A site-differentiated [4Fe–4S] cluster controls electron transfer reactivity of Clostridium acetobutylicum [FeFe]-hydrogenase I

One of the many functions of reduction–oxidation (redox) cofactors is to mediate electron transfer in biological enzymes catalyzing redox-based chemical transformation reactions. There are numerous examples of enzymes that utilize redox cofactors to form electron transfer relays to connect catalytic sites to external electron donors and acceptors. The compositions of relays are diverse and tune transfer thermodynamics and kinetics towards the chemical reactivity of the enzyme. Diversity in relay design is exemplified among different members of hydrogenases, enzymes which catalyze reversible H₂ activation, which also couple to diverse types of donor and acceptor molecules. The [FeFe]-hydrogenase I from Clostridium acetobutylicum (CaI) is a member of a large family of structurally related enzymes where interfacial electron transfer is mediated by a terminal, non-canonical, His-coordinated, [4Fe–4S] cluster. The function of His coordination was examined by comparing the biophysical properties and reactivity to a Cys substituted variant of CaI. This demonstrated that His coordination strongly affected the distal [4Fe–4S] cluster spin state, spin pairing, and spatial orientations of molecular orbitals, with a minor effect on reduction potential. The deviations in these properties by substituting His for Cys in CaI, correlated with pronounced changes in electron transfer and reactivity with the native electron donor–acceptor ferredoxin. The results demonstrate that differential coordination of the surface localized [4Fe–4S]His cluster in CaI is utilized to control intermolecular and intramolecular electron transfer where His coordination creates a physical and electronic environment that enables facile electron exchange between electron carrier molecules and the iron–sulfur cluster relay for coupling to reversible H₂ activation at the catalytic site.

Histidine coordination of the distal [4Fe–4S] cluster in [FeFe]-hydrogenase was demonstrated to tune the cluster spin-states, spin-pairing and surrounding molecular orbitals to enable more facile electron transfer compared to cysteine coordination.     

Structural and electronic properties of the [FeFe] hydrogenase H-cluster in different redox and protonation states. A DFT investigation

The molecular and electronic structure of the Fe 6S 6 H-cluster of [FeFe] hydrogenase in relevant redox and protonation states have been investigated by DFT. The calculations have been carried out according to the broken symmetry approach and considering different environmental conditions. The large negative charge of the H-cluster leads, in a vacuum, to structures different from those observed experimentally in the protein. A better agreement with experimental data is observed for solvated complexes, suggesting that the protein environment could buffer the large negative charge of the H-cluster. The comparison of Fe 6S 6 and Fe 2S 2 DFT models shows that the presence of the Fe 4S 4 moiety does not affect appreciably the geometry of the [2Fe] H cluster. In particular, the Fe 4S 4 cluster alone cannot be invoked to explain the stabilization of the mu-CO forms observed in the enzyme (relative to all-terminal CO species). As for protonation of the hydrogen cluster, it turned out that mu-H species are always more stable than terminal hydride isomers, leading to the conclusion that specific interactions of the H-cluster with the environment, not considered in our calculations, would be necessary to reverse the stability order of mu-H and terminal hydrides. Otherwise, protonation of the metal center and H 2 evolution in the enzyme are predicted to be kinetically controlled processes. Finally, subtle modifications in the H-cluster environment can change the relative stability of key frontier orbitals, triggering electron transfer between the Fe 4S 4 and the Fe 2S 2 moieties forming the H-cluster.     

