Stereochemistry of Optically Active Transition Metal ... - ACS Publications

11

Effect of Solvent o n the Circular D i c h r o i s m of M e t a l

Downloaded by KTH ROYAL INST OF TECHNOLOGY on February 26, 2016 | http://pubs.acs.org Publication Date: May 27, 1980 | doi: 10.1021/bk-1980-0119.ch011

Complexes

TREVOR D. BAILEY and CLIFFORD J. HAWKINS Department of Chemistry, University of Queensland, Brisbane, Australia 4067

Since before the turn of the century it has been known that the optical activity of some chiral compounds is solvent depend ent (1). For example, in 1877 Landolt (2) reported that the specific rotations of (+)-camphor, (-)-nicotine, (+)-diethyl tartrate, and (-)-turpentine varied with solvent and concentration. In the last decade there has been renewed interest in this solvent dependence. A number of different types of organic compounds has been investigated and the results have been interpreted in terms of variations in conformer populations that have resulted from either the effect of the dielectric on coulombic interactions between dipolar groups in the molecule (3) , or from hydrogen-bond interactions between the solvent and the chiral solute (4). Recently papers have appeared dealing specifically with the effects of solvents on the CD of metal complexes (5,6). Work in this field in our laboratory has been directed at elucidating the particular solvent-solute interactions that are responsible for the changes in the CD of complexes. Four general types of interactions have been recognized: (i) stereoselective solvation of the chiral ligand; (ii) solvation of the non-chiral ligand; (iii) ion association between a chiral ion and the counter ion; and (iv) coulombic or hydrogen-bond interactions that change conformer populat ions. Stereoselective Solvation of the Chiral Ligand As Bosnich and Harrowfield first pointed out, the CD of trans-[Co{(R)-pn} Cl ] , where pn is 1,2-propanediamine, is particularly sensitive to solvent ( 5,6). The A -> A tetra gonal component of the lowest energy spin-allowed cubic d-d absorption band ( A -> T )observed to have a positive Cotton effect in some solvents and a negative in others. Although the A -> E component was positive in each of the solvents +

