Metal-Induced Transformations of Carbon Dioxide - American

Chapter

4

Metal-Induced Transformations of Carbon Dioxide

Downloaded by UNIV OF BATH on July 4, 2016 | http://pubs.acs.org Publication Date: December 17, 1988 | doi: 10.1021/bk-1988-0363.ch004

Donald J. Darensbourg, Christopher G. Bauch, and Cesar Ovalles Department of Chemistry, Texas A&M University, College Station, TX 77843 The activation of carbon dioxide by homogeneous and heterogeneous metal catalysts, as well as the nature of the stoichiometric insertion processes of these cata lysts, are examined. The kinetic and mechanistic aspects of CO insertion into the M-X bond of M(CO) X complexes (M = W, Cr and X = H, alkyl, aryl, aryloxy, and alkoxy) i s investigated. The mechanism of CO insertion in these systems is described as an associa tive interchange (I ) type mechanism where prior loss of coordinated CO i s not involved in the insertion process. The homogeneous catalytic transformations of CO involve the formation of alkyl formates from alcohols and alkyl halides using the anionic tungsten complexes, W(CO) Y (Y = -OOCCH3-, -μ-H-W(CO) -, and - C l ) , as catalysts. Alumina supported ruthenium clusters were studied for the effect of cluster nuclearity on the rate of CO methanation. It was found that the reactivity paralleled the nuclearity of the catalyst precursor. -

2

5

2

a

2

-

5

-

5

2

The chemistry of one-carbon molecules (better known as Cj chemistry) i s a very exciting area of research f o r the organometallic chemist. The motivation for these e f f o r t s stems from the b e l i e f that the raw material base f o r commercial organic chemicals w i l l s h i f t from o i l to coal, i n the near future, due to the decline of petroleum reserves. Of the raw materials for the C -based industry, carbon monoxide i s the most commonly used and a great deal of the current research e f f o r t i s designed to investigate the activation of t h i s molecule (1-5). An alternative source of chemical carbon i s carbon dioxide, which i s the cheapest and most abundant of the C molecules (6—12). This single-carbon species has been widely neglected mainly because i t i s regarded as a highly stable molecule. For example, i t i s the thermodynamic end-product of many energy producing processes, the most prominent being the combustion of hydrocarbons. Nevertheless, there are many thermodynamically favorable reactions of C 0 which provide useful organic substances (Equations 1-3). The standard x

l

2

0097-6156/88/0363-0026$06.00/0 © 1988 American Chemical Society

Ayers; Catalytic Activation of Carbon Dioxide ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

4.

DARENSBOURG ET AL.

Metal-Induced

27

Transformations

e n t h a l p y changes a s s o c i a t e d w i t h t h e c o r r e s p o n d i n g r e a c t i o n s i n v o l v i n g carbon monoxide a r e l i s t e d f o r comparison. C 0 ( g ) + H ( g )— H C O O H ( A )

-15.7

kJ/mol

CO(g) + H 0 U ) — · ·

-12.9

kJ/mol

2

2

(1) 2

C0 (g) + 2

HCOOHU)

3H (g)~^CH OH(il) + H 0 U ) 2

-130.9 kJ/mol

2

3

(2) CO(g)

2

co (g) + H (g) +


2

-128.1 kJ/mol

+ 2H (g) — ^ C h ^ O H U )

C H O H U ) - ^ H C O O C H U ) + HgOU) 3

2

3

C0(g) + C h ^ O H U ) — * · HCOOCH^A)

-32.2

kJ/mol

-23.3

kJ/mol

(3)

The two p r o c e s s e s a r e o f c o u r s e r e l a t e d by t h e water-gas s h i f t r e a c t i o n ( E q u a t i o n 4 ) . A l t h o u g h carbon d i o x i d e i s more s t a b l e t h a n carbon monoxide by 283 k J / m o l , f o r m a t i o n o f t h e v e r y s t a b l e water C0(g)

+ H 0U) 2

+2.8

•C0 (g) + H ( g ) 2

2

kJ/mol

(4)

