Infrared Spectroscopy of Organometallic Intermediates - ACS

Chapter 7

Downloaded by UNIV OF PITTSBURGH on December 21, 2014 | http://pubs.acs.org Publication Date: February 26, 1987 | doi: 10.1021/bk-1987-0333.ch007

Infrared Spectroscopy of Organometallic Intermediates James J. Turner, Michael A. Healy, and Martyn Poliakoff Department of Chemistry, University of Nottingham, Nottingham, NG7 2RD, England IR spectroscopy is a powerful spectroscopic technique for examining the structure and behavior of intermediates involved i n organometallic photochemistry. Examples are given of the combination of IR spectroscopy with matrix i s o l a t i o n , with liquid noble gases as solvents, and with flash generation, for probing novel transients and intermediates.

I t i s i m p o r t a n t t o know a s much a s p o s s i b l e a b o u t t h e i n t e r m e d i a t e s i n v o l v e d i n photochemical r e a c t i o n s , p a r t i c u l a r l y those o f r e l e v a n c e t o c a t a l y s i s Π,.2 ) · O v e r t h e p a s t f e w y e a r s o u r a p p r o a c h has i n v o l v e d t h r e e d i f f e r e n t t e c h n i q u e s - m a t r i x i s o l a t i o n , l i q u e f i e d noble gases as s o l v e n t s , and t i m e - r e s o l v e d spectroscopy f r e q u e n t l y i n c o m b i n a t i o n (_3 ) · I n e a c h m e t h o d , i n t e r m e d i a t e s a r e g e n e r a t e d b y UV p h o t o l y s i s a n d p a r t i c u l a r u s e i s made o f I R spectroscopy f o r d e t e c t i o n , c h a r a c t e r i s a t i o n and k i n e t i c measurements. Such emphasis on IR s p e c t r o s c o p y might be t h o u g h t strange given t h e ubiquitous presence o f X-ray c r y s t a l l o g r a p h y and nmr i n o r g a n o m e t a l l i c c h e m i s t r y . However, t h e s e i n t e r m e d i a t e s a r e g e n e r a l l y unstable and f r e q u e n t l y present i n low c o n c e n t r a t i o n s ; m o r e o v e r , IR s p e c t r o s c o p y c a n , w i t h c a u t i o n , be v e r y r e v e a l i n g , p a r t i c u l a r l y i f f u l l u s e i s made o f i s o t o p i c s u b s t i t u t i o n a n d , where a p p r o p r i a t e , f o r c e f i e l d c a l c u l a t i o n s . Inthis a r t i c l e w i l l be d e s c r i b e d some r e c e n t e x a m p l e s w h i c h i l l u s t r a t e t h e a p p l i c a t i o n of these techniques. Matrix

Isolation

The a p p l i c a t i o n o f m a t r i x i s o l a t i o n t o o r g a n o m e t a l l i c c h e m i s t r y h a s b e e n e x t e n s i v e l y d e s c r i b e d e l s e w h e r e (4,5^,6,7) . Two m e t h o d s h a v e g e n e r a l l y been employed. I n t h e f i r s t , based on G.C Pimentel's o r i g i n a l development, t h e s o l i d matrix environment i s a frozen n o b l e g a s - u s u a l l y A r - a t 10-20K a n d t h e u n s t a b l e f r a g m e n t i s generated e i t h e r by p h o t o l y s i s o f a parent molecule already trapped i n t h e m a t r i x , o r by cocondensation from t h e gas phase. In the

0097-6156/87/0333-0110$06.00/0 © 1987 American Chemical Society

In High-Energy Processes in Organometallic Chemistry; Suslick, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