2

2

1

1

1g

1

1

1g

1

2g

1g

1

1g

g

0-8412-0538-8/80/47-119-221$05.00/0 © 1980 American Chemical Society

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

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s t u d i e d , s i g n i f i c a n t v a r i a t i o n s were o b s e r v e d i n i t s r o t a t i o n a l s t r e n g t h {6) (Figure l ) . A H NMR s t u d y (6_) f o u n d t h a t t h e c h e m i c a l s h i f t s o f t h e N-H p r o t o n s were v e r y s e n s i t i v e t o s o l v e n t w i t h t h e s h i f t s b e i n g r e l a t e d t o t h e d o n o r number (j) of the s o l v e n t , i . e . the a b i l i t y o f t h e s o l v e n t t o donate e l e c t r o n s t o form a hydrogen bond w i t h t h e N-H p r o t o n s . F u r t h e r , t h e N-H p r o t o n s t h a t were e s s e n t i a l l y e q u a t o r i a l i n t h e p u c k e r e d f i v e - m e m b e r e d c h e l a t e r i n g showed l a r g e r v a r i a t i o n s i n t h e i r δ values than the a x i a l protons. The d i f f e r e n c e s i n t h e δ v a l u e s f o r t h e e q u a t o r i a l and a x i a l p r o t o n s f o r t h e NH g r o u p s were i n t h e o r d e r : py > dma ~ Me SO > dmf > MeOH > t h f * M e C 0 > s u l « MeCN, where py i s p y r i d i n e , dma d i m e t h y l a c e t a m i d e , dmf d i m e t h y l f o r m a m i d e , t h f t e t r a h y d r o f u r a n , and s u l s u l f o l a n e . The d i f f e r e n c e i n t h e s t r e n g t h o f t h e h y d r o g e n bond f o r m e d b e t w e e n t h e s o l v e n t and t h e e q u a t o r i a l and a x i a l N-H p r o t o n s makes t h e NH g r o u p asym m e t r i c , and t h e d e g r e e o f asymmetry i s s o l v e n t d e p e n d e n t . The r a n k o r d e r o f t h e s o l v e n t s b a s e d on t h e d e g r e e o f asym m e t r y i n t r o d u c e d i n t h e NH g r o u p s was f o u n d t o c o r r e s p o n d c l o s e l y w i t h t h e r a n k o r d e r b a s e d on t h e v a l u e s o f Δε f o r t h e t e t r a g o n a l components o f t h e ^ i g -> * T i g a b s o r p t i o n b a n d . As t h e d e g r e e o f asymmetry i n d u c e d by s o l v a t i o n i n c r e a s e d , t h e * A i g -> *Eg C o t t o n e f f e c t became l e s s p o s i t i v e and t h e A i g -* A g C o t t o n e f f e c t changed from n e g a t i v e t o p o s i t i v e . The d e g r e e o f s t e r e o s e l e c t i v i t y i n t h e s o l v a t i o n a t t h e NH g r o u p s o f a d i a m i n e o r amino a l c o h o l c o m p l e