Pyridyl-Functionalized dmit-Based S₂N₂ Ligand Enables Proton-Shuttling Nickel Electrocatalysts for Hydrogen Evolution

Molecular catalysts built from earth-abundant metals for hydrogen evolution reaction (HER) often struggle to combine high activity, extended lifetimes, and well-defined mechanisms. We reported three nickel(II) complexes, [{dmit(Py')₂}NiX₂] (where X is Cl (1), Br (2), and NCS (3), while dmit(Py')₂ is 4,5-bis((3,4-dimethoxypyridin-2-yl)methylthio)-1,3-dithiole-2-thione), that share an S₂N₂ ligand framework integrating proton-relaying pyridyl groups into the redox-active dmit backbone. X-ray crystallography confirmed a distorted octahedral geometry at nickel. These complexes catalyzed proton reduction with overpotentials (η) of 0.67-0.79 V in 53.80 mM trifluoroacetic acid (TFA), as determined by cyclic voltammetry. Controlled potential electrolysis (CPE) of complex 2 for 8 h achieved a turnover number (TON) of 16.8 with 85.61% Faradaic efficiency. An inverse kinetic isotope effect (KIE = 0.70-0.74) suggested the formation of Ni-H intermediates, while DFT calculations supported a ligand-assisted, metal-centered ECEC mechanism. UV-vis monitoring, postelectrolysis voltammetry, and SEM imaging all pointed to homogeneous catalysis and robust complex stability. These results demonstrate that embedding both proton-shuttling sites and redox noninnocence within the ligand structure enables the development of high-performance HER catalysts, providing a rational design framework for bioinspired catalysts.