Excited States and Reactive Intermediates - American Chemical Society

14 Chemistry of Rhodium and Iridium Phosphine Complexes Flash Photolysis Investigations of Reactive Intermediates

Downloaded by UNIV OF PITTSBURGH on May 4, 2015 | http://pubs.acs.org Publication Date: April 30, 1986 | doi: 10.1021/bk-1986-0307.ch014

David Wink and Peter C. Ford Department of Chemistry, University of California, Santa Barbara, CA 93106

Flash photolysis of the rhodium(I) and iridium(I) complexes MCl(CO)(PPh3)2 in benzene leads to formation of the unsaturated species MCl(PPh3)2, the reaction kinetics of which have been investigated. Reactions with CO to reform MCl(CO)(PPh )2 occur with second order rate constants of 7 x 10 and 2.7 x 108 M-1s-1 for M = Rh and Ir, respectively. The RhCl(PPh )2 species also undergoes fast reactions with PPh (k = 2.6 x 10 M-l-s-1) and with ethylene (>2 x 10 M- s- ) to form RhCl(PPh ) and RhCl(H C=CH )(PPh ) respectively; however, reaction with H2 to form the dihydride is much slower, (l x 10 M- s- ). Also described are flash photolysis studies of the dinitrogen species IrCl(N2)(PPh )2 and the dihydride H2IrCl(CO)(PPh )2. In both cases, the transient IrCl(PPh )2 is formed. These results indicate that CO labilization from the Ir(III) dihydride is a facile photochemical pathway and the photo-reductive elimination of H2 is a more complicated mechanism than previously inferred. 3

7

3

3

6

1

7

1

3

2

2

5

1

2

3

2,

1

3

3

3

Phosphine complexes of low v a l e n t m e t a l complexes have a l o n g h i s t o r y i n the c h e m i s t r y o f homogeneous c a t a l y t i c a c t i v a t i o n of s m a l l molecules(l,2). For example, the c a t a l y s i s c h e m i s t r y of r h o d i u m ( I ) phosphine complexes c o n t i n u e s to h o l d much i n t e r e s t s e v e r a l decades s i n c e the d e s c r i p t i o n of such r e a c t i o n s by W i l k i n s o n ( 3 ) . However, d e s p i t e c o n s i d e r a b l e q u a n t i t a t i v e s c r u t i n y ( 3 - 1 0 ) , the m e c h a n i s t i c d e t a i l s of key c a t a l y t i c s t e p s f o r even the o r i g i n a l W i l k i n s o n ' s c a t a l y s t R h C l ( P P h ) a r e not f u l l y r e s o l v e d ( 7 ^ 1 0 ) . The r e a s o n l i e s w i t h i n the v e r y n a t u r e of c a t a l y t i c p r o c e s s e s , namely t h a t the a c t i v a t i o n of s u b s t r a t e s o f t e n i n v o l v e s r e a c t i o n s of u n s t a b l e t r a n s i e n t s p e c i e s , the p r o p e r t i e s of which can o n l y be i n f e r r e d from k i n e t i c r a t e laws or from s p e c t r a l s t u d i e s under c o n d i t i o n s c o n s i d e r a b l y d i f f e r e n t from t h o s e of an o p e r a t i n g c a t a l y s t . I n some c a s e s i t may be p o s s i b l e to use f l a s h p h o t o l y s i s t o g e n e r a t e s i g n i f i c a n t c o n c e n t r a t i o n s of such a t r a n s i e n t and to i n v e s t i g a t e the r e a c t i o n s of t h a t s p e c i e s more d i r e c t l y . Here we d e s c r i b e some i n v e s t i g a t i o n s 3

3

0097-6156/ 86/ 0307-0197506.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

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

198

EXCITED STATES AND

REACTIVE INTERMEDIATES

u s i n g f l a s h p h o t o l y s i s t e c h n i q u e s t o probe the r e a c t i o n dynamics of r e a c t i v e i n t e r m e d i a t e s i n the c h e m i s t r y of r h o d i u m ( I ) and i r i d i u m ( I ) p h o s p h i n e complexes. Rhodium(I) Complexes A key i n t e r m e d i a t e i n proposed mechanisms f o r W i l k i n s o n ' s c a t a l y s t i s the t r i c o o r d i n a t e s p e c i e s R h C l ( P P t i 3 ) 2 o r i t s s o l v a t e d a n a l o g (7-10) « Thus, i t would be p a r t i c u l a r l y d e s i r a b l e to prepare t h i s s p e c i e s and to i n t e r r o g a t e i t s r e a c t i v i t y under c a t a l y t i c a l l y r e l e v a n t conditions. G i v e n the commonly o b s e r v e d p h o t o l a b i l i t y o f c a r b o n monoxide complexes(11), a l o g i c a l p h o t o c h e m i c a l p r e c u r s o r t o RhCl(PPh ) would be the c a r b o n y l R h C l ( C O ) ( P P h 3 ) 2 . Although e a r l i e r i n v e s t i g a t o r s found t h a t under c o n t i n u o u s p h o t o l y s i s the l a t t e r d i d not d i s p l a y n e t p h o t o c h e m i s t r y ( 1 2 ) , i t appeared l i k e l y t h a t r e v e r s i b l e l i g a n d l a b i l i z a t i o n would be the r e s u l t of f l a s h e x c i t a t i o n . When t r a n s - R h C l ( C O ) ( P P t v ^ i n degassed benzene was s u b j e c t e d to flash photolysis (λ^ > 315 nm, p u l s e d u r a t i o n about 20 y s e c ) , t r a n s i e n t a b s o r p t i o n was o b s e r v e d w i t h the s p e c t r a l c h a r a c t e r i s t i c s i l l u s t r a t e d i n F i g u r e 1. T h i s t r a n s i e n t (when m o n i t o r e d a t X 410 nm) decayed v i a second o r d e r k i n e t i c s over a p e r i o d o f s e v e r a l ms ( F i g u r e 2 ) . When the s o l u t i o n was f l a s h e d under CO (1.0 atm, 0.006 M i n b e n z e n e , ( 1 3 ) ) , no t r a n s i e n t h a v i n g a l i f e t i m e l o n g e r than the f l a s h was d e t e c t e d ; however, a l o n g - l i v e d t r a n s i e n t w i t h the same spectrum as R h C l ( P P h 3 ) 3 was seen when the f l a s h p h o t o l y s i s was c a r r i e d out i n the p r e s e n c e of e x c e s s ΡΡΙΊ3 (0.05 M, see b e l o w ) . Thus CO, not phosphine, p h o t o l a b i l i z a t i o n appears to be the major primary p h o t o r e a c t i o n (Equation 1).


