Mechanistic Insights into Catalytic H_2 Oxidation by Ni Complexes Containing a Diphosphine Ligand with a Positioned Amine Base

摘要

The mixed-ligand complex [Ni(dppp)(P~(Ph)_2N~(Bz)_2)](BF_4)_2, 3, (where P~(Ph)_2N~(Bz)_2 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 H_2 and triethylamine results in the formation of the Ni~0 complex, Ni(dppp)(P~(Ph)_2N~(Bz)_2), 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)(P~(Ph)_2N~(Bz)(μ-H)N~(Bz))](BF_4)_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)(P~(Ph)_2N~(Bz)_2)](BF_4)_2, 5b, and the other is [Ni(dppp)(P~(Ph)_2N~(Bz)_2H_2)](BF_4)_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 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 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.