PNNL

Abstract

The mixed ligand complex [Ni(dppp)(PPh2NBz2)](BF4)2, 3, (where PPh2NBz2 is 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane and dppp is 1,3-bis(diphenylphosphino)propane) has been synthesized. Treatment of this complex with H2 and triethylamine results in the formation of the Ni0 complex, Ni(dppp)(PPh2NBz2), 4, whose structure has been determined by a single crystal X-ray diffraction study. Heterolytic cleavage of H2 by 3 at room temperature forms [HNi(dppp)(PPh2NBz(µ H)NBz)](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)2Ni(dppp)(PPh2NBz2)](BF4)2, 5b, and the other is [Ni(dppp)(PPh2NBz2H2)](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 H2 in the 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 H2 addition for this complex to related catalysts studied previously indicates that for NiII complexes containing two diphosphine ligands, the activation of H2 is favored by the presence of two positioned pendant bases. This work was supported by the US Department of Energy (DOE) Office of Basic Energy Sciences, Chemical Sciences program. Pacific Northwest National Laboratory is operated by Battelle for DOE.