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Interplay of Hemilability and Redox Activity in Models of Hydrogenase Active Sites

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Interplay of Hemilability and Redox Activity in Models of Hydrogenase Active Sites Interplay of hemilability and redox activity in models PNAS PLUS of hydrogenase active sites Shengda Dinga, Pokhraj Ghosha, Marcetta Y. Darensbourga, and Michael B. Halla,1 aDepartment of Chemistry, Texas A&M University, College Station, TX 77843 Edited by Brian M. Hoffman, Northwestern University, Evanston, IL, and approved October 6, 2017 (received for review June 12, 2017) The hydrogen evolution reaction, as catalyzed by two electro- the arrangement shown in Fig. 1B, finding a thiol proton nearby + + + catalysts [M(N2S2)·Fe(NO)2] ,[Fe-Fe] (M = Fe(NO)) and [Ni-Fe] a hydride accommodated in a bridge position between Ni and Fe (M = Ni) was investigated by computational chemistry. As nominal (17), remarkably predicted by density functional theory (DFT) models of hydrogenase active sites, these bimetallics feature two calculations two decades ago (9, 10). Thus, in both hydrogenases kinds of actor ligands: Hemilabile, MN2S2 ligands and redox-active, the hydride-protonation mechanism (HP, also known as het- nitrosyl ligands, whose interplay guides the H2 production mech- erolytic coupling) accounts for H2 production (3). anism. The requisite base and metal open site are masked in the Interestingly, while the major function of nitrogenase (N2-ase) is resting state but revealed within the catalytic cycle by cleavage of nitrogen fixation, it is known that a molecule of H2 is an obligatory – the MS Fe(NO)2 bond from the hemilabile metallodithiolate li- side product as one molecule of N isfixedintoNH (18). Four + 2 3 gand. Introducing two electrons and two protons to [Ni-Fe] pro- equivalents of electrons and four protons are required before the duces H2 from coupling a hydride temporarily stored on Fe(NO)2 H2 is released and the N2 is initially fixed (19, 20); this is Nature’s (Lewis acid) and a proton accommodated on the exposed sulfur of creative mechanism whereby the N2-ase active site can build up – the MN2S2 thiolate (Lewis base). This Lewis acid base pair is initi- sufficient reduction power, stored as hydrides within the expanded ated and preserved by disrupting the dative donation through Fe–S cluster, to reductively activate the strong triple bond of N2. protonation on the thiolate or reduction on the thiolate-bound Hoffman and coworkers (20–23) proposed that such H2 release metal. Either manipulation modulates the electron density of the goes through a reductive elimination mechanism (RE, also known pair to prevent it from reestablishing the dative bond. The as homolytic coupling of two H·) from two hydrides, thus leaving CHEMISTRY electron-buffering nitrosyl’s role is subtler as a bifunctional elec- + two electrons localized within the cluster to fix N2 (Fig. 1C). (Note: tron reservoir. With more nitrosyls as in [Fe-Fe] , accumulated The HP mechanism is also applicable to the H2 production on N2- electronic space in the nitrosyls’ π*-orbitals makes reductions eas- ase in the absence of N2; nevertheless, the capacity for N2 fixation ier, but redirects the protonation and reduction to sites that post- requires the RE mechanism.) pone the actuation of the hemilability. Additionally, two electrons The questions crucial to the development of molecular elec- donated from two nitrosyl-buffered irons, along with two exter- trocatalysts are (i) what conditions lead to the preference for RE nal electrons, reduce two protons into two hydrides, from which vs. HP mechanisms for H2 production assisted by Fe–S clusters, reductive elimination generates H2. and (ii) can these conditions be replicated in small biomimetics of these active sites, using alternate redox-active ligands. actor ligand | biomimetic | computational mechanism | density functional theory | nitrosyl Synthetic Analogs The organometallic characteristics of the hydrogenase active ihydrogen is currently a candidate for energy storage to al- sites have led to a rich area of synthetic chemistry aiming Dleviate problems from electricity produced intermittently by to reproduce core features and delineate structure/function photovoltaic cells or wind turbines (1). Hydrogenases (H2ase) (2, ’ 3) are Nature s masterpiece enzymes for H2 production and its use Significance as an energy vector or chemical substrate; they use abundant base metals in their catalytic active sites. An array of enzymatic and Segmentation of the bimetallic electrocatalysts under in- spectroscopic probes, crowned by modern protein X-ray diffraction vestigation into a metallodithiolate, bidentate S-donor ligand, technology (2, 4), provides opportunities for structure-function and a receiver metal is effective