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s e c o n d , d e r i v e d f r o m G.N. L e w i s a n d G. P o r t e r , t h e m a t r i x i s a f r o z e n o r g a n i c g l a s s a t 77K a n d t h e t r a n s i e n t i s g e n e r a t e d b y p h o t o l y s i s o f a p a r e n t w h i c h h a s been d i s s o l v e d i n t h e h o s t s o l v e n t p r i o r t o f r e e z i n g . The two v a r i a n t s have a d v a n t a g e s a n d d i s a d v a n t a g e s , b u t when a p p l i e d t o o r g a n o m e t a l l i c p h o t o c h e m i s t r y i n w h i c h t h e i n t e r m e d i a t e s a r e d e t e c t e d by IR s p e c t r o s c o p y o f i n t e n s e V(C-O) b a n d s , t h e r e i s l i t t l e t o c h o o s e b e t w e e n them: t h e a d v a n t a g e o f f r o z e n n o b l e g a s e s i s t h a t t h e IR bands a r e u s u a l l y n a r r o w e r a n d hence t h e complex s p e c t r a produced on i s o t o p i c s u b s t i t u t i o n a r e more r e v e a l i n g ; t h e d i s a d v a n t a g e o f t h e n o b l e g a s method i s t h a t the p a r e n t s p e c i e s h a s t o be s u f f i c i e n t l y v o l a t i l e t o be d e p o s i t e d v i a t h egas phase without decomposition. V e r y many C O - c o n t a i n i n g s p e c i e s have been i n v e s t i g a t e d by t h e s e methods. One s p e c i f i c a d v a n t a g e o f t h e f r o z e n n o b l e g a s i s t h a t i t d o e s not a b s o r b IR a n d t h u s i t i s p o s s i b l e , i n p r i n c i p l e , t o examine t h e spectrum o f any c o o r d i n a t e d l i g a n d w i t h i n t h e m a t r i x . In practice, i t i s more d i f f i c u l t b e c a u s e , a l t h o u g h t h e i n t e n s i t y o f V(N-N) a n d V(N-O) b a n d s a r e c o m p a r a b l e w i t h V ( C - O ) b a n d s , m o s t o f t h e v i b r a t i o n s o f o t h e r l i g a n d s g i v e r a t h e r weak I R b a n d s . H o w e v e r , a s an e x a m p l e , i n r e c e n t i m p o r t a n t w o r k , P e r u t z ( 8 ) h a s d e m o n s t r a t e d t h a t p h o t o l y s i s o f C p R h i C ^ H ) i n A r m a t r i c e s a t 20K l e a d s t o r e v e r s i b l e l o s s o f C^H^ t o f o r m C p R h ( C H ) i d e n t i f i e d b y I R s p e c t r o s c o p y , whereas p h o t o l y s i s o f C p I r ( C H ) gives v i n y l hydride complexes. In p r i n c i p l e , IR s p e c t r o s c o p y a l s o o f f e r s a method o f p r o b i n g a g o s t i c i n t e r a c t i o n s i n w h i c h H atoms on a l k y l g r o u p s i n t e r a c t w i t h e m p t y c o o r d i n a t i o n s i t e s o n a m e t a l , a n d C-H a c t i v a t i o n , v i a t h e b e h a v i o r o f t h e s t r e t c h i n g v i b r a t i o n o f CH g r o u p s a t t a c h e d t o t r a n s i t i o n metals. U n f o r t u n a t e l y , t h e p i c t u r e i sc o m p l i c a t e d by c o u p l i n g b e t w e e n CH b o n d s t o g i v e s y m m e t r i c a n d a n t i s y m m e t r i c modes a n d b y F e r m i r e s o n a n c e b e t w e e n t h e s e t w o modes a n d o v e r t o n e s o f b e n d i n g m o d e s . H o w e v e r , M c K e a n (9,_1_0) h a s s h o w n t h a t t h e s e p r o b l e m s a r e r e m o v e d b y o b s e r v a t i o n o f C~H s t r e t c h i n g f r e q u e n c i e s o f t h e CHD g r o u p - t h e V(C-H)^ method. A graph o f v ( C - H ) v e r s u s known C-H b o n d l e n g t h f o r a s e r i e s o f C/H c o m p o u n d s s h o w s a r e m a r k a b l e s t r a i g h t l i n e w i t h a g r a d i e n t c o r r e s p o n d i n g t o a bond l e n g t h change o f - 0 . 0 0 0 1 0 2 3 8 p e r cm s h i f t i n frequency [10]. In a striking a p p l i c a t i o n t o o r g a n o m e t a l l i c c h e m i s t r y , McKean a n d c o l l e a g u e s ( 1 1 ) h a v e shown t h a t i n t h e g a s p h a s e t h e V ( C - H ) . frequencies of C H D M n ( C O ) a n d C H D R e ( C O ) a r e 2 9 5 5 . 0 a n d ^ 3 4 . 6 cm r e s p e c t i v e l y , i m p l y i n g C-H b o n d l e n g t h s o f 1 . 0 9 5 a n d 1 . 0 9 8 8 respectively. 2

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I n c o l l a b o r a t i o n w i t h M c K e a n a n d M c Q u i l l a n we h a v e b e e n a p p l y i n g t h e V(C~H). technique t o unstable species i n matrices. The c o m p l e x p h o t o c h e m i s t r y o f C H ^ M ( C O ) ^ (M = Mn, R e ) h a s a l r e a d y b e e n u n r a v e l l e d b y m a t r i x i s o l a t i o n e x p e r i m e n t s , u t i l i s i n g b o t h IR (V(C-O) a n d V(N-N) v i b r a t i o n s ) a n d U V / v i s i b l e s p e c t r o s c o p y ( 1 2 ) . The p h o t o c h e m i s t r y p a t t e r n i s i l l u s t r a t e d i n 1. F i g u r e 1 s h o w s t h e V(C-H). o f some o f t h e s e s p e c i e s . The f i r s t s t r i k i n g o b s e r v a t i o n i s thaè each V(C-H). i s s p l i t , w i t h some w e a k e r f e a t u r e s ; t h i s b e h a v i o r i s r e l a t e d ! © t h e r o t a t i o n o f C H D g r o u p s a r o u n d t h e M-C b o n d a n d w i l l b e d i s c u s s e d e l s e w h e r e (J_3) . F o r p r e s e n t p u r p o s e s t h e r e a r e some i m p o r t a n t p o i n t s t o n o t e . T h e mean p o s i t i o n s o f the V(C-H). b a n d s o f t h e p a r e n t c o m p o u n d s CHD Mn(CO) a n d S

