Bioinspired Copper(II) Complexes With N,S‐Donor Ligands: Structure–Activity Relationships and Mechanistic Insights for Aqueous Hydrogen Evolution Reaction

The design and development of bioinspired coordination compounds are crucial for laying the foundation to deliver low-cost, energy-efficient materials that promote energy sustainability. In this work, isostructural copper complexes, Cu-L_N2S2 and Cu-L_N3S, in tune with the donor centers in the ligand backbone, (L_N2S2 = 1,2-bis((pyridin-2-ylmethyl)thio)ethane and L_N3S =N-(pyridin-2-ylmethyl)-2-((pyridine-2-ylmethyl)thio)ethane-1-amine) have been prepared, structurally characterized, and evaluated for their electrocatalytic fate towards the sustainable hydrogen production activities in water. Single-crystal X-ray crystallography reveals that the copper centers in both complexes adopt a distorted square pyramidal geometry. Cu-L_N2S2 exhibits superior electrocatalytic performance over Cu-L_N3S in acidic aqueous media, achieving an outstanding turnover frequency (TOF) of 2.90 × 10³ s^-1 with 96% Faradaic efficiency. Detailed mechanistic insights, supported by spectroscopic, analytical, and DFT calculations, reveal divergent HER pathways: Cu-L_N3S follows an ECEC mechanism, while Cu-L_N2S2 operates via a CECE sequence. Protonation and reduction site analyses highlight the critical role of mixed hard-soft donor environments in modulating redox behavior and promoting hydride formation for efficient hydrogen production. The greater structural distortion arises from the higher number of sulfur donors in the ligand environment of Cu-L_N2S2, which in turn promotes the formation of the observed coordination geometry feasible for sustainable electrocatalytic evolution of hydrogen in water.