7 The Importance of Reactions of Oxygen Bases with Metal Carbonyl Derivatives in Catalysis Homogeneous Catalysis of the Water Gas Shift Reaction D O N A L D J . D A R E N S B O U R G and A N D R Z E J
ROKICKI
1
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Department of Chemistry, Tulane University, New Orleans, LA 70118
Base c a t a l y s i s of ligand s u b s t i t u t i o n a l processes of metal carbonyl complexes i n the presence of oxygen donor bases may be apportioned into two d i s t i n c t c l a s sifications. The first category of reactions involves n u c l e o p h i l i c a d d i t i o n of oxygen bases at the carbon center i n metal carbonyls with subsequent oxidation of CO to CO , eqns. 1 and 2 (1, 2 ) . Secondly, there are 2
reactions involving coordination of the oxygen base, with the thus formed metal-oxygen bond greatly lowering the energetics f o r d i s s o c i a t i v e carbon monoxide displacement (eq.3)(3,4).
An essential step in processes utilizing soluble t r a n s i t i o n metal c a t a l y s t s i s the coordination of the substrate to the t r a n s i t i o n metal (5). A corequisite i s the a v a i l a b i l i t y of a vacant s i t e i n the coordinat i o n sphere of the metal for substrate binding, a p r o v i s i o n often met by d i s s o c i a t i o n of a bonded 1
C u r r e n t address: I n s t i t u t e of Organic Chemistry and Technology, Technical u n i v e r s i t y of Warsaw (Politechnika), 00-662 Warszawa, ul. Koszykowa 75, Poland. 0097-6156/81/0152-0107$05.00/0 © 1981 American Chemical Society Ford; Catalytic Activation of Carbon Monoxide ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
CATALYTIC
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108
ACTIVATION
O F CARBON
MONOXIDE
ligand. Hence, processes such as those described i n eqns. 1 and 3 are not only u s e f u l from a synthetic viewpoint but also can serve to a c t i v a t e the metal carbonyl i n c a t a l y t i c reactions. More pertinent to the tenor o f t h i s Symposium, the carbon monoxide oxidation r e a c t i o n depicted i n eq. 2 has received renewed a t t e n t i o n l a r g e l y because of i t s p i v o t a l r o l e during the homogeneous c a t a l y s i s of the water gas s h i f t r e a c t i o n (WGSR) by a v a r i e t y o f metal carbonyls (2,(5). In t h i s correspondence we wish to discuss reaction processes o f importance to the homogeneous c a t a l y s i s o f the WGSR u t i l i z i n g metal car bonyls. P a r t i c u l a r emphasis w i l l be placed on the r e l a t i v e rates of oxygen-exchange vs. metal-hydride bond formation f o r several metal carbonyls; i n c l u d i n g group 6b metal carbonyls and d e r i v a t i v e s thereof, Pe(C0) , and R u ( C 0 ) i . Summarized are our recent investigations on the species present i n s o l u t i o n and t h e i r r e a c t i v i t y patterns during the homogeneous c a t a l y s i s o f the WGSR by group 6b metal carbonyls under mild r e a c t i o n conditions ( l atmosphere CO pres sure and temperature ζ 100°c). 5
2
3
Results and Discussion The l a b i l i t y o f oxygen atoms i n the activated carbon monoxide ligands of R e ( C 0 ) was demonstrated by Muetterties i n 1965 (7). Rhenium hexacarbonyl cation underwent a f a c i l e exchange process with the oxygen atoms i n oxygen-18 enriched water, and the intermediacy of a metallocarboxylic acid was proposed (eq. 4). Deeming and Shaw i s o l a t e d a metallocar boxylic acid a few years l a t e r from the reaction of a +
e
Re(C0)
+ e
+ H0 ^ 2
{Re(C0) C00H} 5
(4)
c a t i o n i c i r i d i u m carbonyl d e r i v a t i v e and water, and showed that upon p y r o l y s i s of t h i s species the corre sponding metal hydride and C0 were produced ( 8 ) . In addition, i t has long been known that some metal car bonyls r e a d i l y react with hydroxide ions to y i e l d metal carbonyl hydride anions and C0 , e.g., Fe(C0) + OH" -+ HFe(C0) " + C0 ( 9 ) . These observations i l l u s t r a t e that there are two transformations open to metallocarboxylic acid i n t e r mediates; r e v e r s i b l e loss of OH" accompanied by oxygen exchange, and metal-hydride formation with expulsion of C0 . Our entry into t h i s area of chemistry was i n 1975 when extensive studies o f oxygen l a b i l i t y i n metal carbonyl cations were i n i t i a t e d (lO). These 2
2
5
4
2
2
Ford; Catalytic Activation of Carbon Monoxide ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
7.
