Excited States and Reactive Intermediates - American Chemical Society

13

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 3, 2016 | http://pubs.acs.org Publication Date: April 30, 1986 | doi: 10.1021/bk-1986-0307.ch013

Photochemical Production of Reactive Organometallics for Synthesis and Catalysis William C. Trogler Department of Chemistry, D-006, University of California at San Diego, La Jolla, CA 92093 Photochemical reactions of unsaturated metallacycles and of platinum and palladium complexes that contain chelating oxalate yield intermediates with two reac tive sites that are either ligand or metal centered. Unsaturated metallacycles exhibit low lying π* excited states that can also function as photoreceptors to pro mote ligand dissociation elsewhere in the molecule. A strong coupling model for excited state reactivity of metal carbonyls is presented. Reactions of photogenerated PtL2 and PdL2 fragments (L = trialkylphosphine) are summarized along with methods of preparing silica attached photocatalysts. S y n t h e t i c a l l y u s e f u l photochemical r e a c t i o n s o f o r g a n o t r a n s i t i o n m e t a l complexes c a n be c l a s s i f i e d a c c o r d i n g t o Scheme I . Scheme I 1)

Ligand

Photodissociation

Cr(C0)

2)

h

V

>»

6

Homolysis o f Metal Ligand

CoMe([l4]aneN )0H2

+

h

4

3)

( 1_) :

V

Cr(C0)

5

+

CO

Bond ( 2 ) :

>

Co([l4]aneN )

2 +

4

+ CH^ +

P h o t o c h e m i c a l H o m o l y s i s o f a M e t a l - M e t a l Bond ( 3 ) : Mn (C0) 2

2Mn(C0)

1 Q

5

or Pd (CNCH )^ 2

3

+

h

V

>

2 Pd(CNCH )^ 3

0097-6156/ 86/ 0307-0177506.00/ 0 © 1986 American Chemical Society

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

178 4)

EXCITED STATES AND Photooxidation

(4):

Fe(n-C H ) 5


5)

REACTIVE INTERMEDIATES

5

2

Photochemical Reductive

g

T

>

[Fe(n-C H ) ] Cl" 5

5

(5):

E l i m i n a t i o n of

hv

^

H

2

2

WW

+

è 16e

18e

These r e a c t i o n s g e n e r a t e a s i n g l e r e a c t i v e s i t e and o c c u r v i a 15e, 16e, 17e, o r 18e i n t e r m e d i a t e s ( 6 - 8 ) . One o f our g o a l s was t o ex amine t h e p h o t o c h e m i c a l b e h a v i o r o f complexes t h a t c o n t a i n an un s a t u r a t e d c h e l a t e chromophore. P h o t o f r a g m e n t a t i o n o f t h e s e systems might l e a d t o two r e a c t i v e c e n t e r s , e i t h e r on t h e l i g a n d o r m e t a l . T h i s c o u l d produce i n t e r m e d i a t e s t h a t e x h i b i t n o v e l c h e m i s t r y . Metallacyclopentadiene, Metalladiazabutadiene, azadiene Photochemistry

and M e t a l l a t e t r a -

C o n s i d e r t h e s e r i e s o f m e t a l l a c y c l e s A-C. These u n s a t u r a t e d r i n g systems were e x p e c t e d t o show low l y i n g e l e c t r o n i c t r a n s i t i o n s

\

/

c—c

1

\

/

Ν —

//

metallacyclo pentadiene

metalladiaza butadiene

Ν

W

metallatetraazabutadiene

because o f t h e u n s a t u r a t e d m e t a l - l i g a n d π system. The photochemis t r y o f CpCo[C4Ph4][PPh3] ( 9 ) , where Cp = n-C^H^ and Ph = C ^ in benzene s o l v e n t i s summarized i n E q u a t i o n 1. I n t h e absence o f 02 phosphine d i s s o c i a t i o n was shown t o y i e l d a 16e i n t e r m e d i a t e t h a t 9

CpCo[C Ph ](PPh ) 4

hv

4

3

CpCo(n-C Ph ) 4

4

CpCo[n -OC(Ph)C(Ph)C(Ph)C(Ph)0]


(1)

13.

179

Photochemical Production of Reactive Organometallics

TROGLER

r e a r r a n g e d t o t h e η - t e t r a p h e n y l ( c y c l o b u t a d i e n e ) complex. In the presence o f O 2 , s t e r e o s p e c i f i c o x i d a t i o n o f t h e t e t r a p h e n y l metall a c y c l e occurred t o y i e l d Z-dibenzoylstilbene. Single crystal X-ray d i f f r a c t i o n ( 9 ) showed t h a t t h i s l i g a n d bound t o CO i n an τΑ-eneone f a s h i o n . S e v e r a l t e t r a a z a b u t a d i e n e complexes t h a t c o n t a i n e d t h e CpCo fragment were s y n t h e s i z e d (10,11), E q u a t i o n 2. A l l t h e d e r i v a t i v e s


CpCo(C0)

2

+

2N R

R = CH =Me, C H 3

6

y

>

CpCo[N(R)NNN(R)]

C ^ , 2,6-Me^H^

(2)

2,4-F^^

were i n t e n s e l y c o l o r e d and c a l c u l a t i o n s (SCF-Xa-DV) (_12) o f t h e model complex CpCo[N(H)NNN(H)], F i g u r e 1, showed t h e p r e s e n c e o f a l o w - l y i n g 30a m e t a l l a c y c l e π* o r b i t a l . S t r o n g π back b o n d i n g t o t h e t e t r a a z a b u t a d i e n e l i g a n d was e v i d e n c e d by s h o r t Co-N bond l e n g t h s and a s i n g l e s h o r t N-N bond l e n g t h i n t h e s t r u c t u r e o f CpCo[N(C6F )NNN(C5F5)], F i g u r e 2. A f u r t h e r i n d i c a t i o n o f t h e s t r o n g π a c c e p t o r c h a r a c t e r o f t h e N 4 R 2 l i g a n d was t h e f o r m a t i o n (13) o f s t a b l e 19e a n i o n s on e l e c t r o c h e m i c a l o r c h e m i c a l (Na/Hg) r e duction. That a d e l o c a l i z e d m e t a l l a c y c l e π o r b i t a l was t h e a c c e p t o r o r b i t a l was s u g g e s t e d by t h e l a r g e v a r i a t i o n i n r e d u c t i o n p o t e n t i a l s on c h a n g i n g t h e s u b s t i t u e n t a s w e l l a s t h e Co h y p e r f i n e s p l i t t i n g i n t h e EPR s p e c t r a ( T a b l e I , ^60% c o b a l t c h a r a c t e r ) . These r e s u l t s 1

5

Table

I.

