Interplay of hemilability and redox activity in models PNAS PLUS of 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 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 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 , 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 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 ]+ = = [ CO (31). The detailed computational mechanistic study tions (E electron addition; C proton addition). Developed described below delineates sequences of protonation and reduction from systematic alterations in such bimetallics is a series shown of the bimetallics and the consequent coupling of electrons and in Fig. 2 of minimal synthetic analogs containing dithiolate- – – protons to H2. Importantly, the increased electron-buffering ca- bridged Fe Fe or Ni Fe cores derived from MN2S2 metal- pacity conveyed by NO was found to influence the hemilability of loligands; all are at least modest electrocatalysts for proton re- the bridging thiolates and essentially change the working mecha- duction/hydrogen production. The MN2S2 metalloligand (26, 27) nism, especially controlling how H2 is produced: HP,orRE. The provides a variable platform according to the carbon connectors connections between these two categories of actor ligands, i.e., within the N2S2 tetradentate ligand, as well as the M itself. + + hemilabile and redox active ligands, provide insight to the The M, as Ni2 , or particularly in the form of [Fe(NO)]2 and + [Co(NO)]2 , may tune the donor properties of the thiolates. The nitrosyl, NO, attached to metal, facilitates redox events because it features π*-orbitals close in energy to a metal’s d orbitals. The energetic proximity enables orbital admixtures so that the M (NO)x moiety shares the electrons during redox activities while the delocalization enables the electrons to flow between M and NO easily. Such orbital mixing creates electronic flexibility in the M(NO)x unit during redox processes, but it prohibits the clear assignments of electrons; this ambiguity was defined to be “non- innocence” by Jørgensen (28). The Enemark–Feltham (29) (E-F) electron count (the metal d electrons plus NO π*-electrons) was introduced to circumvent the partitioning of electrons between the metal and the nitrosyl(s). The receiver groups, i.e., the sec- ond metal (all are iron in our series) bound to the metalloligand MN2S2, may also be modulated to test and verify the results via structure-function analysis. Notably, the hemilability, originally defined by Rauchfuss for P–O bidentate ligands (30), of the metallodithiolate ligands, i.e., their ability to dissociate one arm of the bidentate ligand while main- taining integrity of the bimetallic, was previously determined to + contribute to the catalytic activity of complexes [NiN2S2·Fe(CO)Cp] + 5 + ([Ni-FeCO] ,Cp= η -C5H5) and [Fe(NO)N2S2·Fe(CO)Cp] + ([Fe-FeCO] ) (31) (Fig. 2). The dissociation of one S–Fe(CO)Cp dative bond cleaves the S-donor (Lewis base) and creates a metal Fig. 2. Structural representations of electrocatalysts for proton reduction: open site (Lewis acid). If the components coexist within a convenient + + + [Ni-Fe ] ,[Fe-Fe ] (31), Ni-Fe (38), [Fe-Fe] (32). The background of each distance, the base and acid sites can be used to assist chemical CO CO – species shows the cyclic voltammograms (CVs) before (blue) and after (red) reactions. In fact, reaction-created Lewis acid base pairs, such the addition of acid. The current enhancement in the red scan is determined

as this one, handle the hydrides and the protons, respectively, to relate to H2 production. For Ni-Fe, the CVs were obtained from dimeric 2+ + throughout the catalytic cycle, and account for the catalytic activity [Ni-Fe]2 which was calculated to dissociate into [Ni-Fe] in solution.

