ENDOR/HYSCORE studies of the common intermediate trapped during nitrogenase reduction of N2H2, CH3N2H, and N2H4 support an alternating reaction pathway for N2 reduction

Putative reaction mechanism of nitrogenase with a half-dissociated S2B ligand

We have studied whether dissociation of the S2B sulfide ligand from one of its two coordinating Fe ions may affect the later parts of the reaction mechanism of nitrogenase. Such dissociation has been shown to be favourable for the E2-E4 states in the reaction mechanism, but previous studies have assumed that S2B either remains bridging or has fully dissociated from the active-site FeMo cluster. We employ combined quantum mechanical and molecular mechanical (QM/MM) calculations with two density-functional theory methods, r2SCAN and TPSSh. To make dissociation of S2B possible, we have added a proton to this group throughout the reaction. We study the reaction starting from the E4 state with N2H2 bound to the cluster. Our results indicate that half-dissociation of S2B is unfavourable in most steps of the reaction mechanism. We observe favourable half-dissociation of S2B only when NH or NH2 is bound to the cluster, bridging Fe2 and Fe6. However, the former state is most likely not involved in the reaction mechanism and the latter state is only an intermittent intermediate of the E7 state. Therefore, half-dissociation of S2B seems to play only a minor role in the later parts of the reaction mechanism of nitrogenase. Our suggested mechanism with a protonated S2B is alternating (the two N atoms of the substrate is protonated in an alternating manner) and the substrate prefers to bind to Fe2, in contrast to the preferred binding to Fe6 observed when S2B is unprotonated and bridging Fe2 and Fe6.

Analysis of early intermediate states of the nitrogenase reaction by regularization of EPR spectra

Due to the complexity of the catalytic FeMo cofactor site in nitrogenases that mediates the reduction of molecular nitrogen to ammonium, mechanistic details of this reaction remain under debate. In this study, selenium- and sulfur-incorporated FeMo cofactors of the catalytic MoFe protein component from Azotobacter vinelandii are prepared under turnover conditions and investigated by using different EPR methods. Complex signal patterns are observed in the continuous wave EPR spectra of selenium-incorporated samples, which are analyzed by Tikhonov regularization, a method that has not yet been applied to high spin systems of transition metal cofactors, and by an already established grid-of-error approach. Both methods yield similar probability distributions that reveal the presence of at least four other species with different electronic structures in addition to the ground state E0. Two of these species were preliminary assigned to hydrogenated E2 states. In addition, advanced pulsed-EPR experiments are utilized to verify the incorporation of sulfur and selenium into the FeMo cofactor, and to assign hyperfine couplings of 33S and 77Se that directly couple to the FeMo cluster. With this analysis, we report selenium incorporation under turnover conditions as a straightforward approach to stabilize and analyze early intermediate states of the FeMo cofactor.

Low-temperature trapping of N2 reduction reaction intermediates in nitrogenase MoFe protein-CdS quantum dot complexes

The biological reduction of N2 to ammonia requires the ATP-dependent, sequential delivery of electrons from the Fe protein to the MoFe protein of nitrogenase. It has been demonstrated that CdS nanocrystals can replace the Fe protein to deliver photoexcited electrons to the MoFe protein. Herein, light-activated electron delivery within the CdS:MoFe protein complex was achieved in the frozen state, revealing that all the electron paramagnetic resonance (EPR) active E-state intermediates in the catalytic cycle can be trapped and characterized by EPR spectroscopy. Prior to illumination, the CdS:MoFe protein complex EPR spectrum was composed of a S = 3/2 rhombic signal (g = 4.33, 3.63, and 2.01) consistent with the FeMo-cofactor in the resting state, E0. Illumination for sequential 1-h periods at 233 K under 1 atm of N2 led to a cumulative attenuation of E0 by 75%. This coincided with the appearance of S = 3/2 and S = 1/2 signals assigned to two-electron (E2) and four-electron (E4) reduced states of the FeMo-cofactor, together with additional S = 1/2 signals consistent with the formation of E6 and E8 states. Simulations of EPR spectra allowed quantification of the different E-state populations, along with mapping of these populations onto the Lowe-Thorneley kinetic scheme. The outcome of this work demonstrates that the photochemical delivery of electrons to the MoFe protein can be used to populate all of the EPR active E-state intermediates of the nitrogenase MoFe protein cycle.