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CHD Re(CO),_ a r e v e r y c l o s e t o t h e g a s p h a s e p o s i t i o n s a n d t h e s h i f t (Mn t o R e ) i s a l m o s t i d e n t i c a l i n g a s p h a s e a n d m a t r i x . T h u s , V(C-H). i n t h e m a t r i x w i l l p r o v i d e i n f o r m a t i o n o n t h e C-H b o n d length. On g e n e r a t i o n o f t h e u n s t a b l e _ i y c o m p l e x e s , t h e r e i s a downward s h i f t i n V(C-H). o f 11-14 cm i m p l y i n g a bond l e n g t h e n i n g o f .001 1-.00^§ 8. T h i s i s c l e a r l y a v e r y s e n s i t i v e probe o f t h e e f f e c t of s l i g h t e l e c t r o n i c p e r t u r b a t i o n o f the metal c e n t r e and i s l i k e l y t o be u s e f u l f o r o t h e r s y s t e m s . 2

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L i q u e f i e d Noble Gases as

Solvents

L i q u e f i e d Xe a n d K r h a v e t w o f e a t u r e s w h i c h make t h e m p a r t i c u l a r l y a t t r a c t i v e as low-temperature solvents f o r examining the spectra of unstable organometallics. These s o l v e n t s a r e i n e r t , and t h i s i s important as t h e s i g n i f i c a n t r o l e o f even innocuous s o l v e n t s such as c y c l o h e x a n e i s more r e a d i l y r e c o g n i s e d ; i n c o n t r a s t t o c o n v e n t i o n a l s o l v e n t s f o r IR s p e c t r o s c o p y t h e y h a v e no a b s o r p t i o n s over a wide range o f t h e spectrum, which a l s o permits t h e use o f s p e c i a l l o n g p a t h c e l l s t o overcome problems o f low s o l u b i l i t y . We h a v e u s e d s u c h c e l l s (J_4) t o g e n e r a t e a n d m e a s u r e t h e k i n e t i c s t a b i l i t y over a wide temperature range o f a wide v a r i e t y of unstable s p e c i e s . The e a s i e s t e x p e r i m e n t s - a n d t h e f i r s t p e r f o r m e d - were s i m p l y t h e p h o t o l y s i s o f a m e t a l c a r b o n y l i n l i q u i d x e n o n d o p e d w i t h d i s s o l v e d N a n d h e n c e r e p l a c e m e n t o f CO b y Ν . T h e V ( N - N ) IR b a n d s a r e l i t t l e w e a k e r t h a n V ( C - O ) b a n d s a n d hence d e t e c t i o n and c h a r a c t e r i z a t i o n a r e s t r a i g h t f o r w a r d , p a r t i c u l a r l y when u s e i s made o f p r e v i o u s m a t r i x s t u d i e s . Species e x a m i n e d i n c l u d e C r ( ° ) _ ( N ) ( x = 1-5) (J_5) , N i ( C O ) ( N ) (J_6) W i t h V(N-O) b a n d s i t i s e q u a l l y s t r a i g h t f o r w a r d t o c h a r a c t e r i z e , 2

C

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for

( N

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3

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( x

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1

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(

)

instance, Fe(CO) _ ( °) 2 x ' — ' H o w e v e r , a s i n soîid n o b l e g a s m a t r i c e s , t h e r e a l v a l u e o f l i q u e f i e d n o b l e g a s e s b e c o m e s a p p a r e n t when o t h e r a n d m o r e i n t e r e s t i n g ligands are attached t o the t r a n s i t i o n metal. F o l l o w i n g K u b a s ' s (JJ3) o r i g i n a l s y n t h e s i s o f ( O C ) M o ( P C y ) ( H ) t h e r e has been a g r e a t f l u r r y o f a c t i v i t y i n compounds i n w h i c h H i s sideways bonded, u n d i s s o c i a t e d , t o a t r a n s i t i o n m e t a l (19-22). There i s c o n s i d e r a b l e t h e o r e t i c a l i n t e r e s t (23-27) and i t i s l i k e l y t h a t a w h o l e r a n g e o f c o m p o u n d s w i l l b e made o f w h i c h many w i l l b e unstable. A convenient general route t o such molecules i s s i m i l a r t o t h e N s y n t h e s i s method d e s c r i b e d above b u t r e p l a c i n g t h e N gas by H i n the high pressure c e l l . We c a n c h a r a c t e r i z e t h e H unit by IR s p e c t r o c o p y . F o r i n s t a n c e , F i g u r e 2 s h o w s t h e V(H-H) b a n d s of M(CO) H (M = C r , Mo, W) i n l i q u i d x e n o n a t -70°C ( 2 8 ) ; t h e s i m p l e s t t h e o r e t i c a l a r g u m e n t w o u l d r e l a t e t h e s h i f t i n V(H-H) frequency between f r e e H and c o o r d i n a t e d H , t o t h e degree o f i n t e r a c t i o n of H with the t r a n s i t i o n metal. I t i s gratifying that t h e s t a b i l i t y o f t h e s e t h r e e s p e c i e s i s i n t h e o r d e r W>Cr>Mo, t h e o r d e r o f i n c r e a s i n g V(H-H) f r e q u e n c i e s . I t s h o u l d be n o t e d t h a t i n a n u m b e r o f m a t r i x e x p e r i m e n t s (29,_30) i n v o l v i n g H - d o p e d m a t r i c e s , s p e c t r o s c o p i c e v i d e n c e f o r H complexes has been o b t a i n e d b u t such s t u d i e s have h a d t o r e l y on V(C-O) bands s i n c e i t i s n o t p o s s i b l e t o b u i l d up a s u f f i c i e n t c o n c e n t r a t i o n o f H c o m p l e x i n t h e m a t r i x t o o b s e r v e IR b a n d s a s s o c i a t e d w i t h t h e M-H moiety. 2

