Hemilabile Bridging Thiolates as Proton Shuttles in Bioinspired H₂ Production 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 MNS (NS = bismercaptoethane diazacycloheptane; M = Ni or Fe(NO)) with (η-CH)Fe(CO) (the Fe' component) or (η-CH)Fe(CO), 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 H production in the presence of trifluoroacetic acid. Correlations of electrochemical potentials and H 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)} 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.

Critical computational analysis illuminates the reductive-elimination mechanism that activates nitrogenase for N 2 reduction

“…What differentiates the behavior of S 5B and S 2B in E 4 (4H) is likely to be in some part the presence of the H bond to S 2B from 195 His ; in the low-lying states with broken Fe-S 2B bond, rearrangement of the bridging hydrides makes S 2B unique. We hypothesize that, as proposed for some H 2 -producing catalytic systems (29,30), the hemilability of the Fe-S 2B bond is important to creating an efficient H 2 re pathway with the concomitant binding of N 2 in which Fe 2 first binds both hydrides, and then forms H 2 . We conclude, anticipating the results presented in the next section, that upon release of H 2 and binding of N 2 , the Fe 2 -S 2B bond is restored.…”

“…The resulting structural model is then used to characterize the alternative E 4 "protonation isomers," including, but not limited to, characterization of the ground-state structure of E 4 (4H). In doing this, we are led to consider the hemilability of Fe-S bonds during catalysis (28)(29)(30).…”

Interplay of hemilability and redox activity in models of hydrogenase active sites

“…If the components coexist within a convenient distance, the base and acid sites can be used to assist chemical reactions. In fact, reaction-created Lewis acid-base pairs, such as this one, handle the hydrides and the protons, respectively, throughout the catalytic cycle, and account for the catalytic activity of bimetallic models which do not contain obvious built-in Lewis bases as proton shuttles (31). To stabilize the Lewis pair and to avoid the reinstatement of the dative bond, the pair is protected either by reduction of the Lewis acid or protonation on the Lewis base.…”

An S‐Oxygenated [NiFe] Complex Modelling Sulfenate Intermediates of an O2‐Tolerant Hydrogenase

“…[6,[15][16][17][18] [19] To gain insight into the O 2 tolerance of [NiFe] H 2 ases,the (reversible) Soxygenation of mononuclear Ni and homobimetallic [FeFe] thiolato complexes was investigated in pioneering studies by Darensbourg and co-workers. Thus far,however, extensive modelling of the [NiFe] H 2 ase active site has mainly focused on proposed intermediates of catalytic H 2 cycling and the reactivity of the derived molecular mimics.…”

“…Thus far,however, extensive modelling of the [NiFe] H 2 ase active site has mainly focused on proposed intermediates of catalytic H 2 cycling and the reactivity of the derived molecular mimics. [6,[15][16][17][18] [19] To gain insight into the O 2 tolerance of [NiFe] H 2 ases,the (reversible) Soxygenation of mononuclear Ni and homobimetallic [FeFe] thiolato complexes was investigated in pioneering studies by Darensbourg and co-workers. [20,21] However,t ot he best of our knowledge,heterobimetallic S-oxygenated [NiFe] complexes with mixed thiolato and sulfenato bridging moieties have not been reported to date.…”