Excited States and Reactive Intermediates - American Chemical Society

12 8

8

Photochemistry of Dinuclear d -d Iridium and Platinum Complexes Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 11, 2016 | http://pubs.acs.org Publication Date: April 30, 1986 | doi: 10.1021/bk-1986-0307.ch012

Janet L. Marshall, Albert E. Stiegman, and

Harry B. Gray

Arthur Amos Noyes Laboratory, California Institute of Technology, Pasadena,

3

CA

91125

3

The long-lived ( B and A2 ) excited states of the d - d dimers [Ιr(μ-pz)(COD)] and P t ( p o p ) 4 , respec t i v e l y , undergo a variety of photochemical reactions (pzH is pyrazole; COD i s 1,5-cyclooctadiene; pop is P O H2 , bridging pyrophosphite). Electron transfer reactions to one-electron acceptors such as pyridinium cations or substituted benzophenones are quite f a c i l e with acceptors that have reduction potentials as negative as -2.0 V vs. NHE. With halocarbon acceptors, d -d oxidative addition products are obtained. Several organic substrates with r e l a t i v e l y weak C-H bonds react with the t r i p l e t (dσ*pσ) excited state of Pt (pop)4 by Η-atom transfer. 2

8

U

8

4-

2

2

2-

2

7

5

7

4-

2

An e l e c t r o n i c e x c i t e d s t a t e of a m e t a l complex i s both a s t r o n g e r r e d u c t a n t and o x i d a n t than the ground s t a t e . T h e r e f o r e , complexes w i t h r e l a t i v e l y l o n g - l i v e d e x c i t e d s t a t e s can p a r t i c i p a t e i n i n t e r m o l e c u l a r e l e c t r o n t r a n s f e r r e a c t i o n s t h a t are u p h i l l f o r the c o r r e s p o n d i n g ground s t a t e s p e c i e s . Such e x c i t e d s t a t e e l e c t r o n t r a n s f e r r e a c t i o n s o f t e n p l a y key r o l e s i n m u l t i s t e p schemes f o r the c o n v e r s i o n o f l i g h t to c h e m i c a l energy (1). The redox p o t e n t i a l s o f an e x c i t e d s t a t e can be e s t i m a t e d from i t s s p e c t r o s c o p i c energy (0-0 t r a n s i t i o n , Eo(M/M*)) and the o x i d a t i o n and r e d u c t i o n p o t e n t i a l s o f the ground s t a t e : E°(M+/*M) = Ε°(Μ+·/Μ) - E 3 ] , *[Cr(bpy)$}2+ * [ I r ( M e 2 p h e n ) 2 C l 2 ] , and * [ R e C l ] - (3-11). Our i n t e r e s t i n t h i s a r e a c o n c e r n s the e l e c t r o n t r a n s f e r r e a c t i v i t y o f d i m e r i c d^-d^ complexes o f p l a t i n u m ( I I ) and I r ( l ) . S p e c t r o s c o p i c s t u d i e s i n o u r l a b o r a t o r y i n d i c a t e t h a t the " f a c e - t o f a c e " dimers o f symmetry e x e m p l i f i e d by P t 2 ( p o p ) 4 ^ ~ (pop i s P2O5H2 ", b r i d g i n g p y r o p h o s p h i t e ) I have l o w - l y i n g t r i p l e t ( A 2 ) and s i n g l e t (^A2 ) e m i s s i v e e x c i t e d s t a t e s d e r i v e d from the ( d o ) ( d o * ) * ( ρ σ ) e l e c t r o n i c c o n f i g u r a t i o n (12-16). R e c e n t l y , we have o b s e r v e d f l y o r e s c e n c e and phosphorescence from s i m i l a r e x c i t e d s t a t e s and ^B2) i n lower symmetry ( C ) p y r a z o l y l - b r i d g e d i r i d i u m ( l ) dimers