R e d u c t i o n P o t e n t i a l s v s . NHE i n CH^CN and EPR S p e c t r a l Data i n THF S o l u t i o n f o r (η-C H )CO(1,4-R N,) Complexes. Q

R CH

E°',V

3

2,6-(CH ) C H 3

C

6

3

A

iso(Co),G

iso 2.055

-1.31

2.061

56.3

-1.01

2.078

50.0

-0.97

2.070

51.6

-O.7O

2.066

51.7

57.9

H

6 5 2,4-F C H 2

C

2

S

-1.53

6

3

F

6 5

c o n t r a s t e d w i t h t h o s e f o r complexes t h a t c o n t a i n r i n g System A where l i t t l e π back-bonding t o t h e l i g a n d i s observed (14). I r r a d i a t i o n o f t h e n e u t r a l R = Me d e r i v a t i v e l e d t o slow decom p o s i t i o n ; however, t h e a r y l d e r i v a t i v e s e x t r u d e d N 2 on p h o t o l y s i s (Φ = 10~3-10~ , χ = nm) t o form a s e r i e s o f benzoquinone d i i m i n e complexes (10^11_) i n y i e l d s o f 65-90$, E q u a t i o n s 3-5· Because t h i s r e a c t i o n had no p r e c e d e n t , and because C-F and C-C bond c l e a v a g e was unknown i n t h e o r g a n i c p h o t o c h e m i s t r y o f n i t r e n e s , t h e s t r u c t u r e o f the p e r f l u o r o p h e n y l p h o t o p r o d u c t was v e r i f i e d (V5) by c r y s t a l l o graphy. M e t r i c a l p a r a m e t e r s o f t h e s t r u c t u r e a r e c o n s i s t e n t w i t h s t r o n g Co-N π b o n d i n g i n t h e s e p r o d u c t m e t a l l a c y c l e s . Benzoquinone d i i m i n e s do n o t appear t o be a s good π a c c e p t o r s a s t e t r a a z a b u t a d i e n e s j u d g i n g by t h e i r more n e g a t i v e (13) (by 0.3 t o 0.6 V) r e d u c t i o n p o t e n t i a l s . The mechanism o f f r a g m e n t a t i o n - r e a r r a n g e m e n t t h a t 4



180

EXCITED STATES AND REACTIVE INTERMEDIATES

F i g u r e 1. O r b i t a l energy diagram from SCC-Xa-DV c a l c u l a t i o n s o f t h e CpCo and H-N=N-N=N-H fragments a s w e l l a s t h e CpCo(l,4-H N4) molecule. Reproduced from Ref. 12. C o p y r i g h t 1982, American Chemical S o c i e t y . 2

F i g u r e 2. ORTEP (50% e l l i p s o i d s ) o f CpCo[l ^ - ^ F ^ N / J w i t h s e l e c t e d bond d i s t a n c e s and a n g l e s . Reproduced from R e f . 12. C o p y r i g h t 1982, American C h e m i c a l S o c i e t y .




TROGLER


182



we f a v o r i s shown i n E q u a t i o n 6.

The f i n a l s t e p o f t h e mechanism,

a t t a c k a t an o r t h o r i n g p o s i t i o n , may o c c u r by a r a d i c a l d i s p l a c e ment mechanism ( 1 6 ) * f o r t h e 2,6-Me C5H d e r i v a t i v e t h e e l i m i n a t e d m e t h y l group l e d t o f o r m a t i o n o f methane ( 1 1 ) . 2

3

P h o t o d i s s o c i a t i o n o f CO From T r i c a r b o n y l i r o n D i a z a b u t a d i e n e s and Tetraazabutadienes The i s o l o b a l b e h a v i o r (17) o f t h e CpCo and F e ( C 0 ) fragments i s known. Both F e ( C 0 ) [ l , 4 - R N ] and F e ( C 0 ) [ N ( R ) C ( R ) C ( R ) N ( R ) ] com p l e x e s can be p r e p a r e d and b o t h a r e p h o t o a c t i v e . M o l e c u l a r o r b i t a l c a l c u l a t i o n s (_18) show t h a t s t r o n g back-bonding o c c u r s from t h e F e ( C O ) ^ fragment t o t h e t e t r a a z a b u t a d i e n e l i g a n d π system j u s t as f o r t h e CpCo d e r i v a t i v e . The s i m i l a r i t y between t h e average CO s t r e t c h i n g f r e q u e n c y i n F e ( C 0 ) [ l , 4 - M e N 4 ] and F e i C O ) ^ s u g g e s t s (18) t h a t t h e π a c c e p t o r a b i l i t y o f a t e t r a a z a b u t a d i e n e c h e l a t e compares w i t h t h a t o f two C 0 s . D i a z a b u t a d i e n e complexes, whose c a r b o n y l IR s t r e t c h i n g f r e q u e n c i e s l i e 35-40 cm"'' t o lower energy ( 1 9 ) , a r e weaker π a c c e p t o r s . T h e r e f o r e , t h e r e l a t i v e back-bonding a b i l i t y o f t h e m e t a l l a c y c l e s , C > Β > A, p a r a l l e l s t h e e l e c t r o n e g a t i v i t y o f t h e r i n g atoms. T e t r a a z a b u t a d i e n e (18) and d i a z a b u t a d i e n e (20) complexes c o n t a i n i n g t h e F e ( C O ) ^ moiety e x h i b i t i n t e n s e v i s i b l e a b s o r p t i o n s a t t r i buted t o t r a n s i t i o n s from Fe d o r b i t a l s t o a l o w - l y i n g m e t a l l a c y c l e π* o r b i t a l . A l t h o u g h t h e e x c i t e d s t a t e does n o t d i r e c t l y i n v o l v e Fe-CO bonding o r b i t a l s , e f f i c i e n t CO s u b s t i t u t i o n (21) o c c u r s i n t h e 3

!

3

2

4

!

3

3

2

!


13.

183


TROGLER

p r e s e n c e o f n e u t r a l l i g a n d s , E q u a t i o n s 7, 8, and 9· The a b i l i t y t o generate c o o r d i n a t e l y unsaturated i r o n c e n t e r s with v i s i b l e l i g h t i n

Fe(CO) [ l , 4 - M e N ] 2

+

4

L

- ^ - ^

Fe(C0) L[l,4-Me^]

L = NC H , P ( M e ) , PPl^, P i c - C g H ^ ) ^ 5

5

3

P(0Ph)

3

Fe(C0) [N(Ph)C(Me)C(Me)N(Ph)]

^


3

Fe(C0) (PPh,)[N(Ph)C(Me)C(Me)N(Ph] έ 3 o

P(0Me)

3

+

CO

+

2

3 >

CO

C ^

>

+

(8)

CO

Fe(C0)[P(0Me) ] [l,4-Me N ] 3

+

Fe[P(OMe )]

2

2

(7)

(9)

4

[l,4-Me N ]

3

2

4

t h e p r e s e n c e o f o t h e r UV p h o t o s e n s i t i v e complexes ( e . g . Fe(CO),_) p e r m i t s c o n d e n s a t i o n r e a c t i o n s (21) such as E q u a t i o n 10.