Computational chemical analysis of [FeFe] hydrogenase H-cluster analogues to discern catalytically relevant features of the natural diatomic ligand configuration

Density functional theoretical models of the electronic structure of several configurational isomers and analogues of the [2Fe](H) H-cluster in [FeFe] hydrogenase were analyzed to identify distinguishing features of the canonical cofactor structure potentially relevant to catalysis. Collective analysis of geometric changes over models of oxidized and reduced [2Fe] clusters highlighted movement of the bridging carbonyl and anticorrelation of the proximal and distal Fe-C(terminal) bonds as key explanatory factors for variance over the considered models. Charge and bond order analysis suggest that as the bridging carbonyl favors the distal iron upon reduction, bonding simultaneously becomes more ionic in nature, raising the possibility of simple electrostatic stabilization as a factor in charge accumulation prior to ultimate H(2) creation and release. Frontier orbital energies show cis and trans arrangements of cyanide on the Fe-Fe core to have distinctive energies from the other models, which may be important for redox poise. Altogether, few factors qualitatively distinguish the cis- from the trans-cyano configurations, which may in fact enhance catalytic robustness under conditions leading to exchange of the bridging and terminal carbonyl ligands. However, the naturally occurring trans configuration possesses two distinct donor-metal-acceptor S-Fe-C(O) interactions, which might play a role in enforcing a low-spin ground state for the hydridic mechanism of H(2) production.     

Proton‐Coupled Reduction of the Catalytic [4Fe‐4S] Cluster in [FeFe]‐Hydrogenases

In nature, [FeFe]‐hydrogenases catalyze the uptake and release of molecular hydrogen (H₂) at a unique iron‐sulfur cofactor. The absence of an electrochemical overpotential in the H₂ release reaction makes [FeFe]‐hydrogenases a prime example of efficient biocatalysis. However, the molecular details of hydrogen turnover are not yet fully understood. Herein, we characterize the initial one‐electron reduction of [FeFe]‐hydrogenases by infrared spectroscopy and electrochemistry and present evidence for proton‐coupled electron transport during the formation of the reduced state Hred′. Charge compensation stabilizes the excess electron at the [4Fe‐4S] cluster and maintains a conservative configuration of the diiron site. The role of Hred′ in hydrogen turnover and possible implications on the catalytic mechanism are discussed. We propose that regulation of the electronic properties in the periphery of metal cofactors is key to orchestrating multielectron processes.     

Serine is the molecular source of the NH(CH2)2 bridgehead moiety of the in vitro assembled [FeFe] hydrogenase H-cluster

The active site of [FeFe] hydrogenase, the H-cluster, consists of a canonical [4Fe–4S]_H subcluster linked to a unique binuclear [2Fe]_H subcluster containing three CO, two CN⁻ and a bridging azadithiolate (adt, NH(CH₂S⁻)₂) ligand. While it is known that all five diatomic ligands are derived from tyrosine, there has been little knowledge as to the formation and installation of the adt ligand. Here, by using a combination of a cell-free in vitro maturation approach with pulse electronic paramagnetic resonance spectroscopy, we discover that serine donates the nitrogen atom and the CH₂ group to the assembly of the adt ligand. More specifically, both CH₂ groups in adt are sourced from the C3 methylene of serine.

The CH₂NHCH₂ bridgehead moiety of the [FeFe] hydrogenase H-cluster is derived from serine as revealed by isotope labeling and EPR spectroscopy.

