A Nitrogenase Cluster Model [Fe8S6O] with an Oxygen Unsymmetrically Bridging Two Proto-Fe4S3 Cubes: Relevancy to the Substrate Binding Mode of the FeMo Cofactor

Cubane-type tungsten-iron-sulfur clusters with a nitrogen atom in the core: terminal ligand substitutions and redox behaviors

The lack of M-Fe-S (M = Mo or W) clusters incorporating a second period (2p) atom in the core has resulted in limited investigations and poor understanding of the physical and chemical properties of the M-Fe-S clusters closely related to the FeMo cofactor. In this work, systematic studies have been carried out to explore the chemical reactivities at the terminal ligand sites and the redox properties of a series of clusters comprising a [WFe₃S₃N] cubane core, based on the previously developed cluster [(Tp*)WFe₃S₃(μ₃-NSiMe₃)Cl₃]^1-. Substitutions of the terminal chlorides with ethanethiolate, methanethiolate, thiophenolate, p-thiocresolate and azide occurred smoothly, while the replacement of the chlorides with carbene ligands required the reduction of the precursor into [(Tp*)WFe₃S₃(μ₃-NSiMe₃)Cl₃]^2- first. The reduced cluster core could also be supported by thiophenolates as terminal ligands, but not thiolates or azides. It is remarkable that the thiophenolate ligated reduced cluster can be synthesized from the precursor [(Tp*)WFe₃S₃(μ₃-NSiMe₃)Cl₃]^1-via different synthetic routes, either reduction followed by substitution or substitution followed by reduction, either in situ or stepwise. This work indicates that terminal ligands contribute significantly to determine the chemical and physical properties of the clusters, even though they might affect the cluster core to a limited extent from a structural point of view, which raises the possibility of delicate control in regulating the physical/chemical properties of M-Fe-S clusters with a heteroleptic core incorporating 2p atom(s).

Synthesis of a Fe3-Carbyne Motif by Oxidation of an Alkyl Ligated Iron-Sulfur (WFe3S3) Cluster

The presence of a carbide ligand in the active site of nitrogenases remains an unusual example of organometallic chemistry employed by a protein. Carbide incorporation into the MFe7S9C cluster involves complex biosynthesis, but analogous synthetic methodologies are limited. Herein, we present a new synthetic strategy for incorporating carbon based bridging ligands into iron-sulfur clusters. Starting from a halide precursor, a WFe3S3 cluster displaying three terminal alkyl ligands and an open Fe3 face was prepared. Oxidation results in loss of alkane and formation of a μ3-carbyne. Characterization of these clusters and mechanistic studies are presented.

Stepwise Construction of Mo-Fe-S Clusters Using a LEGO Strategy

The rational synthesis of iron-sulfur clusters with excellent control of the core ligands has been a significant challenge in biomimetic chemistry. In this work, the rational construction of versatile Mo-Fe-S cubane clusters was realized using a LEGO strategy. (LEGO is a line of plastic construction toys consisting of various interlocking plastic bricks which can be assembled and connected in different ways to construct versatile objects. Herein we use "LEGO strategy" as an analogy for the stepwise synthetic methodology, and we use "brick" to represent a corner atom of the cubane structure.) Through careful synthetic control, the ⟨Fe⟩, ⟨S⟩, and ⟨Cl⟩ bricks were mounted piece-by-piece onto the basic ⟨MoS₃⟩ frame to stepwise construct the incomplete cubane core ⟨MoFe₂S₃Cl⟩ and the complete cubane core ⟨MoFe₃S₃Cl⟩. The significantly elongated Fe-Cl bonds for the bridging chlorides in the ⟨MoFe₂S₃Cl⟩ and ⟨MoFe₃S₃Cl⟩ cores permit ligand metatheses to introduce 2p donors at the bridging sites, which used to be a challenge in traditional iron-sulfur chemistry. Therefore, in subsequent controlled reactions, the bridging ⟨Cl⟩ bricks of the ⟨MoFe₂S₃Cl⟩ and ⟨MoFe₃S₃Cl⟩ frames could be easily replaced by ⟨N⟩ , ⟨O⟩, or ⟨S⟩ bricks to generate the ⟨MoFe₂S₃N⟩, ⟨MoFe₂S₃O⟩, ⟨MoFe₃S₃N⟩, and ⟨MoFe₃S₄⟩ cluster cores, demonstrating more choices for the LEGO synthetic strategy. The series of Mo-Fe-S clusters and their derivatives, together with related synthetic strategies, offers a good platform and methodology for biomimetic chemistry in relation to nitrogenase, especially the FeMo cofactor.