Catalysis and structure of nitrogenases

In providing bioavailable nitrogen as building blocks for all classes of biomacromolecules, biological nitrogen fixation is an essential process for all organismic life. Only a single enzyme, nitrogenase, performs this task at ambient conditions and with ATP as an energy source. The assembly of the complex iron-sulfur enzyme nitrogenase and its catalytic mechanism remains a matter of intense study. Recent progress in the structural analysis of the three known isoforms of nitrogenase-differentiated primarily by the heterometal in their active site cofactor-has revealed a degree of structural plasticity of these clusters that suggest two distinct binding sites for substrates and reaction intermediates. A mechanistic proposal based on this finding integrates most of the available experimental data. Furthermore, the first applications of high-resolution cryo-electron microscopy have highlighted further dynamic conformational changes. Structures obtained under turnover conditions support the proposed alternating half-site reactivity in the C2-symmetric nitrogenase complex.

Ammonia from dinitrogen at ambient conditions by organometallic catalysts

Fixation of atmospheric dinitrogen in plants by [Mo-Fe] cofactor of nitrogenase enzyme takes place efficiently under atmospheric pressure and normal temperature. In search for an alternative methodology for the highly energy intensive Haber-Bosch process, design and synthesis of highly efficient inorganic and organometallic complexes by mimicking the structure and function of [Mo-Fe] cofactor system is highly desirable for ammonia synthesis from dinitrogen. An ideal catalyst for ammonia synthesis should effectively catalyse the reduction of dinitrogen in the presence of a proton source under mild to moderate conditions, and thereby, significantly reducing the cost of ammonia production and increasing the energy efficacy of the process. In the light of current research, it is evident that there is a plenty of scope for the development and enhanced performance of the inorganic and organometallic catalysts for ammonia synthesis under ambient temperature and pressure. The review furnishes a comprehensive outlook of numerous organometallic catalysts used in the synthesis of ammonia from dinitrogen in the past few decades.

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.

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.

Heterojunction photocatalysts for artificial nitrogen fixation: fundamentals, latest advances and future perspectives

As an indispensable energy source, ammonia plays an essential role in agriculture and various industries. Given that the current ammonia production is still dominated by the energy-intensive and high carbon footprint Haber-Bosch process, photocatalytic nitrogen fixation represents a low-energy consuming and sustainable approach to generate ammonia. Heterostructured photocatalysts are hybrid materials composed of semiconductor materials containing interfaces that make full use of the unique superiorities of the constituents and synergistic effects between them. These promising photocatalysts have superior performances and substantial potential in photocatalytic reduction of nitrogen. In this review, a wide spectrum of recently developed heterostructured photocatalysts for nitrogen fixation to ammonia are evaluated. The fundamentals of solar-to-ammonia conversion, basic principles of various heterojunction photocatalysts and modification strategies are systematically reviewed. Finally, a brief summary and perspectives on the ongoing challenges and directions for future development of nitrogen photofixation catalysts are also provided.