2of8 | www.pnas.org/cgi/doi/10.1073/pnas.1710475114 Ding et al. Downloaded by guest on September 26, 2021 question as to why Nature settled on the unique configurations The Second Reduction and Associated Geometric and Electronic PNAS PLUS + that are found in redox-active, metalloenzyme active sites. Reorganization. The reduction of [Ni-FeH] at −1.28 V and its further geometric changes are reported in Fig. 3, II. The incoming General Computational Methodology electron is initially shared by both metals of Ni-FeH,asNi’s only All structures were fully optimized with the crystal structures of vacant orbital dx2−y2 is heavily destabilized and Fe already has + Ni-Fe, [Fe-Fe] and the reduced form Fe-Fe, imported as geo- 18 electrons. Nevertheless, the electron-buffering effect of NO li- metric starting points in the computational investigations by gands in the Fe(NO)2 moiety, along with the electron depletion by DFT. Natural bond orbital (NBO) method was applied to certain the hydride, facilitates the acceptance of the second electron at a species for bonding analysis. Further details of the methodology moderate potential. However, to lower the energy of this electron- and optimized coordinates are provided in SI Appendix.The rich species, the hemilabile bridging thiolate easily dissociates the comparison of experimental and computed metric data for S–Fe bond with a concomitant shift of the added electron to Fe + Ni-Fe#H # Ni-Fe, [Fe-Fe] , and Fe-Fe in SI Appendix, Table S1 and selected (NO)2, now a 17-electron species, (S denotes the S of the S–Fe bond that will be broken and, in the text, Fe# denotes the experimental and calculated IR frequencies given in SI Appendix, # Table S2 validate the calculations. The bond-distance error is Fe with the broken S–Fe bond). The Ni-Fe H then rotates the generally less than 1%, with the maximum less than 2%. One hydride beneath the Ni and Fe, creating a semibridging hydride, d = = exception is the metal–metal distance as here there is no co- (Ni-H) 1.691 Å and d(Fe-H) 1.587 Å, and inverting the C2 Ni-H-Fe#′ ′ valent bond between them. It is noteworthy that the oxidized linker, to produce ( indicates the inverted C2 linker). 2+ Ni-H-Fe#′ state of Ni-Fe crystallizes in the dimeric form [Ni-Fe]2 , but the The nickel in has a distorted trigonal bipyramidal ge- τ = dimer was calculated to dissociate in solution (SI Appendix, Ta- ometry ( 0.48) (41), which stabilizes a high-spin state and Ni-Fe]+ results in the antiferromagnetic coupling between the high-spin ble S4); hence the oxidized monomer [ is considered as II 8 9 part of the catalytic cycle and its structure is calculated to be Ni (d )andthe{Fe(NO)2} (SI Appendix,Fig.S3shows spin- analogous to the reduced monomer Ni-Fe. density changes during the geometric reorganizations). The bridg- To stoichiometrically produce a molecule of H on an elec- ing hydride on the mimics of [NiFe]-H2ase has long been featured in 2 – trocatalyst, two protons (i.e., chemical, C, steps) and two elec- the literature (3, 42 45) with a recent interpretation of a model with trons (i.e., electrochemical, E, steps) must be introduced to the the hydride closer to Ni, as it appears to be in the enzyme (46). After catalytic site. After each reduction, any immediate geometric these geometric changes, the second reduction is fully assigned to Fe(NO)2 as the concomitant actuation of hemilability facilitates reorganization is treated as part of the corresponding E step, i.e., CHEMISTRY “ ” the accommodation of the incoming electron, as in the case of a so-called concerted (39) E step. Generally, the E and C steps Fe-Fe ]+ alternate to avoid the accumulation of like charges (31, 40). The [ CO (31). likelihood of each C step is evaluated in our computations by The Second Protonation and the Production of H2. The second comparing the acidities of the protonated species vs. the proton # + protonation on Ni-H-Fe ′ and successive H2 production are provider HOEt2 (as the acid is HBF4·OEt2 or HOEt2·BF4) # # + presented in Fig. 3, III. The S of Ni-H-Fe ′ is an ideal target for (32, 38). A positive ΔpKa (ΔpKa = pKa(CatH) − pKa([HOEt2] ) # # + protonation (ΔpKa of 14.4) producing [Ni-H-Fe ′-S H] . The indicates a thermodynamically favorable C step. Each E step has # # + + thiol–hydride pair in [Ni-H-Fe ′-S H] is already in spatial a calculated redox potential E1/2 (vs. Fc /Fc), which is compared proximity (2.773 Å) and they exothermally couple to H2 over a with the experimentally applied electrode potential derived from 2 barrier of 7.4 kcal/mol without formation of a σ-complex (η -H2) cyclic voltammetry. As a result, the mechanisms will be pre- # + intermediate. The H2 release restores the Fe–S bond in [Ni-Fe′] sented as a set of equilibrium values (G/ΔG, ΔpKa, and E1/2) for and the inverted C2 linker reverses to regenerate the catalyst evaluating the thermodynamic preference of each step. In ad- [Ni-Fe]+ TS . Thus, the [ECEC] catalytic cycle in Fig. 3, III closes with dition, transition-state barriers (G ) for steps other than proton an HP step. and electron transfers are calculated to determine whether such The calculations predict that second reduction event at −1.28 V a step is kinetically allowed. The geometric representations of (calculated) should produce the catalytic wave. However, this cat- species within these electrochemical cycles were based on the alytic wave appears experimentally at approximately −0.70 V and optimized structures. the current increases with additional equivalents of HBF •OEt Ni-Fe Fe-Fe 4 2 The mechanisms for H2 production by and ,as (38). The early appearance of the catalytic wave indicates that the described below, start in parallel with the previous study of second reduction in the mechanism is a proton-coupled electron Ni-Fe ]+ Fe-Fe ]+ [ CO and [ CO (31). But, the mechanisms soon di- transfer (PCET) as in an [ECEC] cycle. The calculated standard verge as the effects of multiple redox-active NO ligands on Ni-Fe potential (see SI Appendix,Fig.S1for more information) of this + and Fe-Fe appear to redirect the protonations and the reductions to proton-coupled reduction from [Ni-FeH] (Fig. 3, I) to the resting # + different recipients. state [Ni-H-Fe ′-S#H] (Fig. 3, III)is−0.32 V, less negative than the calculated potentials for both standalone reduction events, −0.77 – # + Mechanism of H2 Production on the Ni Fe Model Complex and −1.28 V. Of course, the resting state [Ni-H-Fe ′-S#H] differs + + The First Reduction and the First Protonation. Fig. 3, I shows the first from [Ni-FeH] significantly; thus, the actual PCET to [Ni-FeH] reduction at −0.77 V (exp. −0.72 V) on the oxidized monomer Ni-H-Fe#′-S#H]+ + 2+ cannot generate [ without geometric reorga- [Ni-Fe] , dissociated from the dimer [Ni-Fe]2 in solution (see nizations over barriers (Fig. 3). The calculations essentially set SI Appendix, Table S4 for the ΔG of dissociation reactions) and arange,from−0.32 to −1.28 V, for the reduction potential for Ni-Fe]+ + the successive protonation. The Fe(NO)2 moiety of [ a PCET process to [Ni-FeH] ; thus, the existence of an un- accepts the electron and increases the E-F electron count from resolved PCET process may explain the appearance of the cat- 9 10 Ni-Fe]+ {Fe(NO)2} to {Fe(NO)2} . Although [ itself cannot be alytic wave at a less negative potential than that calculated. Ni-Fe] protonated by HBF4·OEt2 (38), the reduced species [ The intermolecular protonation by the acid on the hydride of Δ = # accepts a proton ( pKa 12.9 with respect to HBF4·OEt2). Ni-H-Fe ′ of Fig. 3, II to produce H2 directly from the added Other possible protonation sites are listed in SI Appendix, Table acid is ruled out by a barrier ∼10 kcal/mol higher than that of the # S3. Protonation reduces the E-F electron count of the Fe(NO)2 protonation on S (SI Appendix, Fig. S4). An alternate hydride- moiety to eight as two electrons are consumed by the Fe–H bearing species is presented in SI Appendix, Fig. S5. As in the + + bond, but the overall electron count of the iron in [Ni-FeH] previous work with [Ni-FeCO] (31), a third electron could be [Ni-FeH]+ # # + remains at 18. The , with reduced basicity after the added to [Ni-H-Fe ′-S H] before it releases H2, which would first protonation, cannot accept a second proton. render an E[CECE] catalytic cycle (SI Appendix,Fig.S6). However,

