Sensitization of Olefin Photoreactions by Copper(I) - American

26 Sensitization of Olefin Photoreactions by Copper(I) Compounds

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CHARLES

KUTAL

and

PAUL

A.

GRUTSCH

Department of Chemistry, University of Georgia, Athens, GA 30602

Copper(I) compounds accelerate the rates of a diverse assort ment of olefin photoreactions, including rearrangement, oli gomerization,

and molecular fragmentation.

seeks to provide a basis for understanding

This

effect in terms of the ground- and excited-state of Cu(I).

chapter

this sensitization properties

Thus the ability to generate vacant coordination

sites and form

stable olefin complexes,

as well as the

absence of low-lying vacant d orbitals, are two characteris tics of the metal which have potentially significant conse quences for sensitization. A review of Cu(I)-sensitized olefin photoreactions is presented and a general classification of sensitization processes is proposed.

The potential sensitiza

tion behavior of some previously unstudied

Cu(I) systems

also is explored.

/^Vlefins u n d e r g o a f a s c i n a t i n g assortment of p h o t o c h e m i c a l t r a n s f o r m a tions (1,2,3).

D u r i n g t h e past 15 years there has b e e n a n i n c r e a s i n g

awareness that t h e presence of c e r t a i n t r a n s i t i o n m e t a l c o m p o u n d s c a n i n f l u e n c e t h e course of these processes, i n some cases r e s u l t i n g i n t h e f o r m a t i o n of n o v e l o r o t h e r w i s e d i f l i c u l t - t o - s y n t h e s i z e p r o d u c t s .

I n the

m o s t g e n e r a l sense, t h e r o l e of t h e m e t a l is t o r e n d e r t h e olefin sensitive to t h e a c t i o n of t h e i r r a d i a t i n g l i g h t . C o n s e q u e n t l y , i n this c h a p t e r w e use t h e t e r m s e n s i t i z a t i o n as a g e n e r i c l a b e l to d e n o t e a process i n w h i c h a t r a n s i t i o n m e t a l c o m p o u n d ( t h e sensitizer) accelerates t h e rate of a n olefin p h o t o r e a c t i o n w i t h at least o n e p h o t o n b e i n g r e q u i r e d p e r p r o d u c t molecule formed. P a r t i c u l a r l y p r o m i n e n t i n this r o l e a r e C u ( I ) c o m p o u n d s s e n s i t i z a t i o n of a v a r i e t y of o l e f i n r e a r r a n g e m e n t (14,15,16,17,18),

a n d molecular fragmentation

(4-13), (19)

whose

oligomerization processes is w e l l

0-8412-0429-2/79/33-173-325$05.00/0 © 1979 American Chemical Society King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

326

INORGANIC COMPOUNDS W I T H UNUSUAL PROPERTIES

II

d o c u m e n t e d . W h i l e the m a j o r i t y of systems e x a m i n e d to date h a v e feat u r e d C u X ( w h e r e X is C l , B r , a n d F3CSO3) salts as sensitizers,

recent

results f r o m o u r l a b o r a t o r y h a v e e s t a b l i s h e d that a c o n s i d e r a b l y b r o a d e r r a n g e of C u ( I ) c o m p o u n d s c a n f u n c t i o n i n this c a p a c i t y .

Such com-

p o u n d s possess a p o t e n t i a l l y significant a d v a n t a g e o v e r s i m p l e C u X salts i n that they p r o v i d e a v e h i c l e f o r " t a i l o r i n g " the s e n s i t i z a t i o n p r o p e r t i e s of C u ( I ) b y selection of t h e a p p r o p r i a t e c o m b i n a t i o n of c o o r d i n a t e d Downloaded by NORTH CAROLINA STATE UNIV on December 26, 2017 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch026

ligands. Cu(I)

T h i s a p p r o a c h r e c e n t l y has b e e n u s e d i n attempts to d e v e l o p

sensitizers that absorb s t r o n g l y i n t h e v i s i b l e w a v e l e n g t h r e g i o n

since this w o u l d f a c i l i t a t e t h e use of s u n l i g h t as a p h o t o e x c i t a t i o n source (20,21). I n s e a r c h i n g f o r effective C u ( I ) sensitizers, h o w e v e r , some r a t i o n a l basis f o r selecting t h e m o s t p r o m i s i n g candidates is n e e d e d .

Thus any

c o n s i d e r a t i o n of s e n s i t i z a t i o n b e h a v i o r s h o u l d b e p r e d i c a t e d u p o n a f u n d a m e n t a l u n d e r s t a n d i n g of t h e g r o u n d - a n d excited-state

properties of

C u ( I ) c o m p o u n d s . T h i s v i e w p o i n t has b e e n a d o p t e d i n t h e present c h a p t e r . I n subsequent sections w e s u r v e y t h e s t r u c t u r a l a n d c h e m i c a l properties o f Cu(I)

c o m p o u n d s a n d t h e n c o n s i d e r some p e r t i n e n t features

of t h e i r

excited-state b e h a v i o r . I n this m a n n e r w e h o p e to d e l i n e a t e t h e c h a r a c teristics of C u ( I ) w h i c h c o n t r i b u t e to its effectiveness

as a sensitizer.

