Activation of Carbon Monoxide by Carbon and Oxygen Coordination

1 Activation of Carbon Monoxide by Carbon and Oxygen Coordination Lewis Acid and Proton Induced Reduction of Carbon Monoxide

Downloaded by 80.82.77.83 on December 26, 2017 | http://pubs.acs.org Publication Date: May 5, 1981 | doi: 10.1021/bk-1981-0152.ch001

D. F. SHRIVER Department of Chemistry, Northwestern University, Evanston, IL 60201

In its free state, carbon monoxide is highly resistant to attack by hydrogen and a variety of other common reducing agents. The reactivity of coordinated CO is much greater than that of the free molecule and metal surfaces are in general even more effective than simple coordination compounds in promoting CO reduction. One great challenge to the inorganic chemist is to make the connection between chemistry which occurs on the surfaces of metals and the more readily studied reactions of discrete molecular organometallics. One possible mode of CO activation which has been invoked in heterogenous catalysis is C and O bonding to a surface. This bifunctional activation of CO may lead to CO cleavage and eventual incorporation of a surface carbide into organic products, or to direct incorporation of the C and O coordinated CO into an organic group (1-3). Bifunctional activation also is thought to be important in molecular systems (4,5), but it is fair to say that the evidence for, and understanding of this phenomenon has been very rudimentary. I n t h i s paper we p r e s e n t t h e r e s u l t s o f s t u d i e s a t N o r t h w e s t e r n w h i c h were d e s i g n e d t o p r o v i d e c l e a r - c u t e v i d e n c e f o r b i f u n c t i o n a l CO a c t i v a t i o n i n m o l e c u l a r systems and t o p r o v i d e i n f o r m a t i o n on t h e i m p o r t a n t

0097-6156/81/0152-0001 $05.00/0 © 1981 American Chemical Society Ford; Catalytic Activation of Carbon Monoxide ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

2

CATALYTIC ACTIVATION OF CARBON MONOXIDE c h e m i c a l v a r i a b l e s i n t h e s e r e a c t i o n s . We f i r s t d e s c r i b e L e w i s a c i d p r o m o t i o n o f t h e a l k y l m i g r a t i o n (CO i n s e r t i o n ) r e a c t i o n , i n c l u d i n g r e c e n t r e s u l t s on t h e c o m b i n a t i o n o f t h i s a c i d promoted a l k y l m i g r a t i o n r e a c t i o n w i t h CO r e d u c t i o n . This repetitive s e q u e n c e o f CO i n s e r t i o n and c a r b o n y l r e d u c t i o n p r o v i d e s a means o f b u i l d i n g h y d r o c a r b o n c h a i n s under m i l d c o n d i t i o n s . Finally, proton induced CO r e d u c t i o n w i l l be d e s c r i b e d , and t h e most recent mechanistic i n f o r m a t i o n on t h i s r e a c t i o n w i l l be p r e sented. As a p r e l u d e t o t h e s e d i s c u s s i o n s , we o u t l i n e two f u n d a m e n t a l r e a c t i o n s o f c o o r d i n a t e d CO.


E l e c t r o p h i l i c and N u c l e o p h i l i c A t t a c k o f C o o r d i n a t e d

CO

The a t t a c k on c o o r d i n a t e d c a r b o n monoxide by n u c l e o p h i l e s was f i r s t e x t e n s i v e l y developed i n s y n t h e t i c o r g a n o m e t a l l i c chemistry by E. 0. F i s c h e r and h i s s t u d e n t s (6); as d i s c u s s e d by o t h e r s i n t h i s v o l u m e , t h i s r e a c t i o n p r o v i d e s one r o u t e t o t h e r e d u c t i o n o f c o o r d i n a t e d CO and t o c a t a l y s i s o f t h e w a t e r gas s h i f t r e a c t i o n . Those c a r b o n y l g r o u p s w h i c h a r e s u s c e p t i b l e t o a t t a c k by n u c l e o philes are e l e c t r o n d e f i c i e n t , as j u d g e d by t h e i r h i g h CO s t r e t c i n g frequencies (7). By c o n t r a s t , m e t a l c a r b o n y l s h a v i n g low CO s t r e t c i n g f r e q u e n c i e s a r e s u s c e p t i b l e t o a t t a c k o f the CO oxygen atom by e l e c t r o p h i l e s such as A l ( C H 3 ) 3 , A l B r 3 , o r BF3. T h i s c h e m i c a l evidence and a v a r i e t y of p h y s i c a l evidence i n d i c a t e t h a t a l o w CO s t r e t c h i n g f r e q u e n c y c o r r e s p o n d s t o h i g h e l e c t r o n d e n s i t y on t h e CO i i g a n d ( 8 ) . C a r b o n y l g r o u p s i n t h i s c a t e g o r y i n c l u d e b r i d g i n g c a r b o n y l s , t e r m i n a l c a r b o n y l s i n m e t a l c a r b o n y l a n i o n s , and t e r m i n a l c a r b o n y l s i n donor s u b s t i t u t e d m e t a l c a r b o n y l s , s t r u c t u r e s l a t h r o u g h l c . The a t t a c k on b r i d g i n g c a r b o n y l s by e l e c t r o p h i l i c