Statistical analysis of PN clusters in Mo/VFe protein crystals using a bond valence method toward their electronic structures

Nowadays, large numbers of MoFe proteins have been reported and their crystal data obtained by X-ray crystallography and uploaded to the Protein Data Bank (PDB). By big data analysis using a bond valence method, we make conclusions based on 79 selected PN in all 119 P-clusters of 53 MoFe proteins and 10 P-clusters of 5 VFe proteins from all deposited crystallographic data of the PDB. In the condition of MoFe protein crystals, the resting state PN clusters are proposed to have the formal oxidation state of 2Fe(iii)6Fe(ii), hiding two oxidized electron holes with high electron delocalization. The calculations show that Fe1, Fe2, Fe5, Fe6 and Fe7 perform unequivocally as Fe2+, and Fe3 is remarkably prone to Fe(iii), while Fe4 and Fe8 have different degrees of mixed valences. For PN clusters in VFe protein crystals, Fe1, Fe2, Fe4, Fe5 and Fe6 tend to be Fe2+, but the electron distributions rearrange with Fe7 and Fe8 being more oxidized mixed valences, and Fe3 presenting a little more reductive mixed valence than that in MoFe proteins. In terms of spatial location, Fe3 and Fe6 in P-clusters of MoFe proteins are calculated as the most oxidized and reduced irons, which have the shortest distances from homocitrate in the FeMo-cofactor and [Fe4S4] cluster, respectively, and thus could function as potential electron transport sites. This work shows different electron distributions of PN clusters in Mo/VFe protein crystals, from those obtained from previous data from solution with excess reducing agent from which it was concluded that PN clusters are all ferrous according to Mössbauer and electron paramagnetic resonance spectra.

Partial synthetic models of FeMoco with sulfide and carbyne ligands: Effect of interstitial atom in nitrogenase active site

Nitrogen-fixing organisms perform dinitrogen reduction to ammonia at an Fe-M (M = Mo, Fe, or V) cofactor (FeMco) of nitrogenase. FeMco displays eight metal centers bridged by sulfides and a carbide having the MFe₇S₈C cluster composition. The role of the carbide ligand, a unique motif in protein active sites, remains poorly understood. Toward addressing how the carbon bridge affects the physical and chemical properties of the cluster, we isolated synthetic models of subsite MFe₃S₃C displaying sulfides and a chelating carbyne ligand. We developed synthetic protocols for structurally related clusters, [Tp*M'Fe₃S₃X]^n-, where M' = Mo or W, the bridging ligand X = CR, N, NR, S, and Tp* = Tris(3,5-dimethyl-1-pyrazolyl)hydroborate, to study the effects of the identity of the heterometal and the bridging X group on structure and electrochemistry. While the nature of M' results in minor changes, the chelating, μ₃-bridging carbyne has a large impact on reduction potentials, being up to 1 V more reducing compared to nonchelating N and S analogs.

Structure, reactivity, and spectroscopy of nitrogenase-related synthetic and biological clusters

The reduction of dinitrogen (N2) is essential for its incorporation into nucleic acids and amino acids, which are vital to life on earth. Nitrogenases convert atmospheric dinitrogen to two ammonia molecules (NH3) under ambient conditions. The catalytic active sites of these enzymes (known as FeM-cofactor clusters, where M = Mo, V, Fe) are the sites of N2 binding and activation and have been a source of great interest for chemists for decades. In this review, recent studies on nitrogenase-related synthetic molecular complexes and biological clusters are discussed, with a focus on their reactivity and spectroscopic characterization. The molecular models that are discussed span from simple mononuclear iron complexes to multinuclear iron complexes and heterometallic iron complexes. In addition, recent work on the extracted biological cofactors is discussed. An emphasis is placed on how these studies have contributed towards our understanding of the electronic structure and mechanism of nitrogenases.

