Proposed Ligand-Centered Electrocatalytic Hydrogen Evolution and Hydrogen Oxidation at a Noninnocent Mononuclear Metal–Thiolate

Abstract

The noninnocent coordinatively saturated mononuclear metal–thiolate complex ReL₃ (L = diphenylphosphinobenzenethiolate) serves as an electrocatalyst for hydrogen evolution or hydrogen oxidation dependent on the presence of acid or base and the applied potential. ReL₃ reduces acids to H₂ in dichloromethane with an overpotential of 380 mV and a turnover frequency of 32 ± 3 s^–1. The rate law displays a second-order dependence on acid concentration and a first-order dependence on catalyst concentration with an overall third-order rate constant (<i>k</i>) of 184 ± 2 M^–2 s^–1. Reactions with deuterated acid display a kinetic isotope effect of 9 ± 1. In the presence of base, ReL₃ oxidizes H₂ with a turnover frequency of 4 ± 1 s^–1. The X-ray crystal structure of the monoprotonated species [Re­(LH)­L₂]⁺, an intermediate in both catalytic H₂ evolution and oxidation, has been determined. A ligand-centered mechanism, which does not require metal hydride intermediates, is suggested based on similarities to the redox-regulated, ligand-centered binding of ethylene to ReL₃