Mechanistic Tuning of a Bio-Inspired NiFe Catalyst through Proton Relay Introduction for Enhanced H 2 Production

NiFe hydrogenases achieve efficient hydrogen evolution reaction (HER) under mild conditions, guided by a finely tuned sequence of proton- and electron-transfer events mediated by the first and second coordination spheres. Translating this mechanism into synthetic systems remains a major challenge, as most NiFe models fail to reproduce the enzymatic ECEC-type catalytic sequence (E: electron transfer, C: chemical reaction, here, a proton transfer). Here, we report the rational design of a bio-inspired NiFe complex, LN2S2NiFeCpN(CO)+, that incorporates a pendant amine in the second coordination sphere, thereby acting as a synthetically installed proton relay, by substituting the Cp ligand. This structural modification preserves the redox and electronic features of the parent complex, LN2S2NiFeCp(CO)+, while fundamentally altering its catalytic mechanism. Combined electrochemical, spectroscopic, and DFT studies reveal that the pendant amine facilitates proton-coupled electron transfer and positions a proton in proximity to the metal hydride, enabling an enzyme-like HER pathway. The resulting complex exhibits enhanced H2 production activity in terms of kinetics and operates at a lower overpotential than its nonfunctionalized analog, while the TON remains similar because of stability limitations. This work demonstrates how the precise tuning of the secondary coordination sphere can modulate both mechanism and efficiency, providing a key step toward achieving full biomimicry of NiFe hydrogenases in synthetic electrocatalysts.