Gn. Ilinich et al., THE MECHANISM OF THE CATALYTIC BEHAVIOR OF PLATINUM TRIPHENYLPHOSPHINE COMPLEXES IN THE ETHYLENE HYDROCARBONYLATION, Journal of molecular catalysis. A, Chemical, 101(3), 1995, pp. 221-235
High resolution H-1, C-13 and P-31 NMR was used for step-by-step inves
tigation of ethylene hydrocarbonylation into diethyl ketone (DEK) in t
he 'P(2)PtX(2) (or P4Pt)-CF3COOH/H2O' (P = PPh(3), X = CF3COO-; [H2O]
less than or equal to 30 vol.%) catalytic systems. The so-called 'hydr
ide mechanism' of the reaction was unambigouously determined. Six key
Pt(II) intermediates, namely, [HPtP3](+), 1, trans-[HPt(X)P-2], 2, tra
ns-[HPt(CO)P-2](+) 5, trans-[HPt(C2H4)P-2](+), 6, trans-[C2H5Pt (C2H4)
P-2](+), 7, and trans-[C2H5Pt(CO)P-2](+) , 9, were identified and char
acterized, and in addition their reactivity was studied. The relative
thermodynamic stability of the Pt(II) hydride complexes turned out to
determine the catalytic behaviour of the platinum systems investigated
and was found to increase in CF3COOH/H2O solutions in the order: 2<6<
1<5. In accordance with this, the 'P4Pt-CF3COOH/H2O' system, in which
rapid and quantitative conversion of initially formed complex 1 into t
he extremely inert to C2H4 complex 5, occurred under the reaction cond
itions, exhibited no detectable activity in DEK formation. The initial
activity of the 'P(2)PtX(2)-CF3COOH/H2O' system in DEK formation was
suggested to result from the generation of complex 9, in addition to 5
, at the initial unsteady-state period of the reaction as a result of
the kinetic competition of two rapid reaction sequences: 2 --> 6 --> 7
--> 9 against 2 --> 5. Isomerization of 9 into the corresponding plat
inum propionyl complex appeared to be the rate-determining stage of DE
K formation. The kinetic regularities of the model catalytic reactions
of ethylene hydrogenation and hydrogen isotopic heteroexchange with t
he 'P(2)PtX(2)-CF3COOH/H2O' and 'P4Pt-CF3COOH/H2O' systems were also s
tudied. The former system was found to exhibit a much higher initial a
ctivity dire to formation of a more active platinum hydride complex 2,
while in the latter system a less active complex 1 was formed initial
ly. The activities of both systems. however, leveled off during the re
actions due to 2 reversible arrow 1 equilibration.