relation to current awareness of the importance of C chemistry, the study of compounds having CR groups bridging metal - metal bonds is important. Finally, in the context of the theme of this Conference, perhaps the work described represents a modest step towards placing the synthesis of compounds with metal - metal bonds on a more logical basis than hitherto. 5

5

2

6

4

2

4

3 2

5

2

5

5

5

5

5

2

1

Dimetal Compounds Compounds (3) - (1J) have been i s o l a t e d from r e a c t i o n s between [(η-C H y(0C) W = CR] and the species [Co(C0) (η-C Me )] ( 3 ) , [Rh(C0)L(η-C H )1 (L = CO or PMe ) ( 4 , 5 ) , [Rh(acac)(C0) ] (F), t M ( C O ) ( t h f ) ( n - C H R " ) J (M = Mn, R ' = Me; M = Re, R ' = H) (T), [ C r ( C 0 ) ( t h f ) ( - C M e ) l ( 4 ) , and [ M ( C 0 ) ( n - C H ) 2 ] (M = T i or Zr) ( 7 ) , r e s p e c t i v e l y . The i r o n - t u n g s t e n compound (12) has been obtained by t r e a t i n g [(η-C H )(0C) W = CR] with ~ [ F e ( C 0 ) ] i n ether solvents ( 8 ) . It is e s p e c i a l l y r e a c t i v e , r e a d i l y transforming i n t o d i i r o n tungsten or i r o n d i t u n g s t e n c l u s t e r species i n the presence o f an excess o f one or other o f the r e a c t a n t s . 5

5

2

9

2

7

2

3

5

2

n

6

2

5

2

5

5

2

4

6

5

5

5

2

9

0097-6156/83/0211-0383$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

384

INORGANIC C H E M I S T R Y : TOWARD T H E 2 1 S T C E N T U R Y

R' .C. ^ \

R .PR, Cp(0C) W

Pt'

?

R'

C

P

T

/ \ X

φ

L = PR

(14)

L = CO

3

3

R

3

(2)

R

R

Cp(0C) Wi-

M(C0)Cp

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

p

PR-

Cp(0C) W—Rh(L)(n-C H ) 2

g

7

M (3) (13)

Cp(0C) W-

Co

(4)

CO

Rh

(5)

PMe-

-Rh(acac)(C0)

2

Cp(0C) W-

M(C0) (n-C tyO

2

2

(6)

M

R

(7)

Mn

Me

(8)

Re

H

R

5

R

Cp(0C) W^-^Cr(C0) (n-C Me ) 2

2

6

Cp(0C)W^

6

(9,

MCp *

2

0

M (IS) v

R Cp(0C) W^—^Fe(C0) 2

T i

^

4

(12) Throughout t h i s paper, i n the s t r u c t u r a l formulae, CR = CC Hi*Me-4> Cp = n - C H , Cp* = n - C M e , n - C H = n - i n d e n y l (n = 3 or 5 ) . 6

5

5

5

5

n

5

5

9

7

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

26.

STONE

Alkylidyne

385

Compound

Complex (13) i s one product (see l a t e r ) of the r e a c t i o n o f [ R h ( y - C O ) ( n - i l M e ) ] w i t h [ ( n - C H ) ( 0 C ) W = CRJ ( 9 ) . The compounds (3) - (13) are formulated w i t h dimetallacyclopropene r i n g s , and Tn t h e ' c a s e of (3,), ( 5 ) , (9) and (10) t h i s i s supported by the r e s u l t s of s i n g l e - c r y s t a l ~ X - r a y d i f f r a c t i o n s t u d i e s . Several o f the species c o n t a i n a semi-br'idging CO l i g a n d f o r which there i s e i t h e r i . r . ( v c i r c a 1 850 c n H ) o r X-ray evidence. An i n t e r e s t i n g feature of the s t r u c t u r e o7 (10) i s t h a t the X-ray d i f f r a c t i o n r e s u l t s reveal the presence ofan n - C = 0 F r i d g i n g group. A s i m i l a r l i g a n d i s probably present (U) c e both (10) and ( ] J ) show CO s t r e t c h i n g bands i n the i . r . at 1 638 and 1 578 c m " l , r e s p e c t i v e l y , as w e l l as terminal CO bands at 1 930 and 1 937 cm"!. For the dimetal compounds the most useful s p e c t r o s c o p i c property of a d i a g n o s t i c nature i s the resonance due to the y-C l i g a t e d carbon i n the C n.m.r. spectra. In the precursor t ( n - C H ) ( 0 C ) W = CR] the a l k y l i d y n e carbon atom resonates i n the C n . m . r . spectrum at δ 300 p.p.m. In the dimetal compounds (3) - (13) the s i g n a l f o r the μ-C nucleus i s even more d e s h i e l d e d : t y p i c a l 1 values are 341 ( 3 ) , 327 ( 5 ) , 431 ( 9 ) , and 391 p.p.m. ( Π ) . I t i s becoming i n c r e a s i n g l y apparent t h a t a very extensive"* chemistry i s a s s o c i a t e d w i t h the dimetal species ( 1 ) , and (3) - (1,3). Three types o f r e a c t i o n may be i d e n t i f i e d : ( i ) adiditiorTof other metal l i g a n d fragments, discussed i n the next S e c t i o n , ( i i ) replacement of p e r i p h e r a l l i g a n d s on the metal c e n t r e s , and ( i i i ) r e a c t i o n s at the b r i d g i n g carbon atom. One example of ( i i ) and ( i i i ) w i l l s u f f i c e t o i l l u s t r a t e the scope and p o t e n t i a l f o r new c h e m i s t r y . The PR l i g a n d s t r a n s o i d t o the y-CR group i n (Vj are r e a d i l y d i s p l a c e d by CO gas (1 b a r ) , a f f o r d i n g i n i t i a l l y the dimetal compounds (14)» which subsequently i n s o l u t i o n y i e l d the t r i m e t a l complexes ( l j ) . E v i d e n t l y compounds (1,4) p a r t i a l l y d i s s o c i a t e i n t o [ ( n - C H ) ( 0 C ) W = CR] and [ P t ( C 0 ) ( P R ) j , and the l a t t e r adds to u n d i s s o c i a t e d (14) t o g i v e the complexes (15) 2

2

5

2

5

5

2

c o

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

i n

s i n

1 3

5

5

2

n

3

5

5

2

3

(10).

