Stereochemistry of Optically Active Transition Metal Compounds 9780841205383, 9780841206847, 0-8412-0538-8

Table of contents :
Title Page......Page 1
Half Title Page......Page 3
Copyright......Page 4
ACS Symposium Series......Page 5
FOREWORD......Page 6
PdftkEmptyString......Page 0
PREFACE......Page 7
1 Research in the Stereochemistry of Cobalt Complexes......Page 9
Literature Cited......Page 18
Five-Membered Chelate Rings......Page 21
Six-membered Chelate Rings......Page 25
Seven-Membered Chelate Rings......Page 28
Circular Dichroism Spectra of Tris(diamine)cobalt(III) Complexes......Page 30
Diethylenetriamine Complexes......Page 34
Complexes with a Cyclic Terdentate, R-MeTACN......Page 35
Electron-density Distribution in D3 Complexes......Page 39
The Crystal Structure of Active Racemates and Other Diastereoisomers......Page 40
Structural Study of Asymmetric Hydrogenation......Page 43
Literature Cited......Page 47
3 Circular Dichroic Intensities in the Vibronic Transitions of Chiral Metal Complexes......Page 51
II. Theory......Page 55
III. Examples......Page 74
Literature Cited......Page 79
4 Stereochemical Correlations in the Circular Dichroism of d-d and Charge-Transfer Transitions: Applications toTris(bidentate) Complexes......Page 81
Chromophore Definitions: The Model......Page 83
Circular Dichroism of Charge Transfer Transitions......Page 86
Circular Dichroism of the d-d Transitions......Page 89
Applications......Page 96
Literature Cited.......Page 98
5 Circular Dichroism Spectra of Square Planar Complexes Containing Prochiral Olefins and Their Stereoselective Olefin Exchange......Page 99
CD Pattern of Rhodium(I) and Platinum(II) Complexes......Page 100
Stereo-selectivity on Olefin Exchange......Page 114
Abbreviations......Page 121
Literature Cited......Page 122
6 Chirality Induction in Coordination Complexes......Page 123
Stereospecific hydration of olefins......Page 125
Stereo Specificity directed by the Chelates......Page 131
Stereospecific addition of CN- at a chelated imine - orbital steering......Page 132
Stereospecific Carbinolamine Formation......Page 134
Chiral metal ion cages......Page 136
Literature Cited......Page 138
7 Stereochemistry of Microbial Iron Transport Compounds......Page 140
Model Hydroxamate Complexes......Page 141
Hydroxamate Siderophore Complexes......Page 151
Thiohydroxamate Siderophore Complexes......Page 156
Catecholate Siderophore Complexes......Page 161
Model Catecholate Complexes......Page 165
Summary......Page 168
Literature Cited......Page 172
8 Rational Approaches to Asymmetric Hydrogenation......Page 175
Stereoselection in Metal-Olefin Complexation......Page 176
Mechanistic Studies on Asymmetric Hydrogenation......Page 178
Catalysis and Binding by 5-Ring Chelate Biphosphine Rhodium Complexes......Page 181
Catalysis and Binding by 7-Ring Chelate Biphosphine Rhodium Complexes......Page 183
Catalytic Asymmetric Hydrogenation of other Substrates by 7-Ring Chelate Biphosphines......Page 191
Rhodium Complexes with Larger-ring Chelates of Chiral Biphosphines......Page 193
Rationalization and Summary......Page 196
Literature Cited......Page 198
Azophenol Chelation of Metal Ions as Determined by Ligand Associated Spectral Changes......Page 201
Preparation of Model Azoligand Co(III) Complexes for the Characterization of Co(III) Azoprotein Derivatives......Page 203
Circular Dichroic Properties of Model Azophenol and Azonaphthol Complexes......Page 206
Circular Dichroism as a Monitor of Co (III) Chelation by an Azotyrosine in a Protein......Page 209
Acknowledgment......Page 211
Literature Cited......Page 212
10 Preparation and Circular Dichroism Spectra of Cobalt(III) Complexes Containing Chiral Aminophosphine Chelate Ligands......Page 213
Preparation of Complexes......Page 214
a) trans-[CoCl2(aminophosphine)2]+......Page 216
b) [Co(acac)2L]+......Page 219
Literature Cited......Page 225
Stereoselective Solvation of the Chiral Ligand......Page 226
Solvation of the Non-Chiral Ligand......Page 229
Conformer Population Change......Page 230
Solvent Dependent CD of Pentaamminecobalt(III) Complexes of Chiral Carboxylates......Page 231
Literature Cited......Page 241
12 The Nature of the Equilibrium Displacement Mechanism for the Pfeiffer Effect in Inorganic Chemistry......Page 243
The Equilibrium Displacement Mechanism......Page 244
Absolute Configuration and Equilibrium Displacement......Page 246
Hydrogen Bonding and the Pfeiffer Effect......Page 250
Environment Compounds With No Available Hydrogen for Hydrogen Bonding......Page 251
Literature Cited......Page 257
Additivity of Circular Dichroism Contributions......Page 259
Circular Dichroism of Ethylenediaminediacetic Acid Complexes......Page 261
Literature Cited......Page 275
14 Additivity of Circular Dichroism of d-d Transitions: The Vicinal Effect in a Homologous Series of Triethylenetetraaminecobalt(III) Amino Acid Complexes......Page 277
Discussion......Page 278
Literature Cited......Page 290
Syntheses and Optical Resolutions of Complexes with cis-cis Distribution of Unidentates......Page 292
Synthesis and CD Spectra of Complexes Containing 1,1,1-Tris (aminomethyl) ethane or Hexaniobate Ion as a Terdentate Ligand......Page 304
Synthesis and CD Spectra of a Series of Complexes Containing 1,4,7-Triazacyclononane......Page 306
Literature Cited......Page 316
Complete Resolution of fac-[Co(β-ala)3]......Page 318
Separation and Identification of Isomers of fac-[Co(α-AA)3-n(β-AA)n] (6)......Page 319
Trend of the Separation Factors of Enantiomeric Pairs Eluted with d-Tart......Page 320
Trend of the Seaparation Factors of Enantiomeric Pairs Eluted with Antimony d-Tart......Page 322
Association Model of Sb d-Tart (10)......Page 324
Literature Cited......Page 327
17 Stereoselective Synthesis of Quadridentate Ligands Utilizing a Template Reaction of Metal Complexes......Page 328
Summary......Page 336
Literature Cited......Page 338
18 Stereochemistry of Cobalt(III) β-Ketoaminates and Some Mixed-Ligand Analogues......Page 339
Experimental Section......Page 340
Results and Discussion......Page 344
Literature Cited......Page 357
19 Absolute Configurations from Solution Reactions: The Tris(diimine)iron(II)/Cyanide Inversion Reaction......Page 359
Handedness......Page 361
Activation Parameter Separation Through Stereochemical Results......Page 362
Unsymmetrical Diimine Potential......Page 365
Experimental Section......Page 366
Results and Discussion......Page 368
Addendum......Page 372
Literature Cited......Page 373
20 Photoacoustic Detection of Natural Circular Dichroism in Crystalline Transition Metal Complexes......Page 376
The Nature of the Condensed Phase Photocacoustic Effect......Page 377
The PACD Experiment......Page 379
PAS Theory......Page 380
PACD Theory......Page 382
Experimental......Page 385
Discussion......Page 387
Literature Cited......Page 395
21 Stereochemical Description and Notation for Coordination Systems......Page 397
Ligand Indexing......Page 400
Chemical Abstracts Service Stereochemical Notation......Page 406
More Complex Stereochemical Systems......Page 414
Literature Cited......Page 418
A......Page 420
Β......Page 422
C......Page 423
D......Page 430
E......Page 431
I......Page 432
M......Page 434
N......Page 435
O......Page 436
P......Page 437
R......Page 439
S......Page 440
T......Page 441
V......Page 442
Z......Page 443

Citation preview

Stereochemistry of Optically Active Transition Metal Compounds

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

Stereochemistry

of

Optically

Active

Transition Metal Compounds

Bodie E. D o u g l a s ,

EDITOR

University of Pittsburgh Y o s h i h i k o Saito

EDITOR

University of Tokyo

Based on a symposium sponsored by the Division of Inorganic Chemistry at the A C S / C S J Chemical Congress, Honolulu, Hawaii, April 2-6, 1979.

ACS SYMPOSIUM SERIES

119

AMERICAN CHEMICAL SOCIETY WASHINGTON, D.C. 1980 In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

Data Library of Congress Stereochemistry of optically active transition metal compounds. (ACS Symposium series; 119 ISSN 0097-6156) Includes bibliographies and index. 1. Stereochemistry—Congresses. 2. Transition metal compounds—Congresses. I. Douglas, Bodie Eugene, 1924- . II. Saito, Yoshihiko, 1920- . III. American Chemical Society. Divi­ sion of Inorganic Chemistry. IV. Series: American Chemical Society. ACS symposium series; 119. QD481.S76 541.2'23 80-10816 ISBN 0-8412-0538-8 ACSMC8 119 1-446 1980

Copyright © 1980 American Chemical Society All Rights Reserved. The appearance of the code at the bottom of thefirstpage 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 works, for resale, or for information storage and retrieval systems. 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, repro­ duce, use, or sell any patented invention or copyrighted work that may in any way be related thereto. PRINTED IN THE UNITED STATES AMERICA

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

ACS Symposium Series M . Joa

Advisory Board David L. Allara

W. Jeffrey Howe

Kenneth B. Bischoff

James D. Idol, Jr.

Donald G. Crosby

James P. Lodge

Donald D. Dollberg

Leon Petrakis

Robert E. Feeney

F. Sherwood Rowland

Jack Halpern

Alan C. Sartorelli

Brian M . Harney

Raymond B. Seymour

Robert A. Hofstader

Gunter Zweig

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

FOREWORD The ACS S Y M P O S I U M S E R I E S was founded in 1974 to provide a medium for publishin format of the Series parallels that of the continuing A D V A N C E S I N C H E M I S T R Y S E R I E S 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 Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

PREFACE lfred Werner used the classical approach to prove his intuitive ideas concerning the stereochemistry of coordination compounds. After Werners death there was little interest in inorganic stereochemistry for a long period. One of the groups which helped to revive interest in this field and to keep it active for many years was that of John C. Bailar, Jr. at the University of Illinois. We are pleased that he consented to write Chapter 1, for this volume as a personal account of his years in research. It is particularly appropriat Meeting in Hawaii in observance of his 75th birthday. Optical activity was very important in finally convincing Werner's critics of his theories concerning the stereochemistry of coordination compounds. The study of these optically active compounds has yielded increasingly detailed stereochemical information with improvements in theory and instrumentation and a significant increase in the research effort. The developments in ligand field theory, theories of optical activity, the classical Corey-Bailar paper {see Chapter 1, Ref. 26) on conformational effects, and the establishment of absolute configurations for a large number of metal complexes {see Chapter 2) have been important in stimulating the development of the stereochemistry of metal complexes. Major activities in the study of inorganic stereochemistry are now spread throughout the world. The first joint ACS/CSJ Chemical Congress in Hawaii provided an opportunity to bring together many active investigators from the United States, Japan, and Australia. It has been 25 years since the absolute configuration of the first metal complex was determined; the contributions to this symposium are representative of the achievements in this field since that time. The volume begins with some historical background, the opening lectures of the original symposium, and an invited paper on organometallic chemistry. Stereochemical assignments depend on definitive x-ray work and this work provides a basis for linking theory to experiment. Important new theoretical developments should aid our understanding of and stimulate new work in the study of optically active metal compounds. The stereochemical bases for stereoselectivity in square planar and octahedral complexes and in asymmetric hydrogénation are presented as well as other aspects of synthesis and stereoselectivity. Bioinorganic topics include microbial iron transport compounds and metal ion interactions with azoproteins. Various contributions to C D spectra and the effect of ix In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

solvent on the spectra are considered. Photoacoustic detection of C D and stereochemical description and notation also are discussed and should be instructive for workers in the field. The volume is representative of current work in the stereochemistry of optically active compounds of transition metals. We acknowledge the splendid cooperation of the contributors to the symposium and to this volume. Their efforts and those of the ACS Books Department staff in helping to publish the book soon after the symposium were crucial. We hope that this work will be useful to those active in the field and in arousing further interest in inorganic stereochemistry and its applications. YOSHIHIKO SAITO

BODIE E. DOUGLAS

University of Tokyo Tokyo 106, Japan

Universit Pittsburgh, PA 15260

January 15, 1980

χ In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

1

Research i n the Stereochemistry of Cobalt Complexes

JOHN C. BAILAR, JR. School of Chemical Sciences, University of Illinois, Urbana, IL 61801

Stereochemistry, lon is s t i l l a major area fo methods of studying it have changed, and perhaps because of that, the emphasis on the topics under investigation has changed. Syn­ thetic chemistry has been displaced by, or better, augmented by, physical methods. Both approaches are important and both contri­ bute to the growth of our knowledge. It is most appropriate that we should hold a symposium on the stereochemistry of complexes and compare notes on what is being done in the countries which are represented at this meeting. In introducing the symposium, I should like to recount some developments of the nearly fifty years that I have been involved in this field. Most of the work which I shall describe was done by my students -- not that I think that their work is better than that of others, but because I know it better, and it serves to illustrate some interesting aspects of what I think of as the synthetic approach. Space alone would preclude discussing any more. My entrance into the field of inorganic stereochemistry was brought about in an unusual way. As an organic chemist, my in­ terest lay chiefly in isomerism and isomeric rearrangements. One day in a class in general chemistry which I was teaching, there was a discussion of the hydrolysis of antimony trichloride, which leads to the precipitation of the oxychloride, SbOCl. One of the students referred to this as "antimony hypochlorite". While I was correcting his impression that "0C1" always represents a hypochlorite, it occurred to me that the hypochlorite of a metal in the +1 oxidation state would be isomeric with the oxychloride of that metal in the +3 state. This was the first time I had ever imagined that inorganic compounds could exist in isomeric forms, and it was an exciting idea. That very day, I began efforts to prepare the two isomers of T10C1, only to discover that thallous hypochlorite cannot exist, for the cation is easily oxidized and the anion is a strong oxidizing agent. However, the idea of inorganic isomers persisted and my thinking and reading soon led to the postulation of many other cases; for example, 0-8412-0538-8/80/47-119-001$05.00/0 © 1980 American Chemical Society In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

2

STEREOCHEMISTRY OF

TRANSITION

METALS

s e l o n o s u l f a t e and t h i o s e l e n a t e , p h o s p h o n i u m n i t r a t e and ammonium m e t a p h o s p h a t e and h y d r o x y l a m i n e n i t r i t e and ammonium n i t r a t e . Of c o u r s e , one c a n n o t go v e r y f a r i n l o o k i n g f o r i s o m e r s i n i n organic chemistry without d i s c o v e r i n g the whole, r i c h f i e l d of c o o r d i n a t i o n compounds. And i f one c a n have i s o m e r i c r e a r r a n g e ments i n o r g a n i c m o l e c u l e s , why n o t i n t h e c o m p l e x e s o f c o b a l t , too? I t was t h u s t h a t I became an i n o r g a n i c c h e m i s t . The s t e r e o c h e m i s t r y o f c o o r d i n a t i o n compounds was a p a r t o f c h e m i c a l t h i n k i n g much e a r l i e r t h a n i s g e n e r a l l y assumed. For e x a m p l e , as e a r l y as 1875, v a n H o f f s u g g e s t e d t h a t compounds c o n t a i n i n g s i x g r o u p s a t t a c h e d t o a c e n t r a l m e t a l atom a r e a t t h e c o r n e r s o f an o c t a h e d r o n and t h a t g e o m e t r i c i s o m e r s s h o u l d e x i s t f o r s u i t a b l y s u b s t i t u t e d c a s e s (1). However, i t was A l f r e d W e r n e r who d e m o n s t r a t e d t h a t t h i s i s i n d e e d t h e c a s e , and a l s o t h a t t h e f o u r c o v a l e n t p l a t i n u m ( I I ) compounds a r e c o p l a n a r In those e a r l y days, the m u l t i t u d we now e n j o y was n o t known be made m o s t l y on t h e b a s i s o f s y n t h e s i s and o t h e r c h e m i c a l e v i d e n c e . P r o g r e s s was s l o w , f o r t h i s k i n d o f s t r u c t u r e d e t e r m i n a t i o n r e q u i r e s a g r e a t d e a l o f i m a g i n a t i o n and more e x p e r i m e n t a l w o r k t h a n d e t e r m i n a t i o n o f s t r u c t u r e by modern m e t h o d s . C o n s i d e r , f o r example, Werner's i n g e n i o u s d e t e r m i n a t i o n o f t h e s t r u c t u r e s o f c i s - and t r a n s - [ P t ( N H ) 2 CI 2] ( 2 ) , and h i s d e m o n s t r a t i o n o f t h e o c t a h e d r a l s t r u c t u r e o f s i x - c o o r d i n a t e complexes through the o p t i c a l r e s o l u t i o n of [ C o ( e n ) 2 ( N H 3 ) X ] (X = C l , B r ) Ç3). We a r e now a b l e , by modern t e c h n i q u e s , n o t o n l y t o perform these demonstrations q u i c k l y , b u t , i n the case of c h i r a l c o m p l e x e s , t o show t h e a c t u a l c o n f i g u r a t i o n s o f t h e i s o m e r s (4,5) . A f t e r W e r n e r ' s d e a t h i n 1919, t h e r e was l i t t l e a c t i v i t y i n t h e f i e l d o f s t e r e o c h e m i s t r y o f c o o r d i n a t i o n compounds. An e x c e p t i o n i s f o u n d i n t h e w o r k o f Y u j i S h i b a t a , who had b e e n one o f W e r n e r ' s s t u d e n t s , and who c o n t i n u e d w i t h e x c e l l e n t s t e r e o c h e m i c a l w o r k when he r e t u r n e d t o J a p a n . H i s w o r k on t h e enzyme-like a c t i v i t y o f c o b a l t complexes f u r n i s h e s e s p e c i a l l y i n t e r e s t i n g e x a m p l e s o f s t e r e o s e l e c t i v i t y and o f t h e c a t a l y t i c a c t i o n o f s u c h compounds ( 6 ) . T. P. M c C u t c h e o n and V. L. K i n g , A m e r i c a n s who had done t h e i r t h e s e s on s t e r e o c h e m i c a l t o p i c s under Werner's g u i d a n c e , d i d not c o n t i n u e i n t h a t f i e l d . King, who a c t u a l l y p e r f o r m e d t h e f i r s t r e s o l u t i o n o f an a s y m m e t r i c c o m p l e x ( 7 ) , went i n t o i n d u s t r i a l w o r k . M c C u t c h e o n became a member o f t h e f a c u l t y a t t h e U n i v e r s i t y o f P e n n s y l v a n i a and d i d r e s e a r c h on c o m p l e x compounds, b u t n o t on t h e i r s t e r e o c h e m i s t r y . I n a l o n g p a p e r w h i c h he p u b l i s h e d i n 1912 (_8), Werner d e s c r i b e d many c i s - t r a n s r e a r r a n g e m e n t s w h i c h c o b a l t ( I I I ) compounds u n d e r g o d u r i n g r e p l a c e m e n t r e a c t i o n s . He a l s o d e s c r i b e d some r e a c t i o n s o f o p t i c a l l y a c t i v e c o m p l e x e s , w h i c h g i v e opt i c a l l y a c t i v e p r o d u c t s , s u c h as t h e one shown by t h e e q u a t i o n 3

2 +

levo-cis-[Co(en)2CI2]CI + K C0 2

3

-> d e x t r o - [ C o ( e n ) C 0 ] C l 2

3

+

2KC1.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

1.

BAiLAR

Stereochemistry

of Cobalt

Complexes

3

He had no way o f k n o w i n g w h e t h e r t h i s r e a c t i o n i n v o l v e d a n i n v e r s i o n o f c o n f i g u r a t i o n , but on the b a s i s o f such s t u d i e s , he s u g g e s t e d a mechanism f o r t h e famous W a l d e n i n v e r s i o n o f o r g a n i c c h e m i s t r y , p o s t u l a t i n g t h a t t h e c e n t r a l atom o f a c h i r a l m o l e c u l e exerts a d i r e c t i v e influence. Some y e a r s l a t e r , R o b e r t A u t e n , a n u n d e r g r a d u a t e s t u d e n t a t the U n i v e r s i t y o f I l l i n o i s , a c h i e v e d an o p t i c a l i n v e r s i o n i n the r e a c t i o n j u s t mentioned ( 9 ) . P a t t e r n i n g h i s work on t h a t o f Walden, Auten used s i l v e r carbonate i n s t e a d o f potassium c a r b o n a t e and o b t a i n e d a l e v o - r o t a t o r y c a r b o n a t o complex. I t was soon found t h a t the c h o i c e o f carbonate i s not the i m p o r t a n t f a c t o r , a n d Dwyer, S a r g e s o n a n d R e i d , i n A u s t r a l i a , showed t h a t t h e pH o f t h e s o l u t i o n i s t h e d e c i d i n g f a c t o r ( 1 0 ) . After his i n i t i a l success, Auten looked f o r i n v e r s i o n s i n the r e a c t i o n s o f the d i c h l o r o complex w i t them. I t i s most f o r t u n a t f i r s t , f o r o t h e r w i s e , we w o u l d h a v e c o n c l u d e d t h a t o p t i c a l i n v e r s i o n s do n o t t a k e p l a c e i n t h e r e a c t i o n s o f c o b a l t c o m p l e x e s , and p r o b a b l y w o u l d n o t h a v e t r i e d c a r b o n a t e . W e r n e r ' s d i r e c t i o n s f o r t h e r e s o l u t i o n o f t h e d i c h l o r o comp l e x a r e v e r y s i m p l e a n d q u i t e s p e c i f i c , b u t when A u t e n r e p e a t e d them, he o b t a i n e d no c r y s t a l s o f t h e d i a s t e r e o i s o m e r a t a l l . S e v e r a l t r i a l s a l l gave n e g a t i v e r e s u l t s . Upon r e f l e c t i o n , A u t e n r e a l i z e d t h a t l a b o r a t o r i e s i n E u r o p e a r e n o t h e a t e d as much a s o u r s i n A m e r i c a , s o he r e p e a t e d W e r n e r ' s d i r e c t i o n s w i t h t h e s o l u t i o n c o o l e d t o 16°. T h i s gave a n e x c e l l e n t y i e l d o f p u r e product. E v e r y c h e m i s t knows t h a t sometimes r e p e t i t i o n o f a n o t h e r ' s w o r k does n o t g i v e good r e s u l t s ; many do n o t u n d e r s t a n d , h o w e v e r , t h a t t h e d i f f i c u l t y may l i e i n a m i n o r change i n c o n ditions . I t was s o o n f o u n d t h a t an i n v e r s i o n t a k e s p l a c e a l s o i n t h e reaction [Co(en) Cl ]Cl 2

2

+ NH

3

+ [Co(en) (NH )C1]C1 2

3

2

+

[Co(en) (NH ) ]Cl 2

3

2

3

t h e c o n f i g u r a t i o n o f t h e f i n a l p r o d u c t d e p e n d i n g upon t h e t e m p e r a t u r e at which the r e a c t i o n takes p l a c e (11). T h i s seems t o b e a somewhat more c o m p l i c a t e d r e a c t i o n t h a n t h e o n e w i t h c a r b o n a t e , f o r i t p r o c e e d s i n two d i s t i n c t l y v i s i b l e s t e p s w i t h s h a r p changes i n c o l o r , a n d i n e a c h s t e p t h e p r o d u c t may h a v e t h e o r i g i n a l c o n f i g u r a t i o n , t h e m i r r o r image one, o r t h e t r a n s c o n f i g u r a t i o n . The r o t a t i o n s o b s e r v e d f o r t h e diammine w e r e v e r y s m a l l , b u t w e r e c o n f i r m e d b y o b s e r v a t i o n s made b y s e v e r a l d i f f e r e n t p e o p l e a n d e v e n by s e c r e t l y h a v i n g t h e o b s e r v e r s c h e c k t h e same s a m p l e r e p e a t e d l y . The r e s u l t s a r e shown i n T a b l e I .

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY O F TRANSITION

4

Table I . R e a c t i o n o f Ammonia w i t h l e v o - [ C o ( e n ) 2 C I 2 ] C I [Co(en) (NH )2]Cl3 2

Reagent

Ratio cis/trans in product

[o]

D

-32° -22° +18° +38°

2.5/1 2.85/1 3.7/1 00

-77° -38° +25° +80°

3

to yield

3

Temperature

L i q u i d NH3 ( s o l u t i o n ) L i q u i d NH3 ( s o l u t i o n ) L i q u i d NH (solution) Gaseous NH3

METALS

In t h e r e a c t i o n a t 80°, t h e c o m p l e x r e m a i n e d s o l i d t h r o u g h o u t the r e a c t i o n . E v i d e n t l y v e r y l i t t l e r e a r r a n g e m e n t o f any s o r t took p l a c e , f o r the valu dextro [Co(en) (NH ) ]C1 i n t h i s r e a c t i o n was somewha s u r p r i s i n g , g a t o r s , f o r i t h a d b e e n assumed t h a t t h e two s t e p s w o u l d f o l l o w t h e same m e c h a n i s m , s o t h a t any i n v e r s i o n t h a t t o o k p l a c e i n t h e f i r s t s t e p would be c o u n t e r b a l a n c e d by an i n v e r s i o n i n t h e second. However, E a r l Greenwood was n o t a b l e t o g e t an i n v e r s i o n i n t h e r e a c t i o n (12) 2

3

2

[Co(en) (NH )Cl]Cl 2

3

2

+ NH

+

3

[Co(en) (NH ) ]Cl 2

3

2

3

so i t was assumed t h a t i n v e r s i o n takes place only i n the r e ­ placement o f t h e f i r s t c h l o r o group. R o n a l d A r c h e r l a t e r showed t h a t t h i s i s , i n d e e d , the case ( 1 3 ) . C l o s e l y r e l a t e d t o t h i s i s t h e much l a t e r w o r k o f E i s h i n Kyuno a n d L a u r e n c e B o u c h e r (14) , who s t u d i e d t h e r e a c t i o n s o f t h e t r i e t h y l e n e t e t r a m i n e c o m p l e x , [ C o ( t r i e n ) C l 2 ] C 1 . The c o m p l e x c a t i o n o f t h i s s u b s t a n c e e x i s t s i n t h r e e s t e r e o i s o m e r i c forms

β

trans

b o t h t h e a and β f o r m s b e i n g a s y m m e t r i c a n d r e s o l v a b l e . Treat­ ment w i t h b a s e c o n v e r t s t h e d e x t r o - a - f o r m i n t o i t s l e v o - 3 - i s o m e r . We o n c e t h o u g h t t h a t a l l o f t h e s e r e a r r a n g e m e n t s result f r o m t h e a b s t r a c t i o n o f a p r o t o n f r o m a c o o r d i n a t e d amine a n d t h e m o t i o n o f t h e r e s u l t i n g n e g a t i v e l y c h a r g e d amine group f r o m

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

1.

BAiLAR

Stereochemistry

of Cobalt

5

Complexes

one c o r n e r o f t h e o c t a h e d r o n t o an a d j a c e n t one. T h i s may n o t b e the case, however, f o r A r c h e r and h i s s t u d e n t s a t t h e U n i v e r s i t y o f M a s s a c h u s e t t s h a v e o b s e r v e d a n i n v e r s i o n i n a r e a c t i o n o f an o c t a h e d r a l c o m p l e x i n w h i c h t h e r e a r e no a c i d i c p r o t o n s ( 1 5 ) . [Fe(o-phen) ] 3

3 +

+ 2CN~ + c i s - [ F e ( o - p h e n ) 2 ( C N ) 2 1

+

+ o-phen

Another s t e r e o c h e m i c a l problem concerns the p r e p a r a t i o n o f o c t a h e d r a l c o m p l e x e s o f known c o n f i g u r a t i o n . The R u s s i a n c h e m i s t s , who a r e m a s t e r s i n t h e m a n i p u l a t i o n o f p l a t i n u m c o m p l e x e s , h a v e done i n t e r e s t i n g w o r k i n t h i s a r e a . F o r e x a m p l e , C h e r n y a e v a n d K r a s o v s k a y a (16) p r e p a r e d a l l f i v e isomers o f [ P t ( N H ) C l B r ] : 3

2

2

2

II

III

D r . Anna Gel'man (now u s u a l l y t r a n s l i t e r a t e d a s Hel'man) a n d h e r c o l l e a g u e s h a v e a l s o done e l e g a n t w o r k o f t h i s t y p e a n d h a v e s u c c e e d e d i n m a k i n g t h e compound

I

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

6

STEREOCHEMISTRY OF

TRANSITION

METALS

i n w h i c h the p o s i t i o n o f each group i s f i r m l y e s t a b l i s h e d ( 1 7 ) . T h i s s u b s t a n c e i s c h i r a l , b u t e v i d e n t l y no a t t e m p t has b e e n made to r e s o l v e i t . P l a t i n u m l e n d s i t s e l f r e a d i l y t o these i n v o l v e d syntheses, f o r the t r a n s e f f e c t i n the r e a c t i o n s of p l a n a r p l a t i n u m ( I I ) complexes a l l o w s the p r e p a r a t i o n o f isomers w h i c h o f f e r good s t a r t i n g p l a c e s f o r t h e f o r m a t i o n o f t h e d e s i r e d o c t a h e d r a l p l a t i n u m ( I V ) compounds. I t i s u n f o r t u n a t e t h a t Dr. Hel'man has n o t c o n t i n u e d h e r v e r y i n t e r e s t i n g r e s e a r c h on t h e s y n t h e s i s o f compounds c o n t a i n i n g a m u l t i p l i c i t y o f u n i d e n t a t e g r o u p s , f o r i t i s e x c e l l e n t w o r k . F o r some y e a r s now she has been i n v o l v e d i n the c h e m i s t r y of a c t i n i d e elements. The p r e p a r a t i o n o f t h r e e i s o m e r s o f [ C o ( e n ) ( N H 3 ) 2 C I 2 ] C 1 by Peppard (18), I t h i n k , ranks i n importance w i t h those j u s t mentioned.