3

2

Γ Γ

m o n

hv RhCl(CO) ( P P h ) 3

R h C l ( P P h ) o + CO

2

(1)

3

When the f l a s h p h o t o l y s i s of RhCl(CO) ( P P l v ^ ^ was c a r r i e d out i n the absence o f o t h e r r e a c t a n t s but w i t h X 450 nm, i t was noted t h a t the t r a n s i e n t a b s o r p t i o n decayed i n two s t a g e s ( F i g u r e 3 ) . The r e l a t i v e l y r a p i d second o r d e r decay noted a t 410 nm was f o l l o w e d by a s l o w e r f i r s t o r d e r decay back t o RhCl (CO) (PPI13) 2 w i t h a k ^ of 1.8 s"" (298 K ) . The spectrum of the l o n g e r l i v e d t r a n s i e n t ( F i g u r e 1) i s v e r y c l o s e t o t h a t of t h e dimer [ R h C l ( P P h 3 ^ ] 2 » d e s c r i b e d p r e v i o u s l y ( 1 4 ) and d i s c u s s e d i n the m e c h a n i s t i c schemes f o r W i l k i n s o n ' s c a t a l y s t ( 7 - 1 0 ) . Presumably, [ R h C l i P P t v ^ ^ formed v i a the d i m e r i z a t i o n of R h C K P P l ^ ^ : m o n

Q

s

1

i

k

2 RhCl(PPh ) 3

2

s

2 [RhCl(PPh ) ]2 3

( ) 2

2

and decays by a u n i m o l e c u l a r r a t e - l i m i t i n g s t e p , presumably d i s s o c i a t i o n t o monomers, i . e . , the k_2 s t e p . (The s t o p p e d - f l o w k i n e t i c s of the r e a c t i o n between [ R h C l i P P l v ^ ^ * p r e p a r e d t h e r m a l l y , (14) and CO (P 0.1 - 1.0 atm) i n benzene t o g i v e R h C l ( C O ) ( P P h ) a l s o gave f i r s t o r d e r r a t e s w i t h t h e near l y i d e n t i c a l k , 1.7 s " a t 298 K ) . C a l c u l a t i o n of k_-^ and k 2 from the f l a s h p h o t o l y s i s d a t a r e q u i r e s the e x t i n c t i o n c o e f f i c i e n t o f R h C l ( P P h 3 ) 2 a t >^ i n order c o

3

2

1

o b s

mon



14.

WINK A N D FORD

Chemistry of Rhodium and Iridium Phosphine Complexes

_i

r

400

450

199

ι 500

λ ( in nm )

F i g u r e 1. T r a n s i e n t s p e c t r a r e s u l t i n g from t h e f l a s h p h o t o l y s i s o f RhCl(CO) (PPI13) 2 i n benzene s o l u t i o n . A) Spectrum o b s e r v e d 100 ys a f t e r f l a s h (λ± > 315 nm). P o i n t s i n d i c a t e d r e p r e s e n t a c t u a l e x p e r i m e n t a l o b s e r v a t i o n s ; c u r v e i s drawn f o r i l l u s t r a t i v e purposes. B) Spectrum o b s e r v e d 20 ms a f t e r f l a s h . C) Spectrum o f RhCl(CO)(PPh ) . ντ

3

2



200

EXCITED STATES AND REACTIVE INTERMEDIATES

5

5

10

Time ( i n ms)

10

15

Time ( i n ms)

F i g u r e 2. L e f t : Absorbance changes r e s u l t i n g from the f l a s h p h o t o l y s i s (λ. > 415 nm) of R h C l ( C O ) ( P P h ^ i n benzene solution under Ar at The m o n i t o r i n g wavelength was 410 nm. Right: A l i n e a r second-order p l o t [(A ^ * data i above c u r v e . v

s

o r

t

i e

n

t

n

e

5 Time ( i n ms) Figure 3 . Decay c u r v e f o r t h e f l a s h p h o t o l y s i s o f RhCl(CO) (PPI13)2 i n 25° benzene under Ar showing the f o r m a t i o n o f a n o t h e r i n t e r m e d i a t e s p e c i e s (B) as a p r o d u c t o f t h e s e c o n d - o r d e r decay o f t h e i n i t i a l transient (A).


14.