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I t i s p o s s i b l e under h i g h pressure to photolyse Cr(CO) (H ) f u r t h e r and t o g e n e r a t e C r ( C O ) ( H ) and t h e n t o o b s e r v e exchange a n d s u b s t i t u t i o n r e a c t i o n s (28)· These r e a c t i o n s are summarised i n Scheme 2 w h e r e t h e m o s t s i g n i f i c a n t p r o c e s s i s t h e H /D^ e x c h a n g e . One o f t h e r e a s o n s f o r t h e c u r r e n t i n t e r e s t i n complexes i s connected with c a t a l y s i s . Thus i t i s i m p o r t a n t t o examine compounds i n w h i c h b o t h alkenes (or dienes) are coordinated t o t h e same m e t a l c e n t r e ; i n r e a l c a t a l y t i c s i t u a t i o n s s u c h i n t e r m e d i a t e s a r e l i k e l y t o be v e r y r e a c t i v e . As a f i r s t s t e p we h a v e i n v e s t i g a t e d some u n s t a b l e e t h y l e n e , 1 - b u t e n e a n d butadiene c o m p l e x e s g e n e r a t e d by p h o t o l y s i s o f p a r e n t m o l e c u l e s s u c h as Cr(CO) , F e ( C O ) ( N O ) , C o ( C O ) ( N O ) , d i s s o l v e d i n l i q u i d xenon doped w i t h t h e a p p r o p r i a t e o r g a n i c m o l e c u l e (3J_>32_) . A g a i n , IR s p e c t r o s c o p y o f t h e o r g a n i c l i g a n d has been c r u c i a l i n a s s i g n i n g the s t r u c t u r e of the photoproduct; f o r i n s t a n c e , F i g u r e 3 shows t h e d e r i v a t i o n of t h e bands a s s i g n e d t o c o o r d i n a t e d 1-butene i n C o ( C O ) ( N O ) ( 1 - b u t e n e ) p r o d u c e d on p h o t o l y s i s o f C o ( C O ) ( N O ) i n 1-butene doped l i q u i d xenon. 5

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S i n c e C r ( C O ) ^ i s a w e l l k n o w n hydrogénation p h o t o c a t a l y s t ( 2 ) , an i n t e r e s t i n g e x p e r i m e n t i s t o c h a r a c t e r i z e a C r ( a l k e n e ) ( ) c o m p l e x s t a r t i n g f r o m an a l k e n e c o m p l e x p a r e n t . Until recently t h e r e w e r e no s t a b l e m o n o a l k e n e c o m p l e x e s o f C r , b u t G r e v e l s (33) h a s s y n t h e s i s e d (OC) C r ( t r a n s - c y c l o o c t e n e ) (Ξ ( O O ^ C r i o l ) ) w h i c h i s s t a b i l i s e d because of r e l i e f of r i n g s t r a i n . When t h i s c o m p o u n d was d i s s o l v e d i n l i q u i d Xe a n d p h o t o l y s e d u n d e r h i g h pressure, n o t h i n g h a p p e n e d ; h o w e v e r , i n l i q u i d K r a t -120°C t h e r e was strong evidence f o r the generation of ( O O ^ C r ( o l ) ( D ) , w i t h a h a l f - l i f e of a b o u t a n h o u r , a s s h o w n i n F i g u r e 4 (.34). G i v e n t h e i n s t a b i l i t y o f t h i s s p e c i e s i t was n o t p o s s i b l e t o g e n e r a t e a s u f f i c i e n t c o n c e n t r a t i o n t o o b s e r v e a V(D-D) b a n d b u t f u r t h e r e v i d e n c e f o r t h e c o r r e c t a s s i g n m e n t comes f r o m o b s e r v i n g i t s r e a c t i o n w i t h d i s s o l v e d Ν . An i n t e r e s t i n g q u e s t i o n i s , why d o e s c o o r d i n a t i o n o f t h e o l e f i n s o w e a k e n t h e m e t a l / H i n t e r a c t i o n t o make C r ( C O ) ( o l ) ( ) much l e s s s t a b l e t h a n C r ( C O ) ( H ) ? 2