0

II r / \ ^Ν^Γ j ^ ^ χ ^ \

CH v

Fe(C0Ul,4-(CH ),NJ 3 3 2 4 +

Fe(C0)

i

i

b

e

. ^ ^ > irradiation

J

3

CH 3

(0C) Fe

Fe(C0) [P(CH ) ] 3

3

3

2

hv P(CH ) 3

(11)

3

Fe(C0) [P(CH ) ][N(t-C H )C(CH )C(CH )N(t-C H )] 2

3

3

4

9

3

3

4

9

S t e r i c crowding i n t h e t h e r m a l S t r a n s i t i o n s t a t e f o r E q u a t i o n 11 f a v o r s l o s s o f t h e b u l k y d i a z a b u t a d i e n e l i g a n d , w h i l e under p h o t o chemical c o n d i t i o n s simple s u b s t i t u t i o n occurs. N 2


184

EXCITED STATES AND


R e c e n t l y Kokkes, S t u f k e n s , and Oskam (23) have q u e s t i o n e d whe t h e r CO d i s s o c i a t i o n o c c u r s i n t h e m e t a l l a c y c l e systems we s t u d i e d . T h e i r e v i d e n c e a g a i n s t d i s s o c i a t i o n was t h a t s t e r i c a l l y h i n d e r e d de r i v a t i v e s t h e y examined, Fe(CO)-3[NRCHCHNR], where R = 2 , 6 - i - ( C 3 H ) 6 3 > £ - 6 1 1 > 4-Me(C H4), t-Bu = t-C^Hg, and C H [ C H ( C H 3 ) ] , d i d n o t photodecompose i n s o l u t i o n " ( i n t h e absence o f l i g a n d s ) . They f a i l e d t o n o t i c e our o b s e r v a t i o n (21) ( t h e f i r s t s e n t e n c e under t h e h e a d i n g Photochemical Reactions) t h a t " v i s i b l e l i g h t p h o t o l y s i s of F e ( C O ) y [1,4-Me N4] ( i n hexanes, c y c l o h e x a n e , benzene, C H C 1 , THF, o r CH3CN) r e s u l t s i n t h e l o s s o f CO t o y i e l d an u n s t a b l e s p e c i e s " . Even i f t h e i r statement were t r u e f o r F e ( C 0 ) [ l ^ - ( C H ^ ) ^ ] i t i s d o u b t f u l whether a p h o t o d e c o m p o s i t i o n c r i t e r i o n f o r p h o t o d i s s o c i a t i o n i s meaningful. F o r example, C r ( C O ) ^ does n o t decompose e f f i c i e n t l y when i r r a d i a t e d i n pure h y d r o c a r b o n s o l v e n t s ( i n t h e absence o f l i gands) because o f r a p i d r e v e r s e b i n d i n g (24) o f d i s s o c i a t e d CO. I r o n p e n t a c a r b o n y l e x h i b i t s e f f i c i e n t p h o t o d e c o m p o s i t i o n ( i n t h e ab sence o f l i g a n d s ) because t h e b i m o l e c u l a r r e a c t i o n between F e ( C O ) ^ and p h o t o g e n e r a t e d F e ( C 0 ) 4 y i e l d s i n s o l u b l e Fe (C0)o,. Sterically unhindered Fe(C0) [l,4-Me N4] mimics t h e b e h a v i o r o f F e ( C O ) ^ i n f o r ming a c l u s t e r on i r r a d i a t i o n i n t h e absence o f l i g a n d s . Furthermore s e l e c t i v e p h o t o d i s s o c i a t i o n o f CO from t h e t e t r a a z a b u t a d i e n e complex produces a c o o r d i n a t i v e l y u n s a t u r a t e d s p e c i e s t h a t r e a c t s ( l i k e p h o t o g e n e r a t e d F e ( C 0 ) 4 ) w i t h Fe(CO)^ t o form a dimer, E q u a t i o n 10. 7

C

H

c

H

2

6


2

2

2

2

3

2

2

3

T h e r e f o r e , we a t t r i b u t e t h e p h o t o s t a b i l i t y o f t h e complexes s t u d i e d by Kokkes e t a l . , t o t h e r e v e r s i b l e p r o c e s s o f E q u a t i o n 12.

Fe

(C0) (DAB) 3

DAB

^

Fe(C0) (DAB) 2

+

CO

(12)

= diazabutadiene chelate

The F e ( C 0 ) ( D A B ) s p e c i e s s h o u l d r e s i s t b i n u c l e a r d e c o m p o s i t i o n p a t h ways because o f s t e r i c h i n d r a n c e from t h e b u l k y s u b s t i t u e n t s on t h e DAB l i g a n d . I n a d d i t i o n c o o r d i n a t e l y u n s a t u r a t e d s p e c i e s may be f u r t h e r s t a b i l i z e d by weak c o o r d i n a t i o n t o benzene s o l v e n t employed i n the photochemical s t u d i e s . The a l t e r n a t i v e mechanism t o CO d i s s o c i a t i o n , proposed by S t u f k e n s (23) f o r t h e DAB complexes, i s n o t c o n s i s t e n t w i t h t h e d i f f e r e n c e between t h e r m a l and p h o t o c h e m i c a l r e a c t i o n p r o d u c t s , E q u a t i o n 11. I n s o l u t i o n Kokkes e t a l . propose t h a t one end o f t h e DAB c h e l a t e d i s s o c i a t e s on p h o t o l y s i s . I f t h i s were t h e case i t would be d i f f i c u l t t o u n d e r s t a n d why t h e p h o t o c h e m i c a l r e a c t i o n (where t h e DAB l i g a n d i s h a l f a t t a c h e d ) l e a d s o n l y t o CO d i s p l a c e ment, w h i l e t h e a s s o c i a t i v e t h e r m a l r e a c t i o n l e a d s o n l y t o DAB d i s placement. C o n s i d e r t h e mechanism, E q u a t i o n 13, e s t a b l i s h e d (19) f o r t h e r m a l l o s s o f DAB. The key t o DAB l o s s i s f o r m a t i o n o f t h e monod e n t a t e s p e c i e s D o f E q u a t i o n 13· This intermediate i s i d e n t i c a l to t h a t proposed by Kokkes e t a l . (23) f o r p h o t o c h e m i c a l CO r e p l a c e m e n t . A c c o r d i n g t o t h e i r mechanism, E q u a t i o n 14, the same s p e c i e s D, forms i n a two s t e p p r o c e s s and would t h e r e f o r e be t h e r m a l l y e q u i l i b r a t e d . Thus t h e a l t e r n a t i v e mechanism i s n o t c o n s i s t e n t w i t h t h e r m a l chem i s t r y o f t h e s e systems. 2




13. TROGLER

-CH 0=C—Fe

hv

185

H r

0=C—Fe. sCH I R

R

(14)

L»

R I / ^CH N

^C

L'

I

0r=C~Fe 0

U

I R

0*»

J

„

C a r r y i n g t h e a n a l o g y between t h e p h o t o c h e m i s t r y o f F e ( C O ) ^ and Fe(C0)3[l,4-Me2N4] one s t e p f u r t h e r we n o t e t h a t b o t h compounds (25,26) behave a s p h o t o a s s i s t e d o l e f i n h y d r o s i l a t i o n and i s o m e r i z a tion catalysts. One d i s t i n c t i o n between t h e two c a t a l y s t systems i s t h e l a t t e r (26) o p e r a t e s e f f e c t i v e l y w i t h l o n g wavelength r a d i a t i o n , Table I I . H y d r o s i l a t i o n a c t i v i t y r e q u i r e s continuous p h o t o l y s i s ;


186

EXCITED STATES AND


o l e f i n i s o m e r i z a t i o n a c t i v i t y remains d u r i n g dark r e a c t i o n s a f t e r c a t a l y s t g e n e r a t i o n . F u r t h e r study o f t h e s e c a t a l y t i c r e a c t i o n s i s needed. Table I I .