Hydrogenases catalyze the reversible reactions of H₂ oxidation and proton reduction, and are involved in many microbial metabolic pathways.¹ [FeFe] hydrogenases in particular are hyper-efficient, with turnover rates up to 10⁴/s.² This has led to intense focus on [FeFe] hydrogenases for sustainable production of H₂ and the design of fuel cells.³ The active site of [FeFe] hydrogenases is a six-iron cofactor called the H-cluster (Scheme 1), which consists of a canonical cuboid [4Fe–4S]_H subcluster linked through a bridging cysteine (Cys) residue to a binuclear [2Fe]_H subcluster in which the two iron ions are coordinated by three CO, two CN⁻ and an azadithiolate (adt, NH(CH₂S⁻)₂) bridging ligand. The [2Fe]_H subcluster has been proposed to be the site for H₂ binding and hydride formation,^4,5 which serves as a natural blueprint for designing small molecule catalysts for hydrogen evolution reactions.⁶ The unique structure and catalytic activity has thus raised much interest in the biosynthesis of the H-cluster, which poses a great challenge in cofactor assembly that involves toxic ligands, oxygen sensitivity and an organic adt ligand that has little inherent stability.Open in a separate windowScheme 1Bioassembly of the H-cluster highlighting the source of each moiety.While the [4Fe–4S]_H subcluster in the H-cluster can be formed by the housekeeping gene products that are used to assemble such standard Fe–S clusters, the in vivo bioassembly of the unique [2Fe]_H subcluster requires three special Fe–S “maturase” proteins: HydE, HydF, and HydG.^7,8 Although the functions of HydE and HydF have not been fully elucidated,^9–12 recent studies indicate that HydG is a bifunctional 4Fe–4S radical S-adenosyl-l-methionine (SAM) enzyme which lyses tyrosine to generate CO and CN⁻ and forms a [(Cys)Fe(CO)₂(CN)] organometallic precursor to the H-cluster on a dangler Fe(Cys) site in HydG.^13–16 More recently, by using a synthetic [(Cys)Fe(CO)₂(CN)] carrier we have shown that the two sulfur atoms in the adt ligand are derived from the precursor-bound Cys, but that the CH₂NHCH₂ component is not.¹⁷ Taken together, the biosynthetic origins of the [Fe₂S₂(CO)₃(CN)₂] part of the [2Fe]_H subcluster are depicted in Scheme 1: all five diatomic ligands are tailored from tyrosine by HydG;¹⁸ the two sulfur atoms and the two Fe atoms are from the dangler Fe(Cys) site in HydG (which can be reconstituted with Fe²⁺ and free Cys in solution¹⁹). Remarkably, these components are all delivered to the binuclear cluster assembly in the form of the [(Cys)Fe(CO)₂(CN)] product of HydG. Given these recent advances, the only missing part of the puzzle is the crucial NH(CH₂)₂ moiety: what are its molecular precursors? It has been hypothesized that HydE, which is also a 4Fe–4S radical SAM enzyme, may be involved in the formation of adt, though its physiological substrate and reaction mechanism remains under investigation.^9,10 As for any enzymatic reaction, knowing the actual substrate(s) for the reaction is crucial for unraveling the ultimate mechanism. Therefore, determining of molecular sourcing of the CH₂NHCH₂ component of the adt bridge, currently unknown, is the focus of this work.Assembly of the H-cluster in the lab can be achieved by semi-synthetic and biochemical approaches other than directly co-expressing hydA, hydE, hydF and hydG genes in cells. One very useful method alleviates the need for HydG, HydE, and in some cases, HydF, by using a synthetic [Fe₂(adt)(CO)₄(CN)₂] complex as a direct donor to the [2Fe]_H subcluster assembly.^20–22 Another “cell free synthesis” approach uses HydE/F/G in an in vitro H-cluster maturation reaction developed by the Swartz group.^23,24 The specific in vitro maturation reaction used in our current investigation contains a mixture of E. coli cell lysate containing separately overexpressed HydE, HydF, HydG (all from Shewanella oneidensis), apo-HydA1 (from Chlamydomonas reinhardtii) that harbors the [4Fe-4S]_H subcluster, and a cocktail of low molecular weight cofactors and precursors.²³ This biochemical approach gives us the opportunity to use the same set of enzymes that build the H-cluster in cells, but also enables us to determine the molecular source of each of the components in the H-cluster by using isotope-labeled cofactors/precursors, a procedure that would be very difficult to carry out and fully control in vivo. For example, by supplementing 1-¹³C-Tyr or 2-¹³C-Tyr into the in vitro maturation reaction, the CO or CN⁻ ligands to the diiron subcluster of the maturated HydA1 are respectively labeled with ¹³C.^25,26 The presence of these ¹³C labels can then in turn be detected and analyzed by using advanced electron paramagnetic resonance (EPR) spectroscopy to measure the hyperfine couplings between the magnetic ¹³C nuclei and the unpaired electron spin distributed over the H-cluster in its redox-poised paramagnetic states. In this work, we now search for the source(s) of the CH₂NHCH₂ moiety by using a similar strategy of in vitro maturation coupled to high resolution EPR to screen the assembly products formed with various isotopically labeled small molecule candidates. The presence of nitrogen element in the CH₂NHCH₂ fragment suggests an amino acid origin as one possibility. A systematic screening by pulse EPR of the in vitro maturation products generated with ¹³C, ¹⁵N, and ²H-labeled amino acids reveals that serine (Ser) serves as a molecular source for the NH(CH₂)₂ moiety of the H-cluster.     

