Excited States and Reactive Intermediates - American Chemical Society

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Downloaded by UCSF LIB CKM RSCS MGMT on December 2, 2014 | http://pubs.acs.org Publication Date: April 30, 1986 | doi: 10.1021/bk-1986-0307.ch006

M e t a l - L i g a n d Charge Transfer Photochemistry Metal-Metal Bonded Complexes D. J. Stufkens, A. Oskam, and M. W. Kokkes Anorganisch Chemisch Laboratorium, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands The complexes (CO)MM'(CO)(α-diimine) (M,M'=Mn,Re) show a different photochemistry in 2-Me-THF at T>200K and at T ^ s i g n e d to the s o l v a t e d c a t i o n [ M n ( C O ) ( i - P r - D A B ) ( 2 - M e - T H F ) ] and i n d i c a t e d by φ i n t h e I R s p e c t r u m . T h i s a s s i g n m e n t is b a s e d o n t h e c l o s e s i m i l a r i t y b e t w e e n these bands and those of [ M n ( C O ) ( i - P r - D A B ) ( T H F ) ] [ O T F ] " ( O T F = C F S 0 ) , w h i c h c o m p o u n d h a s b e e n p r e p a r e d s e p a r a t e l y . In t h e I R s p e c t r u m o f t h e p h o t o p r o d u c t s o b t a i n e d at 1 7 3 K e x t r a bands ( i n d i c a t e d w i t h X ) are o b s e r v e d , w h i c h do n o t s h o w u p u p o n p h o t o l y s i s at 2 3 0 K a n d w h i c h a r e v e r y weak in the 1 3 3 K s p e c t r u m . These bands apparently belong to a c o m p l e x w h i c h is o n l y f o r m e d w h e n M n ( C O ) i o i s a m a j o r p h o t o p r o d u c t a n d w h i c h i s unstable at higher t e m p e r a t u r e s . Indeed, these bands disappear w h e n the s o l u t i o n , a f t e r p h o t o l y s i s a t 1 7 3 K , is r a i s e d i n t e m p e r a t u r e . T h e s e b a n d s are therefore assigned to the c o m p l e x M n ( C O ) g ( 2 - M e - T H F ) and this assignment is s u p p o r t e d by t h e c l o s e a g r e e m e n t b e t w e e n these f r e q u e n c i e s and those o f o t h e r M n ( C O ) g L c o m p l e x e s (20). A t higher t e m p e r a t u r e s 2 - M e - T H F i s s u b s t i t u t e d by C O f r o m t h e s o l u t i o n . The ions f o r m e d by p h o t o l y s i s at T < 2 0 0 K a r e not stable at higher temperatures. Raising the temperature causes them to react back to the p a r e n t c o m p o u n d w i t h loss o f 2 - M e - T H F . W h e n , h o w e v e r , P ( n - B u ) is added to a s o l u t i o n of t h e ions, [ M n ( C O ) ( i - P r - D A B ) ( P ( n - B u ) ] [ M n ( C O ) ] " is f o r m e d , w h i c h is a stable c o m p o u n d at r o o m t e m p e r a t u r e . F r o m these results one m i g h t c o n c l u d e t h a t photolysis leads to homolytic or heterolytic splitting of the m e t a l - m e t a l bond, depending on t h e t e m p e r a t u r e o f t h e s o l u t i o n . If i n s t e a d o f ( C O ) M n M n ( C O ) ( i - P r - D A B ) , ( C O ) M n M n ( C O ) ( p h e n ) o r ( C O ) M n M n ( C O ) ( b i p y ) a r e p h o t o l y z e d at 1 3 3 K , i t b e c o m e s c l e a r t h a t h e t e r o l y t i c s p l i t t i n g o f t h e m e t a l - m e t a l b o n d is n o t a p r i m a r y p h o t o p r o c e s s . In t h a t c a s e n e i t h e r M n ( C O ) i o n o r [Mn(CO)5] i s f o r m e d b u t i n s t e a d a p h o t o s u b s t i t u t i o n p r o d u c t . F i g u r e 8. shows t h e I R s p e c t r a l changes upon photolysis of ( C O ) s M n M n ( C O ) ( p h e n ) at 1 3 3 K . O n l y a s m a l l a m o u n t o f [ M n ( C O ) ] ~ is f o r m e d . Instead, f r e e C O shows up w i t h V=2132 c m " . F u r t h e r m o r e , a l l C O - v i b r a t i o n s s h i f t t o l o w e r f r e q u e n c i e s , especially those of the metal fragment w i t h the α-diimine ligand. A t the s a m e t i m e the M L C T band at 520 n m dissappears and a b r o a d band shows up b e t w e e n 5 8 0 a n d 7 0 0 n m . T h e s e d a t a p o i n t t o a p h o t o s u b s t i t u t i o n o f C O o f t h e M n ( C O ) ( p h e n ) m o i e t y by 2 - M e - T H F . T h e p h o t o p r o d u c t ( C O ) M n M n ( C O ) ( p h e n ) ( 2 - M e - T H F ) cannot be i s o l a t e d since it d i s p r o p o r t i o n a t e s upon raising the temperature. A similar disproportionation reaction was observed 5