P h o t o c a t a l y t i c R e a c t i o n s o f Fe(CO) [1 ^ ( C H ^ ) ^ ] O l e f i n s and T r i a l k y l s i l a n e s

with


a

O l e f i n (M) Ethylene (0.12)

S i l a n e (M) HSiEt (0.42)

Fe, M 0.01

3

Irrad. i (min) Conv. 80 65

Products (%) S i E t 4 (98) E t S i C H = C H (2) 3

Ethylene (0.12)

HSiMe (0.36)

0.01

1-Pentene (2)

HSiMe

0.005

3

80

75

2

EtSiife

(90) J

3

90

b

>

95

(2)

Pentene i s o m e r s and Pentane (~85) P e n t y l s i l a n e and Pentenylsilanes (-15)

cis-2-Pentene

HSiMe

0.02

3

58°

>

(0.8)

(8)

95

Pentene i s o m e r s (no h y d r o s i l a t i o n products)

a R e a c t i o n s a t 25°C i n benzene ( o r n e a t ) u s i n g a t o t a l s o l u t i o n v o l ume o f 0.3 mL. The r e a c t i o n s were monitored by p r o t o n NMR and f o r t h e f i r s t t h r e e e n t r i e s , t h e p r o d u c t s were a n a l y z e d by GC-mass spectrometry. A 200W mercury-xenon a r c lamp was used f o r t h e i r r a d i a t i o n s t o g e t h e r w i t h C o r n i n g 3-74 (λ > 400 nm, f i r s t two en t r i e s ) o r 0-52 (λ > 340 nm, l a s t e n t r y ) f i l t e r s . No t h e r m a l r e a c t i o n s were o b s e r v e d p r i o r t o p h o t o l y s i s . b The l a s t 40$ o f t h e r e a c t i o n took p l a c e d u r i n g 10 h i n t h e C o n t i n u e d p h o t o l y s i s f o r 265 min gave no change i n t h e NMR

dark. spectrum.

c P a r t o f t h e r e a c t i o n took p l a c e , a f t e r i r r a d i a t i o n , d u r i n g 20 h i n the dark. d Mostly trans-2-pentene

and < 5%

1-pentene.

S t r o n g C o u p l i n g Model F o r O r g a n o m e t a l l i c

Photoreactions

We n o t e d (21_) t h a t t h e quantum y i e l d f o r p h o t o s u b s t i t u t i o n o f CO i n Fe(C0) [l,4-Me N4], F e ( C 0 ) ( P P h ) [ 1 , 4 - M e N 4 J , and F e ( C 0 ) [ P h N C ( M e ) C (Me)NPh] ( e . g . F i g u r e 3) i n c r e a s e d i n an e x p o n e n t i a l f a s h i o n w i t h i n c r e a s i n g e x c i t a t i o n energy. There was no c o r r e l a t i o n w i t h a b s o r p t i o n s p e c t r a l f e a t u r e s . The h i g h quantum e f f i c i e n c y f o r CO s u b s t i t u ât l o n g wavelengths was unexpected because Χα c a l c u l a t i o n s f o r i r o n t r i c a r b o n y l t e t r a a z a b u t a d i e n e complexes (18) and M0 c a l c u l a t i o n s f o r d i a z a b u t a d i e n e analogues (20) s u g g e s t t h a t t h e l o w e s t e x c i t e d s t a t e s do n o t a l t e r metal-C0 b o n d i n g . Resonance Raman s p e c t r a o f t h e d i a z a b u t a d i e n e complexes (27) s u p p o r t t h i s c o n c l u s i o n . To r a t i o n a l i z e t h e o b s e r v a t i o n s we s u g g e s t e d (21) a s t r o n g c o u p l i n g model f o r ex cited state r e a c t i v i t y . 3

2

2

3

2

3


13.


TROGLER

Most d i s c u s s i o n s (28-32) o f i n o r g a n i c p h o t o c h e m i c a l r e a c t i o n s have f o c u s e d on t h e s p e c i f i c n a t u r e o f e x c i t e d s t a t e s and c o r r e l a tions with photoreactivity. T h i s a s s u m e s t h e weak c o u p l i n g m o d e l (33) f o r e x c i t e d s t a t e r e a c t i v i t y . T h i s a p p r o a c h w i l l be s u c c e s s f u l when t h e e x c i t e d s t a t e t h a t p r e c e d e s t h e p h o t o r e a c t i o n h a s a l o n g l i f e t i m e o r i s l o c a l i z e d ( e . g . , M L C T , a + o* ...). F r e q u e n t l y one f i n d s f l a t Φ vs λ p r o f i l e s and r e a c t i v i t y from the l o w e s t e x c i t e d s t a t e i n t h e weak c o u p l i n g l i m i t . There i s another l i m i t t h a t s h o u l d be c o n s i d e r e d . I f the e x c i t e d state i s short l i v e d , not w e l l l o c a l i z e d , and i f p h o t o c h e m i s t r y competes w i t h v i b r a t i o n a l d e a c t i v a t i o n of an e x c i t e d s t a t e then a s t r o n g c o u p l i n g (33) ( i . e . , s t r o n g c o u p l i n g between t h e i n i t i a l l y p r e p a r e d v i b r o n i c s t a t e and t h e d i s s o c i a t i o n c o n t i n u u m ) m o d e l may b e m o r e a p p r o p r i a t e . We i n t r o d u c e d t h e p r e m i s e t h a t a c o n s t a n t f r a c t i o n o f t h e e x c i t a t i o n e n e r g y i s a v a i l a b l e f o r M-CO d i s s o c i a t i v e p r o c e s s e s . A quasis t a t i s t i c a l ( 3 4 ) p a r t i t i o n i n g o f e x c i t a t i o n e n e r g y w o u l d be f a v o r e d by 1 ) d e n s e m a n i f o l d s o f v i b r o n i c l e v e l s , 2 ) l a c k o f s y m m e t r y s e l e c t i o n r u l e s on n o n r a d i a t i v e decay pathways, and 3) d e l o c a l i z e d e x c i t e d e l e c t r o n i c s t a t e s t h a t do n o t c o u p l e s t r o n g l y w i t h a n y s i n g l e v i b r a t i o n m o d e ; we q u a l i f y t h e l a s t c o n d i t i o n b y n o t i n g t h a t e v e n l o c a l i z e d e x c i t a t i o n s can l e a d (35) t o " s t a t i s t i c a l " b e h a v i o r . The w o r d " s t a t i s t i c a l " i s not used i n a s t r i c t thermal sense, because p a r t i t i o n i n g o f e x c i t a t i o n energy depends on t h e s p e c i f i c s o f i n t r a m o l e c u l a r v i b r o n i c coupling of the i n i t i a l l y prepared s t a t e . Experimental manifestations of strong coupling that areexpected i n c l u d e the f o l l o w i n g : 1) quantum y i e l d s f o r p h o t o d i s s o c i a t i v e p a t h ways t h a t depend on t h e amount b y w h i c h t h e e x c i t a t i o n e n e r g y e x c e e d s the thermodynamic t h r e s h o l d f o r bond b r e a k i n g ; 2) m u l t i p l e r e a c t i o n p a t h w a y s t h a t become a v a i l a b l e a t h i g h e r e x c i t a t i o n e n e r g i e s ; 3) s t r u c t u r e s e n s i t i v i t y t o r e a c t i o n quantum y i e l d s b e c a u s e e n e r g y f l o w r e l i e s o n t h e v i b r a t i o n a l modes t h a t i n i t i a l l y r e c e i v e t h e e n e r g y a n d how t h e y c o u p l e t o o t h e r m o d e s ; 4 ) b o n d i n g c h a r a c t e r o f t h e e x c i t e d s t a t e becomes i r r e l e v a n t . P h o t o r e a c t i o n s o f t h e m e t a l l a c y c l e s d i s c u s s e d ( 2 1 ) show l i n e a r p l o t s o f 1 η Φ ς v s e x c i t a t i o n e n e r g y b e f o r e l i m i t i n g quantum y i e l d s are reached. T h e r e was a c o r r e l a t i o n b e t w e e n t h e d o n o r a t o m s e t about Fe and quantum y i e l d s . Thus F e ( C 0 ) ( P P h 3 ) [ 1 , 4 - M e N 4 ] and F e ( C 0 ) 3 [ l , 4 - M e N 4 ] have s i m i l a r a b s o r p t i o n s p e c t r a , but q u i t e d i f f e r e n t q u a n t u m y i e l d s f o r CO s u b s t i t u t i o n . Absorption spectra of F e ( C 0 ) [ l , 4 - M e N 4 ] and Fe(C0)3[PhNC(Me)C(Me)NPh] are d i f f e r e n t ; how e v e r , b o t h c o m p o u n d s p o s s e s s t h e same d o n o r a t o m s e t a n d e x h i b i t s i m i l a r q u a n t u m y i e l d s f o r CO l o s s . I t i s a l s o noteworthy t h a t i s o e l e c t r o n i c C p C o [ l , 4 - R N 4 ] c o m p l e x e s t h a t do n o t c o n t a i n a n e a s i l y d i s s o c i a b l e group photofragment by N l o s s ( 1 0 , 1 1 ) . T h e r e was a c o r r e l a t i o n b e t w e e n t h e mode o f p h o t o c h e m i c a l d e c o m p o s i t i o n o f t h e F e ( C 0 ) 3 a n d CpCo t e t r a a z a b u t a d i e n e c o m p l e x e s a n d t h e l o w e s t e n e r g y f r a g m e n t i n t h e i r e l e c t r o n i m p a c t mass s p e c t r a (11 ) . For these rea s o n s we f a v o r a s t r o n g c o u p l i n g d e s c r i p t i o n o f t h e p h o t o r e a c t i o n s o f t h e s e compounds where l i g h t e x c i t a t i o n i s r a p i d l y c o n v e r t e d i n t o v i b r a t i o n a l energy t h a t t h e n r e s u l t s i n bond b r e a k i n g as governed by e n e r g e t i c and s t a t i s t i c a l c o n s i d e r a t i o n s . I t s h o u l d be n o t e d t h a t t h e r e are o t h e r e x p l a n a t i o n s (36) f o r wavelength dependencies o f quantum y i e l d s . f