I n t h e s e s y n t h e s e s , P e p p a r d h a d t o r e l y on t h e v e r y weak t r a n s e f f e c t i n t h e c o b a l t c o m p l e x [Co (NH ) ( S 0 ) 2]"· The c i s dichloro-cis-diammineethylenediaminecobalt(III) i o n i s asymmetric, and P e p p a r d p a r t i a l l y r e s o l v e d i t i n t o i t s e n a n t i o m e r i c f o r m s by a d s o r p t i o n on q u a r t z g r o u n d t o 100 mesh. T h i s method o f r e s o l u t i o n i s f r e q u e n t l y , but not a l w a y s , e f f e c t i v e . B e f o r e i t can be f u l l y u t i l i z e d , we w i l l n e e d t o l e a r n a g r e a t d e a l more a b o u t the p r i n c i p l e s of a d s o r p t i o n . The p r e p a r a t i o n o f P e p p a r d ' s i s o m e r ' s i s p a r t i c u l a r l y i n t e r e s t i n g b e c a u s e c o b a l t ( I I I ) comp l e x e s r e a r r a n g e e a s i l y , w h e r e a s t h e p l a t i n u m compounds do n o t . A n o t h e r c a s e i n v o l v i n g c o b a l t c o m p l e x e s was s t u d i e d by Work (19) , who p r e p a r e d , among o t h e r s , a compound c o n t a i n i n g three d i f f e r e n t bidentate amines—ethylenediamine, trimethylened i a m i n e , and n e o p e n t a n e d i a m i n e , [ C o ( e n ) ( t n ) ( d a n ) ] C I 3 , by f o l l o w i n g and e x t e n d i n g a s y n t h e s i s f i r s t d e s c r i b e d by W e r n e r (20): 3

3

[Co(NH ) (N0 )3] -^>[Co(en)(NH )(N0 ) ]-^>[Co(en)(tn) (N0 ) ]N0 3

5£U

3

2

3

t r a n s - [Co (en) ( t n ) C l ] CI 2

2

3

2

2

2

[ Co (en) ( t n ) (dan) ] CI 3

The l a s t s t e p was a c c o m p l i s h e d t h r o u g h t h e c a t a l y t i c i n f l u e n c e o f d e c o l o r i z i n g c a r b o n , a method f i r s t u s e d by B j e r r u m (21) and d e v e l o p e d by Work ( 2 2 ) . D o u b t l e s s , t h i s s y n t h e s i s now c o u l d

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

1.

BAiLAR

Stereochemistry

of Cobalt

7

Complexes

be p e r f o r m e d more e a s i l y t h r o u g h t h e e l e g a n t t r i s - c a r b o n a t o method d e v e l o p e d b y M o r i and h i s c o l l e a g u e s ( 2 3 ) . A g r e a t d e a l c o u l d be l e a r n e d by f u r t h e r s y n t h e t i c s t u d i e s o f u n u s u a l c o m p l e x e s o f p l a t i n u m and c o b a l t , a s w e l l a s o t h e r m e t a l s , but the s y n t h e s e s a r e l o n g and m u l t i s t e p and few chemists h a v e t h e p a t i e n c e t o a t t e m p t them. Knowledge g a i n e d b y w o r k o n t h e c o m p l e x e s o f one m e t a l c a n b e u s e d o n l y i n d i r e c t l y i n s t u d i e s of a n o t h e r ; the c h e m i s t r i e s o f the complexes o f c o b a l t ( I I I ) and c h r o m i u m ( I I I ) , f o r example, a r e q u i t e d i f f e r e n t , a l t h o u g h these substances c l o s e l y resemble each o t h e r i n p h y s i c a l p r o p e r t i e s . Another t o p i c o f i n t e r e s t i n the s t e r e o c h e m i s t r y o f coor­ d i n a t i o n compounds i s t h a t o f s t e r e o s e l e c t i v e r e a c t i o n s . The f i r s t c a s e o f t h i s was d i s c o v e r e d b y T s c h u g a e f f (Chugaev) a n d S o k o l o f f i n 19 07 ( 2 4 ) . They p r e p a r e d t h e t r i s - 1 , 2 - p r o p a n e d i a m i n e c o b a l t ( I I I ) complex fro the ions obtained containe the d e x t r o - base, and t h a t the presence o f t h e s e i n the complex f o r c e d a n o v e r a l l asymmetry upon i t . Thus, they o b t a i n e d [ D i l l ] and [Lddd] c o m p l e x e s , w h e r e t h e c a p i t a l l e t t e r s r e p r e s e n t t h e r o t a t i o n o f t h e complexes, and the s m a l l l e t t e r s , the r o t a t i o n of the molecules o f c o o r d i n a t e d base. Many p e o p l e h a v e w o r k e d i n t h i s f i e l d s i n c e , a n d a good d e a l has b e e n l e a r n e d . Dwyer, S a r g e s o n , and members o f t h e i r g r o u p s h a v e b e e n p a r t i c u l a r l y productive i n this f i e l d . The s u b j e c t was d i s c u s s e d i n d e t a i l b y J a e g e r ( 2 5 ) , who p r e p a r e d t r i s - ( t r a n s - 1 , 2 - c y c l o p e n t a n e d i a m i n e ) r h o d i u m ( I I I ) and c o b a l t ( I I I ) i o n s , and found t h a t , u s i n g the r a c e m i c b a s e , o n l y two o f t h e p o s s i b l e i s o m e r s , ( D i l l a n d L d d d ) , were formed i n e a c h c a s e . Even i f o n l y one o r two m o l e c u l e s o f the o p t i c a l l y a c t i v e base were used a l o n g w i t h a s y m m e t r i c a l base, as i n [ C o ( e n ) ( c p t d i n ) ] and [ C o ( e n ) ( c p t d i n ) ] , a s t e r e o ­ s e l e c t i v e e f f e c t was o b s e r v e d . L i k e w i s e , e f f o r t s t o prepare t r a n s - [ C o ( 1 - c p t d i n ) ( d - c p t d i n ) C l ] were w i t h o u t a v a i l . Jaeger was o f t h e o p i n i o n t h a t t h i s s t e r e o s p e c i f i c i t y was d e p e n d e n t upon t h e symmetry o f t h e c o m p l e x e s — t h e more s y m m e t r i c a l t h e m o l e c u l e , the g r e a t e r i t s s t a b i l i t y . I t i s now g e n e r a l l y a g r e e d , h o w e v e r , t h a t t h e f o r m a t i o n o f some i s o m e r s i n p r e f e r e n c e t o o t h e r s depends upon t h e c o n f o r m a t i o n s o f t h e c h e l a t e r i n g s . This concept i s l a r g e l y due t o P r o f e s s o r E. J . C o r e y ( 2 6 ) . 3 +

3 +

2

2

+

2

The c o m p l e x e s o f p r o p y l e n e d i a m i n e ( 1 , 2 - p r o p a n e d i a m i n e ) o f f e r a somewhat more c o m p l i c a t e d p i c t u r e t h a n t h o s e o f t r a n s - c y c l o ρentanediamine, f o r t h e p r o p y l e n e d i a m i n e m o l e c u l e n o t o n l y c o n t a i n s an asymmetric carbon atom, but i s a l s o u n s y m m e t r i c a l . Werner r e p o r t e d t h e i s o l a t i o n o f a l l e i g h t o f t h e p o s s i b l e i s o m e r s o f c i s - [ C o ( e n ) ( p n ) ( N 0 ) ] , a n d t h i s was s u p p o r t e d b y t h e w o r k o f C o o l e y a n d L i u ( 2 7 ) , who o b t a i n e d f o u r i s o m e r s o f c i s [ C o ( e n ) ( 2 , 3 - b n ) ( N 0 ) ] and f o u r c i s - [ C o ( e n ) ( i s o - b n ) ( N 0 ) ] . (2,3-bn = r a c e m i c N H C H ( C H ) C H ( C H ) N H ; i s o - b n = N H C ( C H ) C H N H ) . The i s o m e r s c o n t a i n i n g 2 , 3 - b u t y l e n e d i a m i n e p r o v e d t o b e o f u n ­ equal s t a b i l i t y — i n f a c t , i n the e a r l y experiments, only three +

2

2

+

2

+

2

2

2

3

3

2

2

3

2

2

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

2

2

STEREOCHEMISTRY OF TRANSITION METALS

8

i s o m e r s were o b t a i n e d , a n d , t o i s o l a t e t h e f o u r t h , t h e e x p e r i m e n t had t o be r e p e a t e d , u s i n g m i l d e r c o n d i t i o n s . In t h e e a r l y b e l i e f t h a t t h e s e r e a c t i o n s were c o m p l e t e l y s t e r e o s p e c i f i c , we f e l t t h a t t h e r e a c t i o n o f [ C o ( 1 - p n ) 2 C I 2 ] w i t h c a r b o n a t e c o u l d n o t g i v e an o p t i c a l i n v e r s i o n u n d e r any e x p e r i ­ mental c o n d i t i o n s . T h i s i s , o f c o u r s e , n o t t r u e , a s was s o o n shown by t h e w o r k o f M c R e y n o l d s (28) and S i s t e r Mary M a r t i n e t t e Hagan (29) . I n a c o b a l t c o m p l e x c o n t a i n i n g a s y m m e t r i c l i g a n d s , one f o r m i s p r e f e r r e d , b u t b o t h forms c a n and do e x i s t . In the same b e l i e f , we hoped t h a t s t e r e o s p e c i f i c e f f e c t s c o u l d be u s e d i n the r e s o l u t i o n of racemic p o t e n t i a l l i g a n d s . T h i s does g i v e p a r t i a l r e s o l u t i o n , as shown by t h e w o r k o f J o h n s o n ( 3 0 ) , J o n a s s e n and G o t t (31,32) and H a m i l t o n ( 3 3 ) . H a m i l t o n ' s r e s u l t s , shown i n T a b l e I I , i n d i c a t e t h i s c l e a r l y . +

[Co a ~ p n )

2

2

( 1 - t a r t ) ] + + d - t a r t " -> [Co (1-pn) 2 ( d - t a r t ) ] Ba + t a r t " -> B a ( t a r t ) 2 +

+

1-tart

2

2

Time o f S t a n d i n g with B a (hours)

[α]ρ o f R e c o v e r e d Tartaric Acid

Y i e l d of Ba t a r t (g)

2 +

0 0.2 0.5 2 4 18 72

-6° 0° +7.5° +7.5° +7.5° +12° +10°

4.7 1.5 0.6 0.9 1.6 0.5 0.2 10.0 (theoretical)

He t r e a t e d [ C o ( 1 - p n ) 2 C O 3 ] C 1 w i t h a 100% e x c e s s o f r a c e m i c t a r t a r i c a c i d t o form the complex [ C o ( 1 - p n ) 2 t a r t ] . The a d d i t i o n o f an e x c e s s o f b a r i u m h y d r o x i d e t o t h e s o l u t i o n gave an i m m e d i a t e p r e ­ c i p i t a t e o f b a r i u m t a r t r a t e w h i c h was removed f r o m t h e s o l u t i o n , and w h i c h was shown t o c o n t a i n an e x c e s s o f 1 - t a r t r a t e . The f i l t r a t e , upon s t a n d i n g , gave more p r e c i p i t a t e , w h i c h was removed a t i n t e r v a l s , and was f o u n d t o c o n t a i n t a r t r a t e o f v a r y i n g r o t a t o r y power, but always d e x t r o . Attempts t o improve the s t e r e o s e l e c t i v i t y by u s i n g v e r y b u l k y a s y m m e t r i c l i g a n d s , s u c h as p h e n y l e t h y l e n e d i a m i n e , h a v e n o t i m p r o v e d t h e s i t u a t i o n ( 3 4 ) . The f i r s t s t e r e o s e l e c t i v e s y n t h e s e s o f c o o r d i n a t i o n com­ pounds w e r e p e r f o r m e d by J o n a s s e n and Huffman (35) who t r e a t e d [Co(en)2C0 ] with d-tartaric acid. T h i s gave a m i x t u r e o f Dand L - [ C o ( e n ) 2 d - t a r t ] , w h i c h , upon t r e a t m e n t w i t h e t h y l e n e d i a m i n e a t 50°, gave a 70% y i e l d o f D - [ C o ( e n ) 3 ] . E i t h e r t h e two t a r t r a t o i s o m e r s w e r e f o r m e d i n u n e q u a l amounts, o r t h e l e s s +

3

+

3 +

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

1.

BAiLAR

Stereochemistry

of Cobalt

9

Complexes

s t a b l e o n e was c o n v e r t e d t o t h e more s t a b l e one d u r i n g t h e r e ­ a c t i o n w i t h ethylenediamine. S i m i l a r l y , treatment o f the mixed t a r t r a t o complexes w i t h h y d r o c h l o r i c a c i d o r w i t h c a l c i u m n i t r i t e gave a p r e p o n d e r a n c e o f d e x t r o - r o t a t o r y - c i s - [ C o ( e n ) 2 C I 2 ] o r dextro-rotatory-cis-[Co(en) (N0 ) 2] , respectively. One o t h e r t o p i c s h o u l d b e m e n t i o n e d — t h e s y n t h e s i s o f com­ p l e x e s i n w h i c h a c h e l a t i n g l i g a n d s p a n s t r a n s p o s i t i o n s . Werner assumed t h a t a c h e l a t i n g l i g a n d must o c c u p y c i s - p o s i t i o n s i n a complex and f o r o r d i n a r y l i g a n d s t h i s i s c e r t a i n l y c o r r e c t . However, s e v e r a l i n v e s t i g a t o r s have p r e s e n t e d e v i d e n c e t h a t u n d e r some c o n d i t i o n s a s i n g l e c h e l a t i n g g r o u p c a n r e a c h a c r o s s t h e t r a n s - p o s i t i o n s o f a p l a n a r complex. S c h l e s i n g e r ( 3 6 ) , and I s s l i e b and H o h l f e l d (37) u s e d b i d e n t a t e l i g a n d s w h i c h were l o n g enough t o r e a c h a c r o s s t h e d i a g o n a l o f a s q u a r e p l a n a r c o m p l e x of Cu(II) o r N i ( I I ) . Venanz s i z e d a r i g i d l i g a n d whic d i a g o n a l o f a p l a t i n u m ( I I ) s q u a r e , but not t o the edge +

+

2

2

M a t t e r n a n d M o c h i d a (40) u s e d q u i t e a d i f f e r e n t a p p r o a c h , a t t a c h i n g a t r i d e n t a t e amine t o p l a t i n u m ( I V ) a n d t h e n d e s t r o y i n g t h e b o n d b e t w e e n t h e m e t a l a n d t h e n i t r o g e n atom i n t h e c e n t e r o f the t r i d e n t a t e l i g a n d by r e d u c i n g the m e t a l t o p l a t i n u m ( I I ) , thus l o w e r i n g t h e c o o r d i n a t i o n number f r o m s i x t o f o u r .

A

Β

B e f o r e t h e e x p e r i m e n t was t r i e d , t h e r e was no a s s u r a n c e i n a n y ­ one's m i n d t h a t t h e d e s i r e d bonds w o u l d b r e a k when t h e m e t a l was r e d u c e d , b u t i t was h o p e d t h a t t h e t r a n s e f f e c t o f t h e c h l o r i d e a n d t h e s e c o n d a r y n a t u r e o f t h e c e n t r a l amine n i t r o g e n

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

10

STEREOCHEMISTRY OF TRANSITION METALS

w o u l d l o o s e n t h o s e atoms f r o m t h e m e t a l . H a p p i l y , t h a t i s what occurred. On r e p e a t e d c r y s t a l l i z a t i o n f r o m w a t e r , t h e r e d u c e d p r o d u c t l o s t ammonia a n d c h l o r i d e , y i e l d i n g

Compound B, i n b o i l i n g h y p o c h l o r i acid dichlorodiamine platinum(II) c h e l a t e on e a c h s i d e o platinu plan ye (41). T h e r e i s c e r t a i n l y no d e a r t h o f i n t e r e s t i n g a n d s i g n i f i c a n t s t e r e o c h e m i c a l problems a w a i t i n g s o l u t i o n . Many f u n d a m e n t a l q u e s t i o n s r e m a i n t o be a n s w e r e d . A l s o , t h e growth o f b i o i n o r g a n i c c h e m i s t r y , t h e u s e o f m e t a l c o m p l e x e s i n c a n c e r chemotherapy, and t h e w i d e s p r e a d c o n c e r n o v e r p o l l u t i o n have opened up a r e a s w h i c h w i l l k e e p a l l o f us b u s y f o r many y e a r s t o come.

Literature Cited 1.

van't Hoff, J . H. Maandblad voor Natuurwetenschappen, 1895, 6, 37. 2. Werner, A. Z. anorg. Chem., 1893, 3, 310. 3. Werner, A. Ber., 1911, 44, 1887. 4. Saito, Yoshihiko. Topics in Stereochemistry, Volume 10, pages 95-174, Ernest L. Eliel and Norman L. Allinger, Eds. John Wiley and Sons, New York (1978). 5. Igi, Kozo; Douglas, Bodie E. J. Coordination Chem., 1978, 7, 155-161 and many other earlier articles in the series by Douglas and his students. 6. Shibata, Y.; Yamasaki, Kazuo. J. Chem. Soc. Japan, 1933, 54, 1207, and several preceding articles. 7. King, V. L. J . Chem. Educ., 1942, 19, 345. 8. Werner, Alfred. Ann., 1912, 386, 1. 9. Bailar, John C., J r . ; Auten, Robert W. J. Am. Chem. Soc., 1934, 56, 774. 10. Dwyer, Francis P.; Sargeson, Alan M.; Reid, Ian K. J. Am. Chem. Soc., 1963, 85, 1215. 11. Bailar, John C., J r . ; Haslam, John H.; Jones, Eldon M. J. Am. Chem. Soc., 1936, 58, 2226. 12. Greenwood, Earl L . , B. S. Thesis, University of Illinois, 1936.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

1.

BAILAR

Stereochemistry of Cobalt Complexes

13.

J.

11

Archer, Ronald D.; Bailar, John C., Jr. J . Am. Chem. Soc., 1961, 83, 812. 14. Kyuno, Eishin; Boucher, L. J.; Bailar, John C . , Jr. J . Am. Chem. Soc., 1965, 87, 4458. 15. Archer, Ronald D.; Suydam, L. Jill; Dollberg, Donald D. Am. Chem. Soc., 1971, 93, 6837. 16. Chernyaev, I. I.; Krasovskaya, Ν. N. Russian J . Inorg. Chem., 1959, 4, 455 (in English). 17. Gel'man, A. D.; Essen, L. N. Doklady Akad. Nauk. S.S.S.R., 1950, 75, 693; C. Α., 1951, 45, 3279h. 18. Bailar, John C., J r . ; Peppard, D. F. J. Am. Chem. Soc., 1940, 62, 105. 19. Bailar, John C., J r . ; Work, J . B. J . Am. Chem. Soc., 1946, 68, 232. 20. Werner, Alfred. Helv 21. Bjerrum, J . "Meta Theory of Reversible Step Reactions"; P. Hasse and Son: Copenhagen, Denmark, 1941. 22. Bailar, John C., J r . ; Work, J . B. J . Am. Chem. Soc., 1945, 67, 176. 23. Mori, M.; Shibata, M.; Kyuno, E . ; Adachi, T. Bull. Chem. Soc. Japan, 1956, 29, 883. See also a review by Shibata, Muraji Proc. Japan Acad., 1974, 50, 779 and the references therein; Bauer, H. F . ; Drinkard, W. C. J . Am. Chem. Soc., 1960, 82, 5031. 24. Tschugaeff, L . ; Sokoloff, W. Ber., 1907, 40, 177; 1909, 42, 55. 25. Jaeger, F. M. "Optical Activity and High Temperature Measurements"; McGraw-Hill: New York, 1930. This contains references to similar work by Lifschitz and by Smirnoff. 26. Corey, E. J.; Bailar, John C., Jr. J . Am. Chem. Soc., 1959, 81, 2620. 27. Cooley, William E . ; Liu, Chui-Fan; Bailar, John C., Jr. J. Am. Chem. Soc., 1959, 81, 4189. 28. Bailar, John C., J r . ; McReynolds, J. P. J. Am. Chem. Soc., 1939, 61, 3199. 29. Sister Mary Martinette, B.V.M.; Bailar, John C., Jr. J . Am. Chem, Soc., 1952, 74, 1054. 30. Johnson, Roy D., Thesis, University of Illinois, 1948. 31. Jonassen, Η. B.; Bailar, John C., J r . ; Gott, A. D. J . Am. Chem. Soc., 1952, 74, 3131. 32. Gott, A. D.; Bailar, John C., Jr. Ibid., 1952, 74, 4820. 33. Hamilton, Nathan, Thesis, University of Illinois, 1947. 34. Hryhorczuk, Lew, Thesis, University of Illinois, 1972. 35. Jonassen, Hans B.; Bailar, John C . , J r . ; Huffman, Ε. H. J. Am. Chem. Soc., 1948, 70, 756. 36. Schlessinger, N. Ber., 1925, 58, 1877. 37. Isslieb, K.; Hohlfeld, G. Z. anorg. allgem. Chem., 1961, 312, 169.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

12

38. 39. 40. 41.

Stefano, N. J.; Johnson, D. Κ.; Lane, R. M.; Venanzi, L. M. Abst. American Chem. Soc. Meeting (Boston, Mass., April 9-14, 1972) INOR 150. Stefano, N. J.; Johnson, D. K.; Venanzi, L. M. Angew. Chem., Int. Ed. Eng., 1974, 13, 133. Mochida, Isao; Mattern, J . Arthur; Bailar, John C., Jr. J. Am. Chem. Soc., 1975, 97, 3021. Fry, Fred; Bailar, John C., J r . ; unpublished work.

RECEIVED September 11, 1979.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

2

Absolute Configuration of Transition M e t a l Complexes

YOSHIHIKO SAITO The Institute for Solid State Physics, The University of Tokyo, Tokyo, Japan

In this paper an overvie studies of optically activ interaction with other fields in coordination chemistry. A quarter of a century has passed away since the first determina­ tion of the absolute configuration of a transition metal complex, [Co(en) ] , was carried out by anomalous scattering of X-rays (1). Since that time the number of transition metal complexes whose absolute configuration have been determined by X-ray method has been growing at an increasing rate and at this time i t has ex­ ceeded 130. In addition to this, numerous optically active organometallic compounds have been studied and their absolute con­ figurations established. At an early stage of the development, the structural informa­ tion was rather fragmentary. Nowadays, however, the accumulation of structural data for isomers has enabled us to understand structural principles and the optical properties of chelate com­ plexes in considerable detail. In this connection, column chroma­ tography on SP Sephadex has played an important role in the sepa­ ration of isomers of coordination compounds (2). In view of the large number of structures, a few basic series of structures will be taken up and discussed. Among the transition metal complexes, the tris(diamine)metal system, particularly tris(ethylenediamine)cobalt(III) and its analogues, has been studied most extensively from both experi­ mental and theoretical sides. 3+

3

Five-Membered Chelate Rings The cause of the isomerism exhibited by this system may be characterized briefly as a combination of configurational and conformational isomerism, the latter arising from non-planarity of metal-ethylenediamine chelate ring. According to IUPAC nomen­ clature (3), the designation of the configurational chirality is based upon the edges of the octahedron spanned by the chelate rings. Any two such edges form a pair of skew lines describing a 0-8412-0538-8/80/47-119-013$07.50/0 © 1980 American Chemical Society In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

14

screw o f the same handedness because o f the presence o f a t h r e e ­ fold axis. The c o n f i g u r a t i o n a l c h i r a l i t y i s designated by Λ ( l e f t - h a n d e d screw) or Δ (right-handed screw). The d e s i g n a t i o n o f conformation i s a l s o based upon the p r i n c i p l e o f a p a i r o f skew lines. For ethylenediamine one o f these l i n e s i s d e f i n e d by the two c h e l a t i n g n i t r o g e n atoms and the other by the two carbon atoms i n the c h e l a t e r i n g . T h i s i s used to c h a r a c t e r i z e the conforma­ t i o n a l c h i r a l i t y S (right-handed) or λ ( l e f t - h a n d e d ) . The con­ cepts symbolized by £ , Λ and S , λ are i n v a r i a n t under proper r o t a t i o n s but are converted i n t o the other under improper r o t a ­ tions. The combination o f these symbols, however, g i v e s r i s e to a c h a r a c t e r i z a t i o n o f the c h e l a t e r i n g which i s c h i r a l i t y i n v a r i ­ ant. T h i s c h a r a c t e r i z a t i o n which i s r e l e v a n t when d i s c u s s i n g the complex o f unknown absolute c o n f i g u r a t i o n , o r the conformational energy, i s designated b B a i l a r (k) · Here, the r e f e r to the d i r e c t i o n o f the bond between the carbon atoms o f the c h e l a t e r i n g r e l a t i v e to the t h r e e f o l d a x i s d e f i n e d by the three edges o f the octahedron spanned by the l i g a n d s . For example,Δ(λ) means a λ conformation a s s o c i a t e d with a c o n f i g u r a t i o n Δ and t h i s is l e i . A l l the f o u r combinations a r e :

Δ ( λ ) o r Λ(ί ) =

Δ(ί)

lei

orA(X) = ob

There are e i g h t p o s s i b l e isomers i n the [M(en)-] system. They comprise two c a t o p t r i c s e r i e s : l e l ^ , l e l p O b , l e l o b and ob_ with Δ and Λ absolute c o n f i g u r a t i o n s , r e s p e c t i v e l y . C r y s t a l s t r u c ­ t u r e s have been determined f o r a number o f [Co(en),] and [Cr(en)_] salts. The l e i - isomers are most f r e q u e n t l y recog­ n i z e d i n these s t r u c t u r e s . Table I l i s t s examples o f conformers of [Coien)-]* " and iCr(en)^} other than l e l . I t seems that 2

1

T

?

5

5

Table I Compounds c o n t a i n i n g complex c a t i o n s with conformations other than l e i , Conformation lel ob

Refs.

[Co ( en) ] [ P b C l ] C l - 3 ^ 0

lel ob

(6)

[Cr(en) ][Ni(CN) ]·1.5H 0

lel ob,

(-)

lelob

Compound [Co(en) ]CSnCl ]Cl 5

3

3

2

9

?

5 8 9

2

2

2

?

[Cr(en) ](SCN) 3

3

2

2

[Cr(en) ][Co(CN) ]·6H 0 ?

g

2

0

( +) g [ C r (en) ^Cl^ 5

9

[Cr (en) ](SCN) ?

2^0

^0.75^0

b

lelob

2

(7) (8)

2

(2)

3

lel , 3

le^ioO^lel+^+OJéob)

lel (70#lel+30&>b) 2

(10) (11)

the conformers other than l e i - , appear more f r e q u e n t l y i n C r ( I I I ) complexes than i n Co(III) complexes. X-ray evidence always i n d i c a t e s that such isomers are favored because they a l l o w more hydrogen bonding i n the c r y s t a l . In ( + ) c o - A - [ C r ( e n ) - ] C l _ » 2 H 0 , Q

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

p

2.

SAITO

Absolute

15

Configuration

one of the three chelate r i n g s e x h i b i t s conformational d i s o r d e r and i t s conformation can be represented as 6($>$ + 4θ# λ. In the case o f [0Γ(βη)-](30Ν)_·0·75Η 0 the d i s o r d e r vanishes and the con­ formation changes to lél,on lowering the temperature to 133K, whereby the u n i t c e l l volume c o n t r a c t s by 2.7#* r e f l e c t i n g the more compact l e l _ conformers, but no change i s observed i n the packing mode ( 1 2 ^ Though to l e s s extent, s i m i l a r conformational d i s o r d e r was r e c e n t l y detected i n c r y s t a l s of (+) o - ( 3 , 3 ' - d i methyl-2,2 -bipyridine)bis(ethylenediamine)cobalt\±II) c h l o r i d e d i p e r c h l o r a t e monohydrate. The absolute c o n f i g u r a t i o n can be desiganted as /\(6»-dmbpy, 90% λ + 10#£") (13). In c r y s t a l s of (+) -gQ-[Co(enV]Cl ·Η2HpO the complex c a t i o n takes the s y n - c h a i ^ - l e l conformation [/\(apê ) and i t s enantiomer] (35)· Recent c a l c u l a t i o n s by c o n s i s t e n t f o r c e f i e l d technique i n d i c a t e d that the C - c h a i r , conformer represents the g l o b a l minimum, supporting the f l e x i b i l i t y o f the c h e l a t e r i n g . ^ The s y n - c h a i r - l e l conformer i s higher i n energy by 1 0 . 9fcJmol" · The observed shape and s i z e o f the complex i o n , [Co(tn),] can be w e l l reproduced by the s t r a i n energy minimization (36)· There a r e three isomers o f 2,4-diaminopentane: R,R, S,S, and R,S. When t h i s molecule forms a six-membered chelate r i n g , the e q u a t o r i a l preference o f the s u b s t i t u t e d methyl groups f i x e s the conformation o f the chelate r i n g as f o l l o w s : R,S-ptn : c h a i R,R-ptn : λ-twist-boa S,S-ptn : S - t w i s t - b o a t . The absolute c o n f i g u r a t i o n o f the l e i , - and ob,-isomers o f [Co(R,R-ptn),Y* are already known [ ( - ) , ^ i l - [ C o f R , R - p t n ) , ] ^ , ( 3 7 ) ; ( + ) ^ o b -[Co(R,R-ptn) ] ^ , ( 3 8 ) 3 · There a r e two p o s s i b l e geometric isomers f o r [ C o ( R , S - p t n ) · fac-(C - c h a i r , ) and mer(C - c h a i r , ) forms. The two isomers were separated ana r e s o l v e d i n t o o p t i c a l isomers ( 3 9 ) · The c r y s t a l s t r u c t u r e o f ( + ) c Q q ~ [Co(R,S-ptn),][Co(CN)g]*5H 0, the isomer which gave c r y s t a l s s u i t a b l e f o r X-ray work, was determined (40)· Figure 3 shows the absolute c o n f i g u r a t i o n o f the complex i o n , (+)egg-CCo(R,S-ptn),] · This i s the f a c i a l isomer and the three chelate r i n g s take the c h a i r conformation with the s u b s t i t u t e d methyl groups i n e q u a t o r i ­ a l p o s i t i o n s . No unusually l a r g e thermal motion o f the r i n g carbon atoms was observed. The c i r c u l a r dichroism s p e c t r a i n the r e g i o n o f the f i r s t a b s o r p t i o n band o f the t r i s - b i d e n t a t e complex i o n s having s i x membered chelate r i n g s are known t o be p a r t i c u l a r l y s e n s i t i v e t o experimental c o n d i t i o n s . F o r example, the CD spectrum o f Δ - l e l , [Co(R,R-ptn) ]C1, i n an aqueous s o l u t i o n shows two peaks: Δ£ = - 0 . 5 8 9 , 5 2 2 nm;A8 = +0.104, 462.5 nm, whereas that o f Δ - l e l , [Co(R,R-ptn),](CIO. ) , i n an aqueous s o l u t i o n g i v e s a negative peak ( Δ ε = - 0 . 5 8 7 ; a t 5 1 8 nm (4l_). The s o l i d s t a t e CD d i f f e r from the s o l u t i o n CD and the s o l u t i o n CD are s e n s i t i v e t o the tem­ perature o f measurement and are a f f e c t e d by the presence o f oxo anions (42, 4 3 , 44). Table V l i s t s the lowest frequency CD s p e c t r a o f t r i s - d i a m i n e c o b a l t ( I I I ) complexes i n the CT region* A l l the absolute c o n f i g u r a t i o n s have been e s t a b l i s h e d by the X-ray method. U n l i k e the CD s p e c t r a i n the f i r s t a b s o r p t i o n r e g i o n , those i n the CT r e g i o n are i n s e n s i t i v e t o the c o n d i t i o n s o f the measurement described above. Using a l l o f the recorded CD data f o r t r i s - d i a m i n e c o b a l t (III) complexes o f known absolute c o n f i g u r a t i o n , an e m p i r i c a l r u l e r e l a t i n g the absolute c o n f i g u r a t i o n t o CD s p e c t r a o f t r i s - d i a m i n e c o b a l t ( I I I ) complexes i n the c h a r g e - t r a n s f e r r e g i o n was estab2

+

e

+

5

2

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

20 Table V

The lowest frequency CD band o f t r i s - d i a m i n e c o b a l t ( I I I ) complexes i n the CT r e g i o n Absolute •ζ -1 configuration χΛ(Τ cm -CCo(en) ]^ 47.4 -31 (45.) Λ +

5 8 9

-[Co(R,R-chxn) ]

3 +

44.1

+48

Δ

g -[Co(S,S-chxn) ]

5 +

43.9

+17

(22)

40.0

-13

40.2

-12

(49)

5 8 9

5

3

3

5

39.4

-8.7

-CCo(R,R-ptn) ]

3+

42.0

+6.5

-CCo(R,R-ptn) ]

3

43.5

-26.2

Α

3 +

3

g -CCo(S-bn) ] 9

3 +

3

g -[Co(R,S-ptn) ] 9

3

5if6

3

5if6

5

3 +

3

-CCo(tn) ]

5 8 9

5

Δ Λ Λ Λ Λ

9

(22)

(46) (42) (48) (46)

3

g -CCo(tmd) ] 9

3 +

3

l i s h e d : a t r i s ( d i a m i n e ) c o b a l t ( I I I ) complex whose s i g n o f the lowest-frequency CD band i n the charge-transfer r e g i o n i s negative has absolute c o n f i g u r a t i o n Λ ; i f i t i s p o s i t i v e , the absolute c o n f i g u r a t i o n i s Δ · In the case o f (+)-^-[0ο(Κ, R-ptn) » the e m p i r i c a l r u l e i s v i o l a t e d , because the p o s i t i v e CD c o n t r i b u t i o n o f the o p t i c a l l y a c t i v e l i g a n d i t s e l f i s superposed i n "this region. Seven-Membered Chelate Rings Only a small number o f s t r u c t u r e s containing seven-membered chelate r i n g s are known ( 5 0 , 5 1 ) · Tris(l,4-diaminobutane)cobalt( I I I ) i o n i s a t h i r d member o f a b a s i c s e r i e s o f s t r u c t u r e s : [Co { H N - ( C H ) - N H j ] (n=1, 2 , 3 · · · ) · This complex i o n has seven-membered chelate r i n g s . Figure 4 shows the absolute con­ f i g u r a t i o n o f (+)^gg-[Co(tmd).j] · I t has D^ symmetry. Table VI 5

2

2

n

Table VI

+

f

Geometry o f the chelate r i n g i n [Co(tmd),]' obs.