Electron Redistribution within the Nitrogenase Active Site FeMo-Cofactor During Reductive Elimination of H2 to Achieve N≡N Triple-Bond Activation

Nitrogen fixation by nitrogenase begins with the accumulation of four reducing equivalents at the active-site FeMo-cofactor (FeMo-co), generating a state (denoted E₄(4H)) with two [Fe-H-Fe] bridging hydrides. Recently, photolytic reductive elimination (re) of the E₄(4H) hydrides showed that enzymatic re of E₄(4H) hydride yields an H₂-bound complex (E₄(H₂,2H)), in a process corresponding to a formal 2-electron reduction of the metal-ion core of FeMo-co. The resulting electron-density redistribution from Fe-H bonds to the metal ions themselves enables N₂ to bind with concomitant H₂ release, a process illuminated here by QM/MM molecular dynamics simulations. What is the nature of this redistribution? Although E₄(H₂,2H) has not been trapped, cryogenic photolysis of E₄(4H) provides a means to address this question. Photolysis of E₄(4H) causes hydride-re with release of H₂, generating doubly reduced FeMo-co (denoted E₄(2H)*), the extreme limit of the electron-density redistribution upon formation of E₄(H₂,2H). Here we examine the doubly reduced FeMo-co core of the E₄(2H)* limiting-state by ¹H, ⁵⁷Fe, and ⁹⁵Mo ENDOR to illuminate the partial electron-density redistribution upon E₄(H₂,2H) formation during catalysis, complementing these results with corresponding DFT computations. Inferences from the E₄(2H)* ENDOR results as extended by DFT computations include (i) the Mo-site participates negligibly, and overall it is unlikely that Mo changes valency throughout the catalytic cycle; and (ii) two distinctive E₄(4H) ⁵⁷Fe signals are suggested as associated with structurally identified "anchors" of one bridging hydride, two others with identified anchors of the second, with NBO-analysis further identifying one anchor of each hydride as a major recipient of electrons released upon breaking Fe-H bonds.

Does the crystal structure of vanadium nitrogenase contain a reaction intermediate? Evidence from quantum refinement

Recently, a crystal structure of V-nitrogenase was presented, showing that one of the µ2 sulphide ions in the active site (S2B) is replaced by a lighter atom, suggested to be NH or NH2, i.e. representing a reaction intermediate. Moreover, a sulphur atom is found 7 Å from the S2B site, suggested to represent a storage site for this ion when it is displaced. We have re-evaluated this structure with quantum refinement, i.e. standard crystallographic refinement in which the empirical restraints (employed to ensure that the final structure makes chemical sense) are replaced by more accurate quantum-mechanical calculations. This allows us to test various interpretations of the structure, employing quantum-mechanical calculations to predict the ideal structure and to use crystallographic measures like the real-space Z-score and electron-density difference maps to decide which structure fits the crystallographic raw data best. We show that the structure contains an OH--bound state, rather than an N2-derived reaction intermediate. Moreover, the structure shows dual conformations in the active site with ~ 14% undissociated S2B ligand, but the storage site seems to be fully occupied, weakening the suggestion that it represents a storage site for the dissociated ligand.

Defining Intermediates of Nitrogenase MoFe Protein during N2 Reduction under Photochemical Electron Delivery from CdS Quantum Dots

Coupling the nitrogenase MoFe protein to light-harvesting semiconductor nanomaterials replaces the natural electron transfer complex of Fe protein and ATP and provides low-potential photoexcited electrons for photocatalytic N₂ reduction. A central question is how direct photochemical electron delivery from nanocrystals to MoFe protein is able to support the multielectron ammonia production reaction. In this study, low photon flux conditions were used to identify the initial reaction intermediates of CdS quantum dot (QD):MoFe protein nitrogenase complexes under photochemical activation using EPR. Illumination of CdS QD:MoFe protein complexes led to redox changes in the MoFe protein active site FeMo-co observed as the gradual decline in the E₀ resting state intensity that was accompanied by an increase in the intensity of a new "g_eff = 4.5" EPR signal. The magnetic properties of the g_eff = 4.5 signal support assignment as a reduced S = 3/2 state, and reaction modeling was used to define it as a two-electron-reduced "E₂" intermediate. Use of a MoFe protein variant, β-188^Cys, which poises the P cluster in the oxidized P⁺ state, demonstrated that the P cluster can function as a site of photoexcited electron delivery from CdS to MoFe protein. Overall, the results establish the initial steps for how photoexcited CdS delivers electrons into the MoFe protein during reduction of N₂ to ammonia and the role of electron flux in the photochemical reaction cycle.

