Studies of the carbonylation of [(η^5-C_5H_5)Co(CO)(R)]_2(R = Me, Et) at temperatures below ambient have revealed that these complexes may be converted into ketones via binuclear diacyl complexes [(η^5-C_5H_5)Co(CO)(COR)]_2; the postulated mechanistic pathway circumvents the earlier-identified intermediate (η^5-C_5H_5)Co(CO)R_2, and involves alkyl transfer from a Co^(II) acyl complex as a critical step

Bergman, Robert G.

White, Mary Anne

English

Caltech Authors - Main

1056 J.C.S. CHEM. COMM., 1979 
Identification of Binuclear Acyl Complexes as Intermediates in the 
CO-induced Conversion of [(Y)5 -C5H5)Co(CO)(Me)] 2 into Acetone, 
and [(Y)5-C5H5)Co(CO)(Et)h into Pentan-3-one 
By MARY ANN WHITE and RoBERT G. BERGMAN* 
(Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125; 
Department of Chemistry, University of California, and Lawrence Berkeley Laboratory, Berkeley, California 94720) 
Summary Studies of the carbonylation of [(1]5-C5H 5)Co(CO)-
(R)]2 (R = Me, Et) at temperatures below ambient have 
revealed that these complexes may be converted into 
ketones via binuclear diacyl complexes [(1]5-C5H 5)Co(CO)-
(COR)]2; the postulated mechanistic pathway circumvents 
the earlier-identified intermediate (1]5-C5H 5)Co(CO)R2, and 
involves alkyl transfer from a con acyl complex as a 
critical step. 
THERMAL decomposition of bis[(1]5-cyclopentadienyl)-
(carbonyl)methylcobalt] [ (1]5-C5H 5)Co(CO)(Me)]2 (1) in a 
number of organic solvents leads to good yields of acetone 
(a process involving the formation of two new carbon-
carbon bonds) and cobalt cluster complexes.1 •2 Examina-
tion of this reaction by n.m.r. spectroscopy demonstrated 
that the transformation involves initial conversion into the 
intermediate mononuclear complex ( 175-C5H 5)Co(CO) (Me)2 
(3, Scheme 1) and crossover experiments showed2 that 
this 'rearrangement' reaction is an inter-, rather than an 
intra-molecular process. 
Carbonylation of (1) leads to acetone and (1] 5-C5H 5)Co(C0)2 
(7) in quantitative yield. Complex (3) can again be 
detected by n.m.r. spectroscopy during the reaction at 
35 oc suggesting that the thermal decomposition and 
carbonylation reactions proceed through the same inter-
@... )l /R 35°C Co-CO' ~
R/1(@ 
0 
(1) R =Me 
(2)R=Et 
(3) R =Me 
. ~ 
0 
II 
R-C-R + 
[_CpCo(COjJ 
~clusters 
mediate. 2 We report here, however, that this apparent 
similarity is deceptive. Lower temperature kinetic and 
n.m.r. studies show that (1) may be converted into acetone 
in a rapid process involving binuclear, acyl-containing 
intermediates, which completely circumvents the mono-
nuclear dimethyl complex (3). 
Upon treatment of a 0·1 M solution of (1) in [2H 8]tetra-
hydrofuran (THF) with an excess of CO, the n.m.r. and i.r. 
bands due to (1) disappeared within a few minutes at room 
temperature and were replaced by several new signals. 
These were due (Scheme 2) to the presence of (3), (7), and 
acetone (all of which had been identified earlier2), as well as 
two new sets of signals due to theE- and Z-isomers (Sa) and 
(5b), formed in a ca. 3: I ratio. t Complex (5) could be 
t Although we believe (Sa) and (Sb) are E- and Z-isomers owing to the similarity of their spectral properties, we do not yet know 
which is E and which Z; the assignments in Scheme 2 are arbitrary. 
J.C.S. CHEM. CoMM., 1979 
co (1) or (2) 25 oc 
c 0 
P\. .)l.._ /R 
R Co-Co -----. 
l{Y \c 
0 0 p 
(4) R =Me 
0 0 
Cp, AA 
R Co-Co R 
YY"'-5 0 Cp 
(5a) R =Me 
(6a} R = Et 
+ 
CP. ~ Cp 
"'/"-/ 
Ryco-Co R 
oY't 
(5b) R =Me 
(6b) R = Et 
isolated as a brown solid by low-temperature column 
chromatography under an inert atmosphere. Although 
n.m.r. and i.r. analysis indicated that the material was 
pure, its extreme thermal instability frustrated attempts at 
elemental analysis. During the carbonylation an even 
more transient material, exhibiting both metal-bound 
methyl and acetyl signals, appeared and then disappeared 
as (5) was formed. Although this material was not isolable, 
its spectral characteristics were consistent with its assign-
ment as the partially carbonylated binuclear complex (4). 
Similar results were obtained when [(7]5-C5H 5)Co(CO)(Et)]2 
(2)t was treated with CO. Again, a 3: 1 ratio of binuclear 
diacyl complexes (6a) and (6b) could be detected spectro-
scopically and these could be isolated by low-temperature 
chromatography. In contrast to the behaviour of (1), 
however, neither a monoacyl intermediate nor (7] 5-C5H 5)Co-
(CO) (Et)2 were observed in the mixture during carbonyla-
tion. 
When either (5) or (6) was allowed to decompose in the 
presence of CO, quantitative formation of (7), and acetone 
and pentan-3-one, respectively, was observed. Decom-
position of a 50: 50 mixture of (5) and (6) gave a 1: 2: 1 
ratio of acetone, butan-2-one, and pentan-3-one, indicating 
(as was observed2 in the decomposition of (1)] that the 
ketone-forming reaction is intermolecular. These reactions 
were surprisingly rapid, proceeding at reasonable rates 