0

0

C 0

0

L

0

C 0

AIR. 3 (la)

(lb)

(lc)

Ford; Catalytic Activation of Carbon Monoxide ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

1.

SHRIVER

3

Lewis Acid and Proton Induced Reduction

reagents i s a common f e a t u r e o f t h e c h e m i s t r y o f p o l y n u c l e a r carbonyls, and i t may l e a d t o a v a r i e t y o f CO r e a r r a n g e m e n t s (8^,9^). One s t r i k i n g p h y s i c a l e f f e c t o f L e w i s a c i d a d d i t i o n t o t h e oxygen end o f CO i s a v e r y l a r g e r e d u c t i o n i n t h e CO s t r e t c h i n g f r e q u e n c y , w h i c h i m p l i e s a l a r g e d e c r e a s e i n CO bond order, Figure 1 (10). T h i s phenomenon w i l l be d i s c u s s e d i n more d e t a i l a t t h e end o f t h e p a p e r , and f o r t h e p r e s e n t i t w i l l s u f f i c e t o point out that the a d d i t i o n o f a Lewis a c i d t o the c a r b o n y l oxygen f a v o r s c a r b e n e - l i k e r e s o n a n c e s t r u c t u r e s , w h i c h a r i s e from the p o l a r i z a t i o n o f the 7r s y s t e m , e q u a t i o n 2. L M-C=0 + A 1 X n

>

3

L^C-0^

(2)


AIX3 The v e r y l a r g e p e r t u r b i n g i n f l u e n c e o f C and 0 b o n d i n g on t h e CO bond o r d e r l e d us t o e x p l o r e the i n f l u e n c e o f L e w i s a c i d and p r o t o n a c i d promoted r e a c t i o n s o f m e t a l c a r b o n y l c o m p l e x e s . A c i d P r o m o t e d CO I n s e r t i o n Owing i n p a r t t o i t s g r e a t c o m m e r c i a l i m p o r t a n c e , t h e CO i n s e r t i o n r e a c t i o n i s p e r h a p s the most t h o r o u g h l y s t u d i e d m e t a l c a r b o n y l r e a c t i o n other than s u b s t i t i o n (11-13). As shown i n e q u a t i o n 3a, the c u r r e n t l y R I

ki

ki

a c c e p t e d mechanism f o r t h i s r e a c t i o n i s t h e m i g r a t i o n o f t h e a l k y l g r o u p o n t o a c o o r d i n a t e d CO, t o y i e l d 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 m e t a l a c y l i n t e r m e d i a t e ( w h i c h p e r h a p s may be s o l v e n t stabilized). T h i s i n t e r m e d i a t e i s t h e n a t t a c k e d b y an i n c o m i n g l i g a n d t o p r o d u c e a s t a b l e a c y l c o m p l e x , e q u a t i o n 3b. When a .R

k

9

X

R

v*- c ; 0 + L' ^ef v-'*-^

(3b)

k

t t i e

s t a b l e product i s formed, k _ « , t h e form g i v e n i n e q u a t i o n 4, w i t h e q u a t i o n s 5a and 5b. 2

k i n e t i c expression takes two l i m i t i n g c o n d i t i o n s ,

2

k ^ f L ' H L ^ C O ) ] r

a

t

e

=

k.

x

(

+ k [L']

f

f

r a t e = K k [ L ] [ L M R ( C 0 ) ] when k [ L ] « 2

n

2

f

r a t e = k [ L M R ( C 0 ) ] when k [ L ] » 1

n

4

)

2

2

k_

k.

x

x


(5a) (5b)


4

CATALYTIC ACTIVATION OF CARBON MONOXIDE

900

Bond Order Figure 1. Bond order vs. CO stretching frequency; the curve was determined from data on organic compounds, data for organometallic compounds have been entered on the curve based on observed CO stretching frequencies.