such as [ I r ( y - p z ) ( C O D ) ] 2 (pzH i s p y r a z o l e , COD i s 1 , 5 - c y c l o o c t a d i e n e 2 (17^). From s p e c t r o s c o p i c and e l e c t r o c h e m i c a l s t u d i e s , the ^ I ^ C d o * ρσ) e x c i t e d s t a t e o f 2 i s p r e d i c t e d t o be a p o w e r f u l r e d u c t a n t w i t h E° ( l r / I r * ) e s t i m a t e d t o be c a . -1.81 V v s . SSCE i n CH3CN. As e x p e c t e d , the r a t h e r l o n g - l i v e d (250 ns (CH CN, 22 ± 2°C)) B ( d o * p o ) e x c i t e d s t a t e r e a d i l y reduces methyl v i o l o g e n ( M V ) a t a d i f f u s i o n l i m i t i n g r a t e ( k * =1.6 χ 1 0 M " s " ; E° (MV +/MV+) = -0.45 V v s . SSCE, CH3CN ( 1 8 ) ) . (Figure 1). We have c o n f i r m e d t h a t t h i s s t a t e i s a p o w e r f u l r e d u c t a n t by an i n v e s t i g a t i o n o f the e l e c t r o n t r a n s f e r quenching o f [ I r ( μ - ρ ζ ) ( C O D ) by a s e r i e s o f p y r i d i n i u m a c c e p t o r s w i t h v a r y i n g r e d u c t i o n p o t e n t i a l s ( F i g u r e 2; T a b l e I ) . F o r a c c e p t o r s w i t h r e d u c t i o n p o t e n t i a l s o f ^ -1.5 t o -1.9 V ( v s . SSCE, CH3CN), the quenching r a t e c o n s t a n t s range from 8 χ 10** M'^-s'l t o 1 χ 10^ M ' ^ s ' l . The important p o i n t i s t h a t , as p r e d i c t e d (_3) > the r a t e o f e l e c t r o n t r a n s f e r d e c r e a s e s markedly as the r e d u c t i o n p o t e n t i a l o f the a c c e p t o r approaches the v a l u e p r e v i o u s l y e s t i m a t e d f o r E° ( I r 2 / l r 2 * ) ( 1 7 ) . While the A 2 (do*po) e x c i t e d state reduction p o t e n t i a l o f P t 2 ( p o p ) 4 ^ ~ cannot be c a l c u l a t e d a c c u r a t e l y due t o the i r r e v e r s i b i l i t y o f the ground s t a t e e l e c t r o c h e m i s t r y , i t can be e s t i m a t e d from quenching e x p e r i m e n t s . In aqueous s o l u t i o n , t h i s e x c i t e d s t a t e r e a d i l y reduces o n e - e l e c t r o n a c c e p t o r s such as [0s(NH3)5Cl]~*" and nicotinamide. From these d a t a , E° ( P t 2 ~ / P t 2 ^ ~ * ) i e s t i m a t e d t o be c a . -1 V v s . NHE i n aqueous s o l u t i o n ( 1_3) · The e x c i t e d s t a t e r e d u c t i o n p o t e n t i a l d e c r e a s e s s i g n i f i c a n t l y i n nonaqueous s o l v e n t s , presumably due t o the d e c r e a s e i n ground s t a t e p o t e n t i a l r e s u l t i n g from p l a c i n g a t e t r a a n i o n i n low d i e l e c t r i c s o l v e n t s . Quenching o f [Pt2(pop)4^~]* i n a c e t o n i t r i l e by a s e r i e s o f s u b s t i t u t e d benzophenone a c c e p t o r s suggests t h a t the e x c i t e d s t a t e redox p o t e n t i a l E° ( P t ~ / P t 2 ~ * ) i s ^ -2 V v s . NHE. T h e r e f o r e , i n nonaqueous s o l v e n t s , the A 2 ( d o * p o ) e x c i t e d s t a t e s h o u l d r e a d i l y reduce a v a r i e t y of substrates (19). 2

2

8

2

3

U

2

U

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 11, 2016 | http://pubs.acs.org Publication Date: April 30, 1986 | doi: 10.1021/bk-1986-0307.ch012