x depends on t h e p r e f e r e n c e o f t h e c h e l a t e r i n g t o e x i s t i n one p a r t i c u l a r c o n f o r m e r , and on t h e non-bonded i n t e r a c t i o n s b e t w e e n t h e h y d r o g e n - b o n d e d solvent molecule i n the a x i a l or e q u a t o r i a l o r i e n t a t i o n s with the g r o u p s on t h e a d j a c e n t c a r b o n a n d , more i m p o r t a n t l y , w i t h t h e a p i c a l l i g a n d s i n an o c t a h e d r a l c o m p l e x . I n c o m p l e x e s s u c h as [ M o { ( R ) - p n } ( C O ) ^ ] where t h e λ and δ c o n f o r m a t i o n s a r e e q u a l l y p o p u l a t e d (jB), s o l v a t i o n o f t h e N-H p r o t o n s does n o t r e s u l t i n t h e p r e p o n d e r a n c e o f an a s y m m e t r i c NH g r o u p o f one c o n f i g u r a t i o n as shown by t h e H NMR s p e c t r u m (9_) · The CD s p e c t r u m c o r r e s p o n d i n g l y does n o t e x h i b i t a marked s o l v e n t dependence. I n s u p p o r t o f t h i s t h e o r y r e l a t i n g t h e s o l v e n t dependence o f t h e CD o f c o m p l e x e s o f c h i r a l d i a m i n e s w i t h s t e r e o s e l e c t i v e s o l v a t i o n a t t h e NH group, the r o t a t i o n a l strengths o f the t e t r a g o n a l components o f t h e * A i g -> £\g a b s o r p t i o n b a n d f o r [ C o { ( R ) t m p n H C N ) ^ ] " , where tmpn i s N,N,N_ , N - t e t r a m e t h y l - l , 2 - p r o p a n e d i a m i n e , do n o t show any marked s o l v e n t dependence a l t h o u g h , as w i l l be d i s c u s s e d l a t e r , t h e p o s i t i o n s o f t h e CD bands do v a r y w i t h s o l v e n t (9.)· For the analogous (R)-pn complex, the A i •> A and &\g + * E C o t t o n e f f e c t s became l e s s n e g a t i v e and l e s s p o s i t i v e , r e s p e c t i v e l y , as t h e donor number o f t h e s o l v e n t i n c r e a s e d (9.) as was f o u n d f o r tvans~-[Co{(R)-pn} Cl ] (6_). Changes i n CD w i t h s o l v e n t o f a s i m i l a r o r d e r o f m a g n i t u d e have been o b s e r v e d f o r o t h e r r e l a t e d systems ( 5 , 1 0 , 1 1 ) .


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550

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Australian Journal of Chemistry Figure 1. CD spectrum of trans-/"CΌ {(Rj-pn^CUBP/ij in various solvents. Abbreviations are as follows: py, pyridine; dma, Ν,Ν-dimethylacetamide; dmf, Ν,Ν-dimethylformamide; thf, tetrahydrofuran; mf, N-methylformamide; sul, sulfolane (6).