Chemistry of Rhodium and Iridium Phosphine Complexes

WINK AND FORD

201

to d e t e r m i n e t h e c o n c e n t r a t i o n s o f t h i s s p e c i e s . T h i s was e s t i m a t e d by assuming t h a t t h e r e a c t i o n w i t h e x c e s s P P h (see above) t r a p p e d a l l RhCl(PPh ) as R h C l ( P P h ) . From t h e known spectrum o f t h e l a t t e r s p e c i e s ( 1 4 ) , t h e i n i t i a l c o n c e n t r a t i o n , thus t h e e x t i n c t i o n c o e f f i c i e n t (8 χ 1 0 M" cm" a t 410 nm, t h e i s o s b e s t i c p o i n t f o r the dimer i n t e r m e d i a t e and t h e s t a r t i n g m a t e r i a l ) , o f R h C l ( P P h ) 2 c o u l d be c a l c u l a t e d . W i t h t h i s e x t i n c t i o n c o e f f i c i e n t , t h e second o r d e r r a t e c o n s t a n t f o r t h e d i s a p p e a r a n c e o f R h C l ( P P h ) 2 was d e t e r mined t o be 1 χ 1 0 M " s " . The amount o f dimer produced by t h e f l a s h p h o t o l y s i s i n t h e absence o f added r e a c t a n t s ( c a l c u l a t e d from the spectrum o f t h e l o n g - l i v e d i n t e r m e d i a t e ) i n d i c a t e d t h a t under t h e s e c o n d i t i o n s about 40% o f R h C l ( P P h ) 2 d i m e r i z e d i n c o m p e t i t i o n w i t h t h e back r e a c t i o n w i t h t h e p h o t o l i b e r a t e d CO t o g i v e RhCl(CO) (PPh )2Thus k _ i and k were e s t i m a t e d a s 6 χ 1 0 M ' ^ " and 4 χ 1 0 M" s~-'-, r e s p e c t i v e l y . 3

3

2

3

2

1

3

1

3

3

8

1

1

3


7

3

1

2

7

1

The above k _ i v a l u e was c o n f i r m e d by c a r r y i n g o u t t h e f l a s h p h o t o l y s i s e x p e r i m e n t s i n t h e p r e s e n c e o f e x c e s s CO, (0.3 - 1 . 2 ) χ 10" M. Under t h e s e c o n d i t i o n s , no dimer was o b s e r v e d and t h e t r a n s i e n t decay was f i r s t - o r d e r w i t h k ^ v a l u e s l i n e a r l y dependent on [CO]. The second o r d e r r a t e c o n s t a n t (k_^) o b t a i n e d from t h e plot of k v s [CO] was (6.9 ± 0.2) χ 1 0 M" s" . The r e a c t i o n o f t h e t r a n s i e n t R h C l ( P P h ) 2 w i t h excess t r i p h e n y l p h o s p h i n e t c g i v e R h C l ( P P h ) 2 d i s p l a y e d f i r s t o r d e r r a t e s dependent on t h e c o n c e n t r a t i o n o f P P h . From t h e s e d a t a , t h e second o r d e r r a t e c o n s t a n t f o r t h e r e a c t i o n d e p i c t e d i n E q u a t i o n 3 was c a l c u l a t e d as 2.8 χ 1θ6 M"* s~* . G i v e n t h e r a t e c o n s t a n t o f 0.71 s~^ d e t e r m i n e d (4) f o r t h e d i s s o c i a t i o n o f P P h from R h C l ( P P h ) i n benzene ( k _ ) , the e q u i l i b r i u m c o n s t a n t f o r d i s s o c i a t i o n ( k _ / k ) i s c a l c u l a t e d t o be 0.25 χ 10~6 M , c o n s i s t e n t w i t h t h e p r e v i o u s e s t i m a t e o f < 10"""* M(4). 0

s

7

1

1

o b s

3

3

3

1

1

3

3

3

3

3

3

k RhCl(PPh ) 3

+ PPh

2

-

3

RhCl(PPh ) 3

(3)

3

k

-3 When t h e f l a s h p h o t o l y s i s o f R h C l ( C O ) ( P P h ) 2 was c a r r i e d o u t as above b u t under d i h y d r o g e n (1.0 atm, 0.0028 M) (5) , t h e i n t e r m e d i a t e RhCl(PPh ) underwent f i r s t o r d e r r e a c t i o n ( k = 2.8 χ 1 0 s " ) t o g i v e a new t r a n s i e n t spectrum h a v i n g an even s m a l l e r absorbance t h a n t h a t o f t h e c a r b o n y l complex over t h e s p e c t r a l range 360-450 nm (Figure 4). T h i s new t r a n s i e n t decayed over a p e r i o d o f seconds t o give RhCl(CO)(PPh ) again. We i n t e r p r e t t h e s e o b s e r v a t i o n s i n terms of t h e r e a c t i o n o f R h C l ( P P h ) with H to give the dihydride ( E q u a t i o n 4) f o l l o w e d by r e a c t i o n o f t h e l a t t e r w i t h CO t o r e g e n e r a t e RhCl(CO)(PPh )2I f a second o r d e r r a t e law f o r E q u a t i o n 4 i s assumed, then t h e c a l c u l a t e d v a l u e o f k^ i s 1.0 χ 10^ M'^-s"!, v e r y much i n agreement w i t h H a l p e r n ' s e s t i m a t e o f k^ (> 7 χ 10^ M~ s~^) drawn from k i n e t i c s a n a l y s i s o f t h e W i l k i n s o n ' s c a t a l y s i s ( 4 ) . F l a s h p h o t o l y s i s under D 2 (1.0 atm) gave i d e n t i c a l s p e c t r a l changes and a c a l c u l a t e d k o f 0.7 χ 10^ M^^s" , i . e . , a k i n e t i c i s o t o p e e f f e c t k£/k^ o f about 1.4. 3

2

3

2

1

Q b s

3

2

3

2

2

3

1

1

3

k

RhCl(PPh ) 3

2

+ H

2

4 >

H RhCl(PPh ) 2

3

(4) 2



202


RhCl(PPh ) 3

·-

2

I

+ H

2

>

H RhCl ( P P h ) 2

3

IT

2

In CA - A ^) H

Abe.