Ding et al. PNAS Early Edition | 3of8 Downloaded by guest on September 26, 2021 + Fig. 3. Computational mechanism of electrocatalytic H2 production on [Ni-Fe] in the presence of HBF4•OEt2: The Gibbs free energies in kcal/mol are scaled to the reference point (G = 0), which resets after every reduction or protonation. The reduction potentials (E1/2) are reported in volts with reference to the +/0 + standard redox couple Fc = 0.0 and the relative acidities (ΔpKa) are reported versus [HOEt2] .

+ for this particular catalyst [Ni-Fe] , the third electron would not does not couple thiol-hydride to generate H2 but transfers the appear to accelerate the catalytic cycle; see SI Appendix for more proton on S to the Fe(NO)2 to create the intermediate [Fe-H- + information. FeH] , featuring one terminal and one (semi)bridging hydride. This dihydride may also be created by the direct protonation of Mechanism of H Production on the Fe-Fe Model Complex 2 Fe-H-Fe on Fe(NO)2, ΔpKa = 13.9. Either protonation has a The First and Second Reduction and the Roaming of the First Proton. negligible barrier (SI Appendix, Fig. S8) and leads to the same + The [Fe-Fe] complex, described here with NO ligands on both productive process. irons, is also an effective electrocatalyst for H2 production with Fig. 4, IV shows the final step, the reductive elimination of H2 + HBF4·OEt2 (32). Fig. 4, I depicts the first two steps of the from two hydrides on [Fe-H-FeH] over a low barrier. The [Fe- 9 + mechanism, the reduction and protonation of the {Fe(NO)2} Fe] is regenerated after the exothermic release of H2 and this + moiety in the [Fe-Fe] , consistent with our earlier experimental [ECEC] catalytic cycle closes with an RE step. The reduction of + and theoretical study (32). Fig. 4, II shows the second reduction [Fe-FeH] (at −1.29 V, calculated) is expected to produce a event and associated geometric changes. The second electron catalytic wave. With only a few equivalents of added acid, the + reduces the iron-mononitrosyl in [Fe-FeH] to {Fe(NO)}8, best experimental catalytic wave appears as early as −0.8 V, which described as antiferromagnetically coupled high-spin FeII (S = 2) overwhelms the shoulder peak at ∼−0.7 V representing the first − and high-spin NO (S = 1) which results in a linear NO (173.1°) electron reduction (32). Again, the discrepancy may be attributed to a (32). Next, hydrogen in Fe-FeH migrates from the iron-dinitrosyl PCET process. This standard reduction potential (SI Appendix,Fig. + to the iron-mononitrosyl while an electron migrates in the op- S1)is−0.28 V from [Fe-FeH] (Fig. 4, I) to the resting state [Fe-H- # # + posite direction, which results in the intermediate Fe-H-Fe with a Fe ′-S H] (Fig. 4, III), if the necessary geometric reorganizations in semibridging hydride. The six-coordinate iron in the{Fe(NO)}7 between are ignored. This value, −0.28 V, is the high limit of the − has a vacant dz2 orbital due to the strong axial hydride ligand, thermodynamic potential of the PCET process with 1.29 V as [H-Fe-Fe-H]+ which forces the unpaired electron into dx2−y2; thus, the nitrosyl the low limit. The addition of a third electron to has no tendency to bend (175.7°) to mix its π*-orbital with Fe dz2. before H2 release is also possible, yielding an E[CECE] cat- + An alternate, but less favorable, roaming route and product is alytic cycle, as in the previous report of [Fe-FeCO] (31). More summarized in SI Appendix, Fig. S7. information is provided in SI Appendix,Fig.S9.

The Second Protonation and the Production of H2. The second Discussion protonation step is summarized in Fig. 4, III. Protonation on one The Factors Controlling the Actuation of Hemilability. The conditions of the two bridging thiolates (ΔpKa = 7.9) of Fe-H-Fe initially wherein the hemilability can be triggered in these bimetallics breaks the S#–Fe(NO) bond but this species rearranges by with bridging thiolates as core structures provide interesting # # breaking the S –Fe(NO)2 bond, restoring the S –Fe(NO) bond, comparisons. As summarized in Fig. 5, as few as one and as many # # + and inverting the C2 linker. The product, [Fe-H-Fe ′-S H] , as four steps may be required. Here, we note that the number of

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+ Fig. 4. Computational mechanism of electrocatalytic H2 production on [Fe-Fe] in the presence of HBF4•OEt2. Note that one transition state was marked with a star as its Gibbs free energy is lower than its immediate precursor, which is caused by the error of solvation and thermal corrections. See the legend of Fig. 3 for more explanation.

steps correlates with the number of NO ligands installed on the the two S–FeI(CO)Cp bonds to reduce the 19-electron count for iron + II 8 catalyst. In the [Ni-FeCO] (Fig. 5), the Ni (d ) within the to 17, as indicated by an irreversible reduction event in the absence metalloligand is unable to hold onto the incoming electron, of the acid (31). See SI Appendix,Fig.S10for the spin-density plot resulting in an internal electron transfer to the 6-coordinate, 18- after the bond dissociation. This type of reduction-actuated hemil- + electron FeII (d6)inFe(CO)Cp , which concomitantly breaks one of ability of a multidentate ligand has precedent in the tridentate

Fig. 5. Conditions required to realize the hemilability of the bridging thiolates on different models. E indicates an electrochemical step (reduction) and C,a chemical step (protonation).

Ding et al. PNAS Early Edition | 5of8 Downloaded by guest on September 26, 2021 + incoming electron. Since for [Fe-Fe] the second electron is again buffered by the Fe(NO), as one saw in the first reduction of + [Fe-FeCO] , a fourth step, the second protonation, which occurs on thiolate sulfur this time, is needed for the S–Fe(NO)2 cleavage, + as in the first protonation for [Fe-FeCO] .