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

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


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

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

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


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

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

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

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79

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

+

3

5

3


5

2

2

3

3

Reaction (CO) MnMn(CO) L + Ρ 5

3

)

[Mn(CO) LP]

>

Mn(CO)

+

+ [Mn(CO) I

3

5

Initiation (CO) MnMn(CO) L 5

—

3

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

> — » »

3

3

5

3

3

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

+ Mn(CO) L

5

3

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

+

3

3

5

3

[Mn(CO) LP]

5

+

+ [Mn(CO) ]~

3

5

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

3

5

3

3

3

5

2

3

5

t t

2

2

6

2

3

3

2

3

3

5

5

3

3

3

+

3

5

3

3


80



Photolysis in rigid m e d i a If t h e p h o t o l y s i s t a k e s p l a c e i n an i n e r t gas m a t r i x , b o t h t h e h o m o l y t i c s p l i t t i n g of the m e t a l - m e t a l bond and the b r e a k i n g of a m e t a l - n i t r o g e n b o n d w i l l be f o l l o w e d by a f a s t b a c k r e a c t i o n t o t h e p a r e n t c o m p o u n d . T h e r a d i c a l s f o r m e d by h o m o l y s i s o f t h e m e t a l - m e t a l b o n d c a n n o t d i f f u s e f r o m the m a t r i x site and w i l l r e c o m b i n e to the parent c o m p o u n d . M o r e o v e r , t h e p h o t o p r o d u c t o b t a i n e d by b r e a k i n g o f a m e t a l n i t r o g e n b o n d , w i l l n o t be s t a b i l i z e d by a c o o r d i n a t i n g s o l v e n t m o l e c u l e a n d t h e r e f o r e r e a c t b a c k to the parent c o m p o u n d . Because of this the p h o t o c h e m i s t r y of some of t h e s e c o m p l e x e s has a l s o b e e n s t u d i e d i n a C h k - m a t r i x at 1 0 K a n d f o r c o m p a r i s o n i n a P V C f i l m , w h i c h is a l e s s r i g i d m e d i u m t h a n t h e m a t r i x e s p e c i a l l y at r o o m t e m p e r a t u r e . W h e n t h e c o m p l e x ( C O ) M n R e ( C O ) ( i - P r - D A B ) is p h o t o l y z e d i n a C H t t - m a t r i x f i v e n e w C O b a n d s s h o w up w i t h s i m u l t a n e o u s l o s s o f C O . T h e n e w b a n d s do n o t b e l o n g t o any o f t h e p h o t o p r o d u c t s o b s e r v e d so f a r . T h e IR spectral changes, shown in F i g u r e 11, exhibit several isosbestic points i n d i c a t i n g a w e l l - d e f i n e d c l e a n r e a c t i o n . T h e M L C T b a n d d i s a p p e a r s a n d no n e w b a n d s h o w s up i n t h e v i s i b l e r e g i o n . A n n e a l i n g a C O - m a t r i x a f t e r t h e p h o t o l y s i s d i d n o t c a u s e a r e a c t i o n w h i c h m e a n s t h a t t h e p h o t o p r o d u c t is c o o r d i n a t i v e l y s a t u r e d . D r a s t i c changes also o c c u r in the 1 200-1500 c m r e g i o n o f t h e I R s p e c t r u m . T h e b a n d at 1479 c m " b e l o n g i n g t o v ( C N ) o f t h e c o o r d i n a t e d i - R r - D A B l i g a n d a n d t h e 1 294 c m band belonging to v ( C C ) d i s a p p e a r