A

C

/

^ •I

0

5

— . — ι1

.L -A

w

d

10

Time ( i n ms)

Figure 4. F l a s h p h o t o l y s i s o f R h C l ( C O ) ( P P h ) 2 i n 25° benzene under H2 (1.0 atm) showing decay o f t h e t r a n s i e n t R h C l ( P P h ) 2 by r e a c t i o n w i t h e x c e s s hydrogen to form a new i n t e r m e d i a t e s p e c i e s (presumably H 2 R h C l ( P P h ) 2 ) h a v i n g an absorbance (Ay) l e s s t h a n t h a t o f t h e s t a r t i n g m a t e r i a l A Q . The c u r v e r e p r e s e n t s t h e t e m p o r a l a b s o r b a n c e changes ( s c a l e t o t h e l e f t ) ; t h e l i n e r e p r e s e n t s a p s e u d o - f i r s t - o r d e r p l o t o f t h i s decay ( s c a l e t o t h e right) . 3

3

3


14.

Chemistry of Rhodium and Iridium Phosphine Complexes 203

WINK AND FORD

F l a s h p h o t o l y s i s o f R h C l ( C 0 ( P P h 3 ) 2 under e t h y l e n e ( 0 . 0 1 atm, 0 . 0 0 1 1 M) l e d t o immediate s p e c t r a l changes c o n s i s t e n t w i t h t h e f o r m a t i o n o f t h e e t h y l e n e complex(_3) R h C l ( H 2 C = C H 2 ) ( P P h ) η w i t h i n t h e duration of the f l a s h . T h i s o b s e r v a t i o n p r o v i d e s a lower l i m i t o f 2 χ 1 0 ^ M ~ l s ~ l f o r t h e second o r d e r r a t e c o n s t a n t f o r t h e r e a c t i o n o f RhCl(PPh )2 with ethylene. The back r e a c t i o n o f t h e e t h y l e n e adduct w i t h CO t o r e f o r m R h C l ( C O ) ( P P h ) 2 was a l s o r a t h e r r a p i d and o c c u r r e d w i t h i n a p e r i o d o f a few m i l l i s e c o n d s . 3

3

3

In summary, t h e f l a s h p h o t o l y s i s o f R h C l ( C O ) ( P P h ) 2 i n benzene leads p r i n c i p a l l y to formation of the c o o r d i n a t i v e l y unsaturated Wilkinson's c a t a l y s t intermediate R h C l ( P P h ) 2 (or i t s solvated analog). The r e a c t i o n s o f t h i s s p e c i e s a r e summarized i n Scheme I . In t h e absence o f o t h e r r e a c t a n t s , t h i s i n t e r m e d i a t e recombines w i t h CO o r d i m e r i z e s v i a v e r y r a p i d r e a c t i o n s , b u t i n t h e p r e s e n c e of added P P h , e t h y l e n e o r d i h y d r o g e n , a d d u c t s o f R h C l ( P P h ) a r e formed. Rate c o n s t a n t s f o r t h e second o r d e r r e a c t i o n s o f R h C l ( P P h ) 2 w i t h v a r i o u s s u b s t r a t e s a r e summarized i n T a b l e I . The r a t e s f o r a d d i t i o n o f t h e two e l e c t r o n d o n o r s CO, C 2 H 4 and P P h a r e s i g n i f i c a n t l y h i g h e r than f o r t h e o x i d a t i v e a d d i t i o n o f d i h y d r o g e n . In a l l c a s e s , however, t h e i n i t i a l a d d u c t s r e a c t e v e n t u a l l y w i t h t h e p h o t o l a b i l i z e d CO t o r e f o r m t h e more s t a b l e s t a r t i n g complex RhCl(CO) (PPh )2« These o b s e r v a t i o n s a r e c o n s i s t e n t w i t h t h e c o n t i n u o u s p h o t o l y s i s s t u d i e s w h i c h r e p o r t e d no n e t p h o t o c h e m i s t r y o f t h i s s p e c i e s i n t h e absence o f oxygen ( 1 2 ) . 3


3

3

3

2

3

3

3

Table I .

Second Order Rate C o n s t a n t s Substrates with RhCl(PPh ) 3

Substrate

2

> 2 χ 10

7

(4 ± 1) χ 1 0

7

(2.8 ± 0.4) χ 1 0

6

(9.8

± 0.5) χ 1 0

4

(7.0

± 0.5) χ 1 0

4

2

3

2

Do

Iridium

7

4

3

H

± 0.2) χ 1 0

(6.9

RhCl(PPh ) PPh

f o r the Reactions of Various i n 25° Benzene S o l u t i o n .

k ( i n M "^"s ^)