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

386

INORGANIC C H E M I S T R Y : TOWARD T H E 21ST C E N T U R Y

I t i s i n t e r e s t i n g to compare the s t r u c t u r e o f the species (15) (IK-ray d i f f r a c t i o n study f o r PR = PMePh ) with those o f the i s o l o b a l bridged alkyne complexes (L = PR or L = c y c l o o c t a - l , 5 - d i e n e ) (11). In both (1^) and (16) the P t - - - - P t vectors ( c i r c a 2.ΤΓΑ) are too great f o r a n y ' a p p r e c i a b l e metal metal i n t e r a c t i o n s , and the platinum atoms can be regarded as having 1 6 - e l e c t r o n c o n f i g u r a t i o n s . We r e f e r r e d e a r l i e r to the s i g n i f i c a n c e of r e a c t i o n s a t the a l k y l i d y n e carbon atoms o f the dimetal s p e c i e s . Our studies i n t h i s area are i n a p r e l i m i n a r y s t a g e , but Schemes 1 and 2 summarise some chemistry at the bridged carbon centres f o r the compounds (Y) and (3^)(12). I t w i l l be noted t h a t protonation o f the neutral Dridged^all C **

6

ô

> ML (d ) 2

M (21)

Mo

(§)

W

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

3

26.

Alkylidyne

STONE

389

Compound

In and (£2) the c e n t r a l metal atoms acquire 1 8 - e l e c t r o n c o n f i g u r a t i o n s by i r - b o n d i n g from a CO l i g a n d on each W ( C 0 ) ( n - C H ) moiety, a feature e s t a b l i s h e d by X-ray d i f f r a c t i o n . The C n . m . r . spectra of (£1) and (22) are a l s o informative showing CO groups i n three environments ( r e l . i n t . 2 : 2 : 2 ) , and resonances f o r the y-CR groups at 6 360 (21) and 376 p.p.m. (22). In an ever i n c r e a s i n g number o f compounds t o l y l i d y n e l i g a n d s t r i p l y - b r i d g e a t r i a n g l e o f metal atoms, one o f which i s tungsten and the other two may or may not i n v o l v e the same element. Thus two types o f core s t r u c t u r e are p o s s i b l e : fM W(y -CR)] (Class A) or [M M W(y -CR)] (Class B ) . Two methods are a v a i l a b l e f o r the synthesis o f c l u s t e r s of Class A. v i z 2

5

5

l 3

2

1

3

2

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

3

( i ) Successive a d d i t i o n o f s i m i l a r metal l i g a n d fragments [ ( n - C H ) ( 0 C ) W = CR] 5

5

to

2

Thus (2£) i s prepared by adding [ R h ^ H O ^ a c a c ) ] to ( 6 ) , the l a t t e r being preformed by t r e a t i n g the mononuclear tungsten a l k y l i d y n e compound with [Rh(C0) (acac)] (6). Similarly, step-wise a d d i t i o n o f [ R h ( C 0 ) ( n - C H ) l to | ( n - C H ) ( 0 C ) W = CR] yields (?5)(9). We r e f e r r e d e a r l i e r t o the r e a c t i v i t y o f ( 1 2 ) . If this species i s generated i n the presence of excess [FeiCO)!,(thf)] i t r a p i d l y affords ( 2 5 ) ( 8 ) . I f , however, (12) i s generated i n the presence o f e x c e s r [ ( n - C H ) ( 0 C ) W = CR] the alkyne bridged i r o n d i t u n g s t e n c l u s t e r (26) i s the major product. This species i s of i n t e r e s t on two counts. F i r s t l y , i t i s an example o f a c l u s t e r i n which a c o u p l i n g o f CR fragments of the carbyne has o c c u r r e d , a r e a c t i v i t y pattern r e f e r r e d to below. Secondly, (26) i s an unsaturated metal c l u s t e r (46 valence e l e c t r o n s ) and t i e W —W d i s t a n c e [2.747(1) A] suggests a degree o f double bonding. 2

2

5

R

L (23)

L

5

9

7

5

5

2

2

R

3

acac

2

( i) (33) OS

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

390

INORGANIC C H E M I S T R Y :

Cp(0C)W

TOWARD THE

21ST CENTURY

W(C0) Cp 2

( i i ) Reaction of [ ( n - C H ) ( 0 C ) W = CR] w i t h dimetal complexes

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

5

5

2

I t has long been known t h a t alkynes r e a c t w i t h dimetal compounds, e . g . [ C o ( C 0 ) l ( 1 7 , 1 8 ) , [ N i ( y - C 0 ) ( n - C H ) 2 ] ( 1 9 , 2 0 ) , or L O 2 ( C 0 ) 7 r n - C H ) 2 ] (21_,27y,To give species w i t h dimetaTTatetrahedrane core s t r u c t u r e s . I t seemed l i k e l y , t h e r e f o r e , t h a t [ ( n - C H ) ( 0 C ) W = CR] would show a s i m i l a r r e a c t i v i t y p a t t e r n , and i n c o n f i r m a t i o n o f t h i s compound (27) i s produced i n q u a n t i t a t i v e y i e l d by r e a c t i n g the a l k y l i d y n e - t u n g s t e n compound w i t h [ C o ( C 0 ) ] i n pentane at room temperature ( 6 J . 2