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.

Thiolate-Bridged Dicobalt Complexes Bearing Hydrazine, Hydrazido, and Diazenido Ligands: Synthesis, Structural Characterization, and Interconversion

Synthetic di- or multimetallic complexes bearing N_xH_y nitrogenous ligands in a sulfur-rich coordination environment have attracted considerable attention due to their importance in evaluating the complex mechanism of biological nitrogen fixation. Herein, we report a series of thiolate-bridged dicobalt N_xH_y species obtained by treatment of Co^IIICo^III precursor with hydrazine and its substituted derivatives at ambient temperature. Remarkably, when the substituent is the cyclohexyl group, the resulting species can interconvert through different pathways. This Co₂S₂ skeleton provides a new model system for obtaining valuable information about the early N₂H_x-bound intermediate species during the catalytic cycle of nitrogenase.

Planar three-coordinate iron sulfide in a synthetic [4Fe-3S] cluster with biomimetic reactivity

Iron-sulfur clusters are emerging as reactive sites for the reduction of small-molecule substrates. However, the four-coordinate iron sites of typical iron-sulfur clusters rarely react with substrates, implicating three-coordinate iron. This idea is untested because fully sulfide-coordinated three-coordinate iron is unprecedented. Here we report a new type of [4Fe-3S] cluster featuring an iron center with three bonds to sulfides. Although a high-spin electronic configuration is characteristic of other iron-sulfur clusters, the planar geometry and short Fe–S bonds lead to a surprising low-spin electronic configuration at the three-coordinate Fe center as determined by spectroscopy and ab initio calculations. In a demonstration of biomimetic reactivity, the [4Fe-3S] cluster reduces hydrazine, a natural substrate of nitrogenase. The product is the first example of NH₂ bound to an iron-sulfur cluster. Our results demonstrate that three-coordinate iron supported by sulfide donors is a plausible precursor to reactivity in iron-sulfur clusters like the FeMoco of nitrogenase.

ENDOR Characterization of (N2)FeII(μ-H)2FeI(N2)-: A Spectroscopic Model for N2 Binding by the Di-μ-hydrido Nitrogenase Janus Intermediate

The biomimetic diiron complex 4-(N₂)₂, featuring two terminally bound Fe-N₂ centers bridged by two hydrides, serves as a model for two possible states along the pathway by which the enzyme nitrogenase reduces N₂. One is the Janus intermediate E₄(4H), which has accumulated 4[e-/H+], stored as two [Fe-H-Fe] bridging hydrides, and is activated to bind and reduce N₂ through reductive elimination (RE) of the hydride ligands as H₂. The second is a possible RE intermediate. ¹H and ¹⁴N 35 GHz ENDOR measurements confirm that the formally Fe(II)/Fe(I) 4-(N₂)₂ complex exhibits a fully delocalized, Robin-Day type-III mixed valency. The two bridging hydrides exhibit a fully rhombic dipolar tensor form, T ≈ [- t, + t, 0]. The rhombic form is reproduced by a simple point-dipole model for dipolar interactions between a bridging hydride and its "anchor" Fe ions, confirming validity of this model and demonstrating that observation of a rhombic form is a convenient diagnostic signature for the identification of such core structures in biological centers such as nitrogenase. Furthermore, interpretation of the ¹H measurements with the anchor model maps the g tensor onto the molecular frame, an important function of these equations for application to nitrogenase. Analysis of the hyperfine and quadrupole coupling to the bound ¹⁴N of N₂ provides a reference for nitrogen-bound nitrogenase intermediates and is of chemical significance, as it gives a quantitative estimate of the amount of charge transferred between Fe and coordinated N, a key element in N₂ activation for reduction.