224


Solvation o f t h e Non-Chiral

Ligand

Changes i n t h e o b s e r v e d CD o f a c o m p l e x w i t h s o l v e n t h a v e b e e n o b s e r v e d where s o l v a t i o n o f t h e c h i r a l l i g a n d i s u n i m p o r t a n t , b u t where NMR a n d , i n some c a s e s , Raman s p e c t r o s c o p i c e v i d e n c e shows t h a t t h e n o n - c h i r a l l i g a n d s i n t h e c o m p l e x a r e b e i n g s o l v a t ed t o v a r y i n g d e g r e e s . Two g e n e r a l e f f e c t s o f t h i s h a v e b e e n observed: (i) t h e s p e c t r o c h e m i c a l p a r a m e t e r , Δ, i s c h a n g e d b y s o l v a t i o n c a u s i n g a s h i f t i n t h e a b s o r p t i o n and CD s p e c t r a , a n d a change i n t h e t e t r a g o n a l o r r h o m b i c s p l i t t i n g o f t h e c u b i c a b s o r p t i o n bands, and (ii) t h e e n e r g y o f t h e c h a r g e t r a n s f e r a b s o r p t i o n bands t h a t are i n v o l v e d i n t h e i n d u c t i o n o f CD i n t h e d-d bands c h a n g e s significantly. S o l v a t i o n o f a c o o r d i n a t e d NH3 l i g a n d , w h i c h c a n be f o l l o w e d by *H M R , does n o t c a u s e a marked c h a n g e i n t h e Δ v a l u e f o r t h a t l i g a n d . However, p r o t o n a t i o n o f some l i g a n d s , f o r e x a m p l e CN~, b y a p r o t i c s o l v e n t c a u s e s a s i g n i f i c a n t c h a n g e i n t h e e n e r g y o f t h e d-d a b s o r p t i o n bands o f t h e m e t a l a n d , i n t h e c a s e o f CN", an i n c r e a s e i n t h e Δ v a l u e ( 1 2 , 1 3 ) . The a b s o r p t i o n bands a n d t h e a s s o c i a t e d C o t t o n e f f e c t s move w i t h t h e a c c e p t o r number (A.N.) (7.) o f t h e s o l v e n t , i . e . t h e a b i l i t y o f t h e p r o t i c s o l v e n t t o accept e l e c t r o n s from a s o l u t e t o form a hydrogen bond. F o r [Co{(R)-pn}(CN)iJ"", A f o r the A Τ a b s o r p t i o n band v a r i e s a s f o l l o w s : hmpa ( h e x a m e t h y l p h o s p h o r t r i a m i d e ) A.N. 1 0 . 6 , λ 3 6 8 nm; M e S 0 , A.N. 1 9 . 3 , λ 3 6 3 nm; MeOH, A.N. U l . 3 , λ 3 5 6 nm; H 0 , A.N. 5 U . 8 , λ 353 nm ( £ ) . Complexes w i t h a t e t r a g o n a l c h r o m o p h o r e , i . e . Σ Δ = Σ Δ ^ Σ Δ , f o r example [ C o - U R j - p n X C N ) ^ ] " , h a v e t h e * Α -* T i a b s o r p t i o n s p l i t i n t o two c o m p o n e n t s , w i t h t h e s e p a r a t i o n r e l a t e d t o t h e d i f f e r e n c e i n t h e s p e c t r o c h e m i c a l p a r a m e t e r s , Δ, f o r t h e l i g a n d s (lît,l£) : 1

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F o r t h e above t e t r a c y a n i d e c o m p l e x , t h e t e t r a g o n a l s p l i t t i n g i s g i v e n b y 0 . 2 5 ( A Q ^ - Δ ρ ) . As A Q ^ c h a n g e s w i t h s o l v e n t , t h e t e t r a g o n a l s p l i t t i n g v a r i e s , a n d t h e r e f o r e , t h e o b s e r v e d CD w i l l change e v e n t h o u g h t h e r o t a t i o n a l s t r e n g t h o f t h e i n d i v i d u a l Cotton e f f e c t s might not a l t e r . A l t h o u g h t h e l i g a n d - f i e l d bands o f ammine c o m p l e x e s a r e n o t v e r y s e n s i t i v e t o s o l v e n t , t h e c h a r g e - t r a n s f e r b a n d s do s h i f t w i t h solvent. F o r example, t h e -> M b a n d i n [ C o ( N H ) ] occurs at 1 9 7 nm i n w a t e r b u t a t 2^0 nm i n M e S 0 ( 9 . ) . The c h a r g e t r a n s f e r t r a n s i t i o n g e n e r a t e s p o s i t i v e c h a r g e o n t h e NH3 l i g a n d s . There f o r e , i n t h e e x c i t e d s t a t e , t h e l i g a n d s w o u l d be a b l e t o f o r m s t r o n g e r h y d r o g e n bonds w i t h a p r o t o n - a c c e p t o r s o l v e n t t h a n i n t h e ground s t a t e , and hence t h e energy s e p a r a t i o n between t h e ground and e x c i t e d s t a t e s w o u l d d e c r e a s e f o r t h e s e s o l v e n t s . S i n c e t h e η

