A bound reaction intermediate sheds light on the mechanism of nitrogenase

Construction of C–N bonds from small-molecule precursors through heterogeneous electrocatalysis

Energy-intensive thermochemical processes within chemical manufacturing are a major contributor to global CO₂ emissions. With the increasing push for sustainability, the scientific community is striving to develop renewable energy-powered electrochemical technologies in lieu of CO₂-emitting fossil-fuel-driven methods. However, to fully electrify chemical manufacturing, it is imperative to expand the scope of electrosynthetic technologies, particularly through the innovation of reactions involving nitrogen-based reactants. This Review focuses on a rapidly emerging area, namely the formation of C-N bonds through heterogeneous electrocatalysis. The C-N bond motif is found in many fertilizers (such as urea) as well as commodity and fine chemicals (with functional groups such as amines and amides). The ability to generate C-N bonds from reactants such as CO₂, NO₃^- or N₂ would provide sustainable alternatives to the thermochemical routes used at present. We start by examining thermochemical, enzymatic and molecular catalytic systems for C-N bond formation, identifying how concepts from these can be translated to heterogeneous electrocatalysis. Next, we discuss successful heterogeneous electrocatalytic systems and highlight promising research directions. Finally, we discuss the remaining questions and knowledge gaps and thus set the trajectory for future advances in heterogeneous electrocatalytic formation of C-N bonds.

Thermodynamically Favourable States in the Reaction of Nitrogenase without Dissociation of any Sulfide Ligand

We have used combined quantum mechanical and molecular mechanical (QM/MM) calculations to study the reaction mechanism of nitrogenase, assuming that none of the sulfide ligands dissociates. To avoid the problem that there is no consensus regarding the structure and protonation of the E₄ state, we start from a state where N₂ is bound to the cluster and is protonated to N₂H₂, after dissociation of H₂. We show that the reaction follows an alternating mechanism with HNNH (possibly protonated to HNNH₂) and H₂NNH₂ as intermediates and the two NH₃ products dissociate at the E₇ and E₈ levels. For all intermediates, coordination to Fe6 is preferred, but for the E₄ and E₈ intermediates, binding to Fe2 is competitive. For the E₄, E₅ and E₇ intermediates we find that the substrate may abstract a proton from the hydroxy group of the homocitrate ligand of the FeMo cluster, thereby forming HNNH₂, H₂NNH₂ and NH₃ intermediates. This may explain why homocitrate is a mandatory component of nitrogenase. All steps in the suggested reaction mechanism are thermodynamically favourable compared to protonation of the nearby His‐195 group and in all cases, protonation of the NE2 atom of the latter group is preferred.

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.

A Mechanism for Nitrogenase Including Loss of a Sulfide

Nitrogenase is the only enzyme in nature that can fix N₂ from the air. The active cofactor of the leading form of this enzyme contains seven irons and one molybdenum connected by sulfide bridges. In several recent experimental studies, it has been suggested that the cofactor is very flexible, and might lose one of its sulfides during catalysis. In this study, the possible loss of a sulfide has been investigated by model calculations. In previous studies, we have shown that there should be four activation steps before catalysis starts, and this study is based on that finding. It was found here that, after the four reductions in the activation steps, a sulfide will become very loosely bound and can be released in a quite exergonic step with a low barrier. The binding of N₂ has no part in that release. In our previous studies, we suggested that the central carbide should be protonated three times after the four activation steps. With the new finding, there will instead be a loss of a sulfide, as the barrier for the loss is much lower than the ones for protonating the carbide. Still, it is suggested here that the carbide will be protonated anyway, but only with one proton, in the E₃ to E₄ step. A very complicated transition state for H₂ formation involving a large structural change was obtained. The combined step, with a loss of H₂ and binding of N₂ , is calculated to be endergonic by +2.3 kcal mol^-1 ; this is in excellent agreement with experiments in which an easily reversible step has been found.

