Mechanistic Insights into Catalytic H2 Oxidation by Ni Complexes Containing a Diphosphine Ligand with a Positioned Amine Base

The mixed-ligand complex [Ni(dppp)(p(2)(Ph)N(2)(Bz))](BF4)(2), 3, (whem (P2N2Bz)-N-Ph is 1,5-dibenzyl-3,7-diplienyl-1,5-diaza-3,7-diphosphacyclooctane and dppp is 1, 3-bis(diphenylphosphino)propane) has been synthesized. Treatment of this complex with H-2 and triethylamine results in the formation of the NO complex, Ni(dppp)((P2N2Bz)-N-Ph), 4, whose structure has been determined by a single-crystal X-ray diffraction study. Heterolytic cleavage of H-2 by 3 at room temperature forms [HNi(dppp)((P2NBz)-N-Ph(mu-H)N-Bz)](BF4)(2), 5a, in which one proton interacts with two nitrogen atoms of the cyclic diphosphine ligand and a hydride ligand is bound to nickel. Two intermediates are observed for this reaction using low-temperature NMR spectroscopy. One species is a dihydride, [(H)(2)Ni(dppp)((P2N2Bz)-N-Ph)](BF4)(2), 5b, and the other is [Ni(dppp) ((P2N2H2)-N-Ph-H-Bz)](BF4)(2), 5c, in which both protons are bound to the N atoms in an endo geometry with respect to nickel. These two species interconvert via a rapid and reversible intramolecular proton exchange between nickel and the nitrogen atoms of the diphosphine ligand. Complex 3 is a catalyst for the electrochemical oxidation of H-2 in Vie presence of base, and new insights into the mechanism derived from low-temperature NMR and thermodynamic studies are presented. A comparison of the rate and thermodynamics of H-2 addition for this complex to related catalysts studied previously indicates that for Ni-II complexes containing two diphosphine ligands, the activation of H-2 is favored by the presence of two positioned pendant bases.