m o l e c u l e i n p r o c e s s e s i n v o l v i n g t h e former C s p e c i e s a c c o u n t s f o r t h e thermodynamic s i m i l a r i t y o f t h e s e r e a c t i o n s . Indeed i t i s t h e r e q u i r e m e n t o f an e x t r a mole o f hydrogen i n t h e C 0 r e a c t i o n s f o r water p r o d u c t i o n which makes i t g e n e r a l l y t h e l e s s a t t r a c t i v e process. I n 1985, over 4 m i l l i o n t o n s o f carbon d i o x i d e were produced from n o n - o i l f i e l d s o u r c e s ( V 3 ) . About t h r e e - f o u r t h s o f t h i s carbon d i o x i d e i s produced a s a c o - p r o d u c t i n the manufacture o f ammonia. Recovery a s a c o - p r o d u c t from g r a i n f e r m e n t a t i o n p r o v i d e s a n o t h e r major s o u r c e o f carbon d i o x i d e . I t i s a l s o o b t a i n e d from r e f i n e r y and c h e m i c a l o p e r a t i o n s and n a t u r a l w e l l s . The major commercial uses o f carbon d i o x i d e a r e d e r i v e d from i t s p h y s i c a l p r o p e r t i e s . These uses i n c l u d e r e f r i g e r a t i o n , beverage c a r b o n a t i o n and f i r e e x t i n g u i s h e r s . Only 10? o f t h e carbon d i o x i d e produced i s used i n c h e m i c a l manufacture. C u r r e n t l y , c a r b o n d i o x i d e i s used a s a c h e m i c a l f e e d s t o c k f o r t h e p r o d u c t i o n o f c a r b o x y l i c a c i d s , c a r b o n a t e s , carbon monoxide, and u r e a (14—16). D e s p i t e t h e f a c t t h a t numerous c h e m i c a l r e a c t i o n s u t i l i z i n g carbon d i o x i d e a r e t h e r m o d y n a m i c a l l y advantageous, t h e r e i s o f t e n a s u b s t a n t i a l k i n e t i c b a r r i e r t o t h e i r occurrence. T r a n s i t i o n metal compounds can s e r v e t o c a t a l y z e r e a c t i o n s o f carbon d i o x i d e , i . e . , i n the u t i l i z a t i o n o f carbon d i o x i d e i n s y n t h e t i c o r g a n i c c h e m i s t r y , t r a n s i t i o n m e t a l complexes can s i m u l t a n e o u s l y a c t i v a t e b o t h carbon d i o x i d e and o t h e r s u b s t r a t e m o l e c u l e s such a s hydrogen o r o l e f i n s . We have i n i t i a t e d i n v e s t i g a t i o n s i n t e n d e d t o c h a r a c t e r i z e homogeneous carbon d i o x i d e r e d u c t i o n p r o c e s s e s and o u r r e s u l t s t o date a r e summarized h e r e i n . S p e c i f i c a l l y , our r e s e a r c h c e n t e r s on m e c h a n i s t i c s t u d i e s o f m e t a l c a r b o n y l a n i o n c a t a l y s t s , which r e a d i l y a c t i v a t e C 0 t o C-H and C-C bond f o r m a t i o n , two o f t h e most i m p o r t a n t p r o c e s s e s i n the s y n t h e s i s o f o r g a n i c m a t e r i a l s . l

2

2


28

CATALYTIC ACTIVATION OF CARBON DIOXIDE

INSERTION REACTIONS OF CARBON DIOXIDE A primary concern i n the i n v e s t i g a t i o n of C0 a c t i v a t i o n c a t a l y s i s i s an e x a m i n a t i o n o f the s t o i c h i o m e t r i c r e a c t i o n s t h i s m o l e c u l e undergoes w i t h t r a n s i t i o n m e t a l complexes. The most i m p o r t a n t o f these r e a c t i o n s are the i n s e r t i o n s of C0 i n t o metal-hydrogen, - c a r b o n , and -oxygen bonds, because t h e s e o f t e n r e p r e s e n t the f i r s t s t e p s i n t h e c o n v e r s i o n o f C 0 i n t o o r g a n i c compounds. 2

2

2

Metal Hydrides. I n s e r t i o n o f C 0 i n t o the metal-hydrogen bond o f cis-HM(CO) L~ (M = W, C r ; L = CO, PMe , P ( 0 M e ) ) has been found t o be an e x t r e m e l y f a c i l e p r o c e s s (17-19). T h i s i s i n c o n t r a s t w i t h the i n a b i l i t y o f C 0 t o i n s e r t i n t o t h e metal-hydrogen bond o f the analogous n e u t r a l manganese h y d r i d e s . A l t h o u g h t h e group 6 and group 7 h y d r i d e s a r e i s o e l e c t r o n i c , t h e y have r a t h e r d i f f e r e n t p r o p e r t i e s . The h y d r i d e i n HMn(C0) i s i n f a c t r a t h e r a c i d i c w i t h a pKa o f *»7. I n c o n t r a s t , HCr(C0) "" i s v e r y h y d r i d i c w i t h a g r e a t d e a l o f e l e c t r o n d e n s i t y l o c a t e d a t the h y d r i d e l i g a n d i t s e l f as i n d i c a t e d by i o n p a i r i n g s t u d i e s (20). T h i s e l e c t r o n d e n s i t y at the hydride i s i m p o r t a n t i n i t s i n t e r a c t i o n w i t h the e l e c t r o p h i l i c carbon o f C 0 . By p r o v i d i n g a h i g h l y l o c a l i z e d n e g a t i v e c h a r g e , the a n i o n i c h y d r i d e a t t r a c t s the carbon d i o x i d e i n c l o s e t o the m e t a l c e n t e r promoting the o r b i t a l o v e r l a p s n e c e s s a r y f o r the f o r m a t i o n o f the i n s e r t i o n product. Another a n i o n i c h y d r i d e w h i c h a l s o undergoes C 0 i n s e r t i o n i s t h e c l u s t e r complex H R u ( C 0 ) " which g i v e s the b r i d g i n g formate complex H C 0 R u ( C 0 ) ~ ( 2 J _ ) . The i n s e r t i o n r e a c t i o n f o r t h e c l u s t e r h y d r i d e i s not n e a r l y as f a c i l e as t h a t o f the group 6 monomers. The ruthenium complex r e q u i r e s h i g h temperatures and p r e s s u r e s o f C 0 i n o r d e r f o r i n s e r t i o n t o o c c u r . The r e a s o n f o r the d i f f i c u l t y may stem from t h e d e r e a l i z a t i o n o f the n e g a t i v e charge between t h r e e m e t a l atoms which would not a l l o w f o r as s t r o n g an i n t e r a c t i o n w i t h C 0 as i s the case f o r t h e monomers. 2