w h i l e t w o n e w b a n d s s h o w up at 1389 a n d 1304 c m , r e s p e c t i v e l y . T h e s e b a n d s a r e a s s i g n e d to v ( C N ) a n d v ( C C ) , r e s p e c t i v e l y , of the i - P r - D A B ligand in the p h o t o p r o d u c t . This low frequency of v ( C N ) p o i n t s t o a l a r g e i n v o l v e m e n t o f t h e 7r*Tevel o f t h e i - P r - D A B l i g a n d i n t h e b o n d i n g s i n c e t h i s o r b i t a l is a n t i - b o n d i n g b e t w e e n C a n d N . A s i m i l a r l o w f r e q u e n c y f o r V ( C N ) has b e e n f o u n d f o r t h e p h o t o p r o d u c t o f F e ( C O ) ( R - D A B ) , i n w h i c h t h e R - D A B l i g a n d is η , η c o o r d i n a t e d t o F e ( 8 ) O n the basis of these results we propose that the c o m p l e x ( C O ) M n ( i - P r - D A B ) R e ( C O ) (30) is f o r m e d ( F i g u r e 12) i n w h i c h t h e i - P r - D A B l i g a n d is σ , σ - c o o r d i n a t e d t o R e a n d η , η t o M n . T h i s p r o p o s a l is s t r o n g l y s u p p o r t e d by t h e o b s e r v a t i o n s o f A d a m s (31) a n d K e i j s p e r (32). A d a m s a c c i d e n t a l l y synthesized the c o m p l e x ( C O ) M n ( M e - D A B ( C H , C H ) ) M n ( C O ) a n d e s t a b l i s h e d t h e s t r u c t u r e by X - r a y d i f f r a c t i o n . K e i j s p e r s y n t h e s i z e d i n l o w y i e l d an a n a l o g o u s c o m p l e x ( C O ) M n ( t - B u - D A B ) M n ( C O ) by t r e a t i n g M n ( C O ) ( t - B u - D A B ) B r w i t h [ C p F e ( C O ) ] " . The C O - s t r e t c h i n g frequencies of these c o m p l e x e s agree very w e l l w i t h those of our photoproduct. P h o t o l y s i s of the o t h e r ( C O ) M M ' ( C O ) ( i - P r - D A B ) ( M , M ' = M n , R e ) c o m p l e x e s in a C h U - m a t r i x did not lead to the f o r m a t i o n of ( C O ) M ( i - P r - D A B ) M ' ( C O ) . If t h e s e c o m p l e x e s a r e h o w e v e r p h o t o l y z e d i n a P V C f i l m , c a s t f r o m T H F , t h r e e of the four c o m p l e x e s ( C O ) M M ' ( C O ) ( i - P r - D A B ) ( M , M ' = M n , R e e x c e p t M = M ' = R e ) c a n be c o n v e r t e d i n t o ( C O ) M ( i P r - D A B ) M ' ( C O ) . F i g u r e 13 s h o w s t h e I R s p e c t r a l c h a n g e s a c c o m p a n y i n g t h e p h o t o l y s i s o f ( C O ) M n M n ( C O ) ( i - P r - D A B ) i n a P V C f i l m at 2 9 3 K . A t t h i s t e m p e r a t u r e no f r e e C O is o b s e r v e d d u e t o t h e b r o a d n e s s o f t h e I R b a n d . N o w , w h e n t h i s c o m p l e x is p h o t o l y z e d i n t h e f i l m at 1 9 3 K , f i v e e x t r a C O b a n d s s h o w up w h i c h d i s a p p e a r u p o n r a i s i n g t h e t e m p e r a t u r e w i t h f o r m a t i o n of the parent c o m p o u n d . A p p a r e n t l y , a T H F substituted c o m p l e x is f o r m e d . T h i s s u b s t i t u t i o n d o e s n o t t a k e p l a c e at t h e M n ( C O ) ( i - P r - D A B ) m o i e t y s i n c e t h e C O - v i b r a t i o n s do not a g r e e w i t h t h o s e o f t h e 2 - M e - T H F s u b s t i t u t e d c o m p l e x f o r m e d in 2 - M e - T H F at 1 3 3 K . The bands are assigned to ( T H F ) ( C O ^ M n M n C O ^ ( i - P r - D A B ) . R a i s i n g the temperature w i l l lead to b a c k s u b s t i t u t i o n o f T H F by C O , s t i l l p r e s e n t i n t h e f i l m . 5