187

0

2

2

2

3

2

2

2


EXCITED STATES AND

188


Recent s t u d i e s (37-39) o f t h e gas phase p h o t o c h e m i s t r y o f FeiCO)^ * C r ( C 0 ) 5 by time r e s o l v e d IR methods shows t h a t e j e c t i o n of two CO s p e r i n c i d e n t photon o c c u r s as t h e e x c i t a t i o n energy i n c r e a s e s . That i s c o n s i s t e n t w i t h t h e s t r o n g c o u p l i n g model p r o p o s e d . I t a l s o appears (40) t h a t few o f t h e e j e c t e d CO groups a r e v i b r a tionally excited. We s p e c u l a t e t h a t t h e energy gap between h i g h f r e q u e n c y CO v i b r a t i o n s and low f r e q u e n c y M-C o r M-M v i b r a t i o n s in simple metal c a r b o n y l s (e.g., Cr(CO)^ or M ^ Î C O ) ^ ) t r a p s e x c i t e d s t a t e energy i n t h e M-C and M-M v i b r a t i o n a l modes. T h i s c o u l d ex p l a i n t h e h i g h quantum e f f i c i e n c i e s f o r CO d i s s o c i a t i o n o r M-M bond h o m o l y s i s i n such compounds. T h i s may a l s o be why quantum e f f i c i e n c i e s f o r CO d i s s o c i a t i o n i n s u b s t i t u t e d c a r b o n y l s (41) d e c r e a s e markedly. I n t r o d u c t i o n o f l i g a n d s w i t h low f r e q u e n c y v i b r a t i o n a l modes p r o v i d e s a s i n k f o r v i b r a t i o n a l e x c i t a t i o n energy and perhaps a b e t t e r p a t h f o r energy m i g r a t i o n t o t h e s u r r o u n d i n g s i n condensed phases. a n c


1

P h o t o c h e m i s t r y o f O x a l a t e and D i t h i o o x a l a t e Complexes o f N i c k e l , P a l l a d i u m , and P l a t i n u m P h o t o o x i d a t i o n o f c o o r d i n a t e d o x a l a t e has been known s i n c e t h e e a r l i e s t s t u d i e s of t r a n s i t i o n metal photochemistry (42). In these r e a c t i o n s o x a l a t e l i g a n d i s p h o t o o x i d i z e d t o C O 2 , and up t o two m e t a l c e n t e r s a r e r e d u c e d by one e l e c t r o n ( e . g . f e r r i o x a l a t e ) . We wondered whether t h e o x a l a t e l i g a n d c o u l d be a t w o - e l e c t r o n p h o t o r e d u c t a n t , by s i m u l t a n e o u s o r r a p i d s e q u e n t i a l e l e c t r o n t r a n s f e r , w i t h m e t a l s prone t o 2e redox p r o c e s s e s . A p p l i c a t i o n o f t h i s c o n c e p t t o 16e square p l a n a r d® complexes, E q u a t i o n 15, was a t t r a c t i v e because i t s h o u l d produce s o l v a t e d 14e m e t a l complexes t h a t a r e i n o r g a n i c analogues o f

M(C 0 )L £

16e

4

-*^->-

2

2C0

2

+

ML

M = N i , Pd, P t

(15)

2

14e

carbenes. The r e p o r t s (43,44) t h a t p l a t i n u m ( O ) complexes c o u l d be i s o l a t e d by i r r a d i a t i n g P t X c ^ Z f ) ( P P h - z ) 2 and t h a t r h o d i u m ( l ) s p e c i e s were o b t a i n e d by i r r a d i a t i n g R h ( C 2 0 ) C l ( p y ) s u g g e s t e d t h a t t h i s p r o c e s s might work. Because 14e P t L fragments can be made t h e r m a l l y (45,46) when L i s a b u l k y phosphine [ e . g . , PCy^ o r P ( t - B u ) ] , we examined (47,48) t h e p h o t o c h e m i s t r y o f s t e r i c a l l y u n h i n d e r e d com plexes. The p h o t o r e a c t i v i t y o f PtiC^O^)(PEt )2, E t = C 2 H 5 , i s sum m a r i z e d i n Scheme I I . P h o t o c h e m i c a l c o n v e r s i o n s a r e h i g h and few s i d e p r o d u c t s ( e . g . , F i g u r e 4) form. A l l the r e a c t i o n s suggest f o r mation o f a r e a c t i v e P t ( P E t ) 2 fragment t h a t can be t r a p p e d as a p l a t i n u m ( O ) s p e c i e s o r combined w i t h o x i d a t i v e a d d i t i o n s u b s t r a t e s t o y i e l d p l a t i n u m ( l l ) compounds. 4

3

2

3

3

3

T h i s c h e m i s t r y has been extended t o produce p a l l a d i u m ( O ) i n t e r mediates ( 4 8 ) . Much o f t h e c h e m i s t r y i s s i m i l a r t o t h a t o f t h e P t analogues e x c e p t t h a t t h e p a l l a d i u m ( O ) complexes a r e more u n s t a b l e and d i f f i c u l t t o i s o l a t e . A reaction c h a r a c t e r i s t i c of palladium i s t h e a d d i t i o n o f a l l y l compounds t o form c a t i o n i c a l l y l complexes, E q u a t i o n 16. T h i s has been p o s t u l a t e d (49) as a key s t e p i n t h e mechanism f o r P d ( d i p h o s ) p c a t a l y z e d r e a c t i o n s o f a l l y l compounds.