lt

3

3


2

5

5

2

2

3

2

3

l l

l 0

2

2

M e t a l A l k y l s and A r y l s . The i n s e r t i o n o f C 0 i n t o m e t a l - c a r b o n bonds a l l o w s f o r the f o r m a t i o n o f carbon-carbon bonds and i s an i m p o r t a n t s t e p i n i t s a c t i v a t i o n . I n an e f f o r t t o f u r t h e r d e f i n e t h e n a t u r e o f the C 0 i n s e r t i o n p r o c e s s , we have examined i t s o c c u r r e n c e i n the a n i o n i c group 6 complexes, cis-RM(CO)„L" (R = -CH , - C H , - C H , - C H C H ; M = W, Cr; L = CO, PMe , P ( 0 M e ) ) ( 2 2 , 2 3 ) . Carbon d i o x i d e was found t o i n s e r t smoothly i n t o t h e m e t a l carbon bond t o form t h e c o r r e s p o n d i n g c a r b o x y l a t e complexes ( E q u a t i o n 5 ) . The i d e n t i t y o f t h e s e c a r b o x y l a t e s was c o n f i r m e d by comparison w i t h an a u t h e n t i c 2

2

3

2

6

5

3

[M]-R

+

2

5

6

5

3

C0 —[M]-0 CR 2

(5)

2

sample p r e p a r e d by the r e a c t i o n o f the m e t a l c h l o r i d e w i t h t h e s i l v e r s a l t of the c a r b o x y l i c a c i d . I n v e s t i g a t i o n s i n t o the k i n e t i c s o f C 0 i n s e r t i o n have r e v e a l e d t h a t the r e a c t i o n i s f i r s t o r d e r i n both m e t a l s u b s t r a t e and C 0 (Equation 6 ) . F i g u r e 1 i l l u s t r a t e s the l i n e a r dependence of t h e 2

2

r a t e = k-[CH W(CO) ] [ C 0 ] = k . [CHJrf(CO) ] d 3 5 2 ODS 3 ο 0

K

o


(6)

4. DARENSBOURG ET AL.

Metal-Induced

29

Transformations

p s e u d o - f i r s t - o r d e r r a t e c o n s t a n t (k ) on C 0 p r e s s u r e f o r t h e c a r b o x y l a t i o n of CH W(C0) ~. A t p r e s s u r e s above 200 p s i however, the c o n c e n t r a t i o n o f C 0 i n c r e a s e s more r a p i d l y w i t h f u r t h e r i n c r e a s e i n p r e s s u r e due t o a breakdown i n Henry*s law a t e l e v a t e d p r e s s u r e s . S e v e r a l f a c t o r s e f f e c t the r a t e o f C 0 i n s e r t i o n i n t o m e t a l carbon bonds. The most n o t a b l e o f these i s the e l e c t r o n d e n s i t y a t the metal center. As i s the case f o r the h y d r i d e (vide supra), the i s o e l e c t r o n i c n e u t r a l manganese and rhenium analogues o f the group 6 a l k y l s do not undergo C 0 i n s e r t i o n . D e m o n s t r a t i o n o f the d r a m a t i c e f f e c t t h a t the e l e c t r o n d e n s i t y , a t the m e t a l c e n t e r , has on C 0 i n s e r t i o n can be seen when the e l e c t r o n d o n a t i n g p r o p e r t y o f the l i g a n d , L, i n cis-CH W(C0)„ΙΓ, i s v a r i e d . T a b l e I shows t h a t the second o r d e r r a t e c o n s t a n t i n c r e a s e s by two o r d e r s of magnitude when CO i s r e p l a c e d by the b e t t e r e l e c t r o n d o n a t i n g l i g a n d s P ( 0 M e ) and PMe . As e x p e c t e d , the more b a s i c PMe l i g a n d has a g r e a t e r e f f e c t than t h a t o f P ( 0 M e ) . 2

3

5

2

2

2

2

3


3

3

3

3

T a b l e I . Second-order r a t e c o n s t a n t s i n t o cis-CH W(C0) L d e r i v a t i v e s .

f o r carbon d i o x i d e i n s e r t i o n

8

3

L

H

k

2

χ 10

CO

3.46

P(OMe)

2.00

PMe^

8.40

(Sec -M

χ 10~

)

2

R e l a t i v e Rates

1.00 57.8 243

R e a c t i o n s c a r r i e d out i n t e t r a h y d r o f u r a n p r e s s u r e o f 760 t o r r .

at 25°C at a carbon d i o x i d e

A l t h o u g h t h e r a t e o f C 0 i n s e r t i o n i n t o the m e t a l carbon bond i s o v e r 200 times f a s t e r than the p a r e n t c a r b o n y l , X—ray c r y s t a l s t r u c t u r e s have determined (22)(Darensbourg D. J . ; Bauch, C. G.; R h e i n g o l d , A. L. I n o r g . Chem., i n p r e s s ) t h a t the M-C bond d i s t a n c e i s somewhat s h o r t e r f o r the phosphine s u b s t i t u t e d complex (Table I I ) . T h i s i n d i c a t e s t h a t the s t r e n g t h of the m e t a l carbon bond, as e v i n c e d by the M-C bond d i s t a n c e , i s o f secondary importance compared t o t h e e l e c t r o n d e n s i t y a t the metal c e n t e r . T h i s i s p l a i n l y e v i d e n t when comparing the n e u t r a l C H R e ( C 0 ) w i t h the a n i o n i c CH W(C0) ". Both were determined t o have n e a r l y i d e n t i c a l m e t a l — a l k y l carbon bond d i s t a n c e s ( w i t h i n esd's) (24)(Darensbourg D. J . ; Bauch, C. G. ; R h e i n g o l d , A. L. I n o r g . Chem., i n p r e s s ) . However, o n l y the a n i o n has been found to undergo C 0 i n s e r t i o n . The mechanism o f the C 0 r e a c t i o n i s b e l i e v e d t o proceed t h r o u g h an a s s o c i a t i v e i n t e r c h a n g e ( I ) type mechanism w i t h a t r a n s i t i o n s t a t e s i m i l a r t o t h a t shown i n F i g u r e 2. The presence of CO d i d not 2

3

5

3

5

2

2



30


k

Figure 1. Plot of the pseudo-first-order rate constant ( h ) as a function of carbon dioxide pressure f o r the carboxylation of CH W(C0) " i n THF at 25°C. 0

Figure 2.