1

2 v

+

3

2

2

3

3

2

2+

1 0

1

1

2

3

+

3

3

U

2

3

3

s

3

3

3

4

2

3

u

Both 1 and 2 luminesce from s i n g l e t and t r i p l e t (do*po) e x c i t e d states. In both c a s e s , the t r i p l e t s t a t e s a r e l o n g - l i v e d and, as j u s t discussed, strong reducing agents. The s i n g l e t e x c i t e d s t a t e s , however, have l i f e t i m e s o f < 10 ps ( 2 0 ) , and r e c e n t experiments s t r o n g l y suggest t h a t the I-B2 (do*po) s t a t e o f 2 does n o t p a r t i c i p a t e in intermolecular electron transfer reactions (21). A l t h o u g h t h e c o n v e r s i o n o f l i g h t energy t o c h e m i c a l energy v i a the e l e c t r o n t r a n s f e r r e a c t i v i t y o f [ I r ( μ - ρ ζ ) ( C O D ) ] 2 * i s r a t h e r f a c i l e , the p h o t o c h e m i c a l p r o d u c t s r a p i d l y r e t u r n t o s t a r t i n g m a t e r i a l s because the back e l e c t r o n t r a n s f e r r e a c t i o n s a r e h i g h l y e x o t h e r m i c . For example, back e l e c t r o n t r a n s f e r between the o x i d i z e d i r i d i u m 3

Lever; Excited States and Reactive Intermediates ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

168


EXCITED STATES AND REACTIVE INTERMEDIATES

Figure

1.

Diagrams

o f [Ιτ(μ-ρζ)(COD)]2 and Pt2(pop>4'



MARSHALL ETAL.

8

8

Dinuclear d -d

-2.0

Iridium and Platinum Complexes

-1.5 E(A /A), +

-1.0 V

vs.

-0.5

SSCE

+

F i g u r e 2. P l o t o f R T l n k ' (V) v s . E ( A / A ) (V) f o r t h e e l e c t r o n t r a n s f e r quenching o f [ i r ( p - p z ) ( C O D ) ] 2 * i n CH3CN (μ = 0.1 M [(n-C4Hq)4NPF6]) a t 22 ± 2°C. The s o l i d l i n e t h a t f i t s p o i n t s 7-13 has a s l o p e o f 0.5. Data and t h e quencher numbering scheme are given i n Table I . 3



^

2

r

R

'

PF

< 6>-

4

9

+

6

p

X 1.,1

+

10

10? 6

8

X

5..8

2..2

8

8

10

10

X

8

8

2. 9

10

X

10

10

10»

X X

10» 10»

X

1010

X X

1010 1010

X

X X

6. 5

6..6

8..1

2. 5

6. 2

5..5

1..5

1..3

- l

X

1 M -1 >

2.,0

q

b

a

( k ' ) are corrected

fordiffusion

effects.

The r a t e c o n s t a n t s

+

-1. .85

k

b.

+

+

3

-1. .67

-1. .57

-1. .52

-1. .52

-1. .49

-1. ,48

-1. .36

-1. .14

-1. .07

-0..93

-0..78

-0..67

a

F o r quenchers 1-3, E ( A / A ) = E ^ ( A / A ) . F o r quenchers 4-13, the r e d u c t i o n s a r e i r r e v e r s i b l e ; t h e r e f o r e , the v a l u e s o f E ( A / A ) a r e the c a t h o d i c peak p o t e n t i a l s , E ( A / A ) , measured a t a c o n s t a n t s c a n r a t e (200 mV/s). Both E ^ ( A / A ) and E ( A / A ) were measured by c y c l i c vo ltammetry (CH3CN, μ - 0.1 M [ ( n - C H ) 4 N P F ] , 22 ± 2 ° C ) .

2

+

E(A /A), V v s . SSCE

quenchers

a.

3

+

2,6-(CH3) -4-OCH

CH3

3

4-C(CH )

3

13.

5

4-CH3

2-OCH3

3

2,4,6-(CH )

CH3

12.

3

CH3

11. 3

2,3,6-(CH )

CH3

10. 2

2,6-(CH )

C H

9. 3

CH3

8.