Calculating standard reduction potentials of [4Fe–4S] proteins

The oxidation–reduction potentials of electron transfer proteins determine the driving forces for their electron transfer reactions. Although the type of redox site determines the intrinsic energy required to add or remove an electron, the electrostatic interaction energy between the redox site and its surrounding environment can greatly shift the redox potentials. Here, a method for calculating the reduction potential versus the standard hydrogen electrode, E°, of a metalloprotein using a combination of density functional theory and continuum electrostatics is presented. This work focuses on the methodology for the continuum electrostatics calculations, including various factors that may affect the accuracy. The calculations are demonstrated using crystal structures of six homologous HiPIPs, which give E° that are in excellent agreement with experimental results. © 2012 Wiley Periodicals, Inc.     

CO disrupts the reduced H-cluster of FeFe hydrogenase. A combined DFT and protein film voltammetry study

Carbon monoxide is often described as a competitive inhibitor of FeFe hydrogenases, and it is used for probing H(2) binding to synthetic or in silico models of the active site H-cluster. Yet it does not always behave as a simple inhibitor. Using an original approach which combines accurate electrochemical measurements and theoretical calculations, we elucidate the mechanism by which, under certain conditions, CO binding can cause permanent damage to the H-cluster. Like in the case of oxygen inhibition, the reaction with CO engages the entire H-cluster, rather than only the Fe(2) subsite.     

Hydroxy and ether functionalized dithiolanes: Models for the active site of the [FeFe] hydrogenase

In attempt to synthesize suitable [FeFe] hydrogenase model complexes, 2-methoxypropane-1,3-dithiole and 4-methyl-4-hydroxy-1,2-dithiolane were reacted with Fe₃(CO)₁₂ to give the respective complexes [Fe₂(CO)₆(H₃COCH(CH₂S)₂)] and [Fe₂(CO)₆(HOC(CH₃)(CH₂S)₂)]. The compounds were characterized by ¹H and ¹³C NMR spectroscopy, mass spectrometry and elemental analysis. In addition, their electrochemical properties were investigated by cyclic voltammetry and compared with that of [Fe₂(CO)₆(HOC(CH₂S)₂)] known from literature.     

Solution-phase photochemistry of a [FeFe]hydrogenase model compound: evidence of photoinduced isomerisation

The solution-phase photochemistry of the [FeFe] hydrogenase subsite model (μ-S(CH(2))(3)S)Fe(2)(CO)(4)(PMe(3))(2) has been studied using ultrafast time-resolved infrared spectroscopy supported by density functional theory calculations. In three different solvents, n-heptane, methanol, and acetonitrile, relaxation of the tricarbonyl intermediate formed by UV photolysis of a carbonyl ligand leads to geminate recombination with a bias towards a thermodynamically less stable isomeric form, suggesting that facile interconversion of the ligand groups at the Fe center is possible in the unsaturated species. In a polar or hydrogen bonding solvent, this process competes with solvent substitution leading to the formation of stable solvent adduct species. The data provide further insight into the effect of incorporating non-carbonyl ligands on the dynamics and photochemistry of hydrogenase-derived biomimetic compounds.     