Transition state f o r C0

o

i n s e r t i o n into W-R

S d

bond.



Metal-Induced

T a b l e I I . Tungsten-CH

Bond D i s t a n c e s .

3

d

c

3


c

3

2.18Â

cis-(Me^P)(CO) W-CH " (C0) Re-CH

M-CH

2.325Â

(CO) W-CH ~ 4

31

Transformations

2.308Â

o

r e t a r d the r a t e o f C 0 i n s e r t i o n (vide supra) i n d i c a t i n g t h a t p r i o r l o s s o f c o o r d i n a t e d CO was not i n v o l v e d . The a c t i v a t i o n parameters f o r C 0 i n s e r t i o n i n t o C H W ( C 0 ) P ( 0 M e ) ~ were determined ( 2 0 ) . The ΔΗ* and AS* o f 42.68 k j m o l e " and -181.2 eu r e s p e c t i v e l y were i n d i c a t i v e o f a mechanism i n v o l v i n g a h i g h l y o r d e r e d t r a n s i t i o n s t a t e w i t h l i t t l e net bond b r e a k i n g . A b a c k s i d e a t t a c k o f C 0 , on the a l k y l c a r b o n , analogous t o t h e S 0 i n s e r t i o n mechanism was r u l e d out by s t u d i e s o f the α-carbon s t e r e o c h e m i s t r y upon i n s e r t i o n ( 2 5 ) . The i n s e r t i o n o f C 0 i n t o t h e m e t a l carbon bond o f threo-cis-W(CO) (L)(CHD-CHD-Ph)" ( L = CO and PMe ) proceeds w i t h r e t e n t i o n o f c o n f i g u r a t i o n a t the α-carbon (Scheme 1) ( 2 6 ) . T h i s i s i n c o n t r a s t t o the i n v e r s i o n o f c o n f i g u r a t i o n a t the a l p h a carbon found i n b a c k s i d e S 0 i n s e r t i o n reactions. A l t h o u g h CO doesn't r e t a r d the r a t e o f C 0 i n s e r t i o n i t does compete w i t h C 0 f o r a c o n c u r r e n t i n s e r t i o n r e a c t i o n i n t o the m e t a l carbon bond ( 2 3 ) . Thus, t h e a n i o n i c a l k y l complexes o f the group 6 m e t a l s p r o v i d e a unique o p p o r t u n i t y t o compare c a r b o n y l a t i o n and c a r b o x y l a t i o n r e a c t i o n under comparable c o n d i t i o n s . The r e a c t i v i t y o f RW(C0) ~ toward CO p a r a l l e l s t h a t o f RMn(C0) (27-31). The m e c h a n i s t i c a s p e c t s o f the c a r b o n y l a t i o n and c a r b o x y l a t i o n r e a c t i o n a r e summarized and compared i n T a b l e I I I . In studies involving s i m u l t a n e o u s i n s e r t i o n s o f b o t h CO and C 0 w i t h CH W(C0) ", i t was found t h a t the r a t e o f d i s a p p e a r a n c e o f the a l k y l complex was a p p r o x i m a t e l y e q u a l t o t h e sum o f the i n d i v i d u a l l y determined r a t e parameters. Hence, the two p r o c e s s e s o c c u r s i m u l t a n e o u s l y and i n d e p e n d e n t l y o f one a n o t h e r . These d i s s i m i l a r i t i e s (dependences on m e t a l , R group, and a n c i l l a r y l i g a n d s ) i n C 0 and CO i n s e r t i o n p r o c e s s e s can be e x p l o i t e d i n p r e f e r e n t i a l l y a f f e c t i n g carbon-carbon bond f o r m i n g r e a c t i o n s i n v o l v i n g carbon d i o x i d e i n the presence o f carbon monoxide. The consequence o f the m e c h a n i s t i c d i f f e r e n c e s s h o u l d be o f major importance i n c a t a l y t i c p r o c e s s e s d e s i g n e d t o u t i l i z e carbon d i o x i d e as a s o u r c e o f c h e m i c a l c a r b o n . 2

2

3

H

3

1

2

2

2

h

3

2

2

2

5

5

2

3

5

2

M e t a l A l k o x i d e s . N o t w i t h s t a n d i n g our u n d e r s t a n d i n g o f t h e m e c h a n i s t i c a s p e c t s o p e r a t i v e i n i n s e r t i o n r e a c t i o n o f carbon d i o x i d e w i t h m e t a l - h y d r i d e and m e t a l - c a r b o n ( a l k y l s o r a r y l s ) bonds has


32


T a b l e I I I . Summary o f M e c h a n i s t i c A s p e c t s o f C a r b o n y l a t i o n vs Carboxylation Reactions.


