Benzylic Complexes of Palladium(II): Bonding Modes and Pentacoordination for Steric Relief

A large variety of alpha-(pentafluorophenylmethyl)-benzylic palladium complexes with different ligands have been synthesized and characterized. Multinuclear NMR spectroscopic data allow to determine the sigma- or eta(3)-benzylic nature of the complexes in solution. The adoption of either coordination mode is a function of the number of ligands coordinated to palladium, and remarkably, the presence of bulky phosphines favors the adoption of a bidentate eta(3)-benzylic mode and pentacoordinated palladium complexes. Experimental data and DFT calculations indicate that this five-coordination could alleviate the steric hindrance of two bulky cis phosphines. The benzylic complexes show a rich fluxional behavior that involves both ligand exchange and sigma- to eta(3)-benzylic interconversion.