M

5

5

8

2

2

5

5

5

5

2

2

8

Co(CO),

Cp(0C) W 2

(27) By r e f l u x i n g the compounds M ( C 0 K ( n - C H ) l (M = Mo or W) with [ ( n - C H ) ( 0 C ) W = CR] i n tot uene, the complexes [2$) and (29) are formed ( 2 3 ) . I n t e r e s t i n g l y , a s i m i l a r r e a c t i o n with L f r ( C 0 K ( n - C H h T affords (30) q u a n t i t a t i v e l y , and no (31) i s produced. Somewhat s i m i l a r behaviour i s shown by [ N i ( y - C 0 ) ( n - C H ) ] which w i t h the a l k y l i d y n e t u n g s t e n compound produces a mixture of (30) and (32). I t should be noted t h a t [ ( n - C H ) ( 0 C ) W = CR] does not d î m e r i s e to (30) when heated by i t s e l f ; indeed such a transformation i s unalfowed. In the reactions of the dimetal compounds with [ ( n - C H ) ( 0 C ) W = CR] i t seems probable t h a t the former reactants d i s s o c i a t e i n i t i a l l y to 2

5

5

2

5

2

2

5

5

5

5

2

2

5

5

5

2

2

5

5

2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

26.

STONE

Alkylidyne

391

Compound

Cp

give mononuclear metal l i g a n d fragments ( 2 4 ) , and t h a t the l a t t e r are captured by [ ( n - C H ) ( 0 C ) W = CR] to give dimetal species which r a p i d l y undergo f u r t h e r r e a c t i o n , a f f o r d i n g the f i n a l products ( 9 ) . In t h i s context i t i s i n t e r e s t i n g t h a t the a l k y l i d y n e t u n g s t e n compound reacts with [ R h ( y - C 0 ) ( n - C M e 5 ) 2 ] to give a mixture of the dimetal species (13) and the t r i m e t a l compound [Rh W(y -CR)(y-C0)(C0) (n-C H5)(n-C5Me5)2] (33). In some instances formation o f (30) may be avoided by employing ' a l k y n e ' displacement r e a c t i o n s . Thus (32) i s formed i n e s s e n t i a l l y q u a n t i t a t i v e y i e l d by r e a c t i n g 5

5

2

2

2

3

2

2

5

5

C l u s t e r s of Class Β may be prepared by adding appropriate metal l i g a n d fragments to the dimetal s p e c i e s . Thus treatment of (3) with [ F e ( C 0 ) l affords ( 3 4 ) , and s i m i l a r l y (4) reacts w i t h " [ F e ( C 0 ) J to give (3§){9).~ The species (V) r e a c t " r e a d i l y with [ F e ( C 0 ) ] i n tetrahydrofuran at'Voom temperature. However, a mixture o f 2

2

9

9

2

9

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

392

INORGANIC C H E M I S T R Y ! TOWARD T H E 2 1 S T C E N T U R Y

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

ML (34)

CoCp*

(35)

Rh(n-C H ) g

7

c l u s t e r s (36) i s produced (25). The t r i m e t a l compounds (3§) r e s u l T f r o m a d d i t i o n of an Fe^tTO)., fragment to Q ) , presumably from [Fe(C0)^(thf)] ( 2 6 ) . Loss o f a molecule o f CO would a f f o r d ( 3 § ) . I s o l a t i o n of the c l u s t e r s (37) can be understood i n the f o l l o w i n g way. I t was mentioned e a r l i e r t h a t the dimetal compounds (V) r e a d i l y r e a c t w i t h CO to g i v e ( 1 4 ) . Enneacarbonyl d i i r o n in~tetrahydrofuran provides a source o f CO (from [ F e ( C 0 ) s ] ) , thereby l e a d i n g to the i n s i t u formation o f (14). A d d i t i o n of an Fe(CO)^ fragment to the l a t t e r , accompanied by l o s s of CO, would y i e l d the c l u s t e r s ( 3 7 ) .

(C0)

3

(38)

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

26.