AND ROKiCKi
DARENSBOURG
Oxygen Bases and Metal Carbonyls
109
investigations were centered around defining the factors that determined whether the oxygen exchange process predominated over the production of metal hydrides and carbon dioxide or vice versa ( 1 1 , 1 2 , 15» 14). For example, the reaction o f Mn(CO) witïTwater l e d to metal hydride formation concomitantly with oxygen exchange, with metal hydride production being much slower (eq. 5 ) . Some o f the other germane observations noted during these studies were the following: e
+
Mn(C0)e + H 0 ^{Mn(C0) C00HJ -*Mn(C0) H + C0
Downloaded by RUTGERS UNIV on March 11, 2016 | http://pubs.acs.org Publication Date: May 5, 1981 | doi: 10.1021/bk-1981-0152.ch007
2
5
5
2
(5)
( l ) The rate of n u c l e o p h i l i c a d d i t i o n of "OH to the metal bound carbon monoxide ligand, as viewed by oxygen exchange, decreased with increasing s u b s t i t u t i o n at the metal center with electron donating ligands, i . e . , MiCOJe* > M(C0) L+ » M(C0) L +. ( 2 ) The more e l e c t r o n - r i c h Ln(C0) _ M(C00H) intermediates were less disposed to C0 elimination with M-H bond formation. (3) Metal-hydride formation was enhanced over oxygen exchange as the b a s i c i t y of the s o l u t i o n increased. In the case of the b i s phosphine derivatives of the group 7 b metals, t h e " I a b i l i t y o f the oxygen atoms was so markedly retarded by s u b s t i t u t i o n that i t was necessary to enhance t h e i r r e a c t i v i t y by means o f base c a t a l y s i s ( 1 3 , 14), a process having mechanistic f e a tures common wi^H" general base c a t a l y s i s of the hydrat i o n o f ketones (eq. 6) ( 1 5 ) . 5
s
4
2
n
2
[M-C*qf
s
[M-£ O]
*~
+
BH*
(6)
We have exploited t h i s base c a t a l y s i s o f the oxygen exchange process to e f f e c t oxygen l a b i l i t y i n the l e s s e l e c t r o p h i l i c carbonyl s i t e s o f n e u t r a l metal carbonyl species. Because [MCOOH] intermediates are r e a d i l y decarboxylated i n the presence of excess hydroxide ion, i n order to observe oxygen exchange processes i n n e u t r a l metal carbonyl complexes i t was convenient to carry out these reactions i n a biphasic system employing phase transfer c a t a l y s i s () (16, 17, 1 8 ) . Under conditions (eq. 7 ) , the (organic phase)
[MCO] + n-Bu^OH" ^ :
(aqueous phase) Na I
H
H
+
+ n-Bu N 0H 4
.
0 [M-C-OH] n-Bu«N
— ^
— Na OH
_
+ n-Bu^K I
Ford; Catalytic Activation of Carbon Monoxide ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
(7)
110
CATALYTIC
ACTIVATION
OF CARBON
MONOXIDE
hydroxide ion concentration i s small i n the organic phase which contains the metal carbonyl, since "*0H i s more h i g h l y hydrated than the halide i o n . In general, the oxygen exchange reactions o f n e u t r a l mono nuclear metal carbonyl species with hydroxide ions p a r a l l e l e d the observations summarized f o r the c a t i onic species (vide supra) with, however, the s i g n i f i cant a d d i t i o n a l developments discussed hereafter. When the reactions of M(C0) (M » Cr, Mo, w) with hydroxide ions were c a r r i e d out under an atmo sphere o f CO, these systems were observed to generate hydrogen c a t a l y t i c a l l y with a low turnover rate (mol. H /mol. c a t a l y s t per day), on the order o f a week a t 7 5 ° (18). However, i n more strongly a l k a l i n e s o l u t i o n s ~ T e . g . , aqueous KOH i n 2-ethoxyethanol) a t 100°C the M(C0) species were observed to be quite a c t i v e c a t a l y s t s f o r the WGSR with a turnover rate of - 30 for C r ( C 0 ) . The p r i n c i p a l metal carbonyl components i n an a l k a l i n e 2-ethoxyethanol s o l u t i o n o f a c a t a l y s t prepared from C r ( C 0 ) were determined to be Cr(C0)ft and Cr(C0) H~ by i n f r a r e d spectroscopy i n the v(co) region (see Figure l ) . I d e n t i f i c a t i o n o f the chromium pentacarbonyl monohydride species was based on spec t r a l comparisons with an authentic sample prepared by protonation o f C r ( C 0 ) 5 ~ ( 1 9 ) . The r e a c t i o n o f hydrox ide ions with Cr(co) e i n aqueous 2-ethoxyethanol to a f f o r d Cr(co) H"" occurs over several hours a t 4 0 - 5 0 ° , even i n the presence o f a large excess o f hydroxide. However, under conditions where the "0H i s not hy drated, KOH i n THF with Crypt 222, the production o f Cr(co) H~ occurs q u a n t i t a t i v e l y over a period o f a few minutes a t ambient temperature employing s t o i c h i o metric q u a n t i t i e s of reagents (Figure 2). When the c a t a l y s t s o l u t i o n described above (Fig ure ΙΑ) was prepared using H 0 , C r ( C 0 ) was shown to undergo the oxygen exchange process a t a much f a s t e r rate than formation o f metal hydride, an observation consistent with the
