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

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)]ClO ₄ , formed using a redox-active N,O-donor Schiff base ligand (H ₂ L). The complex was thoroughly characterized by FTIR, ¹ H NMR, ESI-MS, UV-Vis, and EPR spectroscopy, confirming successful coordination to the Cu ^II center. Electrochemical studies in acetonitrile revealed quasi-reversible Cu ^II /Cu ^I and Cu ^I /Cu ⁰ redox couples, along with a ligand-centered oxidation process. The electrocatalytic hydrogen evolution reaction (HER) activity of [Cu(HL)]ClO ₄ 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 H ₂ (mol _cat ) ^-1 cm ^-2 and a TOF of 1720 mol H ₂ (mol _cat ) ^-1 h ^-1 cm ^-2 . The observed rate constant ( k _obs ) for hydrogen evolution was 1.93 × 10 ³ s ^-1 . Mechanistic studies revealed a pH-dependent behavior: at pH 4.6 and 7, catalysis proceeds via a Cu ^I intermediate; at pH 2.5, Cu ⁰ involvement indicates a mechanistic shift. UV-Vis, FTIR, and DFT analyses confirmed structural stability and supported the proposed catalytic pathway, demonstrating [Cu(HL)]ClO ₄ as a robust and efficient HER catalyst in aqueous media.