New opportunities for efficient N2 fixation by nanosheet photocatalysts

Catalytic ammonia synthesis from dinitrogen (N2) under mild conditions has been considered to be the "holy grail" of N2 fixation, which is one of the most important chemical processes in the agriculture, biological and industrial fields. Given that current artificial N2 fixation is still dominated by the energy-intensive Haber-Bosch process, solar N2 fixation represents an encouraging and fascinating route for carbon-free and energy-saving N2 fixation. However, its practical application is seriously hampered by surface sluggish reaction kinetics. In this minireview, we share our perspectives on the use of two-dimensional (2D) nanosheets for the manipulation of photocatalytic N2 fixation. Nanosheet photocatalysts serve as the perfect platform for the engineering of surface active sites, including defects and iron, all of which can not only bolster photon-exciton interaction toward robust charge carriers generation upon light absorption, but also mimic the function schemes of MoFe-cofactor in nitrogenase toward sufficient N2 binding and activation. These merits endowed by nanosheets photocatalysts provide instructive information on exploring the rich nitrogen photochemistry on solid surfaces and offer new opportunities for the design of novel photocatalysts towards efficient N2 fixation.

A bound reaction intermediate sheds light on the mechanism of nitrogenase

Reduction of N₂ by nitrogenases occurs at an organometallic iron cofactor that commonly also contains either molybdenum or vanadium. The well-characterized resting state of the cofactor does not bind substrate, so its mode of action remains enigmatic. Carbon monoxide was recently found to replace a bridging sulfide, but the mechanistic relevance was unclear. Here we report the structural analysis of vanadium nitrogenase with a bound intermediate, interpreted as a μ²-bridging, protonated nitrogen that implies the site and mode of substrate binding to the cofactor. Binding results in a flip of amino acid glutamine 176, which hydrogen-bonds the ligand and creates a holding position for the displaced sulfide. The intermediate likely represents state E₆ or E₇ of the Thorneley-Lowe model and provides clues to the remainder of the catalytic cycle.

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.

The Role of ESR Spectroscopy in Advancing Catalytic Science: Some Recent Developments

Recent progress is surveyed in regard to the importance of molecular species containing unpaired electrons in catalytic systems, as revealed using ESR spectroscopy. The review begins with studies of enzymes and their role directly in biological systems, and then discusses investigations of various artificially created catalysts with potential human and environmental significance, including zeolites. Among the specific types of catalytic media considered are those for photocatalysis, water splitting, the degradation of environmental pollutants, hydrocarbon conversions, fuel cells, ionic liquids and sensor devices employing graphene. Studies of muonium-labelled radicals in zeolites are also reviewed, as a means for determining the dynamics of transient radicals in these nanoporous materials.

Recent progress in transition-metal-catalyzed reduction of molecular dinitrogen under ambient reaction conditions

This paper describes our recent progress in catalytic nitrogen fixation by using transition-metal-dinitrogen complexes as catalysts. Two reaction systems for the catalytic transformation of molecular dinitrogen into ammonia and its equivalent such as silylamine under ambient reaction conditions have been achieved by the molybdenum-, iron-, and cobalt-dinitrogen complexes as catalysts. Many new findings presented here may provide new access to the development of economical nitrogen fixation in place of the Haber-Bosch process.

Diiron bridged-thiolate complexes that bind N2 at the Fe(II)Fe(II), Fe(II)Fe(I), and Fe(I)Fe(I) redox states