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2

Spectroscopy

A l t h o u g h m a t r i x i s o l a t i o n and l o w - t e m p e r a t u r e s o l v e n t s have g r e a t p o t e n t i a l f o r i d e n t i f y i n g i n t e r m e d i a t e s , and f o r o b t a i n i n g s t r u c t u r a l i n f o r m a t i o n , and - i n t h e c a s e o f t h e s o l v e n t s - f o r o b t a i n i n g k i n e t i c data, i t i s necessary to r e l a t e these o b s e r v a t i o n s t o more o r d i n a r y c o n d i t i o n s , i . e . c o n v e n t i o n a l s o l v e n t s a t room temperature. U V / v i s i b l e s p e c t r o s c o p y o f o r g a n o m e t a l l i c t r a n s i e n t s has been e x t r e m e l y i m p o r t a n t f o r k i n e t i c m e a s u r e m e n t s on p r e v i o u s l y i d e n t i f i e d s p e c i e s , but the s p e c t r a are l e s s v a l u a b l e f o r s t r u c t u r a l i d e n t i f i c a t i o n s i n c e m o s t s p e c t r a show b r o a d a n d f e a t u r e l e s s bands. One way r o u n d t h i s p r o b l e m h a s b e e n t o u t i l i s e m a t r i x IR s p e c t r o s c o p y f o r c h a r a c t e r i s a t i o n a n d t o u s e t h e d a t a obtained from the matrix U V / v i s i b l e spectrum to monitor the room-temperature k i n e t i c s . A more s a t i s f a c t o r y method i s t o r e c o r d t h e IR s p e c t r a o f t r a n s i e n t s d i r e c t l y a n d t h e r e h a s b e e n much a c t i v i t y i n b o t h gas p h a s e and s o l u t i o n o r g a n o m e t a l l i c c h e m i s t r y ; t h i s f i e l d h a s b e e n r e c e n t l y r e v i e w e d (35) . In our l a b o r a t o r y ,



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1

3120

I

»

»

1

3050

cm"

I

•

»

2750

J

1

2680

1

F i g u r e 2. I R s p e c t r a s h o w i n g t h e s u b s t a n t i a l w a v e n u m b e r s h i f t b e t w e e n b a n d s a s s i g n e d t o t h e V(H-H) v i b r a t i o n o f c o o r d i n a t e d molecular hydrogen i n M o ( C O ) ( H ) , C r ( C 0 ) ( H ), and W(CO) (H) i n l i q u i d Xe a t -70°C. Note t h a t t h e t h r e e t r a c e s were r e c o r d e d i n separate experiments and that t h e absorbance s c a l e r e f e r s t o t h e Mo t r a c e [ 2 8 ] . 5

2

5

Scheme 2


5


116


WAVENUM8ERS

F i g u r e 3. S e r i e s o f I R s p e c t r a i l l u s t r a t i n g how t h e a b s o r p t i o n s ( s p e c t r u m e) o f c o o r d i n a t e d 1-butene were i d e n t i f i e d : ( a ) s p e c t r u m o f L X e b e f o r e a d d i t i o n o f r e a c t a n t s (weak b a n d s d u e t o t r a c e i m p u r i t i e s ) ; (b) s p e c t r u m a f t e r a d d i t i o n o f 1-butene; ( c ) s p e c t r u m a f t e r a d d i t i o n o f Co(CO)~NO t o t h e b u t e n e / L X e m i x t u r e ( b a n d s d u e t o C o ( C O ) NO m a r k e d w i t h o ) ; ( d ) s p e c t r u m o b t a i n e d a f t e r UV p h o t o l y s i s ( b a n d s d u e t o C o ( C O ) ( N O ) ( η - 1 - b t n ) m a r k e d w i t h Ψ^; ( e ) s p e c t r u m o f c o o r d i n a t e d 1 - b u t e n e i n Co(CO) (NO)(η - 1 - b u t e n e ) o b t a i n e d f r o m s p e c t r u m d b y c o m p u t e r s u b t r a c t i o n o f t h e b a n d s o f C o ( C O ) NO a n d 1 - b u t e n e ( l a r g e a r r o w m a r k s t h e V(C=C) b a n d ) [ 3 1 ] . 2

2


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117



7.

2Û80

2055

2 0 3 0

2QG5

1980

1955

193Q

1905

NAVENUMBER

F i g u r e 4. I R s p e c t r a i n D ^ d o p e d l i q u i d K r a t -120°C o f (0C) Cr(ol) and o f the p h o t o l y s i s product (OC) Cr(ol)(D ) [ o l= trans-cyclooctene]. was u s e d r a t h e r t h a n t o avoid p o t e n t i a l s p e c t r a l o v e r l a p o f a n y V(H-H) b a n d s w i t h b a n d s d u e t o the hydrocarbon l i g a n d . 5