CO C H

2

Complexes

Vaska's complex t r a n s - I r C l ( C O ) ( P P h ) 9 has s e r v e d as an i m p o r t a n t model f o r m e c h a n i s t i c i n v e s t i g a t i o n o f c a t a l y t i c a l l y r e l e v a n t r e a c t i o n s such as t h e o x i d a t i v e a d d i t i o n and r e d u c t i v e e l i m i n a t i o n of s m a l l m o l e c u l e s ( 1 5 ) . The l a t t e r p r o c e s s e s have a l s o been t h e s u b j e c t o f some p h o t o c h e m i c a l i n v e s t i g a t i o n . F o r example, t h e r e d u c t i v e e l i m i n a t i o n o f H 2 d e p i c t e d i n E q u a t i o n 5, w h i c h i s a r e l a t i v e l y s l o w t h e r m a l l y a c t i v a t e d p r o c e s s ( k ^ = 3.8 χ 10"^ s ~ l i n 25° benzene s o l u t i o n ( 1 5 ) ) , has been shown t o o c c u r r e a d i l y when t h e d i h y d r i d e complex was s u b j e c t e d t o c o n t i n u o u s p h o t o l y s i s w i t h 366 nm light(16). However, Vaska's compound i t s e l f was r e p o r t e d t o be 3




204

RhCl(CO)(PPh ) 3

2

Scheme I . A summary o f t h e r e a c t i o n s o f RhCl(PPH3)2 as s t u d i e d by the k i n e t i c f l a s h p h o t o l y s i s o f RhCl(CO)(PPh3>2.


14.


WINK AND FORD

p h o t o i n e r t under c o n t i n u o u s p h o t o l y s i s ( _ 1 6 ) . I n t h e s e c o n t e x t s , we were i n t e r e s t e d i n e s t a b l i s h i n g whether V a s k a s compound would show the same type o f t r a n s i e n t b e h a v i o r upon f l a s h p h o t o l y s i s as d i d t h e rhodium a n a l o g (above) and whether t r a n s i e n t s c o u l d be d e t e c t e d i n the p h o t o r e a c t i o n s o f t h e r e l a t e d I r ( I I I ) o x i d a t i v e a d d u c t s . T

H IrCl(C0)(PPh ) -* trans-IrCl(CO)(PPh ) + H 2

3

2

3

2

(5)

2

Flash p h o t o l y s i s of t r a n s - I r C l ( C O ) ( P P h ) i n s t r i n g e n t l y deaerated benzene s o l u t i o n under an argon atmosphere ( λ ^ > 254 nm) r e s u l t e d i n the formation o f a t r a n s i e n t with strong absorption i n the s p e c t r a l r e g i o n 390-550 nm ( * - 430 nm) which decayed t o t h e i n i t i a l b a s e l i n e v i a c l e a n l y second o r d e r k i n e t i c s ( F i g u r e 5 ) . The r e t u r n t o t h e i n i t i a l spectrum i s c o n s i s t e n t w i t h t h e e a r l i e r r e p o r t (16) t h a t c o n t i n u o u s p h o t o l y s i s o f t r a n s - I r C l ( C O ) ( P P h 3 ) 2 gave no n e t photoreaction. When s i m i l a r f l a s h experiments were c a r r i e d o u t under v a r i o u s p r e s s u r e s o f CO, t h e t r a n s i e n t decay k i n e t i c s were f i r s t o r d e r w i t h t h e observed r a t e c o n s t a n t s k l i n e a r l y dependent on P . Thus, we c o n c l u d e t h a t t h e i n t e r m e d i a t e s p e c i e s o b s e r v e d i s t h e p r o duct o f CO p h o t o d i s s o c i a t i o n ( E q u a t i o n 6) and t h a t t h e decay p r o c e s s i s t h e r e c o m b i n a t i o n t o t h e s t a r t i n g complex ( E q u a t i o n 7 ) . The second o r d e r r a t e c o n s t a n t k-j = (2.7 ± 0.7) χ 10^ M ~ l s ~ l was d e t e r mined from t h e p l o t o f k v s [CO]. These r e s u l t s a r e v e r y s i m i l a r to t h e c h e m i s t r y i n d u c e d by t h e f l a s h p h o t o l y s i s o f t h e r h o d i u m ( I ) analogue above, a l t h o u g h ky i s about a f a c t o r o f f o u r f a s t e r f o r t h e Ir(I) transient. 3

2

Γ Γ


m a x

O D S

c o

o b s

hv trans-IrCl(CO)(PPh ) 3

k

IrCl(PPh ) 3

+ CO

2

>

2

IrCl(PPh ) 3

2

+ CO

(6)

7 >

trans-IrCl(CO)(PPh ) 3

(7)

2

F l a s h p h o t o l y s i s o f t h e analogous d i n i t r o g e n complex t r a n s - I r C l (N )(PPh ) (17) demonstrates t h a t f l a s h p h o t o l y s i s l e a d s i n b o t h c a s e s t o immediate appearance o f a t r a n s i e n t spectrum t h e same, w i t h i n e x p e r i m e n t a l u n c e r t a i n t y , as t h a t a t t r i b u t e d above t o IrCl(PPh ) . Continuous p h o t o l y s i s o f I r C l ( N ) ( P P h ) i n C^D, under o t h e r w i s e analogous c o n d i t i o n s r e s u l t s i n t h e d i s a p p e a r a n c e or t h e i n f r a r e d band a t t r i b u t e d t o t h e c o o r d i n a t e d N ( F i g u r e 6 ) , and t h e p r o d u c t s o l u t i o n d i s p l a y s a p r o t o n nmr resonance a t -22.5 ppm i n d i c a t i n g t h e f o r m a t i o n o f an i r i d i u m h y d r i d e ( 1 8 ) . Thus, p h o t o l a b i l i zation of N to give I r C l ( P P h ) i s i r r e v e r s i b l e ( E q u a t i o n 8 ) , and, i n t h e absence o f t h e o t h e r r e a c t a n t s , t h i s r e a c t i v e i n t e r m e d i a t e a p p a r e n t l y undergoes i n t e r n a l o r t h o m e t a l l a t i o n o f a t r i p h e n y l phos p h i n e t o g i v e an i r i d i u m ( I I I ) h y d r i d e p r o d u c t ( s ) . I n t h e f l a s h p h o t o l y s i s experiment, t h e l a t t e r p r o c e s s i s e v i d e n c e d by slow absorbance changes i n t h e 340 t o 550 nm range w i t h i s o s b e s t i c p o i n t s at 460 and 334 nm c o n s i s t e n t w i t h f o r m a t i o n o f I r ( I I I ) p r o d u c t s . 2