All known nitrogenase cofactors are rich in both sulfur and iron and are presumed capable of binding and reducing N2. Nonetheless, synthetic examples of transition metal model complexes that bind N2 and also feature sulfur donor ligands remain scarce. We report herein an unusual series of low-valent diiron complexes featuring thiolate and dinitrogen ligands. A new binucleating ligand scaffold is introduced that supports an Fe(μ-SAr)Fe diiron subunit that coordinates dinitrogen (N2-Fe(μ-SAr)Fe-N2) across at least three oxidation states (Fe(II)Fe(II), Fe(II)Fe(I), and Fe(I)Fe(I)). The (N2-Fe(μ-SAr)Fe-N2) system undergoes reduction of the bound N2 to produce NH3 (∼50% yield) and can efficiently catalyze the disproportionation of N2H4 to NH3 and N2. The present scaffold also supports dinitrogen binding concomitant with hydride as a co-ligand. Synthetic model complexes of these types are desirable to ultimately constrain hypotheses regarding Fe-mediated nitrogen fixation in synthetic and biological systems.

Identification of a key catalytic intermediate demonstrates that nitrogenase is activated by the reversible exchange of N₂ for H₂

Freeze-quenching nitrogenase during turnover with N2 traps an S = ½ intermediate that was shown by ENDOR and EPR spectroscopy to contain N2 or a reduction product bound to the active-site molybdenum-iron cofactor (FeMo-co). To identify this intermediate (termed here EG), we turned to a quench-cryoannealing relaxation protocol. The trapped state is allowed to relax to the resting E0 state in frozen medium at a temperature below the melting temperature; relaxation is monitored by periodically cooling the sample to cryogenic temperature for EPR analysis. During -50 °C cryoannealing of EG prepared under turnover conditions in which the concentrations of N2 and H2 ([H2], [N2]) are systematically and independently varied, the rate of decay of EG is accelerated by increasing [H2] and slowed by increasing [N2] in the frozen reaction mixture; correspondingly, the accumulation of EG is greater with low [H2] and/or high [N2]. The influence of these diatomics identifies EG as the key catalytic intermediate formed by reductive elimination of H2 with concomitant N2 binding, a state in which FeMo-co binds the components of diazene (an N-N moiety, perhaps N2 and two [e(-)/H(+)] or diazene itself). This identification combines with an earlier study to demonstrate that nitrogenase is activated for N2 binding and reduction through the thermodynamically and kinetically reversible reductive-elimination/oxidative-addition exchange of N2 and H2, with an implied limiting stoichiometry of eight electrons/protons for the reduction of N2 to two NH3.

Misconception of reductive elimination of H2, in the context of the mechanism of nitrogenase

Calculated atom partial charges reveal misconceptions of reductive elimination of H₂.

Nitrite and hydroxylamine as nitrogenase substrates: mechanistic implications for the pathway of N₂ reduction

Investigations of reduction of nitrite (NO₂^–) to ammonia (NH₃) by nitrogenase indicate a limiting stoichiometry, NO₂^– + 6e^– + 12ATP + 7H⁺ → NH₃ + 2H₂O + 12ADP + 12P_i. Two intermediates freeze-trapped during NO₂^– turnover by nitrogenase variants and investigated by Q-band ENDOR/ESEEM are identical to states, denoted H and I, formed on the pathway of N₂ reduction. The proposed NO₂^– reduction intermediate hydroxylamine (NH₂OH) is a nitrogenase substrate for which the H and I reduction intermediates also can be trapped. Viewing N₂ and NO₂^– reductions in light of their common reduction intermediates and of NO₂^– reduction by multiheme cytochrome c nitrite reductase (ccNIR) leads us to propose that NO₂^– reduction by nitrogenase begins with the generation of NO₂H bound to a state in which the active-site FeMo-co (M) has accumulated two [e^–/H⁺] (E₂), stored as a (bridging) hydride and proton. Proton transfer to NO₂H and H₂O loss leaves M–[NO⁺]; transfer of the E₂ hydride to the [NO⁺] directly to form HNO bound to FeMo-co is one of two alternative means for avoiding formation of a terminal M–[NO] thermodynamic “sink”. The N₂ and NO₂^– reduction pathways converge upon reduction of NH₂NH₂ and NH₂OH bound states to form state H with [−NH₂] bound to M. Final reduction converts H to I, with NH₃ bound to M. The results presented here, combined with the parallels with ccNIR, support a N₂ fixation mechanism in which liberation of the first NH₃ occurs upon delivery of five [e^–/H⁺] to N₂, but a total of seven [e^–/H⁺] to FeMo-co when obligate H₂ evolution is considered, and not earlier in the reduction process.