The Lewis Acid–Base Pair Generated from the Cleavage of the S–Fe Bond. Fig. 6 summarizes the key mechanistic aspects of the ac- tuation of the hemilability of the bridging thiolate in the reactive centers to assist the electrocatalytic process of H2 production. The first reaction (Fig. 6, Left) shows a simple dative-bond dis- Fig. 6. Actuation of the hemilability of the thiolate. Either the reduction on ruption, which would be expected to be short-lived as it is ther- the dissociated metal, or the protonation on the dissociated sulfur, finalizes modynamically advantageous to reestablish the dative bond. In the dative-bond dissociation and preserves the reactive sites. our mechanistic study, it was discovered that this bond dissoci- ation is synergic with either reduction or protonation. In other Me2 +/0 words, either of these manipulation modulates the electron trispyrazolborate ligand of [Rh(CO)(PPh3)Tp ] and derivatives by Geiger et al. (39) and Connelly et al. (47, 48). density on the Lewis acid or the Lewis base to quench their + The complex [Fe-FeCO] (Fig. 5) places the incoming electron acidity or basicity and prevent the reformation of the dative bond 7 on {Fe(NO)} in the N2S2 site, which has electron-buffering (Fig. 6, Right). capacity and does not initiate bond dissociation. Here a second Inspection of the correlation between the number of steps to step, the protonation of S, which is quite basic after the re- trigger the hemilability and the number of NO ligands (Fig. 5) leads duction, removes electron density from S, reduces the donation to the conclusion that the electron-buffering capacity demonstrated + to Fe(CO)Cp , and leads to the S–Fe bond rupture (31). NBO by NO ligand prevents an earlier S–Fe bond dissociation as it in- analysis in SI Appendix, Fig. S11 shows that both the S–Fe bond terferes with either endeavor to make the bond dissociation irre- (before the protonation) and the S–H bond (after the pro- versible, by redirecting the reduction or protonation. The more tonation) primarily use the 3p orbital of the three-coordinate nitrosyls the complex has, the more electron-buffering capacity it S, while the remaining lone pair is dominated by the less ac- receives such that more reduction/protonation steps are needed cessible 3s orbital. before the dative-bond dissociation occurs. + + + In contrast to [Ni-FeCO] and [Fe-FeCO] ,the[Ni-Fe] and + H Production Step: HP vs. RE. [Fe-Fe] complexes, whose H2 production mechanisms are pre- 2 The increasing number of nitrosyls sented in this work, require additional electrons and protons to on the model also makes formation of the hydride(s) easier. For Fe-Fe + dissociate the S–Fe(NO)2 bond. The reason lies in the nitrosyl [ ] , two external electrons are enough to reduce two protons + + + ligand’s electron-buffering capacity. In both [Ni-Fe] and [Fe-Fe] , and create two hydrides, one on each iron of [Fe-H-Fe-H] , with 9 the {Fe(NO)2} moiety accepts the first electron to produce the two additional internal electrons coming from the Fe(NO)x 10 π {Fe(NO)2} ; this reduction does not require the dissociation of a fragments buffered by NO *-orbitals. In comparison, other μ S–Fe(NO)2 bond to accommodate the incoming electron. Since [FeFe]-H2ase models, such as (dppv)(CO)Fe( -pdt)Fe(CO)(dppv) the NO’s electron-buffering capacity supports the electron-rich Fe, (49, 50) and (dppv)(CO)Fe(μ-edt)Fe(CO)3 (edt = ethane-1,2- 10 the first proton goes to the {Fe(NO)2} moiety to create an iron dithiolate) (51) that lack an electron reservoir such as NO, ex- hydride, rather than to the sulfur. Still these two steps do not elicit perimentally achieve a dihydrido derivative by the addition of an the S–Fe(NO)2 bond dissociation. However, the second reduction to exogenous hydride. + HP [Ni-Fe] triggers the S–Fe(NO)2 bond dissociation as neither the The coupling to produce H2 varies mechanistically. The 8 now-saturated, five-coordinate {Fe(NO)2} -hydride nor the square- mechanism only needs one hydride, which means it can occur on II RE planar Ni can accept the electron easily. Thus, the S–Fe(NO)2 less electron-rich metal complexes, while the needs two hy- HP bond now cleaves after three steps, ECE, so that the FeH(NO)2 drides to produce H2.Thus,the process operates in the [ECEC] + + moiety becomes four-coordinate and has the vacancy for the second and E[CECE] cycles of both [Ni-FeCO] and [Fe-FeCO] (31). For

Fig. 7. H2 production by either HP or RE coupling and their immediate precursors.

