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) ([(DPBPh)-D-Ar]; Ar = Ph, Mes). The [(DPBPh)-D-Ar] framework supports pseudo-tetrahedral nickel complexes featuring eta(2)-B,C coordination from the ligand backbone. For the B-phenyl derivative, the adduct [(DPBPh)-D-Ph]Ni(THF) has been characterized by X-ray diffraction and features a very short interaction between nickel and the eta(2)-B,C ligand. For the B-mesityl derivative, the reduced nickel complex [(DPBPh)-D-Mes]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, [(DPBPh)-D-Mes]Ni is an efficient catalyst for the hydrogenation of olefin substrates under mild conditions.