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

Abstract

The hydrogen evolution reaction, as catalyzed by two electrocatalysts [M(NS)·Fe(NO)], [-Fe] (M = Fe(NO)) and [Ni-Fe] (M = Ni) was investigated by computational chemistry. As nominal models of hydrogenase active sites, these bimetallics feature two kinds of actor ligands: Hemilabile, MNS ligands and redox-active, nitrosyl ligands, whose interplay guides the H production mechanism. The requisite base and metal open site are masked in the resting state but revealed within the catalytic cycle by cleavage of the MS-Fe(NO) bond from the hemilabile metallodithiolate ligand. Introducing two electrons and two protons to [Ni-Fe] produces H from coupling a hydride temporarily stored on Fe(NO) (Lewis acid) and a proton accommodated on the exposed sulfur of the MNS thiolate (Lewis base). This Lewis acid-base pair is initiated and preserved by disrupting the dative donation through protonation on the thiolate or reduction on the thiolate-bound metal. Either manipulation modulates the electron density of the pair to prevent it from reestablishing the dative bond. The electron-buffering nitrosyl's role is subtler as a bifunctional electron reservoir. With more nitrosyls as in [-Fe], accumulated electronic space in the nitrosyls' π*-orbitals makes reductions easier, but redirects the protonation and reduction to sites that postpone the actuation of the hemilability. Additionally, two electrons donated from two nitrosyl-buffered irons, along with two external electrons, reduce two protons into two hydrides, from which reductive elimination generates H.

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).…”

Structural and Electronic Responses to the Three Redox Levels of Fe(NO)N2S2‐Fe(NO)2

“…[19][20][21] We introduced NO into models of the [FeFe]-hydrogenase enzyme active site with the expectation that it might reproduce the function of the electron reservoir [Fe 4 S 4 ]sub-cluster in the H-cluster.C learly the binding capability of the N 2 S 2 unit to both Fe(NO) and Fe(NO) 2 persists throught wo reductionsi ncreasing the overall electronc ount by two. The XRD analysis, solid-state structures, and IR data are consistentw ith data from EPR spectroscopy, magnetic susceptibility and computational modeling that define the electronic structures of theses pecies.…”

“…[12][13][14][15] In the latter,M eyer et al pre-sentedX -ray crystal structures in three redox levels. [19][20][21] Specifically,r edox reactivity in the NO of the Fe(NO)u nit might mimic the 4Fe4S cluster of the H-cluster.G enerally,t he diiron complex offers opportunity to examine the mutual influence of redox activeu nits as mediated by sulfur bridges.As indicated in Figure 1, multiple chemical routes led to the cationic,o xidized form, {Fe(NO)} 7 -{Fe(NO) 2 } 9 ,t hat is,1 + + or 1* + + , implicating particular thermodynamic stability and as elf-assembly process. [16] While mostly explored with M = Ni 2 + ,other dications such as [Fe(NO)] 2 + ,[Co(NO)] 2 + ,and [VO] 2 + are also included in this new class of metallo-ligands.…”

“…[17] An ACS-related structure, [Fe(NO)N 2 S 2 ·Fe(NO) 2 ] + /0 was revealed by us during studies of metallodithiolates as ligands in dinitrosyliron complexes, DNICs, Figure 1. [19][20][21] Specifically,r edox reactivity in the NO of the Fe(NO)u nit might mimic the 4Fe4S cluster of the H-cluster.G enerally,t he diiron complex offers opportunity to examine the mutual influence of redox activeu nits as mediated by sulfur bridges. [19][20][21] Specifically,r edox reactivity in the NO of the Fe(NO)u nit might mimic the 4Fe4S cluster of the H-cluster.G enerally,t he diiron complex offers opportunity to examine the mutual influence of redox activeu nits as mediated by sulfur bridges.…”