W i t h i n this f r a m e w o r k the results of several studies d e a l i n g w i t h C u ( I ) sensitization of olefin photoreactions are r e v i e w e d , a n d some m e c h a n i s t i c generalizations are p r o p o s e d . F i n a l l y , t h e p o t e n t i a l s e n s i t i z a t i o n b e h a v i o r s of some n o v e l C u ( I ) systems are c o n s i d e r e d . Important

Features of Cu(I)

Chemistry

T h e r e exists a n extensive l i t e r a t u r e o n t h e p r e p a r a t i o n a n d c h a r a c t e r i z a t i o n of C u ( I ) c o m p o u n d s (22,23);

a representative

s a m p l i n g of

reactions is p r e s e n t e d i n S c h e m e 1. A l t h o u g h C u d i s p r o p o r t i o n a t e s t o +

Cu

+ 2

a n d C u ° i n aqueous s o l u t i o n , the + 1 o x i d a t i o n state c a n b e s t a b i l i z e d

v i a c o o r d i n a t i o n of s t r o n g l y b i n d i n g l i g a n d s . T h u s t h e s o l u b l e complexes Cul ~ and C u ( N H ) 2

3

2

+

are stable i n aqueous s o l u t i o n , a n d i n s o l u b l e salts

s u c h as the C u ( I ) h a l i d e s exist i n the s o l i d state o r i n contact w i t h w a t e r . L i g a n d s possessing l o w l y i n g , v a c a n t TT* orbitals w h i c h are c a p a b l e of a c c e p t i n g e l e c t r o n d e n s i t y f r o m the m e t a l also t e n d to s t a b i l i z e t h e + 1 state; c o m m o n examples are l , 1 0 - p h e n a n t h r o l i n e ( p h e n ) ,

2,2 -bipyridine /

( b i p y ) , p h o s p h i n e s , arsines, a n d olefins. W h i l e discrete t w o - a n d t h r e e - c o o r d i n a t e

C u ( I ) compounds

have

b e e n r e p o r t e d , the f a v o r e d c o o r d i n a t i o n n u m b e r of t h e m e t a l is f o u r . Q u i t e f r e q u e n t l y , t w o or m o r e c o p p e r atoms w i l l share l i g a n d s , r e s u l t i n g i n t h e f o r m a t i o n of p o l y n u c l e a r clusters.

C o m p l e x e s of s t o i c h i o m e t r y

&14X4L4 ( X = C1, B r , I; L = P R , A s R ) f o r e x a m p l e , exist as t e t r a m e r i c 3

3

units w h o s e s t r u c t u r e d e p e n d s u p o n t h e steric d e m a n d s of X a n d R . I n

King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

26.

KUTAL

A N D GRUTSCH

Sensitization

327

of Olefin Photoreactions

Scheme 1 CuI(PPh ) 3

60,000 c m " ) . 1

Thus the absorption

characteristics o f C u ( I ) c o m p o u n d s c a n b e a t t r i b u t e d to o n e o r m o r e o f the f o l l o w i n g transitions: ( 1 ) l i g a n d - t o - m e t a l charge transfer; ( 2 ) m e t a l t o - l i g a n d c h a r g e transfer; (4)

( 3 ) c h a r g e transfer to solvent ( C T T S ) ; a n d

i n t r a l i g a n d . Some repersentative s p e c t r a l assignments a r e l i s t e d i n

T a b l e I. D i f f e r e n t types of b e h a v i o r are e x p e c t e d d e p e n d i n g u p o n the i d e n t i t y of the e x c i t e d state p o p u l a t e d . T h e p h o t o - i n d u c e d r e d i s t r i b u t i o n o f elect r o n d e n s i t y i n a l i g a n d - t o - m e t a l C T state f o r m a l l y generates a r e d u c e d m e t a l a n d o x i d i z e d l i g a n d r a d i c a l ; the latter c a n b e s u s c e p t i b l e to n u c l e o p h i l i c attack b y solvent or other species i n s o l u t i o n . F o r a m e t a l - t o - l i g a n d C T state, p r o m o t i o n o f e l e c t r o n d e n s i t y t o the p e r i p h e r y o f t h e c o m p l e x c a n f a v o r b i m o l e c u l a r e l e c t r o n transfer.

Thus M c M i l l i n a n d co-workers

r e c e n t l y h a v e r e p o r t e d that v i s i b l e i r r a d i a t i o n of C u ( 2 , 9 - M e p h e n ) the p r e s e n c e o f d 5 - b i s ( i m i n o d i a c e t a t o ) c o b a l t a t e ( I I I )

a l c o h o l m i x t u r e results i n t h e p r o d u c t i o n o f C u ( I I ) a n d C o ( I I ) suggests that t h e C u ( I )

+

in

ioni n a water-

T h i s p h o t o - i n d u c e d r e d o x b e h a v i o r m i m i c s that of R u ( b i p y ) tant.

2

3

+ 2

(40).