Single-Step Sulfur Insertions into Iron Carbide Carbonyl Clusters: Unlocking the Synthetic Door to FeMoco Analogues

The one-step syntheses, X-ray structures, and spectroscopic characterization of synthetic iron clusters, bearing either inorganic sulfides or thiolate with interstitial carbide motifs, are reported. Treatment of iron carbide carbonyl clusters [Fe_n (μ_n -C)(CO)_m ]^x (n=5,6; m=15,16; x=0,-2) with electrophilic sulfur sources (S₂ Cl₂ , S₈ ) results in the formation of several μ₄ -S dimers of clusters, and moreover, iron-sulfide-(sulfocarbide) clusters. The core sulfocarbide unit {C-S}^4- serves as a structural model for a proposed intermediate in the radical S-adenosyl-L-methionine biogenesis of the M-cluster. Furthermore, the electrophilic sulfur strategy has been extended to provide the first ever thiolato-iron-carbide complex: an analogous reaction with toluylsulfenyl chloride affords the cluster [Fe₅ (μ₅ -C)(SC₇ H₇ )(CO)₁₃ ]^- . The strategy described herein provides a breakthrough towards developing syntheses of biomimetic iron-sulfur-carbide clusters like FeMoco.

Construction of Synthetic Models for Nitrogenase-Relevant NifB Biogenesis Intermediates and Iron-Carbide-Sulfide Clusters

The family of nitrogenase enzymes catalyzes the reduction of atmospheric dinitrogen (N2) to ammonia under remarkably benign conditions of temperature, pressure, and pH. Therefore, the development of synthetic complexes or materials that can similarly perform this reaction is of critical interest. The primary obstacle for obtaining realistic synthetic models of the active site iron-sulfur-carbide cluster (e.g., FeMoco) is the incorporation of a truly inorganic carbide. This review summarizes the present state of knowledge regarding biological and chemical (synthetic) incorporation of carbide into iron-sulfur clusters. This includes the Nif cluster of proteins and associated biochemistry involved in the endogenous biogenesis of FeMoco. We focus on the chemical (synthetic) incorporation portion of our own efforts to incorporate and modify C1 units in iron/sulfur clusters. We also highlight recent contributions from other research groups in the area toward C1 and/or inorganic carbide insertion.

The Spectroscopy of Nitrogenases

Nitrogenases are responsible for biological nitrogen fixation, a crucial step in the biogeochemical nitrogen cycle. These enzymes utilize a two-component protein system and a series of iron-sulfur clusters to perform this reaction, culminating at the FeMco active site (M = Mo, V, Fe), which is capable of binding and reducing N2 to 2NH3. In this review, we summarize how different spectroscopic approaches have shed light on various aspects of these enzymes, including their structure, mechanism, alternative reactivity, and maturation. Synthetic model chemistry and theory have also played significant roles in developing our present understanding of these systems and are discussed in the context of their contributions to interpreting the nature of nitrogenases. Despite years of significant progress, there is still much to be learned from these enzymes through spectroscopic means, and we highlight where further spectroscopic investigations are needed.

Controlled Incorporation of Nitrides into W-Fe-S Clusters

Incorporation of monatomic 2p ligands into the core of iron-sulfur clusters has been researched since the discovery of interstitial carbide in the FeMo cofactor of Mo-dependent nitrogenase, but has proven to be a synthetic challenge. Herein, two distinct synthetic pathways are rationalized to install nitride ligands into targeted positions of W-Fe-S clusters, generating unprecedented nitride-ligated iron-sulfur clusters, namely [(Tp*)₂ W₂ Fe₆ (μ₄ -N)₂ S₆ L₄ ]^2- (Tp*=tris(3,5-dimethyl-1-pyrazolyl)hydroborate(1-), L=Cl^- or Br^- ). ⁵⁷ Fe Mössbauer study discloses metal oxidation states of W^IV ₂ Fe^II ₄ Fe^III ₂ with localized electron distribution, which is analogous to the mid-valent iron centres of FeMo cofactor at resting state. Good agreement of Mössbauer data with the empirical linear relationship for Fe-S clusters indicates similar ligand behaviour of nitride and sulfide in such clusters, providing useful reference for reduced nitrogen in a nitrogenase-like environment.