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r o t a t i o n a l s t r e n g t h i n d u c e d i n a n e l e c t r i c d i p o l e f o r b i d d e n d- A a n d A]_ -> E ( D ) c o m p o n e n t s , r e s p e c t i v e l y , o f the A + T a b s o r p t i o n band ( 1 7 - 2 2 ) . This e f f e c t has b e e n a s c r i b e d t o t h e g e n e r a t i o n o f asymmetry a t t h e Ν d o n o r s v i a hydrogen bonding t o t h e a n i o n ( 2 3 ) . The d o n o r a n d a c c e p t o r p r o p e r t i e s o f t h e s o l v e n t a s w e l l as i t s d i e l e c t r i c c o n t r o l t h e degree o f i o n a s s o c i a t i o n . I f t h i s e f f e c t needs t o b e m i n i m i z e d f o r a c a t i o n i c s o l u t e , a n i o n s s u c h as p e r c h l o r a t e o r t e t r a p h e n y l b o r a t e s h o u l d be used w i t h s o l v e n t s t h a t possess a d i e l e c t r i c greater t h a n , say, 1 5 . F o r a n a n i o n i c com p l e x i n a p o l a r s o l v e n t , l i t h i u m c o u l d be used as a c o u n t e r i o n . E f f e c t s o f i o n a s s o c i a t i o n a r e more n o t i c a b l e f o r c o m p l e x e s t h a t c a r r y a h i g h c h a r g e , f o r example 3+» t h a n f o r i o n s t h a t c a r r y a s i n g l e charge. l

x

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The f r e e e n e r g y d i f f e r e n c e s b e t w e e n c o n f o r m e r s o f a s o l u t e a r e i n some c a s e s s o l v e n t d e p e n d e n t , e s p e c i a l l y w h e r e (a) one c o n f o r m e r o f t h e s o l u t e p o s s e s s e s s t r o n g e r i n t r a m o l e c u l a r hydrogen-bonding p r o p e r t i e s than o t h e r s , and t h e hydrogen-bonding c a p a b i l i t i e s o f t h e s o l v e n t s can modify t h e degree o f i n t r a m o l e c u l a r hydrogen b o n d i n g and t h u s change t h e c o n f o r m a t i o n a l e n e r g y d i f f e r e n c e s (h). A l t e r n a t i v e l y , t h e v a r i o u s c o n f o r m e r s may d i f f e r i n t h e i r a b i l i t y t o hydrogen bond t o t h e s o l v e n t , and t h e conformer p o p u l a t i o n s w i l l v a r y depending on t h e hydrogen-bonding character o f the solvent. (b) t h e m o l e c u l e p o s s e s s e s two n e i g h b o r i n g d i p o l e s t h a t a r e a t d i f f e r e n t o r i e n t a t i o n s t o one a n o t h e r i n t h e v a r i o u s c o n f o r m e r s ( 3 ) . The d i p o l a r i n t e r a c t i o n s , w h i c h c o n t r i b u t e t o t h e c o n f o r m a t i o n a l f r e e e n e r g y d i f f e r e n c e s , depend o n t h e d i e l e c t r i c o f t h e medium, b e i n g a t t e n u a t e d i n h i g h d i e l e c t r i c m e d i a , a n d enhanced i n l o w d i e l e c t r i c m e d i a . F o r s y s t e m s where h y d r o g e n - b o n d i n g i n t e r a c t i o n s a r e n o t o f g r e a t i m p o r t a n c e as f a r a s t h e CD i s c o n c e r n e d , b u t d i p o l a r i n t e r a c t i o n s a r e important, i t i s t h e o r e t i c a l l y p o s s i b l e t o de t e r m i n e t h e s t r a i n - e n e r g y d i f f e r e n c e and t h e d i p o l a r - i n t e r a c t i o n