6of8 | www.pnas.org/cgi/doi/10.1073/pnas.1710475114 Ding et al. Downloaded by guest on September 26, 2021 + PNAS PLUS [Ni-Fe] , the [ECEC] cycle involves the HP coupling, while with a strategy used by Nature to enable nonnoble metals to do + one more electron in the E[CECE] cycle [Ni-Fe] switches to the multielectron chemistry. RE coupling process (SI Appendix,Fig.S5). In other words, the RE coupling is made possible by the electronic flexibility of NO Summary ligands attached to metals. Finally, the RE mechanism takes over Our theoretical investigation highlights the role of the hemil- completely in either the [ECEC] cycle or the E[CECE] cycle (SI ability of the bridging dithiolate and the electron-buffering ca- + Appendix,Fig.S7) of the trinitrosyl species [Fe-Fe] .Fig.7sum- pacity of ligands such as NO in bimetallic electrocatalysts. Upon marizes the H2 production steps and corresponding immediate the actuation of the hemilability by dative-bond dissociation, precursors of all four electrocatalysts. From the aspect of Lewis acid and base sites are created and serve as reactive cen- RE structure-function analysis, the coupling requires one more ters. To maintain the availability of the Lewis acid–base pair, it Lewis acid to park the additional hydride while eliminating the must be protected from reformation of the dative bond. The necessity of a Lewis base to store the proton, which is however protection for our systems is achieved by the modification of HP required by coupling. Although the hemilability of the thi- the electron densities either on the acid (by reduction) or on the olate may still be important in molecular isomerization and the base (by protonation) to prohibit a stable donation from the base resting states, the Lewis base required to hold a proton is no Fe-Fe + to the acid. longer mandatory for [ ] , as the incoming proton can al- The electron buffering by the NO provides metal sites with ways be stored as a hydride, whose production uses the electrons flexibility such that the protonation and reduction are directed to held by the irons and their nitrosyl(s). sites other than the potential Lewis base and acid sites. The The relevance between electron availability and H2 coupling mechanism is also present in the enzymes: [NiFe]-H ase (with no realization of the hemilability of the bridging thiolate is modu- 2 lated by its interplay with the NO ligand. Thus, NO can interfere immediate buffering) and [FeFe]-H2ase (with the [Fe4S4] sub- cluster to buffer one electron) can provide two and three elec- with the early (upon reduction) hemilability of the MN2S2 ligand and postpones the creation of the Lewis acid–base pair. The trons on the most reduced Ni-L and Hsred states, respectively. In other words, they are unable to generate two hydrides, even if electron-buffering NO ligands also provide an electron-rich metal there are enough vacant sites for two hydrides; therefore, they site that facilitates conversion of protons to hydrides, pivoting the can only proceed through the HP mechanism (2). Intriguingly, mechanism from HP to RE as multiple hydrides become available. nitrogenase, whose electron buffering can be attributed to the TheultimateroleofNOinthemodelsisrecognizedtobeabi- extensive delocalization of the FeS cluster (Fig. 1), takes in four functional electron reservoir and it (at least partially) reproduces CHEMISTRY electrons to create two hydrides on the E4 state and it can exe- the function of the [Fe4S4]subclusterof[FeFe]-H2aseactivesite. cute an RE step to produce H2 concomitantly with N2 fixation. This RE step strategically deposits two reduction equivalences ACKNOWLEDGMENTS. We are grateful for computing resources provided by needed for the initial uptake and activation of N in the nitro- the Laboratory for Molecular Simulation and the High-Performance Re- 2 search Computing Facility at Texas A&M University. This work was funded genase FeMo cofactor (19, 21–23). The E4 state of nitrogenase HP by the National Science Foundation (CHE-1300787, CHE-1664866 to M.B.H. can actually be discharged with an step to produce H2, but in and CHE-1266097, CHE-1665258 to M.Y.D.) and the Robert A. Welch Foun- the absence of N2 (20). In conclusion, such electron delocalization is dation (A-0648 to M.B.H. and A-0924 to M.Y.D.).