2

CH3

2

7.

5

2

2

C H

3-CONH

3-CONH

6.

5

C H CH

3

CH3

2

5.

6

2

4-C0NH

4-C0 CH

4.

2

C H

3.

3

CH

CH

4-CN

5

N

R'

3

+

3

t r a n s f e r quenching o f [ I r ( y - p z ) ( C O D ) ] 2 * by a l k y l a t e d

R

-

1.

R

Data f o r e l e c t r o n

2.

p

Table I.


12. MARSHALL ET AL.

8

8

Dinuclear d -d

dimer and reduced methyl v i o l o g e n can be m o n i t o r e d by f l a s h and the r a t e c o n s t a n t i s n e a r t h a t o f t h e d i f f u s i o n l i m i t : [lr(u-pz)(COD)]

+ 2

+ MV+

k

» [Ir(y-pz)(COD)]

b

171


photolysis

2

+ MV +

2

(3)

S i m i l a r l y , the back e l e c t r o n t r a n s f e r r e a c t i o n s i n v o l v i n g a l k y l a t e d p y r i d i n i u m a c c e p t o r s a r e v e r y r a p i d and no n e t p h o t o c h e m i s t r y i s observed. To u t i l i z e t h e s t r o n g r e d u c i n g power o f t h e ( d o * p o ) e x c i t e d s t a t e s o f the p l a t i n u m and i r i d i u m dimers, the n o n p r o d u c t i v e back e l e c t r o n t r a n s f e r r e a c t i o n s need t o be i n h i b i t e d . An e f f e c t i v e way to a c c o m p l i s h t h i s i s t o use a c c e p t o r s t h a t a r e t h e r m a l l y unstable a f t e r the i n i t i a l e l e c t r o n t r a n s f e r . R e d u c t i o n o f a l k y l h a l i d e s has been shown t o l e a d t o s h o r t - l i v e d r a d i c a l a n i o n s RX , which r a p i d l y decompose t o g i v e R- and X" ( k = 3 χ 1 0 s " f o r Ο Η β Ο Ι , k 3 χ 1 0 s ' f o r CH3Br ) (22). The t r i p l e t e x c i t e d s t a t e s o f I and 2 are c a p a b l e o f r e d u c i n g a number o f h a l o c a r b o n s , and t h i s onee l e c t r o n r e d u c t i o n s t e p l e a d s t o n e t p h o t o c h e m i s t r y because o f r a d i c a l anion fragmentation (23).


3

T

7

1

7

d

8

1

T

C

E x c i t a t i o n o f t h e Ai -*~ Â , 2 U < 4h) ( 1 + 2 > 2 ( 2v^ e l e c t r o n i c t r a n s i t i o n s o f P t ( p o p ) 4 ^ ~ and [ I r ( u - p z ) ( C 0 D ) ] j r e s p e c t i v e l y , i s f o l l o w e d i n each case by a r a p i d r e a c t i o n w i t h 1,2d i c h l o r o e t h a n e (DCE) t o form a d - d d i h a l i d e dimer ( P t ( p o p ) 4 ( C l ) " or [ l r ( y - p z ) ( C 0 D ) ( C l ) ] ) and e t h y l e n e : l

3 a

D

1 a

1 β

3 β

c

2 U

g

2

2

7

7

h

2

2

2

Pt (pop) 2

4 4

-

+ ClCH CH Cl-r^:

[lr(y-pz)(C0D)]

2

2

*C1-Pt

2

+ C1CH CH C1 > 4 2

2

λ

> 5 0

n

m

Pt-Cl + C H 2

C

1

^

I

r

ΐ Γ

—

\

C

l

+

(4)