13.

TROGLER


189

F i g u r e 3· E l e c t r o n i c a b s o r p t i o n spectrum o f F e ( C 0 ) - z ( l ,4-Me2N ) and quantum y i e l d s f o r p h o t o s u b s t i t u t i o n o f CO by PPh3. Repro duced from Ref. 21. C o p y r i g h t 1981, American C h e m i c a l S o c i e t y . 4

Pt < P E t > ( C H > 3

6 19.6 ppM,

'j

2

2

_p

p t

4

* 3486 Hz

I

Pt(PEt ) 6 4 . 3 ppM,

3

2

P t

_p

, J

2

4

*

3

5

0

3

H

z

F i g u r e 4. S u c c e s s i v e P{ H}NMR s p e c t r a (109 MHz) showing t h e p h o t o c h e m i c a l c o n v e r s i o n under 1 atm e t h y l e n e o f P t ( P E t ^ ) 2 ( 0 2 0 4 ) t o P t ( P E t ^ ) 2 ( C H 4 ) . I r r a d i a t i o n t i m e s a r e shown a t r i g h t . The s y m m e t r i c a l l y d i s p o s e d s a t e l l i t e peaks r e s u l t from t h o s e mole c u l e s t h a t c o n t a i n ^95pt (33·8$ abundance, I = § ) . S i g n a l s marked by an a s t e r i s k i n t h e f i n a l spectrum a r e u n i d e n t i f i e d s i d e p r o d u c t s , which form a t l o n g i r r a d i a t i o n t i m e s . Reproduced from Ref. 48. C o p y r i g h t 1985, American C h e m i c a l S o c i e t y . 2



190

Scheme I I Reactions o f P t ( C 0 ^ ) ( P E t ) 2

3

2

On U l t r a v i o l e t I r r a d i a t i o n

t r a n s - P t ( C H 0)H(PEt 3

2Pt(PEt ) 3

P

t

3

(

P

E

) 3

1

* *·>

trans-Pt(R)X(PEt ) 5 £

•» CH 0H

n

3


r

PtCl (PEt ) 2

3

2

V 4

2

RX = C>-H_C1, 6 5 CH C l , C H I 3

hv >

Pt

/ 3

3

olefin =

2

C H 2

Pt(C0) (PEt ) 2

3

2

V

[Pt(H 0)H(PEt ) ]0H^ 2

cis-Pt(R,Si)H(PËt )

RjSiH

4

2

and t r a n s - P t H ( P E t ) 2

C0

3

2

-*-|— Pt(olefin)(PEt ) ?

o

PEt^ Η—PtL

-Pt H

[0 CH] 2

PEt^

X = OAc, OPh, OH OEt, C l L = diphos or [ P ( n - B u ) ] 2

3

2

Presumably P d ( d i p h o s ) i s g e n e r a t e d i n t h e c a t a l y t i c c y c l e by decom p o s i t i o n o r l i g a n d d i s s o c i a t i o n from t h e b i s ( d i p h o s ) complex. The r e a c t i v i t y o f p h o t o g e n e r a t e d P d L d i f f e r s from P t L s i n c e t h e l a t t e r s p e c i e s does n o t add a l l y l s u b s t r a t e s c l e a n l y . P h o t o c h e m i c a l r o u t e s 2

2


2

13.


TROGLER

191

t o PtL2 and P d L s p e c i e s , t h a t c o n t a i n l e s s s t e r i c a l l y h i n d e r e d p h o s p h i n e s , complements t h e r m a l c h e m i s t r y known (45,46,50) f o r b u l k y ML2 s p e c i e s (M = Pd and P t , L = PCV3 o r P ( t - B u ) 3 ) . H i g h r e a c t i v i t y o f the p h o t o g e n e r a t e d s p e c i e s i s i l l u s t r a t e d by the f o l l o w i n g com p a r i s o n (49,51,52). 2


Ph-Cl

+

Pt(C 0 )(PEt ) 2

4

3

Ph-Cl

+

Pt(PEt )

Ph-Cl

+

Pt(PCy )

3

>

2

3

1

1

Q

0

C

trans-PtClPh(PEt ) 3

>

trans-PtClPh(PEt ) 3

14 days 20° C ^

3

2

2

trans-PtClPh(PCy )

Our attempts t o p r e p a r e Ni(0204)!^ complexes i n v a r i a b l y l e d t o f o r m a t i o n o f i n s o l u b l e Ni(C204). T h i s may be a t t r i b u t e d t o the i n a b i l i t y o f the weak f i e l d o x a l a t e l i g a n d t o s t a b i l i z e square p l a n a r Ni(il). We t h o u g h t t h a t a s t r o n g f i e l d o r s o f t v e r s i o n o f the oxa l a t e l i g a n d might be u s e f u l . I t seemed t h a t the d i t h i o o x a l a t e (S2C20 ~) l i g a n d would e x h i b i t p h o t o c h e m i s t r y analogous t o c h e l a t i n g oxalate. T h e r e f o r e the s e r i e s o f d i t h i o o x a l a t e complexes M(S2C2~ 02)L.2 have been p r e p a r e d (53) where L = PMe3 o r , L2 = d i p h o s = Ph2~ PCH -CH PPh2 and depe = Et PCH -CH PEt2»and M = N i , Pd, and P t . The IR s t r e t c h o f the C=0 group (1680-1750 cm-1 ) p r o v e s s u l f u r c o o r d i n a t i o n f o r the S2C2O2" l i g a n d . I r r a d i a t i o n o f the d i p h o s d e r i v a t i v e s i n CH2CI2 produced f r e e SCO and MCI2(diphos). T h e r e f o r e i t appears t h a t t h e d i t h i o o x a l a t e l i g a n d can a l s o be r e d u c t i v e l y e l i m i n a t e d by photolysis. The c h e m i s t r y o f t h e s e systems i s c o m p l i c a t e d by s e c o n dary r e a c t i o n s w i t h SCO and i s under i n v e s t i g a t i o n . 2

2

2

2

2

S y n t h e s i s and P h o t o r e a c t i v i t y

2

o f Surface-Bound P l a t i n u m O x a l a t e s

The p h o t o c h e m i c a l l y produced Pt(PEt3)2 fragment, s t a b i l i z e d a s t h e c i s - and t r a n s - P t H p ( P E t ^ ) ? complexes, has proved (54,55,56) t o be an e f f i c i e n t and l o n g l i v e d homogeneous c a t a l y s t f o r H2/D2 exchange ( E q u a t i o n 17), d e u t e r a t i o n o f acetone o r a c e t o n t r i l e ( E q u a t i o n s 18 and 19), d e c o m p o s i t i o n o f f o r m i c a c i d ( E q u a t i o n 20), and h y d r o l y s i s o f a c e t o n i t r i l e ( E q u a t i o n 21). Because o f the c a t a l y t i c promise Η,

3D.