Reaction Variables

Carbonylation

Carboxylation

3

K i n e t i c order i n C0 or CO

First-order i n C0

Nature of Metal

T h i r d row more r e a c t i v e than f i r s t row

F i r s t row more r e a c t i v e than t h i r d row

R dependence

S m a l l dependence on R group, a l k y l s f a s t e r than a r y l s

Reaction g r e a t l y retarded by e l e c t r o n - w i t h d r a w i n g R substituents

Ancillary

S t e r i c a l l y nonencumb e r i n g phosphorus donor l i g a n d s greatly accelerate reaction

Little

Retention of configuration

Retention of configuration

2

2

ligands

Stereochemi stry at α-carbon

Mixed-order i n CO; independent o f CO a t h i g h CO p r e s s u r e s

effect

These o b s e r v a t i o n s have been e x t e n s i v e l y noted f o r RMn(C0) , and t h e more l i m i t e d s t u d y on t h e group 6 a n i o n i c a n a l o g s r e p o r t e d h e r e i n i s i n complete agreement w i t h these g e n e r a l i z a t i o n s . 5


4.

DARENSBOURG ET AL.

Metal-Induced

33

Transformations

undergone s i g n i f i c a n t advancement, analogous p r o c e s s e s i n v o l v i n g m e t a l - a l k o x i d e s have been l e s s w e l l - i n t e r p r e t e d (32-34). We have r e c e n t l y s y n t h e s i z e d W(C0) 0R~ (R = Ph, C h\CH -m) d e r i v a t i v e s from W(C0) THF and [Et„N][OR] s a l t s (Darensbourg, D. J . ; Sanchez, Κ. M.; R h e i n g o l d , A. L. J . Am. Chem. S o c , i n p r e s s ) . I n s o l u t i o n t h e s e s p e c i e s a r e e x t r e m e l y CO l a b i l e , decomposing r e a d i l y t o W„(CO) ( y - 0 R ) ~ i n t h e absence o f a CO atmosphere ( 3 5 ) . Treatment o f [Et„N][W(CO) 0R] w i t h C 0 i n THF under m i l d c o n d i t i o n s (< 1 atmosphere C 0 a t ambient temperature) r e s u l t s i n r a p i d p r o d u c t i o n o f W(CO) 0C(0)0R~, t h e product o f C 0 i n s e r t i o n i n t o the W-OR bond. The t u n g s t e n p e n t a c a r b o n y l a r y l carbonate complex was c h a r a c t e r i z e d by I R , *H, and C ΝMR s p e c t r o s c o p i e s . A d d i t i o n o f s m a l l q u a n t i t i e s o f water t o s o l u t i o n s o f W(CO) 0C(0)0R" causes b r i g h t orange c r y s t a l s o f [Et N] [W(C0) (n -C0 )]·Η 0 t o p r e c i p i t a t e from s o l u t i o n . The m o l e c u l a r s t r u c t u r e o f t h e t u n g s t e n t e t r a c a r b o n y l carbonate was unambiguously a s s i g n e d on t h e b a s i s o f an X-ray c r y s t a l l o g r a p h i c d e t e r m i n a t i o n . T h i s c h e m i s t r y i s summarized i n Scheme 2 (Darensbourg, D. J . ; Sanchez, Κ. M.; R h e i n g o l d , A. L. J . Am. Chem. S o c , i n p r e s s ) . Because o f t h e extreme CO l a b i l i t y o f t h e W(C0) 0R" s p e c i e s , CO l o s s might be a p r e r e q u i s i t e f o r C 0 i n s e r t i o n . However, t h e r a t e o f C 0 i n s e r t i o n i s n o t i n h i b i t e d by t h e presence o f carbon monoxide. Hence, we b e l i e v e t h a t an open c o o r d i n a t i o n s i t e i s unnecessary f o r the i n s e r t i o n p r o c e s s t o o c c u r . The r e a c t i o n i s thought t o i n v o l v e a c o n c e r t e d i n s e r t i o n p r o c e s s , s i m i l a r t o t h a t proposed and w e l l documented f o r t h e i n s e r t i o n o f C 0 i n t o CH W(C0) -. N e v e r t h e l e s s , i n s e r t i o n o f C 0 i n t o t h e W-OR bond i s more f a c i l e than t h e c o r r e s p o n d i n g p r o c e s s i n v o l v i n g W-R. Attempts t o induce t h e i n s e r t i o n o f CO i n t o t h e W-OPh bond o f (C0) W0Ph~ have f a i l e d a t CO p r e s s u r e s up t o 500 p s i . I n s i t u h i g h p r e s s u r e FTIR measurements r e v e a l e d o n l y t h e presence o f t h e s t a r t i n g phenoxide t u n g s t e n complex. 5

6

3

5

, f

1 2

3

l t

5

2

2

5

2

1 3

5

2


H

2

H

3

2

5

2

2

2

3

5

2

5

C a t a l y t i c P r o c e s s e s U s i n g CO, Promoted by T r a n s i t i o n M e t a l Complexes I n t h i s s e c t i o n we w i s h t o i n c o r p o r a t e o u r knowledge o f t h e scope o f t r a n s i t i o n m e t a l - C 0 c h e m i s t r y i n e x p l o r i n g p o t e n t i a l uses o f C 0 as a f e e d s t o c k i n t h e s y n t h e s i s o f f i n e and b u l k c h e m i c a l s . 2