A [2Fe-1S] Complex That Affords Access to Bimetallic and Higher-Nuclearity Iron-Sulfur Clusters

Small, coordinatively unsaturated iron-sulfur clusters are conceived as building blocks for the diverse set of shapes of iron-sulfur clusters in biological and synthetic chemistry. Here we describe a synthetic method for preparing [2Fe-1S] clusters containing two iron(II) ions, which are supported by a relatively unhindered β-diketiminate supporting ligand. The [2Fe-1S] cluster can be isolated in the presence of trimethylphosphine, and the compound with one PMe₃ on each iron(II) ion has been crystallographically characterized. The PMe₃ ligands may be removed with B(C₆F₅)₃ to give a spectroscopically characterized species with solvent ligands. This species is a versatile synthon for [2Fe-2S], [4Fe-3S], and [10Fe-8S] clusters. Crystallographic characterization of the 10Fe cluster shows that it has all iron(II) ions, and the core has two [4Fe-4S] cubes that share a face in a novel arrangement. This cluster also has two iron sites that are coordinated to solvent donors, suggesting the potential for using this type of cluster for reactivity in the future.

Chemical Synthesis of an Asymmetric Mimic of the Nitrogenase Active Site

The synthetic inorganic chemistry of metal-sulfur (M-S, M = metals) clusters has played an important, complementary role to the biochemical analyses of nitrogenase toward a better understanding of the enzyme active site. The active site of nitrogenase (designated the M-cluster) can be extracted from the protein in a solvent-stabilized form, [(cit)MoFe₇S₉C] (cit = (R)-homocitrate). One important finding of the extracted M-cluster is its catalytic activity toward the reduction of C₁-substrates (CN^-, CO, CO₂) into C₁-C₅ hydrocarbons in solution. This catalytic property poses challenges for chemists to reproduce the function with synthetic mimics, not only because of the biochemical interests but also due to the potential significance in green chemistry and catalysis research. In this context, our successful synthesis of an asymmetric Mo-Fe-S cluster, [Cp*MoFe₅S₉(SH)]^3-, is one of the recent important achievements in synthetic M-S chemistry, as this cluster catalyzes the reduction of C₁-substrates in a similar manner to the extracted M-cluster. Even though the synthetic protocol for this cluster has been described in the literature, there are plenty of pitfalls for researchers unfamiliar with synthetic M-S chemistry. In this chapter, we provide general precautionary statements and detailed protocols for the synthesis of [Cp*MoFe₅S₉(SH)]^3-, with a brief discussion of the experimental tips based on the authors' experience in both biochemical and synthetic chemical fields.

A Thermodynamic Model for Redox-Dependent Binding of Carbon Monoxide at Site-Differentiated, High Spin Iron Clusters

Binding of N2 and CO by the FeMo-cofactor of nitrogenase depends on the redox level of the cluster, but the extent to which pure redox chemistry perturbs the affinity of high spin iron clusters for π-acids is not well understood. Here, we report a series of site-differentiated iron clusters that reversibly bind CO in redox states FeII4 through FeIIFeIII3. One electron redox events result in small changes in the affinity for (at most ∼400-fold) and activation of CO (at most 28 cm-1 for νCO). The small influence of redox chemistry on the affinity of these high spin, valence-localized clusters for CO is in stark contrast to the large enhancements (105-1022 fold) in π-acid affinity reported for monometallic and low spin, bimetallic iron complexes, where redox chemistry occurs exclusively at the ligand binding site. While electron-loading at metal centers remote from the substrate binding site has minimal influence on the CO binding energetics (∼1 kcal·mol-1), it provides a conduit for CO binding at an FeIII center. Indeed, internal electron transfer from these remote sites accommodates binding of CO at an FeIII, with a small energetic penalty arising from redox reorganization (∼2.6 kcal·mol-1). The ease with which these clusters redistribute electrons in response to ligand binding highlights a potential pathway for coordination of N2 and CO by FeMoco, which may occur on an oxidized edge of the cofactor.

Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation

A new series of tetranuclear Fe clusters displaying an interstitial μ₄-F ligand was prepared for a comparison to previously reported μ₄-O analogues. With a single nitric oxide (NO) coordinated as a reporter of small-molecule activation, the μ₄-F clusters were characterized in five redox states, from Fe^II₃{FeNO}⁸ to Fe^III₃{FeNO}⁷, with NO stretching frequencies ranging from 1680 to 1855 cm^-1, respectively. Despite accessing more reduced states with an F^- bridge, a two-electron reduction of the distal Fe centers is necessary for the μ₄-F clusters to activate NO to the same degree as the μ₄-O system; consequently, NO reactivity is observed at more positive potentials with μ₄-O than μ₄-F. Moreover, the μ₄-O ligand better translates redox changes of remote metal centers to diatomic ligand activation. The implication for biological active sites is that the higher-charge bridging ligand is more effective in tuning cluster properties, including the involvement of remote metal centers, for small-molecule activation.

Redox behaviour of ([fc(NPiPr2)2]Fe)2, formation of an iron–iron bond and cleavage of azobenzene

The redox behaviour of the dimeric tetrairon complex, ([fc(NPⁱPr₂)₂]Fe)₂ (where fc(NPⁱPr₂)₂ = 1,1′-(C₅H₄NPⁱPr₂)₂Fe) has been investigated.

Structure of a pentamanganese(II)-phenoxide cluster with a central five-coordinate oxide: MnII5(μ-OPh)6(μ3-OPh)2(μ5-O)(Py)6·Py (Py is pyridine)

Polynuclear metal clusters frequently feature geometric structural features not common in traditional coordination chemistry. These structures are of particular interest to bioinorganic chemists studying metallocluster enzymes, which frequently possess remarkably unusual inorganic structures. The structure of the manganese cluster μ₅-oxido-di-μ₃-phenoxido-hexa-μ-phenoxido-hexakis(pyridine-κN)hexamanganese(II) pyridine monosolvate, [Mn₅(C₆H₅O)₈O(C₅H₅N)₆]·C₅H₅N or Mn^II₅(μ-OPh)₆(μ₃-OPh)₂(μ₅-O)(Py)₆·Py, containing an unusual trigonal bipyramidal central oxide, is described. The compound was isolated from a reaction mixture containing bis(trimethylsilylamido)manganese(II) and phenol. The central O atom is presumed to have originated as adventitious water. The molecule crystalizes in a primitive monoclinic crystal system and is presented in the centrosymetric P2/n space group. The molecule possesses crystallographically imposed twofold symmetry, with the central O atom centred on the twofold axis and surrounded by a distorted trigonal bipyramidal arrangement of Mn atoms, which are further bridged by phenoxide ligands, and terminally ligated by pyridine. A pyridine solvent molecule resides nearby, also situated on a crystallographic twofold axis. The cluster is compared to three closely related previously reported structures.

Exploring secondary-sphere interactions in Fe-N x H y complexes relevant to N2 fixation

Hydrogen bonding and other types of secondary-sphere interactions are ubiquitous in metalloenzyme active sites and are critical to the transformations they mediate. Exploiting secondary sphere interactions in synthetic catalysts to study the role(s) they might play in biological systems, and to develop increasingly efficient catalysts, is an important challenge. Whereas model studies in this broad context are increasingly abundant, as yet there has been relatively little progress in the area of synthetic catalysts for nitrogen fixation that incorporate secondary sphere design elements. Herein we present our first study of Fe-N x H y complexes supported by new tris(phosphine)silyl ligands, abbreviated as [SiPNMe3] and [SiPiPr2PNMe], that incorporate remote tertiary amine hydrogen-bond acceptors within a tertiary phosphine/amine 6-membered ring. These remote amine sites facilitate hydrogen-bonding interactions via a boat conformation of the 6-membered ring when certain nitrogenous substrates (e.g., NH3 and N2H4) are coordinated to the apical site of a trigonal bipyramidal iron complex, and adopt a chair conformation when no H-bonding is possible (e.g., N2). Countercation binding at the cyclic amine is also observed for anionic {Fe-N2}- complexes. Reactivity studies in the presence of proton/electron sources show that the incorporated amine functionality leads to rapid generation of catalytically inactive Fe-H species, thereby substantiating a hydride termination pathway that we have previously proposed deactivates catalysts of the type [EPR3]FeN2 (E = Si, C).