Co-N N-C C-C N-Co-N Co-N-C N-C-C C-C-C Co-N-C-C N-C-C-C C-C-C-C

1.991(5) 1.506(11) 1.512(13) 89.2(2)

122.9(3) 113.6(5) 111.6(6) 96.2

75.9 56.3

+

calc. 2.004 A Q

1-515 1.527

88.4° 120.4 113.3 113.7 101.1 79.0 56.1

compares the observed and c a l c u l a t e d geometries o f the complex i o n

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

2. SAITO

Absolute

Configuration

21

Acta Crystallographica Figure 4. A perspective drawing of the complex ion (+) 89-[Co(tmd) ] (51) 5

s

3+

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

22

(29)· The chelate r i n g takes on a skew conformation and i t i s s t r a i n e d : a l l the bond angles i n the chelate r i n g are g r e a t e r than the normal t e t r a h e d r a l angle. The minimized s t r u c t u r e agrees reasonably w e l l with the observation. The chelate r i n g i s c h i r a l and the conformation can be designated as λ provided that the h e l i c i t y i s defined by the l i n e j o i n i n g nitrogen atoms and the l i n e j o i n i n g the two carbon atoms next to the n i t r o g e n atoms. The c e n t r a l C-C bond i n the chelate r i n g i s i n c l i n e d by about 0.6 with respect to the t h r e e f o l d a x i s of the complex i o n . Hence t h i s i s the l e i - isomer and the absolute c o n f i g u r a t i o n i s Δ(λλλ). A marked d i f f e r e n c e i n the geometry of t h i s complex i o n from that of [Co(en)_r i s that three of the s i x methylene groups bonded to the n i t r o g e n atoms are above the upper t r i g o n a l plane of the three n i t r o g e n atoms and the remaining three are below the lower t r i g o ­ n a l plane of the nitroge On the other hand, the ethylen r i n g i s 3·83 A d i s t a n t from the t h r e e f o l d a x i s , compared to 2.81 A i n the case of [Co(en),] . These c h a r a c t e r i s t i c features i n the arrangement of the n o n - l i g a t i n g atoms a f f e c t the magnitudes of r o t a t o r y strengths R(E) and R(A ) . This point w i l l be discussed i n the next s e c t i o n . o

C i r c u l a r Dichroism Spectra of T r i s ( d i a m i n e ) c o b a l t ( I I I ) Complexes T r i s ( d i a m i n e ) c o b a l t ( I I I ) complexes u s u a l l y give two c i r c u l a r dichroism bands with opposite s i g n and d i f f e r e n t magnitudes i n the absorption r e g i o n around 20x10 cm" i n aqueous s o l u t i o n (the f i r s t absorption r e g i o n ) . Th^se bands are a s c r i b e d tçj the d. ^ e l e c t r o n t r a n s i t i o n from the A^ ground s t a t e to the E and e x c i t e d l e v e l s o f octahedral parentage i n a D, environment. McCaffery and Mason measured the s i n g l e c r y s t a l c i r c u l a r dichroism spectrum of (+) gg-CCoCen^J^Clg^NaCl^oH^O with l i g h t propagated p a r a l l e l to the o p t i c a x i s f i n which a l l the complex ions are a r ranged with t h e i r t h r e e f o l d axes p a r a l l e l to the o p t i c a x i s (45). Under t h i s c o n d i t i o n only the Ε component i s e x c i t e d . This c r y s ­ t a l measurement showed that the i n t r i n s i c r o t a t o r y strength o f the A j p * E t r a n s i t i o n i s p o s i t i v e and s u b s t a n t i a l l y l a r g e r than that of s o l u t i o n c i r c u l a r ^ichrogysm. T h i s means that the i n t r i n s i c r o t a t o r y strength of A^—> A^ must be negative and almost as l a r g e as that of the Ε component, s i n c e the t r i g o n a l s p l i t t i n g i s s m a l l . Kuroda and S a i t o showed that the r o t a t o r y strengths o f the Ε and A^ components can be separated by combining the c i r c u l a r dichroism s p e c t r a o f a s i n g l e u n i a x i a l c r y s t a l and i n i t s microc r y s t a l l i n e s t a t e (52). Even i f the t h r e e f o l d a x i s of the complex i o n i s not o r i e n t e d p a r a l l e l to the o p t i c a x i s , i t i s p o s s i b l e to r e s o l v e the observed s o l i d s t a t e CD spectra i n t o the Ε and A^ com­ ponents by making use of the known c r y s t a l s t r u c t u r e . Jensen and Galsbjil measured the c r y s t a l CD s p e c t r a of ( + ) g Q - [ C o ( e n ) , ] ^ i o n doped i n a host c r y s t a l of racemic [ i H e n ^ ^ C I g i N a C l - ô H ^ O with l i g h t propagated both p a r a l l e l and perpendicular to the three&

a

+

5

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

2.

SAiTo

Absolute

Configuration

23

f o l d a x i s of [Co(en),]3+ (53)· This was f i r s t achieved by the phase modulation technique i n p o l a r i z e d spectroscopy (5^)· Before then, measurements o f CD were r e s t r i c t e d to s o l u t i o n , g l a s s e s , f i n e powder and u n i a x i a l c r y s t a l s with l i g h t propagated along the o p t i c a x i s , since the s i g n a l was otherwise d i s t o r t e d by i n t e r f e r e n c e from l i n e a r b i r e f r i n g e n c e and l i n e a r dichroism. Table VII summarizes the observed r o t a t o r y strengths o f some t r i s diamine c o b a l t ( I I I ) complexes. As seen from the t a b l e , R(E) i s p o s i t i v e and R(A ) i s negative f o r the absolute c o n f i g u r a t i o n Λ t while R(E) i s negative and R(A ) i s p o s i t i v e f o r Δ c o n f i g u r a t i o n , i n agreement with the well-known e m p i r i c a l r u l e f o r the s o l u t i o n CD spectrum. The values l i s t e d i n Table VII of R(E) and R(A ) are c o r r e c t e d f o r random o r i e n t a t i o n f a c t o r s of 2/3 and 1/3, r e ­ s p e c t i v e l y . Thus the net r o t a t o r y strength K(T^) = R(E) + R(A ) may be compared d i r e c t l which are l i s t e d i n the ry s t r e n g t h RiT..) changes s i g n on going from s o l i d to s o l u t i o n i n the case o f CCo?S,S-cptn),]^ and [ C o ( S , S - p t n ) J · This observa­ t i o n may be a s c r i b e d to tne i o n a s s o c i a t i o n or conformational change i n s o l u t i o n (41). The absolute values of the observed R(E) and R(A ) possess nearly the same magnitudes with opposite signs. |R(E7| has the major r o t a t o r y strength f o r the complexes with five-membered chelate r i n g s l i k e [Co(en),] , [Co(pn),] , [Co(chxn),;r and [ C o ( c p t n ) _ r t whereas R(A 7 possesses tne major r o t a t o r y strength f o r ^ t h e complexes CCo(ptn),] and [Co(tmd),] · This trend appears to be r e l a t e d to the s p a c i a l arrangement of the n o n - l i g a t i n g atoms around the cobalt atom. Figure 5 shows p r o j e c t i o n s of the chelate r i n g s o f these complex­ es upon a plane through the t h r e e f o l d a x i s and the twofold a x i s . As seen from the f i g u r e , a l l the n o n - l i g a t i n g atoms are between the t r i g o n a l planes formed by the three nitrogen atoms f o r those complexes whose |R(E)| i s g r e a t e r than |R(A )|. On the contrary, i n the case of the complexes with |R(E)| < |R(A >|, some o f the n o n - l i g a t i n g atoms are above and below the t r i g o n a l planes and those atoms l y i n g between the t r i g o n a l planes are smaller i n number and l o c a t e d more d i s t a n t than those i n the f i r s t group. The d-d t r a n s i t i o n s are magnetic dipole-allowed but e l e c t r i c d i p o l e forbidden. I f a coulombic c o r r e l a t i o n between t^e compo­ nents o f the e l e c t r i c hexadecapole moment o f the A ^ — » Τ d e l e c t r o n t r a n s i t i o n and a t r a n s i t i o n d i p o l e moment induced i n each l i g a n d group i s considered, the c o r r e l a t e d d i p o l e moment o f the l i g a n d group g i v e s r i s e to a non-zero s c a l a r product with a component of the magnetic t r a n s i t i o n moment (55)* Thus such a d i s p o s i t i o n as w e l l as the geometry o f the chelate r i n g system might give a greater |R(A^)I than |R(E)| f o r the complex ions of the second group. 2

2

2

2

p

2

2

These D, complexes have played a prominent r o l e as model systems i n the t h e o r e t i c a l s t u d i e s of n a t u r a l o p t i c a l a c t i v i t y , since the high symmetry of the complexes makes tedious c a l c u l a ­ t i o n s more or l e s s f e a s i b l e and a l o t of experimental data are

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

3

3

+31.1

A-CCo(tmd) ]Br

2

+12.5

3

3

Λ- [Co(S,S-ptn) 3C1 '2H 0

T

+57.3

+41.5

/V-[Co(S,S-cptn)_]Cl ·4Η_0 3 3 ^

3

2

+56.5

3

3

Λ-CCo(S,S-chxn) ]CI,·5H-0

3

+38.1

+43

+2

-38.7

-2.0

-14.5

-7.6

+2.8

+5.4

+1.5

+4.2

-54.5

-51.1

-36.6

-55.7

-41

(52)

(£2)

(52)

(52)

(102)

(52)

(52)

(53)

(102)

(52)

Ref.

+50.9

+4.3

R(T.,)

(45.)

-58.6

2

B(A )

+52.6

+62.9

A-CCo(S-pn) ]Br

2

+59-9

3

Λ-[Οο(βη) ]ΒΓ ·Η 0

3

A-2CCo(en) ]Cl .NaCl»6H 0

R(E)

f a c t o r s o f 2/3and 1/3 r e s p e c t i v e l y . n

cgs)

-4.9

+1.9

-4.3

+3.9

+4.2

+4.4

R(T„) 1 soin +4.4

Values o f R(E) and R(A^) are c o r r e c t e d by the f i x e d o r i e n t a t i o n

Table V I I . Rotatory s t r e n g t h s o f t r i s ( d i a m i n e ) c o b a l t ( I I I ) complexes (10~

2.

sAiTO

Absolute

Configuration

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

25

STEREOCHEMISTRY OF TRANSITION METALS

26

a v a i l a b l e f o r comparison. The t h e o r e t i c a l study was c a r r i e d out by a number of workers, notably by Richardson (56-61), Evans, Schreiner and Hauser (62) and Mason and Seal (557· Two models are now employed to c a l c u l a t e the o p t i c a l r o t a t o r y s t r e n g t h : one i s the c r y s t a l f i e l d model admitting only s t a t i c coupling between a metal d - e l e c t r o n and the charge d i s t r i b u t i o n i n the p e r t u r b i n g l i g a n d i n the ground s t a t e and the other i s the dynamic coupling model t a k i n g i n t o account o f the coupling of the t r a n s i t i o n i n the chromophore with e l e c t r i c d i p o l e t r a n s i t i o n induced i n the l i g a n d by the t r a n s i t i o n charge d i s t r i b u t i o n . Both models can account f o r the observed f e a t u r e s o f the CD s p e c t r a with considerable success. For [Co(en)_] , the values f o r observed R(E)'s i n Table VII should be compared with t h e o r e t i c a l values o f 35·7 and 63.8, the former being based on the c r y s t a l f i e l d model (62) and the l a t t e r on the dynami good. Diethylenetriamine

Complexes

There are three d i f f e r e n t ways o f c o o r d i n a t i n g two d i e t h y l ­ enetriamine molecules to a c o b a l t ( I I I ) i o n . Among three geometric isomers, the u - f a c i a l - and mer-isomers are o p t i c a l l y a c t i v e and have p a i r s o f enantiomers r e s p e c t i v e l y , whereas the s _ - f a c i a l isomer i s o p t i c a l l y i n a c t i v e . A l l geometric and o p t i c a l isomers i n t h i s system were i s o l a t e d , and the geometric c o n f i g u r a t i o n s were assigned f o r the o p t i c a l l y a c t i v e isomers from the d i f ­ ference i n racemization behavior o f the o p t i c a l l y a c t i v e uf a c i a l - and mer-isomers (63)· A l l the c r y s t a l s t r u c t u r e s o f these isomers were determined. The s - f a c i a l isomer has approxi­ mately C symmetry. The conformations o f the two fused c h e l a t e r i n g s are enantiomeric (64). In c r y s t a l s of (-) ,-g^-u-fac[ C o ( d i e n ) ] [ C o ( C N ) g > 2 H 0 , there e x i s t two d i f f e r e n t conformers i n an asymmetric u n i t . They both have a twofold a x i s and the absolute c o n f i g u r a t i o n can be designated as skew chelate p a i r s ΔΛΔ. However, the conformations o f the two chelate r i n g s formed by a d i e n molecule i n one complex i o n are SX while those i n the other are λλ (65). 2 h

2

2

f

B r

1

The absolute c o n f i g u r a t i o n o f (+) .Q >-mer-[Co(dien) 2 ^ 7 * E^O has r e c e n t l y been determined (66). Figure 6 shows a per­ s p e c t i v e drawing o f the complex i o n . The complex i o n can be des­ ignated as trans-λ-ΝΗ, p r o v i d i n g that the c h i r a l i t y i s d e f i n e d by the l i n e j o i n i n g the two Η atoms and the l i n e j o i n i n g the two secondary n i t r o g e n atoms i n t r a n s - p o s i t i o n s (3, 60). The two terdentate molecules coordinate to the c e n t r a l c o b a l t atom i n mer p o s i t i o n s with three n i t r o g e n atoms forming a d i s t o r t e d o c t a ­ h e d r a l complex. The complex i o n has an approximate twofold sym­ metry. The Co-secondary Ν bond o f 1.940 A i s s i g n i f i c a n t l y s h o r t e r than the Co-terminal Ν bond o f 1.981 A. The angle sub­ tended a t the c e n t r a l cobalt atom i s 187 · The three l i g a t i n g n i t r o g e n atoms of the l i g a n d and the c o b a l t atom are n e a r l y c o p i a r

r

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

2.

Absolute

sAiTo

Configuration

27

nar and the two planes formed by the cobalt and the three n i t r o g e n atoms make an angle o f 89·6 · The five-membered chelate r i n g takes an envelope form, one o f the two methylene carbon atoms that i s bonded to the secondary n i t r o g e n atom i s s h i f t e d by 0.64 A from the plane formed by the remaining four atoms. The conformations of the two fused chelate r i n g s are S and λ, r e s p e c t i v e l y . The geometry o f the complex c a t i o n agrees w e l l with the r e s u l t o f conformational a n a l y s i s (67)t as shown i n Table V I I I . The net c h i r a l i t y o f t h i s complex i o n i s zero. The o p t i c a l a c t i v i t y o f the complex i o n a r i s e s from the dissymmetric d i s p o s i ­ t i o n o f the methylene groups with respect to the c o o r d i n a t i o n plane and the c h i r a l arrangement o f the two trans N-H bonds. R i c h a r d s o n s s e c t o r r u l e (60) was t e s t e d on the b a s i s o f the f i n a l atomic parameters. In h i s d e r i v a t i o n the p e r t u r b a t i o n treatment 1

Table VIII

Observe mer-[Co(dien) y* o

+

obs.

calc.

1.981

1.976

Co-N(H)

1.940

1.942

C-N(H )

1.493

1.495

C-N

1.482

1.486

N-Co-N

85.1

Co-N(H ) 2

2

86.0°

116.2

114.5

Co-N(H )-C

109.3

109.3

Co-N(H)-C

109.5

107.8

N(H )-C-C

108.7

109.1

N(H)-C-C

104.7

105.9

C-N-C 2

2

I

was c a r r i e d out to the second order i n both the wave f u n c t i o n and r o t a t o r y s t r e n g t h . A negative net r o t a t o r y strength i n the r e g i o n o f the f i r s t absorption, A —> T^ was p r e d i c t e d . The CD s p e c t r a of the complex i o n i n aqueous s o l u t i o n agreed with t h i s . e x p e c t a t i o n ^ = +0.096 a t 19·5χ10^ cm , d£ = -Ο.181 a t 21.9x10^ 1

cm" ,

(68)).

Complexes with a C y c l i c Terdentate,

R-MeTACN

Mason and Peacock synthesized the c y c l i c terdentate, R - ( - ) 2-methyl-l,4,7-triazacyclononane and i t s Co(III) complex, [Co(RMeTACN) ]^ (69). Figure 7 shows a perspective drawing o f the complex i o n , T^j-gQ-CCoiR-MeTACN)^^ i n i t s i o d i d e pentahydrate c r y s t a l s . The complex i o n has D symmetry by the requirement o f 2

T

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

28

Figure 7.

A perspective drawing of the complex ion ( -) -[Co(R-MeTACN) ] + 589

2

3

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

2.

Absolute

sAiTo

Configuration

29

the space group. The methyl group i s attached to one of the three c h e l a t e r i n g s o f the c y c l i c terdentate l i g a n d , so that the complex i o n e x h i b i t s o r i e n t a t i o n a l d i s o r d e r . The observed e l e c t r o n - d e n s i t y d i s t r i b u t i o n i n d i c a t e s that the methyl group i s attached i n e q u a t o r i a l p o s i t i o n s with respect to the c h e l a t e r i n g . There are three p o s s i b l e geometrical isomers f o r the complex i o n i n respect of the p o s i t i o n s of the two methyl groups. No con­ c l u s i o n can be drawn concerning the isomerism, due to the o r i e n ­ t a t i o n a l d i s o r d e r , s i n c e the X-ray a n a l y s i s only i n d i c a t e s the average s t r u c t u r e . Two molecules o f the c y c l i c terdentate co­ o r d i n a t e to the cobalt atom with s i x secondary n i t r o g e n atoms from above and below the metal atom to form an octahedral complex. A MeTACN molecule spans a face o f the octahedron. The s i x f i v e membered chelate r i n g s take λ conformation (70). Nonomiya sepa­ r a t e d the c o b a l t ( I I I ) comple Sephadex column chromatography isomers (71)· Figure 8 shows two modes of c o o r d i n a t i o n o f R MeTACN. In the mode " b , the l i g a n d i s coordinated to the metal atom with the nine-membered r i n g upside down compared to the mode a . The absolute c o n f i g u r a t i o n o f N(1) i s S i n the mode a while i t i s R i n the b mode. There are three ways o f combining the modes a and b : aa, ab and bb. In a d d i t i o n to t h i s there are three geometrical isomers i n respect o f the p o s i t i o n s of the methyl groups. Accordingly, nine isomers are p o s s i b l e as a whole. The c r y s t a l subjected to X-ray s t r u c t u r e a n a l y s i s i n c o r p o r a t e s three aa type isomers. Nonomiya found that aa and ab type isomers can be separated, r e s p e c t i v e l y , i n t o two components: one isomer and a mixture of the remaining two. On the other hand the bb type isomers were separated as one component by h i s method. A l l these complexes show very strong p o s i t i v e CD extremes i n the r e g i o n o f the f i r s t s p i n allowed d-d transition of o c t a h e d r a l parentage. The geometric a r r a y o f c h e l a t e groups i n t h i s complex i o n d i f f e r from that o f [Co(en) "] i n the same manner as d e s c r i b e d before. I t has the n o n - l i g a t i n g atoms above and below the t r i g o ­ n a l planes o f the l i g a t i n g n i t r o g e n atoms and none o f the nonl i g a t i n g atoms e x i s t s i n the e q u a t o r i a l plane. The c r y s t a l s o f [CoiR-MeTACN^Dl-z^H^O are o p t i c a l l y u n i a x i a l and each complex ion i s arranged with the t h r e e f o l d a x i s p a r a l l e l to the o p t i c a x i s . The l i g h t propagated along the o p t i c a x i s can only e x c i t e the Ε component o f the t r a n s i t i o n . The s i n g l e c r y s t a l c i r c u l a r dichroism spectrum shows a s i n g l e negative peak a t 487 nm, while that i n aqueous s o l u t i o n has a p o s i t i v e peak a t about 487 nm. The s o l u t i o n c i r c u l a r dichroism s p e c t r a a l s o r e v e a l s R(A_), g i v i n g the sum, R(T^) = R(E) + R(A^). A comparison o f the s i n g l e c r y s t a l and the s o l u t i o n CD s p e c t r a i n d i c a t e s that R(E) has minor r o t a t o r y s t r e n g t h o f -0.16 ϋ β and R(A ) +0.32 DP (72). T h i s r e s u l t sup­ p o r t s the observed r e l a t i o n between the r e l a t i v e magnitudes o f R(E) and R(A^) and the arrangement o f n o n - l i g a t i n g atoms i n t r i s b i d e n t a t e complexes (Table V I I ) . The p o l a r capping o f ^ " " ( " ^ c S ^ w

w

w

w

,f

w

w

Μ

w

M

w

2

M

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

2.

SAITO

Absolute

31

Configuration

[Co(en)-/r with the phosphate i o n by hydrogen bonding i n s o l u t i o n i s known to enhance R(A > at the expense o f R(E) ( 7 3 » 7 4 ) . The p o l a r capping of (+)cgQ-[Co(en)^] by the c o v a l e n t l y bonded t r i s (methyleneamino) g r o u p ^ r e s u l t s i n ^ - ) g g - [ C o ( l , 3 » 6 , 8 , 1 0 , 1 3 , 1 6 , 1 9 octa-aza-bicyclo[6.6.6]-eicosane] (75/· This complex i o n g i v e s R(T ) o f - 0 . 0 6 8 DP , i n c o n t r a s t to +0.047 o f A-[Co(en),] . These observations i l l u s t r a t e the general enhancement o i R(A^) a t expense o f R(E) by the a d d i t i o n o f atoms or atomic groups to the p o l a r r e g i o n o f the [CoNg] chromophore o f D^ symmetry. 2

5

>|

M

E l e c t r o n - d e n s i t y D i s t r i b u t i o n i n D^

Complexes

Recent improvements i n experimental and computational t e c h ­ niques i n X-ray c r y s t a l l o g r a p h y have made i t p o s s i b l e to estimate atomic charge-density i complex, based on accurat y example d e s c r i b e d here. The c r y s t a l s t r u c t u r e s ofA-le3.T*CCo(S,S-chxn)^](NO ) ·3ΗρΟ and Δ-ob -[Co(S,S-chxn) ](N0 ) , ^ ^ Ο have been * determines ( ^ 6 ) . The number o f e l e c t r o n s w i t h i n a sphere o f r a d i u s 1 . 2 2 A (covalent r a d i u s o f cobalt) are l i s t e d i n Table IX, together with other r e l a t e d complex i o n s . The c e n t r a l metal atom i s n e u t r a l i z e d l a r g e l y by donation o f e l e c t r o n s from the l i g a t i n g n i t r o g e n atoms, i l l u s t r a t i n g that P a u l i n g s e l e c t r o n e u t r a l i t y r u l e holds f o r these t r a n s i t i o n metal complexes. Larsson and h i s c o l Table IX. E f f e c t i v e charge on the c e n t r a l metal atom Complex C(R) E f f e c t i v e charge Ref. 1

a

(77)

(ZD (78) (76) (76)

a: Number o f e l e c t r o n s w i t h i n a sphere o f r a d i u s 1 · 2 2

X

l a b o r a t o r y estimated the e f f e c t i v e charge o f cobalt i n iCo(en)J} and iCo(W)/] by ESCA and obtained the value o f + 0 . 7 ( 3 ) and + 0 . 6 ( 4 ) , r e s p e c t i v e l y ( 7 9 ) · Non-bonding m may be w r i t t e n a s R

om

= Σ Σ R Ν (Τ) ο ν,my ν

where R v,my ^ 0

R

ον,πιμ

s

a

vibronic

rotatory

= Ιπι·

(2)

and N ( T ) i s a n o r m a l i z e d B o l t z m a n n w e i g h t i n g f a c t o r r e f l e c t i n g the p o p u l a t i o n o f t h e v - t h v i b r a t i o n a l l e v e l o f t h e ground e l e c ­ t r o n i c s t a t e ( o ) a t t e m p e r a t u r e T. I n E q . ( 2 ) , μ_ a n d m a r e t h e e l e c t r i c and m a g n e t i c d i p o l e moment o p e r a t o r s , r e s p e c t i v e l y , φ and φ ^ a r e v i b r a t i o n a l wave f u n c t i o n s , and ψ a n d ty a r e e l e c ­ t r o n i c wave f u n c t i o n s . I n w r i t i n g E q . ( 2 ) , we h a v e assumed t h e B o r n - O p p e n h e i m e r a d i a b a t i c a p p r o x i m a t i o n a n d we t a k e ψ a n d ψ V

ο ν

0

m

0

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

πι

3.

