Hemilabile Bridging Thiolates as Proton Shuttles in Bioinspired H2 Production Electrocatalysts

Synthetic analogues and computationally assisted structure–function analyses have been used to explore the features that control proton–electron and proton–hydride coupling in electrocatalysts inspired by the [NiFe]-hydrogenase active site. Of the bimetallic complexes derived from aggregation of the dithiolato complexes MN2S2 (N2S2 = bismercaptoethane diazacycloheptane; M = Ni or Fe­(NO)) with (η5-C5H5)­Fe­(CO)+ (the Fe′ component) or (η5-C5H5)­Fe­(CO)2+, Fe″, which yielded Ni–Fe′+, Fe–Fe′+, Ni–Fe″+, and Fe–Fe″+, respectively, both Ni–Fe′+ and Fe–Fe′+ were determined to be active electrocatalysts for H2 production in the presence of trifluoroacetic acid. Correlations of electrochemical potentials and H2 generation are consistent with calculated parameters in a predicted mechanism that delineates the order of addition of electrons and protons, the role of the redox-active, noninnocent NO ligand in electron uptake, the necessity for Fe′–S bond breaking (or the hemilability of the metallodithiolate ligand), and hydride-proton coupling routes. Although the redox active {Fe­(NO)}7 moiety can accept and store an electron and subsequently a proton (forming the relatively unstable Fe-bound HNO), it cannot form a hydride as the NO shields the Fe from protonation. Successful coupling occurs from a hydride on Fe′ with a proton on thiolate S and requires a propitious orientation of the H–S bond that places H+ and H– within coupling distance. This orientation and coupling barrier are redox-level dependent. While the Ni–Fe′ derivative has vacant sites on both metals for hydride formation, the uptake of the required electron is more energy intensive than that in Fe–Fe′ featuring the noninnocent NO ligand. The Fe′–S bond cleavage facilitated by the hemilability of thiolate to produce a terminal thiolate as a proton shuttle is a key feature in both mechanisms. The analogous Fe″–S bond cleavage on Ni–Fe″ leads to degradation.