STONE

Alkylidyne

I n t e r e s t i n g l y , compounds o f Class A or Class Β have 50 c l u s t e r valence e l e c t r o n s i f the three metal atoms are i n d i v i d u a l l y to have 1 8 - e l e c t r o n c o n f i g u r a t i o n s . * Several o f the compounds meet t h i s requirement, e . g . ( 2 5 ) , ( 2 7 ) , ( 3 1 ) , (34) or (36). However, compound (23) w itT T ïë c î u s t e r ~ v a l e n c e ~ e l e c t r o n s , and compounds (37) and (38) with 48 c l u s t e r valence e l e c t r o n s , possessing four^and f i v e ' s k e l e t a l bond e l e c t r o n p a i r s , r e s p e c t i v e l y , r e f l e c t the tendency of rhodium or platinum atoms to adopt 1 6 - e l e c t r o n c o n f i g u r a t i o n s . By r e f l u x i n g [ R u ( C 0 ) i ] with [ ( n - C H ) ( C 0 ) W == CR] i n toluene the isomeric c l u s t e r s (39) are produced ( 8 ) . The osmium analogs (40) form i n t h e " r e a c t i o n between [Os (y-H)2(y-CH y(CO)io] and the a l k y l i d y n e t u n g s t e n compound i n t h f at 60 ° C . In the formation o f (39) and (40) a l k y l i d y n e groups have coupled to produce t r i m e t a l species^with co-ordinated RC R groups. In the context o f i s o l o b a l r e l a t i o n s h i p s , i t i s i n t e r e s t i n g to regard (39) and ($Q) as d i m e t a l l a c y c l o b u t a d i e n e s s t a b l i s e d by complexatioh w i t h M(C0) o r W ( C 0 ) ( n - C H ) groups. A f u r t h e r p o i n t o f i n t e r e s t i s t h e i r ready i n t e r c o n v e r s i o n i n s o l u t i o n , which we have s t u d i e d by v a r i a b l e temperature n . m . r . measurements ( 8 ) . Reaction o f [(n-C H5)(0C) W = CR] with polynuclear metal carbonyl species i s l i k e l y t o a f f o r d many new heteronuclear c l u s t e r compounds c o n t a i n i n g tungsten. I l l u s t r a t i v e of t h i s i s the formation o f (41) i n the r e a c t i o n with [Os (CO)io(cyclo-CeHi0 ] (£) 3

3

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

393

Compound

2

5

5

2

2

2

3

2

5

3

5

2

2

/Π\ Cp(0C) W 2

5

. '

/w\ 'W(C0) Cp

Cp(0C) W

2

2

^

M(C0)

3

^w(co) cp ?

(CO), *

M (3?)

Ru

(40)

0s

We have chosen to t r e a t the y - C atom as part o f the c l u s t e r c o r e , and the RC group (5 valence e l e c t r o n s ) as c o n t r i b u t i n g three e l e c t r o n s f o r c l u s t e r bonding. A l t e r n a t i v e l y , the t r i m e t a l compounds, i n which each metal atom has an 1 8 - e l e c t r o n c o n f i g u r a t i o n , can be regarded as 48 e l e c t r o n species having three-electron y -CR ligands. By adopting t h i s formalism, compound (23) would be a 4 4 - e l e c t r o n c l u s t e r . 3

3

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

394

INORGANIC C H E M I S T R Y :

TOWARD T H E 21 ST

CENTURY

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

Conclusion We seem to have been the f i r s t to appreciate t h a t W ( C 0 ) ( n - C H ) (W,d ) i s i s o l o b a l with CH ( 2 7 ) , and to use t h i s concept i n designed syntheses o f compounds w i t h bonds between tungsten and other metals. We are hoping to extend the method to other systems c o n t a i n i n g LpM = CR groups, i n v o l v i n g both e a r l y and l a t e t r a n s i t i o n m e t a l s , e . g . M U ( d ) { 0 s C l ( C 0 ) ( P P h ) (28) } a n 3 ~ R L ( d ) { T a C l ( P M e ) ( n - C M e ) (29J1. 5

2

5

5

7

3

3

3

2

5

2

6

5

Acknowledgments I thank my very t a l e n t e d co-workers c i t e d i n the references who c a r r i e d out the work, and the U . K . Science and Engineering Research Council f o r f i n a n c i a l support. LITERATURE CITED

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

Ashworth, T.V., Howard, J.A.K., and Stone, F.G.A., J.Chem.Soc.,Chem.Commun., 1979, 42; J.Chem.Soc.,Dalton Trans., 1980, 1609. Fisher, E.O., Lindner, T.L., Huttner, G., Friedrich, P., Kreissl, F.R., and Besenhard, J.O., Chem.Ber., 1977, 110, 3397. Razay, H., Ph.D. Thesis, Bristol University, 1982. Chetcuti, M.J., Green, M., Jeffery, J . C ., Stone, F.G.A., and Wilson, Α.Α., J.Chem.Soc.,Chem.Commun., 1980, 948. Jeffery, J . C ., Schmidt, Μ., and Stone, F.G.A., unpublished results. Chetcuti, M.J., Chetcuti, P.A.M., Jeffery, J . C ., Mills, R.M., Mitrprachachon, P., Pickering, S.J., Stone, F.G.A., and Woodward, P., J.Chem.Soc.,Dal ton Trans., 1982, 699. Dawkins, G.M., Green, M., Salaun, J.-Y., and Stone, F.G.A., unpublished results. Busetto, L., Green, M., Howard, J.A.K., Hessner, B., Jeffery, J . C., Mills, R.M., Stone, F.G.A., and Woodward, P., J.Chem.Soc.,Chem.Commun., 1981, 1101.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

26. STONE Alkylidyne Compound

9. 10. 11.

12. 13. Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29.