TDDFT modeling of the CO‐photolysis of Fe2(S2C3H6)(CO)6, a model of the [FeFe]‐hydrogenase catalytic site

Upon irradiation with ultraviolet wavelengths, Fe₂(S₂C₃H₆)(CO)₆, a simple model of the [FeFe]‐hydrogenase active site, undergoes CO dissociation to form the unsaturated Fe₂(S₂C₃H₆)(CO)₅ species and successively a solvent adduct at the vacant coordination site. In the present work, the CO‐photolysis of Fe₂(S₂C₃H₆)(CO)₆ was investigated by density functional theory (DFT) and time‐dependent DFT (TDDFT). Trans Fe₂(S₂C₃H₆)(CO)₅ form and the corresponding trans heptane or acetonitrile solvent adducts are the lowest energy ground state forms. CO dissociation barriers computed for the lowest triplet state are roughly halved with respect to those for the ground state suggesting that some low‐lying excited potential energy surface (PES) could be loosely bound with respect to Fe? C bond cleavage. The TDDFT excited state PESs and geometry optimizations for the excited states likely involved in the CO‐photolysis suggest that the Fe? S bond elongation and the partial isomerization toward the rotated form could take place simultaneously, favoring the trans CO photodissociation. © 2014 Wiley Periodicals, Inc.     

In silico evaluation of proposed biosynthetic pathways for the unique dithiolate ligand of the H-cluster of [FeFe]-hydrogenase

The biosynthesis of the active site of the [FeFe]-hydrogenases (H-cluster) remains a tantalizing puzzle due to its unprecedented and complex ligand environment. It contains a [2Fe] cluster ([2Fe](H)) bearing cyanide and carbon monoxide ligands attached to low-valence Fe ions and an abiological dithiolate ligand (SCH(2)XCH(2)S)(2-) that bridges the two iron centers. Various experimentally testable hypotheses have already been put forward regarding the precursor molecule and the biosynthetic mechanism that leads to the formation of the dithiolate ligand. In this work, we report a density functional theory-based theoretical evaluation of these hypotheses. We find preference for a mechanistically simple and energetically favorable pathway that includes known radical-SAM (S-adenosylmethionine) catalyzed reactions. We modeled this pathway using a long alkyl chain precursor molecule that leads to the formation of pronanadithiolate (X = CH(2)). However, the same pathway can be readily adopted for the biosynthesis of the dithiomethylamine (X = NH) or the dithiomethylether (X = O) analog, provided that the proper precursor molecule is available.     

The electronic structure of the H-cluster in the [FeFe]-hydrogenase from Desulfovibrio desulfuricans: a Q-band 57Fe-ENDOR and HYSCORE study

The active site of the (57)Fe-enriched [FeFe]-hydrogenase (i.e., the "H-cluster") from Desulfovibrio desulfuricans has been examined using advanced pulse EPR methods at X- and Q-band frequencies. For both the active oxidized state (H(ox)) and the CO inhibited form (H(ox)-CO) all six (57)Fe hyperfine couplings were detected. The analysis shows that the apparent spin density extends over the whole H-cluster. The investigations revealed different hyperfine couplings of all six (57)Fe nuclei in the H-cluster of the H(ox)-CO state. Four large 57Fe hyperfine couplings in the range 20-40 MHz were found (using pulse ENDOR and TRIPLE methods) and were assigned to the [4Fe-4S](H) (cubane) subcluster. Two weak (57)Fe hyperfine couplings below 5 MHz were identified using Q-band HYSCORE spectroscopy and were assigned to the [2Fe](H) subcluster. For the H(ox) state only two different 57Fe hyperfine couplings in the range 10-13 MHz were detected using pulse ENDOR. An (57)Fe line broadening analysis of the X-band CW EPR spectrum indicated, however, that all six (57)Fe nuclei in the H-cluster are contributing to the hyperfine pattern. It is concluded that in both states the binuclear subcluster [2Fe](H) assumes a [Fe(I)Fe(II)] redox configuration where the paramagnetic Fe(I) atom is attached to the [4Fe-4S](H) subcluster. The (57)Fe hyperfine interactions of the formally diamagnetic [4Fe-4S](H) are due to an exchange interaction between the two subclusters as has been discussed earlier by Popescu and Münck [Popescu, C.V.; Münck, E., J. Am. Chem. Soc. 1999, 121, 7877-7884]. This exchange coupling is strongly enhanced by binding of the extrinsic CO ligand. Binding of the dihydrogen substrate may induce a similar effect, and it is therefore proposed that the observed modulation of the electronic structure by the changing ligand surrounding plays an important role in the catalytic mechanism of [FeFe]-hydrogenase.     