RICHARDSON

Circular

Dichroic

45

Intensities

to be e i g e n f u n c t i o n s o f t h e e l e c t r o n i c H a m i l t o n i a n o f t h e system. I n t h e Born-Oppenheimer a d i a b a t i c a p p r o x i m a t i o n , e l e c t r o n i c mo­ t i o n i s f u l l y c o r r e l a t e d w i t h the nuclear p o s i t i o n s (instantaneous s t a t i c c o n f i g u r a t i o n s ) , b u t i s n o t c o r r e l a t e d w i t h n u c l e a r motion. I n t h i s a p p r o x i m a t i o n , t h e v i b r o n i c wave f u n c t i o n s h a v e w e l l d e f i n e d e l e c t r o n i c a n d v i b r a t i o n a l quantum numbers. I n c a s e s where t h e a d i a b a t i c Born-Oppenheimer a p p r o x i m a t i o n b r e a k s down, E q s . (1) and (2) a r e n o l o n g e r a p p r o p r i a t e s i n c e t h e system cannot e x i s t i n s t a t i o n a r y s t a t e s w i t h w e l l - d e f i n e d e l e c ­ t r o n i c quantum numbers. I n t h e s e c a s e s , t h e s y s t e m c a n be v i e w e d as s a m p l i n g d i f f e r e n t p a r t s of t h e e l e c t r o n i c c o n f i g u r a t i o n a l space as t h e n u c l e i v i b r a t e . These c a s e s c a n b e e x p e c t e d t o o c c u r when electron-nuclear v i b r a t i o n a l coupling (vibronic coupling) i s large and when t h e r e a r e d e g e n e r a t e o r n e a r l y d e g e n e r a t e e l e c t r o n i c s t a t e s i n t h e s y s t e m . A b r e a k d o w i t h a d i a b a t i Born-Oppenheime approximation f o r degenerat (JT) e f f e c t , and a breakdow approximatio presenc o f n e a r l y d e g e n e r a t e s t a t e s i s commonly r e f e r r e d t o a s t h e pseudo Jahn-Teller(PJT) e f f e c t . In the nonadiabatic approximation, t h e v i b r o n i c wave f u n c t i o n s o f t h e s y s t e m must b e e x p r e s s e d a s Ψ. = Σ Σ C. ψ φ (3) j j,nvVnv η ν * where, now, t h e c o m p o s i t e v i b r o n i c quantum number j i s t h e o n l y "good" quantum number f o r d e s i g n a t i n g m o l e c u l a r s t a t e s , a n d t h e ίψηΦην^ t h e a d i a b a t i c B o r n - O p p e n h e i m e r v i b r o n i c wave f u n c ­ tions. The e x p a n s i o n c o e f f i c i e n t s , C j v , r e f l e c t e l e c t r o n nuclear v i b r a t i o n a l coupling. I n c a s e s where t h e a d i a b a t i c a p p r o x i m a t i o n b r e a k s down, t h e only well-defined t r a n s i t i o n s are vibronic t r a n s i t i o n s with r o t a ­ t o r y s t r e n g t h s g i v e n by J

J

a

r

e

> n

R.. = Ιπι·

(4)

(for the v i b r o n i c t r a n s i t i o n i j ) . I fstrong v i b r o n i c coupling e x i s t s only w i t h i n a s m a l l , w e l l - d e f i n e d subset of molecular e l e c ­ t r o n i c s t a t e s , t h e n i t may be p o s s i b l e t o c a l c u l a t e a n e t e l e c ­ t r o n i c r o t a t o r y s t r e n g t h f o r t r a n s i t i o n s t o t h e s e s t a t e s by summing over a l l the v i b r o n i c r o t a t o r y strengths a s s o c i a t e d w i t h v i b r o n i c l e v e l s f a l l i n g w i t h i n the manifold of coupled e l e c t r o n i c s t a t e s . Only i n t h i s c o n t e x t does t h e term, e l e c t r o n i c r o t a t o r y s t r e n g t h , have any m e a n i n g i n t h e p r e s e n c e o f v e r y s t r o n g v i b r o n i c c o u p l i n g (and a c o n s e q u e n t breakdown o f t h e a d i a b a t i c Born-Oppenheimer approximation). This excursion into e l e c t r o n i c versus vibronic rotatory s t r e n g t h s and t h e a d i a b a t i c v e r s u s t h e n o n a d i a b a t i c a p p r o x i m a t i o n may be h i g h l y r e l e v a n t t o t h e d e t a i l e d i n t e r p r e t a t i o n o f t r a n s i ­ t i o n m e t a l complex c h i r o p t i c a l s p e c t r a . I n most o p t i c a l l y a c t i v e t r a n s i t i o n m e t a l c o m p l e x e s , t h e symmetry o f t h e m e t a l i o n - d o n o r atom c l u s t e r r e m a i n s r a t h e r h i g h ( n e a r l y 0 o r n e a r l y Di* ) and, a s a r e s u l t , i t i s common t o f i n d many n e a r - d e g e n e r a c i e s ( o r e v e n e x a c t d e g e n e r a c i e s ) among t h e s p e c t r o s c o p i c s t a t e s o f i n t e r e s t . Even i n n

n

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

46

the p r e s e n c e of low-symmetry l i g a n d f i e l d s , t h e r e remains u n c e r ­ t a i n t y regarding the r e l a t i v e p e r t u r b a t i v e strengths of v i b r o n i c c o u p l i n g v e r s u s low-symmetry l i g a n d f i e l d p o t e n t i a l s i n i n f l u e n c i n g the d e t a i l e d n a t u r e of the s p e c t r o s c o p i c s t a t e s i n a m e t a l complex. I n f a c t , i n some c a s e s s t r o n g v i b r o n i c c o u p l i n g c a n s e r v e t o q u e n c h o r m o d e r a t e l i g a n d f i e l d e f f e c t s , and i n o t h e r c a s e s i t c a n l e a d to a m p l i f i c a t i o n of l i g a n d f i e l d e f f e c t s (13). Clearly, in d e a l i n g w i t h t h e c h i r o p t i c a l s p e c t r a o f t r a n s i t i o n m e t a l com­ p l e x e s i t i s i m p o r t a n t t o be c o g n i z a n t o f t h e p o s s i b l e i n f l u e n c e s o f v i b r o n i c c o u p l i n g upon t h e s p e c t r a l d e t a i l s . In p a r t i c u l a r , g r e a t c a r e must be e x e r c i s e d i n m a k i n g s p e c t r a - s t r u c t u r e c o r r e l a ­ t i o n s b a s e d on t h e a s s i g n m e n t o f s p e c i f i c s p e c t r a l f e a t u r e s t o well-defined (pure)electronic transitions. Concern about the p o s s i b l e i n f l u e n c e of v i b r o n i c i n t e r a c t i o n s upon t h e CD s p e c t r a o f t r a n s i t i o by R.G. D e n n i n g ( 1 4 ) . D e n n i n of C o ( e n ) undergoes a s t r o n g ( t e t r a g o n a l ) J a h n - T e l l e r d i s t o r t i o n v i a c o u p l i n g t o an eg v i b r a t i o n a l mode o f t h e C o N c l u s t e r . This s t r o n g t e t r a g o n a l JT d i s t o r t i o n was t h e n presumed t o be e f f e c t i v e i n "quenching" the c r y s t a l f i e l d induced t r i g o n a l s p l i t t i n g of t h e T x g s t a t e ( a m a n i f e s t a t i o n o f t h e s o - c a l l e d Ham e f f e c t ) i n C o ( e n ) 3 + . D e n n i n g (14) f u r t h e r s u g g e s t e d t h a t t h e two CD b a n d s observed i n the r e g i o n of the A •> T t r a n s i t i o n i n Co(en)^"** a r i s e f r o m two d i f f e r e n t JT v i b r o n i c s t a t e s d e r i v e d f r o m ^ l g - e g c o u p l i n g , r a t h e r t h a n f r o m t h e two t r i g o n a l components (*Ε and A) of the T e l e c t r o n i c s t a t e . The i n f l u e n c e o f J a h n - T e l l e r and pseudo J a h n - T e l l e r i n t e r a c t i o n s upon t h e CD s p e c t r a o f t h e d-d t r a n s i t i o n s i n t r a n s i t i o n m e t a l c o m p l e x e s has been s t u d i e d i n c o n s i d e r a b l e d e t a i l ( t h e o r e t i c a l l y ) by R i c h a r d s o n and c o w o r k e r s ( 1 5 , 16, 17, 18, 1 9 ) . These l a t t e r s t u d i e s i n c l u d e d c o n s i d e r a t i o n of m e t a l complexes belonging to t r i g o n a l l y symmetric s t r u c t u r a l c l a s s e s ( 1 6 ) , as w e l l a s m e t a l c o m p l e x e s o f pseudo-tetragonal symmetry ( 1 5 , 17, 1 8 ) . The m a i n c o n c l u s i o n o f t h e s e s t u d i e s was t h a t w h e r e a s v i b r o n i c i n t e r a c t i o n s o f t h e JT and PJT t y p e s ( w i t h i n t h e m a n i f o l d o f d-d e x c i t e d s t a t e s ) w i l l n o t , i n g e n e r a l , a l t e r t h e n e t ( o r t o t a l ) d-d_ r o t a t o r y s t r e n g t h for a given sys­ tem, t h e y c a n p l a y a d o m i n a n t r o l e i n d e t e r m i n g how CD i n t e n s i t y i s d i s t r i b u t e d t h r o u g h o u t t h e d.-d t r a n s i t i o n r e g i o n . I t was f o u n d t h a t i n t h e p r e s e n c e o f s t r o n g JT and PJT i n t e r a c t i o n s among t h e d-d_ s t a t e s , i t becomes i m p o s s i b l e ( o r m e a n i n g l e s s ) t o a s s i g n s p e c i f i c f e a t u r e s i n t h e CD s p e c t r a t o s p e c i f i c ( i - d e l e c ­ tronic transitions. The i n d i v i d u a l CD b a n d s , i n s u c h c a s e s , w i l l g e n e r a l l y r e f l e c t "mixed" e l e c t r o n i c parentage. +

3

6

1

x

x

l g

l

l g

x

2

l g

I n t h o s e c a s e s where t h e a d i a b a t i c a p p r o x i m a t i o n can be a s ­ sumed t o h o l d , t h e i n f l u e n c e o f v i b r o n i c c o u p l i n g on t h e s p e c t r o s ­ c o p i c p r o p e r t i e s o f a s y s t e m c a n be t r e a t e d w i t h i n t h e H e r z b e r g T e l l e r ( p e r t u r b a t i v e ) f o r m a l i s m (20) f o r v i b r o n i c i n t e r a c t i o n s . T h i s f o r m a l i s m i s a p p l i c a b l e when t h e v i b r o n i c i n t e r a c t i o n e n e r g i e s a r e s m a l l compared t o t h e e n e r g y s p a c i n g s between t h e

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

3.

RICHARDSON

Circular

Dichroic

Intensities

47

coupled e l e c t r o n i c s t a t e s . V i b r o n i c r o t a t o r y strengths and c i r c u l a r dichroism spectra, considered w i t h i n the a d i a b a t i c a p p r o x i mation and the Herzberg-Teller(HT) formalism, have been treated i n considerable d e t a i l by Weigang and coworkers ( 2 1 , 2 2 , 2 3 ) . Weigang concentrated p r i m a r i l y on the CD of organic chromophores i n h i s a p p l i c a t i o n s of the theory. V i b r o n i c a l l y induced coupling of the d_-d spectroscopic s t a t e s t o odd-parity (ungerade) e l e c t r o n i c states of t r a n s i t i o n metal complexes plays a s i g n i f i c a n t (and sometimes dominant) r o l e i n determining the observed d i p o l e strengths and absorption i n t e n s i t i e s of d-d t r a n s i t i o n s . The poss i b l e i n f l u e n c e of these v i b r o n i c i n t e r a c t i o n s upon d-d r o t a t o r y strengths has been considered q u a l i t a t i v e l y by M.J. Harding ( 2 4 ) using Weigang s theory. However, no d e t a i l e d or q u a n t i t a t i v e studies have been reported on t h i s problem. Bray, Ferguson, and Hawkins (25) have a p p l i e d the v i b r o n i c coupling model o f P e r r i t i o n o f the CD spectra produce (molecular exciton) t r a n s i t i o n s i n t r i s complexes of 1,10-phenant h r o l i n e and 2 , 2 - b i p y r i d i n e with Zn(II) and N i ( I I ) . This problem i n v o l v i n g a trimer comprised of three t r i g o n a l l y disposed i n t e r a c t i n g monomer u n i t s ( l i g a n d chromophores) i s formally analogous t o the problem i n v o l v i n g a t r i g o n a l l y perturbed T i g s t a t e (treated by Richardson, e t a l . , (16) f o r Co(en)3+). In the weak ( v i b r o n i c ) coupling l i m i t , e l e c t r o n i c r o t a t o r y strengths are well-defined; whereas i n the strong ( v i b r o n i c ) coupling l i m i t , one can only speak of v i b r o n i c r o t a t o r y strengths of mixed e l e c t r o n i c composition. In the present study, we s h a l l re-examine the theory of v i b r o n i c coupling i n c h i r a l t r a n s i t i o n metal complexes as i t pert a i n s t o r o t a t o r y strength c a l c u l a t i o n s and to the i n t e r p r e t a t i o n of the observed CD spectra f o r these systems. We s h a l l focus p r i m a r i l y on the d-d ( l i g a n d - f i e l d ) t r a n s i t i o n s , and s h a l l cons i d e r v i b r o n i c coupling both w i t h i n the manifold of and Q6(t2 )« a t r i g o n a l l y d i s t o r t e d (D ) MLe c l u s t e r , ten normal coordinates are required to describe the 15 v i b r a t i o n a l degrees-of-freedom. We s h a l l denote the normal coordinates o f the n

g

t

F

o

u

r

U

3

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

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; Washington, D. C. Society: 20036Washington, DC, 1980. ACS Symposium Series; American Chemical

u

STEREOCHEMISTRY

48

e

t r i g o n a l l y d i s t o r t e d MLg system by: e

a

OF TRANSITION

METALS

a

Qi(ai)-, Q2( )> Q3a^ 2)>

a

e

a

a

n

d

Q3e( >> Q ^ ( 2 ) , Qi* (> Q5 ( >> Q6 < l)> Qee(e). If the t r i g o n a l d i s t o r t i o n i s assumed small, then we can expect the t r i g o n a l modes to r e f l e c t strong octahedral parentage. Thus, f o r example, the Q 3 ( a ) and Q 3 ( e ) t r i g o n a l modes are expected to c o r r e l a t e s t r o n g l y with the Q 3 ( t ] ) octahedral mode. We s h a l l w r i t e the v i b r a t i o n a l - e l e c t r o n i c ( v i b r o n i c ) Hamiltonian of the system as a

e

a

a

e

2

a

e

u

H(r,Q) = H (r,Q) + T ( Q ) , e

(5)

V

where T ( Q ) i s the k i n e t i c energy operator f o r n u c l e a r v i b r a t i o n ­ a l motion w i t h i n the MLg c l u s t e r , and H (r,Q) i s the e l e c t r o n i c Hamiltonian operator d e f i n e d by V

£

H (r,Q) = T ( r ) + V(r,Q) e

e

In Eq. ( 6 ) , T ( r ) i s the k i n e t i c energy operator f o r the e l e c ­ trons and V(r,Q) i s the t o t a l p o t e n t i a l energy operator f o r the system. The c o l l e c t i o n of e l e c t r o n coordinates i s denoted by {r} and the c o l l e c t i o n of normal coordinates f o r the MLg c l u s t e r i s denoted by {Q}. Expanding V(r,Q) i n the normal coordinates {Q} about the e q u i l i b r i u m nuclear c o n f i g u r a t i o n of the MLg c l u s t e r , we may w r i t e e

V(r,Q) = V°(r) + Σ V^Q

a

+ ft) Σ Σ V£ Q Qg+ a

a

,

(7)

a 3 2

where V£ - [3V(r,Q)/3Q ] , VJJ = [8 V(r,Q)/3Q 9Qg] , and α and 3 l a b e l normal coordinates of the MLg system. The term V ( r ) represents the p o t e n t i a l energy of the complete system with the n u c l e i clamped i n t h e i r e q u i l i b r i u m p o s i t i o n s . The operator V°(r) has t r i g o n a l d i h e d r a l ( D 3 ) symmetry and may be w r i t t e n as a

Q

V°(r) = V° + V°,

3

a

Q

(8)

where V§ i s the octahedral (Oh) p a r t of V°(r) and v£ i s the t r i g o n a l d i h e d r a l ( D 3 ) part of V°(r). The ungerade components of VÇ r e f l e c t the c h i r a l i t y of the system. The second and t h i r d terms i n Eq. ( 7 ) are v i b r o n i c c o u p l i n g terms and t h e i r i n f l u e n c e on the e l e c t r o n i c p r o p e r t i e s of*the system w i l l be t r e a t e d by p e r t u r b a t i o n techniques. In our perturbat i o n treatment we define the zeroth-order e l e c t r o n i c Hamiltonian to be H

0 ( ) = T ( r ) + V°(r), r

£

(9)

with e i g e n f u n c t i o n s ψη obtained as s o l u t i o n s to the Schrodinger equation Η°(Γ)Ψ°(Γ) ε η

= EηVη( r ) .

(10)

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

3.

RICHARDSON

Circular

Dichroic

49

Intensities

In the s o - c a l l e d "crude a d i a b a t i c a p p r o x i m a t i o n " , the o r d e r v i b r o n i c wave f u n c t i o n s may be w r i t t e n a s

where t h e v i b r a t i o n a l wave f u n c t i o n s φ tions to [ T ( Q ) + V°(r)

η

ν

zeroth-

(Q) a r e f o u n d a s s o l u ­

+ ft) Σ (£] * ( Q ) = W ^ Q ) . (12) α I n w r i t i n g Eq. (12) we h a v e assumed t h e h a r m o n i c a p p r o x i m a t i o n f o r v i b r a t i o n a l m o t i o n s o f t h e MLg n u c l e i . In o u r p e r t u r b a t i o n treatment o f v i b r o n i c i n t e r a c t i o n s t h e wave f u n c t i o n s d e f i n e d b y Eq. (11) c o m p r i s e o u r z e r o t h - o r d e r b a s i s s e t , and the p e r t u r b a t i o n H a m i l t o n i a n i s g i v e n by v

n v

H'(r,Q) = H ( r , Q ) £

or, H'(r,Q) = Σ ν ; Q + ft) Σ Σ Q Q (14) a a 3 i n c l u d i n g terms l i n e a r and q u a d r a t i c i n the normal c o o r d i n a t e s {Q}. The " p e r t u r b e d " v i b r o n i c wave f u n c t i o n s may be e x p r e s s e d a s a

V J

Σ

a

*

η ν

( 1 5 ) J

'

where t h e e x p a n s i o n c o e f f i c i e n t s , C J , a r e t o b ef o u n d b y d i a g o n a l i z i n g (H9 + H ) i n t h e b a s i s s e t {ψ£ ( r ) φ (Q) }. j I 1 V

T

η ν

B. C o ( I I I ) S y s t e m s . The m o d e l d e s c r i b e d a b o v e ( i n S e c t i o n II.A.) i s a p p l i c a b l e t o any s i x - c o o r d i n a t e system o f t r i g o n a l d i h e d r a l ( D 3 ) symmetry. To i l l u s t r a t e t h e a p p l i c a t i o n s o f t h e m o d e l , we c o n s i d e r h e r e s i x - c o o r d i n a t e C o ( I I I ) c o m p l e x e s o f D3 symmetry i n w h i c h t h e C o ( I I I ) i o n r e s i d e s i n a " s t r o n g " c r y s t a l field. I n t h i s c a s e , the ground s t a t e o f the complex i s nond e g e n e r a t e w i t h A i ( A i g ) symmetry a n d t h e ^ - e l e c t r o n ( s i n g l e t ) e x ­ c i t e d s t a t e s a r e o f symmetry t y p e s A ( T i g ) , E ( T i ) , Α χ ( T ) a n d E(T ). A schematic energy l e v e l diagram i s given i n F i g u r e 1 f o r t h e s e d - e l e c t r o n s t a t e s a n d f o r two a d d i t i o n a l s t a t e s o f T i o c t a h e d r a l p a r e n t a g e , A ( T i ) a n d E ( T i ) . The l a t t e r s t a t e s may be assumed t o b e d e r i v e d e i t h e r f r o m metal«-*\Ligand c h a r g e - t r a n s f e r e x c i t a t i o n s o r f r o m a d ^ m e t a l i o n c o n f i g u r a t i o n . The e l e c t r i c d i p o l e (ED) a n d m a g n e t i c d i p o l e (MD) s e l e c t i o n r u l e s g o v e r n i n g t r a n s i t i o n s between t h e A i ( A i g ) g r o u n d s t a t e and t h e e x c i t e d s t a t e s shown i n F i g u r e 1 a r e summarized a s f o l l o w s : 2

g

2 g

2 g

u

2

u

u

5

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

Octahedral (0 ) h

Figure 1.

Trigonal

Dihedral

3

Schematic energy-level diagram for a Co(III) complex

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

3.

RICHARDSON

Circular

Excited

Dichroic

State

2

D3

Ok

A (Ti ) Ε (TxJ) Ai(T ) Ε (Tog) A (Ti ) Ε (T ) g

51

Intensities

MD MD

MD(z), ED(z) MD(x,y), ED(x,y)

ED ED

MD(x,y), ED(x,y) MD(z), E D ( z ) MD(x,y), ED(x,y)

2 g

2

u

l u

where t h e p o l a r i z a t i o n d e s i g n a t i o n s ( x , y , z ) r e f e r t o a t r i g o n a l c o o r d i n a t e s y s t e m ( w i t h t h e ζ-axis c o i n c i d e n t w i t h t h e C3 sym­ metry a x i s o f t h e system). Assuming s t r o n g o c t a h e d r a l parentage, t h e A i ( A i g ) - > A 2 ( T i ) a n d E ( T i ) t r a n s i t i o n s may be e x p e c t e d t o e x ­ h i b i t r e l a t i v e l y s t r o n g o p t i c a l a c t i v i t y a n d o n l y weak a b s o r p ­ t i v i t y , w h e r e a s t h e k\ ( A i ) - > A 2 ( T i ) a n d E ( T i ) t r a n s i t i o n s may be e x p e c t e d t o e x h i b i t tivity. The Αχ(Aig)->Ai p e c t e d t o e x h i b i t r e l a t i v e l y weak o p t i c a l a c t i v i t y a n d weak a b ­ sorptivity. These e x p e c t a t i o n s b a s e d on symmetry s e l e c t i o n r u l e s r e f l e c t i n g s t r o n g o c t a h e d r a l parentage a r e g e n e r a l l y borne o u t by e x p e r i m e n t a l o b s e r v a t i o n . I n o u r a n a l y s i s o f t h e m o d e l C o ( I I I ) s y s t e m s we s h a l l assume s t r o n g o c t a h e d r a l parentage f o r t h e e l e c t r o n i c s t a t e s and f o r t h e CoLe v i b r a t i o n a l modes. We s h a l l f u r t h e r assume t h a t t h e g r o u n d e l e c t r o n i c s t a t e A i ( A i g ) remains u n a f f e c t e d by v i b r o n i c i n t e r a c ­ tions. Given these assumptions, t h e p r i n c i p a l i n f l u e n c e s o f t h e Q 2 ( e ) a n d Q s ( t 2 ) v i b r a t i o n a l modes a r e t o ( 1 ) c a u s e J a h n - T e l l e r (JT) and pseudo j a h n - T e l l e r (PJT) d i s t o r t i o n s w i t h i n t h e T i , T , and T i e x c i t e d s t a t e s , a n d ( 2 ) i n d u c e m i x i n g between t h e T i and T2ç e x c i t e d s t a t e s . These e f f e c t s w i l l b e m a n i f e s t e d i n t h e i n t e n s i t y d i s t r i b u t i o n s w i t h i n theAi -*Ti a n d A i - * T 2 g d.-d t r a n s i t i o n s , b u tthey w i l l n o t s i g n i f i c a n t l y a l t e r t h e n e t (or t o t a l ) CD a n d a b s o r p t i o n i n t e n s i t i e s o f t h e s e t r a n s i t i o n s . On t h e o t h e r hand, t h e Q 3 ( t ) , Q i * ( t i ) , a n d Q e ( t 2 u ) v i b r a t i o n a l modes w i l l b e e f f e c t i v e i n m i x i n g t h e T i e x c i t e d s t a t e w i t h t h e T^g a n d T?_g excited states. This w i l l lead t o a r e d i s t r i b u t i o n of e l e c t r i c d i p o l e i n t e n s i t y o u t o f t h e Aig->-Ti t r a n s i t i o n a n d i n t o t h e Aig-KTig a n d Aig->T2g d_-d t r a n s i t i o n s . I n what f o l l o w s , we s h a l l f i r s t e x a m i n e t h e i n f l u e n c e o f T * ( t g + e g ) c o u p l i n g on t h e CD s p e c t r u m o f t h e C o ( I I I ) A - > T t r a n s i t i o n (neglecting a l l other v i b r o n i c i n t e r a c t i o n s ) . Secondl y , we s h a l l e x a m i n e t h e i n f l u e n c e o f t h e t 2 ( Q s ) a n d e ( Q 2 ) v i b r a t i o n a l modes o n t h e A i - > T 2 CD s p e c t r u m v i a v i b r o n i c a l l y i n duced Tig-T2g m i x i n g s . F i n a l l y , we s h a l l c o n s i d e r T - T a n d 2 g ~ l u mixings under t h e i n f l u e n c e o f v i b r o n i c i n t e r a c t i o n s w i t h t h e t i ( Q 3 a n d Qi+) a n d t 2 ( Q 6 ) v i b r a t i o n a l modes. g

g

g

g

u

u

g

g

u

2 g

g

g

l u

g

u

u

u

l g

2

lg

g

g

g

g

l g

T

lg

l u

T

u

U

C Tig*(t?g+eg) Coupling. To t e r m s l i n e a r i n Q , t h e v i b r o n i c H a m i l t o n i a n f o r t h i s c a s e may be w r i t t e n a s a

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

52

STEREOCHEMISTRY

H ( r , Q ) = H°(r) + ( h V

+ 5e Q e +

5

2

+ h

5

+ Vj _Q

+

e

e

5 e

+ h

)

5 a

+ (V£

_ + V| Q e

5 a

+

OF TRANSITION METALS

Q

+ V _Q _

2 +

2

2

),

(16)

w h e r e H°(r) i s d e f i n e d by E q . ( 9 ) , h i s the harmonic o s c i l l a t o r H a m i l t o n i a n f o r t h e α-th t r i g o n a l v i b r a t i o n a l mode, and V^C^ i s t h e l i n e a r v i b r o n i c c o u p l i n g t e r m f o r t h e Q mode. The + and s u b s c r i p t s d e n o t e components o f t h e d o u b l y d e g e n e r a t e v i b r a t i o n a l modes, Q and Q5 . I t i s understood that a l l of the operators a p p e a r i n g i n Eq. (16) a r e d e f i n e d f o r t h e t r i g o n a l l y d i s t o r t e d T i g ( A + E) e l e c t r o n i c s t a t e o f t h e C o ( I I I ) m o d e l s y s t e m . Having d e f i n e d our v i b r a t i o n a l c o o r d i n a t e s w i t h respect t o t h e t r i g o n a l l y d i s t o r t e d e q u i l i b r i u m g e o m e t r y o f t h e CoLg c l u s t e r , V £ = 0 a n d t h e l i n e a r c o u p l i n g t e r m V"5 Q v a n i s h e s i n E q . ( 1 6 ) . We d e n o t e t h e t r i g o n a s t a t e by ψ ° ( Α ) , ψ°_(Ε) f u n c t i o n s o f t h e o p e r a t o r H ° ( r ) , a n d t h e y have t h e symmetry p r o ­ p e r t i e s C * g = i|/g, 0 ψ°_ = ωψ°_, and 0 ψ ° = ω*ψ°_, where ω = e x p ( 2 i r i / 3 ) and C i s t h e t h r e e f o l d r o t a t i o n o p e r a t o r o f t h e D p o i n t group. The d o u b l y d e g e n e r a t e v i b r a t i o n a l modes may be c o n ­ v e n i e n t l y expressed i n p o l a r c o o r d i n a t e form as a

a

2

e

2

a

a

5a

2

3

3

3

3

Q

3

= p exp(i), Q -

2 +

2

2

= p exp(-i(|>) 2

and, Q5e+

=

e x

f

P5e P(^ )»

=

Q5 -

P5e

e

e x

!

P(-^ )

with C Q = o)Q +, C Q _ = ω*(} _, a n d s i m i l a r l y f o r Q s + a n d Qse-The e f f e c t s o f t h e Q and Q s v i b r a t i o n a l modes w i l l be t o r e n d e r t h e Ε(ψ°_,ψ°_) e l e c t r o n i c s t a t e J a h n - T e l l e r u n s t a b l e and t o c o u p l e t h e Ε(φ°_,ψ9:) a n d Α ( ψ ° ) s t a t e s v i a a p s e u d o J a h n - T e l l e r mechanism. The v i b r o n i c wave f u n c t i o n f o r t h e l g * ( 2 g g ) s y s t e m may be e x p r e s s e d a s 3

2 +

2

3

2

2

2

e

e

2

T

* = + + X ^

}

t

+

+

xs?

) .

e

+

xS?) (17)

The v i b r o n i c e n e r g y l e v e l s and v i b r a t i o n a l a m p l i t u d e f u n c t i o n s Οία » Χ α ^ > * Χα ^ » where α = 2,5e, o r 5a) may be f o u n d by s o l ­ v i n g t h e s e c u l a r e q u a t i o n , Eq. ( 1 8 ) . I n t h i s m a t r i x e q u a t i o n , +

and

a n (

Η°(χ)ψ? = Ε?φ2,

(19)

Η ? ( Γ ) Ψ ° = E°X

(20)

t h e i n t e r a c t i o n m a t r i x elements, E^j and T y , E

ij

=

< ψ

°

V

Q

I * + 2 + + ν£_0. _|ψ°>, 2

a r e g i v e n by (21)

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

RICHARDSON

Ο w CM , Χ

Circular

+ ^ CM Χ

I VCM X

Dichroic

w

Intensities

0 0 > + < l ) l < l ) O c O + c O m w m w i n w i n w i n X X X X X

I cO

m

cO

m f ο ο w ι

+

κ­

+

ω m

m

ο ο

+ +

α) m

ί ο ο

CN

eg

+ ο

Ο I

ι° °

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

54 and, T

=

ij

22

^i^e+Qse-H + n _Q e-U°>e

< >

5

Synnnetry a r g u m e n t s r e q u i r e t h a t = Ε = o-

E

*>

τ

+

-o

=

= Τ = ο-

+0

E

E

-

E

o+

=

Y 2 Î ! 2 +

= T*

=

ex

i


Y2P2 P( t )»

(24)

= Y Q5e+ - Ϊ5 Ρ5ββχρ(1Φ'),

+

5e

β

(25)

k Q _ = k p exp(-i), 2

2

2

2

and,

* (26)

k5 Q5 e

where t h e l i n e a r c o u p l i n are d e f i n e d by Ύα = < Φ + Ι ( ν / 3 Ρ ) | φ ο > = < Ψ ? Κ | Ψ Ο > , « = 2 o r 5e. 3

α+

(27)

+

0

The l i n e a r c o u p l i n g c o n s t a n t d e f i n e d by

f o r the JT interactions,

k , are a

= , α Ξ 2 o r 5e.

k a - a

(28)

α

The m a t r i x e q u a t i o n ( 1 8 ) i s b l o c k - d i a g o n a l ( a s shown) o n l y i f t h e c o u p l i n g modes, α = 2,5e, a n d 5 a , a r e m u t u a l l y o r t h o g o n a l . The t w o - d i m e n s i o n a l h a r m o n i c o s c i l l a t o r H a m i l t o n i a n s f o r t h e α = 2 a n d 5e modes a r e g i v e n b y h

Ι 0 / 3 ρ £ ) + U/Pa> 0/*Ρα>

ω

a

2

- (^> αΡα "

+ (1/ρ ) 0 /3φ )]. α

The

2

2

2

eigenfunctions of h Va.vA

=

ε

X

may be o b t a i n e d f r o m s o l u t i o n s t o

a

α,ν)1 a , v A

(29)

=

(v

a

+1)

K

\,vi

(

3

0

)

where t h e quantum numbers ν a n d £ c a n t a k e on t h e v a l u e s , ν = 0,1,2,··· and £ = v,v-2,···,-v. F o r α = 5 a , we w r i t e

h x

Α

α α,ν

- ε χ α,ν α , ν

= ( v + %)ηω χ α ' α α,ν

Λ

Λ

(31)

where h i s a o n e - d i m e n s i o n a l h a r m o n i c o s c i l l a t o r H a m i l t o n i a n expressed i n terms o f Q s . I t i s now c o n v e n i e n t t o e x p a n d t h e v i b r a t i o n a l a m p l i t u d e f u n c t i o n s (χ(°), χ ^ , a n d X^~0 a p p e a r i n g i n E q s . ( 1 7 ) a n d ( 1 8 ) i n terms o f b a s i s s e t s c o n s t r u c t e d from t h e e i g e n s o l u t i o n s o f Eqs. ( 3 0 ) a n d ( 3 1 ) . Upon d o i n g t h i s , t h e e i g e n s o l u t i o n s o f E q . (18) may b e e x p r e s s e d a s a

a

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

3.

RICHARDSON

ψ

= J

Circular

ΑΣ

A

Dichroic

(Ο

(o) X

jvil 2,v&

+

Σ

vil

° A

55

Intensities

Β^

θ )

χί

+

θ )

A

jv£ 5e,vil

Σ

c< x< o )

0 )

A

j v 5a,v

ν

V

v +ψ°(Σ A jvil 2,vil v£

+

+

+

( + )

( + )

A

Ψ°(Σ

A - y jviT2,v£ (

vil

)

(

)

Σ

vil Σ

vil

+

A

jvil 5e,v£

Σ

c

ν

+

A

jvil 5e,v£

Σ

ν

W

x
^ (where λ = o,-, or + correspond to e l e c t r o n i c states ψ§, ψ ° , and ψ°_, r e s p e c t i v e l y ) are found by s o l v i n g Eq X^vSL (a = 2 or 5e), a r e obtaine λ(ο,-, or +) and the v i b r a t i o n a l f u n c t i o n s , X ^ ^ v from Eq. (31) f o r the appropriate λ. For a more complete d e s c r i p t i o n of the v i b r o n i c l e v e l s associated with the T i ( A 2 + E) e l e c t r o n i c e x c i t e d s t a t e , the wave functions »j[Eq. 132)] must be augmented to include v i b r a ­ t i o n a l wave functions associated with a l l of the normal modes other than α = 2,5a, and 5e. Here we s h a l l r e s t r i c t our a t t e n ­ t i o n to j u s t those v i b r o n i c l e v e l s derived from the v i b r a t i o n a l modes α = 2,5a, and 5e. Assuming no v i b r o n i c couplings i n v o l v i n g the ground e l e c t r o n i c s t a t e of our model system, the r o t a t o r y strength of a t r a n s i t i o n between the lowest v i b r a t i o n a l l e v e l o f the ground e l e c t r o n i c s t a t e and the j - t h v i b r o n i c l e v e l of the T}g*(t2g + eg) coupled s t a t e may be w r i t t e n as B

a n
- j g go'- j j '-'Vgo r

Y

1

>

(33)

where ψ? i s the e l e c t r o n i c wave f u n c t i o n f o r the Α ι ( Α ^ ) ground state o f our model system and g i s the v i b r a t i o n a l wave f u n c ­ t i o n f o r the ground v i b r a t i o n a l l e v e l o f the ground e l e c t r o n i c state. In expanded form, Eq. (33) may be w r i t t e n as 0

β

R. - Σ Η < > [ Σ Σ λ α,ν,Α α · , ν \ 4 ' J

λ

X

D< > ^

ν Α ( α )

X

J

DÎ >* ,, * > V

(

α

Χ S^golvDS^v'A'Igo)],

W

where, R.