3

3

2

2

2

3

2

2

2

3

2

hv IrCl(N )(PPh ) 2

3

IrCl(PPh ) 3

2

2

a* I r C l ( P P h ) 3

^lr(III)

2

+ N

2

hydride


(8) (9)


206


irCl (C0)L

2

F i g u r e 5. Top: Absorbance changes r e s u l t i n g from t h e f l a s h p h o t o l y s i s o f I r C l ( C O ) ( P P h ) i n 25° benzene under A r . The m o n i t o r i n g w a v e l e n g t h was 420 nm. Bottom: A l i n e a r s e c o n d - o r d e r p l o t f o r t h e d a t a i n the above c u r v e . 3

2


WINK AND FORD



14.

F i g u r e 6. Top: IR absorbance changes r e s u l t i n g from t h e CW photolysis ( X = 405 nm) o r I r C l ( N ) (ΡΡΐΐβ^ i n 25 benzene under Ar. Bottom: E l e c t r o n i c s p e c t r a l changes r e s u l t i n g from a s i m i l a r p h o t o l y s i s o f I r C l ( N ) (PPI13) . The top c u r v e i n each c a s e r e p r e s e n t s t h e spectrum o f t h e s t a r t i n g m a t e r i a l . i r r

2

2

2


EXCITED STATES AND

208

REACTIVE INTERMEDIATES

In agreement w i t h the e a r l i e r r e p o r t ( 1 6 ) , we found t h a t H 2 l r C l ( C 0 ) ( P P h ) undergoes f a c i l e p h o t o e l i m i n a t i o n of H (Equation 5) when i r r a d i a t e d . The quantum y i e l d was 0.56 m o l e s / e i n s t e i n f o r c o n t i n u o u s p h o t o l y s i s a t 313 nm, a v a l u e c l o s e to t h a t r e p o r t e d f o r the s i m i l a r complex H I r C l ( P P h ) (0.55). However, a p a r t i c u l a r l y s t r i k i n g o b s e r v a t i o n was t h a t f l a s h p h o t o l y s i s of H I r C l ( C 0 ) ( P P h ) i n benzene under 1 atm. H (λ^ > 254 nm) r e s u l t e d i n t r a n s i e n t absorbance i n the s p e c t r a l r e g i o n 400-550 nm e x p e r i m e n t a l l y i n d i s t i n g u i s h a b l e from t h a t seen f o r the f l a s h p h o t o l y s e s of t r a n s IrCl(CO)(PPh ) and of t r a n s - I r C l ( N ) ( P P h ) . T h i s t r a n s i e n t decayed v i a second o r d e r k i n e t i c s t o a p r o d u c t h a v i n g the spectrum of V a s k a s compound. Over a p e r i o d of 10 m i n u t e s , the l a t t e r underwent s u b s e quent r e a c t i o n w i t h H t o r e f o r m the s t a r t i n g complex a c c o r d i n g t o E q u a t i o n 10 f o r which the r a t e c o n s t a n t 1.2 Μ" ^"" has p r e v i o u s l y been r e p o r t e d ( 1 5 ) . 3

2

2

2

3

3

2

2

3

3

2

Γ Γ

2

2

3

2

T

2


-

trans-IrCl(CO)(PPh ) 3

2

+ H

>

2

1

H IrCl(C0)(PPh ) 2

3

2

(10)

The f l a s h p h o t o l y s i s of D I r C l ( C 0 ) ( P P h ) under 1 atm D demonstrated no i s o t o p e e f f e c t on the r e l a x a t i o n of the f i r s t t r a n s i e n t to g i v e t r a n s - I r C l ( C O ) ( P P h ) . However, f l a s h p h o t o l y s i s of H I r C l ( C 0 ) ( P P h ) under H /C0 m i x t u r e s gave decay r a t e s l i n e a r l y dependent on P . P l o t s of k v v s [CO] as above gave the second o r d e r r a t e c o n s t a n t (2.6 ± 0.7) χ 10° M"~ls~"^ w i t h i n e x p e r i m e n t a l u n c e r t a i n t y o f the ky v a l u e r e p o r t e d above. These r e s u l t s i n d i c a t e t h a t H I r C l ( C O ) ( P P h ) f i r s t undergoes p h o t o d i s s o c i a t i o n o f CO ( E q u a t i o n 11) f o l l o w e d by e l i m i n a t i o n of H from the r e s u l t i n g p e n t a c o o r d i n a t e d i n t e r m e d i a t e ( E q u a t i o n 12) to g i v e the I r C l ( P P h ) t r a n s i e n t formed d i r e c t l y v i a f l a s h p h o t o l y s i s of t r a n s - I r C l ( C O ) ( P P h ) . hv H IrCl(C0)(PPh ) > H IrCl(PPh ) + CO (11) 2

3

3

2

3

2

2

2

2

2

C Q

Q

s

2

3

2

2

3

2

3

2

3

2

2

2

k-, H IrCl(PPh ) 2

3

3

2

7

IrCl(PPh )

2

3

+ H

2

(12)