2

A l k y l Formate P r o d u c t i o n . I n t h e past few y e a r s , formate e s t e r s have become an i m p o r t a n t c l a s s o f o r g a n i c compounds m a i n l y because o f t h e i r v e r s a t i l i t y as c h e m i c a l f e e d s t o c k (16,36—42), and as raw m a t e r i a l s f o r t h e perfume and f r a g r a n c e i n d u s t r y (43—46). S p e c i f i c a l l y , formate e s t e r s ( m e t h y l , e t h y l , p e n t y l , e t c ) have been used as s t a r t i n g m a t e r i a l f o r t h e p r o d u c t i o n o f aldehydes ( 3 6 ) , ketones ( 3 6 ) , c a r b o x y l i c a c i d s (37-40), and amides (4,2). F o r example, methyl formate can be h y d r o l y z e d t o f o r m i c a c i d (39,40) o r c a t a l y t i c a l l y i s o m e r i z e d t o a c e t i c a c i d ( 3 8 ) . On t h e o t h e r hand, a l k y l formates have been employed i n t h e perfume and f r a g r a n c e i n d u s t r y i n amounts o f a p p r o x i m a t e l y 1000 t o 3000 l b / y e a r (43-46). Among t h e formates t h a t have been commonly used f o r these purposes a r e : o c t y l (4j3), h e p t y l ( 4 4 ) , e t h y l ( 4 5 ) , and amyl (46) f o r m a t e s . Our r e c e n t i n t e r e s t i n t h e c h e m i s t r y o f carbon d i o x i d e (9,12) has i n c l u d e d i n v e s t i g a t i o n s o f t h e s y n t h e s i s o f a l k y l formates u t i l i z i n g C 0 as a source o f c h e m i c a l carbon ( E q u a t i o n 7) (47,48). 2


34


Ph

H

Ph

N02CW(C0)(L)J i t ) •* 4

•PNPCQT»)

Να*

NoCOTtJ

-NoCOTs)

OTs

W(C0) (L) 4

êrythro

threo

Ph

Ph

•co

PNP*

2

400 pti W(C0) lL)


PNP* C-0-W(C0) (L)

4

II

threo

0 L =. CO

threo

Me P

}

3

Scheme 1.

ROW(CO) " • C0 — R O C 0 W ( C O ) " 5

2

2

5

1/2 H 0

-CO +C0

2

\

[R0W(C0) )^ 3

COjWiCO)^

3

3 [ROW(CO) ] 3

-4

cc^wico)^

M

Scheme 2.


4

4. DARENSBOURG ET AL. C0

2

+ H

Metal-Induced

2

+ ROH

L

U

d

U

35

Transformations

V

HC0 R + H 0 2

(7)

2

The c a t a l y s t s o r c a t a l y s t s p r e c u r s o r s employed i n t h e s e s t u d i e s were a n i o n i c group 6 c a r b o n y l complexes (50) o r group 8 m e t a l c a r b o n y l c l u s t e r s (37-40), where r e a c t i o n c o n d i t i o n s were 500 p s i ( C 0 / H ) and 125° C. For t h e group 6 m e t a l c a t a l y s t s , t h e t u r n - o v e r numbers o b t a i n e d f o r t h e m e t h y l f o r m a t e p r o d u c t i o n were c a . 15 u s i n g methanol as s o l v e n t f o r a 24 hour p e r i o d . The a n i o n i c m e t a l c a r b o n y l s examined a s c a t a l y s t s p r e c u r s o r s i n c l u d e d : H M ( C 0 ) ~ , H C 0 M ( C 0 ) " , and 0Η 00 Μ(00) ·" a s t h e i r PPN s a l t s (PPN - b i s ( t r i p h e n y l p h o s p h i n e ) iminium and M - Cr o r W). The proposed r e a c t i o n pathway i s d e p i c t e d i n Scheme 3. Congruous w i t h t h e c a t a l y t i c c y c l e r e p r e s e n t e d i n Scheme 3 t h e m e t a l l o f o r m a t e d e r i v a t i v e s a r e e x t r e m e l y CO l a b i l e , s p e c i f i c a l l y a t e q u a t o r i a l CO s i t e s a s demonstrated by C 0 l a b e l l i n g s t u d i e s ( e q u a t i o n 8 ) . The d i s s o c i a t i o n o f CO i s i m p o r t a n t i n c a t a l y s i s , f o r the a d d i t o n o f CO i n h i b i t s t h e c a t a l y t i c hydrogénation o f carbon d i o x i d e . The n a t u r e o f t h e s p e c i e s (boxed) i n Scheme 3 i s thought t o i n v o l v e no change i n t h e m e t a l ' s o x i d a t i o n s t a t e , i . e . , a l i g a n d a s s i s t e d h e t e r o l y t i c s p l i t t i n g o f d i h y d r o g e n (49-51). That t h e d i s t a l oxygen atom o f monodentate f o r m a t e s o r c a r b o x y l a t e s i s e x t r e m e l y b a s i c , i s seen i n i t s a b i l i t y t o i n t e r a c t w i t h k r y p t o f i x - 2 2 1 e n c a p s u l a t e d sodium i o n ( F i g u r e 3) ( 5 2 ) . 2

2

3

2

1 0

2

2

5

5


9

1 3

C a r b o x y l a t i o n o f methanol ( E q u a t i o n 9 ) , where carbon monoxide o r g i n a t e s from t h e r e v e r s e water-gas s h i f t r e a c t i o n ( E q u a t i o n 4 ) , was r u l e d out a s a p o s s i b l e r o u t e t o methyl formate i n t h i s i n s t a n c e . T h i s c o n c l u s i o n i s based on t h e o b s e r v a t i o n t h a t when ( C 0 ) W 0 C H " was u t i l i z e d a s t h e c a t a l y s t i n t h e presence o f C 0 , o n l y H C 0 M e was d e t e c t e d i n i t i a l l y v i a GC-MS. A d d i t i o n a l l y , when C H C 0 W ( C 0 ) " was employed a s t h e c a t a l y s t t h e f i r s t formed product i n q u a n t i t a t i v e y i e l d was CH C0 H, w i t h subsequent e s t e r i f i c a t i o n a f f o r d i n g CH C0 CH . 13