(41) a n d

c o m p o u n d f u n c t i o n s as a n excited-state r e d u c -

Q u a l i t a t i v e l y s i m i l a r b e h a v i o r has b e e n f o u n d b y H u r s t a n d c o -

w o r k e r s (42,43)

i n the b i n u c l e a r b r i d g e d systems

(NH ) Co-L-Cu (L 3

5

is 0 C ( C H ) C H — C H R or N H ( C H ) C H = C H ) w h e r e i r r a d i a t i o n 2

2

n

2

2

n

2

i n d u c e s C u ( I ) - t o - C o ( I I I ) e l e c t r o n transfer m e d i a t e d b y the 7r-delocalized orbitals o f the b r i d g i n g l i g a n d s . Stevenson a n d D a v i s h a v e r e p o r t e d that p o p u l a t i o n o f a C T T S state i n C u C l " 3

of Some C u ( I )

6.3 7.8 3.3 1.9

X X X X

c a n result i n p r o d u c t i o n o f a

Compounds

c (M' cm' ) 1

2

Assignment

1

10 10 10 10

3 3 3 4

N B D - » C u or C u -> N B D Cu 2,9-Mephen CTTS n(P)

->TT*

Ref. 11 38 39 13


330


solvated electron w h i c h , i n turn, can b e scavenged b y H h y d r o g e n ) o r other f a c i l e e l e c t r o n acceptors (44).

+

(producing

I n t r a l i g a n d TT-TT* o r

n - r r * e x c i t e d states are e x p e c t e d to e x h i b i t the r e a c t i v i t y patterns teristic of t h e u n c o o r d i n a t e d m o l e c u l e .

II

charac-

T h e p o s s i b i l i t y exists, h o w e v e r ,

that c o o r d i n a t i o n to t h e m e t a l c a n orient the l i g a n d i n a p a r t i c u l a r l y reactive (or nonreactive)

c o n f o r m a t i o n or p e r h a p s i n d u c e a h i g h degree of

stereospecificity i n p r o d u c t f o r m a t i o n . F u r t h e r m o r e , t h e m e t a l c a n p e r Downloaded by NORTH CAROLINA STATE UNIV on December 26, 2017 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch026

t u r b t h e r a d i a t i v e a n d n o n r a d i a t i v e processes of t h e l i g a n d e x c i t e d states. A d d i t i o n a l possibilities arise w h e n c o n s i d e r i n g the b e h a v i o r of c o p p e r cluster systems.

T h u s transitions w h i c h i n v o l v e the s t a b i l i z i n g b r i d g -

i n g l i g a n d s c a n result i n one o r m o r e of the f o l l o w i n g p a t h w a y s : ( 1 ) m u l t i p l e l i g a n d loss; ( 2 ) s t r u c t u r a l changes i n the cluster f r a m e w o r k ; a n d (3) declusterification. R e l a t i v e l y f e w cases of l u m i n e s c e n c e f r o m C u ( I ) c o m p o u n d s h a v e b e e n r e p o r t e d . Z i o l o , et a l . o b s e r v e d e m i s s i o n f r o m ( P P h ) C u X 3

a halogen)

m

n

i n solid-state a n d l o w - t e m p e r a t u r e glasses (45).

n

( X is

H a r d t re-

p o r t e d that c u p r o u s halides react w i t h v a r i o u s n i t r o g e n bases (e.g., p y r i d i n e , p i c o l i n e s ) to f o r m c o m p o u n d s w h o s e e m i s s i o n m a x i m a are s t r o n g l y temperature 47).

dependent

( s o - c a l l e d fluorescence

thermochromism)

(46,

S o m e 1:1 c u p r o u s h a l i d e complexes w i t h m e t h y l o r e t h y l i s o n i c o -

t i n a t e h a v e b e e n n o t e d to e m i t w h i l e the c o r r e s p o n d i n g 1:2 species d o n o t (48). (PPh ) 3

2

M c M i l l i n a n d B u c k n e r have observed emission f r o m C u ( b i p y ) b o t h i n t h e s o l i d state a n d i n a glass at 77 ° K a n d suggest that i t

originates f r o m a m e t a l - t o - b i p y charge transfer state (49).

Recently w e

r e p o r t e d some rare examples of l u m i n e s c e n c e f r o m C u ( I ) complexes i n room-temperature (diphos)BH

4

fluid

s o l u t i o n (13).

Thus

Cu(PPh ) BH 3

2

( d i p h o s is 1 , 2 - b i s ( d i p h e n y l p h o s p h i n o ) e t h a n e )

4

and C u -

both exhibit

a b r o a d e m i s s i o n i n the v i s i b l e r e g i o n w h i c h is t e n t a t i v e l y assigned as ligand localized.

Survey

of Cu (I)-Sensitized

Olefin

I n a series of papers (14,15,16) studies

of t h e c u p r o u s

Photoreactions T r e c k e r et a l . r e p o r t e d t h e i r d e t a i l e d

h a l i d e - s e n s i t i z e d d i m e r i z a t i o n of

norbornene.