1. Cook TR, et al. (2010) Solar energy supply and storage for the legacy and nonlegacy 16. Evans RM, et al. (2016) Mechanism of hydrogen activation by [NiFe] hydrogenases. worlds. Chem Rev 110:6474–6502. Nat Chem Biol 12:46–50. 2. Lubitz W, Ogata H, Rüdiger O, Reijerse E (2014) Hydrogenases. Chem Rev 114: 17. Ogata H, Nishikawa K, Lubitz W (2015) detected by subatomic resolution 4081–4148. protein crystallography in a [NiFe] hydrogenase. Nature 520:571–574. 3. Siegbahn PEM, Tye JW, Hall MB (2007) Computational studies of [NiFe] and [FeFe] 18. Simpson FB, Burris RH (1984) A nitrogen pressure of 50 atmospheres does not prevent hydrogenases. Chem Rev 107:4414–4435. evolution of hydrogen by nitrogenase. Science 224:1095–1097. 4. Mulder DW, et al. (2011) Insights into [FeFe]-hydrogenase structure, mechanism, and 19. Hoffman BM, Lukoyanov D, Yang Z-Y, Dean DR, Seefeldt LC (2014) Mechanism of maturation. Structure 19:1038–1052. nitrogen fixation by nitrogenase: The next stage. Chem Rev 114:4041–4062. 5. Cao Z, Hall MB (2001) Modeling the active sites in metalloenzymes. 3. Density func- 20. Hoffman BM, Lukoyanov D, Dean DR, Seefeldt LC (2013) Nitrogenase: A draft – tional calculations on models for [Fe]-hydrogenase: Structures and vibrational fre- mechanism. Acc Chem Res 46:587 595. quencies of the observed redox forms and the reaction mechanism at the diiron active 21. Lukoyanov D, et al. (2016) Reductive elimination of H2 activates nitrogenase to re- ≡ center. J Am Chem Soc 123:3734–3742. duce the N N triple bond: Characterization of the E4(4H) Janus intermediate in wild- – 6. Fan H-J, Hall MB (2001) A capable bridging ligand for Fe-only hydrogenase: Density type enzyme. J Am Chem Soc 138:10674 10683. functional calculations of a low-energy route for heterolytic cleavage and formation 22. Lukoyanov D, et al. (2016) Reversible photoinduced reductive elimination of H2 from the nitrogenase dihydride state, the E( (4H) Janus intermediate. J Am Chem Soc 138: of dihydrogen. J Am Chem Soc 123:3828–3829. 4) 1320–1327. 7. Liu Z-P, Hu P (2002) Mechanism of H2 on Fe-only hydrogenases. J Chem 23. Lukoyanov D, et al. (2015) Identification of a key catalytic intermediate demonstrates Phys 117:8177–8180. that nitrogenase is activated by the reversible exchange of N for H . J Am Chem Soc 8. Liu Z-P, Hu P (2002) A density functional theory study on the active center of Fe-only 2 2 137:3610–3615. hydrogenase: Characterization and electronic structure of the redox states. JAm 24. Tard C, Pickett CJ (2009) Structural and functional analogues of the active sites of the Chem Soc 124:5175–5182. [Fe]-, [NiFe]-, and [FeFe]-hydrogenases. Chem Rev 109:2245–2274. 9. Niu S, Hall MB (2001) Modeling the active sites in metalloenzymes 5. The heterolytic 25. Schilter D, Camara JM, Huynh MT, Hammes-Schiffer S, Rauchfuss TB (2016) Hydrogenase bond cleavage of H( ) in the [NiFe] hydrogenase of Desulfovibrio gigas by a nucleo- 2 enzymes and their synthetic models: The role of metal hydrides. Chem Rev 116: philic addition mechanism. Inorg Chem 40:6201–6203. 8693–8749. 10. Niu S, Thomson LM, Hall MB (1999) Theoretical characterization of the reaction in- 26. Denny JA, Darensbourg MY (2015) Metallodithiolates as ligands in coordination, bi- − termediates in a model of the nickel iron hydrogenase of Desulfovibrio gigas. JAm oinorganic, and organometallic chemistry. Chem Rev 115:5248–5273. – Chem Soc 121:4000 4007. 27. Denny JA, Darensbourg MY (2016) Approaches to quantifying the electronic and 11. Siegbahn PEM (2004) Proton and electron transfers in [NiFe] hhydrogenase. Adv Inorg steric properties of metallodithiolates as ligands in coordination chemistry. Coord – Chem 56:101 125. Chem Rev 324:82–89. − 12. Pavlov M, Siegbahn PEM, Blomberg MRA, Crabtree RH (1998) Mechanism of H H 28. Jørgensen CK (1966) Differences between the four halide ligands, and discussion re- − – activation by nickel iron hydrogenase. J Am Chem Soc 120:548 555. marks on trigonal-bipyramidal complexes, on oxidation states, and on diagonal ele- 13. Mulder DW, Guo Y, Ratzloff MW, King PW (2017) Identification of a catalytic iron- ments of one-electron energy. Coord Chem Rev 1:164–178. hydride at the H-cluster of [FeFe]-hydrogenase. J Am Chem Soc 139:83–86. 29. Enemark JH, Feltham RD (1974) Principles of structure, bonding, and reactivity for 14. Reijerse EJ, et al. (2017) Direct observation of an iron-bound terminal hydride in metal nitrosyl complexes. Coord Chem Rev 13:339–406. [FeFe]-hydrogenase by nuclear resonance vibrational spectroscopy. J Am Chem Soc 30. Jeffrey JC, Rauchfuss TB (1979) Metal complexes of hemilabile ligands. Reactivity and 139:4306–4309. structure of dichlorobis(o-(diphenylphosphino)anisole)ruthenium(II). Inorg Chem 18: 15. Adamska A, et al. (2012) Identification and characterization of the “super-reduced” 2658–2666.

state of the H-cluster in [FeFe] hydrogenase: A new building block for the catalytic 31. Ding S, et al. (2016) Hemilabile bridging thiolates as proton shuttles in bioinspired H2 cycle? Angew Chem Int Ed Engl 51:11458–11462. production electrocatalysts. J Am Chem Soc 138:12920–12927.