4

C

H

2 4

5

This binuclear photooxidative addition reaction i s general for a number o f h a l o c a r b o n s ( F i g u r e 3 ) . While DCE and 1,2-dibromoethane r e a c t c l e a n l y t o g i v e the d i h a l i d e m e t a l dimers and e t h y l e n e , s u b s t r a t e s such as bromobenzene o r methylene c h l o r i d e r e a c t through an a l k y l o r a r y l i n t e r m e d i a t e . This intermediate reacts further to y i e l d the d i h a l i d e d - d m e t a l complexes. A mechanism t h a t a c c o u n t s f o r the o x i d a t i v e a d d i t i o n o f h a l o carbons has been proposed f o r the two dimers ( F i g u r e 4) ( 2 3 ) . The mechanism i n v o l v e s t h e o x i d a t i v e quenching o f t h e t r i p l e t e x c i t e d s t a t e o f t h e m e t a l dimer as the p r i m a r y p h o t o p r o c e s s . This gives a r a d i c a l a n i o n s p e c i e s t h a t d i s s o c i a t e s a h a l i d e , t h e r e b y p r o d u c i n g an organic r a d i c a l . The d i s s o c i a t e d h a l i d e adds t o the p a r t i a l l y o x i d i z e d m e t a l dimer t o form a mixed v a l e n c e I r * - I r * * - X o r P t * Pt***-X i n t e r m e d i a t e . T h i s i n t e r m e d i a t e r e a c t s f u r t h e r w i t h the r e m a i n i n g o r g a n i c r a d i c a l (presumably i n a second, t h e r m a l e l e c t r o n t r a n s f e r s t e p ) t o form t h e f i n a l d - d d i h a l i d e dimer. T h i s mechanism i s s u p p o r t e d by t h e p r o d u c t d i s t r i b u t i o n found f o r the p h o t o c h e m i c a l r e a c t i o n s w i t h 1,2-bromochloroethane. A t h i g h c o n c e n t r a t i o n s o f t h i s s u b s t r a t e , the r e s u l t i n g p l a t i n u m and i r i d i u m dimers a r e the d i b r o m i d e s p e c i e s , P t ( p o p ) 4 ( B r ) ^ ~ and [Ir(μ-ρζ)(COD) 7

7

1

7

7

2

2


172


[lr(/x-pz)(COD)] Tr

2

+ XCH CH X

2

2

2

— — ^ X - I r — Tr -X + C H 2

4

(X = CI, Br)


Ir

+ CH CI

2

2

^CICH

2

4Pt (pop) 2

Pt

- Ir—Ir-CI

^ C l -Ir-Ir-CI

(non-aqueous solvents)

4

2

2

+ XCH CH X — — 2

2

^X-Pt-Pt-X

+

C H 2

4

(X- CI, Br) Pt

2

+ 2C H X 6

—

5

»-X-Pt—Pt-X + biphenyl

^ C H P t —Pt-X 6

5

(X = CI, Br)

hi/ CH CI 2

Pt

2

+

^ C I C H - Pt—Pt-CI

2

2

or CR CI 2

or hl/ 2

5-CI-Pt—Pt-CI

> CICR - Pt—Pt-CI 2

3

3

4

F i g u r e 3. R e a c t i v i t y o f [ I r ( μ - ρ ζ ) ( C O D ) ] * and [ P t ( p o p ) 4 ~ ] * with halocarbons. 2

2

2

F i g u r e 4. Proposed g e n e r a l m e c h a n i s t i c scheme f o r h a l o c a r b o n p h o t o o x i d a t i v e a d d i t i o n t o b i n u c l e a r I r ( l ) and P t ( l l ) .


12.

8

H

Dinuclear d -d

MARSHALL ET AL.