5



118


t r a n s i e n t s a r e g e n e r a t e d by f l a s h p h o t o l y s i s u s i n g e i t h e r a x e n o n f l a s h l a m p o r a n X e C l e x c i m e r l a s e r ; t h e r a p i d l y c h a n g i n g IR s i g n a l i s m o n i t o r e d a t a p a r t i c u l a r IR f r e q u e n c y b y a l i n e - t u n a b l e CO laser. The e x p e r i m e n t i s r e p e a t e d o v e r a w h o l e s e r i e s o f CO l a s e r l i n e s s e p a r a t e d b y a p p r o x i m a t e l y 4 cm . Fortunately, there i s a good o v e r l a p b e t w e e n t h e V(C-O) a b s o r p t i o n bands o f o r g a n o m e t a l l i c c o m p o u n d s a n d t h e o u t p u t o f a CO l a s e r . F u l l d e t a i l s of the a p p a r a t u s h a v e b e e n d e s c r i b e d e l s e w h e r e (3J3,_36) . A s i m p l e a p p l i c a t i o n o f t i m e - r e s o l v e d IR s p e c t r o s c o p y (TRIR) is d i s t i n g u i s h i n g between C O - b r i d g e d and n o n - b r i d g e d d i n u c l e a r m e t a l c a r b o n y l s i n s o l u t i o n ( 3J7 ) - F o r e x a m p l e , p h o t o l y s i s o f MM'(CO) (Μ,Μ' = Mn, Re) i n s o l u t i o n i n v o l v e s two p a t h w a y s , b e l i e v e d t o be t h e r a d i c a l M ( C O ) ^ a n d t h e C O - l o s s s p e c i e s MM'(CO)g. A c o m b i n a t i o n o f m a t r i x i s o l a t i o n a n d T R I R s h o w s t h a t Mn ( C O ) ^ and MnRe(CO) ( 3 8 - 4 2 ) h a v e CO b r i d g e s b u t t h a t Re (CO) (42-44) does n o t a n d t h a t t h e r a t e o f r e a c t i o n o f CO w i t h MnRe(CO) i s greater t h a n w i t h Re^(CO)^. E x t e n s i v e u s e h a s b e e n made o f t h i s m e t h o d i n i d e n t i f y i n g a n d m o n i t o r i n g t h e b e h a v i o r o f t h e two p h o t o t r a n s i e n t s f r o m [ C p F e ( C O ) ] , i . e . the r a d i c a l CpFe(CO) and t h e n o v e l t r i p l y - b r i d g e d s p e c i e s CpFe(p-CO) FeCp ( 4 5 , 4 b ) , t h e s t r u c t u r e o f w h i c h has been f i r m l y i d e n t i f i e d by m a t r i x e x p e r i m e n t s (47,48). When [ C p F e ( C O ) ] i s p h o t o l y s e d i n CH CN s o l u t i o n , the major p r o d u c t i s ( C O ) C p F e ( M - C O ) F e C p C H CN. An i n t e r e s t i n g q u e s t i o n a r i s e s : W h i c h o f t h e two i n t e r m e d i a t e s i s r e s p o n s i b l e f o r t h e o v e r a l l photochemistry? F i g u r e 5 shows t h e TRIR s p e c t r a f r o m a f l a s h e x p e r i m e n t i n v o l v i n g [ C p F e ( C O ) ] a n d CH CN i n c y c l o h e x a n e s o l u t i o n ( 4 6 ) . From t h e s e s p e c t r a i t i s c l e a r t h a t t h e p r o d u c t g r o w s i n a t t h e same r a t e a s t h e t r i p l y b r i d g e d i n t e r m e d i a t e d e c a y s ; t h e r a d i c a l d i s a p p e a r s much f a s t e r . Hence t h e mechanism o f t h i s r e a c t i o n i n v o l v e s the r e a c t i o n of the t r i p l y bridged species w i t h CH CN. 2

2

3

2

2

2

3

I n r e c e n t w o r k i n c o l l a b o r a t i o n w i t h G.R. D o b s o n , we h a v e examined t h e ^ f l a s h p h o t o l y s i s of W ( C O ) ( p i p e r i d i n e ) L (L = PPh > P ( O E t ) , P ( 0 P r ) ) i n η-heptane s o l u t i o n . T h e r e was previous e v i d e n c e f o r t h e p r o d u c t i o n o f two i n t e r m e d i a t e s t e n t a t i v e l y i d e n t i f i e d a s W(CO) L ( S ) (S = s o l v e n t ) , w i t h L a n d S c i s a n d t r a n s t o e a c h o t h e r (49,5ÎÎ) . I t h a s now p r o v e d p o s s i b l e t o o b t a i n f i r m i d e n t i f i c a t i o n o f t h e s e two s p e c i e s i n r o o m t e m p e r a t u r e s o l u t i o n f r o m t h e i r V(C-O) a b s o r p t i o n s p e c t r a . I t c o u l d be p r o v e d t h a t t h e i r s t r u c t u r e s w e r e i n d e e d c i s a n d t r a n s W(CO)^L b y g e n e r a t i n g e x a c t l y t h e same s p e c i e s i n l o w - t e m p e r a t u r e m a t r i c e s . One noteworthy f e a t u r e of the experiments i s t h a t , even w i t h such a w e a k l y c o o r d i n a t i n g s o l v e n t as h e p t a n e , t h e c i s and t r a n s i n t e r m e d i a t e s d o n o t i n t e r c o n v e r t on a m i l l e s e c o n d t i m e s c a l e [.53]. I t w i l l be c l e a r t h a t i n t h e s e e x a m p l e s t h e s h o r t - l i v e d t r a n s i e n t s have been d e t e c t e d by t h e i r V(C-O) v i b r a t i o n s . Church (51) has managed t o o b s e r v e V(N-N) o f C r ( C O ) ( N ) and W e i t z has o b s e r v e d V(N-O) o f t r a n s i e n t s p r o d u c e d i n t h e r e a c t i o n o f Fe(CO),. w i t h NO ( 5 2 ) . However, a t t h i s s t a g e i t l o o k s as though d e t e c t i o n o f o r g a n o m e t a l l i c i n t e r m e d i a t e s v i a , f o r e x a m p l e , V(C-H) o r V(C=C) a b s o r p t i o n s , w i l l r e q u i r e e x t e n s i v e improvements i n s e n s i t i v i t y . 4