(e)

= Ι*·

(35)

Φ

—

—

Λ g Λ Λ g defines the purely e l e c t r o n i c r o t a t o r y strength the ψ° -> ψ° (λ = ο,-, or +) t r a n s i t i o n and S λ S (go|viQ = < ! χ > ( X )

( g )

X

( λ )

associated

with

(36)

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

56

i s a Franck-Condon o v e r l a p i n t e g r a l o v e r the ground s t a t e v i b r a ­ t i o n a l f u n c t i o n XQ[ ο the e x c i t e d s t a t e v i b r a t i o n a l f u n c t i o n *a^v£ f ° a - t h ' n o r m a l mode ( a = 2,5a, o r 5e and λ = ο,-, o r +)! For α = 5a, £ = £ = o f o r a l l v a l u e s of v. The c o e f f i c i e n t s a

r

t

n

n

d

e

f

D

jv!(a)

a

D

( X )

D

( X )

r

ed

e

f

i

n

=

jv£(2)

e

d b

:

( λ )

A

Α jv£'

=

jv£(5e)

v

Β

( λ )

*jvV

and, η

(λ) jv£(5a)

β

Γ L

(λ) jv '

where t h e c o e f f i c i e n t s A j Eq.

(32).

The

complex c o n j u g a t

D

gv^(a)' I f we make t h e s i m p l i f y i n g a s s u m p t i o n t h a t t h e v i b r a t i o n a l wave f u n c t i o n s o f t h e g r o u n d ( g ) and e x c i t e d ( X ) e l e c t r o n i c s t a t e s a r e i d e n t i c a l , t h e n Eq. (34) r e d u c e s t o R.

J

= R

( e )

[|Af

ο

0 )

joo'

1

|2

+ | f B

0

)

| 2 + |C^ |2]

joo

1

0 )

1

JO

1

1

^ l„ T i ( A + E) e l e c t r o n i c t r a n s i t i o n i s d i s t r i b u t e d among t h e v i b r o n i c l e v e l s d e r i v e d f r o m ( A + E ) * ( a i + 2e) c o u p l i n g s . The t o t a l (ornet) r o t a t o r y strength of the A i T i transition i s , t h e r e f o r e , given by i g

g

2

2

g

R(Aig

T

l g

) = Σ R. = R

( e )

+ RJ

e )

g

e )

+ R^ ,

(39)

e;

where R j i s d e f i n e d b y Eq. ( 3 4 ) a n d t h e R J ^ ( X = o,-, o r +) a r e d e f i n e d b y E q . ( 3 5 ) . The " s t a t i c " s t e r e o c h e m i c a l a n d s t r u c t u r a l f e a t u r e s o f t h e m e t a l complex d e t e r m i n e t h e s i g n and magnitude o f R ( A j g -> T i g ) a s w e l tronic rotatory strength aspects o f these " s t a t i c s t r u c t u r a l f e a t u r e s a r e d e s c r i b e d by t h e p o t e n t i a l e n e r g y o p e r a t o r , V ^ ( r ) , o f E q . ( 8 ) . The t o t a l ( o r net) A i g T i g r o t a t o r y strength i s i n v a r i a n t t o T i * ( t + e ) vibronic coupling. Only t h e d i s t r i b u t i o n o f e l e c t r o n i c r o t a t o r y s t r e n g t h among t h e component v i b r o n i c t r a n s i t i o n s i s a f f e c t e d by the T i g * ( t g + e ) c o u p l i n g s . The v i b r o n i c c o u p l i n g m o d e l d e s c r i b e d i n t h i s s e c t i o n ( U . C . ) i s h i g h l y r e s t r i c t e d i n s o f a r a s o n l y l i n e a r c o u p l i n g terms have been i n c l u d e d e x p l i c i t l y , a n d o n l y two e - t y p e t r i g o n a l modes (those d e r i v e d from t h e t a n d e g o c t a h e d r a l CoLg c l u s t e r modes) h a v e been c o n s i d e r e d . I t i s r e l a t i v e l y easy t o extend o u r t r e a t ­ ment t o i n c l u d e q u a d r a t i c c o u p l i n g t e r m s . However, t h e i n c l u s i o n o f a d d i t i o n a l e - t y p e t r i g o n a l modes ( e . g . , t h o s e d e r i v e d f r o m t h e t a n d t i u o c t a h e d r a l modes) w o u l d d r a s t i c a l l y c o m p l i c a t e o u r model. T h i s l a t t e r e x t e n s i o n o f t h e model s h o u l d n o t prove n e c e s s a r y so l o n g a s t h e CoLc c l u s t e r r e t a i n s v e r y n e a r l y o c t a h e ­ d r a l symmetry ( i . e . , VÇ T i t r a n s i t i o n i n CoLg s y s t e m s i s m a g n e t i c d i p o l e a l l o w e d w h e r e a s t n e A i g -> T g t r a n s i t i o n i s m a g n e t i c dipole forbidden. U s i n g t h e n o t a t i o n o f F i g u r e 1, t h e t r i g o n a l (D3) s e l e c t i o n r u l e s s p e c i f y t h a t t h e A i ( A i ) -> A ( T i ) , E ( T i ) , and E b ( T g ) t r i g o n a l t r a n s i t i o n s a r e m a g n e t i c d i p o l e a l l o w e d , w h i l e t h e A i ( A i ) - A i ( T ) t r a n s i t i o n remains magnetic d i p o l e forbidden. I n the usual p e r t u r b a t i o n treatments of o p t i c a l a c t i v i t y i n t r i g o n a l d i h e d r a l C o ( I I I ) c o m p l e x e s , i t i s assumed t h a t t h e A i -*· E^ component o f t h e e r s t w h i l e A i -> T transition "borrows"magnetic d i p o l e c h a r a c t e r (and, t h e r e f o r e , r o t a t o r y s t r e n g t h ) f r o m t h e A i + E component o f t h e A ^ -> T i g t r a n s i t i o n . T h i s i s assumed t o o c c u r v i a T i - T m i x i n g induced by t h e g e r a d e components o f t h e t r i g o n a l f i e l d p o t e n t i a l , VS. By t h i s m o d e l , t h e s i g n a n d m a g n i t u d e o f t h e A i ( A i ) •+ E b ( T ; r o t a t o r y s t r e n g t h ( a n d CD) may b e c o r r e l a t e d d i r e c t l y t o t h a t o f t h e g

2

2 £

g

g

2

g

2

g

a

2

>

g

2 g

g

2 g

a

g

2 g

g

2 g

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

g

58

STEREOCHEMISTRY OF TRANSITION METALS

A

A

E

T

l( lg) a( lg) transition. The A T CD w o u l d n o t r e f l e c t any a s p e c t s o f t h e Αχ -* A component o f t h e A i g T i g t r a n s i t i o n . T h i s p i c t u r e i s a l t e r e d d r a s t i c a l l y when v i b r o n i c c o u p l i n g e f f e c t s a r e taken i n t o account. V i b r o n i c c o u p l i n g may e n t e r i n t o t h i s p r o b l e m i n s e v e r a l ways. F i r s t , b o t h t h e T i and T states are subject t o " i n t r a - s t a t e " c o u p l i n g s o f t h e types ( f o r example): T i * ( t g + e ) and T g * ( t + e ). Secondly, " i n t e r - s t a t e " c o u p l i n g o f t h e t y p e ( T i g + T ) * ( t g + e g ) may become i m p o r t a n t . We s h a l l e x a m i n e t h e p o s s i b l e i n f l u e n c e o f t h i s l a t t e r ( " i n t e r ­ s t a t e " ) type o f c o u p l i n g here. T r i g o n a l e - t y p e modes ( d e r i v e d , f o r e x a m p l e , f r o m t h e t g o r e g o c t a h e d r a l modes) c a n e f f e c t t h e f o l l o w i n g " i n t e r - s t a t e c o u p l i n g s o f t r i g o n a l e l e c t r o n i c components: ( E + E )*e, ( E + A i ) * e , and ( A + Eb)*e. T h i s has t h e consequence t h a t v i b r o n i c components o f t h e A i ( A i g ) > A i ( T ) t r a n s i t i o exhibit v a n i s h i n g CD w h i c h h a transition. Furthermore ( 2g) t r a n s i t i o n may e x h i b i t CD " b o r r o w e d " f r o m b o t h t h e A i •> E and A i -* A t r i g o n a l components o f t h e A i g -> T i g t r a n s i t i o n . H e r e t o f o r e , t h e a p p e a r a n c e o f m u l t i p l e components i n t h e A i g •> T CD r e g i o n h a s a l w a y s been a t t r i b u t e d t o t h e p r e s e n c e o f m u l ­ t i p l e s p e c i e s t y p e s o r t o a r e d u c t i o n o f symmetry ( f r o m D3 t o C 3 ) i n t h e complex under study. Strong T i - T m i x i n g under t h e i n f l u e n c e o f a n e - t y p e t r i g o n a l v i b r a t i o n a l mode i s , p e r h a p s , a more l i k e l y c a u s e f o r t h e a p p e a r a n c e o f m u l t i p l e components in the A -* T CD s p e c t r u m . Detailed studies of (Tig + T ) * ( t + e ) coupling to octa­ h e d r a l systems and o f ( A + E + A i + E )*e coupling i n trigonal s y s t e m s h a v e n o t y e t been c a r r i e d o u t . Such s t u d i e s i n c l u d i n g n u m e r i c a l c o m p u t a t i o n s w o u l d be e x t r e m e l y c o m p l e x . However, t h e q u a l i t a t i v e c o n s e q u e n c e s o f s u c h c o u p l i n g s o n t h e CD s p e c t r a o f c h i r a l t r i g o n a l s y s t e m s may be d i s c e r n e d r e a d i l y f o l l o w i n g t h e arguments g i v e n above. I n b r i e f , ( A + E + A i + E ) * e c o u p l i n g c a n be e x p e c t e d t o l e a d t o CD o f m i x e d s i g n and m i x e d p o l a r i ­ z a t i o n i n t h e A i -> T transition region, reflecting the sign and p o l a r i z a t i o n p r o p e r t i e s o f t h e A i ( A i ) ->• A ( T i ) + E ( T i ) CD bands. l g

i g

2

g

g

2

g

2

2 g

2 g

g

2 g

2

2

a

D

a

2

T

a

2

2 g

g

i g

2 g

2 g

2

2

g

2

a

g

2

g

g

D

a

D

2 g

g

2

g

a

g

E. ( T i g + T i ) * ( t i u + t ) Coupling. This type o f coupling i s t h e b a s i s f o r t h e s o - c a l l e d H e r z b e r g - T e l l e r (HT) v i b r o n i c t h e o r y o f d-d i n t e n s i t i e s i n o c t a h e d r a l (Oh) t r a n s i t i o n m e t a l c o m p l e x e s . I n t h i s t h e o r y a p p l i e d t o CoL,6, t h e A i g Tigtransition i s as­ sumed t o g a i n e l e c t r i c d i p o l e c h a r a c t e r v i a v i b r o n i c a l l y i n d u c e d T i g - T i m i x i n g u n d e r t h e i n f l u e n c e o f an u n g e r a d e v i b r a t i o n a l mode ( o f e i t h e r t i or t symmetry). ( H e r e we s h a l l i g n o r e v i b r o n i c a l l y induced m i x i n g o f t h e A i g ground s t a t e w i t h ungerade e x c i t e d s t a t e s , and c o n s i d e r o n l y v i b r o n i c p e r t u r b a t i o n s on t h e excited state Ti .) The l o w - t e m p e r a t u r e A i + T i electric d i p o l e (absorption) spectrum i s , then, p r e d i c t e d t o c o n s i s t o f t h r e e p r o g r e s s i o n s based on f a l s e o r i g i n s l o c a t e d a t 0 - 1 ( 0 ) 3 ) , u

2 u

u

u

g

2

u

g

g

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

3.

RICHARDSON

Circular

Dichroic

59

Intensities

0 - 1 ( 0 ) ^ ) , a n d 0-1 (ως), where 0 ) 3 , ω^, a n d ως a r e t h e f u n d a m e n t a l f r e q u e n c i e s o f t h e Q 3 ( t i ) , Q t + ( t i ) , a n d Q6(t2u) n o r m a l modes, respectively. Strong Jahn-Teller d i s t o r t i o n s w i t h i n t h e T and/or T s t a t e s would, o f course, tend t o complicate t h i s simple t h r e e - p r o g r e s s i o n spectrum. I n t r i g o n a l d i h e d r a l c o m p l e x e s o f " n e a r " o c t a d e d r a l sym­ metry: T -> A + E a n d T i •+ + E (using the notation of F i g u r e 1 ) . I n these systems, t h e A (Αχ) -*· T ( A + E ) t r a n s i t i o n h a s i n h e r e n t e l e c t r i c d i p o l e s t e n g t h due t o t h e u n g e r a d e components o f t h e " s t a t i c " t r i g o n a l f i e l d p o t e n t i a l vÇ. However, a d d i t i o n a l e l e c t r i c d i p o l e c h a r a c t e r may be i n t r o d u c e d i n t o t h i s t r a n s i t i o n v i a ( A + E + A + E ' ) * e c o u p l i n g , where t h e c o u p l i n g mode i s an e - t y p e t r i g o n a l v i b r a t i o n . A complete treatment o f ( A + E + A + E')*e c o u p l i n g would r e q u i r e s i m u l taneous c o n s i d e r a t i o n o ( J a h n - T e l l e r a n d pseud and T i ) a n d i n t e r - s t a t e v i b r o n i c i n t e r a c t i o n s ( o t h e H e r z b e r g T e l l e r t y p e between T i a n d T i ) . F u r t h e r m o r e , a l l e - t y p e t r i g o n a l modes s h o u l d be c o n s i d e r e d i n d e p e n d e n t o f t h e i r o c t a h e d r a l parentage. I n t h e p r e s e n t t r e a t m e n t , we s h a l l a d o p t a much more r e s t r i c t e d m o d e l i n w h i c h o n l y i n t e r - s t a t e ( T i - T i ) i n t e r a c t i o n s a r e c o n s i d e r e d and i n which o n l y those e-type t r i g o n a l modes o f t i or t o c t a h e d r a l ancestry a r e taken i n t o account. T h i s l a t t e r r e s t r i c t i o n t o v i b r a t i o n a l modes d e r i v e d from t h e t i and t o c t a h e d r a l v i b r a t i o n s s h o u l d be a c c e p t a b l e so l o n g a s t h e ΟοΙ,ς c l u s t e r r e m a i n s v e r y n e a r l y o c t a h e d r a l a n d Ί° « V§. u

u

l g

l u

1

l g

2

a

u

L

2

2

&

a

G

L G

2

a

2

2

u

g

u

g

u

u

2

2

u

u

u

Ύ

Given | A ) , and electronic We f u r t h e r 2

t h e a p p r o x i m a t i o n s c i t e d a b o v e , we t a k e | A ) , | E ) , | E ) t o be e i g e n f u n c t i o n s o f t h e t r i g o n a l l y s y m m e t r i c H a m i l t o n i a n d e f i n e d i n E q . ( 9 ) , H°(r) = T ( r ) + V°(r). define a ( l i n e a r ) v i b r o n i c i n t e r a c t i o n operator 2

a

?

£

H'(r,Q) = Σ v ; Q (40) a where Σ i s t a k e n o v e r t h e e - t y p e t r i g o n a l modes d e r i v e d f r o m t h e Q 3 ( t i ) , Q i + ( t i ) , a n d Q 6 ( t 2 ) o c t a h e d r a l modes. T h a t i s , a=3e,4e, a n d 6e ( s e e S e c t i o n I I . A . f o r n o t a t i o n ) . Restricting our c o n s i d e r a t i o n t o T i - T i i n t e r - s t a t e m i x i n g s and u s i n g f i r s t - o r d e r t i m e - i n d e p e n d e n t p e r t u r b a t i o n t h e o r y , we may e x p r e s s t h e p e r t u r b e d wave f u n c t i o n s o f i n t e r e s t a s f o l l o w s : a

α

u

u

u

g

u

|A >

= | A ) + Σ λ (Ε' , A ) | E ' ) Q a

I V

- |E ) + Σ À ( E \ E ) | E ' ) Q a

2

2

a

α

a

2

a

a

(41)

a

+ Σ X (A»,E )|A£)Q a a

a

a

(42)

where, (43)

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

60

i s a perturbation expansion c o e f f i c i e n t . (Note t h a t t h e p e r ­ t u r b e d wave f u n c t i o n s a r e d e n o t e d b y p o i n t e d k e t s !>, w h e r e a s t h e u n p e r t u r b e d wave f u n c t i o n s a r e d e n o t e d b y r o u n d e d k e t s I ) ) . Having r e s t r i c t e d our p e r t u r b a t i o n treatment t o T i - T i i n t e r ­ s t a t e i n t e r a c t i o n s , i t i s e a s y t o s e e t h a t t h e wave f u n c t i o n s e x p r e s s e d by E q s . ( 4 1 ) a n d (42) a r e a c t u a l l y good t o f i r s t - a n d second-order i n H . We s h a l l i g n o r e a l l v i b r o n i c p e r t u r b a t i o n s on t h e g r o u n d e l e c t r o n i c s t a t e o f o u r C o ( I I I ) m o d e l s y s t e m , so that: g

u

f

|A > = I Α χ ) .

(44)

X

U t i l i z i n g E q s . ( 4 1 ) , ( 4 2 ) , a n d ( 4 4 ) , t h e e l e c t r i c a n d magne­ t i c d i p o l e t r a n s i t i o n moments f o r t h e Αχ A and E t r a n s i t i o n s may be e x p r e s s e d ( t o s e c o n d - o r d e r ) 2

1

2

a

= (k \0\A l

α and = ( A i | 0 | E ) + Σ X ( E ' , E ) Q ( A | 0 | E ' ) 1

a

a

a

a

a

1

+ Σ X (A|,E )Q (A |Ô|Aj), a

Λ

a

a

(46)

1

a

Λ

where 0 = μ o r m. The r o t a t o r y s t r e n g t h s e x p r e s s e d t o s e c o n d - o r d e r , a r e g i v e n by R(A

X

-* A ) = R

R °Ul (

+

( o )

2

f o r these t r a n s i t i o n s ,

( A ! -> A ) 2

+ Ε')Σ Σ

α α

λ (E\A )X 2

Ut

•

OC

(E\A )Q 2

Ut

Q .

(47)

ΙΛ

and,

R(A - E ) = R °Ul + E ) (

X

a

a

+

R °Ul

+ E f ) Z Σ λα(Ε',Ε&)λαΙ (Ef ,Ea)QaQa,

(

+ R ^ U l + Α^)Σ Σ X ( A ^ , E ) X ( A ^ , E ) Q Q a a + I m [ ( A | î | E ) - ( E |m|A!) a

a

a f

a

a

a î

1

l

a

a

,

,

+ (A!|y|E )-(E |m|A )]ZX (E ,E )Q , α a

1

a

a

a

(48)

where R °U (

-v j ) = Ι*(ψ°|ί|ψ°)·(ψ°|η|ψ°).

(49)

Now l e t u s c o n s i d e r a s p e c i f i c v i b r o n i c t r a n s i t i o n l e a d i n g from t h e ground v i b r a t i o n a l l e v e l o f t h e ground e l e c t r o n i c s t a t e (go) t o t h e ν (β) v i b r a t i o n a l l e v e l o f t h e e x c i t e d e l e c t r o n i c

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

3.

RICHARDSON

Circular

Dichroic

Intensities

61

state (ev(3)). I n t h e p r e s e n t c o n t e x t , g r e f e r s t o t h e Αχ e l e c t r o n i c g r o u n d s t a t e and e may be e i t h e r t h e A o r E e l e c ­ t r o n i c e x c i t e d s t a t e o f our C o ( I I I ) model system. 3 denotes t h e v i b r a t i o n a l mode c o n t a i n i n g t h e ν q u a n t a i n t h e v i b r o n i c l e v e l ev(3). D e n o t i n g t h e i n i t i a l a n d f i n a l v i b r a t i o n a l wave f u n c t i o n s b y Φ^ a n d Φ ( β ) , r e s p e c t i v e l y , we may w r i t e f o r t h e vibronic rotatory strengths 2

0

R

o,v(3Î

A l

β ν

* *> A

=

r

° *

(

A

-

i

2 Ι

Α

< Φ

)

8

+R^°Ui ·> Ε')Σ Σ λ (Ε',Α )λ


Φ


| 2

2

01

α α g o I Qa 'Ι e v /( 3^ ) e v/( 3 J) Qα Ι Ι go > , x

| Φ

,(Ε',Α )

2

ι Χ

a

Φ

Φ

(

»

0

5

0

>

and, R

, (Αι -> Ε ) = R ^ A i o,v(3; a 0

+

/

R °Ul (

+

Χ

1




I Q go' ^ a Ι e v/( 3\ ) 1




>

-

r

° ^ 1 -

(

Α

$ g o

2>Ι ν

| Φ


|

>|2

5 a e

,

(53)

F o r t h e c a s e 3=a, we

>

βν(α) ! α

>|^

$ E V ( E )

$ e v ( B )

- Ε')|λ (Ε·,Α )|2|.

(60)

0

I n t h e s p e c i a l c a s e where R ( U l

A ) = - R < ° U l -»• Ε a) y i n M o f f i t t ' s t h e o r y o f d-d o p t i c a l a c t i v i t y i n t r i g o n a l ( D 3 ) C o ( I I I ) complexes ( 9 , 1 2 ) , Eq. (60) r e d u c e s t o R

net χ

+

(Α ι x

8

2

+

Ti^) =

A

g'

[|λ (Ε»,Α )| α

R °Ul (

^

2

Σ { R

a ' go

α

2

(

° 1U I

+

a

s

f

E )

2

+ |λ (Ε',Ε )| ]

2

α

α

+ Ap|X (A^,E )| }. 2

a

a

(61)

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

64

Making t h e f u r t h e r assumption ( i n t h e s p i r i t o f M o f f i t t ' s simple model) t h a t R ( ° } A I + E ) = - R ( ° U X + A p , Eq. ( 6 1 ) f u r t h e r r e ­ duces t o f

R

net -

( A

V

*

lg

|λ (Ε·,Ε )| α

" 2

r

(

° ^

-

A

< $

^

o «'V l Q

g

net

( A

T

lg

- lg>

« 2'Vl ( A

- |λ (Ε',Α )|2].

β

α

+

R

net

( A

lg

2

(62)

2

N o t e t h a t t h e a s s u m p t i o n s made i n o b t a i n i n g quire that

R

[ | X

T

" l«> =

Eq. ( 6 2 ) a l s o r e ­

0

(

6

3

)

The s i m p l e f o r m a l a n a l y s i s o u t l i n e d a b o v e r e v e a l s several i n t e r e s t i n g aspects o C o ( I I I ) complexes o f t r i g o n a (A + E + A + E ) * e coupling can lead t o mixed p o l a r i z a t i o n w i t h i n t h e v i b r o n i c CD band e n v e l o p s o f b o t h t h e Αχ A and i E t r a n s i t i o n s . I n t h e absence o f v i b r o n i c c o u p l i n g , t h e Ai A t r a n s i t i o n would be p o l a r i z e d p a r a l l e l t o t h e t r i g o n a l a x i s o f t h e s y s t e m , w h e r e a s t h e Αχ ->• E t r a n s i t i o n w o u l d b e polarized perpendicular t o the trigonal axis. I n t h e presence o f v i b r o n i c c o u p l i n g , v i b r o n i c components o f b o t h p a r a l l e l a n d p e r p e n d i c u l a r p o l a r i z a t i o n may a p p e a r i n b o t h t h e Αχ •> A a n d Αχ •> E CD b a n d s . S e c o n d l y , f r o m E q s . ( 6 1 ) a n d ( 6 2 ) we s e e t h a t t h e n e t CD a s s o c i a t e d w i t h t h e A x T x t r a n s i t i o n need n o t v a n i s h e v e n when t h e z e r o t h - o r d e r n e t e l e c t r o n i c r o t a t o r y strength, R(°UI + A ) + R T x CD s p e c t r u m . f

2

a

2

2

A

a

2

a

2

a

g

2

g

a

2

a

f

g

g

F. Summary o f Q u a l i t a t i v e C o n c l u s i o n s . I nconsidering the i n f l u e n c e o f v i b r o n i c c o u p l i n g i n t h e d-d. CD s p e c t r a o f t r i g o n a l d i h e d r a l C o ( I I I ) s y s t e m s , we h a v e e x a m i n e d t h r e e g e n e r a l c a s e s . F i r s t we e x a m i n e d v i b r o n i c i n t e r a c t i o n s w i t h i n t h e T x ( A + E ) excited state o f Co(III). These i n t e r a c t i o n s a r e o f t h e J a h n T e l l e r a n d p s e u d o J a h n - T e l l e r t y p e s and r e q u i r e a n o n - a d i a b a t i c v i b r o n i c coupling formalism. The p r i n c i p a l e f f e c t s o f t h e s e i n t e r a c t i o n s a r e r e d i s t r i b u t i o n s o f r o t a t o r y s t r e n g t h among t h e v i b r o n i c components o f t h e Αχ A and E t r a n s i t i o n s . Secondly, we e x a m i n e d v i b r o n i c a l l y i n d u c e d m i x i n g s b e t w e e n t h e T x ( A + E ) and T ( A x + Εχ>) e x c i t e d s t a t e s . Q u a l i t a t i v e l y we showed t h a t ( A + E + Αχ + E ) * e c o u p l i n g c o u l d l e a d t o n o n v a n i s h i n g r o t a ­ t o r y s t r e n g t h i n b o t h t h e Αχ Αχ a n d Αχ -> E v i b r o n i c a l l y p e r ­ t u r b e d t r i g o n a l c o m p o n e n t s o f t h e A x •> T t r a n s i t i o n s . Cong

2

2

a

a

g

2

2 g

2

a

D

D

g

2 g

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

a

3.

RICHARDSON

Circular

Dichroic

65

Intensities

sequently, the A i g T g CD c o u l d e x h i b i t m i x e d p o l a r i z a t i o n . F i n a l l y , we e x a m i n e d t h e p o s s i b l e c o n s e q u e n c e s o f v i b r o n i c a l l y i n d u c e d T i ( A + E ) - Ti (A£ + E ) m i x i n g . H e r e we f o u n d t h a t v i b r o n i c components o f m i x e d p o l a r i z a t i o n c a n b e e x p e c t e d w i t h i n b o t h t h e A i -> A a n d Αχ E CD b a n d s a s s o c i a t e d w i t h t h e A i g •> Tig t r a n s i t i o n . I n t h i s c a s e , an a d i a b a t i c v i b r o n i c c o u p l i n g f o r m a l i s m was assumed v a l i d s i n c e t h e e n e r g y s e p a r a t i o n between t h e c o u p l e d e l e c t r o n i c s t a t e s c o u l d b e assumed t o b e l a r g e com­ pared t o t h e v i b r o n i c c o u p l i n g energy. I t i s , o f course, h i g h l y a r t i f i c i a l t o consider each o f t h e above-mentioned c o u p l i n g cases i n i s o l a t i o n . F o r example, i n c o n s i d e r i n g T i g - T i i n t e r a c t i o n s r i g o r o u s l y i t would be n e c e s ­ sary t o take i n t o account simultaneously t h e l i k e l i h o o d t h a t b o t h T i ( A + E ) a n d Ti (A£ + E ) a r e b o t h J T a n d P J T d i s t o r t e d . The same w o u l d h o l d t r u e f o r v i b r o n i c a l l y i n d u c e d T i T in teractions. Such c o n s i d e r a t i o n tedious and complicated p u t a t i o n a l r e q u i r e m e n t s , i f o n e ' s o b j e c t i v e was t o e x p l i c a t e quantitatively the vibronic d e t a i l s of the A i Ti + T CD s p e c t r a o f t r i g o n a l C o ( I I I ) systems. A t present i t would appear n e c e s s a r y t o be c o n t e n t w i t h t h e q u a l i t a t i v e c o n c l u s i o n s e l i c i t e d from t h e less-tban-complete treatments given i n t h i s section. The v i b r o n i c c o u p l i n g a n a l y s i s g i v e n h e r e h a s a v e r y i m ­ portant i m p l i c a t i o n f o r spectra-structure c o r r e l a t i o n studies u s i n g CD. The u s u a l p r a c t i s e i n s u c h s t u d i e s i s t o a s s i g n s p e c i ­ f i c f e a t u r e s i n t h e CD s p e c t r a t o s p e c i f i c e l e c t r o n i c t r a n s i t i o n s o f w e l l - d e f i n e d e l e c t r o n i c s t a t e p a r e n t a g e . The i n t e n s i t i e s o f t h e s e f e a t u r e s a r e t h e n assumed t o b e d i r e c t l y a n d s i m p l y r e ­ l a t e d t o pure e l e c t r o n i c r o t a t o r y s t r e n g t h s , and t h e s i g n s and magnitudes o f these e l e c t r o n i c r o t a t o r y strengths a r e then c o r ­ r e l a t e d w i t h s p e c i f i c s t e r e o c h e m i c a l f e a t u r e s o f t h e system ( e i t h e r by u s e o f v a r i o u s models and t h e o r i e s o r a c c o r d i n g t o some s e t o f e m p i r i c a l r u l e s ) . I n the presence o f n o n - n e g l i g i b l e v i b r o n i c i n t e r a c t i o n s t h e s e p r o c e d u r e s c a n , a t b e s t , be m i s ­ l e a d i n g and, a t w o r s t , l e a d t o erroneous s t r u c t u r a l c o n c l u s i o n s . Under c o n d i t i o n s o f n o n - n e g l i g i b l e v i b r o n i c i n t e r a c t i o n s i t i s n o t p o s s i b l e , i n g e n e r a l , t o a s s i g n s p e c i f i c CD f e a t u r e s t o t r a n s i t i o n s o f w e l l - d e f i n e d e l e c t r o n i c parentage. I s i t p o s s i b l e t o make c l e a r - c u t s p e c t r a - s t r u c t u r e c o r r e l a ­ t i o n s i n t h e absence o f a complete v i b r o n i c a n a l y s i s o f t h e s p e c t r a and o f t h e systems under study? I t seems l i k e l y t h a t t h e n e t CD i n t e n s i t y ( a n d n e t e l e c t r o n i c r o t a t o r y s t r e n g t h ) a s ­ s o c i a t e d w i t h a l l o f t h e d-ci t r a n s i t i o n s i n a g i v e n c o m p l e x w i l l , i n l a r g e p a r t , be c o n s e r v e d i n t h e p r e s e n c e o f n o n - n e g l i ­ g i b l e v i b r o n i c i n t e r a c t i o n s . T h i s assumes, o f c o u r s e , t h a t v i ­ b r o n i c a l l y induced T i g - T i mixing w i l l g e n e r a l l y be s m a l l . S p e c t r a - s t r u c t u r e c o r r e l a t i o n s s h o u l d b e b a s e d , t h e n , on n e t d_-d_ r o t a t o r y s t r e n g t h s r a t h e r t h a n o n t h e r o t a t o r y s t r e n g t h s a s s o c i a t e d w i t h s p e c i f i c CD bands ( o f u n c e r t a i n o r m i x e d e l e c ­ t r o n i c parentage). 2

1

g

2

a

u

2

a

u

1

g

2

a

u

g

g

2 g

u

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

66

STEREOCHEMISTRY OF TRANSITION METALS

The v i b r o n i c c o u p l i n g t h e o r y p r e s e n t e d i n t h i s s e c t i o n was developed s p e c i f i c a l l y f o r c h i r a l s i x - c o o r d i n a t e systems o f t r i g o n a l d i h e d r a l ( D 3 ) symmetry, a n d i t was a p p l i e d t o t h e d_-d CD s p e c t r a o f C o ( I I I ) c o m p l e x e s . I t i s , o f c o u r s e , s i m i l a r l y a p p l i c a b l e t o t r i g o n a l d i h e d r a l complexes o f any t r a n s i t i o n m e t a l i o n . The f o r m a l i s m a p p r o p r i a t e f o r a n a l y z i n g v i b r o n i c c o u p l i n g i n c h i r a l s y s t e m s o f o t h e r symmetry t y p e s i s s i m i l a r t o t h a t g i v e n h e r e i n a l l g e n e r a l a s p e c t s , a l t h o u g h t h e symmetrydetermined f e a t u r e s w i l l , o f course, be d i f f e r e n t . III.

