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.