3

3

3

5

2



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119

F i g u r e 5. T i m e - r e s o l v e d I R s p e c t r a o b t a i n e d a f t e r UV f l a s ^ h p h o t o l y s i s o f [ C p F e ( C O ) ] , A, ( 6 x 1 0 M) a n d CH CN ( 6 x 1 0 ~ M) i n c y c l o h e x a n e s o l u t i o n a t 25°C. The b a n d s a r e l a b e l l e d Β [CpFe(CO) ], C [Cp Fe (M-CO) ] a n d D [ C p F e ( C O ) ^ ( C H ^ C N ) . The s p e c t r a have been r e c o n s t i t u t e d f r o m ~70 k i n e t i c t r a c e s recorded a t i n t e r v a l s o f ~4 cm a c r o s s t h e wavenumber region illustrated. The s p e c t r a a r e drawn b y i n t e r p o l a t i o n between t h e s e d i s c r e t e p o i n t s . The f i r s t t h r e e s p e c t r a c o r r e s p o n d t o t h e d u r a t i o n o f t h e f i r i n g o f t h e UV f l a s h l a m p a n d s u b s e q u e n t s p e c t r a a r e shown a t i n t e r v a l s o f 10 p s e c . The n e g a t i v e p e a k s i n t h e f i r s t spectrum a r e due t o m a t e r i a l d e s t r o y e d byt h e f l a s h . These n e g a t i v e peaks have been o m i t t e d from t h e s u b s e q u e n t t r a c e s t o a v o i d undue c o n f u s i o n i n t h e f i g u r e . Reproduced w i t h p e r m i s s i o n from Ref. 46. C o p y r i g h t 1986, The Chemical Society. 2

2

2

2

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Conclusions T h e s e e x a m p l e s show t h a t I R s p e c t r o s c o p y c o n t i n u e s t o b e a p o w e r f u l technique f o rthe characterisation o f unstable intermediates important i norganometallic photochemistry.


Acknowledgments Work a t N o t t i n g h a m h a s b e e n s u p p o r t e d b y t h e SERC, t h e D o n o r s o f t h e P e t r o l e u m R e s e a r c h Fund a d m i n s t e r e d by t h e American C h e m i c a l S o c i e t y , t h e P a u l I n s t r u m e n t Fund o f t h e R o y a l S o c i e t y a n d t h e S c i e n t i f i c S t i m u l a t i o n P r o g r a m m e o f t h e E E C . We a l s o g r a t e f u l l y a c k n o w l e d g e t h e h e l p o f many c o l l e a g u e s b o t h a t N o t t i n g h a m a n d elsewhere.

Literature Cited 1. Geoffroy, G.L.; Wrighton, M.S. 'Organometallic Photochemistry', Academic Press: New York, 1979. 2. Moggi, L.; Juris, Α.; Sandrini, D.; Manfrin, M.F. Rev. Chem. Intermed. 1981, 4, 171. 3. For further background discussion see e.g. Turner, J.J.; Poliakoff, M. 'Photochemical Intermediates' in Chisholm, M.A. Ed. 'Inorganic Chemistry Towards the 21st Century', American Chemical Society, 1983: Fresenius Z. Anal. Chem. 1986, 324, 819. 4. Hitam, R.B.; Mahmoud, Κ.A.; Rest, A.J. Coord. Chem. Rev. 1984, 55, 1. 5. Burdett, J.K. Coord. Chem. Rev. 1978, 27, 1. 6. Wrighton, M.S.; Graff, J.L.; Kazlauskas, R.J.; Mitchener, J.C.; Reichel, C.L. Pure Appl. Chem. 1982, 54, 161. 7. Perutz, R.N. Chem. Rev. 1985, 85, 77, 97. 8. Haddleton, D.M.; Perutz, R.N. J. Chem. Soc. Chem. Commun. 1985, 1372, and in press. 9. McKean, D.C. Chem. Soc. Rev. 1978, 7, 399. 10. McKean, D.C. J. Mol. Struct. 1984, 113, 251. 11. Long, C.; Morrison, A.R.; McKean, D.C.; McQuillan, G.P. J. Am. Chem. Soc. 1984, 106, 7418. 12. Horton-Mastin, Α.; Poliakoff, M.; Turner, J.J. Organometallics 1986, 5, 405. 13. Horton-Mastin, Α.; McKean, D.C.; McQuillan, G.P.; Poliakoff, M.; Turner, J.J. to be published. 14. Maier, W.B.II;Freund, S.M.; Holland, R.F.; Beattie, W.H. J . Chem. Phys. 1978, 69, 1961. 15. Turner, J.J.; Poliakoff, M.; Maier, W.B.; Simpson, M.B. Inorg. Chem. 1983, 22, 911. 16. Turner, J.J.; Simpson, M.B.; Poliakoff, M.; Maier, W.B. J. Am. Chem. Soc. 1983, 105, 3898. 17. Gadd, G.E.; Poliakoff, M.; Turner, J.J. Inorg. Chem. 1984, 23, 630. 18. Kubas, G.J.; Ryan, R.R.; Swanson, B.I.; Vergamini, P.J.; Wasseman, J.J. J. Am. Chem. Soc. 1984, 106, 451. 19. Morris, R.H.; Sawyer, J.F.; Shiralian, M.; Zubkowski, J.D. J. Am. Chem. Soc. 1985, 107, 5381.


