Reversible H2 Addition across a Nickel–Borane Unit as a Promising Strategy for Catalysis

We report the synthesis and characterization of a series of nickel complexes of the chelating diphosphine-borane ligands ArB­(o-Ph2PC6H4)2 ([ArDPBPh]; Ar = Ph, Mes). The [ArDPBPh] framework supports pseudo-tetrahedral nickel complexes featuring η2-B,C coordination from the ligand backbone. For the B-phenyl derivative, the THF adduct [PhDPBPh]­Ni­(THF) has been characterized by X-ray diffraction and features a very short interaction between nickel and the η2-B,C ligand. For the B-mesityl derivative, the reduced nickel complex [MesDPBPh]Ni is isolated as a pseudo-three-coordinate “naked” species that undergoes reversible, nearly thermoneutral oxidative addition of dihydrogen to give a borohydrido-hydride complex of nickel­(II) which has been characterized in solution by multinuclear NMR. Furthermore, [MesDPBPh]Ni is an efficient catalyst for the hydrogenation of olefin substrates under mild conditions.