Dynamic Carboxylic Acid Arms Enable Proton Shuttling in Iron-Based Hydrogen Catalysis

Devising artificial electrocatalyst with smartly installed proton relay motifs, as present in natural hydrogenase enzyme has long been proved to be an effective strategy. Proton responsive groups properly positioned near the active center act as a proton shuttle site and facilitate H₂ generation via an easy hydride/proton coupling step. Herein we investigated a series of Fe (III) complexes of substituted picolinic acid which undergoes reversible dechelation and chelation in presence of acids and base respectively, making free carboxylic acid arms available near the metal center in acid conditions (CF₃COOH/HBF₄). Investigations on the electrocatalytic activity of these complexes showcased the involvement of these free carboxylic acid arms in electrocatalytic hydrogen evolution reaction (HER). Further, detailed mechanistic analysis reveals sequential two-electron reductions followed by protonation, enabled by pendant carboxylic acid arms, allow the formation of a metal hydride intermediate which facilitates efficient H₂ generation. The catalyst showed efficient HER activity achieving maximum turnover frequency (TOF_max) of 10,000 s^-1 with faradaic efficiency above 90%. These findings underscore the importance of tailored secondary-sphere interactions in designing efficient, earth-abundant electrocatalysts and these insights could be useful for future design principles of catalyst for several small molecule activation reactions.