C o m p e l l i n g e v i d e n c e w a s p r e s e n t e d f o r t h e i n v o l v e m e n t of a p r e f o r m e d m e t a l - o l e f i n c o m p l e x i n the m e c h a n i s m of sensitization. T h u s ether s o l u tions of n o r b o r n e n e a n d C u B r separately d i s p l a y e d n o m a x i m u m i n t h e i r e l e c t r o n i c spectra d o w n to 220 n m whereas a m i x t u r e of t h e t w o e x h i b i t e d a n intense a b s o r p t i o n at 239 n m . T h e a p p e a r a n c e of this b a n d w a s a t t r i b u t e d to the f o r m a t i o n of a 1:1 B r C u - n o r b o r n e n e c o m p l e x .

U p o n 254-nm

i r r a d i a t i o n , p r e d o m i n a n t p r o d u c t i o n of the exo-trans-exo d i m e r 1 ( R e a c t i o n 1) e n s u e d . T h e o b s e r v e d s e c o n d - o r d e r d e p e n d e n c e of t h e q u a n t u m y i e l d f o r this process u p o n t h e n o r b o r n e n e c o n c e n t r a t i o n w a s i n t e r p r e t e d



26.

KUTAL AND GRUTSCH

Sensitization

of Olefin

331

Photoreactions

i n terms of a t e r m o l e c u l a r i n t e r a c t i o n b e t w e e n t w o ground-state o l e f i n m o l e c u l e s a n d the p h o t o - e x c i t e d B r C u - n o r b o r n e n e c o m p l e x . S a l o m o n a n d K o c h i r e e x a m i n e d this d i m e r i z a t i o n process

(among

o t h e r s ) b y u s i n g t h e v e r y attractive sensitizer, C u ( F C S 0 ) (17).

Earlier

3

w o r k (30,31)

3

b y these authors h a d e s t a b l i s h e d that C u ( I ) complexes

c o n t a i n i n g one, t w o , three, a n d even f o u r c o o r d i n a t e d olefinic b o n d s c o u l d b e f o r m e d b y d i s p l a c e m e n t of t h e w e a k l y c o o r d i n a t i n g F C S 0 3

3

a n i o n . I n contrast, c u p r o u s h a l i d e s e x h i b i t l i t t l e t e n d e n c y to c o o r d i n a t e m o r e t h a n one o l e f i n i c b o n d (30, 50).

I r r a d i a t i o n of solutions c o n t a i n i n g

C u ( F C S 0 ) and norbornene resulted i n preferential formation ( ^ 9 0 % ) 3

3

of d i m e r 1 b u t w i t h a q u a n t u m y i e l d that d i s p l a y e d a p e n d e n c e u p o n olefin c o n c e n t r a t i o n .

first-order

W h i l e no completely

de-

satisfactory

e x p l a n a t i o n f o r this d i s c r e p a n c y w i t h the results of T r e c k e r , et a l . ( v i d e supra)

c o u l d b e offered, i t w a s n o t e d that

s o l u b i l i t y considerations

a l l o w e d t h e q u a n t u m y i e l d d e p e n d e n c e to b e tested o v e r a m u c h w i d e r olefin c o n c e n t r a t i o n i n the C u ( F C S 0 ) system. T h e p h o t o c h e m i c a l d a t a 3

3

w e r e e x p l i c a b l e i n terms of the m e c h a n i s t i c sequence o u t l i n e d i n S c h e m e 2. T h u s w h i l e b o t h 1:1 a n d 1:2 C u ( I ) - n o r b o r n e n e complexes are present i n s o l u t i o n , d i m e r 1 is p r o d u c e d e x c l u s i v e l y b y p h o t o e x c i t a t i o n of t h e latter species. M i n i m a l l y , ground-state c o m p l e x f o r m a t i o n b e t w e e n C u ( I ) a n d n o r bornene

facilitates

a b s o r b i n g olefin.

the a b s o r p t i o n of l i g h t b y t h e o t h e r w i s e

weakly

T h e r e is, i n a d d i t i o n , t h e p o s s i b i l i t y that the close

p r o x i m i t y of t w o ( o r m o r e ) n o r b o r n e n e m o l e c u l e s i n t h e c o m p l e x p r e disposes

them t o w a r d dimerization. A m p l e precedent

exists

f o r this

t e m p l a t e effect of t r a n s i t i o n metals i n olefin c y c l o a d d i t i o n reactions 52, 53).

(51,

A n o t h e r , q u i t e i n t r i g u i n g role w h i c h has b e e n p r o p o s e d f o r C u ( I )

i n v o l v e s the p h o t o - i n d u c e d f o r m a t i o n of a c a r b e n i u m i o n i n t e r m e d i a t e 2 f r o m the i n i t i a l l y f o r m e d c o m p l e x ( R e a c t i o n 2 ) . S u c h " p h o t o c u p r a t i o n " r e c e n t l y has b e e n i m p l i c a t e d i n a series of n o v e l C u ( I ) - s e n s i t i z e d r e a r r a n g e m e n t a n d f r a g m e n t a t i o n processes of 7 - m e t h y l e n e n o r c a r a n e

(19).

A d d i t i o n a l studies are c l e a r l y w a r r a n t e d t o test t h e generality of this process.


332

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES


*


II


26.