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energy d i f f e r e n c e f o r two conformers i n e q u i l i b r i u m v i a a study o f t h e s o l v e n t dependence o f t h e CD. The o b s e r v e d r o t a t i o n a l s t r e n g t h i s r e l a t e d t o t h e s e two e n e r g y t e r m s b y e q u a t i o n 2 (3.) · RT i n { ( R ^ g

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where R i s t h e r o t a t i o n a l s t r e n g t h f o r a p a r t i c u l a r t r a n s i t i o n , A G i s t h e s t r a i n - e n e r g y d i f f e r e n c e , AV i s t h e d i p o l a r - i n t e r a c t i o n energy d i f f e r e n c e , and D i s t h e d i e l e c t r i c c o n s t a n t . I f both Rj and R j j a r e known, A G a n d AV c a n b e d e t e r m i n e d b y p l o t t i n g t h e left-hand side o f equation 2 against l/D. T h i s s h o u l d be a s t r a i ght l i n e w i t h s l o p e AV a n d i n t e r c e p t A G . I f o n l y R^ i s known, t h e above p l o t i s r e p e a t e d f o r v a r i o u s v a l u e s o f R J J - The v a l u e t h a t g i v e s t h e best s t r a i g h t l i n e r e l a t i o n s h i p i s chosen f o r R J J . I n t h e r e m a i n d e r o f t h i s p a p e r t h e s o l v e n t - d e p e n d e n t CD o f a p a r t i c u l a r c o m p l e x w i l l b e c o n s i d e r e d i n some d e t a i l . The example has b e e n c h o s e n b e c a u s e t h e v a r i a t i o n i n CD w i t h s o l v e n t i s r e l a t e d t o a s o l v e n t - i n d u c e d change i n t h e c o n f o r m e r p o p u l a t ions o f a c h i r a l l i g a n d .


S

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S o l v e n t Dependent CD o f P e n t a a m m i n e c o b a l t ( I I I ) C h i r a l Carboxylates

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The CD s p e c t r a o f c a r b o x y l i c a c i d s and e s t e r s h a v e b e e n studied extensively ( 2 Î - 3 5 ) . I n t h e r e g i o n o f t h e η -> π* t r a n s i t i o n o f t h e c a r b o x y l a t e g r o u p , two C o t t o n e f f e c t s o f o p p o s i t e s i g n have been o b s e r v e d , t h e i n t e n s i t i e s o f w h i c h v a r y i n v e r s e l y w i t h one a n o t h e r a s t h e s o l v e n t a n d / o r t h e t e m p e r a t u r e a r e changed. T h e s e two C o t t o n e f f e c t s h a v e b e e n a s s o c i a t e d w i t h two r o t a m e r s o f t h e c a r b o x y l a t e s , t h e p o p u l a t i o n s o f which a r e s o l v e n t and t e m p e r a t u r e dependent ( 2 8 - 3 1 ) . F o r a 2 - s u b s t i t u t e d p r o p i o n i c a c i d o r i t s e s t e r t h e most s t a b l e r o t a m e r s a r e shown i n F i g u r e 2 . R o t a m e r H I i s g e n e r a l l y c o n s i d e r e d n o t t o make a m a j o r c o n t r i b u t i o n t o t h e o b s e r v e d CD s p e c t r a b e c a u s e o f t h e p o t e n t i a l l y l o w r o t a t i o n a l s t r e n g t h o f i t s t r a n s i t i o n s and because o f i t s postulated low r e l a t i v e concentration. The CD s p e c t r a o f p e n t a a m m i n e c o b a l t ( I I I ) complexes o f ( S ) a-amino a c i d s b i n d i n g t h r o u g h t h e c a r b o x y l a t e g r o u p , a n d o f ( S ) 2 - s u b s t i t u t e d p r o p i o n a t e s have e i t h e r a p o s i t i v e o r a n e g a t i v e Cotton e f f e c t or both i n t h e region o f the A -* * E ( D ^ ) com p o n e n t o f t h e A i „ ->• T i g a b s o r p t i o n . P r e v i o u s l y t h e s e two bands h a v e b e e n a s c r i b e d t o t h e t w o components o f t h e t r a n s i t i o n r e n d e r e d n o n d e g e n e r a t e b y t h e l o w symmetry o f t h e o v e r a l l m o l e c u l e (36,37>38). However, i t h a s b e e n o b s e r v e d t h a t t h e changes i n t h e s i z e s o f t h e s e p o s i t i v e and n e g a t i v e C o t t o n e f f e c t s w i t h s o l v e n t and t e m p e r a t u r e a r e s i m i l a r t o t h o s e c h a n g e s f o u n d f o r t h e f r e e a c i d s o r t h e i r e s t e r s , and t h e s e changes c a n a l s o be r a t i o n a l i z e d , a t l e a s t i n p a r t , i n t e r m s o f changes i n r o t a m e r p o p u l a t i o n s . As an example o f t h i s , t h e CD s p e c t r u m o f p e n t a a m m i n e { ( S ) - 2 - c h l o r o p r o p i o n a t o ) c o b a l t ( i l l ) } w i l l b e c o n s i d e r e d i n some d e t a i l . x

