Oxidative addition of Me1 to [PdMe2(bpy)] (bpy = 2,2'-bipyridine) occurs by the S N~ 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)]+I-, in CD3CN solvent. The reaction occurs more slowly than the analogous reaction of [PtMe2(bpy)], but the same mechanism operah. Reductive elimination from [PdIMe,(bpy)] to give ethane and [PdIMeCbpy)] follows good firsborder 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 [PdMe,(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 [PdIMe,(bpy)]. The reductive elimination step is therefore concerted, and possible mechanisms, which may involve direct CC coupling or CC coupling after an agostic CHPd interaction, are discussed. This work is relevant to catalytic CC coupling reactions using palladium complex catalysts.
1) in which the imine acts as a bidentate [N,NЈ] 2) has been obtained by refluxing a methanol solution of complex 1 in the presence of base; both complexes 1 and 2 have been characterized crystallographically. The cyclometallation process has been studied kinetically at variable temperature and pressure as a function of the base added in methanol and ethanol solutions. In all cases, the actual C-H bond activation is preceded by the Z to E isomerization of the imine; detection of this species by proton NMR has also been achieved. The kinetic, thermal and baric activation parameters associated with the two processes are indicative of a different reaction mechanism operating for the reactions when the "external" base added (NaCH 3 CO 2 ) can, in fact, react with the starting material to form a new acetato complex which allows for the reactions to be intramolecular.
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