A guide to secondary coordination sphere editing

This tutorial review showcases recent (2015-2021) work describing ligand construction as it relates to the design of secondary coordination spheres (SCSs). Metalloenzymes, for example, utilize SCSs to stabilize reactive substrates, shuttle small molecules, and alter redox properties, promoting functional activity. In the realm of biomimetic chemistry, specific incorporation of SCS residues (e.g., Brønsted or Lewis acid/bases, crown ethers, redox groups etc.) has been shown to be equally critical to function. This contribution illustrates how fundamental advances in organic and inorganic chemistry have been used for the construction of such SCSs. These imaginative contributions have driven exciting findings in many transformations relevant to clean fuel generation, including small molecule (e.g., H⁺, N₂, CO₂, NO_x, O₂) reduction. In most cases, these reactions occur cooperatively, where both metal and ligand are requisite for substrate activation.

Combining metal-metal cooperativity, metal-ligand cooperativity and chemical non-innocence in diiron carbonyl complexes

Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton-electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal-metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand's naphthyridine core enables both heterolytic metal-ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp3)-H bonds by ∼25 kcal mol-1. H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic (S = ) complex containing a mixed-valent iron(0)-iron(i) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity.

Microbial methane emissions from the non-methanogenesis processes: A critical review

Methane, a potent greenhouse gas of global importance, has traditionally been considered as an end product of microbial methanogenesis of organic matter. Paradoxically, growing evidence has shown that some microbes, such as cyanobacteria, algae, fungi, purple non-sulfur bacteria, and cryptogamic covers, produce methane in oxygen-saturated aquatic and terrestrial ecosystems. The non-methanogenesis process could be an important potential contributor to methane emissions. This systematic review summarizes the knowledge of microorganisms involved in the non-methanogenesis process and the possible mechanisms of methane formation. Cyanobacteria-derived methane production may be attributed to either demethylation of methyl phosphonates or linked to light-driven primary productivity, while algae produce methane by utilizing methylated sulfur compounds as possible carbon precursors. In addition, fungi produce methane by utilizing methionine as a possible carbon precursor, and purple non-sulfur bacteria reduce carbon dioxide to methane by nitrogenase. The microbial methane distribution from the non-methanogenesis processes in aquatic and terrestrial environments and its environmental significance to global methane emissions, possible mechanisms of methane production in each open water, water-to-air methane fluxes, and the impact of climate change on microorganisms are also discussed. Finally, future perspectives are highlighted, such as establishing more in-situ experiments, quantifying methane flux through optimizing empirical models, distinguishing individual methane sources, and investigating nitrogenase-like enzyme systems to improve our understanding of microbial methane emission from the non-methanogenesis process.

Comparisons of bond valences and distances for CO- and N2-bound clusters of FeMo-cofactors

By comparisons of N2 and isoelectronic substrate CO bound FeMo-cofactors (FeMo-cos) in nitrogenases, we have used a classical bond valence method to calculate the oxidation states of the iron and molybdenum atoms in FeMo-cos.

A thiolate-bridged FeIVFeIV μ-nitrido complex and its hydrogenation reactivity toward ammonia formation

Iron nitrides are key intermediates in biological nitrogen fixation and the industrial Haber-Bosch process, used to form ammonia from dinitrogen. However, the proposed successive conversion of nitride to ammonia remains elusive. In this regard, the search for well-described multi-iron nitrido model complexes and investigations on controlling their reactivity towards ammonia formation have long been of great challenge and importance. Here we report a well-defined thiolate-bridged Fe^IVFe^IV μ-nitrido complex featuring an uncommon bent Fe-N-Fe moiety. Remarkably, this complex shows excellent reactivity toward hydrogenation with H₂ at ambient conditions, forming ammonia in high yield. Combined experimental and computational studies demonstrate that a thiolate-bridged Fe^IIIFe^III μ-amido complex is a key intermediate, which is generated through an unusual two-electron oxidation of H₂. Moreover, ammonia production was also realized by treating this diiron μ-nitride with electrons and water as a proton source.

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

Iron valences of 129 P-clusters from FeMo/V proteins were analyzed using a bond valence method, supposing the existence of Fe3+ in a generally considered all-ferrous PN cluster in solution with excess reducing agent.

