Pyridyl Aroyl Hydrazone-Based Metal Complexes Showing a Ligand-Centered Metal-Assisted Pathway for Electrocatalytic...

In the past few decades, the concern over the excessive use of fossil fuels and consequent environmental damage has triggered the search for cleaner and renewable energy resources. This has led to the consideration of hydrogen as our future fuel. It can be produced from various sources, and if used as fuel, it generates only water as a byproduct. Electrocatalytic reduction of protons (2H⁺ + 2e^- → H₂) is one of the available methods for producing hydrogen gas at large scales. Currently, the most efficient electrocatalysts are Pt-based complexes. In order to make the entire process more cost-effective, it has become necessary to find electrocatalysts that are derived from earth-abundant metals such as Ni, Zn, Fe, etc. Herein, we have demonstrated the electrocatalytic hydrogen evolution reaction (HER) catalyzed by pyridyl aroyl hydrazone ligand (HL)-based metal complexes (M = Fe, Co, Ni, Cu, and Zn). Rate calculations using controlled potential electrolysis revealed the optimal overall catalytic performance of NiL₂ among the investigated complexes. For NiL₂, a maximum turnover frequency of 7040 s^-1 with a 0.42 V overpotential was obtained when triethylamine hydrochloride was used as a proton source. Both experimental and density functional theory (DFT) calculations suggested a ligand-centered metal-assisted catalysis pathway for NiL₂ in the presence of triethylammonium chloride.