1.

SHRIVER

5


The simple second-order kinetics o f equation 5a a p p l y f o r Mn(CO)5(0113) when L=CO a t s u b a t m o s p h e r i c p r e s s u r e s . I t i s under t h i s s e t o f c o n d i t i o n s t h a t we have s t u d i e d t h e L e w i s a c i d p r o moted CO i n s e r t i o n r e a c t i o n , see F i g u r e 2. P r i o r t o o u r s t u d i e s i t was r e c o g n i z e d t h a t i o n p a i r i n g w i t h a n i o n i c m e t a l c a r b o n y l s c o u l d promote CO i n s e r t i o n and r e l a t e d reactions (14-16). Both kinetic and n o n - k i n e t i c evidence suggests the importance o f i o n p a i r s i n these types o f r e a c t i o n s (14,17). F o r e x a m p l e , a s m a l l c a t i o n was found t o g r e a t l y a c c e l e r a t e t h e CO i n s e r t i o n r e a c t i o n r e l a t i v e t o t h e same r e a c t i o n with a large cation, equation 6 (14). THF +

M [RFe(CO) ~] + L

1

>


4

+

rate

+

(M = (Ph P) N ) 3

2

M [L Fe(CO)3(CRO)] +

+

1

+

: rate (M = L i )

1:10

(6)

3

Our work on t h e b i f u n c t i o n a l a c t i v a t i o n o f CO i n s e r t i o n was prompted b y t h e t h o u g h t t h a t s t r o n g m o l e c u l a r L e w i s a c i d s s h o u l d be more e f f e c t i v e and more g e n e r a l t h a n s i m p l e c a t i o n s . It a l r e a d y had been o b s e r v e d t h a t m o l e c u l a r L e w i s a c i d s w o u l d p r o mote a m o l e c u l a r F i s c h e r - T r o p s c h t y p e r e a c t i o n 05), and t h a t i r o n d i e n e c o m p l e x e s c a n be c o n v e r t e d t o p o l y c y c l i c k e t o n e s b y t h e a c t i o n o f aluminum h a l i d e s , e q u a t i o n 7 , ( 1 8 ) , b u t i n f o r m a t i o n on t h e c o u r s e o f t h e s e r e a c t i o n s was s k e t c h y .

(7)

In the f i r s t s t u d i e s performed a t Northwestern, Steven S t r a u s s found t h a t Α1ΒΓ3 b r o u g h t about a m o d e r a t e i n c r e a s e i n t h e r a t e o f CO u p t a k e b y Mn(CO)5(0113). Susan B u t t s t h e n d i s c o v e r e d t h a t t h e r e a c t i o n o c c u r s i n two s t e p s . The f i r s t i s a v e r y r a p i d CO i n s e r t i o n t o y i e l d a c y c l i c p r o d u c t , e q u a t i o n 8 a , w h i c h i s f o l l o w e d b y t h e much s l o w e r u p t a k e o f CO, e q u a t i o n 8 b . 0 C

1/

OC — 0

C

? c°

C°

Mn—CO /ι C 0

(8a)

0

+ AlBr* 0

> fast

1/ / « 3 O C — M n — C. /Ι ·> 0

0

' / B r — A l Br 2


6



TIME CMIN) Journal of the American Chemical Society

Figure 2.

Second-order kinetic plot for the reaction of CO at subatmospheric pressures with Mn(CO) (CCH OAlBrBr ) in toluene solution (19) k

i

î


1.