Structures and reaction dynamics of N2 and H2 binding at FeMo-co, the active site of nitrogenase

The chemical reactions occurring at the Fe₇MoS₉C(homocitrate) cluster, FeMo-co, the active site of the enzyme nitrogenase (N₂ → NH₃), are enigmatic. Experimental information collected over a long period reveals aspects of the roles of N₂ and H₂, each with more than one type of reactivity. This paper reports investigations of the binding of H₂ and N₂ at intact FeMo-co, using density functional simulations of a large 486 atom relevant portion of the protein, resulting in 27 new structures containing H₂ and/or N₂ bound at the exo and endo coordination sites of the participating Fe atoms, Fe2 and Fe6. Binding energies and transition states for association/dissociation are determined, and trajectories for the approach, binding and separation of H₂/N₂ are described, including diffusion of these small molecules through proximal protein. Influences of surrounding amino acids are identified. FeMo-co deforms geometrically when binding H₂ or N₂, and a procedure for calculating the energy cost involved, the adaptation energy, is introduced here. Adaptation energies, which range from 7 to 36 kcal mol^-1 for the reported structures, are influenced by the protonation state of the His195 side chain. Seven N₂ structures and three H₂ structures have negative binding free energies, which include the estimated entropy penalties for binding of N₂, H₂ from proximal protein. These favoured structures have N₂ bound end-on at exo-Fe2, exo-Fe6 and endo-Fe2 positions of FeMo-co, and H₂ bound at the endo-Fe2 position. Various postulated structures with N₂ bridging Fe2 and Fe6 revert to end-on-N₂ at endo positions. The structures are also assessed via the calculated potential energy barriers for association and dissociation. Barriers to the binding of H₂ range from 1 to 20 kcal mol^-1 and barriers to dissociation of H₂ range from 3 to 18 kcal mol^-1. Barriers to the binding of N₂, in either side-on or end-on mode, range from 2 to 18 kcal mol^-1, while dissociation of bound N₂ encounters barriers of 3 to 8 kcal mol^-1 for side-on bonding and 7 to 18 kcal mol^-1 for end-on bonding. These results allow formulation of mechanisms for the H₂/N₂ exchange reaction, and three feasible mechanisms for associative exchange and three for dissociative exchange are identified. Consistent electronic structures and potential energy surfaces are maintained throughout. Changes in the spin populations of Fe2 and Fe6 connected with cluster deformation and with metal-ligand bond formation are identified.

Structural analysis of the reductase component AnfH of iron-only nitrogenase from Azotobacter vinelandii

Biological nitrogen fixation, the conversion of atmospheric dinitrogen into bioavailable ammonium, is exclusively catalyzed by the enzyme nitrogenase that is present in nitrogen-fixing organisms, the diazotrophs. So far, three different nitrogenase variants, encoded in their corresponding, distinct gene clusters, have been found in nature. Each one of these consists of a catalytic dinitrogenase component and a unique, ATP-dependent reductase, the Fe protein. The three variant nitrogenases differ in the composition of the active site and contain either molybdenum, vanadium or only iron in the dinitrogenase component. Here we present the 2.0 Å resolution crystal structure of the ADP-bound reductase component AnfH of the iron-only nitrogenase from the model diazotroph Azotobacter vinelandii. A comparison of this structure with the ones reported for the two other Fe protein homologs NifH and VnfH in the ADP-bound state shows that all are adopting the same conformation. However, cross-reactivity assays with the three nitrogenase homologs revealed AnfH to be compatible with iron-only nitrogenase and to a lesser degree with the vanadium-containing enzyme, but not with molybdenum nitrogenase.

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.

Sulfide and transition metals - A partnership for life

Sulfide and transition metals often came together in Biology. The variety of possible structural combinations enabled living organisms to evolve an array of highly versatile metal-sulfide centers to fulfill different physiological roles. The ubiquitous iron‑sulfur centers, with their structural, redox, and functional diversity, are certainly the best-known partners, but other metal-sulfide centers, involving copper, nickel, molybdenum or tungsten, are equally crucial for Life. This review provides a concise overview of the exclusive sulfide properties as a metal ligand, with emphasis on the structural aspects and biosynthesis. Sulfide as catalyst and as a substrate is discussed. Different enzymes are considered, including xanthine oxidase, formate dehydrogenases, nitrogenases and carbon monoxide dehydrogenases. The sulfide effect on the activity and function of iron‑sulfur, heme and zinc proteins is also addressed.