5

l 2

2

3

3

3

2

2

l2

2

2

5

2

3

CO + MeOH

Ψ~

HC0 Me 2

(9)

A l t h o u g h we have been a b l e t o demonstrate t h a t methyl formate i s d e r i v e d d i r e c t l y from carbon d i o x i d e , i t i s p o s s i b l e , employing t h e same m e t a l c a r b o n y l c a t a l y s t p r e c u r s o r s , t o c a t a l y z e t h e p r o d u c t i o n of methyl formate from t h e r e a c t i o n o f CO and methanol ( E q u a t i o n 9 ) . A c a t a l y t i c c y c l e c o n s i s t e n t w i t h our c u r r e n t knowledge i n t h i s a r e a i s r e p r e s e n t e d i n Scheme 4 ( 5 3 ) ( D a r e n s b o u r g , D. J . ; Gray, R. L.; O v a l l e s , C. J . M o l e c . C a t a l . , i n p r e s s ) . I t i s r e l e v a n t t o note h e r e i n t h a t carbon d i o x i d e i s a p o i s o n t o t h e c a t a l y t i c p r o c e s s diagrammed i n Scheme 4 i n t h a t i t r e a c t s w i t h t h e c o - c a t a l y s t , methoxide, t o produce m e t h y l c a r b o n a t e .




36


Metal-Induced

Transformations



Scheme 4.


37

38


M e c h a n i s t i c a s p e c t s o f carbon d i o x i d e hydrogénation p r o c e s s e s ( E q u a t i o n 7) c a r r i e d out i n s o l v e n t s o t h e r t h a n methanol, e.g., e t h a n o l o r p r o p a n o l , were c o m p l e t e l y c o n s i s t e n t w i t h those noted i n methanol s o l v e n t . However, the c a t a l y t i c a c t i v i t i e s were c o n s i d e r a b l y lower i n t h e s e a l c o h o l s as compared w i t h methanol, by a f a c t o r ofA#3. T h i s e f f e c t was a t t r i b u t e d t o a s o l v e n t i n h i b i t i o n o f the a d d i t i o n o f d i h y d r o g e n t o the u n s a t u r a t e d m e t a l s p e c i e s . For t h i s r e a s o n , we have t u r n e d our a t t e n t i o n towards the r e a c t i o n o f a l k y l h a l i d e s w i t h C 0 and H i n o r d e r t o p r o v i d e a more e f f e c t i v e pathway t o h i g h e r m o l e c u l a r weight a l k y l formates ( b u t y l or o c t y l ) u s i n g C 0 as a s o u r c e of carbon ( E q u a t i o n 10). A n i o n i c group 6 m e t a l complexes were used as c a t a l y s t s , and the presence o f a sodium s a l t (NaHC0 or NaOCH ) was r e q u i r e d i n o r d e r t o r e g e n e r a t e the c a t a l y t i c a l l y a c t i v e i n t e r m e d i a t e s (Darensbourg, D. J . ; O v a l l e s , C. J . Am. Chem. S o c , i n p r e s s ) . A g e n e r a l c a t a l y t i c c y c l e f o r the p r o d u c t i o n o f formate e s t e r s s t a r t i n g from a l k y l h a l i d e s , C 0 , and H i s shown i n Scheme 5. 2

2

2


3

3

2

RX + C 0

H

2

+ 2

^aY^

H C Q

2

R

+

N a X

+

H Y

2

( 1 0 )

M e c h a n i s t i c a s p e c t s o f t h i s c a t a l y t i c p r o c e s s have been f o r t h c o m i n g from k i n e t i c i n v e s t i g a t i o n s o f the component r e a c t i o n s which comprise the proposed c y c l e . The r a t e - l i m i t i n g p r o c e s s i n the c a t a l y t i c c y c l e i s the r e a c t i o n between the a n i o n i c h a l i d e complexes,

Scheme 5.


4.

DARENSBOURG ET AL.

Metal-Induced

39

Transformations

M(C0) X", and H i n the presence o f a g e n e r a l base t o p r o v i d e a n i o n i c m e t a l h y d r i d e s . T h i s p r o c e s s was shown t o be f i r s t - o r d e r i n both m e t a l complex and d i h y d r o g e n and was not i n h i b i t e d by a d d i t i o n o f carbon monoxide. C o n s i s t e n t w i t h the r d s i n c a t a l y s i s b e i n g f o r m a t i o n o f the m e t a l h y d r i d e i n t e r m e d i a t e , t h e m e t a l c a t a l y z e d r e a c t i o n o f RX/C0 /H t o p r o v i d e HCOOR i s not i n h i b i t e d by CO. The w e l l - e s t a b l i s h e d f o r m a t i o n o f m e t a l l o f o r m a t e , M ( C 0 ) 0 C H " , from M(C0) H~ and C 0 i s f o l l o w e d by a l e s s f a c i l e p r o c e s s i n v o l v i n g t h e r e a c t i o n o f the m e t a l l o f o r m a t e w i t h RX. T h i s l a t t e r r e a c t i o n i s f i r s t - o r d e r i n both m e t a l complex and a l k y l h a l i d e and i s i n h i b i t e d by carbon monoxide. 5

