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
King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
330
INORGANIC COMPOUNDS W I T H UNUSUAL PROPERTIES
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
King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
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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.
King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
332
INORGANIC
COMPOUNDS
WITH
UNUSUAL PROPERTIES
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*
King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
II
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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
King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
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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
King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
26.
KUTAL AND GRUTSCH
Sensitization
335
of Olefin Photoreactions
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
King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
26.
KUTAL AND GRUTSCH Table II.
Sensitization
339
of Olefin Photoreactions
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
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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
King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
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-
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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 -
King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.
26.
KUTAL A N D GRUTSCH
Sensitization
of Olefin
Photoreactions
341
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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).
Literature Cited 1. Fonken, G. J., "Organic Photochemistry," O. L. Chapman, Ed., p. 197, Marcel Dekker, New York, 1967. 2. Kropp, P. J., PureAppl.Chem. (1970) 24, 585. 3. Coyle, J. D., Chem. Soc. Rev. (1974) 3, 329. 4. Srinivasan, R., J. Am. Chem. Soc. (1963) 85, 3048. 5. Srinivasan, R., J. Am. Chem. Soc. (1964) 86, 3318. 6. Baldwin, J. E., Greeley, R. H., J. Am. Chem. Soc. (1965) 87, 4514. 7. Haller, I., Srinivasan, R., J. Am. Chem. Soc. (1966) 88, 5084. 8. Nozaki, H., Nisikawa, Y., Kawanisi, M., Noyori, R., Tetrahedron (1967) 23, 2173. 9. Whitesides, G. M., Goe, G. L., Cope, A. C., J. Am. Chem. Soc. (1969) 91, 2608. 10. Deyrup, J. A., Betkouski, M., J. Org. Chem. (1972) 37, 3561. 11. Schwendiman, D. P., Kutal, C., Inorg. Chem. (1977) 16, 719. 12. Schwendiman, D. P., Kutal, C., J. Am. Chem. Soc. (1977) 99, 5677. 13. Grutsch, P. A., Kutal,C.,J.Am. Chem. Soc. (1977) 99, 6460. 14. Arnold, D. R., Trecker, D. J., Whipple, E. B., J. Am. Chem. Soc. (1967) 87, 2596. 15. Trecker, D. J., Henry, J. P., McKeon, J. E., J. Am. Chem. Soc. (1965) 87, 3261. 16. Trecker, D. J., Foote, R. S., Henry, J. P., McKeon, J. E., J. Am. Chem. Soc. (1966) 88, 3021.
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II
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26.
KUTAL AND GRUTSCH
Sensitization
of
Olefin
Photoreactions
343
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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
RECEIVED February 22, 1978.
King; Inorganic Compounds with Unusual Properties—II Advances in Chemistry; American Chemical Society: Washington, DC, 1979.