The E2 state of FeMoco: Hydride Formation versus Fe Reduction and a Mechanism for H2 Evolution

The iron‐molybdenum cofactor (FeMoco) is responsible for dinitrogen reduction in Mo nitrogenase. Unlike the resting state, E₀, reduced states of FeMoco are much less well characterized. The E₂ state has been proposed to contain a hydride but direct spectroscopic evidence is still lacking. The E₂ state can, however, relax back the E₀ state via a H₂ side‐reaction, implying a hydride intermediate prior to H₂ formation. This E₂→E₀ pathway is one of the primary mechanisms for H₂ formation under low‐electron flux conditions. In this study we present an exploration of the energy surface of the E₂ state. Utilizing both cluster‐continuum and QM/MM calculations, we explore various classes of E₂ models: including terminal hydrides, bridging hydrides with a closed or open sulfide‐bridge, as well as models without. Importantly, we find the hemilability of a protonated belt‐sulfide to strongly influence the stability of hydrides. Surprisingly, non‐hydride models are found to be almost equally favorable as hydride models. While the cluster‐continuum calculations suggest multiple possibilities, QM/MM suggests only two models as contenders for the E₂ state. These models feature either i) a bridging hydride between Fe₂ and Fe₆ and an open sulfide‐bridge with terminal SH on Fe₆ (E₂‐hyd) or ii) a double belt‐sulfide protonated, reduced cofactor without a hydride (E₂‐nonhyd). We suggest both models as contenders for the E₂ redox state and further calculate a mechanism for H₂ evolution. The changes in electronic structure of FeMoco during the proposed redox‐state cycle, E₀→E₁→E₂→E₀, are discussed.

Transition Metal Carbide Complexes

Carbide complexes remain a rare class of molecules. Their paucity does not reflect exceptional instability but is rather due to the generally narrow scope of synthetic procedures for constructing carbide complexes. The preparation of carbide complexes typically revolves around generating L_nM-CE_x fragments, followed by cleavage of the C-E bonds of the coordinated carbon-based ligands (the alternative being direct C atom transfer). Prime examples involve deoxygenation of carbonyl ligands and deprotonation of methyl ligands, but several other p-block fragments can be cleaved off to afford carbide ligands. This Review outlines synthetic strategies toward terminal carbide complexes, bridging carbide complexes, as well as carbide-carbonyl cluster complexes. It then surveys the reactivity of carbide complexes, covering stoichiometric reactions where the carbide ligands act as C₁ reagents, engage in cross-coupling reactions, and enact Fischer-Tropsch-like chemistry; in addition, we discuss carbide complexes in the context of catalysis. Finally, we examine spectroscopic features of carbide complexes, which helps to establish the presence of the carbide functionality and address its electronic structure.

The crystalline state as a dynamic system: IR microspectroscopy under electrochemical control for a [NiFe] hydrogenase

Controlled formation of catalytically-relevant states within crystals of complex metalloenzymes represents a significant challenge to structure-function studies. Here we show how electrochemical control over single crystals of [NiFe] hydrogenase 1 (Hyd1) from Escherichia coli makes it possible to navigate through the full array of active site states previously observed in solution. Electrochemical control is combined with synchrotron infrared microspectroscopy, which enables us to measure high signal-to-noise IR spectra in situ from a small area of crystal. The output reports on active site speciation via the vibrational stretching band positions of the endogenous CO and CN- ligands at the hydrogenase active site. Variation of pH further demonstrates how equilibria between catalytically-relevant protonation states can be deliberately perturbed in the crystals, generating a map of electrochemical potential and pH conditions which lead to enrichment of specific states. Comparison of in crystallo redox titrations with measurements in solution or of electrode-immobilised Hyd1 confirms the integrity of the proton transfer and redox environment around the active site of the enzyme in crystals. Slowed proton-transfer equilibria in the hydrogenase in crystallo reveals transitions which are only usually observable by ultrafast methods in solution. This study therefore demonstrates the possibilities of electrochemical control over single metalloenzyme crystals in stabilising specific states for further study, and extends mechanistic understanding of proton transfer during the [NiFe] hydrogenase catalytic cycle.