Site-selective X-ray spectroscopy on an asymmetric model complex of the [FeFe] hydrogenase active site

The active site for hydrogen production in [FeFe] hydrogenase comprises a diiron unit. Bioinorganic chemistry has modeled important features of this center, aiming at mechanistic understanding and the development of novel catalysts. However, new assays are required for analyzing the effects of ligand variations at the metal ions. By high-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection (XAS/XES = XAES) and density functional theory (DFT), we studied an asymmetrically coordinated [FeFe] model complex, [(CO)(3)Fe(I)1-(bdtCl(2))-Fe(I)2(CO)(Ph(2)P-CH(2)-NCH(3)-CH(2)-PPh(2))] (1, bdt = benzene-1,2-dithiolate), in comparison to iron-carbonyl references. Kβ emission spectra (Kβ(1,3), Kβ') revealed the absence of unpaired spins and the low-spin character for both Fe ions in 1. In a series of low-spin iron compounds, the Kβ(1,3) energy did not reflect the formal iron oxidation state, but it decreases with increasing ligand field strength due to shorter iron-ligand bonds, following the spectrochemical series. The intensity of the valence-to-core transitions (Kβ(2,5)) decreases for increasing Fe-ligand bond length, certain emission peaks allow counting of Fe-CO bonds, and even molecular orbitals (MOs) located on the metal-bridging bdt group of 1 contribute to the spectra. As deduced from 3d → 1s emission and 1s → 3d absorption spectra and supported by DFT, the HOMO-LUMO gap of 1 is about 2.8 eV. Kβ-detected XANES spectra in agreement with DFT revealed considerable electronic asymmetry in 1; the energies and occupancies of Fe-d dominated MOs resemble a square-pyramidal Fe(0) for Fe1 and an octahedral Fe(II) for Fe2. EXAFS spectra for various Kβ emission energies showed considerable site-selectivity; approximate structural parameters similar to the crystal structure could be determined for the two individual iron atoms of 1 in powder samples. These results suggest that metal site- and spin-selective XAES on [FeFe] hydrogenase protein and active site models may provide a powerful tool to study intermediates under reaction conditions.     

Controlled radical polymerization of styrene‐based models of the active site of the [FeFe]‐hydrogenase

Within this study, we report on the first controlled radical polymerization of styrene‐based models of the active site of the [FeFe]‐hydrogenase. Three different model complexes based on styrene were prepared including propanedithiolato‐bridged, 2‐azapropanedithiolato‐bridged, and bifunctional styrene iron complex. These model complexes were copolymerized with styrene using free radical and the reversible addition‐fragmentation chain transfer polymerization method. The polymerization behavior of the hydrogenase models is discussed and analyzed in detail. It could be shown that the model complex can be incorporated into copolymers. The obtained copolymers exhibit narrow molar mass distributions. The presence of the [FeFe]‐hydrogenase models were proven by atomic absorption spectrometry, NMR and IR spectroscopy as well as cyclovoltammetric measurements. It could be shown that the [FeFe]‐hydrogenase mimic copolymers, as well as the monomeric originating complexes exhibit electrocatalytic proton reduction at a low potential of –2.2 V. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013     

Modeling [FeFe] hydrogenase: Synthesis and protonation of a diiron dithiolate complex containing a phosphine-N-heterocyclic-carbene ligand

The ligand exchange reaction of I_Me-(CH₂)₂-PPh₂ (I_Me = 1-methyimidazol-2-ylidene) and the hexacarbonyl complex [{Fe₂{μ-S(CH₂)₃S}(CO)₆] (1) resulted in the formation of the chelated complex [{Fe₂{μ-S(CH₂)₃S}(CO)₄(I_Me-(CH₂)₂-PPh₂)] (2). The molecular structure of 2 was confirmed by spectroscopic and X-ray analyses. This complex catalyzes proton reduction. Low temperature NMR studies on the protonation of 2 revealed the formation of a terminal hydride intermediate.     