395

Green, M., Jeffery, J.C., Porter, S.J., Razay, H., and Stone, F.G.Α., J.Chem.Soc.,Dalton Trans., in press. Chetcuti, M.J., Marsden, Κ., Moore, I., Stone, F.G.Α., and Woodward, P., J.Chem.Soc.,Dalton Trans., in press. Boag, N.M., Green, M., Howard, J.A.K., Spencer, J.L., Stansfield, R.F.D., Thomas, M.D.O., Stone, F.G.Α., and Woodward, P., J.Chem.Soc.,Dalton Trans., 1980, 2182; Boag, N.M., Green, Μ., Howard, J.A.K., Stone, F.G.Α., and Wadepohl, H., ibid., 1981, 862. Jeffery, J.C., Moore, I., Razay, H., and Stone, F.G.Α., J.Chem.Soc.,Chem.Commun., 1981, 1255. Hodgson, D., Jeffery, J.C., Marsden, K., Stone, F.G.Α., Went, M.J., and Woodward, P., unpublished results. Ashworth, T.V., Chetcuti, M.J., Howard, J.A.K., Stone, F.G.Α., Wisbey, S.J., and Woodward, P., J.Chem.Soc.,Dalton Trans., 1981, 763. Boag, N.M., Howard, J.A.K., Green, M., Grove, D.M., Spencer, J.L., and Stone, F.G.Α., J.Chem.Soc.,Dalton Trans., 1980, 2170. Carriedo, G.A., Marsden, Κ., Stone, F.G.Α., and Woodward, P., unpublished results. Sternberg, H.W., Greenfield, H., Friedel, R.A., Wotiz, J . , Markby, R., and Wender, I., J.Am.Chem.Soc., 1954, 76, 1457. Dickson, R.S., and Fraser, P.J., Adv.Organomet.Chem., 1974, 12, 323. Tilney-Bassett, J.F., J.Chem.Soc., 1961, 577; 1963, 478. Jolly, P.W., and Wilke, G., "The Organic Chemistry of Nickel", Vol. 1, Academic Press, N.Y.C., 1974. Nakamura, Α., and Hagahara, N., Nippon Kagaku Kaishi, 1963, 84, 344. Knox, S.A.R., Stansfield, R.F.D., Stone, F.G.Α., Winter, M.J., and Woodward, P., J.Chem.Soc.,Dalton Trans., 1982, 173; and references cited therein. Green, M., Porter, S.J., and Stone, F.G.Α., to be published. Madach, T., and Vahrenkamp, H., Chem.Ber., 1980, 113, 2675. Chetcuti, M.J., Howard, J.A.K., Mills, R.M., Stone, F.G.A., and Woodward, P., J.Chem.Soc.,Dalton Trans., in press. Cotton, F.A., and Troup, J.M., J.Am.Chem.Soc., 1974, 96, 3438. Hoffmann, R., 'Les Prix Nobel 1981', Almqvist and Wiksell, Stockholm, 1982. Clark, G.R., Marsden, Κ., Roper, W.R., and Wright, L.J., J.Am.Chem.Soc., 1980, 102, 6570. Fellmann, J.D., Turner, H.W., and Schrock, R.R., J.Am.Chem.Soc., 1980, 102, 6608.

RECEIVED August

24, 1982

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

396

INORGANIC C H E M I S T R Y : TOWARD T H E

21ST

CENTURY

Discussion

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

A.W. Adamson, U n i v e r s i t y o f S o u t h e r n C a l i f o r n i a ; You demon­ s t r a t e d t h e u s e f u l n e s s of t h e i s o l o b a l a p p r o a c h as a c o r r e l a ­ t i n g one. Can y o u , however, g i v e some examples o f how the a p p r o a c h has l e d t o e x p e r i m e n t a l c h e m i s t r y t h a t w o u l d n o t have b e e n s u g g e s t e d by o t h e r r a t i o n a l e s ?

F.G.A. S t o n e : I had t h o u g h t t h a t t h e answer t o y o u r q u e s t i o n had been embodied i n t h e l e c t u r e . However, p e r h a p s I failed t o s t r e s s t h a t t h e i s o l o b a l a p p r o a c h i n our w o r k h a s b e e n p r e d i c t i v e r a t h e r t h a n a mere c o r r e l a t i o n o f a p p a r e n t ­ l y u n r e l a t e d r e s u l t s . I w o u l d h i g h l i g h t two e x p e r i m e n t s w h i c h emphasize the ' p r e d i c t i v e ' c h a r a c t e r of the r e s e a r c h . ( i ) We w e r e l e d t o p r e p a r e t h e t r i m e t a l compound ( 2 0 ) as a c o n s e q u e n c e of t h e e a r l i e r p r e p a r a t i o n o f t h e compounds [ P t ( a l k y n e ) ] (15) from [ Ρ ^ 0 Η „ ) ] and RC^CR. The isolobal relationship b e t w e e n [ ( n - C H ) ( 0 C ) W ^ C R ] and RC=CR prompted us to investigate the corresponding reaction between the tungsten compound and [Pt(C H ) ], as discussed elsewhere (14). (ii) I t was the long known o b s e r v a t i o n that alkynes w i t h [ C o ( C 0 ) ] a f f o r d the d i c o b a l t species [ C o ( u - a l k y n e ) ( C 0 ) ] w h i c h prompted our d i s c o v e r y o f t h e c l u s t e r compound ( 2 7 ) . It seemed likely (6) t h a t [(η-C H )(0C) W=CR] would react w i t h [ C o ( C 0 ) ] i n a s i m i l a r manner t o RC^CR b e c a u s e of t h e r e l a t i o n s h i p CR W ( C 0 ) (η -C H ) . 2

2

5

3

s

2

2

2

k

3

8

2

5

2

5

6

2

8

2

5

5

W.L. Gladfelter, U n i v e r s i t y o f M i n n e s o t a ; What i s t h e possibility of f o r m i n g c a r b o n - c a r b o n bonds u s i n g two m e t a l a l k y l i d y n e fragments?

F.G.A. S t o n e : W i t h increasing f r e q u e n c y , as our w o r k i n t h i s a r e a d e v e l o p s , we a r e o b s e r v i n g r e a c t i o n s i n w h i c h t h e m e t a l a l k y l i d y n e f r a g m e n t s l i n k t o f o r m C-C b o n d s . P l e a s e r e f e r t o compounds ( 3 9 ) and ( 4 0 ) , and t h e r e l e v a n t r e f e r e n c e s to these species.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

26.