3

1

1

s

1

x

s

s

S

3

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2

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STUFKENS ETAL.

MLCT

Photochemistry


0.50τ

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

3

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




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

3



6. STUFKENS ET AL. MLCT Photochemistry

83

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

3

3

3

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

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

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



84


16. R.R. Andréa, D.J. Stufkens and A. Oskam, J.Organomet.Chem. 1985,290,63. 17. R.R. Andréa, A. Terpstra, D.J. Stufkens and A. Oskam, Inorg. Chim. Acta 1985, 96, L57. 18. L.H. Staal, G. van Koten and K. Vrieze, J. Organomet. Chem. 1979, 175, 73. 19. N. Flitcroft, D.K. Huggins and H.D. Kaesz, Inorg. Chem. 1964, 3, 1123. 20. M.L. Ziegler, H. Haas and R.K. Sheline, Chem. Ber. 1965, 98, 2454. 21. R.J. Kazlauskas and M.S. Wrighton, J. Am. Chem. Soc. 1982, 104, 5748. 22. H. van der Poel, G. van Koten and G.C. van Stein, J. Chem. Soc., Dalton Trans. 1981, 2164. 23. P.C. Ford in "Progress in Inorganic Chemistry"; Lippard, S.J., Ed.; John Wiley: New York, 1983; vol. 30, pp. 213-271. 24. A.J. Lees and A.W. Adamson, J. Am. Chem. Soc. 1982, 104, 3804. 25. D.R. Tyler, J. Photochem. 1982, 20, 101. 26. A.E. Stiegman and D.R. Tyler,J. Photochem. 1984, 24, 311. 27. A.E. Stiegman and D.R. Tyler, Inorg. Chem. 1984, 23, 527. 28. A.S. Goldmann and D.R. Tyler, J. Am. Chem. Soc. 1984, 106, 4067. 29. A.E. Stiegman, M. Stieglitz and D.R. Tyler, J. Am. Chem. Soc. 1983, 105, 6032. 30. This notation is used to indicate that the i-Pr-DAB ligand is σ-Ν, σ-Ν coordinated to Re an η -CN, η -CN to Mn. 31. R.D. Adams, J. Am. Chem. Soc. 1980, 102, 7476. 32. J. Keijsper, G. van Koten, K. Vrieze, M. Zoutberg and C.H. Stam, Organometallics 1985, 4, 1306. 33. H. Yeasaka, T. Kobayashi, H. Yasufuko and S. Nagakuru, J. Am. Chem. Soc. 1983, 105, 6249. 34. Q.Z. Shi, T. Richmond, W.C. Trogler and F. Basolo, J. Am. Chem. Soc. 1984, 106, 71. 2

2

RECEIVED November 8, 1985


Excited States and Reactive Intermediates - American Chemical Society

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