SHRIVER 0

C

c

,CH Mnrrrr:C

o

c

I

AlBr

c

o C H

/ / 3 OC — M n r m C /| * 0A1 Br 0 C

->

CO

3

1

/

Br


(8b)

0

c o 1/

OC

7


2

The s t r u c t u r e o f the c y c l i c r e a c t i o n p r o d u c t o f e q u a t i o n 8a has been d e t e r m i n e d b y Dr E l i z a b e t h H o l t a t t h e U n i v e r s i t y o f G e o r g i a , and a s i m i l a r c y c l i c s t r u c t u r e i s i n d i c a t e d b y s p e c t r o s c o p i c d a t a f o r other m e t a l systems ( 1 9 ) . R e a c t i o n 8a i s so r a p i d t h a t the r a t e has e l u d e d measurement by c o n v e n t i o n a l k i n e t i c methods, but b a s e d on the i m m e d i a t e c o l o r change upon m i x i n g we e s t i m a t e t h a t the r a t e i s a t l e a s t 10^ g r e a t e r t h a n the f o r ward r a t e i n r e a c t i o n 3a. Thus the r o l e o f the L e w i s a c i d i s n o t simply t o capture the c o o r d i n a t i v e l y unsaturated intermediate, M n ( C 0 ) ( C 0 R ) , b u t r a t h e r t o promote the a l k y l m i g r a t i o n process. A t t a c k b y the L e w i s a c i d must o c c u r p r i o r t o o r v e r y e a r l y i n t h e sequence o f a l k y l m i g r a t i o n . One p o s s i b l e v a r i a t i o n on t h i s g e n e r a l p i c t u r e i s p r i o r e q u i l i b r i u m complex f o r m a t i o n between Mn(C0)5R and ΑΙΒΓβ, eq. 9a, f o l l o w e d b y a l k y l m i g r a t i o n , eq. 9 b . We have no e x p e r i m e n t a l e v i d e n c e on w h e t h e r o r not the r e a c t i o n 4

RMn(C0)

5

+ AlBr3

^

N

RMn(CO) (COAlBr ) 5

RMn(CO) (COAlBr ) 4

»

3

(9a)

3

Mn(C0) (CR0AlBrBr ) 4

2

(9b)

r a t e i s i n c r e a s e d b y s i m u l t a n e o u s b r o m i d e a t t a c k on Mn, b u t t h e t h e o r e t i c a l s t u d i e s o f B e r k e and Hoffmann s u g g e s t t h a t n u c l e o p h i l i c a t t a c k on the c e n t r a l m e t a l i s not l i k e l y t o a s s i s t the a l k y l m i g r a t i o n r e a c t i o n ( 1 3 ) . T h e r e i s no d i r e c t e v i d e n c e f o r the equl i b r i u m complex f o r m a t i o n d e p i c t e d i n e q u a t i o n 9a; i n d e e d the h i g h CO s t r e t c h i n g f r e q u e n c i e s o f M n i C O ^ i C ^ ) are i n a range f o r w h i c h s t a b l e c o m p l e x e s have n o t been o b s e r v e d between c a r b o n y l s and L e w i s a c i d s such as A l B ^ . However, the f r e q u e n c y - b a s i c i t y c o r r e l a t i o n does not e x c l u d e the p o s s i b i l i t y o f m i n u t e but k i n e t i c a l l y i m p o r t a n t amounts o f t h e complex b e i n g formed. I t also i s p o s s i b l e t h a t a p r e - e q u i l i b r i u m does not e x i s t but i n s t e a d a l k y l m i g r a t i o n occurs simultaneously with Lewis a c i d a t t a c k , equation 10. Whatever the f i n e r d e t a i l s o f t h i s s t e p , t h e r e i s no doubt o f the great a c c e l e r a t i o n o f a l k y l m i g r a t i o n by m o l e c u l a r Lewis acids.

(0C) Mn-C0 4

9 (some CH^)

3

H00CCC1 H

7

H00CCC1H

2

2

none

Rate

2

1


1.

SHRIVER

9


K i n e t i c s t u d i e s b y B u t t s and Richmond i n d i c a t e t h a t b o t h t h e monomer and d i m e r o f d i c h l o r o a c e t i c a c i d promote t h e r e a c t i o n i n an a r o m a t i c s o l v e n t , e q u a t i o n s 12a and 12b, ( 2 0 ) . Mn(C0)c(CHo) D

+ CO

J

Ί

»

Mn( CO) c ( C C H o O )

((Me s o i n .