Uncovering Redox Non-innocent Hydrogen-Bonding in Cu(I)-Diazene Complexes

The life-sustaining reduction of N₂ to NH₃ is thermoneutral yet kinetically challenged by high-energy intermediates such as N₂H₂. Exploring intramolecular H-bonding as a potential strategy to stabilize diazene intermediates, we employ a series of [^xHetTpCu]₂(μ-N₂H₂) complexes that exhibit H-bonding between pendant aromatic N-heterocycles (^xHet) such as pyridine and a bridging trans-N₂H₂ ligand at copper(I) centers. X-ray crystallography and IR spectroscopy clearly reveal H-bonding in [^pyMeTpCu]₂(μ-N₂H₂) while low-temperature ¹H NMR studies coupled with DFT analysis reveals a dynamic equilibrium between two closely related, symmetric H-bonded structural motifs. Importantly, the ^xHet pendant negligibly influences the electronic structure of ^xHetTpCu^I centers in ^xHetTpCu(CNAr^2,6-Me₂) complexes that lack H-bonding as judged by nearly indistinguishable ν(CN) frequencies (2113-2117 cm^-1). Nonetheless, H-bonding in the corresponding [^xHetTpCu]₂(μ-N₂H₂) complexes results in marked changes in ν(NN) (1398-1419 cm^-1) revealed through resonance Raman studies. Due to the closely matched N-H BDEs of N₂H₂ and the pyH⁰ cation radical, the aromatic N-heterocyclic pendants may encourage partial H-atom transfer (HAT) from N₂H₂ to ^xHet through redox-non-innocent H-bonding in [^xHetTpCu]₂(μ-N₂H₂). DFT studies reveal modest thermodynamic barriers for concerted transfer of both H-atoms of coordinated N₂H₂ to the ^xHet pendants to generate tautomeric [^xHetHTpCu]₂(μ-N₂) complexes, identifying metal-assisted concerted dual HAT as a thermodynamically favorable pathway for N₂/N₂H₂ interconversion.

Substrate Specific Metal-Ligand Cooperative Binding: Considerations for Weak Intramolecular Lewis Acid/Base Pairs

Metal-ligand cooperative binding modes were interrogated in a series of zinc bis(thiophenoxide) complexes. A weak B-S binding interaction is observed in solution between the weakly Lewis basic thiophenoxide ligands and an appended trialkylborane. The energy of this binding event is dependent upon the strength of the Lewis acid and its proximity to the zinc thiophenoxide.

Deuterium equilibrium isotope effects in a supramolecular receptor for the hydrochalcogenide and halide anions

We highlight a convenient synthesis to selectively deuterate an aryl C-H hydrogen bond donor in an arylethynyl bisurea supramolecular anion receptor and use the Perrin method of competitive titrations to study the deuterium equilibrium isotope effects (DEIE) of anion binding for HS-, Cl-, and Br-. This work highlights the utility and also challenges in using this method to determine EIE with highly reactive and/or weakly binding anions.

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.

Dinitrogen binding and activation at a molybdenum-iron-sulfur cluster

The Fe-S clusters of nitrogenases carry out the life-sustaining conversion of N₂ to NH₃. Although progress continues to be made in modelling the structural features of nitrogenase cofactors, no synthetic Fe-S cluster has been shown to form a well-defined coordination complex with N₂. Here we report that embedding an [MoFe₃S₄] cluster in a protective ligand environment enables N₂ binding at Fe. The bridging [MoFe₃S₄]₂(μ-η¹:η¹-N₂) complex thus prepared features a substantially weakened N-N bond despite the relatively high formal oxidation states of the metal centres. Substitution of one of the [MoFe₃S₄] cubanes with an electropositive Ti metalloradical induces additional charge transfer to the N₂ ligand with generation of Fe-N multiple-bond character. Structural and spectroscopic analyses demonstrate that N₂ activation is accompanied by shortened Fe-S distances and charge transfer from each Fe site, including those not directly bound to N₂. These findings indicate that covalent interactions within the cluster play a critical role in N₂ binding and activation.

