Rapid oxidative hydrogen evolution from a family of square-planar nickel hydride complexes

Abstract

A series of square-planar nickel hydride complexes supported by bis(phosphinite) pincer ligands with varying substituents (-OMe, -Me, and -Bu t ) on the pincer backbone have been synthesized and completely characterized by NMR spectroscopy, IR spectroscopy, elemental analysis, and X-ray crystallography. Their cyclic voltammograms show irreversible oxidation peaks (peak potentials from 101 to 316 mV vs. Fc + /Fc) with peak currents consistent with overall one-electron oxidations. Chemical oxidation by the one-electron oxidant Ce(NBu 4 ) 2 (NO 3 ) 6 was studied by NMR spectroscopy, which provided quantitative evidence for post-oxidative H 2 evolution leading to a solvent-coordinated nickel(ii) species with the pincer backbone intact. Bulk electrolysis of the unsubstituted nickel hydride (3a) showed an overall one-electron stoichiometry and gas chromatographic analysis of the headspace gas after electrolysis further confirmed stoichiometric production of dihydrogen. Due to the extremely high rate of the post-oxidative chemical process, electrochemical simulations have been used to establish a lower limit of the bimolecular rate constant (k f > 10 7 M -1 s -1 ) for the H 2 evolution step. To the best of our knowledge, this is the fastest known oxidative H 2 evolution process observed in transition metal hydrides. Quantum chemical calculations based on DFT indicate that the one-electron oxidation of the nickel hydride complex provides a strong chemical driving force (-90.3 kcal mol -1 ) for the production of H 2 at highly oxidizing potentials.