2

T h i s v i e w c o n t r a s t s t o the p r o p o s a l t h a t the d i h y d r i d e p h o t o e l i m i n a t i o n o c c u r s by a s i n g l e c o n c e r t e d s t e p but i s c o n s i s t e n t w i t h t h e o r e t i c a l arguments(19) and e x p e r i m e n t a l o b s e r v a t i o n s ( 2 0 , 2 1 ) t h a t r e d u c t i v e e l i m i n a t i o n from complexes o f t e n o c c u r s much more r e a d i l y a f t e r l i g a n d d i s s o c i a t i o n from the o r i g i n a l hexacoordinate species to give a pentacoordinate intermediate. Given that f o r m a t i o n of I r C l ( P P h ) was complete w i t h i n the l i f e t i m e of the f l a s h (20 u s ) , a lower l i m i t f o r k ^ can be e s t i m a t e d as 5 χ 10^ s . Thus, we c o n c l u d e t h a t d i s s o c i a t i o n of CO a c c e l e r a t e s d i h y d r o g e n e l m i n a t i o n by n i n e or more o r d e r s of magnitude. Notably, t h i s r a t e a c c e l e r a t i o n o c c u r s d e p s i t e the d i s s o c i a t i o n of the π-acid CO w h i c h would be e x p e c t e d t o f a v o r the lower o x i d a t i o n s t a t e o f the m e t a l center. (*The f o l l o w i n g o b s e r v a t i o n argues a g a i n s t the p o s s i b i l i t y o f a s e q u e n t i a l two-photon p r o c e s s i n v o l v i n g i n i t i a l H photolabiliz a t i o n to generate I r C l ( C O ) ( P P h ) f o l l o w e d by s e c o n d a r y p h o t o l y s i s of t h i s p r o d u c t to g i v e " I r C l ( P P h ) " . The r e l a t i v e p u l s e i n t e n s i t y r e q u i r e d to g e n e r a t e the same c o n c e n t r a t i o n of the l a t t e r t r a n s i e n t was f i v e times l a r g e r when the i n i t i a l s u b s t r a t e was trans-IrCl(CO) PPh ) (under argon) than when H I r C l ( C O ) ( P P h ) (under H ) was the 3

2

- 1

2

2

3

2

3

3

2

2

2

3

2

2


14.


WINKANDFORD

i n i t i a l substrate. However, an a l t e r n a t i v e mechanism s h o u l d a l s o be c o n s i d e r e d , namely, t h e p o s s i b i l i t y t h a t b o t h CO and H l o s s o c c u r s i m u l t a n e o u s l y o r s e q u e n t i a l l y from t h e e x c i t e d s t a t e o f H 2 l r C l ( C 0 ) (PPh3)2A t p r e s e n t such a mechanism, a l t h o u g h u n p r e c e d e n t e d , c a n not be d i f f e r e n t i a t e d from t h e s t e p w i s e pathway p r o p o s e d i n E q u a t i o n s 11 and 12.)


2

Preliminary i n v e s t i g a t i o n s of the I r ( I I I ) species HlrCl(CO) (PPI12C6H4) ( P P h 3 ) 2 , t h e o r t h o m e t a l l a t e d isomer o f Vaska's compound(22), are c l o s e l y analogous. Continous p h o t o l y s i s i n t h i s case leads to the f o r m a t i o n o f I r C l ( C O ) (ΡΡΙΊ3) 2However, f l a s h p h o t o l y s i s l e a d s to a t r a n s i e n t spectrum q u a l i t a t i v e l y t h e same a s t h a t a t t r i b u t e d t o I r C l ( P P h 3 ) 2 and t h i s t r a n s i e n t decays by a second o r d e r pathway ([CO] dependent, i . e . E q u a t i o n 7) t o form t r a n s - I r C l ( C O ) ( P P t v j ) 2 a s t h e f i n a l product. Thus, a g a i n i t appears t h a t t h e s t a r t i n g complex has undergone CO p h o t o d i s s o c i a t i o n f o l l o w e d by H / a r y l e l i m i n a t i o n t o form I r C l ( P P h 3 ) 9 w i t h i n the 20 ys l i f e t i m e of the f l a s h . This leads to a 5 χ 1 0 s " l lower l i m i t f o r t h e r a t e constant of r e d u c t i v e e l i m i n a t i o n from t h e p e n t a c o o r d i n a t e i n t e r m e d i a t e , a v a l u e a t l e a s t e i g h t o r d e r s o f magnitude f a s t e r t h a n t h e r a t e o f about 3 χ 1 0 ~ s ~ l we have measured f o r t h e t h e r m a l r e a c t i o n o f H l r C l ( C O ) (PC^H^Ph) ( P P t v j ^ t o give trans-IrCl(CO)(PPh ) i n 70° benzene. 4

4

3

2

In summary, we have o b s e r v e d t h a t a common t r a n s i e n t i s produced i n t h e f l a s h p h o t o l y s i s o f f o u r s p e c i e s , t r a n s - I r C l (CO) (PPI13) 2 , trans-IrCl(N )(PPh )2, H I r C l ( C O ) ( P P h C H ) ( P P h ) and H I r C l ( C 0 ) ( P P h ) . These r e s u t l s s u g g e s t t h a t t h e p h o t o - i n d u c e d v e r s i o n o f E q u a t i o n 5 i s a s t e p w i s e mechanism i n v o l v i n g i n i t i a l CO d i s s o c i a t i o n as t h e primary photoreaction of H 2 l r C l ( C 0 ) (PPti3)2. The r e s u l t i n g p e n t a c o o r d i n a t e d I r ( I I I ) i n t e r m e d i a t e a p p e a r s t o be d r a m a t i c a l l y a c t i v a t e d toward H2 r e d u c t i v e e l i m i n a t i o n as p r e d i c t e d i n t h e o r e t i c a l t r e a t ments. These t r a n s f o r m a t i o n s a r e i l l u s t r a t e d i n Scheme I I . 3

