Kinetic aspects of the influence of CO pressure on cyclohexene hydrocarbomethoxylation catalyzed by a diphosphine palladium system. Thermodynamic characteristics of some ligand exchange reactions

Abstract

The effects of temperature and CO pressure on the rate of cyclohexene hydrocarbomethoxlyation catalyzed by the system of Pd(OAc)2/trans-2,3-bis(diphenylphosphinemethyl)norbornane (TBDPN)/p-toluenesulfonic acid (TsOH) system were studied. It was found that in the 358–373 K temperature range, reaction rates correlated with CO pressure with maximums in the range of 2.1–3.1 MPa. The results were interpreted via a hydride mechanism including diphosphine palladium complexes as intermediates, augmented with ligand exchange reactions, which lower palladium catalyst activity. The ascending branches of the reaction rate versus CO pressure curves are due to the CO acting as a reagent in the catalytic cycle, while the descending branches were interpreted in the framework of low reactivity Pd(TBDPN)(CO)2 complex formation at high CO pressures. Several effective rate constants were estimated at the 343–373 K range using least squares method. The effective activation energy was determined based on the temperature correlation with one of these constants. By analyzing the effective activation energies of cyclohexene hydrocarbomethoxylation, we were able to gauge the changes in enthalpy, entropy and Gibbs free energy in ligand exchange reactions between “ballast” palladium complexes, such as Pd(TBDPN)3, Pd(TBDPN)(CO)2 and [HPd(TBDPN)(CH3OH)]TsO. The relative stability ranges and formation entropies of these complexes were determined under the conditions of cyclohexene hydrocarbomethoxylation. Based on our understanding of the heating effect of ligand exchange reactions between Pd(TBDPN)(CO)2 and [HPd(TBDPN)(CH3OH)]TsO complexes, we determined that the binding energy of CO molecules is 7 kJ/mol higher than the binding energy of diphosphine TBDPN molecules (without chelate formation). Based on the entropy changes in ligand exchange reactions, we propose that the [HPd(TBDPN)(CH3OH)]TsO complex is practically undissociated in these conditions. The analysis of changes in Gibbs free energy showed that in palladium ligand exchange, under the conditions in question, Pd(TBDPN)3 formation is thermodynamically favorable.