Electrocatalytic hydrogen evolution by a novel copper(II) complex: influence of pH and mechanistic insight via DFT

Abstract The growing global energy crisis and the depletion of fossil fuel resources have intensified the search for sustainable, affordable, and environmentally benign energy alternatives. Hydrogen, as a clean energy carrier, offers a viable solution to reduce dependence on carbon-based fuels. In this study, we report the synthesis and hydrogen evolution activity of a novel copper(II) complex, [Cu(HL)]ClO₄, formed using a redox-active N,O-donor Schiff base ligand (H₂L). The complex was thoroughly characterized by FTIR, ¹H NMR, ESI-MS, UV-Vis, and EPR spectroscopy, confirming successful coordination to the CuII center. Electrochemical studies in acetonitrile revealed quasi-reversible CuII/CuI and CuI/Cu⁰ redox couples, along with a ligand-centered oxidation process. The electrocatalytic hydrogen evolution reaction (HER) activity of [Cu(HL)]ClO₄ was evaluated in phosphate buffer (PBS) at pH 2.5, 4.6 and 7. The catalyst showed efficient proton reduction with onset overpotentials of 892 mV (pH 2.5), 678 mV (pH 4.6) and 517 mV (pH 7). Bulk electrolysis at -1.5 V for 130 minutes at pH 2.5 produced 95 C of charge, corresponding to a TON of 3710 mol H₂ (molcat)⁻¹ cm⁻² and a TOF of 1720 mol H₂ (molcat)⁻¹ h⁻¹ cm⁻². The observed rate constant (kobs) for hydrogen evolution was 1.93 × 10³ s⁻¹. Mechanistic studies revealed a pH-dependent behavior: at pH 4.6 and 7, catalysis proceeds via a CuI intermediate; at pH 2.5, Cu⁰ involvement indicates a mechanistic shift. UV-Vis, FTIR, and DFT analyses confirmed structural stability and supported the proposed catalytic pathway, demonstrating [Cu(HL)]ClO₄ as a robust and efficient HER catalyst in aqueous media.