In-depth insight into the electronic and steric effects of phosphine ligands on the mechanism of the R-R reductive elimination from (PR3)2PdR2

The aim of this study was to investigate both the electronic and steric effects of the ancillary phosphine ligand L on the reductive elimination of Me-Me from a series of L2PdMe2 and LPdMe2 complexes. Density functional theory was used to study these processes with the model ligands L = PMe3, PH3, PCl3 and with the experimentally reported ligands L = PPh3, PPh2Me, PPhMe2. For the model ligands we confirm that electron donation from L affects the barrier for reductive elimination from L2PdMe2 but not from LPdMe2. In the former case the greater the electron donation or basicity of L, the greater the barrier and the later the transition state. This is because electron donation increases the sigma* antibonding between Pd and L in the transition structure. On the other hand, if L is a good pi acceptor this stabilizes the occupied d(pi) orbital of Pd in the transition structure and lowers the barrier to reductive elimination. In the case of the reactions involving LPdMe2 as the intermediate, it is the loss of the first L (L2PdMe2 -> LPdMe2 + L) which determines the differences in the barrier height. Greater electron donation leads to greater L-to-Pd sigma donation and a stronger Pd-L bond, and thus a greater overall barrier. A comparison of these results with the reductive elimination of 1,3-butadiene from divinyl palladium complexes L2PdR2 shows that the barriers are lower in the vinyl case because of a mix of orbital factors. Our results show that there is a significant stabilizing interaction between the Pd d(pi) orbital and the vinyl-vinyl hybrid sigma*/pi* orbitals in the reductive elimination transition structure. At the same time this Pd-R-2 orbital stabilization alleviates the potential antibonding interactions between Pd and L and makes the vinyl elimination much less susceptible to ancillary ligand effects. Energy-decomposition analyses have been used to elucidate the contributing factors to the activation energies for the reductive eliminations with the model phosphine ligands. These analyses have also been used to disentangle the electronic and steric effects involved in the larger ligand systems. The electronic effects of the experimentally reported ligands are found to be very similar to each other. On the other hand, steric effects lead to a destabilization of the reactant L2PdMe2 complexes but not the transition structures, which results in a decrease in the barriers to reductive elimination compared to the smaller phosphine ligands. These steric effects do not play a role in reductive elimination from LPdMe2. These detailed analyses of the electronic and steric factors may be used to assist the design of systems which enhance or retard reductive elimination behaviour. (c) 2009 Elsevier B.V. All rights reserved.