THE MECHANISM OF THE CATALYTIC BEHAVIOR OF PLATINUM TRIPHENYLPHOSPHINE COMPLEXES IN THE ETHYLENE HYDROCARBONYLATION

High resolution H-1, C-13 and P-31 NMR was used for step-by-step investigation of ethylene hydrocarbonylation into diethyl ketone (DEK) in the '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 'hydride mechanism' of the reaction was unambigouously determined. Six key Pt(II) intermediates, namely, [HPtP3](+), 1, trans-[HPt(X)P-2], 2, trans-[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 characterized, 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 the extremely inert to C2H4 complex 5, occurred under the reaction conditions, 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 platinum propionyl complex appeared to be the rate-determining stage of DEK formation. The kinetic regularities of the model catalytic reactions of ethylene hydrogenation and hydrogen isotopic heteroexchange with the 'P(2)PtX(2)-CF3COOH/H2O' and 'P4Pt-CF3COOH/H2O' systems were also studied. The former system was found to exhibit a much higher initial activity dire to formation of a more active platinum hydride complex 2, while in the latter system a less active complex 1 was formed initially. The activities of both systems. however, leveled off during the reactions due to 2 reversible arrow 1 equilibration.