• >

(12a)

3

r a t e = k{[HA] + a [ H A ] } [ M n ( C O ) ( C H ) ] [ C O ] 2

3


where HA

2

5

Cl HCOOH and 2

(12b)

3

0.2

A v e r y e x c i t i n g r e c e n t development i n j o i n t r e s e a r c h w i t h P r o f e s s o r B u r w e l l i s t h e o b s e r v a t i o n b y C o r r e a , Nakamura and S t i m s o n , t h a t m e t a l o x i d e s u r f a c e s a l s o promote CO i n s e r t i o n (21). I n t h i s s t u d y i t was p o s s i b l e t o c h a r a c t e r i z e t h e n a t u r e o f t h e r e a c t i o n p r o d u c t s formed b y t h e i n t e r a c t i o n o f a l k y l m e t a l c a r b o n y l s w i t h m e t a l o x i d e s u r f a c e s , w h i c h h a d been a c t i v a t e d t o p r o d u c e s u r f a c e a c i d and b a s e s i t e s . The r e a c t i o n was f o l l o w e d by two methods, i n t h e f i r s t i n s t a n c e t h e e v o l u t i o n o f CO, H , CH4 and o t h e r l i g h t m o l e c u l e s was measured i n t h e c o u r s e o f t h e i n t e r a c t i o n o f the organoraetallic molecule with the surface. S e c o n d l y t h e n a t u r e o f t h e r e a c t i o n p r o d u c t on t h e s u r f a c e was deduced from F o u r i e r t r a n s f o r m i n f r a r e d s p e c t r o s c o p y o f t h e s u r face species. F o r t u n a t e l y we have a v a i l a b l e m o l e c u l a r a n a l o g u e s o f t h e s u r f a c e s p e c i e s so t h e s t r u c t u r a l i n f e r e n c e s from i n f r a r e d spectroscopy are quite strong. F o r example a sample o f γ-alumina was h e a t e d t o 9 0 0 C i n h i g h p u r i t y h e l i u m , c o o l e d , and t h e n exposed t o M n ( C O ) 5 ( ^ 3 ) . The r e s u l t i n g i n f r a r e d s p e c t r u m , when compared w i t h t h e m o l e c u l a r species, i n d i c a t e s that a surface a c e t y l i s formed w i t h a c y c l i c s t r u c t u r e a n a l o g o u s t o t h o s e seen i n t h e m o l e c u l a r L e w i s a c i d promoted r e a c t i o n s , e q u a t i o n 1 3 . 2

e

A1-0-A1-0

/ ////

+

Mn(C0K(CHo 5

J

> (0C) Mnrr^.C A

I Al—0

(13)

> — A l — 0

I I I I I I I T h i s i n s e r t i o n r e a c t i o n i s v e r y f a s t , as j u d g e d b y t h e i m m e d i a t e color change upon e x p o s u r e o f t h e alumina surface to the organometallic, and i n k e e p i n g w i t h these i n t e r p r e t a t i o n s no a p p r e c i a b l e amounts o f g a s e o u s p r o d u c t s a r e e v o l v e d on t h e t i m e s c a l e o f the measurements. One i m p l i c a t i o n o f t h i s o b s e r v a t i o n i s t h a t t h e m e t a l o x i d e s , w h i c h a r e f r e q u e n t l y employed t o s u p p o r t c o n v e n t i o n a l m e t a l l i c h e t e r o g e n o u s c a t a l y s t s , may p l a y an a c t i v e r o l e i n p r o m o t i n g CO r e d u c t i o n b y t h e i n t e r a c t i o n o f s u r f a c e L e w i s a c i d s i t e s w i t h t h e o x y g e n end o f CO. Indeed s u p p o r t e f f e c t s a r e w e l l documented f o r t h e c o n v e r s i o n o f CO t o h y d r o c a r bons ( 2 2 - 2 3 ) , and t h e r e f o r e we s p e c u l a t e t h a t i n t e r a c t i o n s , s u c h as t h o s e s u g g e s t e d i n e q u a t i o n 14, may be i m p o r t a n t . The u s u a l



10 /V

/>/

Η ν

/-^x

A1-0-A1-0-A1


I

CH

3

A1-0-A1-0-A1

I I I I I I

I I I I I

(14)