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From t h e H NMR w o r k o f K a r a b a t s o s a n d c o w o r k e r s o n s u b s t i t u t e d acetaldehydes (39,^0)? and from t h e i n f r a r e d s t u d i e s o f Brown (hi) a n d L a a t o a n d I s o t a l o (h2) w i t h α-substituted c a r b o x y l i c a c i d e s t e r s , i t can be concluded t h a t i n s o l u t i o n the e q u i l i b r i u m b e t w e e n t h e r o t a m e r s i n F i g u r e 2 where R i s CH3 a n d X i s C I favours rotamer I I i n s o l v e n t s o f low d i e l e c t r i c w i t h rotamer I I I l e a s t favoured, and favors rotamer I i n solvents o f high dielectric. The s o l v e n t s w i t h h i g h d i e l e c t r i c a t t e n u a t e t h e d i p o l e - d i p o l e r e p u l s i o n b e t w e e n t h e C - C l a n d C=0 g r o u p s , w h i c h i s maximized i n rotamer I , and hence s h i f t t h e e q u i l i b r i u m towards rotamer I . I f the temperature o f the s o l u t i o n i s decreased, t h e a s s o c i a t e d i n c r e a s e i n d i e l e c t r i c would tend t o s h i f t t h e e q u i l i b r i u m towards rotamer I . As f o u n d f o r s i m i l a r c h i r a l c a r b o x y l a t e s , t h e CD s p e c t r a o f 2 - c h l o r o p r o p i o n i c a c i d and o t h e r 2 - c h l o r o c a r b o x y l i c a c i d s i n t h e region o f the η π a b s o r p t i o n have a p o s i t i v e and a n e g a t i v e Cotton e f f e c t w i t h t h e p o s i t i v e a t lower wavelength f o r the (S) c o n f i g u r a t i o n ( 2 8 ) . The l o w w a v e l e n g t h b a n d was a s s o c i a t e d w i t h r o t a m e r I a n d t h e h i g h w a v e l e n g t h band w i t h I I ( 2 8 ) . G a f f i e l d a n d G a l e t t o s t u d i e d t h e s o l v e n t - i n d u c e d changes i n t h e η ->· π r e g i o n o f t h e CD s p e c t r u m o f ( S ) - 2 - c h l o r o v a l e r i c a c i d , a n d f o u n d t h e g e n e r a l t r e n d t h a t , as t h e d i e l e c t r i c o f t h e s o l v e n t i n c r e a s e d , the p o s i t i v e Cotton e f f e c t i n c r e a s e d i n s i z e , and the n e g a t i v e d e c r e a s e d ( 2 8 ) . T h i s i s c o n s i s t e n t w i t h t h e above p r o p o s a l t h a t t h e e q u i l i b r i u m s h i f t s t o w a r d s I as t h e d i e l e c t r i c i s i n c r e a s e d . The v a r i a b l e t e m p e r a t u r e CD s p e c t r a f o r t h i s compound were a l s o r e p o r t e d w i t h EPA ( e t h e r : isopentane: e t h a n o l i n a volume r a t i o o f 5 : 5 : 2 ) as s o l v e n t . B o t h t h e n e g a t i v e a n d p o s i t i v e C o t t o n e f f e c t s i n c r e a s e d i n s i z e as t h e t e m p e r a t u r e was l o w e r e d . Since the d i e l e c t r i c increases w i t h decreasing temperature, a decrease i n t h e n e g a t i v e band might have been e x p e c t e d i f t h e d i e l e c t r i c e f f e c t was d o m i n a n t . T h i s was i n f a c t o b s e r v e d f o r t h e a n a l o g o u s ( S ) - a - c h l o r o c a r b o x y l i c a c i d s w i t h i s o p r o p y l and t e r t - b u t y l groups on t h e α-carbon: t h e p o s i t i v e C o t t o n e f f e c t i n c r e a s e d w h e r e a s t h e n e g a t i v e became d i m i n i s h i n g l y s m a l l a t - l 8 5 ° C . R i c h a r d s o n a n d S t r i c k l a n d (h3) have e s t i m a t e d t h e r o t a t i o n a l s t r e n g t h s o f t h e t r a n s i t i o n s o f t h e c a r b o x y l a t e chromophore f o r 2 - c h l o r o p r o p i o n i c a c i d d i r e c t l y from m o l e c u l a r o r b i t a l s c a l c u l a t e d u s i n g INDO a n d CNDO s e m i - e m p i r i c a l MO m o d e l s , a n d e x c i t e d - s t a t e wave f u n c t i o n s c o n s t r u c t e d i n t h e v i r t u a l o r b i t a l - c o n f i g u r a t i o n i n t e r a c t i o n approximation. Their c a l c u l a t i o n s , which, o f course, do n o t t a k e i n t o a c c o u n t s o l v e n t e f f e c t s , r e s u l t e d i n b i n d i n g energies f o r t h e rotamers i n c r e a s i n g i n the order I I < I I I < I which i s i n disagreement w i t h t h e order o f s t a b i l i t i e s concluded f r o m t h e NMR a n d i n f r a r e d w o r k . F u r t h e r , t h e c a l c u l a t i o n s y i e l d e d r o t a t i o n a l s t r e n g t h s o f t h e η -> π t r a n s i t i o n f o r r o t a m e r s I , I I , and I I I o f - 7 - 9 2 , +13.98, and - 1 7 - 5 , r e s p e c t i v e l y , i n c o n f l i c t w i t h t h e above c o n c l u s i o n s o f G a f f i e l d a n d G a l e t t o ( 2 8 ) . R i c h a r d s o n a n d S t r i c k l a n d (h3) q u e s t i o n e d t h e a s s i g n m e n t o f t h e h i g h w a v e l e n g t h n e g a t i v e b a n d t o t h e η -> π* t r a n s i t i o n ( 2 8 )