Examples

A number o f v i b r o n i c CD c a l c u l a t i o n s have b e e n r e p o r t e d i n the l i t e r a t u r e f o r t r a n s i t i o n metal complexes. C a l i g a and R i c h a r d s o n (15) t r e a t e d t h e g e n e r a l c a s e o f p a i r s o f n e a r l y degenerate e l e c t r o n i c s y m m e t r i c v i b r a t i o n a l mode was b a s e d on a n o n - a d i a b a t i c c o u p l i n g r e p r e s e n t a t i o n , a n d t h e i r c a l c u l a t i o n s i n c l u d e d a wide range o f values f o r t h e s p e c t r o s c o p i c and v i b r o n i c c o u p l i n g parameters i n h e r e n t t o t h e i r model. R i c h a r d s o n a n d c o w o r k e r s ( 1 7 , 18) t h e n e x t e n d e d t h i s m o d e l t o t r e a t d i s s y m m e t r i c pseudo t e t r a g o n a l ( n e a r l y D^h) m e t a l complexes i n which three n e a r l y degenerate e l e c t r o n i c t r a n s i t i o n s a r e c o u p l e d v i a pseudo J a h n - T e l l e r i n t e r a c t i o n s i n v o l v i n g e i t h e r two o r t h r e e v i b r a t i o n a l modes o f t h e s y s t e m . A g a i n , m o d e l CD s p e c t r a and v i b r o n i c r o t a t o r y s t r e n g t h s were c a l c u l a t e d f o r a wide v a r i e t y o f parameter s e t s r e f l e c t i n g t h e s p e c t r o s c o p i c p r o p e r t i e s and v i b r o n i c c o u p l i n g d e t a i l s o f t h e system. R i c h a r d s o n and Hilmes (19) i n c o r p o r a t e d v i b r o n i c e f f e c t s i n t o t h e i r t r e a t m e n t o f t h e C u ( I I ) E •> T CD o b s e r v e d i n s i n g l e c r y s t a l s o f ZnSeOi+'ol^O doped w i t h C u ( I I ) i o n s . I n t h e C u ( I I ) :ZnSeOi -6H 0 doped s y s t e m , t h e C u ( I I ) i o n s a r e e a c h c o o r d i n a t e d t o s i x w a t e r m o l e c u l e s w i t h t h e CuOg c l u s t e r s b e i n g v e r y n e a r l y o c t a h e d r a l ( 0 ^ ) . However, t h e e x a c t s i t e symmetry f o r e a c h C u ( I I ) i s C so t h a t t h e E g r o u n d s t a t e i s s p l i t i n t o two n o n d e g e n e r a t e o r b i t a l components a n d t h e T excited state i s s p l i t i n t o t h r e e n o n d e g e n e r a t e o r b i t a l components. I n c a l c u l a t i n g t h e t e m p e r a t u r e d e p e n d e n c e o f t h e n e t ( t o t a l ) CD i n t e n s i t y a s s o c i a t e d w i t h t h e E -> T Cu(II) t r a n s i t i o n , R i c h a r d s o n and H i l m e s ( 1 9 ) e x p l i c i t l y i n c l u d e d pseudo J a h n - T e l l e r t y p e i n t e r a c t i o n s w i t h i n t h e s p l i t E ground s t a t e assuming a s i n g l e p e r t u r b i n g v i b r a t i o n a l mode. The e x p e r i m e n t a l l y o b s e r v e d temp e r a t u r e d e p e n d e n c e o f t h e E •> T CD c o u l d be a c c o u n t e d f o r q u a n t i t a t i v e l y o n l y by t h e i n c l u s i o n o f these v i b r o n i c i n t e r a c t i o n s w i t h i n t h e E ground s t a t e . As was m e n t i o n e d p r e v i o u s l y , two c o m p u t a t i o n a l s t u d i e s o f v i b r o n i c e f f e c t s i n c h i r a l t r i g o n a l s y s t e m s h a v e been r e p o r t e d (18, 2 8 ) . B o t h o f t h e s e s t u d i e s were a d d r e s s e d t o t h e g e n e r a l p r o b l e m o f v i b r o n i c p e r t u r b a t i o n s on t h e c h i r o p t i c a l s p e c t r a o f t r i g o n a l m e t a l c o m p l e x e s . However, t h e c a l c u l a t i o n s r e p o r t e d i n 2

2

g

+

2 g

2

2

2

g

Z

2 g

2

2

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2 g

2

g

2

2

g

2 g

2

g

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

3.

RICHARDSON

Circular

Dichroic

Intensities

67

e a c h study were based on parameter s e t s c l o s e l y r e l a t e d t o t h e A i g -> T i g CD o f C o ( I I I ) c o m p l e x e s s u c h a s C o ( e n ) and C o ( o x ) ~ . I n b o t h s t u d i e s c o n s i d e r a t i o n o f v i b r o n i c i n t e r a c t i o n s was c o n ­ f i n e d t o J a h n - T e l l e r a n d pseudo J a h n - T e l l e r t y p e c o u p l i n g s w i t h i n a t r i g o n a l l y s p l i t excited state o f T i ( o r T ) octahedral paren­ tage. I n e a c h s t u d y o n l y a s i n g l e v i b r a t i o n a l p e r t u r b i n g mode o f e - t y p e t r i g o n a l symmetry was i n c l u d e d i n t h e c o m p u t a t i o n a l m o d e l . The g e n e r a l a s p e c t s o f t h e v i b r o n i c c o u p l i n g m o d e l s u s e d i n t h e s e two s t u d i e s were i d e n t i c a l t o t h o s e d e s c r i b e d i n S e c ­ t i o n s II.Α., II.Β., a n d U . C . o f t h e p r e s e n t p a p e r e x c e p t t h a t o n l y a s i n g l e p e r t u r b i n g mode was c o n s i d e r e d . In the earlier study (by R i c h a r d s o n , e t a l . ( 1 6 ) ) , o n l y l i n e a r J a h n - T e l l e r and pseudo J a h n - T e l l e r c o u p l i n g s were i n c l u d e d i n t h e n u m e r i c a l calculations. I n t h e more r e c e n t s t u d y b y Z g i e r s k i a n d Pawlikowski (28), quadrati Jahn-Telle coupling als i n cluded i n the c a l c u l a t i o n s To d e m o n s t r a t e ho simples typ coupling may i n f l u e n c e t h e A i ( A i ) - > T i ( A + E) CD o f a c h i r a l t r i g o n a l d i h e d r a l ( D 3 ) m e t a l c o m p l e x , we s h a l l p r e s e n t some r e s u l t s o b ­ t a i n e d b y a c t u a l c o m p u t a t i o n s . We s h a l l assume a s i n g l e p e r ­ t u r b i n g v i b r a t i o n a l mode o f e - t y p e t r i g o n a l symmetry a n d o f f u n d a m e n t a l f r e q u e n c y ω*ρ ( e x p r e s s e d i n c m ) . We s h a l l d e n o t e the t r i g o n a l s p l i t t i n g energy by Δ (cm" ) = ( E ^ - E ) / h c . Only the l i n e a r J T and PJT c o u p l i n g terms w i l l be i n c l u d e d i n t h e c o m p u t a t i o n a l m o d e l . The l i n e a r J T c o u p l i n g c o n s t a n t , k, w i l l be d e f i n e d a c c o r d i n g t o E q . ( 2 8 ) , a n d t h e l i n e a r P J T c o u p l i n g c o n s t a n t , γ, w i l l b e d e f i n e d a c c o r d i n g t o E q . ( 2 7 ) . The r a t i o of pure e l e c t r o n i c r o t a t o r y s t r e n g t h s w i l l be t a k e n i n a l l c a s e s t o b e R ^ : R Î : R o ^ = 1 : 1 : - l , where +

3

g

g

g

3

2 g

2

-1

1

0

E

e )

R^

e)

R^

e)

e )

e

= Im(Ai|y|E )-(E |m|Ai)

(64a)

= Im(Ai|y|A ).(A |m|Ai).

(64b)

+

±

and, 2

2

The v i b r o n i c wave f u n c t i o n s f o r t h e ( A + E ) * e c o u p l e d s t a t e s o f o u r m o d e l s y s t e m may be w r i t t e n a s 2

Ψ

( θ )

( θ )

= Ψ° Σ Α γ J ° ν ' Λ ^ ν

+

ψ

}

+ Ψ° +

( + )

Σ Α v ^ * V

X

ν

( + )

v

*

excited

(

6

5

)

°- γ 5 ν 1 4 > ν,£ where t h e n o t a t i o n i s s i m i l a r t o t h a t u s e d i n E q . (32) o f S e c ­ t i o n U . C . e x c e p t t h a t h e r e we a r e c o n s i d e r i n g j u s t o n e c o u p l i n g mode ( a n e - t y p e t r i g o n a l mode). The e x p r e s s i o n s a n d p r o c e d u r e s o u t l i n e d i n R e f . _16 a n d i n S e c t i o n U . C . o f t h e p r e s e n t p a p e r may now b e u s e d t o c a l c u l a t e t h e r o t a t o r y s t r e n g t h s o f t r a n s i ­ t i o n s o r i g i n a t i n g i n t h e ground v i b r a t i o n a l l e v e l o f t h e ground e l e c t r o n i c s t a t e ( A i ) and t e r m i n a t i n g i n the j v i b r o n i c l e v e l s J

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF

68

TRANSITION METALS

d e r i v e d from the (A + E)*e coupled e x c i t e d s t a t e s . I n a l l o f t h e c a l c u l a t i o n s r e p o r t e d h e r e we h a v e assumed i d e n t i c a l o s c i l l a t o r f r e q u e n c i e s f o r t h e p e r t u r b i n g mode i n t h e g r o u n d and e x c i t e d e l e c t r o n i c s t a t e s . Furthermore, i n con­ s t r u c t i n g t h e v i b r o n i c wave f u n c t i o n s g i v e n by Eq. (65) we h a v e employed a harmonic o s c i l l a t o r b a s i s s e t w h i c h i n c l u d e d a l l f u n c t i o n s w i t h ν £ 10. A s s u m i n g G a u s s i a n - s h a p e d v i b r o n i c CD b a n d s , t h e CD s p e c t r u m i n t h e v i c i n i t y o f t h e A i ( A i ) - * T i g ( A + E) t r a n s i t i o n o f o u r m o d e l s y s t e m may be s y n t h e s i z e d a c c o r d i n g t o 2

g

2

2

Δε(ϋΓ) = C Σ (ÔT.R./6) exp [-(ω j 3

e

2

- ω.) /ό ]

3

(66)

3

e

1

where Δε = L - R, uTj i s t h e t r a n s i t i o n f r e q u e n c y ( i n cm" ) for a t r a n s i t i o n to the j - t rotatory strength define w i d t h p a r a m e t e r ( e x p r e s s e d i n cm" u n i t s ) , and C i s a c o n s t a n t f a c t o r whose n u m e r i c a l v a l u e d e p e n d s , i n p a r t , on t h e u n i t s chosen f o r R j . F o r our p r e s e n t p u r p o s e s , i t w i l l be more c o n ­ v e n i e n t t o e x p r e s s Eq. (66) as f o l l o w s : 1

Δε(Ω)

= C Σ (ô£ j 1

2

2

+ n . ) ( R . / 6 ) exp[-(Ω - Ω . ) / 6 ] , J J 3 - 1

(67)

where Ω (cm"" ) = ÔT - ôJg, Ω. ( c m ) = ôTj = and = (Eg- I^O/hc. The new f r e q u e n c y v a r i a b l e ^ Ω, i s d e f i n e d t o be z e r o a t t h e resonance frequency of the unperturbed, pure e l e c t r o n i c t r a n s i ­ t i o n Αχ •> E. I n t h e c a l c u l a t i o n s r e p o r t e d h e r e we have s e t aig = 20,000 cm" . CD s p e c t r a f o r t h e Αχ + ( A + E ) * e t r a n s i t i o n s o f o u r t r i g o n a l d i h e d r a l (D3) m o d e l s y s t e m w e r e c a l c u l a t e d f o r a number o f p a r a m e t e r s e t s i n w h i c h t h e v a l u e s o f Δ, γ, and k were v a r i e d . These s p e c t r a a r e d i s p l a y e d i n F i g u r e s 2-5. In these f i g u r e s , t h e CD i n t e n s i t y s c a l e (Δε) i s g i v e n i n a r b i t r a r y u n i t s and t h e frequency s c a l e i s e x p r e s s e d i n terms of Ω(cm" ). The c a l c u l a t e d s p e c t r a shown i n F i g u r e s 2-5 d e m o n s t r a t e t h e p r o f o u n d i n f l u e n c e w h i c h v i b r o n i c c o u p l i n g may have on b o t h t h e q u a l i t a t i v e and q u a n t i t a t i v e a s p e c t s o f t h e CD o b s e r v e d w i t h ­ i n t h e Αχ -> A + Ε t r a n s i t i o n r e g i o n . Although the net ( t o t a l ) r o t a t o r y s t r e n g t h and CD i n t e n s i t y r e m a i n i n v a r i a n t t o t h e v a l u e s o f Δ, γ, and k, t h e d i s t r i b u t i o n o f CD i n t e n s i t y and t h e s i g n p a t t e r n s w i t h i n t h e CD s p e c t r u m a r e shown t o be v e r y s e n s i t i v e t o t h e s e p a r a m e t e r s . The v a l u e s o f Δ, γ, and k w i l l , o f c o u r s e , be d e t e r m i n e d t o some e x t e n t b y - t h o s e s t e r e o c h e m i c a l and e l e c ­ t r o n i c s t r u c t u r a l f e a t u r e s o f a c o m p l e x w h i c h one w i s h e s t o e l u c i d a t e v i a CD s p e c t r a l s t u d i e s . However, i n n e a r l y a l l s p e c t r a - s t r u c t u r e c o r r e l a t i o n studies reported to date, the i n ­ f l u e n c e o f v i b r o n i c c o u p l i n g on t h e CD s p e c t r a l f e a t u r e s has been i g n o r e d o r n e g l e c t e d . The s p e c t r a shown i n F i g u r e s 2-5 r e ­ f l e c t o n l y the simplest k i n d s of v i b r o n i c i n t e r a c t i o n s ( i n t r a ­ s t a t e JT and PJT c o u p l i n g s ) w h i c h may i n f l u e n c e t h e d e t a i l s o f 1

2

1

2

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

3.

RICHARDSON

Circular

Dichroic

1

1

Intensities

I

-600

I

0

I

69

I

I

600 iKcrrT )

I

1200

I

.

1800

1

Figure 2. Calculated CD spectra for the A, -» (A + E)*e transitions using the parameter sets R :R. :R = 1:1:—1 (in arbitrary units), Δ = ώ = 300 cm' , k = 0 and y = 0 ( ), y = 0.5 ώ ( ), and y = 1.0 ω ( ). Δε is ex­ pressed in arbitrary units and δ was taken to be 0.6 ω . 2

+

(e)

(e)

(e)

0

1

ρ

ρ

ρ

ρ

ÎMcm" ) 1

Figure 3. Calculated CD spectra for the A -> (A + E)*e transitions using the parameter sets R :R. :R = 1:1:—1 (in arbitrary units), Δ = k = ω = 300 cm , and y = 0 ( ), y = 0.5ω ( ), and y = 1.0 ω ( ). Acts expressed in arbitrary units and δ was taken to be 0.6 ω . t

(e) +

1

(e)

0

2

(e)

ρ

p

ρ

ρ

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

70

Figure 4. Calculated CD spectra for the A -> (A + E)*e transitions using the parameter sets R :R_ :R/ = 1:1:—1 (in arbitrary units), Δ = y = ( ) and k = 1.25 m ( ). Δε is expressed in arbitrary units and δ was taken to be 0.6 w . t

fe;

fe,

2

e;

p

+

1

p

p

p

Figure 5. Calculated CD spectra for the A i -» (A + E)*e transitions using the parameter sets R/ ;R. :R/ = 1:1:—1 (in arbitrary units), y = k = ω = 300 cm , and Δ = 0.5 τ> ( ), Δ = 1.5 ΊΠ ( ), and Δ = 2.5 m (- · - -). Δε is expressed in arbitrary units and δ was taken to be 0.6 m . t

eJ

fe:)

eJ

ρ

1

ρ

ρ

p

p

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

3.

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Circular

Dichroic

Intensities

71

t h e CD s p e c t r a a s s o c i a t e d w i t h c h i r a l t r a n s i t i o n m e t a l c o m p l e x e s . M u l t i p l e mode c o u p l i n g a n d i n t e r - s t a t e c o u p l i n g s w i l l , i n general, introduce a d d i t i o n a l complications. IV.

Conclusions

Modest v i b r o n i c c o u p l i n g s t r e n g t h s i n m e t a l c o m p l e x e s may: (1) l e a d t o s i g n i f i c a n t m i x i n g s o f " f i x e d - n u c l e i " e l e c t r o n i c states; (2) a l t e r t h e e n e r g y l e v e l s p a c i n g s a n d o r d e r i n g s w i t h in spectroscopic state manifolds; (3) c a u s e s i g n i f i c a n t d i s t o r t i o n s w i t h i n degenerate o r n e a r l y degenerate spectroscopic s t a t e s ; and, (4) p r e c l u d e t h e a s s i g n m e n t o f t r a n s i t i o n s t o i n i t i a l and f i n a l s t a t e s o f w e l l - d e f i n e d e l e c t r o n i c i d e n t i t i e s (quantum n u m b e r s ) . The c o n s e q u e n c e s o f n o n - n e g l i g i b l e v i b r o n i c c o u p l i n g i n c h i r a l m e t a l c o m p l e x e s may b e t o : (1) p r o d u c e s i g n i ficant alterations in C sign patterns, r e l a t i v (2) r e q u i r e t h a t v i b r o n i c c o u p l i n g mechanisms a n d s t r e n g t h s be taken i n t o account along w i t h stereochemical s t r u c t u r a l f a c t o r s in constructing detailed spectra-structure correlation rules; and, (3) a b r o g a t e s e c t o r ( o r r e g i o n a l ) r u l e s b a s e d o n t h e CD o b s e r v e d f o r s p e c i f i c bands o r s p e c t r a l f e a t u r e s . V i b r o n i c c o u p l i n g e f f e c t s on the c h i r o p t i c a l s p e c t r a o f o p t i c a l l y a c t i v e m e t a l complexes tend t o obscure the i n h e r e n t r e l a t i o n s h i p s between t h e CD o b s e r v a b l e s a n d s t r u c t u r a l f e a t u r e s such a s a b s o l u t e c o n f i g u r a t i o n , l i g a n d c o n f o r m a t i o n , and l i g a n d spatial distributions. For t h i s reason, s p e c t r a - s t r u c t u r e r e l a t i o n s h i p s based on the " f i x e d - n u c l e i " approximation and on the a s s u m p t i o n o f w e l l d e f i n e d " e l e c t r o n i c " t r a n s i t i o n s must b e a p plied with considerable caution. A great wealth of spectrosc o p i c a l l y a n d s t r u c t u r a l l y i m p o r t a n t i n f o r m a t i o n may be o b t a i n e d f r o m d e t a i l e d v i b r o n i c a n a l y s e s o f CD s p e c t r a . However, v e r y few a n a l y s e s o f t h i s s o r t h a v e been r e p o r t e d t o d a t e . Acknowledgments T h i s w o r k was s u p p o r t e d b y 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 ( G r a n t CHE77-02150) a n d b y t h e C a m i l l e a n d H e n r y D r e y f u s F o u n d a t i o n ( t h r o u g h a T e a c h e r - S c h o l a r Award t o F . R . ) .

Literature Cited 1. 2. 3.

"Alfred Werner, 1866-1919", Hevl. Chim. Acta, 1966, Com­ memoration Volume IX, ICCC, Zurich. Jaeger, F.M., "Spatical Arrangements of Atomic Systems and Optical Activity", George Fisher Baker Lectures, Vol. 7, Cornell University, McGraw-Hill, New York, 1930. Mathieu, J.P., "Les Theories Moleculaires du Pouvoir Rotatoire Naturel", Gauthier-Villars, Paris, 1946.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

72 4. 5. 6. 7. 8.

9. 10. 11.

12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

STEREOCHEMISTRY OF TRANSITION METALS

Kuhn, W. and Bein, Κ., Z. Physik. Chem., 1934, B24, 335. Kuhn, W. and Bein, Κ., Z. Anorg. Allg. Chem., 1934, 216, 321. Kuhn, W., Naturwissenschaften, 1938, 19, 289. Saito, Y.; Nakatsu, K.; Shiro, M.; and Kuraya, Η., Acta Crystallogr., 1955, 8, 729. For the accepted nomenclature and notation regarding ab­ solute configuration designations for coordination com­ pounds, see: (a) "I.U.P.A.C. Information Bulletin No. 33", 1968, p. 68; (b) Inorg. Chem., 1970, 9, 1. Moffitt, W., J . Chem. Phys., 1956, 25, 1189. Condon, E.U.; Alter, W.; and Eyring, Η., J . Chem. Phys., 1937, 5, 753. Mason, S.F. in "Fundamental Aspects and Recent Developments in Optical Rotatory Dispersion and Circular Dichroism", edited by J . Ciardell Ltd., New York, 1973 Richardson, F.S., Chem. Rev., 1979, 79, 17. See, for example: (a) Ham, F.S., Phys. Rev., 1965, A138, 1727; (b) Sturge, M.D., Solid State Physics, 1967, 20, 91; (c) Stephens, P . J . , J . Chem. Phys., 1969, 51, 1995. Denning, R.G., Chem. Comm., 1967, 120. Caliga, D. and Richardson, F.S., Mol. Phys., 1974, 28, 1145. Richardson, F.S.; Caliga, D.; Hilmes, G.; and Jenkins, J., Mol. Phys., 1975, 30, 257. Richardson, F.S.; Hilmes, G.; and Jenkins, J., Theoret. Chim. Acta (Berl.), 1975, 39, 75. Hilmes, G.; Caliga. D.; and Richardson, F.S., Chem. Phys., 1976, 13, 203. Richardson, F.S. and Hilmes, G., Mol. Phys., 1975, 30, 237. Herzberg, G. and Teller, Ε., Z. Physik. Chem., 1933, B21, 410. Weigang, O.E., J . Chem. Phys., 1965, 43, 3609. Harnung, S.E.; Ong, E.C.; and Weigang, O.E., J . Chem. Phys., 1971, 55, 5711. Weigang, O.E. and Ong, E . C . , Tetrahedron, 1974, 30, 1783. Harding, M.J., J.C.S. Faraday II, 1972, 68, 234. Bray, R.G.; Ferguson, J.; and Hawkins, C . J . , Aust. J . Chem., 1969, 22, 2091. Perrin, M.H. and Gouterman, Μ., J . Chem. Phys., 1967, 46, 1019. See, for example: (a) Englman, R., "The Jahn-Teller Effect in Molecules and Crystals", Wiley-Interscience, New York, 1972, Chapter 3; (b) references 13 and 16 cited above. Zgierski, M.Z. and Pawlikowski, Μ., J . Chem. Phys., 1979, 70, 3444.

RECEIVED September 13, 1979.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

4

Stereochemical Correlations i n the Circular D i c h r o i s m of d-d

and Charge-Transfer Transitions: Applications to

Tris(bidentate) Complexes PIETER E. SCHIPPER Department of Theoretical Chemistry, The University of Sydney, NSW 2006, Australia The problem of distinguishin not peculiar to Alice(1) ations of chiral metal complexes, apart from being of basic chemical significance, has proved an intriguing challenge to a range of techniques. Circular dichroism in particular, because of the ease of measurement, has enjoyed considerable popularity as a stereochemical probe, albeit with incommensurate success. The major difficulty lies in the interpretation of the CD spectrum in terms of some theoretical model of the complex, so that the reliability of the technique is determined largely by the validity or relevance of the theoretical model. Empirical correlation rules and a complete molecular orbital approach may be considered to constitute the two extremes of the model approach. The many empirical rules (see, e.g. discussions and references in Hawkins' book (2)) cannot be considered modelindependent in that they make the implicit assumption that the complexes to which they apply have a similarity of electronic structure and subsequently of CD spectra; the rigorous definition of the nature of the "similarity" would ultimately lead to a welldefined model. Such rules thus require a minimum specification of the model, but also result in a limited predictive power. The other extreme, namely the molecular orbital approach, requires a detailed specification of the model, and the predictive power is restricted theoretically only by computing costs and the transience of the human species. The most fruitful approach steers the middle course of establishing simple theoretical models of the complex in terms of suitable transferable parameters which may be calculated, or determined empirically by some other technique which is, in itself, independent of the chirality of the complex. The empirical rules may then be assessed theoretically, and the range of their valid­ ity defined explicitly in terms of such parameters. The critical discussion of such a simple, unified model in terms of transfer­ able, well-defined parameters is the main aim of this paper.

0-8412-0538-8/80/47-119-073$05.00/0 © 1980 American Chemical Society In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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Separable Chromophore Models. The normal absorption s p e c t r a o f most metal complexes conveniently comprise three d i s t i n c t regions: the d-d r e g i o n , derived from t r a n s i t i o n s between perturbed metal d s t a t e s ; the charge t r a n s f e r r e g i o n , comprising t r a n s i t i o n s i n v o l v i n g an e l e c t r o n t r a n s f e r from a predominantly metal d s t a t e to some l i n e a r combination o f l i g a t i n g atom s t a t e s (or v i c e v e r s a ) ; and f i n a l l y , those bands (the chelate region) l a r g e l y d e r i v e d from the f r e e l i g a n d t r a n s i t i o n s . Each such s p e c t r a l region may be a s c r i b e d t o emanating from a p a r t i c u l a r s e c t i o n o f the o v e r a l l complex, c a l l e d a chromophore, such t h a t a good d e s c r i p t i o n ( i n t e n s i t y and energies) o f the absorption spectrum i n that region may be obtained by c o n s i d e r i n g wave f u n c t i o n s l o c a l i z e d wholly on that chromophore. By d e f i n i n g the chromophores i n t h i s way, i t becomes p o s s i b l e t o parametrize such q u a n t i t i e s as e l e c t r i c t r a n s i t i o n moments and energies unambiguousl normal absorption spectru o f n o n - i n t e r a c t i n g chromophores. Such a d e f i n i t i o n i s a c r u c i a l assumption i n the approach t h a t follows and, somewhat s u r p r i s i n g l y , i s not commonly e x p l o i t e d i n a l t e r n a t i v e approaches. Consider, f o r example, the d-d transitions. I f the metal i o n i s taken as the chromophore, i t i s g e n e r a l l y impossible to get r e a l i s t i c energies and i n t e n s i t i e s f o r the normal absorption because o f the importance of the metall i g a t i n g atom overlap. Thus i t i s u n l i k e l y t o provide a s a t i s ­ f a c t o r y b a s i s f o r a CD model. On the other hand, i f the chromo­ phore i s taken to i n c l u d e both the metal and the l i g a t i n g atom system, both the d-d and charge t r a n s f e r regions i n the normal absorption spectrum may be e x p l o i t e d to parametrize the chromo­ phore as much as p o s s i b l e . The remainder o f the complex (the chelate system) then c o n s t i t u t e s another chromophoric system. The importance o f t h i s t o the development o f a CD model a r i s e s i n the f o l l o w i n g way. Consider the complex to comprise two chromophores, A and B, such that A i s a c h i r a l , and Β i s the c h i r a l p e r t u r b e r . The CD at the t r a n s i t i o n s o f A a r i s e s through simple c o u p l i n g o f A and Β as d e s c r i b e d by p e r t u r b a t i o n theory, y i e l d i n g expressions c o n t a i n i n g the t r a n s i t i o n moments and energies o f the unperturbed chromophore. The u t i l i t y o f the p e r t u r b a t i o n theory t h e r e f o r e depends c r i t i c a l l y on the d e f i n i t i o n of the unperturbed chromophores. The dominant terms i n the p e r t u r b a t i o n expansion may then be extracted unambiguously. Thus the aim o f a good CD model i s t o d e f i n e the chromophores i n such a way t h a t , i d e a l l y , (i) a s i n g l e p e r t u r b a t i o n term dominates, and ( i i ) the q u a n t i t i e s appearing i n t h i s term may be c a l c u l a t e d r e a l i s t i c a l l y o r determined e m p i r i c a l l y (e.g. from the normal absorption spectrum). Returning t o the d-d t r a n s i t i o n s as an example, the choice of a metal i o n chromophore leads t o a number o f problems. F i r s t l y , most o f the p e r t u r b a t i o n terms w i l l i n i t i a l l y e f f e c t adjustments

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simply t o give c l o s e r agreement with the absorption spectrum. Even t h i s w i l l be done inadequately i f the d-d s p e c t r a depend d i r e c t l y on the m e t a l - l i g a n d bonding, f o r then the assumption o f a separable metal i o n chromophore i s , i n i t s e l f , a bad one. Secondly, those p e r t u r b a t i o n terms d e s c r i b i n g the CD w i l l be u n n e c e s s a r i l y complex, and i n f a c t those terms a r i s i n g from metall i g a n d bonding ( i n v o l v i n g , f o r example, charge t r a n s f e r states) w i l l not appear i n the expressions a t a l l , because they do not appear i n the chromophore d e f i n i t i o n s . The p e r t u r b a t i o n theory i n such a case has t o patch up the bad chromophore, before even t r y i n g t o i s o l a t e the l i m i t e d CD c o n t r i b u t i o n s t h a t can a r i s e f o r t h a t chromophore. The choice o f the m e t a l - l i g a t i n g atom chromophore, on the other hand, leaves the p e r t u r b a t i o n theory f r e e t o i s o l a t e the CD d i r e c t l y , as the unperturbed chromophore already should give an absorption spectrum. The a r t o f developing a good CD model thus l i e s t o t a l l y i n the chromophore d e f i n i t i o n s , a f a c t o r u s u a l l y swamped by complex p e r t u r b a t i o n expressions. The importance o f t h i s p o i n t cannot be o v e r s t r e s s e d , as the f a i l u r e o f some CD models stems u l t i m a t e l y from an o v e r s i m p l i f i e d chromophore d e f i n i t i o n , a d e f i c i e n c y t h a t , i n most cases, cannot be remedied by any subsequent t h e o r e t i c a l refinements (e.g. going t o higher order i n p e r t u r b a t i o n theory). No attempt w i l l be made i n t h i s paper t o review the l a r g e body o f l i t e r a t u r e on CD models o f complexes, and the reader i s e s p e c i a l l y r e f e r r e d to a recent, comprehensive review by Richardson {3) . The d i s c u s s i o n here w i l l be r e s t r i c t e d mainly t o the approaches developed r e c e n t l y by t h i s author f o r the CD o f charge t r a n s f e r (4) and d-d t r a n s i t i o n s (50, and the s p e c i f i c a p p l i c a t i o n to t r i s ( b i d e n t a t e s ) and determination o f t h e i r absolute c o n f i g u r a t i o n . Chromophore D e f i n i t i o n s :

The Model

The complexes which w i l l be considered here are those o f D^ symmetry, such that the chelates and l i g a t i n g atoms are separa t e l y i d e n t i c a l . A p p l i c a t i o n s t o lower symmetry complexes f o l l o w s i m i l a r l i n e s . The chromophores f o r such systems may be d e f i n e d i n the f o l l o w i n g way. The A c h i r a l M e t a l - L i g a t i n g Atom Chromophore (A). It is u n r e a l i s t i c t o ignore m e t a l - l i g a t i n g atom bonding i n the d e s c r i p t i o n o f e i t h e r the d-d o r charge t r a n s f e r (CT) t r a n s i t i o n s , so t h a t both o f these are considered t o a r i s e from a s i n g l e a c h i r a l chromophore encapsulating the metal i o n and the l i g a t i n g atoms. The exact s p e c i f i c a t i o n o f the chromophore proceeds through s p e c i f y i n g the t r a n s i t i o n moments and energies o f the chromophore t r a n s i t i o n s . In f a c t , we w i l l be r e p r e s e n t i n g the e n t i r e complex by two such moment and energy r e p r e s e n t a t i o n s , each r e p r e s e n t a t i o n c o n s t i t u t i n g a s i n g l e chromophore (A o r B ) .