173


(Br)J2» e x c l u s i v e l y . Low c o n c e n t r a t i o n s o f 1,2-bromochloroethane, however, y i e l d the mixed h a l i d e m e t a l dimers P t 2 ( p o p > 4 ( B r ) ( C l ) ~ and Ir2(y-pz)2(C0D)2(Br)(Cl). T h i s r e s u l t i s p r e d i c t e d by the proposed mechanism ( F i g u r e 5 ) . P h o t o l y s i s r e s u l t s i n f o r m a t i o n o f Pt2(pop)4~ ( B r ) ~ or Ir2(u-pz)2(COD>2(Br) as i n t e r m e d i t a e s . The i n t e r m e d i a t e can r e a c t w i t h another bromochloroethane m o l e c u l e , as i t does when the l a t t e r s p e c i e s i s i n h i g h c o n c e n t r a t i o n , to y i e l d the dibromide dimer or i t can r e a c t w i t h the c h l o r o e t h a n e r a d i c a l t o y i e l d the mixed h a l i d e m e t a l s p e c i e s . The l a t t e r pathway becomes c o m p e t i t i v e at low h a l o c a r b o n c o n c e n t r a t i o n s . In g e n e r a l , the o x i d a t i v e a d d i t i o n of h a l o c a r b o n s i s t y p i c a l o f the p h o t o c h e m i s t r y a r i s i n g from e l e c t r o n t r a n s f e r from d - d m e t a l dimers w i t h the f i n a l p r o d u c t b e i n g the s t a b l e d - d m e t a l - m e t a l bonded dimers (24-25). 4


4

8

7

8

7

B i m o l e c u l a r quenching o f the e x c i t e d s t a t e s o f m e t a l complexes g e n e r a l l y i n v o l v e s e l e c t r o n t r a n s f e r or energy t r a n s f e r p r o c e s s e s (_1). R e c e n t l y , however, Pt2(pop>4^~ has been found to undergo a photo c h e m i c a l r e a c t i o n i n v o l v i n g atom a b s t r a c t i o n as a p r i m a r y p h o t o p r o cess ( 2 6 ) . The r e a c t i o n i n v o l v e s the c a t a l y t i c c o n v e r s i o n o f i s o p r o p a n o l to acetone: Pt (pop) 2

4 4

" +

h v

(CH ) CHOH 3

3

2

> Pt (pop) 2

4 4

" + H

2

+

(CH ) CO 3

2

(6)

4

While [ P t 2 ( p o p ) 4 ~ ] * i s a s t r o n g r e d u c t a n t , i t i s not s u f f i c i e n t l y r e d u c i n g to t r a n s f e r one e l e c t r o n to an a l c o h o l ( 2 7 ) . E x t r a c t i o n o f a hydrogen i n a primary photoprocess would produce an i s o p r o p y l r a d i c a l t h a t c o u l d undergo d i s p r o p o r t i o n a t i o n to y i e l d acetone ( 2 8 ) : 2(CH ) COH 3

»

2

(CH ) CHOH + ( C H ) C O 3

2

3

or f u r t h e r r e a c t w i t h a m e t a l dimer 3

[Pt (pop) 2

4 4

-]* +

(7)

2

(29). 4

(CH ) COH—*Pt (pop) (H) - + 3

2

2

4

(CH ) CO 3

2

(8)

Our k i n e t i c i n v e s t i g a t i o n o f t h i s r e a c t i o n p r o v i d e s c o m p e l l i n g e v i dence f o r an atom a b s t r a c t i o n mechanism. Quenching o f the t r i p l e t e x c i t e d s t a t e o f 1 by v a r i o u s a l c o h o l s o c c u r s o n l y when an α-hydrogen i s p r e s e n t (19). No quenching o c c u r s w i t h _t-butanol o r t r i p h e n y l carbinol. Furthermore, completely deuterated i s o p r o p a n o l y i e l d s a k i n e t i c i s o t o p e e f f e c t o f 1.5 ( T a b l e I I ) . Table

3

II.

4

Data f o r the quenching o f [Pt2(pop)4 ~* ] * by o r g a n i c s t r a t e s i n CH CN s o l u t i o n at 22 ± 2°C.

sub-

3

1

q

(CH ) CHOH

10

3

(C H )CH

10

4

3

6

2

5

3

(C H ) CHOH 6

5

2

k (2-propanol)/k (2-propanol(d-8)) q

1

k (M" s" )

Quencher

q

ΙΟ* =1.5.