2

2

2

5

5

2

2

C 0 M e t h a n a t i o n . F i n a l l y , we have s t u d i e d the c a t a l y t i c a c t i v i t y and s e l e c t i v i t y toward methanation o f carbon d i o x i d e u s i n g s e v e r a l a l u m i n a s u p p o r t e d ruthenium c l u s t e r s i n c l u d i n g R u ( C 0 ) , KHRu (C0) , [PPN][HC0 Ru (C0) ], H R u ( C 0 ) , KH Ru„(CO) , [PPN][H Ru„(C0) ], and R u C ( C 0 ) ( 5 4 ) . Comparative s t u d i e s were made w i t h the mononuclear complexes R u C l and R u ( C 0 ) . The l a t t e r s p e c i e s p r o v i d e s a l o w - v a l e n t , o r g a n o m e t a l l i c , mononuclear ruthenium s o u r c e . C a t a l y s t s were s u p p o r t e d by i m p r e g n a t i o n over alumina ( p a r t i a l l y d e h y d r o x y l a t e d a t 150° C i n vacuo) and a c t i v a t e d i n hydrogen a t 200° C. C a t a l y s t c h a r a c t e r i z a t i o n i n c l u d e d d i f f u s e r e f l e c t a n c e i n f r a r e d s p e c t r o s c o p y , s u r f a c e a r e a d e t e r m i n a t i o n , and e l e c t r o n m i c r o s c o p y . I n g e n e r a l , t h e c l u s t e r d e r i v e d c a t a l y s t s were more a c t i v e than t h e a n a l o g o u s l y prepared c a t a l y s t o b t a i n e d from R u C l , e.g., a t 180° C t h e c a t a l y s t d e r i v e d from R u C ( C 0 ) was 22 t i m e s more a c t i v e than t h a t d e r i v e d from R u C l . The a c t i v i t y o f t h e s u p p o r t e d n e u t r a l s p e c i e s was observed t o i n c r e a s e a s the number o f ruthenium atoms p r e s e n t i n the p r e c u r s o r complex i n c r e a s e , i . e . , Ru(C0) < R u ( C 0 ) < Ru C(C0) .


2

3

3

l I

2

3

3

12

1 0

H

6

k

1 2

l 2

3

l2

1 7

3

5

3

6

1 7

3

5

3

1 2

6

1 7

Acknowledgments. The a u t h o r s a r e most g r a t e f u l t o the 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 , whose s u p p o r t has made p o s s i b l e t h e i r c o n t r i b u t i o n s t o t h e r e s e a r c h d e s c r i b e d h e r e i n . They a r e l i k e w i s e a p p r e c i a t i v e t o a l l t h e i r c o l l e a g u e s mentioned i n t h e r e f e r e n c e s , whose many o r i g i n a l c o n t r i b u t i o n s have made t h i s such an e x c i t i n g a r e a o f r e s e a r c h t o be i n .

Literature Cited 1. Henrici-Olive, G.; Olive, S. J. Mol. Cat. 1978, 3, 443. 2. Henrici-Olive, G.; Olive, S. Angew. Chem. Int. Ed. 1976, 15, 136. 3. Masters, C. Adv. Organomet. Chem. 1979, 17, 61. 4. Henrici-Olive, G.; Olive, S. The Chemistry of the Catalyzed Hydrogenation of Carbon Monoxide, Springer-Verlag; Berlin, Heidelberg, Germany,1984. 5. Keim, W. Catalysis in C Chemistry; Keim W., Ed.; Reidel D. Publishing Co., Dorecht, Holland, 1983, p. 5. 6. Eisenberg, R.; Hendricksen, D. E. Adv. Catal. 1979, 28, 79. 7. Denise, Β.; Sneeden, R. P. Chem. Tech. 1982, 12, 108. 8. Lapidus, A. L.; Ping, Y. Y. Russ. Chem. Rev. 1981, 50, 63. 9. Darensbourg, D. J . ; Kudaroski, R. A. Adv. Organomet. Chem. 1983, 22, 129. 1




40

10. Sneeden, R. P. A. In Comprehensive Organometallic Chemistry; Wilkinson, G.; Stone, F. G. Α.; Abel, E. W., Eds.; Pergamon Press: Oxford, 1982; Vol. 8, p.225. 11. Ito, T.; Yamamoto, A. Organic and Bio-Organic Chemistry of Carbon Dioxide; Inoue, S.; Yamazaki, Ν., Eds.; Kondonsha, Ltd.: Toyko, Japan, 1982, p 79. 12. Darensbourg, D. J . ; Ovalles, C. Chem. Tech. 1985, 15, 636. 13. Chem. Eng. News, 1985, 63, 10. 14. Czoukowski, M. P.; Bayne, A. R. Hydrocarbon Process. 1980, 59, 103. 15. Leonard, J. D. U.S. Patent 4 299 981 (1981). 16. Ikaraski, T. Chem. Economy & Eng. Rev. 1980, 12, 31 17. Darensbourg, D. J.; Rokicki, A. Organometallics 1982, 1, 1685. 18. Slater, S. G.; Lusk, R.; Schumann, B. F.; Darensbourg, M. Organometallics 1982, 1, 1662. 19. Darensbourg, D. J . ; Rokicki, A.; Darensbourg, M. Y. J. Am. Chem. Soc.

1981,

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Metal-Induced Transformations of Carbon Dioxide - American

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