π-π Interactions supporting outer-sphere hydride transfer – A new direction in metal-ligand cooperativity

A series of new phosphine-free Ru(II)-CNC complexes with benzimidazolylidenes have been found to have the unique ability to operate via inner-sphere and outer-sphere mechanisms at different stages of the same catalytic cycle. All new complexes have been characterised using multinuclear NMR spectroscopy and mass analyses while the solid-state structure of 1a has been determined using single-crystal X-ray diffraction technique. The new complexes serve as excellent catalysts for α-alkylation of ketones using primary alcohols. DFT calculations reveal these systems follow an inner-sphere mechanism for the alcohol dehydrogenation step, but the subsequent hydrogenation of the enone proceeds via an outer-sphere mechanism. An unprecedented crossover between inner- and outer-sphere mechanisms was observed to occur via structural rearrangement of the Ru-hydride intermediates. The outer-sphere path is found to be supported by π-π interaction between the extended aromatic ring system of the benzimidazolylidene-based CNC-ligand and aromatic rings of the substrates in a new, unprecedented case of metal-ligand cooperativity.