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-Ph(2)PC(6)H(4))(2) ([(Ar)DPB(Ph)]; Ar = Ph, Mes). The [(Ar)DPB(Ph)] framework supports pseudo-tetrahedral nickel complexes featuring η(2)-B,C coordination from the ligand backbone. For the B-phenyl derivative, the THF adduct [(Ph)DPB(Ph)]Ni(THF) has been characterized by X-ray diffraction and features a very short interaction between nickel and the η(2)-B,C ligand. For the B-mesityl derivative, the reduced nickel complex [(Mes)DPB(Ph)]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, [(Mes)DPB(Ph)]Ni is an efficient catalyst for the hydrogenation of olefin substrates under mild conditions.