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20. 21. 22. 23. 24.


25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35.

36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47.


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Crabtree, R.H.; Lavin, M. J. Chem. Soc. Chem. Commun. 1985, 794: 1985, 1661. Crabtree, R.H.; Hamilton, D.G. J. Am. Chem. Soc. 1986, 108, 3124. Crabtree, R.H.; Lavin, M.; Bonneviot, L. J. Am. Chem. Soc. 1986, 108, 4032. Hay, P.J. Chem. Phys. Lettrs. 1984, 103, 466. Saillard, J.Y.; Hoffmann, R. J. Am. Chem. Soc. 1984, 106, 2006. Jean, Y.; Lledos, A. Nouv. J. Chim. in press. Jean, Y.; Eisentein, O.; Volatron, F.; Maouche, B.; Sefta, F. J. Am. Chem. Soc. in press. Burdett, J.K. private communication. Upmacis, R.K.; Poliakoff, M.; Turner, J.J. J. Am. Chem. Soc. 1986, 108, 3645. Perutz, R.N. private communication. e.g. Sweany, R.L., J. Am. Chem. Soc. 1985, 107, 2374: private communication. Gadd, G.E.; Poliakoff, M.; Turner, J.J. Inorg. Chem. 1986, 3604. Gregory, M.F.; Jackson, S.A.; Poliakoff, M.; Turner, J.J. J. Chem. Soc. Chem. Commun. 1986, 1175. Grevels, F-W.; Skibbe, V. J. Chem. Soc. Chem. Commun. 1984, 681. S.A. Jackson, unpublished observations. Poliakoff, M.; Weitz, E. 'Detection of Transient Organometallic Species by Fast Time-Resolved IR Spectroscopy', in Stone, F.G.A.; R. West, Eds 'Advances in Organometallic Chemistry', 1986, 25, 277. Dixon, A.J.; Healy, M.A.; Hodges, P.M.; Moore, B.D.; Poliakoff, M.; Simpson, M.B.; Turner, J.J.; West, M.A. J. Chem. Soc. Faraday Trans. 2 in press. For an excellent review of the photochemistry of binuclear metal species see Meyer, T.J.; Caspar, J.V. Chem.Rev. 1985, 85, 187. Hepp, A.F.; Wrighton, M.S. J. Am. Chem. Soc. 1983, 105, 5934. Dunkin, I.R.; Harter, P.; Shields, C.S. J. Am. Chem. Soc. 1984, 106, 7248. Church, S.P.; Hermann, H.; Grevels, F-W.; Schaffner, K. J. Chem. Soc. Chem. Commun. 1984, 785. Sedar, T.A.; Church, S.P.; Weitz, E. J. Am. Chem. Soc. 1986, 108, 1084. Firth, S.; Hodges, P.M.; Poliakoff, M.; Turner, J.J. Inorg. Chem. in press. Church, S.P.; Grevels, F-W.; Schaffner, K. Organometallics in press. Church, S.P., private communication. Moore, B.D.; Simpson, M.B.; Poliakoff, M.; Turner, J.J. J. Chem. Soc. Chem. Commun. 1984, 972. Dixon, A.J.; Healy, M.A.; Poliakoff, M.; Turner, J.J. J. Chem. Soc. Chem. Commun. 1986, 994. Hepp, A.F.; Blaha, J.P.; Lewis, C.; Wrighton, M.S. Organometallics 1984, 3, 174.


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48. Hooker, R.H.; Mahmoud, Κ.A.; Rest, A.J. J. Chem. Soc. Chem. Commun. 1983, 1022. 49. Dobson, G.R.; Bernai, I.; Reisner, M.G.; Dobson, C.B.; Mansour, S.E. J. Am. Chem. Soc. 1985, 107, 525. 50. Dobson, G.R.; Dobson, C.B.; Mansour, S.E. Inorg. Chim. Acta 1985, 100, C7. 51. Church, S.P.; Grevels, F-W.; Hermann, H.; Schaffner, K. Inorg. Chem. 1984, 23, 3830. 52. Weitz, E. private communication. 53. Dobson, G.R.; Hodges, P.M.; Firth, S.; Healy, M.A.; Poliakoff, M.; Turner, J.J.; Asali, K.J. J. Am. Chem. Soc., submitted. RECEIVED

November 3,

1986


Infrared Spectroscopy of Organometallic Intermediates - ACS

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