I

e x p l a n a t i o n i s t h a t The AI2O3 s u r f a c e w i t h d r a w s e l e c t r o n d e n s i t y f r o m the m e t a l p a r t i c l e , t h e r e b y c h a n g i n g i t s r e a c t i v i t y . We do not b e l i e v e t h a t t h i s i s a c h e m i c a l l y reasonable explanation, b e c a u s e AI2O3 l a c k s low l y i n g c o n d u c t i o n bands w h i c h m i g h t a c c e p t electron density. More l o c a l i z e d i n t e r a c t i o n s , such as t h o s e b e t w e e n the s u r f a c e atoms and the m e t a l p a r t i c l e s h o u l d be i n e f f e c t i v e i n g r e a t l y a l t e r i n g the c h a r g e on t h e m e t a l p a r t i c l e , b e c a u s e the p r e s e n c e on the s u r f a c e o f b o t h e l e c t r o n a c c e p t o r s , Al ^, and donors, 0~ , should yield mutually compensating effects. +

2

A d d i t i o n o f MX^

A c r o s s CO M u l t i p l e Bonds

The o r g a n i c a n a l o g u e s o f the r e a c t i o n s t o be d i s c u s s e d h e r e a r e the b o r a n e r e d u c t i o n s o f a l d e h y d e s and k e t o n e s and the a d d i t i o n o f m e t a l a l k y l s a c r o s s k e t o n i c c a r b o n y l s , e q u a t i o n 15. In c o n t r a s t t o the ease o f t h e s e o r g a n i c r e a c t i o n s , q u a l i t a t i v e d a t a w h i c h has a c c u m u l a t e d i n our l a b o r a t o r y o v e r the l a s t decade d e m o n s t r a t e s t h a t the c a r b o n y l group i n o r g a n o m e t a l l i c s i s f a i r l y r e s i s t a n t to a d d i t i o n across CO. F o r e x a m p l e , many s t a b l e adducts of organometallic carbonyls w i t h aluminum a l k y l s are known, eq. l c , but under s i m i l a r c o n d i t i o n s a k e t o n e w i l l q u i c k l y r e a c t by a d d i t i o n o f the aluminum a l k y l a c r o s s the CO bond. A s i m i l a r r e a c t i v i t y p a t t e r n i s seen w i t h b o r o n h a l i d e s . T h e r e i s good e v i d e n c e t h a t a d d i t i o n across ketonic CO g r o u p s i s p r e c e e d e d by s i m p l e a d d u c t f o r m a t i o n ( 2 4 - 2 6 ) , and i t i s t h o u g h t t h a t t h i s adduct f o r m a t i o n p o l a r i z e s the c a r b o n y l , m a k i n g t h e c a r b o n s u s c e p t i b l e t o a t t a c k by the R", H~, o r X" n u c l e o p h i l e s , e q u a t i o n 15.

C=0

AlRq

R» :C=OAI^ R» ^

0»

Jfec-OAlRj

(15)

The r e s i s t a n c e o f m e t a l c a r b o n y l s t o a d d i t i o n a c r o s s the CO bond may r e f l e c t the i n f l u e n c e o f the a d j a c e n t e l e c t r o n r i c h m e t a l c e n t e r , w h i c h can d e l o c a l i z e e l e c t r o n d e n s i t y o n t o the c a r -


1.

11


SHRIVER

bonyl carbon. The e l e c t r o n d e n s i t y s h i f t from t h e m e t a l t o t h e c a r b o n y l carbon w i l l thereby p a r t i a l l y o f f s e t the p o l a r i z a t i o n o f t h e c a r b o n y l by t h e L e w i s a c i d , and t h u s m o d e r a t e t h e r e a c t i v i t y o f the c a r b o n y l carbon toward n u c l e o p h i l e s . V i b r a t i o n a l s p e c t r o s c o p i c e v i d e n c e f o r t h i s e l e c t r o n d e r e a l i z a t i o n upon a d d u c t f o r m a t i o n has been c i t e d i n an e a r l i e r s e c t i o n . The d e p r e s s e d r e a c t i v i t y o f t h e CO bond i n m e t a l c a r b o n y l s r e l a t i v e t o o r g a n i c c a r b o n y l s i s not a p p a r e n t i n t h e c a s e o f BH3 and AIH3. F o r example, M a s t e r s and c o w o r k e r s have o b s e r v e d t h a t ^ B ' T H F r e d u c e s m e t a l a c y l compounds t o t h e c o r r e s p o n d i n g a l k y l s , e q . 16. A l t h o u g h no m e c h a n i s t i c s t u d i e s have been r e p o r t e d , i t


