Tuning Hydrogen Evolution Pathways through Ligand Design and Proton Sources for Mixed-Ligand Cobalt(II) Catalysts

In the pursuit of an effective molecular catalyst for electrocatalytic proton reduction, herein, we have reported two mixed-ligand-based cobalt(II) complexes, namely, [Co(L)(Bpy)Cl](ClO4) (1) and [Co(L)(dmBpy)Cl](ClO4) (2) (where L = 4-methoxy-N-((1-methyl-1H-benzo[d]imidazol-2-yl)methyl)-N-(pyridin-2-ylmethyl)aniline, Bpy = 2,2/-bipyridine, and dmBpy = 4,4/-dimethyl-2,2/-bipyridyl). Both complexes are well characterized using different spectroscopic and analytical techniques, including single-crystal X-ray diffraction for structure elucidation. The introduction of two methyl groups into complex 2 was aimed at elucidating the electronic effects of these substituents on the electrochemical properties of the complex and determining their impact on its electrocatalytic hydrogen evolution reaction (HER) performance. The electrocatalytic HER studies were performed using acetic acid and trifluoroacetic acid (TFA) as weak and strong proton sources, respectively, in an acetonitrile medium. The mechanism has been drawn based on experimental observations and the reported literature. This study shows the effect of the strength of two proton sources along with the electronic effect of ligands on the mechanism being followed by them. The involvement of cobalt hydride was confirmed by the electrochemical reduction of styrene to ethylbenzene. Postcatalytic studies confirmed the molecular integrity of both complexes, confirming the homogeneous nature of the catalysts.