KUTAL AND GRUTSCH

Sensitization of Olefin Photoreactions

333

(2)

2 R e c e n t w o r k (20, 21) i n o u r l a b o r a t o r y has f o c u s e d u p o n the use of t r a n s i t i o n m e t a l c o m p o u n d s to sensitize the e n e r g y - s t o r i n g v a l e n c e i s o m e r i z a t i o n of n o r b o r n a d i e n e , N B D , to q u a d r i c y c l e n e , Q ( R e a c t i o n 3 ) . I n p a r t i c u l a r w e h a v e f o u n d that a c a t a l y t i c a m o u n t of C u C l f u n c t i o n s as a n effective a n d q u i t e specific sensitizer f o r this t r a n s f o r m a t i o n . C o n v e r s i o n s of greater t h a n 9 0 % h a v e b e e n a c h i e v e d since C u ( I ) is ineffective as a catalyst f o r t h e e n e r g y - r e l e a s i n g reverse r e a c t i o n .

Spectral a n d photo-

c h e m i c a l e v i d e n c e s u p p o r t a m e c h a n i s m w h i c h features a 1:1 C I C u — N B D c o m p l e x as the p h o t o a c t i v e species. A s i l l u s t r a t e d i n F i g u r e 3, a n o b v i o u s c o n s e q u e n c e of c o m p l e x a t i o n is a shift of t h e a b s o r p t i o n s p e c t r u m of t h e system i n t o a r e g i o n accessible to the 3 1 3 - n m i r r a d i a t i o n u s e d .

Possible

p a t h w a y s b y w h i c h the p h o t o - e x c i t e d c o m p l e x relaxes to Q h a v e b e e n discussed

(12).

NBD

Q



334

INORGANIC

200

COMPOUNDS

250

WITH UNUSUAL

300

PROPERTIES

II

350

\,nm Inorganic Chemistry

Figure

The

3. Spectral evidence for complex formation tween CuCl and NBD in ethanol (11)

C u ( I ) sensitized rearrangement

C O D , to t r i c y c l o [ 3 . 3 . 0 . 0 ' ] o c t a n e , 2

6

of

be-

ci,s,ds-l,5-cyclooctadiene,

3 ( R e a c t i o n 4 ) , represents a n interest-

i n g contrast i n b e h a v i o r to the systems c o n s i d e r e d thus f a r . I n h i s s t u d y of this process

( 4 , 5 ) , S r i n i v a s a n n o t e d that the s p e c t r u m of a n ether

s o l u t i o n c o n t a i n i n g b o t h C u C l a n d t h e o l e f i n is essentially t h e s u m of t h e individual component

spectra,

cis,cis- 1 , 5 - C O D

suggesting

(although not conclusively

3


26.

KUTAL AND GRUTSCH

Sensitization

335


p r o v i n g ) the absence of a p p r e c i a b l e c o m p l e x f o r m a t i o n . S i n c e

cis,cis-l,5-

C O D was present i n c o n s i d e r a b l e excess i n the p h o t o c h e m i c a l studies, i t c o n s t i t u t e d the m a j o r a b s o r b i n g species. I n fact, a reasonable v a l u e (
C u + o l e f i n '

Class 1

C u + olefin — C u -

Class 2

olefin

Class 3

hp olefin Cu -» Cu * > [ C u * + olefin] -> C u + o l e f i n '

1

1

1

olefin*

1

Cu

1

1

1

> [ C u + olefin*] - » C u + olefin' 1

1

1

1

C l a s s 1 — P h o t o e x c i t a t i o n of a p r e f o r m e d c o p p e r - o l e f i n c o m p l e x . C o m p l e x a t i o n p l a y s a k e y role b y s h i f t i n g t h e a b s o r p t i o n s p e c t r u m of t h e system into a w a v e l e n g t h r e g i o n accessible to the i r r a d i a t i n g l i g h t a n d / o r b y p r o v i d i n g a sterically o r e l e c t r o n i c a l l y f a v o r e d p a t h w a y to t h e p h o t o product. C l a s s 2 — B i m o l e c u l a r i n t e r a c t i o n b e t w e e n a n e x c i t e d state of t h e olefin a n d t h e ground-state C u ( I ) c o m p o u n d . T h e r o l e of C u ( I ) c a n b e to s t a b i l i z e t h e e x c i t e d state of t h e olefin v i a c o m p l e x a t i o n ( 9 ) o r p e r h a p s t o f o r m a n i n t e r m e d i a t e ( s u c h as that p o s t u l a t e d f o r p h o t o c u p r a t i o n (19)) w h i c h is p a r t i c u l a r l y a m e n a b l e to p r o d u c t f o r m a t i o n . C l a s s 3 — B i m o l e c u l a r i n t e r a c t i o n b e t w e e n a n e x c i t e d state of t h e C u ( I ) c o m p o u n d a n d ground-state olefin. P o s s i b l e interactions i n c l u d e e l e c t r o n i c energy transfer a n d exciplex f o r m a t i o n (56, 57). A l t h o u g h most r e p o r t e d examples of C u ( I ) sensitization c a n b e a c c o m m o d a t e d w i t h i n a single class ( T a b l e I I ) , a m o r e c o m p l i c a t e d situa t i o n m a y result i n systems w h e r e m u l t i p l e p h o t o c h e m i c a l steps are i n v o l v e d . T h u s t h e o v e r a l l C u C l - s e n s i t i z e d c o n v e r s i o n of cis,cis- 1 , 5 - C O D to 3 ( S c h e m e 3 ) appears to feature b o t h C l a s s 1 a n d C l a s s 2 s e n s i t i z a t i o n processes. T h e most o b v i o u s d i s t i n c t i o n a m o n g t h e three classes rests u p o n t h e i d e n t i t y of t h e l i g h t - a b s o r b i n g species. O u r c h o i c e i n o r g a n i z i n g t h e e x p e r i m e n t a l results a l o n g these lines r a t h e r t h a n , f o r e x a m p l e , o n t h e basis of t h e identities of possible g r o u n d - o r excited-state i n t e r m e d i a t e s