Hydrocarbons w i t h r e l a t i v e l y weak C-H bonds a l s o r e a c t w i t h [ P t 2 ( p o p ) 4 " ] * by Η-atom t r a n s f e r . For example, t o l u e n e can be p h o t o c a t a l y t i c a l l y c o n v e r t e d t o b i b e n z y l by a b s t r a c t i o n o f a m e t h y l 3

4




hi/

-M + BrCH CH CI 2

3

2

M — M * + BrCH CH CI 2

2

M—M , CICH CH Br" +

2

2

M—M-Br, CICH CH • 2

BrCH CH CI 2

2

1

2

CICH CH 2

2

M—M-Br Figure 5 . Proposed pathways f o r B r C I ^ C I ^ C l a d d i t i o n t o b i n u c l e a r I r ( l ) and P t ( l l ) .

Cl-M—M-Br photooxidative


8

8

Dinuclear d -d

12. MARSHALL ET AL.

hydrogen and c o u p l i n g o f t h e r a d i c a l Pt (pop) 2

4 4

~

+ C H -CH J ^ P t ( p o p ) 6

5

3

175


2

products: 4 4

" + C H -CH -CH -C H 6

5

2

2

6

5

+ H

2

(9)

An i n t e r m e d i a t e has been observed f o r t h e r e a c t i o n i n a l c o h o l s . Narrow band i r r a d i a t i o n (370 nm) o f P t ( p o p ) ~ i n i s o p r o p a n o l r e s u l t s i n the d i s a p p e a r a n c e o f P t ( p o p ) ~ and t h e appearance o f an i n t e r m e d i a t e w i t h a maximum a t 313 nm. T h i s i n t e r m e d i a t e w i l l t h e r m a l l y b a c k - r e a c t ( t i ^ = 85 min) by f i r s t - o r d e r k i n e t i c s t o r e f o r m P t ( p o p ) ~ o r w i l l c o n v e r t back immediately upon 313 nm i r r a d i a t i o n . Long-term p h o t o l y s i s a t low energy (370 nm) does n o t produce acetone, w h i l e e q u a t i o n (6) o c c u r s r e a d i l y w i t h broadband i r r a d i a t i o n . The exact n a t u r e o f t h e i n t e r m e d i a t e has n o t been d e t e r m i n e d ; however, i t s a b s o r p t i o n spectrum s t r o n g l y resembles t h a t o f a (metal-metal-bonded) dimer. One p o s s i b l e s t r u c t u r e would be t h a t d e r i v e d from t h e o x i d a t i v e a d d i t i o n o f i s o p r o p a n o l : ( C H ) C ( H ) - 0 - P t Pt-H. Another p o s s i b i l i t y i s a d i h y d r i d o s p e c i e s , H-Pt-Pt-H. Irra d i a t i o n o f e i t h e r o f these i n t e r m e d i a t e s c o u l d y i e l d o r g a n i c p r o d u c t s and r e g e n e r a t e P t ( p o p ) ~ . In summary, t h e t r i p l e t (do*po) e x c i t e d s t a t e s o f t h e d 8 - 8 m e t a l dimers [ I r ( μ - ρ ζ ) ( C O D ) ] and P t ( p o p ) ~ undergo a v a r i e t y o f photo c h e m i c a l r e a c t i o n s . E l e c t r o n t r a n s f e r t o o n e - e l e c t r o n quenchers such as p y r i d i n i u m c a t i o n s o r h a l o c a r b o n s r e a d i l y o c c u r s w i t h a c c e p t o r s t h a t have r e d u c t i o n p o t e n t i a l s as n e g a t i v e as -2.0 V. With the l a t t e r r e a g e n t s , n e t t w o - e l e c t r o n , photoinduced e l e c t r o n t r a n s f e r y i e l d s d^-d^ o x i d a t i v e a d d i t i o n p r o d u c t s . A d d i t i o n a l l y , the t r i p l e t (do*po) e x c i t e d s t a t e o f P t ( p o p ) ~ a p p a r e n t l y i s a b l e t o r e a c t by e x t r a c t i n g a hydrogen atom from a C-H bond o f an o r g a n i c s u b s t r a t e . 4

2

4

4

2

4

4


2

4

3

2

4

2

4

d

4

2

2

4

4

2

4

Acknowledgment. J . L. M. thanks t h e Sun Co. f o r a g r a d u a t e f e l l o w ship. T h i s r e s e a r c h was supported by 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 Grant CHE84-19828.