+ 2

+

>

2 HD

>

3H

CH CN 3

=>•

li

HC00H -

>

H

H0

>

0 II •NH H_C-C3

D

2

-

CH C(0)CH 3 3

1| D 2

Η C-C=N

+

+

2

-

2

(17)

+

2

H

2

+

CD C(0)CD 3 3

+

CD CN 3

co

(18)

(19)

2

(20)

2

(21)


192

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


o f t h e s e systems we d e c i d e d t o s y n t h e s i z e s u r f a c e a t t a c h e d o x a l a t e derivatives· F o r c e r t a i n c a t a l y s t a p p l i c a t i o n s ( e . g . , ease o f s e p a r a t i o n ) i t i s d e s i r a b l e t o have h e t e r o g e n e o u s r a t h e r t h a n homogeneous c a t a l y s t s . From a f u n d a m e n t a l view t h e r e i s i n t e r e s t i n comparing t h e s u r f a c e e f f e c t on c a t a l y t i c r a t e s and mechanisms o f s u r f a c e - a t t a c h e d homo geneous c a t a l y s t s . Of t h e two commonly used s u p p o r t s ( 5 7 ) , o r g a n i c polymers o r s i l i c a , we chose s i l i c a o f h i g h pore d i a m e t e r (140A) be cause o f i t s r i g i d i t y and p e r m e a b i l i t y i n p o l a r media. Most p r e v i o u s (57) s t u d i e s o f phosphine s u p p o r t e d t r a n s i t i o n m e t a l complexes have employed a r y l p h o s p h i n e l i g a n d s . T h i s p r e s e n t s problems s i n c e a r y l p h o s p h i n e s o f t e n d i s s o c i a t e and c a t a l y s t l e a c h i n g poses a p r o b l e m . S m a l l t r i a l k y l p h o s p h i n e s , by c o n t r a s t , a r e among t h e most d i f f i c u l t l i g a n d s t o d i s p l a c e from a m e t a l c e n t e r . Aryl p h o s p h i n e s a r e b u l k y and h i n d e r s u b s t r a t e a c c e s s t o t h e m e t a l c e n t e r . C l e a v a g e o f t h e P-C bond ( i . e . , d e g r a d a t i o n ) as w e l l as o r t h o m e t a l l a t i o n o c c u r s more r e a d i l y w i t h a r y l p h o s p h i n e a n a l o g s ( 5 8 ) . Several s y n t h e t i c p r o c e d u r e s were e x p l o r e d f o r s y n t h e s i z i n g s u r f a c e a t t a c h e d o x a l a t e complexes. The b e s t p r o c e d u r e (59) i s o u t l i n e d i n Scheme I I I . I n t h e s y n t h e s i s o f Scheme I I I we used D a v i s o n S i l i c a (Grade 62, 1402 p o r e d i a m e t e r . 340 m^/g) and a c h i e v e d a maximum s u r f a c e c o v e r a g e o f 1 molecule/113% which amounts t o 70% f u n c t i o n a l i z a t i o n o f t h e s u r f a c e ( w i t h t h e assumption t h a t 6 s u r f a c e h y d r o x y l s a n c h o r one platinum complex). Key p o i n t s o f t h e s y n t h e s i s i n c l u d e : 1) t h e v o l a t i l i t y o f r e a c t a n t s i n s t e p 1 and t h e h i g h y i e l d (97%) of the photo c h e m i c a l a d d i t i o n make i t p o s s i b l e t o p r e p a r e t h e L - P E t l i g a n d i n g r e a t e r t h a n 99% p u r i t y ; 2) t h e v o l a t i l e SMe l i g a n d can be removed i n s t e p 2 and t h e d e r i v a t i z e d p l a t i n u m complex, which i s an o i l , can be i s o l a t e d i n h i g h p u r i t y ; 3) c a p p i n g r e m a i n i n g s u r f a c e h y d r o x y l groups w i t h h e x a m e t h y l d i s i l a z a n e i n s t e p 4 p r e v e n t s r e a c t i o n s o f p h o t o g e n e r a t e d P t ( 0 ) w i t h t h e s u p p o r t ; 4) p u t t i n g t h e complex on t h e s u p p o r t as a s t a b l e P t ( l l ) s p e c i e s p r o t e c t s t h e b a s i c p h o s p h i n e l i gand from o x i d a t i o n ( 6 0 ) . B e s i d e s t h e a n a l y t i c a l d a t a ( P t / P r a t i o = 1/2) t h a t c h a r a c t e r i z e t h e s u p p o r t e d complex t h e IR spectrum e x h i b i t s s t r e t c h e s t h a t a r e i d e n t i c a l t o t h o s e i n t h e homogeneous analogue (49) P t ( C 0 4 ) ( P E t 3 ) . I f t h e sample i s i r r a d i a t e d (as a n u j o l m u l l ) t h e o x a l a t e s t r e t c h e s d i s a p p e a r and a new peak a p p e a r s a t 2330 c m , a t t r i b u t e d to CO2· Thus, E q u a t i o n 22 o c c u r s on t h e s u r f a c e . R e c a l l (Scheme I I ) t h a t 9

2

2

2

2

-1

Et? "CHhv 2C0 {

< y

CH.

-Ρ EΡ t

A

2

(22)

. υ'

p h o t o g e n e r a t e d P t ( P E t 3 ) c o u l d be t r a p p e d w i t h CO t o form P t ( P E t 3 ) (C0) . S i n c e t h e c a r b o n y l s t r e t c h e s (1930 and 1973 cm~1) a r e c h a r a c t e r i s t i c (47) o f t h i s complex we i r r a d i a t e d t h e s u r f a c e s u p p o r t e d complex under CO. O b s e r v a t i o n o f t h e peaks a t 1929 and 1965 cm~1 2

2


2

193


13. TROGLER

Scheme I I I MeO^ (1)

MeO—Si—CH==CH

+ HPEt^

2

MeO „ \ MeO-Si-(CH ) -PEt =

hv .

Λ

2

MeO^


(2)

2

2

L-PEt

MeO^

2L—PEt

6

+ Pt(C 0 )(SMe )

2

2

4

6

2

> SMe

2

+ +

Pt(C 0 )(L—PEt ) 2

4

£

(3) 4 Silica —0' —0' +

Pt(C 0 )(L-PEt) 2

4

r 2

f

> 6 6

—0'

—o,

Silica^

-o. \ -0-Si-CH —

E

t 2

CH— Ρ 2

-0 -0.

Pt

-O-Si-CH—CH—P

3 (4) HN(SiMe ) 3

2

caps o f f f r e e s u r f a c e OH w i t h i n e r t SlMe., group, and wash w e l l .

suggests t h a t t h e surface generated species o f Equation t r a p p e d , E q u a t i o n 23·

22 c a n be


2

2

194

EXCITED STATES AND

>

Et

Et.