even at 0 °C in [2H 8]THF. Decomposition in the absence 
of CO gave similar results, except that some cobalt clusters 
were observed as final organometallic products in place of 
(7). Kinetic studies demonstrated that the rate of car-
bonylation of (5) is first order in complex concentration; 
addition of excess of CO has essentially no effect on the rate 
[at 0·5 atm CO, kobs = 5·5 X I0-5 s-1 ; at 9·5 atm CO, 
kobs = 3·23 X I0-5 s-1 (0 °C}]. The most critical feature 
of these observations is that none of the mononuclear 
dimethyl complex (3) is formed during the decomposition 
of (5), and the rate of conversion of (5) into acetone is much 
faster than the rate at which (3) is converted into acetone. 
From these data we conclude that carbonylation of (1) can 
1057 
take place by two distinct routes: one (slower) process 
involves the intermediacy of the mononuclear complex (3), 
and another, more rapid reaction involves the binuclear 
diacyl complexes (Sa) and (5b). 
co CpCo(C0)2 
(1)~2CpCo(CO)Me- CpCo(CO)Me2+ CpCo(CO>- (7) (A) (3) (B) + 
coJf-co st~~·Jf-co Me2co 
2 Cp~~:col2 Me o~• 1 
fastJt Cp-C,-Me +CO _ 
0 0 -<y CO (E) 
(S)-Cpc:'Me MeAC C ;!iAl 
' + 0 p~ () co oc.,........ -cAl 4 
(D) (0) 
These seemingly disparate results are accommodated by 
the mechanism outlined in Scheme 3. As suggested earlier, 2 
we believe that the binuclear complex (1) is in rapid 
equilibrium with small amounts of the mononuclear Coli 
species (A). In the absence of large amounts of CO, 
methyl transfer from one molecule of (A) to another occurs,§ 
leading to (3) and (B), which either forms clusters or reacts 
rapidly with CO to give (7). As the amount of CO available 
increases, (A) is diverted to the dicarbonyl (C). At partial 
conversion, reaction between (C) and (A) may occur, 
leading directly to (3) and (7), which is undoubtedly why 
(3) is often produced along with (5) during the carbonylation 
of (1). At higher CO pressures, (A) is scavenged more 
efficiently, and larger amounts of (C) are produced. As a 
19-electron (or perhaps a 17-electron, 7] 3-cyclopentadienyl) 
intermediate, there should be a strong driving force for CO 
insertion, leading to the 17-electron acyl species (D) which 
dimerizes to give (5). 
We believe that (D) is the critical species which accounts 
both for the formation of (5) from (1) and for the rapid 
production of acetone from (5). As with the dialkyl 
complexes (1) and (2), small concentrations of the mono-
nuclear Co11 species (D) are in equilibrium with (5). It is 
possible that acyl transfer occurs leading to the diacyl 
analogue of (3); however, the complete absence of biacetyl 
in the product mixtures makes this seem unlikely. Appar-
ently (D) operates as a methyl transfer reagent (in analogy 
to the hydride transfer propensity shown by the anionic 
formyl complexes of Casey3 and Gladysz4). Disproportiona-
tion of (D) by this route leads to (7) and the acylmethyl 
complex (E), the direct precursor of acetone. In this way, 
the route from (5) to acetone avoids the apparently slower 
CO insertion step in (3). 
In summary, our results show that complexes such as (1) 
may be converted into the same products [acetone and (7)] 
by two different mechanisms which, though related to one 
another, involve distinctly different intermediates. They 
t Prepared :;tnalogously to (1) by treatmentl of Na[(7J5-C5H 5)Co(C0)]2 with Etl. Satisfactory spectral and analytical data were 
obtained for this material; on thermal decomposition it is converted rapidly (room temperature) into ethylene, ethane, and pentan-
3-one, but carbonylation leads quantitatively to pentan-3-one. 
§ This is a somewhat simpler version of the chain mechanism suggested earlier ;2 transfer of a methyl group between two molecules 
of the intermediate (A) now seems more likely in view of the first-order kinetics observable here. 
1058 J.C.S. CHEM. COMM., 1979 
also emphasize that reactions of multinuclear complexes 
[even those that produce products of the same nuclearity, 
such as the conversion of (1) into (5) and (2) into (6)] may 
proceed via reactive mononuclear fragments. 
We are grateful to the National Science Foundation for 
support of this work. 
1 N. E. Schore, C. S. Ilenda, and R. G. Bergman,]. A mer. Chem. Soc., 1977, 99, 1781. 
2 N. E. Schore, C. S. Ilenda, and R. G. Bergman,]. A mer. Chem. Soc., 1976, 98, 7436. 
3 C. P. Casey and S.M. Neumann,]. A mer. Chem. Soc., 1978, 100, 2544. 
4 J. Gladysz and W. Tam,]. A mer. Chem. Soc., 1978, 100, 2545. 
(Received, 17th july 1979; Com. 769.) 


Identification of Binuclear Acyl Complexes as Intermediates in the CO-induced Conversion of [(η^5-C_5H_5)Co(CO)(Me)]_2 into Acetone, and [(η^5-C_5H_5)Co(CO)(Et)]_2 into Pentan-3-one

https://authors.library.caltech.edu/32765/1/WHIcc79.pdf

Identification of Binuclear Acyl Complexes as Intermediates in the CO-induced Conversion of [(η^5-C_5H_5)Co(CO)(Me)]_2 into Acetone, and [(η^5-C_5H_5)Co(CO)(Et)]_2 into Pentan-3-one

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