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(NBu₄)₂(NO₃)₆ was studied by NMR spectroscopy, which provided quantitative evidence for post-oxidative H₂ 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 > 10⁷ M⁻¹ s⁻¹) for the H₂ evolution step. To the best of our knowledge, this is the fastest known oxidative H₂ 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 H₂ at highly oxidizing potentials.