26.

KUTAL AND GRUTSCH Table II.

Sensitization

339


Classification of Some C u (I)-Sensitized Olefin Photoreactions Example

D i m e r i z a t i o n of norbornene i n presence of C u B r ( R e a c t i o n 1) or C u ( F C S 0 ) (Scheme 2) R e a r r a n g e m e n t a n d f r a g m e n t a t i o n of 7 - m e t h y l e n e norcarane i n the presence of C u ( F C S 0 ) V a l e n c e i s o m e r i z a t i o n of n o r b o r n a d i e n e to q u a d r i cyclene i n presence of C u C I ( R e a c t i o n 3) R e a r r a n g e m e n t of cis,trans-l,5-C0T> to 3 i n presence of C u C I (Scheme 3) 3

Class 1 Downloaded by NORTH CAROLINA STATE UNIV on December 26, 2017 | http://pubs.acs.org Publication Date: May 5, 1979 | doi: 10.1021/ba-1979-0173.ch026

Ref. 16,17

3

3

Class 2

I s o m e r i z a t i o n of cis,cis-l,5-C0D to cis,trans-l,5-C0T> i n presence of C u C I (Scheme 3)

Class 3

V a l e n c e i s o m e r i z a t i o n of n o r b o r n a d i e n e to q u a d r i cyclene i n presence of C u ( P P h ) B H a n d r e l a t e d compounds 3

2

11,12 9 9 13, 59

4

f o r m e d subsequent to l i g h t a b s o r p t i o n reflects the p r e s e n t l y u n d e r s t a n d i n g of m a n y of the m e c h a n i s t i c

19

3

details of

incomplete

Cu(I)-sensitized

processes. F u t u r e w o r k a l o n g these lines is c l e a r l y n e e d e d . B a s e d u p o n the g e n e r a l classification i n S c h e m e 4, h o w e v e r , it is p o s s i b l e t o d e l i n e a t e some g r o u n d - a n d excited-state characteristics

of

C u ( I ) w h i c h c a n c o n t r i b u t e to its effectiveness as a sensitizer. T h u s t h e a b i l i t y of a C u ( I )

c o m p o u n d to generate v a c a n t c o o r d i n a t i o n sites a n d

to f o r m stable olefin complexes is c l e a r l y advantageous f o r C l a s s 1 sensitiz a t i o n ; a l t h o u g h less certain, i t is p r o b a b l y i m p o r t a n t i n C l a s s 2 processes as w e l l . C o n v e r s e l y , C l a s s 3 sensitization w i l l b e m o r e p r o m i n e n t a m o n g c o o r d i n a t i v e l y saturated C u ( I ) c o m p o u n d s w h o s e l i g a n d s are not r e a d i l y d i s p l a c e d b y olefins. T h e absence of l o w - l y i n g v a c a n t d orbitals o n C u ( I ) has p o t e n t i a l l y significant consequences i n that the energy of a n a b s o r b e d p h o t o n , w h i c h m i g h t o t h e r w i s e b e d i s s i p a t e d v i a the l i g a n d s u b s t i t u t i o n processes characteristic of l i g a n d - f i e l d e x c i t e d states ( 5 8 ) , neled into chemically more productive pathways.

can be chan-

T h i s feature is p a r -

t i c u l a r l y c r u c i a l f o r C l a s s 3 sensitization w h i c h requires that a n e x c i t e d state characteristic of the C u ( I ) c o m p o u n d u n d e r g o a b i m o l e c u l a r intera c t i o n w i t h a ground-state olefin m o l e c u l e . Future

Directions

T h e a v a i l a b i l i t y of a diverse assortment of reasonably stable a n d , i n m a n y cases, h i g h l y c o l o r e d C u ( I ) c o m p o u n d s (23) of C u ( I )

n o v e l results o b t a i n e d f o r C u ( P P h ) B H 3

(59)

suggests that studies

s e n s i t i z a t i o n n e e d n o t b e l i m i t e d to s i m p l e C u X salts. 2

4

(13)

The

and related compounds

r e i n f o r c e this v i e w . C o n s e q u e n t l y , i n this final s e c t i o n w e b r i e f l y


340

INORGANIC COMPOUNDS W I T H UNUSUAL

PROPERTIES

II

c o n s i d e r the p o t e n t i a l s e n s i t i z a t i o n p r o p e r t i e s of some p r e v i o u s l y u n studied, b u t rather i n t r i g u i n g C u ( I )

systems.

