Proton Reduction by Amino Acid-Based Iron Complexes: Impact of Proton Residue Concentration at Catalyst Sites

The microenvironment of a catalyst plays a vital role in the electrocatalytic proton reduction reaction, especially in aqueous media where proton sources like buffer ions, water, and hydronium ions compete. In natural enzymes, these difficulties have been overcome by judicial design of protein scaffolds, such as a proton channel, hydrophobic pocket, or preposition of the suitable amino acid residue. In the current study, a series of iron amino acid complexes ([Fe(O∩NR)2], where O∩N represents ortho imino methyl phenolate and R is glycine (C1) and aspartic acid (C2) with and without a proton channel have been studied to highlight the role of proton relay to proton shuttle during aqueous proton reduction. Complexes C1 and C2 show pH-dependent proton reduction in aqueous buffer solution. Complex C2, equipped with multiple side chain carboxylic acid groups, shows highest rate for proton reduction in pH 3 MES solution. Coordination of α-carboxylate to iron center plays a crucial role, changing the pH of the aqueous solution below 5.0 protonates the -COO- group to -COOH functionality and activates the iron Schiff base complexes, for HER catalysis. This study illuminates the introduction of a proton-shuttling channel within the catalyst's microenvironment by enhancing ∼5 times higher efficiency of the HER and paving the way for innovative advancements in sustainable energy.