Mechanism-Guided Kinetic Analysis of Electrocatalytic Proton Reduction Mediated by a Cobalt Catalyst Bearing a Pendant Basic Site

Cobalt polypyridyl complexes stand out as efficient catalysts for electrochemical proton reduction, but investigations into their operating mechanisms, with broad-reaching implications in catalyst design, have been limited. Herein, we investigate the catalytic activity of a cobalt­(II) polypyridyl complex bearing a pendant pyridyl base with a series of organic acids spanning 20 pK a units in acetonitrile. Structural analysis, as well as electrochemical studies, reveals that the Co­(III) hydride intermediate is formed through reduction of the Co­(II) catalyst followed by direct metal protonation in the initial EC step despite the presence of the pendant base, which is commonly thought of as a more kinetically accessible protonation site. Protonation of the pendant base occurs after the Co­(III) hydride intermediate is further reduced in the overall ECEC pathway. Additionally, when the acid used is sufficiently strong, the Co­(II) catalyst can be protonated, and the Co­(III) hydride can react directly with acid to release H2. With thorough mechanistic understanding, the appropriate electroanalytical methods were identified to extract rate constants for the elementary steps over a range of conditions. Thermodynamic square schemes relating catalytic intermediates proposed in the three electrocatalytic HER mechanisms were constructed. These findings reveal a full description of the HER electrocatalysis mediated by this molecular system and provide insights into strategies to improve synthetic fuel-forming catalysts operative through metal hydride intermediates.