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I t should be noted here t h a t t h i s r e p r e s e n t a t i o n i s q u i t e d i s t i n c t from the s p e c i f i c a t i o n o f the wave f u n c t i o n s . I t may be p o s s i b l e , f o r example, t o get a good estimate o f a t r a n s i t i o n moment and energy from the normal absorption spectrum, without ever e x p l i c ­ i t l y d e f i n i n g a wave f u n c t i o n ; on the other hand, i n some other cases the t r a n s i t i o n moment may be c a l c u l a t e d from an approximate wave f u n c t i o n , i n which case the moment i t s e l f i s only as good as the wave f u n c t i o n employed. T h i s may, however, be viewed simply as a p r a c t i c a l d e t a i l o f how b e s t t o put values t o the v a r i a b l e s i n the moment/energy r e p r e s e n t a t i o n . Any mention o f wave functions i n t h i s paper should be i n t e r p r e t e d i n t h i s context (For chemists who are weaned on wave f u n c t i o n s , t h i s i s not always an easy task.) In order t o d e f i n e the moments appearing i n the p e r t u r b a t i o n expansions, we denote the ground s t a t e o f A by ψ^, and l e t Ψ 'Ψ represent d c o n f i g u r a t i o n s , so tha t r a n s i t i o n with r e s p e c t i v e t r a n s i t i o n energies ( r e l a t i v e t o the ground state) ε^, e^. Charge t r a n s f e r t r a n s i t i o n s are g e n e r a l l y e l e c t r i c d i p o l e allowed, with a corresponding e l e c t r i c t r a n s i t i o n moment Α

Α

where i s the e l e c t r i c d i p o l e operator centred a t the symmetry (metal! o r i g i n . Under D , t h i s moment w i l l be e i t h e r z - p o l a r i z e d or x , y - p o l a r i z e d , so t h a t the r e p r e s e n t a t i o n by the t o t a l i t y o f CT e l e c t r i c moments i s a c h i r a l , and may be taken t o have symmetry without any l o s s o f g e n e r a l i t y . (This i l l u s t r a t e s an important f e a t u r e o f moment r e p r e s e n t a t i o n s ; they may be o f higher symmetry than the system they represent. This i s because they are an approximation t o the system, and an exact represent­ a t i o n r e q u i r e s an i n f i n i t e moment (multipolar) r e p r e s e n t a t i o n . They can never, however, have a lower symmetry.) The d-d t r a n s i t i o n s may be e i t h e r magnetic d i p o l e allowed or forbidden, b u t are g e n e r a l l y (by symmetry) e l e c t r i c d i p o l e forbidden. T h i s may seem c o n t r a d i c t o r y as they appear i n the normal absorption spectrum with a f i n i t e but small i n t e n s i t y , but we s h a l l consider t h i s i n d e t a i l l a t e r . Thus a t t h i s stage, the d-d t r a n s i t i o n s are c h a r a c t e r i z e d s o l e l y by t h e i r magnetic t r a n s ­ i t i o n moments do M

^ ιdι =

ι ,o.

V ? A ' V

where m^ i s the magnetic d i p o l e operator origin. In a d d i t i o n , the moments

(

2

)

d e f i n e d a t the metal

connecting the e x c i t e d CT and d c o n f i g u r a t i o n s appear i n the p e r t u r b a t i o n expansion. A l l these moments under again are

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

4.

SCHIPPER

Tris(bidentate)

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77

e i t h e r z- o r x , y - p o l a r i z e d , c o n s t i t u t e an a c h i r a l r e p r e s e n t a t i o n , and may be taken without l o s s o f g e n e r a l i t y to have covering symmetry. The above r e p r e s e n t a t i o n i s s u f f i c i e n t to d e s c r i b e complete­ l y the CT absorption. I t might be argued, however, t h a t the d-d absorption i n t e n s i t i e s bear no r e l a t i o n to the moments d e f i n e d above - and t h i s i s p e r f e c t l y t r u e . That t h i s does not i n v a l ­ i d a t e the r e p r e s e n t a t i o n f o r the d-d region f o l l o w s from n o t i n g t h a t absorption i n t e n s i t i e s are i n v a r i a b l y e l e c t r i c d i p o l e i n character, whereas the d-d CD predominantly a r i s e s from magnetic t r a n s i t i o n moments which are symmetry allowed. The d-d t r a n s ­ i t i o n s gain i n t e n s i t y i n normal absorption through such mechan­ isms as s p i n - o r b i t and v i b r o n i c c o u p l i n g . Taking the l a t t e r as an example, i t w i l l be mainly the m e t a l - l i g a t i n g atom system vibrations that contro be accommodated withou reducing i t s symmetry from D ^ . F o r magnetic d i p o l e forbidden t r a n s i t i o n s , the moment r e p r e s e n t a t i o n i s then simply i n terms of the e f f e c t i v e d i p o l e moment Δμ as determined from the absorption spectrum. F o r magnetic d i p o l e allowed d-d t r a n s ­ i t i o n s , t h i s e l e c t r i c moment may be neglected, as discussed later. The P e r t u r b i n g Chelate Chromophore System (B). The chelate system Β may be considered e f f e c t i v e l y as three separate chelate chromophores indexed by I such that I = 1,2,3. Each i n d i v i d u a l chelate i s considered t o have a gound s t a t e φ and two e l e c t r i c d i p o l e allowed (from the ground s t a t e ) e x c i t e d s t a t e s φ^, φ^, so t h a t the moment r e p r e s e n t a t i o n becomes or I OS I rs I

(4)

(6)

ι

ι

where i s the e l e c t r i c d i p o l e operator d e f i n e d a t the o r i g i n of the~chelate (which i s taken t o l i e on the C^ a x i s o f I a t the p o i n t o f the v e c t o r r from the metal o r i g i n o f A). The ~MI t r a n s i t i o n energies o f the e x c i t e d chelate s t a t e s are ε , ε ι r' s respectively. T h i s s p e c i f i c a t i o n o f the B-system i s t o t a l l y adequate f o r the CD work that f o l l o w s , and n e g l e c t s any c o u p l i n g between the chelates themselves. To e x p l a i n adequately the energy s p l i t t i n g (though not the i n t e n s i t i e s ) o f the chelate system i n the complex, i t i s necessary t o consider the i n t e r c h e l a t e c o u p l i n g , but t h i s does not n e c e s s i t a t e extending the moment r e p r e s e n t a t i o n d e f i n i n g the chromophoric system. I t w i l l be shown l a t e r that the p e r t u r b a t i o n o f the A chromophore by the Β system i s independent o f t h i s c o u p l i n g , so t h a t i t w i l l not be considered

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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STEREOCHEMISTRY OF TRANSITION METALS

further. Assumptions o f the Model. There are c e r t a i n assumptions inherent i n these chromophore d e f i n i t i o n s which e x p l i c i t l y exclude c e r t a i n CD mechanisms. For example, the higher order t r a n s i t i o n moments of the metal ion are neglected t o t a l l y , a u t o m a t i c a l l y excluding any c o n t r i b u t i o n s t o the d-d CD from f i r s t order p e r t urbation theory such as discussed i n the dynamic coupling approach o f Mason (6) , and the s t a t i c c o u p l i n g approach i n i t i a t e d by M o f f i t t Ç 7 ) and extended by Richardson (8) . Furthermore, the chelate system i s represented s o l e l y by i t s t r a n s i t i o n moments, and any c h i r a l e f f e c t s of the s t a t i c charge d i s t r i b u t i o n are neglected. The j u s t i f i c a t i o n f o r both these assumptions has i t s source i n the high symmetry of the chromophores, and i s discussed i n d e t a i l elsewhere With regard to th only e l e c t r i c moments at a s i n g l e o r i g i n assumes that the CT chromophore i s a c h i r a l . This assumption e x p l i c i t l y excludes the mechanism d i s c u s s e d by Mason ( 1 0 ) , f o r reasons discussed i n the next s e c t i o n . This i s not to imply, however, that the l i g a t i n g atoms are t o t a l l y i n s e n s i t i v e to the chelate s t r u c t u r e ; i n f a c t , the chelates perturb the l i g a t i n g atoms s u f f i c i e n t l y to l e a d to a unique z - d i r e c t i o n (C^ axis) o f the a c h i r a l chromophore, manifesting as an appreciable energy s p l i t t i n g o f the z- and x,yp o l a r i z e d t r a n s i t i o n s (which would be degenerate i f the a c h i r a l chromophore were of o c t a h e d r a l symmetry). T h i s e f f e c t on the l i g a t i n g atoms i s not, however, p r i m a r i l y due to the c h i r a l i t y of the chelate system, but to i t s having a unique C^ a x i s . T h i s i s of course r e t a i n e d i n the D ^ symmetry of A. T h i s r a t h e r exhaustive d e f i n i t i o n o f the chromophores should i l l u s t r a t e the importance of a d e t a i l e d d e s c r i p t i o n of the model, because a l l the assumptions of the CD model to be described have already been made. Any d e f i c i e n c i e s i n i t s d e s c r i p t i o n of r e a l systems may be t r a c e d d i r e c t l y back to the preceding d i s c u s s i o n . We now turn to d i s c u s s i n g those p e r t u r b a t i o n terms which l e a d to the CD of the CT and d-d t r a n s i t i o n s of such chromophores. ( 4 _ , 9

C i r c u l a r Dichroism

o f Charge T r a n s f e r T r a n s i t i o n s

The CD o f the CT t r a n s i t i o n s has a t t r a c t e d l i t t l e d i r e c t i n t e r e s t i n the l i t e r a t u r e . Mason (10) has p o s t u l a t e d t h a t i t a r i s e s from an e x c i t o n mechanism i n which the CT t r a n s i t i o n i s broken up i n t o three degenerate o s c i l l a t o r s or chromophores with d i f f e r e n t o r i g i n s . Such a procedure i s u n j u s t i f i e d quantum mechanically, as each chromophore must have n e g l i g i b l e e l e c t r o n exchange with any other chromophore; c o n s i d e r i n g that a common metal d s t a t e i s i n v o l v e d i n the d e f i n i t i o n of each such chromophore or o s c i l l a t o r , such a model i s , despite i t s p i c t o r i a l appeal, t h e o r e t i c a l l y i n c o n s i s t e n t . Mason supports h i s model by n o t i n g t h a t the CT CD e x h i b i t s the e x c i t o n s t r u c t u r e c h a r a c t e r i s t i c of

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

4.

Tris(bidentate)

scHippER

Complexes

79

the chelate CD, which i s determined i n an analogous way. It will be shown here, however, t h a t the experimental r e s u l t s ( i n c l u d i n g the c h a r a c t e r i s t i c e x c i t o n s t r u c t u r e o f the CT bands) are r e a d i l y explained by a mechanism t h a t a r i s e s p u r e l y from the e l e c t r i c moments of the CT s t a t e s , defined a t t h e n a t u r a l symmetry o r i g i n and of D , symmetry, due to p e r t u r b a t i o n by the c h i r a l chelate system {bf. In d e s c r i b i n g the CT CD, only the moments y and the corresponding t r a n s i t i o n energies ε o f the chromophore A need be considered. Under D ^ symmetry, these moments are e i t h e r z - p o l a r i z e d ( f o r t r a n s i t i o n s of A^ symmetry) or x , y - p o l a r i z e d (for transitions). The p o l a r i z a t i o n and magnitude of the moments, and the t r a n s i t i o n energies, are a l l i n p r i n c i p l e access­ i b l e through normal absorption experiments, so that i n t h i s instance the chromophor The CT t r a n s i t i o n s becom each o f the chelate moments y . Only one e x c i t e d s t a t e need be s p e c i f i e d i n t h i s case, the f i n a l CD being summed over a l l the chelate s t a t e s . Each chelate I acts independently (any i n t e r ­ a c t i o n s between the chelates being neglected) i n g i v i n g a coupledo s c i l l a t o r (Kirkwood-Kuhn) c o n t r i b u t i o n o f the form (11) u

c

r

r

R? = C° V ( y ° , M ° ) y ° % ° x r ι ε ~ ~ι ~ ~ i ~MI M

where

T

(7)

ε ε

c

c r

ε

2

η(ε*-ε ) r

c

and

oc -or

·ν

= y

τ

with the cap denoting the u n i t v e c t o r . S i m p l i f i c a t i o n o f t h i s expression may be e f f e c t e d by e x p l o i t ­ i n g the g e n e r a l i z e d s e l e c t i o n r u l e s ( 9 ) which we s h a l l use throughout t h i s work. These r u l e s have been formulated i n such a way that p e r t u r b a t i o n expressions such as the above may be reduced t o the simplest p o s s i b l e form c h a r a c t e r i s t i c of the symmetry of the chromophores i n v o l v e d . In p a r t i c u l a r , under V^d' the CT chromophore moment products appearing i n the expanded • oc oc oc form o f equation 7 above, utz^ μ (α) y (β) (where y (α) i s the a ^ t h c a r t e s i a n component^of y ), may be replaced by Ρ [y (α)y (β)] where Ρ i s ~ t h e t o t a l l y symmetric p r o j e c t i o n operator under D ^, d e f i n e d as o

I Rr (10) ξ=ι The (ξ = 1, ... , h) are the h symmetry operations o f D ^ i n t h i s case, and the transformed products become

P°

=

^

ς

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

80

STEREOCHEMISTRY OF TRANSITION METALS

oc

oc

P°[y (a)y (f3)] = 0 and

i f

P°[y (z)]

O C

2

= [y° (z)]

o c

2

= P°[y (y)]

P°[y (x)]

C

2

o c

2

α^β Α

=

|μ |

= \ [y

o c

2

(11)

(x)]

2 +

o c

i[y

(y)]

2

E

2

=|y | -

S u b s t i t u t i o n leads t o the complete f a c t o r i z a t i o n o f the A and I terms, and a l s o leads t o a separate c o n t r i b u t i o g f o r each symm­ etry: f o r z - p o l a r i z e d CT t r a n s i t i o n s , and R f o r x,y-polari z e d t r a n s i t i o n s . These have the form J

\

r

= C^[yY

ι R

E

= (ξ

r

Γ

Γ

[ y ° ( z ) y ° ( x ) r ( y ) - μ° (ζ) μ ° (y) r (χ) ] (12) I I MI I I MI

ε ~ E

ty ]

2

r

r

r

r

[y° (y)y° (z)r(x) - y? (x)y° (z)r(y)].(13) ι ε ~ I Summation over I and s u b s t i t u t i o (assuming the m e t a l - l i g a t i n g atom geometry i s octahedral) leads t o the f o l l o w i n g expressions f o r the CD o f the CT t r a n s i t i o n s , with r being the value o f any one o f the r : ML _ MI T

I Ει R

2

—

= 2

2

(ε -ε ) r r

Ε

ΐξ(Γ)

.

(15)

3

ML

A Ε R (r) , Rg(r) are the CD strengths o f the A^, Ε bands o f the chelate system as derived from M o f f i t t ' s e x c i t o n m o d i f i c a t i o n (12) of the Kirkwood-Kuhn mechanism, a p p l i e d t o t r i s ( b i d e n t a t e s ) by Mason ( 1 0 ) . F o r the D c o n f i g u r a t i o n , they reduce t o *>>

=

r

" *B< >

+

=~ %

r

r

ML l H ° |

2

(">

with the + s i g n f o r l o n g - a x i s , - f o r perpendicular axis p o l a r i z e d t r a n s i t i o n s o f the chelate. For the L c o n f i g u r a t i o n , a l l the signs are reversed. Thus the CT CD terms may be i n t e r p r e t e d as simply due t o s t e a l i n g o f the chelate CD; a CT t r a n s i t i o n o f a given p o l a r i z a t i o n s t e a l s i t s CD from the chelate band o f the same p o l a r i z a t i o n , and has the same s i g n , provided o f course t h a t the CT t r a n s i t i o n l i e s a t lower energies than the chelate bands. The r e s u l t o f t h i s i s t h a t a p a i r o f c l o s e - l y i n g A ^ , E ^ t r a n s f e r s t a t e s l e a d t o a band system t h a t mimics the e x c i t o n shape o f the chelate system. The r a t i o s o f the CT CD t o that o f the chelate system have the value Δ

Α

= R X < r )

- 2K ^

(17)

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

4.

scHippER

A

E

Tris(bidentate)

E

= R /^

( r

)

Complexes

81

E

=

|y |Vr^ r

5 -1 with Κ =^.6> cW B n d >

(

·

c

We s h a l l r e f e r i n a general way t o the A term inducibility

of A,

and t o the Β term

2

6

)

(Ω^) as the

(Λ ) as the inducing

power

of B. In p r i n c i p l e , t h i s expression should be summed over a l l CT s t a t e s o f the appropriate symmetry, and over a l l s t a t e s r , s o f the chelate system. However, we s h a l l see t h a t there are p h y s i c a l reasons which lead t o dominant c o n t r i b u t i o n s from p a r t i c u l a r states. The Inducibility of A. The Ω are functions p u r e l y o f the a c h i r a l chromophore A. I t i s impossible t o estimate these empir­ i c a l l y by other techniques, so t h a t the only approach would seem to be through simple model c a l c u l a t i o n s . These are beyond the scope o f t h i s paper, so t h a t the i n d u c i b i l i t y w i l l be d i s c u s s e d i n a general way, with a p a r t i c u l a r emphasis on i t s r o l e i n estab­ l i s h i n g CD/stereochemical c o r r e l a t i o n s . Some i n s i g h t i n t o the nature o f the i n d u c i b i l i t y may be gained^through c o n s i d e r i n g a p a r t i c u l a r d-d t r a n s i t i o n , say ψ° ψ (symmetry Γ ^ ) . Supposing f o r purposes o f i l l u s t r a t i o n that a l l s t a t e s may be described by s i n g l e c o n f i g u r a t i o n s o r determinants, then each t r a n s i t i o n may be considered as *a s i n g l e e l e c t r o n jump between one e l e c t r o n o r b i t a l s . The d-d t r a n s i t i o n may then be w r i t t e n i n the simpler n o t a t i o n d ^ d ^ . Denoting the CT s t a t e by c , i t follows from the f a c t t h a t each d i p o l e operat­ or i s a one e l e c t r o n operator t h a t the i n d u c i b i l i t y i s only f i n i t e i f the matrix elements «

U

U

A

I

V

^

I

H

A

I

V

^

I

I

H

A

I

V

do not vanish by symmetry. T h i s i m p l i e s immediately that the charge t r a n s f e r t r a n s i t i o n that acts as an intermediate i s e i t h e r an e l e c t r o n t r a n s f e r from d -*c. o r from c + d , . (See reference ο 1 1 1

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

4. SCHIPPER

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Complexes

85

(15) f o r f u r t h e r d e t a i l s . ) This s e v e r e l y r e s t r i c t s the CT i n t e r ­ mediate s t a t e s , so t h a t i n most cases only one need be considered. In the one e l e c t r o n p i c t u r e , CT t r a n s i t i o n s i n v o l v i n g any other d s t a t e s cannot c o n t r i b u t e t o the i n d u e i b i l i t y o f the d -+d^ t r a n s ­ ition. The r e s u l t o f t h i s i s that the i n d u c i b i l i t y o¥ t r a n s i t i o n s t o other d s t a t e s w i l l g e n e r a l l y be q u i t e d i f f e r e n t , as q u i t e d i f f e r e n t CT s t a t e s are i n v o l v e d , l e a d i n g t o q u i t e d i f f e r e n t CD strengths f o r each d-d t r a n s i t i o n . There are a l s o e x t r a symmetry r e s t r a i n t s inherent i n equation 24 r e s t r i c t i n g the nature o f the CT s t a t e s t h a t can a c t as intermediates. This leads t o a number of important c o n c l u s i o n s , which are l a b e l l e d f o r each reference in l a t e r sections. C(7) The value, ο I the induclbituty depends cnJjticaJLly on the nature of the d-d transition, and in turn on the nature of the

intermediate charge transfer state.

C(2) Von. tris [bidentates], fore have. quite different induciOilyUX.es, because quite different intenmexLiate charge than*fen.λ teuton axe. Involved. C(3) Any changes In the. charge transfer states (e.g. through peMurbation of the. legating atom) will lead directly to changes In the. inducibility and thus to changes In the. d-d CD. C(4) Any correlations be.tKse.en the. inductbitities ο I two d-d transitions [e.g. the. £JAn ratio) axe. transferable from complex to complex only if the acbuyùxl chromophobe is strictly the same tn each case; I.e. such correlations should only be expected fen. complexe* with similar d electron configurations and similar charge transfer states. There i s another conclusion t h a t follows from the c o n s i d e r a t i o n s of the next s e c t i o n , b u t i s best c o l l e c t e d here as i t r e l a t e s d i r e c t l y t o the i n d u c i b i l i t y . C(5) The relative intensities of the CD ο I d-d transitions ο I K > £ symmetry is determined largely by the indacibilUu.es, and tkus iAe nature of the achiral chromophore. Q

The Inducing Power. The inducing power i s c h a r a c t e r i s t i c o f the chelate system, and because o f the dependence on t r a n s i t i o n moments connecting the e x c i t e d s t a t e s a l s o cannot s t r i c t l y be determined e m p i r i c a l l y . Some conclusions may, however, be drawn immediately.

The inducing power is of opposite sign fer the V and L configurations of the chelate system.

C[6)

C(7) A series of complexes with the same chelate system will be characterized by the same inducing powens. The i n d u c i n g power may be d i r e c t l y i n t e r p r e t e d as a hyperp o l a r i z a b i l i t y term o f the chelate system, so that i t i s u n l i k e l y to be much a f f e c t e d by the d e t a i l s o f the metal-ligand bonding.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

86

I t i s a c t u a l l y p o s s i b l e to s i m p l i f y the form o f the inducing power i n terms o f the moment representations d e f i n e d e a r l i e r . Assuming an o c t a h e d r a l geometry o f the m e t a l - l i g a t i n g atom system, and c o n s i d e r i n g i n i t i a l l y extended pi-system c h e l a t e s f o r which the t r a n s i t i o n s are mainly long- or s h o r t - a x i s p o l a r i z e d , the inducing powers are r e a d i l y shown t o reduce t o the form A ( A , E ) = - A ( E , E ) = -2JÏ 2 g

u

g

y

u

o

r

y

r

s

y

s o

Λ(Ε ,A ) = 0 g 2u or so i f both y , y are long-axis p o l a r i z e d , and Λ(Α

r

r S

/ r ^ ; (27)

S O

Ε ) = -A(E ,A ) = 2 / 2 y ° y y / r l ; A(E ,E ) = 0 (28) u g u ML g u .- o r s if y i s long-axis, y d e f i n e d f o r the D c o n f i g u r a t i o n sign moments are determined by using the reference d i r e c t i o n s o f Figure 1 as being p o s i t i v e f o r the d i p o l e operator. For l i g a n d systems such as dipy, phen, the CD s p e c t r a o f the chelate systems suggest t h a t the the terms o f equation 28 should be the dominant f a c t o r s l e a d i n g t o the d-d CD. The e q u a l i t y ( i n magnitude) o f the inducing powers i n equation 27, and a l s o o f those i n equation 28, f o l l o w s from the D^ symmetry o f the chelate system, and may be taken as a general feature o f D^ systems. I t i s t h i s f a c t o r that leads t o c o n c l u s i o n C(5) o f the previous section. I t i s d i f f i c u l t t o determine simply from l o o k i n g a t the previous equations how the inducing powers o f d i f f e r e n t l i g a n d systems w i l l be r e l a t e d . However, i f the h y p e r p o l a r i z a b i l i t y type terms are determined l a r g e l y by arrangement o f the p i systems r a t h e r than t h e i r i n t e r n a l d e t a i l s (which i s q u i t e p l a u s ­ i b l e i f one considers somewhat n a i v e l y t h a t each i s r e a l l y a "sea" of p i - e l e c t r o n s f l o a t i n g around each chelate frame), then i t would be most l i k e l y t h a t the o v e r a l l i n d u c i b i l i t i e s would a t l e a s t be o f the same s i g n f o r a range o f c h e l a t e systems. T h i s appears t o be the case even f o r s a t u r a t e d l i g a n d systems. We s h a l l t h e r e f o r e make the f o l l o w i n g assumption, s t r e s s i n g t h a t i t need not be s t r i c t l y t r u e even w i t h i n the confines o f our model (C(l)-C(7) are conclusions based d i r e c t l y on the CD model, and are not assumptions). A(7) Thz J^LQVI o£ thz inducing povozn ÂJ> tndzpzndznt ο I thz dztxUMd ^tAucXu/tz ol thz zhzlatz, and dztznmivizd only by thz ovzhaZZ confitgu/iation ofi thz complzx. Magnitude of the Second-Order Terms. Before d i s c u s s i n g the a p p l i c a t i o n s o f these c o n c l u s i o n s , i t i s important t o e s t a b l i s h t h a t the mechanism d i s c u s s e d here can account f o r the magnitude of the d-d CD found experimentally. Using equation 16 and the u n i t s d i s c u s s e d f o r equations 17, 18, the magnitude o f the r a t i o

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

SCHIPPER

Tris(bidentate)

Complexes

2

3 Figure 1.

Definition of dipole operator directions and axis system: (a) long-axis chelate transitions; (b) short-axis chelate transitions.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

88

of the d-d CD (magnetic d i p o l e allowed) has the form (see equation 22) Δ

= 2.1X10

4

c ε

A

α

ε

B

t o t h a t o f the chelate CD

α

° ι or ι r υ r ML '~

(29) 2

1

ε

ε

5

=

1 ( ) 4

1

P u t t i n g r = 3 Â, Δε^-ε^/3 ~ ( " )/ cm" , and a l l chelate d i p o l e moments equal t o l e Â, the r a t i o reduces t o α

Δ = 0.02Ω (Γ.,Γ ) A d c

3

ά

(30)

with m i n BM. Assuming the CT moments f o r A are o f the order o f 1/3 eA, and the magnetic moment about 3 BM, the r a t i o i s s t i l l about 1 0 " , i n good agreement with the magnitudes found experim­ e n t a l l y . Considering t h a t th abov value t unrealistic such a s i m p l i s t i c c a l c u l a t i o evidence f o r the importanc questionabl whether many other d-d CD models could meet t h i s r a t h e r s t r i n g e n t t e s t o f g i v i n g an absolute magnitude i n the r i g h t b a l l park f o r r e a l i s t i c parameter values. Applications I t may seem that the previous d i s c u s s i o n leads t o so many r e s t r i c t i o n s t h a t one d e s p a i r s a t the use o f d-d CD as a s t e r e o ­ chemical probe a t a l l . The source o f t h i s apparent dilemma i s t h a t the CD i s s e n s i t i v e not only t o the absolute c o n f i g u r a t i o n of the chelate system, but a l s o t o any v a r i a t i o n s i n the a c h i r a l chromophore. Such a negative conclusion i s r e a d i l y a l l e v i a t e d , however, i f the r e s t r i c t i o n s are considered together with estab­ lished empirical rules. In a d d i t i o n , the s e n s i t i v i t y t o the a c h i r a l chromophore, though a p a r t i a l hindrance t o the e s t a b l i s h ­ ment o f a simple CD-stereochemical c o r r e l a t i o n t o cover a l l complexes, i s a c t u a l l y an appreciable advantage t o the study o f the e f f e c t s o f , f o r example, added anions on the m e t a l - l i g a n d bonding. We now i l l u s t r a t e these f e a t u r e s with some examples. 3 8 The Dominant Ε Rule. The band i n d , d and s p i n - p a i r e d d t r i s ( b i d e n t a t e ) complexes i s g e n e r a l l y s p l i t i n t o d i s t i n c t 2σ' P° f i t i s found e m p i r i c a l l y that the Ε band has the stronger CD. The absolute c o n f i g u r a t i o n i s thus determ­ ined d i r e c t l y through the sign o f the dominant Ε band (see e.g. references i n Hawkins (_2) ) . This i s c o n s i s t e n t with A ( l ) , C(2) , but should only apply f o r s i m i l a r chromophores ( C ( 4 ) ) . The r e l a t i v e l y wide a p p l i c a b i l i t y o f t h i s r u l e must stem from an u n d e r l y i n g s i m i l a r i t y o f the e l e c t r o n i c c o n f i g u r a t i o n s o f such chromophores. Perhaps the b e s t example when such a r u l e should apply r i g o r o u s l y on t h e o r e t i c a l grounds are the t r i s ( d i a m i n e ) complexes, f o r which the a c h i r a l chromophore should not be e x t e n s i v e l y perturbed by s u b s t i t u t i o n s a t the chelate carbons. 6

Α

E

c o m

n e n t s

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

4.