Altering Hydrogenation Pathways in Photocatalytic Nitrogen Fixation by Tuning Local Electronic Structure of Oxygen Vacancy with Dopant

To avoid the energy-consuming step of direct N≡N bond cleavage, photocatalytic N₂ fixation undergoing the associative pathways has been developed for mild-condition operation. However, it is a fundamental yet challenging task to gain comprehensive understanding on how the associative pathways (i.e., alternating vs. distal) are influenced and altered by the fine structure of catalysts, which eventually holds the key to significantly promote the practical implementation. Herein, we introduce Fe dopants into TiO₂ nanofibers to stabilize oxygen vacancies and simultaneously tune their local electronic structure. The combination of in situ characterizations with first-principles simulations reveals that the modulation of local electronic structure by Fe dopants turns the hydrogenation of N₂ from associative alternating pathway to associative distal pathway. This work provides fresh hints for rationally controlling the reaction pathways toward efficient photocatalytic nitrogen fixation.

Two ligand-binding sites in CO-reducing V nitrogenase reveal a general mechanistic principle

Besides its role in biological nitrogen fixation, vanadium-containing nitrogenase also reduces carbon monoxide (CO) to hydrocarbons, in analogy to the industrial Fischer-Tropsch process. The protein yields 93% of ethylene (C₂H₄), implying a C-C coupling step that mandates the simultaneous binding of two CO at the active site FeV cofactor. Spectroscopic data indicated multiple CO binding events, but structural analyses of Mo and V nitrogenase only confirmed a single site. Here, we report the structure of a two CO-bound state of V nitrogenase at 1.05 Å resolution, with one μ-bridging and one terminal CO molecule. This additional, specific ligand binding site suggests a mechanistic route for CO reduction and hydrocarbon formation, as well as a second access pathway for protons required during the reaction. Moreover, carbonyls are strong-field ligands that are chemically similar to mechanistically relevant hydrides that may be formed and used in a fully analogous fashion.

A Molecular Low-Coordinate [Fe-S-Fe] Unit in Three Oxidation States

A [Fe-S-Fe] subunit with a single sulfide bridging two low-coordinate iron ions is the supposed active site of the iron-molybdenum co-factor (FeMoco) of nitrogenase. Here we report a dinuclear monosulfido bridged diiron(II) complex with a similar complex geometry that can be oxidized stepwise to diiron(II/III) and diiron(III/III) complexes while retaining the [Fe-S-Fe] core. The series of complexes has been characterized crystallographically, and electronic structures have been studied using, inter alia, 57 Fe Mössbauer spectroscopy and SQUID magnetometry. Further, cleavage of the [Fe-S-Fe] unit by CS2 is presented.

The electronic structure of FeV-cofactor in vanadium-dependent nitrogenase

The electronic structure of the active-site metal cofactor (FeV-cofactor) of resting-state V-dependent nitrogenase has been an open question, with earlier studies indicating that it exhibits a broad S = 3/2 EPR signal (Kramers state) having g values of ∼4.3 and 3.8, along with suggestions that it contains metal-ions with valencies [1V3+, 3Fe3+, 4Fe2+]. In the present work, genetic, biochemical, and spectroscopic approaches were combined to reveal that the EPR signals previously assigned to FeV-cofactor do not correlate with active VFe-protein, and thus cannot arise from the resting-state of catalytically relevant FeV-cofactor. It, instead, appears resting-state FeV-cofactor is either diamagnetic, S = 0, or non-Kramers, integer-spin (S = 1, 2 etc.). When VFe-protein is freeze-trapped during high-flux turnover with its natural electron-donating partner Fe protein, conditions which populate reduced states of the FeV-cofactor, a new rhombic S = 1/2 EPR signal from such a reduced state is observed, with g = [2.18, 2.12, 2.09] and showing well-defined 51V (I = 7/2) hyperfine splitting, a iso = 110 MHz. These findings indicate a different assignment for the electronic structure of the resting state of FeV-cofactor: S = 0 (or integer-spin non-Kramers state) with metal-ion valencies, [1V3+, 4Fe3+, 3Fe2+]. Our findings suggest that the V3+ does not change valency throughout the catalytic cycle.