Literature Cited 1. Balzani, V.; Bolletta, F.; Gandolfi, M. T.; Maestri, M. Top. Curr. Chem. 1978, 75, 1-64. 2. Meyer, T. G. Prog. Inorg. Chem. 1938, 30, 389-441 and references therein. 3. Bock, C. R.; Connor, J. Α.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 4815-4824. 4. Bock, C. R.; Whitten, D. G.; Meyer, T. J. J. Am. Chem. Soc. 1975, 97, 2909-2911. 5. Lin, C. T.; Bottcher, W.; Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1976, 98, 6536-6544. 6. Toma, H. E.; Creutz, C. Inorg. Chem. 1977, 16, 545-550. 7. Bock, C. R.; Connor, J. Α.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. Chem. Phys. Lett. 1975, 61, 522-525. 8. Ballardini, R.; Varani, G.; Indelli, M. T.; Scandola, F.; Balzani, V. J. Am. Chem. Soc. 1978, 100, 7219-7223. 9. Sutin, N. J. Photochem. 1979, 10, 19-40. 10. Sutin, N.; Creutz, C. Pure and Appl. Chem. 1980, 52, 2717-2738.



176

E X C I T E D STATES A N D R E A C T I V E I N T E R M E D I A T E S

11. Nocera, D. G.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 74397350. 12. The Pt2(pop)44- anion can be made with a variety of counterions. Spectroscopic and aqueous photochemical studies were performed with the barium and potassium salts, respectively. The tetrabutylammonium salt is required for photochemistry in nonaqueous solvents. 13. Che, C.-M.; Butler, L. G.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 7796-7797. 14. Fordyce, W. Α.; Brummer, J. G.; Crosby, G. A. J. Am. Chem. Soc. 1981, 103, 7061-7064. 15. Rice, S. F.; Gray, Η. B. J. Am. Chem. Soc. 1983, 105, 4571-4575. 16. Parker, W. L.; Crosby, G. A. Chem. Phys. Lett. 1984, 105, 544546. 17. Marshall, J. L.; Stobart, S. R.; Gray, Η. B. J. Am. Chem. Soc. 1984, 106, 3027-3029. 18. Bock, C. R.; Meyer, T. J.; Whitten, D. G. J. Am. Chem. Soc. 1974, 96, 4710-4712. 19. Heinrichs, Μ. Α.; Stiegman, A. E.; Gray, Η. B. unpublished results. 20. Miskowski, V.; Stiegman, A. E.; Gray, Η. B. unpublished results. 21. Winkler, J. R.; Marshall, J. L.; Netzel, T. L.; Gray, Η. B. J. Am. Chem. Soc. submitted. 22. Kochi, J. K. "Organometallic Mechanisms and Catalysis"; Academic Press: New York, 1978; Chapter 7. 23. Caspar, J. V.; Gray, Η. B. J. Am. Chem. Soc. 1984, 106, 30293030. 24. Fukuzumi, S.; Nishizawa, N.; Tanaka, T. Bull. Chem. Soc. Jpn. 1983, 56, 709-714. 25. Fukuzumi, S.; Hironaka, K.; Nishizawa, N.; Tanaka, T. Bull. Chem. Soc. Jpn. 1983, 56, 2220-2227. 26. Roundhill, D. M. J. Am. Chem. Soc. 1985, 107, 4354-4356. 27. Bard, A. J.; Lind, H. "Encyclopedia of Electrochemistry of the Elements"; Marcel Dekker: New York, 1973; Vol. XI, p. 181. 28. Hay, J. M. "Reactive Free Radicals"; Academic Press: New York, 1974; pp. 96-102. 29. Nonhebel, D. C.; Walton, J. C. "Free Radical Chemistry"; Cambridge University Press, 1974; p. 19. RECEIVED November 8, 1985


Excited States and Reactive Intermediates - American Chemical Society

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