DFT studies on the distinct mechanisms of C–H activation and oxidation reactions mediated by mononuclear- and binuclear-palladium

Abstract

A series of density functional theory calculations have been carried out to investigate the detailed mechanisms of C–H activation and oxidation reactions, and further to disclose the distinct effects of mononuclear- and binuclear-palladium on these reaction pathways. The results of calculations demonstrated that the C–H activation of 2-phenylpyridine with mononuclear Pd(OAc)₂ prefers the inner-shell proton-abstraction mechanism, while that with binuclear Pd₂(μ-OAc)₄ is biased to the outer-shell proton-abstraction mechanism. The rate-determining free-energy barriers of the two mechanisms were calculated to be 24.2 and 24.8 kcal mol⁻¹, respectively. More importantly, we have simulated the oxidation pathways of Pd^II → Pd^III and Pd^II → Pd^IV with strong oxidants including PhI(OAc)₂, PhICl₂ and NCS, and found that a binuclear Pd^II-precursor would be oxidized to the corresponding binuclear Pd^III-complex while a mononuclear Pd^II-precursor was deemed to evolve to the corresponding mononuclear Pd^IV-complex. In addition, the oxidation of Pd^II with PhI(OAc)₂ has been characterized as a radical mechanism, in sharp contrast to the ion-pair mechanism prevalent for the oxidation of Pd^II with PhICl₂. The calculated kinetic and thermodynamic parameters could be qualitatively consistent with the related experimental observations. This molecular modelling can provide valuable insights into the understanding of the distinct effects of the resting state and oxidant on these important transformations.