2

Experimental

2

6

4

3

2

3

2

Section

The f l a s h p h o t o l y s i s a p p a r a t u s was t h a t d e s c r i b e d previously(23) m o d i f i e d by t h e u s e o f B i o m a t i o n 805 t r a n s i e n t d i g i t i z e r i n t e r f a c e d t o a H e w l e t t P a c k a r d 86 computer f o r d a t a c o l l e c t i o n , a n a l y s i s and plotting. Wavelength s e l e c t i o n was a c c o m p l i s h e d by u s e o f an aqueous NaNÛ3 s o l u t i o n a s a UV and IR f i l t e r . The benzene used i n t h e s e s t u d i e s was s c r u p u l o u s l y d e a e r a t e d by freeze/pump/thaw c y c l e s and d r i e d by d i s t i l l a t i o n from a Na/K amalgam. S o l u t i o n s were p r e p a r e d by vacuum m a n i f o l d t e c h n i q u e s . S t o p p e d - f l o w k i n e t i c s were c a r r i e d out u s i n g a Gibson-Durram D110 s p e c t r o p h o t o m e t e r e q u i p p e d f o r t h e h a n d l i n g o f s o l u t i o n s under d e a e r a t e d c o n d i t i o n s ( 2 4 ) . The d a t a s t a t i o n d e s c r i b e d above was used f o r c o l l e c t i o n , a n a l y s i s and p l o t t i n g of d i g i t a l data. Acknowledgments T h i s r e s e a r c h was s p o n s o r e d by 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 . The rhodium and i r i d i u m used i n t h e s e s t u d i e s was p r o v i d e d on l o a n by Johnson Matthey, I n c .


210



Scheme I I . A summary o f t h e p h o t o r e a c t i o n s intermediate ΐΓ01(ΡΡη ) . 3

leading

t o t h e common

2


14.

WINK A N D FORD

Chemistry of Rhodium and iridium Phosphine Complexes 211


Literature Cited 1. Pignolet, L.H., Ed. "Homogeneous Catalysis with Metal Phosphine Complexes"; Plenum Press: New York, N.Y., 1983. 2. Halpern, J. Acc. Chem. Res. 1970, 3, 386-392. 3. Osborn, J.A.; Jardine, F.H.; Young, J.F.; Wilkinson, G. J. Chem. Soc.(A) 1966, 1711-1732. 4. Halpern, J.; Wong, C.S. J.C.S. Chem. Commun. 1973, 629-630. 5. Tolman, C.A.; Meakin, P.Z.; Lindner, D.L.; Jesson, J.P. J. Amer. Chem. Soc. 1974, 96, 2762-2774. 6. Halpern, J.; Okamoto, T.; Zakhariev, A. J. Mol. Catal. 1976, 2, 65-69. 7. Tolman, C.A.; Faller, J.W. Chapt. 2 in ref. 1. 8. Halpern, J. Trans. Amer. Cryst. Assoc. 1978, 14, 59-70. 9. Jardine, F.H. Prog. Inorg. Chem. 1982, 28, 63-201. 10. James, B.R. "Homogeneous Hydrogenation"; Wiley-Interscience: New York, N.Y. 1973, pp. 204-250. 11. Geoffroy, G.L.; Wrighton, M.S. "Organometallic Photochemistry"; Academic Press, New York, N.Y. 1979. 12. Geoffroy, G.L.; Denton, D.A.; Keeney, M.E.; Bucks, R.R. Inorg. Chem. 1976, 15, 238202385. 13. Braker, W.; Mossman, A.L. "The Matheson Unabridged Gas Databook: A Compilation of Physical and Thermodynamic Properties of Gases"; Matheson Gas Products: East Rutherford, NJ, 1974, Vol. I, p. 11. 14. Geoffroy, G.L.; Keeney, M.E. Inorg. Chem. 1977, 16, 205-207. 15. Vaska, L. Acc. Chem. Res. 1968, 1, 335. 16. Geoffroy, G.L.; Hammond, G.S.; Gray, H.B. J. Amer. Chem. Soc. 1975, 97 3933-3936. 17. Collman, J.P.; Kubota, M.; Vastine, F.D.; Sun, J.Y.; Kang, J.W. J. Amer. Chem. Soc. 1968, 90, 5430-5437. 18. Bennett, M.A.; Milner, D.L. J. Amer. Chem. Soc. 1969. 91, 69836994. 19. Tatsumi, K.; Hoffman, R.; Yamamoto, Α.; Stille, J.K. Bull. Chem. Soc. Japan 1981, 51, 1857-1867. 20. Basato, M.; Morandini, F.; Longato, B.; Bresadola, S. Inorg. Chem. 1984, 23, 649-653. 21. Milstein, D.; Calabrese, J.C. J. Amer. Chem. Soc. 1982, 104, 3773-3774; 5227-5228. 22. Valentine, J. J.C.S. Chem. Commun. 1973, 857-858. This paper reports that the orthometallated product is a mixture of two geometric isomers. 23. Durante, V.A.; Ford, P.C. Inorg. Chem. 1979, 18, 588-593. 24. Trautman, R.J.; Gross, D.C.; Ford, P.C. J. Amer. Chem. Soc. 1985, 107, 2355. RECEIVED November 8, 1985


Excited States and Reactive Intermediates - American Chemical Society

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