Reactivity and Mechanism in Oxidative Addition to Palladium(II) and Reductive Ellmlnation from Palladium(IV) and an Estimate of the Palladium-Methyl Bond Energy

Abstract

Oxidative addition of methyl iodide to [PdMe2(bpy)]( bpy = 2,2'-bipyridine) occurs by the SN2 mechanism. Evidence includes the observation of second-order kinetics in acetone solvent, with a large negative value for the entropy of activation, and the observation of a cationic species, [PdMe3(bpy)(CD3CN)]+ in, CD3CN solvent. The reaction occurs more slowly than the analogous reaction of [PtMe2(bpy)], but the same mechanism operates. Reductive elimination from [PdIMe3(bpy)] to give ethane and [PdIMe(bpy)] follows good first order kinetics, occurs more rapidly in polar solvents, and is strongly retarded by added iodide. These observations are interpreted in terms of a mechanism that involves preliminary ionization of iodide followed by reductive elimination from the cation [PdMe3(bpy)]+. Studies by differential scanning calorimetry allow an estimate of the Pd-C bond energy of -130 kJ mo1-l to be obtained, and this value is considerably higher than the activation energy for reductive elimination of ethane from [PdIMe3(bpy)]. The reductive elimination step is therefore concerted, and possible mechanisms, which may involve direct C-C coupling or C-C coupling after an agostic CHPd interaction, are discussed. This work is relevant to catalytic C-C coupling reactions using palladium complex catalysts