Hydrogen Evolution Catalyzed by Corrole-Chelated Nickel Complexes, Characterized in all Catalysis-Relevant Oxidation States

Designing catalysts that are not based on precious metals remains a major research priority worldwide, particularly for the feasible production of hydrogen gas. Given the increasing interest in metallocorroles as electrocatalysts, three nickel corroles were explored as catalysts for the hydrogen evolution reaction (HER) in the current study. The precatalysts were fully characterized as square planar d8 nickel(II) coordination compounds in which the extra negative charge is stabilized by electron-withdrawing substituents on the corrole and a robust countercation. This unique feature leads to an about 0.8 V shift in redox potentials and different reaction pathways compared to trivalent and hence neutral metallocorroles, as well as increased reactivity toward weak acid relative to a neutral nickel(II) porphyrin analogue. Only singly reduced and singly oxidized reaction intermediates are shown to be involved in the catalytic cycle for the conversion of protons to hydrogen, by both experimental and computational analyses; the nickel corroles were prepared and characterized in all of the corresponding oxidation states. The nickel corrole with two sulfonamido groups on its skeleton catalyzes the HER with an onset potential of about −1 V vs Fc+/0 and thousands of turnovers per second.