Insight into the Iron-Molybdenum Cofactor of Nitrogenase from Synthetic Iron Complexes with Sulfur, Carbon, and Hydride Ligands

Nitrogenase enzymes are used by microorganisms for converting atmospheric N2 to ammonia, which provides an essential source of N atoms for higher organisms. The active site of the molybdenum-dependent nitrogenase is the unique carbide-containing iron-sulfur cluster called the iron-molybdenum cofactor (FeMoco). On the FeMoco, N2 binding is suggested to occur at one or more iron atoms, but the structures of the catalytic intermediates are not clear. In order to establish the feasibility of different potential mechanistic steps during biological N2 reduction, chemists have prepared iron complexes that mimic various structural aspects of the iron sites in the FeMoco. This reductionist approach gives mechanistic insight, and also uncovers fundamental principles that could be used more broadly for small-molecule activation. Here, we discuss recent results and highlight directions for future research. In one direction, synthetic iron complexes have now been shown to bind N2, break the N-N triple bond, and produce ammonia catalytically. Carbon- and sulfur-based donors have been incorporated into the ligand spheres of Fe-N2 complexes to show how these atoms may influence the structure and reactivity of the FeMoco. Hydrides have been incorporated into synthetic systems, which can bind N2, reduce some nitrogenase substrates, and/or reductively eliminate H2 to generate reduced iron centers. Though some carbide-containing iron clusters are known, none yet have sulfide bridges or high-spin iron atoms like the FeMoco.

A [3Fe–3S]3+ cluster with exclusively μ-sulfide donors

A [3Fe–3(μ-S)]³⁺ cluster is reported in which each ferric center has a distorted trigonal pyramidal geometry, with an S = 1/2 ground state for the cluster and unusually anisotropic hyperfine coupling constants as determined by variable temperature magnetometry and Mössbauer spectroscopy.

The discovery of Mo(III) in FeMoco: reuniting enzyme and model chemistry

Biological nitrogen fixation is enabled by molybdenum-dependent nitrogenase enzymes, which effect the reduction of dinitrogen to ammonia using an Fe₇MoS₉C active site, referred to as the iron molybdenum cofactor or FeMoco. In this mini-review, we summarize the current understanding of the molecular and electronic structure of FeMoco. The advances in our understanding of the active site structure are placed in context with the parallel evolution of synthetic model studies. The recent discovery of Mo(III) in the FeMoco active site is highlighted with an emphasis placed on the important role that model studies have played in this finding. In addition, the reactivities of synthetic models are discussed in terms of their relevance to the enzymatic system.

A convenient route to synthetic analogues of the oxidized form of high-potential iron-sulfur proteins

An amide-bound [Fe4S4](3+) cluster, [Fe4S4{N(SiMe3)2}4](-) (1), was found to serve as a convenient precursor for synthetic analogues of the oxidized form of high-potential iron-sulfur proteins. Treatment of 1 with 4 equiv of bulky thiols led to replacement of the amide ligands with thiolates, giving rise to a series of [Fe4S4(SR)4](-) clusters (R = Dmp (2a), Tbt (2b), Eind (2c), Dxp (2d), Dpp (2e); Dmp = 2,6-di(mesityl)phenyl, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl, Dxp = 2,6-di(m-xylyl)phenyl, Dpp = 2,6-diphenylphenyl). These clusters were characterized by the mass spectrum, the EPR spectrum, and X-ray crystallography. The redox potentials of the [Fe4S4](3+/2+) couple, -0.82 V (2a), -0.86 V (2b), -0.84 V (2c), -0.74 V (2d), and -0.63 V (2e) vs Ag/Ag(+) in THF, are significantly more negative than that of [Fe4S4(SPh)4](-/2-) (-0.21 V).