SCHIPPER

Tris(bidentate)

Complexes

89

T h i s i s found t o be the case (2) . An i l l u s t r a t i o n o f the s e n s i t i v i t y o f t h i s r u l e to changes i n the a c h i r a l chromophore i s t h a t the a d d i t i o n o f p o l a r i z a b l e oxyanions have a dramatic e f f e c t on the CD spectrum (16),the A band g a i n i n g i n t e n s i t y a t the expense o f the Ε band (17,18) , f o r the diamine complexes discussed above. T§e e f f e c t i s only pronounced f o r those complexes which can lead t o hydrogen bonding of the anions to the n i t r o g e n hydrogens. Such e f f e c t s are frequ­ e n t l y r a t i o n a l i z e d as due t o e x t r a p e r t u r b a t i o n s on an a c h i r a l chromophore ( i . e . they are absorbed i n t o the c h i r a l p e r t u r b e r ) , but i n t h i s model i t i s probably due t o the hydrogen bonding having s u f f i c i e n t e f f e c t on the nitrogens t h a t the a c h i r a l chromo­ phore d e f i n i t i o n must incorporate these changes at the outset. The anion e f f e c t thus e f f e c t i v e l y leads t o a new a c h i r a l chromo­ phore, because o f the l a r g e f f e c t th charg t r a n s f e s t a t e s T h i s has been e x p l o i t e and thus e f f e c t i n g an assignmen configâration Such e f f e c t s serve to i l l u s t r a t e how e a s i l y the d-d CD can be a l t e r e d through v a r i a t i o n s i n the a c h i r a l chromophore ( C ( s ) ) . The

Rule.

I t i s tempting t o s i m p l i f y the symmetry o f

the a c h i r a l chromophore, and thus i n turn s i m p l i f y the CD-stereochemical c o r r e l a t i o n s . F o r example, i f the l i g a t i n g atoms are taken to be i s o t r o p i c , the symmetry o f the a c h i r a l chromophore becomes o c t a h e d r a l . The CD should then reduce t o that o f a s i n g l e T^ band. E m p i r i c a l l y , the Τ CD could be approximated as a simple sum o f the A^ , Ε bands*, l e a d i n g t o a "T " r u l e . Under such o c t a S e d r l l symmetry o f the achiraS chromophore, the g e n e r a l i s e d s e l e c t i o n r u l e s reduce the CD strength f o r the band to a s i n g l e c o n t r i b u t i o n f o r each chelate I : d

ι

.

. lg

=

1 d 6 ε

I

m

[

ϋ

oc ϋ

cd * m

do., - r s -so -or. ϋ ι Hi "Hi *

] [

1

( 3 1 )

For a c h i r a l c h e l a t e s , the t r i p l e products o f chelate moments on a s i n g l e chelate I i s zero, so that R (T ) I lg

=

0

f o r each I

(32)

f o r uncoupled c h e l a t e s . [There i s a small c o n t r i b u t i o n i f a degree o f coupling between the chelates i s introduced, but i t i s at l e a s t an order o f magnitude smaller than that derived e a r l i e r f o r the s p l i t A^ , Ε bands.] Thus, under Ο , the CD vanishes, so that the T, In f a c t , i t l g ?ule^would seem t o be o f l i t t l e use. i s a good i l l u s t r a t i o n o f ricw an u n r e a l i s t i c choice o f chromo­ phore can lead to a quite misleading r e s u l t . In these complexes, the Ε , A^ s p l i t t i n g i s c r u c i a l to g e t t i n g an appreciable CD, so that ?he achiralchromophore must accommodate t h i s c l e a r s p l i t t i n g of the T band a t the outset. A d d i t i o n o f the CD o f the Ε , A^ component! cannot (except i n s p e c i a l cases perhaps where t h i y 1

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

90

are equal and opposite i n sign) be i d e n t i f i e d as t h a t o f the band. A d d i t i v i t y Rules. In a d d i t i o n t o these d i r e c t CD-stereochemical c o r r e l a t i o n r u l e s , there are a s e t o f e m p i r i c a l a d d i t i v i t y r u l e s determined e m p i r i c a l l y by Douglas (see, e.g., reference {19) and c o l l a t e d references i n Hawkins ( 2 ) ) . An example o f such a r u l e i s t h a t f o r cases i n which the c h e l a t e i t s e l f has an asymmetric carbon. The d-d CD c o n t r i b u t i o n s from the asymmetric carbon atom and t h a t o f the chelate system as a whole are found t o be a d d i t i v e . The t h e o r e t i c a l j u s t i f i c a t i o n f o r t h i s a d d i t i v i t y f o l l o w s d i r e c t l y from the model d i s c u s s e d i n t h i s paper, and has been d i s c u s s e d i n some d e t a i l elsewhere (4). ( I t i s worth n o t i n g t h a t we i m p l i c i t l y used t h i s a d d i t i v i t y i n c o n s i d e r i n g the d-d CD the c h e l a t e s I , with th symmetry.)

Literature Cited. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

Carroll,L., "The Annotated Alice", (ed. M. Gardner), Penguin (1977). Hawkins,C.J., "Absolute Configuration of Metal Complexes", Wiley-Interscience, New York, N.Y. (1971). Richardson,F.S., Chem.Reviews (1979) 79 17. Schipper,P.E., J.Am.Chem.Soc. (1978) 100 1433. Schipper,P.E., J.Am.Chem.Soc. (1979) 000 0000. Mason,S.F. and Seal,R.H., Mol.Phys. (1976) 31 755. Moffitt,W., J.Chem.Phys. (1956) 25 1189. Richardson,F.S., J.Phys.Chem. (1971) 75 692. Schipper, P.E., J.Am.Chem.Soc. (1978) 100 3658. Mason,S.F., Inorg.Chim.Acta Revs. (1968) 2 89. Schellman,J.A., Accts.Chem.Res. (1968) 1 144. Moffitt,W., J.Chem.Phys. (1956) 25 467. Norden, B. and Tjerneld,F., F.E.B.S. Letters (1976) 67 368. Schipper,P.Ε., Chem.Phys. (1976) 12 15. Schipper,P.Ε., J.Am.Chem.Soc. (1976) 98 7938. Smith,H.L. and Douglas,Β.Ε., Inorg.Chem. (1966) 5 784. Gollogly,J.R. and Hawkins,C.J., Chem.Comm. (1968) 689. Mason,S.F. and Norman,B.J., Chem.Comm. (1964) 339. Douglas, B . E . , Inorg.Chem. (1965) 4 1813.

RECEIVED September 1 3 , 1 9 7 9 .

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

5

Circular D i c h r o i s m Spectra of Square Planar Complexes Containing Prochiral Olefins and T h e i r

Stereoselective

Olefin Exchange

KAZUO SAITO Chemistry Department, Faculty of Science, Tohoku University, Sendai, 980 Japan

The relationship between the absolute configuration and CD spectrum has been widely discussed, but mostly for octahedral complexes. (1) Asymmetric coordination of η -olefins was first demonstrated by Cope et al. (2) with trans-[PtCl (1-phenylethylamine)(trans-cyclo-octene)] and extended by Paiaro and Panunzi (3) by the preparation of a pair of diastereoisomers, such as transdichloro(R or S-α-phenylethylamine) (trans-2-butene)platinum(II) , trans-[PtCl (1-phenylethylamine) (tbn)]. Since then several complexes with such an asymmetry have been prepared, and the relationship between the absolute configuration and CD pattern has been discussed for platinum(II) complexes. (4) Scott and Wrixon (5) reported that S,S-η and R,R-η2 configuration give CD peaks with positive and negative signs in the d-d transition region at ca. 27,000 cm-1. Less information is available for the complexes with other metal ions, and only palladium(II) (5) and iron(0) (6) complexes were discussed. A change in the kind of olefin does not cause significant changes in absorption spectra, so long as the other ligands remain unchanged. Hence the replacement of one olefin ligand by another cannot be detected by absorption spectrometry. Olefin exchange in platinum(II) complexes is an important elementary reaction related to their catalytic action in homogeneous systems, but kinetic studies have not been made because of such experimental difficulty. NMR studies gave only limited information. Measurement of the change in the CD spectrum of the complexes with prochiral olefins in the presence of an excess of free prochiral or non-prochiral olefins enables the estimation of the substitution rate. This method is useful for examining the influence of other ligands upon the rate of olefin exchange, (e.g. trans effect, (7)), but is also useful for elucidating the stereo-selctivity involved in the olefin exchange. This paper deals with the relationship between the absolute configuration of r) -olefins and the CD pattern of new platinum(II) 2

2

2

2

2

0-8412-0538-8/80/47-119-091$06.00/0 © 1980 American Chemical Society In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

92

and rhodium(I) complexes of the types shown i n Figure 1, and the source of s t e r e o s e l e c t i v i t y f o r o l e f i n exchange.

with

CD P a t t e r n of Rhodium(I) and P l a t i n u m ( I I ) Complexes CD of Rhodium(I) Complexes. Figure 2 shows the v i s i b l e and UV absorption of r | - o l e f i n complexes of rhodium(I). The synthesis of these complexes has been reported elsewhere. (8) I t i s seen t h a t r | - o l e f i n s give absorption peaks or shoulders i n the regions 20,000 t o 30,000 cm-1 and around 40,000 cm" . These are the same regions with those where n . - o l e f i n platinum(II) complexes give c h a r a c t e r i s t i c absorption, which i s exemplified i n Figure 3. (9) (Table I) Figure 4 i l l u s t r a t e s the CD s p e c t r a of a few rhodium(I) complexes. The p a t t e r n i Two c h a r a t e r i s t i c and commo where the r ] - o l e f i n s cause absorption peaks:i.e. negative peaks between 20,000 and 30,000 cm"! and l a r g e p o s i t i v e peaks at ca. 40,000 cm" . The p a t t e r n i n the r e g i o n between these two i s too complicated and s e n s i t i v e to the other l i g a n d s and no systematic trend i s seen. Figure 5 shows the d i f f e r e n c e i n CD p a t t e r n between doubly bridged b i n u c l e a r complexes of platinum(II) and rhodium(I) c o n t a i n i n g S,£-£ratts-cyclo-octene (coe), which can be coordinated only i n t h i s c o n f i g u r a t i o n . Whenever one looks a t the peaks at ca. 22,000 cm" , the peaks have opposite s i g n s . On the other hand, the s i g n of the peak a t around 40,000 cm"l i s the same, although the i n t e n s i t y d i f f e r s g r e a t l y . The molar e x t i n c t i o n c o e f f i c i e n t i n the former r e g i o n i s ca. 10 M"lcm"l, and cannot be reckoned to be a p u r e l y d-d t r a n s i t ­ ion. However, the i n f l u e n c e of the c e n t r a l metal i o n i s remarkable. The ε value of the peak at ca. 40,000 cm-1 i s more than 104M~ cm- , and the CD s i g n i s independent of the metal i o n . Hence t h i s must r e f l e c t the absolute c o n f i g u r a t i o n of the p r o c h i r a l o l e f i n i t s e l f . 2

2

1

2

2

1

1

3

1

1

CD of Platinum(II) Complexes. Scott and Wrixon discussed the r e l a t i o n s h i p between the CD and the s t r u c t u r e of various o l e f i n s i n c l u d i n g many terpene d e r i v a t i v e s , but t h e i r s i s l i m i t e d to the low energy d-d t r a n s i t i o n r e g i o n . (5) The CD p a t t e r n i n the r e g i o n 30,000 to 40,000 cm" i s very complicated and cannot be understood s y s t e m a t i c a l l y . We have synthesized various complexes c o n t a i n i n g S,S-trans-2-butene (tbn) or /S'-2-methyl-2-butene (mbn) as a source of asymmetry and other l i g a n d s i n c l u d i n g L - p r o l i n a t e , 4-substituted a n i l i n e s and 4 - s u b s t i t u t e d p y r i d i n e s . Among them [PtCl(0-phenylenediamine)(5-mbn)] i s the f i r s t o p t i c a l l y a c t i v e square planar complexes with p o s i t i v e charge. The CD data are given i n Table I I . 1

Importance of the c i s Ligand. Figure 6 shows the CD of the two geometrical isomers of [ P t C l ( L - p r o l i n a t e ) ( S , S - t b n ) ] . Both have r a t h e r l a r g e negative CD peaks a t ca. 36,000 cm" , but the c i s isomer has one shoulder a t ca. 32,500 cm" , whereas the trans 1

1

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

SAiTO

Square

Planar

Complexes

Figure 2. Absorption spectra of rhodium(I) complexes containing olefins and other ligands in hexane: acac, enolate anion of acetylacetone; coe, cyclooctene; dbm~, enolate anion of dibenzoylmethane.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

Figure 3.

UV absorption and CD spectra of [PtCl(L-prolinate)-(tra.ns-2-butene)]: ( λ (+)s8o* ; ( λ (-) 8o ; ( λ vincinal effect. €

3

A€

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

5.

SAITO

Square Planar

95

Complexes

Table I*Absorption Data o f Rhodium(I) and Platinum(II) Complexes Containing r| -01efins 2

Complexes

V 10 cm" a

b

[Rh(acac)(C0) 1 ' [Rh(acac)(ethylene)2] [Rh(dbm) (ethylene) ] [Rh(acac)(trans-coe)2] [Rh(dbm)(trans-coe) ] [Rh2Cl (trans-coe) ] [PtCl(L-pro)(trans-coe)] f [PtCl(L-pro)(trans-coe)] g K[PtCl2(L-pro)] 2

2

c

c

2

c

2

2

V

loge

3

25.00 25.00 27.40 25.32 27.40 22.73* 24.8 * 27.03*

3

ΙΟ ^"

1

41.00 41.32 40.00 40.98* 40.00 40.98 40.00 40.00*

1.91 3.31 3.98 2.95 3.96 2.97 1.54 1.73

loge 1

3.68 4.00 4.08 3.94 4.38 4.48 3.20 3.32

h

a) c) e) g) *)

i n diethylether i n hexane cyclooctene i n acetonitrile shoulders

b d) enolate anion of dibenzoylmethane f ) L - p r o l i n a t e , data i n Ref. h) i n Ref.

Figure 4. CD spectra of rhodium(I) and platinum(ll) complexes containing S,Strains-2-butene (tbn) and S^S-trans-cyclooctene (coe) in hexane: ( ), P(C H ) [PtCla(S S-tbn)J in acetonitrile; (- - -) [Rh Cl (S,S-coe)] ( ), [Rh(acac)(S,Scoe) ]; ( ), [Rh(dbm)(S$-coe) ]. 6

9

2

t

2

;

2

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

5 r

STEREOCHEMISTRY OF TRANSITION METALS

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

5.

SAITO

Figure 6.

Square Phnar

Complexes

CD spectra of cis- and triins(NJ/)-[PtCl(L-prolinate)-(S,S-tTims-2butene)] in acetonitrile: ( j , cis isomer; ( j , trans isomer.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

97

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

6

5

5

4

4

2

e

d

f

f

f

5

a

h

d

/

,

e

6

c

2

22 9(+0.46) 24 0(+D.76) 23. 6(+0.51) 23, 6(+0.44) 23. 6(+0.41)

3

V/10 cm-l ( Δε )

* 34. 3(--1.90) 39 7(-0. 54) 26 .8(+1.08) ** 37.0( -2 26) 33.2(-0 .57) 27 .6(+0.92) 26 .5(-0.21) 29.4(+0.13) 34. 3(--1.05) 35. K- -1.32) 39 ,5(+3. 20) 28 *0(-0.14) 34. 7(--0.86) 27 .0(-0.28) 34. 3(--0.76) 27 .2(-0.30) 34. 5(--0.70) 27 .K-0.26) ** 28 .3(+0.36) 32.8(-0.03) 37. 0(--0.59) 40 3(-0. 35) 37.4( -1 .19) **35.0(-0 .72) 27 .3(+1.32)

CD peaks

a) D i f f e r e n c e spectrum between trans^Ni//)"[PtCl(L-pro)(5,£-tbn)] and transiN,//)-[PtCl(L-pro)(ethylene)] b) D i f f e r e n c e spectrum between cis(N,//)-[PtCl(L-pro)(S,S-tbn)] and ois(N,//)-[PtCl(L-pro)(C2H4)] c) D i f f e r e n c e spectrum between ois-[PtCl (S,£-tbn) (,9-1-phenyl-ethylamine)] and ois(N,//)- [ P t C l 2 (ethylene) (i5-l-phenylethylamine) ] d) i n a c e t o n i t r i l e h) S-l-phenylethylamine; i n acetone e) i n dichloromethane *) peaks are broad f) a n i l , a n i l i n e ; i n benzene **) shoulders g) i n ethanol

2

3

trans (N,//)- [PtCl (L-pro) (5,5-tbn) ] ' cis (N,//y [PtCl (L-pro) (5,5-tbn) ] b,d P(C H ) [PtCl3(S-mbn)] P(C6H )4[PtCl (£,S-tbn)] trans-[PtCl (S-mbn)(4-Cl-anil)] trans-[PtCl2(S-mbn)(anil)] trans-[PtCl2(S-mbn)(4-CH3-anil)] [PtCl(S-mbn){o-C H (NH )2>1 [ B ( C H ) 4 ] 9 cis-[PtCl2('S' /S -tbn) (5-l-PhEtNH2)]

Complexes

2

Table II. Peaks i n the C i r c u l a r Dichroism Spectra o f n - 0 1 e f i n complexes of Platinum(II)

5.

SAiTO

Square

Planar

99

Complexes

isomer gives one broad peak s h i f t e d t o longer wave lengths. This f a c t suggests the importance o f other l i g a n d s which are c i s and trans t o the o l e f i n . Figure 7 i l l u s t r a t e s the CD patterns o f the mbn complexes c o n t a i n i n g c i s - d i c h l o r o l i g a n d s , and a trans-chloro or 4-substituted a n i l i n e l i g a n d . Despite the d i f f e r e n c e i n b a s i c ­ i t y and even i n the c o o r d i n a t i n g atom, the negative CD peaks a t ca. 34,500 c m have almost equal p o s i t i o n s and i n t e n s i t i e s . I t seems as i f the v a r i a t i o n of the c i s l i g a n d i s more important i n determining the CD p a t t e r n i n t h i s region than the trans l i g a n d . This c o n s i d e r a t i o n i s f u r t h e r v e r i f i e d by comparing the CD o f those complexes having c h l o r i n e and n i t r o g e n i n the c i s p o s i t i o n and other donors i n the t r a n s . (Figure 8) The solvents were chosen i n accordance with the s o l u b i l i t y and the p a t t e r n cannot be compared i n one solvent. Here again the l o c a t i o n o f the negat­ ive CD peaks are not ver or the presence o f anothe amine and p r o l i n a t e , which have asymmetric n i t r o g e n upon coordinat­ ion) . A l l these f a c t s suggest t h a t the c i s - i n f l u e n c e i s more s i g n i f i c a n t than the t r a n s - i n f l u e n c e f o r determining the l o c a t i o n of CD peaks around 35,000 cm" . Because o f the very strong t r a n s i n f l u e n c e o f the asymmetric o l e f i n , l i g a n d s trans t o the o l e f i n would have only a small i n f l u e n c e on the p l a t i n u m ( I I ) , e s p e c i a l l y when the trans l i g a n d s are mere e l e c t r o n p a i r donors. Figure 9 gives the CD p a t t e r n o f the complexes trans-[PtCl2(£-mbn)(4s u b s t i t u t e d - p y r i d i n e ) ] . The peaks below 30,000 c n r are alomost i d e n t i c a l , but the negative peaks a t ca. 35,000 cm" s h i f t s as the s u b s t i t u e n t on the p y r i d i n e r i n g changes. P y r i d i n e d e r i v a t i v e s can have d^-d-ji i n t e r a c t i o n s with platinum(II) and may perturb the electronic state. -1

1

1

1

Asymmetric Influence from the trans Ligand. As shown l a t e r , when an asymmetric n i t r o g e n i s i n the trans p o s i t i o n of the com­ plexes o f the type trans (N,//)-[PtCl(L-am)(ethylene)], asymmetry i s introduced by the s u b s t i t u t i o n o f tbn f o r the ethylene i n organic s o l v e n t s . (10.) L-Alanine, L-phenylalanine, and L - v a l i n e f a i l t o introduce asymmetry on the incoming tbn i n the e q u i l i b ­ rated s t a t e . However, L - p r o l i n e , N- and C - s u b s t i t u t e d L - p r o l i n e and even ^ - s u b s t i t u t e d L - v a l i n e induce asymmetry. (Table V) There must be some e l e c t r o n i c i n t e r a c t i o n from the asymmetric n i t r o g e n upon the o l e f i n through p l a t i n u m ( I I ) . Figure 10 shows the CD p a t t e r n o f some o f these complexes. Complexes without asymmetric n i t r o g e n give only very weak CD i n the r e g i o n from 20,000 t o 40,000 cm" . E s p e c i a l l y between 27,000 and 40,000 c m the CD i s much weaker than those with asymmetric nitrogens (Figure 10-D). Since the complexes with L - p r o l i n a t e , L-hydroxyprolinate and αΖΖ-ο-L-hydroxyprolinate have very s i m i l a r patterns t o one another, the asymmetric carbon atoms on the p y r r o l i d i n e r i n g do not seem t o give s i g n i f i c a n t c o n t r i b u t i o n s . (Figure 10-B) On the other hand, i n t r o d u c t i o n o f a methyl o r benzyl s u b s t i t u e n t on the n i t r o g e n changes the CD p a t t e r n t o a 1

-1

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

100

log ε

Figure 7. UV absorption and CD spectra of phtinum(II) complexes containing S-2-methyl-2-butene (mbn) and 4-substituted anilines: ( ), X = Η in trans[PtCl (S-mbn)(4-X-anline)] (in benzene); (- · ·), X = CI in trans-[PtCl (S-mbn)(4-X-aniline)] (in benzene); ( ), (X = CH ) in trans-[PtCl (S-mbn)(4-X-aniline)] (in benzene); (----), P(C HrJJPtCl^S-mbn)] in dichloromethane. 2

2

3

2

6

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

5.

SAITO

Square Planar

101

Complexes

Λ / \

25

3 t > X 35

1 AO

Ïï/I0 cm" 3

i M

1

\J

Figure 8. CD spectra of platinum(II) complexes containing S-2-methyl-2-butene and various amines on the cis site: ( ), [PtCl(o-phenylenediamine)(S-mbn)] in ethanol; ( ), cisfN,//)[PtCl(L-prolinate)(S,S-tbn)] in acetonitrile; (-'-'), cis(Cl)[PtCl (S-l-phenylethyhmineXS-mbn)] in acetone. 2

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

102

Figure 9. CD spectra of phtinum(II) complexes containing S-2-methyl-2-butene (mbn) and 4-substituted pyridines on the trans site in dichloromethane: ( ), X = NH ; ( X = H; and( j , X = C0 C H in trans-[PtCl (4-X-py)(S-mbn)]. 2

2

2

5

2

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

SAiTO

Square Planar

Complexes

log£

Figure 10. UV absorption (A) and CD spectra of transfN,/'/ )-[PtCl(L-aminocarboxyhte)(ethylene)] in acetonitrile (10). B: ( ), L-prolinate; ( ), L-hydroxyprolinate; (· · -), al\o-L-hydroxyprolinate. C: ( ), N-methyl-L-pro; ( ), N-methyl-L-hyp; (· · -), N-benzyl-L-pro. D: ( ), L-ahninate; ( ), L-phenyhlaninate; ( · · -), L-valinate. E: ( ),N-benzyl-L-Oalinate. f

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

104

marked extent, e s p e c i a l l y i n the region from 25,000 to 40,000 cm"! (Figure 10-C). In p a r t i c u l a r l y tf-benzyl-L-valinate gives a l a r g e negative peak at ca. 34,000 c i r r i , which i s s i m i l a r to that of Nb e n z y l - L - p r o l i n a t e (Figure 10-E). This f a c t i n d i c a t e s a marked s t e r e o s e l e c t i v i t y on c o o r d i n a t i o n of #-benzyl-L-valinate. trans(N,//)-[PtCl(tf-bz-L-val)(ethylene)] can e x i s t as a p a i r of d i a stereoisomers, S(N)S(C) and R(N)S(C). I t s CD spectrum i s very s i m i l a r to t h a t of tf-benzyl-L-prolinato complex, which can have only R{N)S (C) c o n f i g u r a t i o n . The #-benzyl-L-valinato complex seems to be formed almost e x c l u s i v e l y i n the R(N)S(C) form, when Zeise's s a l t undergoes s u b s t i t u t i o n with f r e e #-benzyl-L-valine i n a s l i g h t l y a c i d i c s o l u t i o n . The preference f o r the i?-conf i g u r a t i o n around the coordinated n i t r o g e n adjacent to an 5-carbon i s r a t h e r common f o r octahedral complexes. (11) Molecular model s t u d i e s show that s t e r i c hindrance betwee th s u b s t i t u e n t th n i t r o g e d 5-carbon would be r e s p o n s i b l Figures 10-B and -C i n d i c a t e that the CD curves i n the 33,000 and 37,000 c i r r i regions have reversed s i g n s . On the assumption of the a d d i t i v i t y law, the d i f f e r e n c e s of CD's between the #-benzylL - p r o l i n a t o and L - p r o l i n a t o , and between #-benzyl-L-valinato and L - v a l i n a t o complexes are p l o t t e d against the wave number i n Figure 11-A. S i m i l a r p l o t s of όΔε'ε bentween #-methyl-L-prolinato and L - p r o l i n a t o , and between tf-methyl-L-hydroxyprolinato and Lhydroxyprolinato complexes are shown i n Figure 11-B. The δΔε curves are very s i m i l a r to each other r e g a r d l e s s of the aminocarboxylate moiety. Hence the a d d i t i v i t y law should hold between the c o n t r i b u t i o n s of N-substituent and of the aminocarboxylate, the former being independent of the c h e l a t e framework. Usefulness of the quadrant r u l e f o r the i n t e r p r e t a t i o n of the CD signs of n - o l e f i n complexes of platinum(II) i n 25,000 cirri r e g i o n was demonstrated by Scott and Wrixon (5) . We have a p p l i e d t h i s r u l e f o r i n t e r p r e t i n g the c o n t r i b u t i o n of the asymmetric n i t r o g e n . Figure 12 shows the p r o j e c t i o n of trans(N //)-[PtCl( t f - a l k y l - L - p r o ) ( e t h y l e n e ) ] . The square plane i s represented by the h o r i z o n t a l l i n e , and the Pt-N bond i s perpendicular to the paper plane. The asymmmetric n i t r o g e n i s beneath platinum(II) (large dotted c i r c l e ) . The c o n t r i b u t i o n of the minus quadrant at below l e f t s i d e behind the paper should depend on the s i z e of the s u b s t i ­ tuent on n i t r o g e n ( t r i a n g l e ) . With an increase i n s i z e of t h i s s u b s t i t u e n t (H, methyl and benzyl) the c o n t r i b u t i o n of t h i s minus component should increase to give the c a l c u l a t e d curves shown i n Figure 11. The UV absorption curves of a l l the present complexes have peaks with e=ca.ΙΟ^ΙΓ^-αχΤ from 31,000 to 45,000 cm" . Denning, Hartley and Venanzi assigned the absorption bands of Zeise's s a l t i n t h i s region t o d-π*(ethylene) t r a n s i t i o n . (12) We have observed CD peaks with Δ ε s -1.3 and +3.3 a t ca.35,000 and 39,500 cm" for the tetraphenylphosphonium s a l t of [PtCl3 (S,S-tbn) ] " i n a c e t o n i t r i l e . (13) The peak a t 35,000 cm" must correspond to the same t r a n s i t i o n as that of the main CD band of Figure 10 (and Figure 2

r

1

1

1

1

1

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

5.

SAiTO

Square

Planar

105

Complexes

-0.5}

Figure 11. Difference in CD between two complexes of the type transfN,//)[PtCl(L-aminocarboxylate)( ethylene ) with and without substituent on the nitrogen (10)

Figure 12.

Projection of the square planar complexes (10): A, tr3u\s(N,//)-[PtCl(-substituted L-am)(ethylene)]; B, S,S-trans-2-fewiene moiety.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

STEREOCHEMISTRY OF TRANSITION METALS

106

1 1 ) . Figure 12 a l s o shows the p r o j e c t i o n of t h i s Zeise-type complex; the C-C moiety i s placed across the square plane o f platinum(II) behind the paper. A common c o n t r i b u t i o n o f the minus r e g i o n t o the lower l e f t behind the paper seems t o predominate f o r the Zeise-type and the present complexes. The d-n* t r a n s i t i o n may be perturbed by the asymmetric n i t r o g e n trans t o ethylene t o give a marked CD peak i n t h i s r e g i o n . The c i s - i s o m e r cis (N,//)-[PtCl(L-pro)(ethylene)] gives only a weak CD i n t h i s r e g i o n (Figure 1 3 ) .

S t e r e o - s e l e c t i v i t y on O l e f i n Exchange Selectivity

on the Exchange of Prochiral

Olefins.

Substitut-

2

ion o f o l e f i n s f o r the coordinated r | - o l e f i n was f i r s t s t u d i e d k i n e t i c a l l y by PMR spectroscop ( acac, enolate anion o i n such a r e a c t i o n was pointed out f i r s t by C o r r a d i n i , Paiaro and Panunzi f o r the e q u i l i b r i u m o f cis-[PtCl2(5-amine)(olefin)] i n organic s o l v e n t , the i ? - c o n f i g u r a t i o n being p r e f e r r e d by 5 t o 50 %. (15) Many s t u d i e s have d e a l t with the s e l e c t i v i t y o f r e a c t i o n s o f coordinated l i g a n d s , (16) but nothing has been r e ported concerning the s t e r e o s e l e c t i v i t y f o r the s u b s t i t u t i o n o f coordinated o l e f i n s . We examined s e v e r a l years ago the r a t e o f the f o l l o w i n g r e a c t i o n s by use o f CD measurements and the i s o t o p i c l a b e l l i n g method. 3

trans (N,//)-[PtCl(L-pro)(S,S-tbn[ #])]

+ tbn

trans (N,//h [PtCl (L-pro) (R,R or 5,5-tbn)]

3

+ tbn[ #]

(1)

and found a s i g n i f i c a n t s e l e c t i v i t y i n favor o f s u b s t i t u t i o n with r e t e n t i o n o f c o n f i g u r a t i o n (Table I I I ) . Table I I I Second Order Rtae Constants o f the S u b s t i t u t i o n of 2-Butene f o r the 5,5-2-Butene i n trans(N,//)[PtCl(L-prolinate)(5S -tpans-2-butene[3iï] )] i n Acetone. (17) #

,

Olefin

trans - 2-butene*

eis-2-butene

Temp/°C -λ -1 -1 k _/io M s k. / i o " V s

8.0

-20.0

8.0

-20.0

347

70.9±7.6

6.2

0.9

70.2±4.1

32.3

5.6

ISO

* The c a l c u l a t e d k , -3 -1 -1 roo

3.1 χ 10

M

s

and k

r o n i

a t 8.0°C are 29.1 and

, r e s p e c t i v e l y , the r a t i o k ^ g j / f c

^

being 9.4.

In Stereochemistry of Optically Active Transition Metal Compounds; Douglas, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

5.

SAITO

Square

Vlanar

Complexes

107

The i d e n t i c a l r a t e o f s u b s t i t u t i o n o f non-prochiral cis-2-butene f o r the coordinated S,5-tbn measured by the two methods i n d i c a t e s the absence o f other r e a c t i o n s such as l o c a l proton exchange between