W h i l e t h e d i s c u s s i o n is

a d m i t t e d l y s p e c u l a t i v e , w e h o p e that i t is e q u a l l y suggestive. W e h a v e r e c e n t l y n o t e d (21) series, C u X P R ( N - N ) 3

=

t h e a t t r a c t i v e characteristics

of

the

( t y p i c a l l y X — C l , B r ; R = a l k y l or p h e n y l , N - N

p h e n or b i p y ) . D e p e n d i n g u p o n t h e l a b i l i t y of P R , these c o m p o u n d s 3

c a n c o n c e i v a b l y f u n c t i o n as either C l a s s 1 or C l a s s 3 sensitizers.

Further-


m o r e , t h e i r s t r o n g a b s o r p t i o n i n t h e v i s i b l e w a v e l e n g t h r e g i o n raises t h e p o s s i b i l i t y of h a r n e s s i n g s u n l i g h t to d r i v e u s e f u l p h o t o c h e m i c a l reactions. T h e r e a d y a c c e s s i b i l i t y of o n l y o n e c o o r d i n a t i o n ( v i a loss of C O ) site a b o u t c o p p e r makes C u [ H B ( p z ) ] C O ( F i g u r e 2) p o t e n t i a l l y attrac3

t i v e as a specific sensitizer f o r p h o t o r e a r r a n g e m e n t s of olefins. F o r e x a m p l e , e n d o - d i c y c l o p e n t a d i e n e i n the p r e s e n c e of C u ( 0 S C F ) 3

undergoes

3

v i r t u a l l y exclusive p h o t o d i m e r i z a t i o n ( R e a c t i o n 7, p a t h i ) , p r e s u m a b l y v i a p r i o r f o r m a t i o n of a 2:1 o l e f i n : C u ( I ) c o m p l e x (17).

Since the analo-

gous b i s - o l e f i n c o m p l e x i n v o l v i n g the C u [ H B ( p z ) ] m o i e t y is s t e r i c a l l y 3

i m p r o b a b l e , the most l i k e l y photoprocess is i n t e r n a l c y c l i z a t i o n ( R e a c t i o n 7, p a t h i i ) . P r e l i m i n a r y studies i n o u r l a b o r a t o r y i n d i c a t e that

Cu[HB-

( p z ) ] C O sensitizes t h e N B D - t o - Q c o n v e r s i o n ( R e a c t i o n 3) w i t h respec3

t a b l e q u a n t u m efficiency

(21).

T h e sensitization properties of cluster systems s u c h as [ C u X L ] u r e 1)

are p a r t i c u l a r l y i n t r i g u i n g since t h e p o s s i b i l i t y of

4

(Fig-

multicenter

sensitization exists. T h u s the p h o t o c h e m i c a l or t h e r m a l ( l i k e l y w h e n L is b u l k y ) s u b s t i t u t i o n of t w o or m o r e L groups b y olefins generates a species i n w h i c h the olefin m o l e c u l e s are s i t u a t e d i n close p r o x i m i t y to e a c h other. O n e l i k e l y c o n s e q u e n c e of this t e m p l a t e effect of the cluster is efficient p h o t o - i n d u c e d olefin o l i g o m e r i z a t i o n . A n o t h e r i n t e r e s t i n g cluster system is [ C u H P P h ] , w h o s e 3

6

structure

contains t h e c o p p e r atoms at t h e apices of a d i s t o r t e d o c t a h e d r o n ( F i g u r e 5)

(60).

A l t h o u g h t h e exact locations of t h e h y d r i d e l i g a n d s are u n c e r -


26.

KUTAL A N D GRUTSCH

Sensitization

of Olefin

Photoreactions

341


0 = Cu

Advanced Inorganic Chemistry

Figure

5.

Structure of (26)

[CuH(PPh )] 3

6

t a i n , t h e y most l i k e l y b r i d g e t h e six l o n g C u - C u distances i n d i c a t e d b y s o l i d lines.

T w o p o t e n t i a l p h o t o r e a c t i v e p a t h w a y s o f t h e cluster a r e

e l i m i n a t i o n of h y d r o g e n (61, 62) a n d h y d r o g e n a t i o n o f a d d e d u n s a t u r a t e d m o l e c u l e s . T h e latter process c a n h a v e some c a t a l y t i c v a l u e i f a n a p p r o p r i a t e source to r e p l e n i s h the h y d r o g e n c o n s u m e d c a n b e f o u n d . Acknowledgment W e w i s h to a c k n o w l e d g e t h e c o n t r i b u t i o n s of D . P . S c h w e n d i m a n a n d E . M . Sweet.

O u r studies d e s c r i b e d h e r e i n h a v e b e e n s u p p o r t e d b y

t h e D e p a r t m e n t of E n e r g y ( E Y - 7 6 - S - 0 9 - 0 8 9 3 ) a n d t h e N a t i o n a l S c i e n c e F o u n d a t i o n ( M P S 75-13752).