Postbiosynthetic modification of a precursor to the nitrogenase iron-molybdenum cofactor

Nitrogenases utilize Fe-S clusters to reduce N₂ to NH₃ The large number of Fe sites in their catalytic cofactors has hampered spectroscopic investigations into their electronic structures, mechanisms, and biosyntheses. To facilitate their spectroscopic analysis, we are developing methods for incorporating ⁵⁷Fe into specific sites of nitrogenase cofactors, and we report herein site-selective ⁵⁷Fe labeling of the L-cluster-a carbide-containing, [Fe₈S₉C] precursor to the Mo nitrogenase catalytic cofactor. Treatment of the isolated L-cluster with the chelator ethylenediaminetetraacetate followed by reconstitution with ⁵⁷Fe²⁺ results in ⁵⁷Fe labeling of the terminal Fe sites in high yield and with high selectivity. This protocol enables the generation of L-cluster samples in which either the two terminal or the six belt Fe sites are selectively labeled with ⁵⁷Fe. Mössbauer spectroscopic analysis of these samples bound to the nitrogenase maturase Azotobacter vinelandii NifX reveals differences in the primary coordination sphere of the terminal Fe sites and that one of the terminal sites of the L-cluster binds to H35 of Av NifX. This work provides molecular-level insights into the electronic structure and biosynthesis of the L-cluster and introduces postbiosynthetic modification as a promising strategy for studies of nitrogenase cofactors.

Structural Characterization of Two CO Molecules Bound to the Nitrogenase Active Site

As an approach towards unraveling the nitrogenase mechanism, we have studied the binding of CO to the active-site FeMo-cofactor. CO is not only an inhibitor of nitrogenase, but it is also a substrate, undergoing reduction to hydrocarbons (Fischer-Tropsch-type chemistry). The C-C bond forming capabilities of nitrogenase suggest that multiple CO or CO-derived ligands bind to the active site. Herein, we report a crystal structure with two CO ligands coordinated to the FeMo-cofactor of the molybdenum nitrogenase at 1.33 Å resolution. In addition to the previously observed bridging CO ligand between Fe2 and Fe6 of the FeMo-cofactor, a new ligand binding mode is revealed through a second CO ligand coordinated terminally to Fe6. While the relevance of this state to nitrogenase-catalyzed reactions remains to be established, it highlights the privileged roles for Fe2 and Fe6 in ligand binding, with multiple coordination modes available depending on the ligand and reaction conditions.

CowN sustains nitrogenase turnover in the presence of the inhibitor carbon monoxide

Nitrogenase is the only enzyme capable of catalyzing nitrogen fixation, the reduction of dinitrogen gas (N₂) to ammonia (NH₃). Nitrogenase is tightly inhibited by the environmental gas carbon monoxide (CO). Nitrogen-fixing bacteria rely on the protein CowN to grow in the presence of CO. However, the mechanism by which CowN operates is unknown. Here, we present the biochemical characterization of CowN and examine how CowN protects nitrogenase from CO. We determine that CowN interacts directly with nitrogenase and that CowN protection observes hyperbolic kinetics with respect to CowN concentration. At a CO concentration of 0.001 atm, CowN restores nearly full nitrogenase activity. Our results further indicate that CowN's protection mechanism involves decreasing the binding affinity of CO to nitrogenase's active site approximately tenfold without interrupting substrate turnover. Taken together, our work suggests CowN is an important auxiliary protein in nitrogen fixation that engenders CO tolerance to nitrogenase.

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

Dinitrogen (N2 ) is the most abundant gas in Earth's atmosphere, but its inertness hinders its use as a nitrogen source in the biosphere and in industry. Efficient catalysts are hence required to ov. ercome the high kinetic barriers associated to N2 transformation. In that respect, molecular complexes have demonstrated strong potential to mediate N2 functionalization reactions under mild conditions while providing a straightforward understanding of the reaction mechanisms. This Review emphasizes the strategies for N2 reduction and functionalization using molecular transition metal and actinide complexes according to their proposed reaction mechanisms, distinguishing complexes inducing cleavage of the N≡N bond before (dissociative mechanism) or concomitantly with functionalization (associative mechanism). We present here the main examples of stoichiometric and catalytic N2 functionalization reactions following these strategies.

