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

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 (k) 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.