STONE

Alkylidyne

397

Compound

G.A. O z i n , U n i v e r s i t y of Toronto; In view of the c u r r e n t i n t e r e s t i n e l e c t r i c a l l y c o n d u c t i n g , undoped and doped p o l y a c e t y l e n e t h i n f i l m s , do y o u e n v i s a g e any p o s s i b i l i t y o f i n i t i a t i n g a controlled p o l y m e r i z a t i o n of a m e t a l - m e t a l t r i p l y bonded o r g a n o m e t a l l i c complex t o produce a s p e c i e s of t h e form: L ,n

w

L n

w

L n

w

L n

\

/

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch026

L

L

η

M' L η

η

L

M L

η

, ( c i s or tr a n s )

η

F.G.A. S t o n e : T h i s i s a v e r y i n t e r e s t i n g t h o u g h t , but it seems more likely t h a t t h e model compound e m p l o y e d i n o u r work [ ( η - C H ) ( 0 C ) W ^ C R ] w i l l f o r m c y c l i c o l i g o m e r s r a t h e r than afford l i n e a r p o l y m e r s , when t r e a t e d w i t h a p p r o p r i a t e t r a n s i t i o n m e t a l s a l t s . You w i l l r e c a l l t h a t many l o w - v a l e n t m e t a l complexes r e a c t w i t h a l k y n e s t o produce arenes c a t a l y t i c a l l y . We h a v e n o t as y e t s u c c e e d e d i n t r i m e r i s i n g [ ( n - C H ) ( 0 C ) W E C R ] , but t h e l a t t e r i n t h e p r e s e n c e o f c e r t a i n c o m p l e x e s a f f o r d s t h e b r i d g e d - a l k y n e d i t u n g s t e n compound ( 3 0 ) . I r e f e r you t o our p a p e r s : Green, M.; Porter, S . J . ; S t o n e , F.G.A. J . Chem. Soc. Dalton Trans., i n press. G r e e n , M.; J e f f r e y , J.C.; P o r t e r , S . J . ; R a z a y , H.; Stone, F.G.A. J . Chem. Soc., D a l t o n T r a n s . , i n p r e s s . 5

5

2

5

5

2

L. L e w i s , G e n e r a l E l e c t r i c : F o l l o w i n g y o u i s o l o b a l a r g u ­ ment, w o u l d you e x p e c t o r do you o b s e r v e any m e t a l c a r b o n bond scission reactions? Vollhardt e t a l . (_1) o b s e r v e C^C bond s c i s s i o n t o form carbynes u s i n g C p C o ( C 0 ) . P r o f e s s o r Schrock j u s t t o l d us how t o b r e a k C^C u s i n g h i g h v a l e n t m e t a l s . Does y o u r W=C show any s i m i l a r r e a c t i v i t y ? ( 1 ) F r i t c h , J.R.; V o l l h a r d t , K.P.C. Angew. Chem. 1980, 19. 2

F.G.A. S t o n e : Y e s , I w o u l d e x p e c t c l e a v a g e o f t h e WEC bond i n [ ( n - C H ) ( 0 C ) W E C R ] to occur i n c e r t a i n reactions. Indeed, we b e l i e v e we have o b s e r v e d C=W bond c l e a v a g e i n a r e a c t i o n o f t h e t u n g s t e n compound w i t h a c a r b i d o ( c a r b o n y l ) i r o n c l u s t e r . I n t e r e s t i n g l y , [ ( n - C H ) ( 0 C ) W E C R ] reacts with sulphur t o y i e l d [ ( n - C H ) ( 0 C ) · W S C R : S ] ( I . Moore, B r i s t o l U n i v e r s i t y ) . 5

5

2

5

5

2

e

5

5

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

27 Structure of [{Os (μ-H) (CO) C(μ-O)} (B O )] 3

3

9

3

3

3

SHELDON G. SHORE

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch027

The Ohio State University, Department of Chemistry, Columbus, OH 43210

Professor Stone's paper points out that the reactivity of [(η-C H )(OC)W=CR] towards transition metal complexes is similar to that of an alkyne. It would be of interest to examine this compound and several of its derivatives which contain C=W double bonds with respect to their reactivity patterns towards the BH3 group to determine if reactions analogous to the hydroboration reaction of alkynes and olefins would occur (1) or reactions simi­ lar to the attempted hydroboration described below would take place. In our laboratory we have examined the reactivity pattern of [Os (μ-H) (CO) ], an unsaturated cluster which can be represented as possessing an osmium-osmium double bond in its classical valence bond representation. We find (2,3) that this compound undergoes a number of reactions with metal carbonyls which in some cases can be formulated as proceeding through intermediates analo­ gous to metal olefin complexes. We have also examined the reaction of THFBH3 with [Os (μ-Η) (CO) ](4,5). Our purpose was to determine if a reac­ tion analogous to hydroboration would occur to give a compound 5

5

2

3

2

3

10

2

10

i s o e l e c t r o n i c w i t h the known [ 0 s ( y - H ) ( y - C H ) ( C O ) ] ( 6 ) . f o l l o w i n g r e a c t i o n was observed. 3