Response to Comment on "Structural evidence for a dynamic metallocofactor during N2 reduction by Mo-nitrogenase"

Peters et al comment on our report of the dynamic structure of the nitrogenase metallocofactor during N₂ reduction. Their claim that their independent structural refinement and consideration of biochemical data contradict our finding is incorrect and is strongly refuted by our biochemical and structural data that collectively and conclusively point to the binding of dinitrogen species to the nitrogenase cofactor.

Synthesis and Reactivity of Iron Complexes with a Biomimetic SCS Pincer Ligand

Recent experimental evidence suggests that the FeMoco of nitrogenase undergoes structural rearrangement during N₂ reduction, which may result in the generation of coordinatively unsaturated iron sites with two sulfur donors and a carbon donor. In an effort to synthesize and study small-molecule model complexes with a one-carbon/two-sulfur coordination environment, we have designed two new SCS pincer ligands containing a central NHC donor accompanied by thioether- or thiolate-functionalized aryl groups. Metalation of the thioether ligand with Fe(OTf)₂ gives 6-coordinate complexes in which the SCS ligand binds meridionally. In contrast, metalation of the thiolate ligand with Fe(HMDS)₂ gives a four-coordinate pseudotetrahedral amide complex in which the ligand binds facially, illustrating the potential structural flexibility of these ligands. Reaction of the amide complex with a bulky monothiol gives a four-coordinate complex with a one-carbon/three-sulfur coordination environment that resembles the resting state of nitrogenase. Reaction of the amide complex with phenylhydrazine gives a product with a rare κ¹-bound phenylhydrazido group which undergoes N-N cleavage to give a phenylamido complex.

Cleavage of cluster iron-sulfide bonds in cyclophane-coordinated FenSm complexes

Reaction of the tri(μ-sulfido)triiron(iii) tris(β-diketiminate) cyclophane complex, Fe3S3LEt/Me (1), or of the di(μ-sulfido)diiron(iii) complex Fe2S2HLEt/Me (5), with the related tri(bromide)triiron(ii) complex Fe3Br3LEt/Me (2) results in electron and ligand redistribution to yield the mixed-ligand multiiron complexes, including Fe3Br2SLEt/Me (3) and Fe2Br2SHLEt/Me (4). The cleavage and redistribution observed in these complexes is reminiscent of necessary Fe-S bond cleavage for substrate activation in nitrogenase enzymes, and provides a new perspective on the lability of Fe-S bonds in FeS clusters.

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.

Mixed-Valent Diiron μ-Carbyne, μ-Hydride Complexes: Implications for Nitrogenase

Binding of N₂ by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 e^- and 4 H⁺ equivalents to the active site. Although pulse EPR studies indicate the presence of two Fe-(μ-H)-Fe moieties, the structural and electronic features of this mixed valent intermediate remain poorly understood. Toward an improved understanding of this bioorganometallic cluster, we report herein that diiron μ-carbyne complex (P₆ArC)Fe₂(μ-H) can be oxidized and reduced, allowing for the first time spectral characterization of two EPR-active Fe(μ-C)(μ-H)Fe model complexes linked by a 2 e^- transfer which bear some resemblance to a pair of E_n and E_n+2 states of nitrogenase. Both species populate S = 1/2 states at low temperatures, and the influence of valence (de)localization on the spectroscopic signature of the μ-hydride ligand was evaluated by pulse EPR studies. Compared to analogous data for the {Fe₂(μ-H)}₂ state of FeMoco (E₄(4H)), the data and analysis presented herein suggest that the hydride ligands in E₄(4H) bridge isovalent (most probably Fe^III) metal centers. Although electron transfer involves metal-localized orbitals, investigations of [(P₆ArC)Fe₂(μ-H)]⁺¹ and [(P₆ArC)Fe₂(μ-H)]^-1 by pulse EPR revealed that redox chemistry induces significant changes in Fe-C covalency (-50% upon 2 e^- reduction), a conclusion further supported by X-ray absorption spectroscopy, ⁵⁷Fe Mössbauer studies, and DFT calculations. Combined, our studies demonstrate that changes in covalency buffer against the accumulation of excess charge density on the metals by partially redistributing it to the bridging carbon, thereby facilitating multielectron transformations.