Catalytic reduction of N2 to NH3 by an Fe-N2 complex featuring a C-atom anchor

While recent spectroscopic studies have established the presence of an interstitial carbon atom at the center of the iron-molybdenum cofactor (FeMoco) of MoFe-nitrogenase, its role is unknown. We have pursued Fe-N2 model chemistry to explore a hypothesis whereby this C-atom (previously denoted as a light X-atom) may provide a flexible trans interaction with an Fe center to expose an Fe-N2 binding site. In this context, we now report on Fe complexes of a new tris(phosphino)alkyl (CP(iPr)3) ligand featuring an axial carbon donor. It is established that the iron center in this scaffold binds dinitrogen trans to the C(alkyl)-atom anchor in three distinct and structurally characterized oxidation states. Fe-C(alkyl) lengthening is observed upon reduction, reflective of significant ionic character in the Fe-C(alkyl) interaction. The anionic (CP(iPr)3)FeN2(-) species can be functionalized by a silyl electrophile to generate (CP(iPr)3)Fe-N2SiR3. (CP(iPr)3)FeN2(-) also functions as a modest catalyst for the reduction of N2 to NH3 when supplied with electrons and protons at -78 °C under 1 atm N2 (4.6 equiv NH3/Fe).

Cleaving the n,n triple bond: the transformation of dinitrogen to ammonia by nitrogenases

Biological nitrogen fixation is a natural process that converts atmospheric nitrogen (N2) to bioavailable ammonia (NH3). This reaction not only plays a key role in supplying bio-accessible nitrogen to all life forms on Earth, but also embodies the powerful chemistry of cleaving the inert N,N triple bond under ambient conditions. The group of enzymes that carry out this reaction are called nitrogenases and typically consist of two redox active protein components, each containing metal cluster(s) that are crucial for catalysis. In the past decade, a number of crystal structures, including several at high resolutions, have been solved. However, the catalytic mechanism of nitrogenase, namely, how the N,N triple bond is cleaved by this enzyme under ambient conditions, has remained elusive. Nevertheless, recent biochemical and spectroscopic studies have led to a better understanding of the potential intermediates of N2 reduction by the molybdenum (Mo)-nitrogenase. In addition, it has been demonstrated that carbon monoxide (CO), which was thought to be an inhibitor of N2 reduction, could also be reduced by the vanadium (V)-nitrogenase to small alkanes and alkenes. This chapter will begin with an introduction to biological nitrogen fixation and Mo-nitrogenase, continue with a discussion of the catalytic mechanism of N2 reduction by Mo-nitrogenase, and conclude with a survey of the current knowledge of N2- and CO-reduction by V-nitrogenase and how V-nitrogenase compares to its Mo-counterpart in these catalytic activities.

Unpaired electrons as probes of catalytic systems

An overview is provided of the importance of molecular species containing unpaired electrons in catalytic systems, as revealed using ESR spectroscopy. The review aims to demonstrate the considerable extent of scientific progress that has been made in this broad topic during the past few decades. Studies of catalytically active surfaces, including zeolites, are surveyed, and the detection of radical species, formed as intermediates in their reactions, using matrix isolation and spin-trapping techniques. Radical cation formation in zeolites is discussed, and the employment of muon spin rotation and relaxation techniques to study the mobility of labelled radicals in various porous and catalytic media. Among the specific types of catalytic media considered are those for photocatalysis, water splitting, degradation of environmental pollutants, hydrocarbon conversions, fuel cells and sensor devices employing graphene. The review concludes with recent developments in the study of enzymes and their reactions, using ESR-based methods.