Synthesis, structure and reactivity of Ni site models of [NiFeSe] hydrogenases

Abstract

A series of structural models of the Ni centre in [NiFeSe] hydrogenases has been developed which exhibits key structural features of the Ni site in the H₂ cycling enzyme. Specifically, two complexes with a hydrogenase-analogous four-coordinate ‘NiS₃Se’ primary coordination sphere and complexes with a ‘NiS₂Se₂’ and a ‘NiS₄’ core are reported. The reactivity of the complexes towards oxygen and protons shows some relevance to the chemistry of [NiFeSe] hydrogenases. Exposure of a ‘NiS₃Se’ complex to atmospheric oxygen results in the oxidation of the selenolate group in the complex to a diselenide, which is released from the nickel site. Oxidation of the selenolate ligand on Ni occurs approximately four times faster than oxidation with the analogous sulfur complex. Reaction of the complexes with one equivalent of HBF₄ results in protonation of the monodentate chalcogenolate and the release of this ligand from the metal centre as a thiol or selenol. Unrelated to their biomimetic nature, the complexes serve also as molecular precursors to modify electrodes with Ni–S–Se containing particles by electrochemical deposition. The activated electrodes evolve H₂ in pH neutral water with an electrocatalytic onset potential of −0.6 V and a current density of 15 μA cm⁻² at −0.75 V vs. NHE.