Photocatalytic hydrogen evolution by two comparable [FeFe]‐hydrogenase mimics assembled to the surface of ZnS

Two photocatalytic hydrogen evolution systems were constructed by assembling [FeFe]‐hydrogenase mimics, either carboxyl group‐containing ( C1 ) or not ( C2 ), on to the surface of ZnS using triethanolamine as electron donor in DMF‐H₂O (9/1, v/v) solution. Upon irradiation for 30 h, the turnover numbers of hydrogen evolution were 3400 and 4950 for the hybrid system C1 /ZnS and C2 /ZnS, respectively. The photocatalytic activity of the C2 /ZnS system was five times higher than the activity of the pristine ZnS, suggesting that the [FeFe]‐hydrogenase mimics are crucial toward improving the activity of ZnS. On the basis of the spectroscopic studies and analyses, the photogenerated electron transfer from ZnS to the mimics is probably responsible for the activity enhancement of ZnS. The time dependence of hydrogen generation shows that the mimic C2 is more active than C1 . The different hydrogen evolution activity can be attributed to the different adsorption modes of the two [FeFe]‐hydrogenase mimics on the surface of ZnS. Copyright © 2014 John Wiley & Sons, Ltd.     

Structural insight into [Fe–S2–Mo] motif in electrochemical reduction of N2 over Fe1-supported molecular MoS2

The catalytic synthesis of NH₃ from the thermodynamically challenging N₂ reduction reaction under mild conditions is currently a significant problem for scientists. Accordingly, herein, we report the development of a nitrogenase-inspired inorganic-based chalcogenide system for the efficient electrochemical conversion of N₂ to NH₃, which is comprised of the basic structure of [Fe–S₂–Mo]. This material showed high activity of 8.7 mg_NH3 mg_Fe⁻¹ h⁻¹ (24 μg_NH3 cm⁻² h⁻¹) with an excellent faradaic efficiency of 27% for the conversion of N₂ to NH₃ in aqueous medium. It was demonstrated that the Fe₁ single atom on [Fe–S₂–Mo] under the optimal negative potential favors the reduction of N₂ to NH₃ over the competitive proton reduction to H₂. Operando X-ray absorption and simulations combined with theoretical DFT calculations provided the first and important insights on the particular electron-mediating and catalytic roles of the [Fe–S₂–Mo] motifs and Fe₁, respectively, on this two-dimensional (2D) molecular layer slab.

A nitrogenase-inspired inorganic-based chalcogenide system containing [Fe–S₂–Mo] motif is developed for the efficient electrochemical conversion of N₂ to NH₃.     

Electron delocalization from the fullerene attachment to the diiron core within the active-site mimics of [FeFe]hydrogenase

Attachment of the redox-active C(60)(H)PPh(2) group modulates the electronic structure of the Fe(2) core in [(μ-bdt)Fe(2)(CO)(5)(C(60)(H)PPh(2))]. The neutral complex is characterized by X-ray crystallography, IR, NMR spectroscopy, and cyclic voltammetry. When it is reduced by one electron, the spectroscopic and density functional theory results indicate that the Fe(2) core is partially spin-populated. In the doubly reduced species, extensive electron communication occurs between the reduced fullerene unit and the Fe(2) centers as displayed in the spin-density plot. The results suggest that the [4Fe4S] cluster within the H cluster provides an essential role in terms of the electronic factor.