0 L M-C-CH n

3

+ H B'THF

>

3

L M-CH CH n

2

(16)

3

was p r o p o s e d t h a t t h i s r e d u c t i o n i s p r e c e e d e d b y t h e f o r m a t i o n o f a BH3 a d d u c t a t t h e a c y l oxygen ( 4 ) . R e c e n t l y , AIH3 r e d u c t i o n s o f m e t a l c a r b o n y l s t o p r o d u c e h y d r o c a r b o n s have been r e p o r t e d as w e l l (27,28). I n v e s t i g a t i o n s i n o u r l a b o r a t o r y b y R e b e c c a S t i m s o n have d e m o n s t r a t e d t h a t i t i s p o s s i b l e t o combine t h e b o r a n e r e d u c t i o n o f a m e t a l a c y l w i t h t h e L e w i s a c i d promoted CO i n s e r t i o n r e a c t i o n w h i c h h a s been d i s c u s s e d e a r l i e r i n t h i s paper ( 2 9 ) . I n t h i s r e a c t i o n , w h i c h i s presumed t o p r o c e e d b y e q u a t i o n 17, t h e

R I L M-CO

"BH3" — ^

n

>

0

0

in

in

C \ ,R V-MrC ΌΒΗ3

>

C \ I^M-C^-R

, _ (17)

BH3 a c t s b o t h as a r e d u c i n g agent f o r t h e a c y l c a r b o n y l and as a p r o m o t i n g agent f o r s u b s e q u e n t CO i n s e r t i o n i n t o t h e m e t a l - a l k y l bond. As y e t t h e p r o c e s s h a s been c a r r i e d as f a r as C^Hg, w i t h M n ( C O ) ( C H ) , CO, and H B-THF as r e a c t a n t s . 5

3

3

CO C o n v e r s i o n t o M e t h y l i d y n e s As p r e v i o u s l y i l l u s t r a t e d i n F i g u r e 1, t h e CO bond o r d e r i s g r e a t l y r e d u c e d b y t h e a d d i t i o n o f an a c c e p t o r t o t h e c a r b o n y l oxygens o f a m e t a l c a r b o n y l . The CO s t r e t c h i n g f r e q u e n c i e s and s t r u c t u r e s o f these adducts bear c l o s e resemblances t o those i n compounds w h i c h a r e r e g a r d e d as m e t h y l e n e ( c a r b e n e ) and m e t h y l idyne (carbyne) complexes. C o m p a r i s o n s between some o f t h e s e a n a l o g u e s a r e g i v e n i n 18a and 18b. ( F o r t h e sake o f c l a r i t y , ^0-AlBr L M^C CH n

N

N

3

3

;0-CH L^H — C CH ;

vs

3

3


(

1

8

a

)


12 0-Α1ΒΓ3 C

O-CHq C

J

Fe

J

/

1 — Fe

vs

\

Co —

\

(18b)

Co

Co

Fe


t e r m i n a l CO l i g a n d s w i l l be o m i t t e d from t h e s t r u c t u r a l r e p r e s e n t a t i o n s of the polynuclear carbonyls throughout the r e s t of the text.) The a n a l o g y w i t h t h e m e t h y l i d e n e s i s s t r e n g t h e n e d by t h e recent discovery that b r i d g i n g carbonyls i n anionic polynuclear c a r b o n y l a n i o n s a r e s u s c e p t i b l e t o a l k y l a t i o n and p r o t o n a t i o n , e q u a t i o n s 19a and 19b ( 3 0 - 3 5 ) . These r e a c t i o n s c o n t r a s t

Fe^

F

e

(19a)

Fe:

CH3SO3F

9H

Fe „ *

. CO-CHq . FeC/

CO"

F

'

Activation of Carbon Monoxide by Carbon and Oxygen Coordination

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