Ding et al. PNAS Early Edition | 7of8 Downloaded by guest on September 26, 2021 32. Hsieh C-H, et al. (2014) Redox active iron nitrosyl units in proton reduction electro- benzimidazol-2’-yl)-2,6-dithiaheptane]copper(II) perchlorate. J Chem Soc Dalton . Nat Commun 5:3684. Trans, 1349–1356. 33. Hsieh C-H, Darensbourg MY (2010) A Fe(NO)310 trinitrosyliron complex stabilized by 42. Ogo S, et al. (2007) A dinuclear Ni(mu-H)Ru complex derived from H2. Science 316: an n-heterocyclic carbene and the cationic and neutral Fe(NO)2(9/10) products of its 585–587. NO release. J Am Chem Soc 132:14118–14125. 43. Ogo S, et al. (2013) A functional [NiFe]hydrogenase mimic that catalyzes electron and

34. Hsieh C-H, Chupik RB, Pinder TA, Darensbourg MY (2013) Dinitrosyl iron adducts of hydride transfer from H2. Science 339:682–684. (N2S2)M(NO) complexes (M = Fe, Co) as metallodithiolate ligands. Polyhedron 58: 44. Barton BE, Whaley CM, Rauchfuss TB, Gray DL (2009) Nickel-iron dithiolato hydrides 151–155. relevant to the [NiFe]-hydrogenase active site. J Am Chem Soc 131:6942–6943. 35. Pulukkody R, Darensbourg MY (2015) Synthetic advances inspired by the bioactive 45. Barton BE, Rauchfuss TB (2010) Hydride-containing models for the active site of the dinitrosyl iron unit. Acc Chem Res 48:2049–2058. nickel-iron hydrogenases. J Am Chem Soc 132:14877–14885. 36. Lyaskovskyy V, de Bruin B (2012) Redox non-innocent ligands: Versatile new tools to 46. Brazzolotto D, et al. (2016) Nickel-centred proton reduction catalysis in a model of control catalytic reactions. ACS Catal 2:270–279. [NiFe] hydrogenase. Nat Chem 8:1054–1060. 37. Luca OR, Crabtree RH (2013) Redox-active ligands in catalysis. Chem Soc Rev 42: 47. Connelly NG, Emslie DJH, Metz B, Orpen AG, Quayle MJ (1996) Redox-induced κ2-κ3 1440–1459. isomerisation in rhodium hydrotris(3,5-dimethylpyrazolyl)borate chemistry: The sta- 38. Ghosh P, et al. (2017) A matrix of heterobimetallic complexes for interrogation of bilisation of square-pyramidal rhodium(II). Chem Commun, 2289–2290. hydrogen evolution reaction electrocatalyst. Chem Sci, 10.1039/C7SC03378H. 48. Connelly NG, et al. (2001) Redox-induced κ2-κ3 isomerisation in hydrotris(pyrazolyl)borator- 39. Geiger WE, Ohrenberg NC, Yeomans B, Connelly NG, Emslie DJH (2003) Reversible hodium complexes: Synthesis, structure and ESR spectroscopy of stabilised rhodium(II) species. sequence of intramolecular associative and dissociative electron-transfer reactions in J Chem Soc Dalton Trans, 670–683. hydrotris(pyrazolylborate) complexes of rhodium. J Am Chem Soc 125:8680–8688. 49. Barton BE, Rauchfuss TB (2008) Terminal hydride in [FeFe]-hydrogenase model has

40. Surawatanawong P, Tye JW, Darensbourg MY, Hall MB (2010) Mechanism of elec- lower potential for H2 production than the isomeric bridging hydride. Inorg Chem 47: trocatalytic hydrogen production by a di-iron model of iron-iron hydrogenase: A 2261–2263. density functional theory study of proton dissociation constants and electrode re- 50. Wang W, Rauchfuss TB, Zhu L, Zampella G (2014) New reactions of terminal hydrides duction potentials. Dalton Trans 39:3093–3104. on a diiron dithiolate. J Am Chem Soc 136:5773–5782. 41. Addison AW, Rao TN, Reedijk J, van Rijn J, Verschoor GC (1984) Synthesis, structure, 51. Heiden ZM, Zampella G, De Gioia L, Rauchfuss TB (2008) [FeFe]-hydrogenase models and spectroscopic properties of copper(II) compounds containing nitrogen–sulphur and hydrogen: Oxidative addition of dihydrogen and silanes. Angew Chem Int donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methyl- Ed Engl 47:9756–9759.

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