for understanding the proton analysis of the intricate H2ase active-site molecular machinery. andelectronuptakeinH2-evolution reactions. Coexisting actor/ Strategically placed acid and base functionalities in the active site reaction-involved ligands, i.e., electron-buffering NO and hemi- guide and store protons and electrons for their efficient processing labile, chelating metallodithiolate, subtly cooperate to control into H , or the reverse, H oxidation, reaction. Currently favored 2 2 electrocatalytic H2-production mechanisms. Two mechanisms mechanisms are based on earlier proposals from computational emerge in a single catalyst to yield H : protonation of a hydride – – 2 modeling of [FeFe]- (3, 5 8) and [NiFe]-H2ase (3, 9 12). or reductive elimination from a metal dihydride. A Lewis acid– In the active site of [FeFe]-H2ase (Fig. 1A), a diiron unit takes base pair appears by cleaving the hemilabile thiolate from the up protons via an amine base strategically placed to hold and metal and serves as the reactive centers to process electrons and transfer that proton to the available open site on a reduced iron protons; a protonation or a reduction on the Lewis pair modu- to create an iron hydride, whose existence was recently spec- lates their electron densities and protects these reactive centers troscopically confirmed (13, 14); the amine then accepts another + − from converting back to a dative bond. proton (15). Importantly, the H /H components of H2 are positioned within a convenient distance for coupling over a low Author contributions: M.Y.D. and M.B.H. designed research; S.D. and P.G. performed barrier (6) (Fig. 1A). A similar strategy appears to be operative research; and S.D., M.Y.D., and M.B.H. wrote the paper. in the [NiFe]-H2ase active site; whether a guanidine base from The authors declare no conflict of interest. R509 (16), which hovers over the NiFe core and is required for This article is a PNAS Direct Submission. full enzyme activity, is the proton delivery agent itself, or a cys- Published under the PNAS license. teine (C546) thiolate sulfur, bound to the Ni (17), facilitates the 1To whom correspondence should be addressed. Email: [email protected]. + − ultimate H /H coupling, is not firmly established. Structural This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. evidence from recent high-resolution X-ray diffraction indicates 1073/pnas.1710475114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1710475114 PNAS Early Edition | 1of8 Downloaded by guest on September 26, 2021 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 pro- tected either by reduction of the Lewis acid or protonation on the Lewis base. As shown in Fig. 2, multiple electron-buffering NO ligands have been introduced into the N2S2-based bimetallic models, in attempts to reproduce the electron-buffering function of the [Fe4S4] subcluster of [FeFe]-hydrogenase (32–35). The elec- tronic flexibility introduced by NO raises the optimistic expec- tation that buffer ligands might convert first-row transition − − metals that are 1e catalysts into 2e catalysts(36,37).Another prospect is that such electron-buffering ligands prevent dramatic structural reorganization or a change in coordination number dur- ing redox activities, consistent with the structurally constrained ac- tive sites of enzymes; in this way, they might contribute to catalyst longevity. Precisely how such delocalization might affect the mechanistic behavior or the individual steps/events of our models remains a question, and is the topic of this article. Fig. 1. Active sites of (A) [FeFe]-hydrogenase (H state), (B) [NiFe]- In this work, two bimetallic complexes known to be elec- hyd + hydrogenase (Ni-R state), and (C)theH-producing intermediate of nitro- · 2 trocatalysts for proton reduction, [Fe(NO)N2S2 Fe(NO)2] + genase (E4/H4 state that fixes N2 following RE of H2), displayed with bound H ([Fe-Fe] )(32)and[NiN2S2·Fe(NO)2](Ni-Fe) (38) (Fig. 2) were and indicating coupling routes. Charges are not explicitly assigned. Hydrides investigated via computational chemistry as they contain poten- and protons are colored red and blue, respectively. tially hemilabile bridging thiolates as well as multiple electron- + buffering NO ligands. For clarity, the two irons in [Fe-Fe] are differentiated as follows: The former Fe (underlined as shown in relationships in bimetallic complexes that are electrocatalysts for Fig. 2) refers to the iron in Fe(NO)N2S2, and the latter Fe refers H2 production (2, 24, 25). Even without the enzymatic intricate to the receiver unit, Fe(NO)2.) These reaction mechanisms are positioning of proton and electron relay functions, many model + compared with a previous theoretical study of [Ni-FeCO] and complexes show positive responses to appropriate E/C condi- Fe-Fe
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