228

b e c a u s e t h e i r c a l c u l a t i o n s h a d y i e l d e d two a l m o s t d e g e n e r a t e t r a n s i t i o n s , one l o c a l i z e d o n t h e C l atom, and one i n v o l v i n g C l -> COOH c h a r g e t r a n s f e r , t h a t w e r e b e l o w t h e η -*· π* t r a n s i t i o n i n e n e r g y o c c u r r i n g a t a b o u t 260 nm. However, R i c h a r d s o n a n d S t r i c k l a n d a c k n o w l e d g e d t h a t t h e i r t h e o r e t i c a l model h a d d i f f i c u l t i e s w i t h m o l e c u l e s c o n t a i n i n g s e c o n d r o w atoms s u c h a s C l . F o r t h e o t h e r m o l e c u l e s t h e y s t u d i e d , t h e r e s u l t s were i n good agreement w i t h t h e CD s p e c t r a . One s t r u c t u r e o f p e n t a a m m i n e { ( S ) - 2 - c h l o r o p r o p i o n a t o } c o b a l t ( i l l ) i s g i v e n i n F i g u r e 3 . The l i g a n d i s shown a s t h e r o t a m e r I , w i t h t h e c a r b o x y l a t e g r o u p i n a p l a n e b i s e c t i n g ANCoN a s was f o u n d i n t h e X - r a y d e t e r m i n e d s t r u c t u r e (hk) o f a c e t a t o p e n t a a m m i n e cobalt(lll). The e n e r g y l e v e l d i a g r a m i s p r e s e n t e d i n F i g u r e U , and t h e a b s o r p t i o n s p e c t r u m o f t h e c o m p l e x f o r a r a n g e o f s o l v e n t s i n F i g u r e 5 · F o u r bands a r e o b s e r v e d a t t h e f o l l o w i n g a p p r o x i m a t e positions: A -> Τ ( 0 ) , 505 nm; Α -> T g ( O h ) , 350 nm; πρ(θ) d c h a r g e t r a n s f e r , 2h0 nm; a n d οι, -> d charge t r a n s f e r , 200 nm. The σ -> d _ y c h a r g e t r a n s f e r h a s A symmetry t h a t i s e l e c t r i c - d i p o l e forbidden. The t e t r a g o n a l l i g a n d f i e l d s p l i t s t h e A •> T i d-d b a n d i n t o A -> E a n d Δ\σ + * A components, w h i c h h a v e b e e n e s t i m a t e d t o o c c u r a t a b o u t 507 nm a n d 1*57 nm, r e s p e c t i v e l y ( 3 7 ) . The π-interaction removes t h e d e g e n e r a c y o f the A Eg t r a n s i t i o n t o g i v e components o f B a n d B i symmetry. I n s o l v e n t s o f r e l a t i v e l y l o w d i e l e c t r i c t h a t a r e a l s o p o o r donors, some i n t e r a c t i o n a p p e a r s t o o c c u r b e t w e e n t h e c o m p l e x a n d t h e t e t r a p h e n y l b o r a t e c o u n t e r - i o n as e v i d e n c e d b y t h e i n c r e a s e d a b s o r p t i o n i n t h e r e g i o n a b o u t 300 nm. The e f f e c t i s most m a r k e d f o r t h e t e t r a h y d r o f u r a n s o l u t i o n t h a t has t h e lowest d i e l e c t r i c o f the solvents studied. A c c o r d i n g t o S c h i p p e r ' s AICD t h e o r y ( l 6 ) , t h e d-d^ t r a n s i t i o n w i t h B symmetry g a i n s i t s r o t a t i o n a l s t r e n g t h v i a t h e c h a r g e t r a n s f e r t r a n s i t i o n s w i t h Β ( π ( θ ) -> d a n d a _ y -> d ) a n d Αχ symmetry, a n d t h e d-d t r a n s i t i o n w i t h B\ symmetry v i a t h e B\ (σ d ) a n d Αχ c h a r g e - t r a n s f e r t r a n s i t i o n s , w h e r e a s t h e (1-d t r a n s i t i o n w i t h A symmetry i s i n a c t i v e . This i sconsistent w i t h t h e o b s e r v e d CD s p e c t r a o f t h i s t y p e o f c o m p l e x as t h e components w i t h E p a r e n t a g e d o m i n a t e t h e CD s p e c t r a ( 3 6 , 3 7 , 3 8 ) . According t o t h e AICD t h e o r y , t h e i n d u c i b i l i t i e s o f t h e * A i •> B a n d t h e * A i -* * B components v i a t h e σ·^ -»· d charge-transfer transitions a r e o f t h e same s i g n (1+5), b u t more w o r k i s r e q u i r e d b e f o r e i t w i l l b e known w h e t h e r t h e AICD t h e o r y p r e d i c t s t h a t t h e two components h a v e t h e same s i g n f o r t h e i r r o t a t i o n a l s t r e n g t h s b e c a u s e t h e c o n t r i b u t i o n f r o m t h e π ( θ ) -> d 2 c h a r g e t r a n s f e r w i t h B symmetry h a s t o be d e t e r m i n e d a n d t h e o r i e n t a t i o n s o f t h e t r a n s i t i o n s w i t h r e s p e c t t o t h e i n d u c i n g moments must b e e v a l u a t e d . The CD s p e c t r a o f t h e c o m p l e x u n d e r t h e A i -> T a b s o r p t i o n a t 293 Κ i n a r a n g e o f s o l v e n t s a r e p r e s e n t e d i n F i g u r e 6, and t h e r e l a t i o n s h i p between t h e t o t a l r o t a t i o n a l s t r e n g t h and the d i e l e c t r i c constant o f the solvent i s p l o t t e d i n Figure 7· As e x p e c t e d f r o m t h e work w i t h t h e c a r b o x y l i c a c i d s a n d e s t e r s , t h e 1

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BAILEY AND HAWKINS

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