title: Hemilabile Bridging Thiolates as Proton Shuttles in Bioinspired H2Production Electrocatalysts.
abstract: 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 H2production in the presence of trifluoroacetic acid. Correlations of electrochemical potentials and H2generation 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)}7moiety 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.
authors.affiliation: Department of Chemistry, Texas A & M University , College Station, Texas 77843, United States.
authors.forename: Shengda
authors.initials: S
authors.lastname: Ding
authors.affiliation: Department of Chemistry, Texas A & M University , College Station, Texas 77843, United States.
authors.forename: Pokhraj
authors.initials: P
authors.lastname: Ghosh
authors.affiliation: Department of Chemistry, Texas A & M University , College Station, Texas 77843, United States.
authors.forename: Allen M
authors.initials: AM
authors.lastname: Lunsford
authors.affiliation: Department of Chemistry, Texas A & M University , College Station, Texas 77843, United States.
authors.forename: Ning
authors.initials: N
authors.lastname: Wang
authors.affiliation: Department of Chemistry, Texas A & M University , College Station, Texas 77843, United States.
authors.forename: Nattamai
authors.initials: N
authors.lastname: Bhuvanesh
authors.affiliation: Department of Chemistry, Texas A & M University , College Station, Texas 77843, United States.
authors.forename: Michael B
authors.initials: MB
authors.lastname: Hall
authors.affiliation: Department of Chemistry, Texas A & M University , College Station, Texas 77843, United States.
authors.forename: Marcetta Y
authors.initials: MY
authors.lastname: Darensbourg