2

2

The

1 0

3 [ 0 s ( y - H ) ( C 0 ) ] + 3 C/»H 0BH —> [{0s (μ-Η) (CO) C(y-0) } ( B 0 ) ] 3

2

l o

8

3

3

3

9

3

3

3

+ C4H10

3

The x-ray s t r u c t u r e of [ { 0 s ( y - H ) ( C 0 ) C ( y - 0 ) } ( B 0 ) ] (Figure 1) was determined by Dr. J . C. Huffman (Molecular Structure Center, Indiana U n i v e r s i t y ) . I t contains a boroxine r i n g ( B 0 ) and to each boron there i s attached, through a b r i d g i n g oxygen between boron and carbon, the c l u s t e r u n i t 0 s ( μ - Η ) ( C 0 ) C . The proton nmr spectrum (τ28.5) of t h i s compound i s c o n s i s t e n t with the presence of b r i d g i n g hydrogens around the 0 s plane. 3

3

9

3

3

3

3

3

3

3

9

3

0097-6156/83/0211-0399$06.00/0 © 1983 American Chemical Society In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

400

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch027

INORGANIC C H E M I S T R Y : TOWARD T H E 21 ST C E N T U R Y

Figure 1.

The molecular structure of

[{OS (fi-H) (CO) C(p-0)} (B O )}. s

s

9

s

s

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

s

27.

SHORE

Structure

of

[{Os (^H)s(CO) C(^0)}3(B O )] s

9

s

401

s

The above r e a c t i o n i s c l o s e t o being q u a n t i t a t i v e . Stoichiometry was determined by v a r y i n g molar r a t i o s of r e a c t a n t s . The Ci»Hio was i d e n t i f i e d by mass spectrometry. I n t h i s r e a c t i o n oxygen i s abstracted from C l

absorption

2

(D

where represent the i n i t i a l and f i n a l s t a t e wavefunc­ t i o n s , ê i s a u n i t vector p o i n t i n g i n the d i r e c t i o n of the p o l a r i z a t i o n , r=(x,y,z) i s a u n i t vector p o i n t i n g i n the d i r e c t i o n of the x, y or ζ axes, and the sum i s taken over a l l o r i e n t a t i o n s (20). The EXAFS, χ, i s p r o p o r t i o n a l to absorption c r o s s - s e c t i o n . Equation 1 can be s i m p l i f i e d to χ = Σ Ki|

r |f>|2

2

C O

s e

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

(2)

29.

HAHN AND

HODGSON

Polarized

x-Ray Absorption

433

Spectroscopy

where θ i s the angle between ê and r . The matrix element can be evaluated to give the f a m i l i a r i s o t r o p i c EXAFS expression (c.f. references 10-14). In the s i n g l e - e l e c t r o n approximation, t h i s matrix element i s o n l y s i g n i f i c a n t i f r p o i n t s i n the d i r e c t i o n of the unoccupied s t a t e of i n t e r e s t (edge s t r u c t u r e ) or the b a c k s c a t t e r e r of i n t e r e s t (EXAFS). Thus, we can i d e n t i f y θ i n Equation 2 w i t h the angle between e and an unoccupied o r b i t a l (edge s t r u c t u r e ) or an absorber-backscatterer vector (EXAFS). T h i s i s i l l u s t r a t e d f o r EXAFS i n Figure 1. For a r i g o r o u s development of the theory of PXAS, the reader i s r e f e r r e d t o , f o r example, reference 20. For c r y s t a l l i n e samples of cubic or higher symmetry, f o r p o l y c r y s t a l l i n e samples, or f o r amorphous m a t e r i a l s (e.g. for most XAS experiments), symmetry r e q u i r e s that the observed XAS be s p h e r i c a l l y symmetric (20,21). In these cases the angular dependence i n Equation 2 must be averaged over a l l o r i e n t a t i o n s , g i v i n g a f a c t o r of 1/3.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch029

9

Edges Although there are notable exceptions, most a p p l i c a t i o n s of edge s t r u c t u r e f o r determination of molecular geometry have concentrated on an e m p i r i c a l c o r r e l a t i o n of absorber s i t e sym­ metry and chemical environment with edge s t r u c t u r e . This i s a r e s u l t of the d i f f i c u l t y i n q u a n t i t a t i v e l y i n t e r p r e t i n g much of the edge s t r u c t u r e . When the i n c i d e n t photon has an energy just barely above the absorption threshold, the resulting photoelectron can undergo numerous m u l t i p l e s c a t t e r i n g events, g r e a t l y complicating the t h e o r e t i c a l a n a l y s i s (22). Thus, while t r a n s i t i o n s to bound s t a t e s are reasonably w e l l understood, a complete t h e o r e t i c a l t r e a t m e n t o f the edge s t r u c t u r e i s difficult. Oriented samples allow the p o s s i b i l i t y of g r e a t l y s i m p l i ­ f y i n g the i n t e r p r e t a t i o n of the edge s t r u c t u r e . The information obtained from p o l a r i z e d edge s t u d i e s i s i n many ways analogous to the information obtained from p o l a r i z e d o p t i c a l spectroscopy. S i n c e edge s t r u c t u r e i n c l u d e s t r a n s i t i o n s i n t o h i g h - l y i n g molecular o r b i t a l s ( f o r d i s c r e t e molecular systems) or h i g h - l y i n g bands ( f o r extended s o l i d - s t a t e s t r u c t u r e s ) , the p o l a r i z a t i o n dependence o f t h e s e t r a n s i t i o n s can, i n p r i n c i p l e , p r o v i d e i n f o r m a t i o n about the symmetry p r o p e r t i e s of s p e c i f i c o r b i t a l s or bands. The observed symmetry i s a product of the symmetries of the i n i t i a l s t a t e , the f i n a l s t a t e , and the operator c o u p l i n g these s t a t e s . Κ and L ( I ) absorption edges thus have an important s i m p l i f i c a t i o n over o p t i c a l spectroscopy s i n c e the i n i t i a l s t a t e (Is or 2s) i s s p h e r i c a l l y symmetric and does not i n f l u e n c e the observed orientational dependence of the transition. U n f o r t u n a t e l y , the i d e n t i f i c a t i o n of x - r a y a b s o r p t i o n edge t r a n s i t i o n s i s not always unambiguous, e s p e c i a l l y f o r t r a n s i t i o n s i n t o s t a t e s that are near the continuum.

