Critical effect of phosphane ligands on the mechanism of carbon-carbon bond formation involving palladium(II) complexes: A theoretical investigation of reductive elimination from square-planar and T-shaped species

A theoretical ONIOM study has been carried out to understand the influence of phosphane ligands on the structure of Pd complexes and their reactivity in C-C bond formation. The calculations were performed for Me-Me reductive elimination with the ligands L = PPh3, PCy3, PMe3, PH3, and vinyl-vinyl, Ph-Ph, ethynyl-ethynyl, vinyl-Me, vinyl-Ph and vinyl-ethynyl couplings with L = PPh3 for [PdR2Ln] complexes (n = 1, 2). The calculations revealed critical changes in the reactivity of palladium complexes depending on the mechanism and ligand type. In the case of the standard four-coordinate pathway (n = 2) the relative reactivity in carbon-carbon bond formation follows the order: L = PPh3 > PH3 > PCy3 > PMe3. However, for reductive elimination involving T-shaped complexes by the ligand predissociation pathway (n = 1), the relative reactivity changes in the order: L = PCy3 > PPh3 > PH3 > PMe3. The theoretical study suggested that the steric effect of phosphane ligands has the largest impact on the structure of the initial palladium complexes, while the electronic effect is most influential on the transition states of C-C coupling in these complexes. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)