-CFT

1

Pt

-CH„


9

-CH+

2C0

(23)

/- \ P t

-CH-

-P Et

Eto

x

c

2

In c o n t r a s t t o homogeneous analogues (48,61) t h e s i l i c a bound P t L fragment does n o t c a t a l y z e a c e t o n i t r i l e h y d r o l y s i s . Initial experiments showed h y d r o l y s i s a c t i v i t y < 1/1000 t h a t o f t h e homo geneous system. T h i s p u z z l e d us u n t i l we found t h a t homogeneous c a t a l y s t systems where t h e phosphine l i g a n d s a r e c o n s t r a i n e d t o be c i s [ e . g . , P t ( C 0 4 ) ( d i p h o s ) ] show s i m i l a r low a c t i v i t y . Molecular modeling s t u d i e s (CPK models) o f the s u r f a c e a t t a c h e d r e a g e n t o f Scheme I I I s u g g e s t t h a t t h e p l a t i n u m c e n t e r cannot adopt a t r a n s c o n f i g u r a t i o n necessary f o r e f f e c t i v e c a t a l y s i s . P r e v i o u s work (48) w i t h homogeneous analogues showed t h a t S i - H oxidative additions y i e l d c i s products. A c i s geometry o f h y d r i d e and s i l y l may be a l l o w e d i n c a t a l y t i c h y d r o s i l a t i o n . Because t h e i n d u s t r a l homogeneous h y d r o s i l a t i o n c a t a l y s t (62) i s H P t C l £ we t e s t e d t h e a c t i v i t y o f our s u r f a c e g e n e r a t e d r e a g e n t f o r t h e r e a c t i o n of E q u a t i o n 24. A s u s p e n s i o n o f the c a t a l y s t was i r r a d i a t e d i n 1-


2

2

2

heptene and a v i o l e n t r e a c t i o n ensured (400 t u r n o v e r s / P t ) on a d d i t i o n of d i c h l o r o m e t h y l s i l a n e . The h y d r o s i l a t i o n p r o d u c t formed i n over 97$ y i e l d and was pure by gc and % i NMR a f t e r f i l t r a t i o n from t h e catalyst. On a p e r p l a t i n u m b a s i s the c a t a l y s t has c a 1/100 t h e a c t i v i t y of H P t C l ^ . P r e s e n t work f o c u s e s on c a t a l y t i c mechanisms o f photo and t h e r m a l g e n e r a t e d c a t a l y s t s . 2

2

Conclusions P h o t o c h e m i c a l r e a c t i o n s o f t r a n s i t i o n metal complexes t h a t c o n t a i n u n s a t u r a t e d c h e l a t e s f a l l i n t o t h r e e c a t e g o r i e s : 1) f r a g m e n t a t i o n o f t h e l i g a n d t o y i e l d two r e a c t i v e f u n c t i o n a l i t i e s ; 2) e l i m i n a t i o n o f t h e l i g a n d t o g e n e r a t e two r e a c t i v e s i t e s a t t h e m e t a l ; 3) c h e l a t e l o c a l i z e d e x c i t e d s t a t e s can f u n c t i o n as p h o t o r e c e p t o r s t o promote p h o t o d i s s o c i a t i o n o f o t h e r m e t a l - l i g a n d bonds i n t h e complex. These p r o c e s s e s can be used as an e n t r y t o new r e a c t i v e i n t e r m e d i a t e s and catalysts. Acknowledgments I thank the s t u d e n t s (C.E. Johnson, M.E. G r o s s , R.S. Paonessa, A.L. P r i g n a n o , D. P o u r r e a u , R.L. Cowan), and p o s t d o c t o r a l s (C.E. J e n s e n , M.J. Maroney) who c o n t r i b u t e d t o t h e r e s e a r c h program d e s c r i b e d . F i n a n c i a l s u p p o r t o f our r e s e a r c h by the A i r F o r c e O f f i c e o f


13. TROGLER


S c i e n t i f i c R e s e a r c h , Army R e s e a r c h Foundation i s a p p r e c i a t e d .

195

O f f i c e , and N a t i o n a l S c i e n c e

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1. 2. 3.

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30. Vanquickenborne, L.G.; Ceulemans, A. Coord. Chem. Rev. 1983, 48, 157. 31. Ford, P.C. Coord. Chem. Rev. 1982, 44, 61. 32. Adamson, A.W. Coord. Chem. Rev. 1968, 3, 169· 33. Robinson, G.W.; Frosch, R.P. J. Chem. Phys. 1962, 37, 1962; 1963, 38, 1187. 34. Chock, D.P.; Jortner, J.; Rice, S.A. J. Chem. Phys. 1968, 49, 610. 35. Jortner, J.; Rice, S.A.; Hochstrasser, R.M. Adv. Photochem. 1969, 7, 149. 36. Langford, C.H. Acc. Chem. Res. 1984, 17, 96. 37. Ouderkirk, A.J.; Wermer, P.; Schultz, N.L.; Weitz, E. J. Am. Chem. Soc. 1983, 105, 3354. 38. Seder, T.A.; Church, S.P.; Ouderkirk, A.J.; Weitz, E. J. Am. Chem. Soc. 1985, 107, 1432. 39. Tumas, W.; Gitlin, B.; Rosan, A.M.; Yardley, J.T. J. Am. Chem. Soc. 1982, 104, 55. 40. Poliakoff, M.; Weitz, E. Adv. Organomet. Chem. 1985, in press. 41. von Gustorf, E.A.K.; Leenders, L.H.G.; Fischler, I.; Perutz, R. Adv. Inorg. Chem. Radiochem. 1976, 19, 65. 42. Balzani, V.; Carassiti, V. "Photochemistry of Coordination Com pounds"; Academic Press: New York, 1970. 43. Blake, D.M.; Nyman, C.J. J. Am. Chem. Soc. 1970, 92, 5359. 44. Addison, A.W.; Gillard, R.S.; Sheridan, P.S.; Tipping, L.R.H. J. Chem.Soc.,Dalton Trans. 1974, 709. 45. Otsuka, S. J. Organomet. Chem. 1980, 200, 191. 46. Shaw, B.L. ACS Symp. Ser. 1982, 196, 101. 47. Paonessa, R.S.; Trogler, W.C. Organometallics 1982, 1, 768. 48. Paonessa, R.S.; Prignano, A.L.; Trogler, W.C. Organometallics 1985, 4, 647. 49. Trost, B.M. Acc. Chem. Res. 1980, 13, 385. 50. Stone, F.G.A. Angew. Chem., Intl. Ed. Engl. 1984, 23, 89. 51. Gerlach, D.H.; Kane, A.R.; Parshall, G.W.; Jesson, J.P.; Muetterties, E.L. J. Am. Chem. Soc. 1971, 93, 3543. 52. Fornies, J.; Green, M.; Spencer, J.L.; Stone, F.G.A. J. Chem. Soc., Dalton Trans. 1977, 1006. 53. Cowan, R.L.; Pourreau, D.; Trogler, W.C., to be published. 54. Paonessa, R.S.; Trogler, W.C. J. Am. Chem. Soc. 1982, 104, 1138. 55. Paonessa, R.S.; Trogler, W.C. J. Am. Chem. Soc. 1982, 104, 3529. 56. Paonessa, R.S.; Trogler, W.C. Inorg. Chem. 1983, 22, 1038. 57. Bailey, D.C.; Langer, S.H. Chem. Rev. 1981, 81, 109. 58. Parshall, G.W. Acc. Chem. Res. 1970, 3, 139. 59. Prignano, A.L.; Trogler, W.C., to be published. 60. Bemi, L.; Clark, H.C.; Davies, J.A.; Fyfe, C.A.; Wasylishen, R.E. J. Am. Chem. Soc 1982, 104, 438. 61. Jensen, C.M.; Trogler, W.C., submitted. 62. Parshall, G.W. "Homogeneous Catalysis"; Wiley: New York, 1980. RECEIVED November 8, 1985


Excited States and Reactive Intermediates - American Chemical Society

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