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch029

434

INORGANIC C H E M I S T R Y : TOWARD T H E 21 ST C E N T U R Y

Figure 1. Polarized XAS. The orientations of ê and k and the angle θ (see Equa­ tion 2) are shown for a typical PXAS experiment. For this orientation, there can be no Cu-S contribution to the EXAFS (& — 90°). The Cu-N contribution to the EXAFS will be 3cos e-times that observed for an isotropic sample. 2

In Inorganic Chemistry: Toward the 21st Century; Chisholm, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

29.

HAHN AND

HODGSON

Polarized

x-Ray Absorption

435

Spectroscopy

E l e c t r o n i c S t r u c t u r e Determination* Cox and Beaumont have studied the p o l a r i z e d x-ray a b s o r p t i o n edge of a s i n g l e c r y s t a l of ZnF^ (23), i n which the Zn has t e t r a g o n a l ( D ^ ) s i t e symmetry. The observed a n i s o t r o p i c K-absorption edges were explained i n terms of a ls+4p and a ls-K4p ,4p ) t r a n s i t i o n . Templeton and Templeton rfave observed a n i s o t r o p i c edges f o r the "oxo" species [V=0] and [ U 0 ] (24,25). In the vanadium case, a very strong pre-edge t r a n s i t i o n was observed only when the e l e c t r i c vector of the p o l a r i z a t i o n was o r i e n t e d p a r a l l e l to the V=0 bond. From t h i s ρ type symmetry and from the strength of t h e f e a t u r e ( i n d i c a t i n g an a l l o w e d t r a n s i t i o n ) , i t was suggested that t h i s peak i s due to a t r a n s i t i o n i n t o a molecular o r b i t a l c o n t a i n i n g vanadium 3d 2 d 4s o r b i t a l s and a sigma o r b i t a l (having ρ symmetry) from the vanadyl oxygen. In the u r a n y l case, the L ( I ) (2s i n i t i a l s t a t e ) and the L ( I I I ) (2p i n i t i a l s t a t e ) edges were both s t u d i e d (25). Because of t h e i r d i f f e r e n t i n i t i a l - s t a t e symmetries, these two edges give mutually e x c l u s i v e d.ipole s e l e c t i o n r u l e s . In agreement with t h i s , the authors observed not only o r i e n t a t i o n a l dependence f o r both edges, but a l s o a d i f f e r e n t o r i e n t a t i o n a l dependence f o r the d i f f e r e n t edges. y

2

2

2

a n

Publication Date: March 3, 1983 | doi: 10.1021/bk-1983-0211.ch029

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O r i e n t a t i o n Determination. While p o l a r i z e d edge s t u d i e s , together with a known sample o r i e n t a t i o n , can provide information about the e l e c t r o n i c s t r u c t u r e of the absorber, one can a l s o use p o l a r i z e d edges to probe ordered systems of unknown o r i e n t a t i o n . T h i s s o r t of approach was used i n a study of B ^ adsorbed on graphite (26,27). In t h i s case, the o r i e n t a t i o n a l dependence of an edge t r a n s i t i o n was used t o c a l c u l a t e t h e d e g r e e of o r i e n t a t i o n a l p u r i t y of the graphite s u r f a c e . Another example of the p o t e n t i a l u t i l i t y of p o l a r i z e d edge spectra f o r s t r u c t u r e determination i s found f o r [ M o 0 ^ 2 Î " (28)· T h i s molecule has C £ symmetry and the C2 axes of a l l of the molecules i n the u n i t c e l l are c o l l i n e a r . Thus, when the c r y s t a l i s o r i e n t e d with the p o l a r i z a t i o n p a r a l l e l to the S-S i n t e r a t o m i c v e c t o r , the p o l a r i z a t i o n i s perpendicular to the Mo-O bonds and n e a r l y p a r a l l e l to the Mo-S bonds. S i m i l a r l y , the c r y s t a l can be o r i e n t e d with the p o l a r i z a t i o n perpendicular to the Mo-S bonds and n e a r l y p a r a l l e l to the Mo-O bonds. For both o r i e n t a t i o n s , e x c e l l e n t agreement was obtained with SCF-X α c a l c u l a t i o n s of the edge s t r u c t u r e (8). Moreover, e x c e l l e n t agreement was a l s o observed between the p o l a r i z e d edge s p e c t r a of [MoO £ 1 ~ * , corresponding "pure" i s o t r o p i c s p e c t r a of [MoO^] ~ and [MoS^] " ( e*g., w i t h the p o l a r i z a t i o n p a r a l l e l to the 0-0 v e c t o r , an [MoO^] spectrum was observed) (8). In general, i f a molecule contains l i g a n d s which i n d i v i d u a l l y g i v e very d i f f e r e n t edge s t r u c t u r e s , i t may be pos­ s i b l e to determine the o r i e n t a t i o n of these l i g a n d s u s i n g p o l a r ­ i z e d absorption edges. As shown here, the i s o t r o p i c spectrum of a "pure" compound, c o n t a i n i n g only one of the l i g a n d s , can be used to determine the ligand-dependent edge s t r u c t u r e s . 2

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