Theoretical insight into the C-C coupling reactions of the vinyl, phenyl, ethynyl, and methyl complexes of palladium and platinum

The mechanism and controlling factors of the C-C reductive elimination reactions of vinyl, phenyl, ethynyl, and methyl ligands from the Pd and Pt complexes RR'M(PH3)(2) were studied with a density functional method. The barrier of C-C coupling from the symmetrical R2M-(PH3)(2) (where M = Pd, Pt) complex decreases in the order R = methyl > ethynyl > phenyl > vinyl, and the exothermicity of the reaction increases in the same order. That is, the methyl-methyl coupling has the highest barrier and smallest exothermicity, while the vinyl-vinyl coupling has the smallest barrier and largest exothermicity. For the asymmetrical RR'M(PH3)(2) complexes, the activation and reaction energies are found to be approximately the average of the corresponding parameters of symmetrical coupling reactions, and this simple rule is expected to be valid for other asymmetrical coupling reactions involving different substituted alkyl, vinyl, phenyl, and ethynyl groups as well as different transition-metal complexes. These C-C coupling reactions occur much more easily in Pd than in Pt complexes, because the Pd-R bonds are weaker than the Pt-R bonds. The major thermodynamic and kinetic factors determining the C-C coupling in these complexes have been discussed. For reactions with similar exothermicities, the kinetics of C-C bond formation is mainly determined by the orientation effect that includes the directionality of the M-C bond and the steric interaction between R and the other ligand (phosphine in the present case), which favors vinyl over phenyl over methyl. However the activation barrier is strongly dominated by exothermicity when it is very different between reactions.