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17. Salomon, R. G., Kochi, J. K., J. Am. Chem. Soc. (1974) 96, 1137. 18. Salomon, R. G., Folting, K., Streib, W. E., Kochi, J. K., J. Am. Chem. Soc. (1974) 96, 1145. 19. Salomon, R. G., Salomon, M. F., J. Am. Chem. Soc. (1976) 98, 7454. 20. Kutal, C., Schwendiman, D. P., Grutsch, P. A., Solar Energy (1977) 19, 651. 21. Kutal, C., ADV. CHEM. SER. (1978) 168, 158. 22. Quin, H. W., Tsai, J. H., Adv. Inorg. Chem. Radiochem. (1968) 12, 217. 23. Jardine, F. H., Adv. Inorg. Chem. Radiochem. (1975) 17, 115. 24. Churchill, M. R., Kalra, K. L., Inorg. Chem. (1974) 13, 1065. 25. Churchill, M. R., Kalra, K.L.,Inorg. Chem. (1974) 13, 1427. 26. Cotton, F. A., Wilkinson, G., "Advanced Inorganic Chemistry," 3rd ed., Chap. 25, John Wiley, New York, 1972. 27. Fackler, J. P., Prog. Inorg. Chem. (1976) 21, 55. 28. Dewar, M. J. S., Bull. Soc. Chim. Fr. (1951) 18, C79. 29. Chatt, J., Duncanson, L. A., J. Chem. Soc. (1953) 2939. 30. Salomon, R. G., Kochi, J. K., J. Am. Chem. Soc. (1973) 95, 1889. 31. Salomon, R. G., Kochi, J., J. Organomet. Chem. (1974) 64, 135. 32. Tsuda, T., Habu, H., Horiguchi, S., Saegusa, T., J. Am. Chem. Soc. (1974) 96, 5930. 33. Bruce, M. I., Ostazewski, A. P. P., J. Chem.Soc.,Chem. Commun. (1972) 1124. 34. Bruce, M. I., Ostazewski, A. P. P., J. Chem. Soc., Dalton Trans. (1973) 2433. 35. Churchill, M. R., DeBoer, B. G., Rotella, F. J., Salah, O. M. A., Bruce, M. I., Inorg. Chem. (1975) 14, 2051. 36. Muetterties, E. L., Alegranti, C. W., J. Am. Chem. Soc. (1970) 92, 4114. 37. Moore, C. E., "Atomic Energy Levels," NBS Circular 467, p. 117, 1952. 38. Day, P., Sanders, N., J. Chem. Soc. A (1967) 1536. 39. Sugasaka, K., Fujii,A.,Bull.Chem. Soc. Jpn. (1976) 49, 82. 40. McMillin, D. R., Buckner, M.T.,Ahn, B. T., Inorg. Chem. (1977) 16, 943. 41. Creutz,C.,Sutin, N., Inorg. Chem. (1976) 15, 496. 42. Hurst, J. K., Lane, R.H.,J.Am. Chem. Soc (1973) 95, 1703. 43. Farr, J. K., Hulett, L. G., Lane, R. H., Hurst, J. K., J. Am. Chem. Soc. (1975) 97, 2654. 44. Stevenson, K. L., Davis, D. D., "Abstracts of Papers," 174th "National Meeting, ACS, Chicago, August, 1977, INOR 121. 45. Ziolo, R. F., Lipton, S., Dori, Z., J. Chem.Soc.,Chem. Commun. (1970) 1124. 46. Hardt, H. D., Naturwiss. (1974) 61, 107. 47. Hardt, H. D., Pierre, A., Inorg. Chim. Acta (1977) 25, L59. 48. Goner, M. A. S., Drátovský, M., Naturwiss. (1915) 62, 96. 49. Buckner, M. T., McMillin, D. R., "Abstracts of Papers," 174th National Meeting, ACS, Chicago, August, 1977, INOR 183. 50. Manahan, S. E., Inorg. Chem. (1966) 5, 2063. 51. Schrauzer, G. N., Adv. Catal. (1968) 18, 373. 52. Fraser, A. R., Bird, P. H., Bezman, S. A., Shapley, J. R., White, R., Osborn, J. A.,J.Am. Chem. Soc. (1973) 95, 597. 53. Hill, B., Math, K., Pillsbury, D., Voecks, G., Jennings, W., Mol. Photo chem. (1973) 5, 195. 54. Lippard, S. J., Melmed, K. M., Inorg. Chem. (1967) 6, 2223. 55. Gill, J. T., Lippard, S. J., Inorg. Chem. (1975) 14, 751. 56. Lamola, A. A., "Techniques of Organic Chemistry," Vol. 14, P. A. Leermakers, A. Weissberger, Eds., Chap. 2, John Wiley, New York, 1969. 57. Balzani, V., Moggi, L., Manfrin, M. F., Bolletta, F., Laurence, G. S., Coord. Chem. Rev. (1975) 15, 321.


26.

KUTAL AND GRUTSCH

Sensitization

of

Olefin

Photoreactions

343

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RECEIVED February 22, 1978.


Sensitization of Olefin Photoreactions by Copper(I) - American

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