Catalytic Reduction of Hydrazine to Ammonia with MoFe(3)S(4)-Polycarboxylate Clusters. Possible Relevance Regarding the Function of the Molybdenum-Coordinated Homocitrate in Nitrogenase

Computational Study on the Influence of Mo/V Centers on the Electronic Structure and Hydrazine Reduction Capability of [MFe3S4]3+/2+ Complexes

[MFe3S4] cubanes have for some time been of interest for their ability to mimic the electronic and geometric structure of the active site of nitrogenase, the enzyme responsible for fixing N2 to NH3. Nitrogenase naturally occurs in three forms, with the major difference being that the metal ion present in the cofactor active site is either molybdenum (FeMoco), vanadium (FeVco), or iron. The molybdenum and vanadium versions of these cofactors are more closely studied, owing to their larger abundance and rate of catalysis. In this study, we compare free energy profiles and electronic properties of the Mo/V cubanes at various stages during the reduction of N2H4 to NH3. Our findings highlight the differences in how the complexes facilitate the reaction, in particular, vanadium's comparatively weaker ability to interact with the Fe/S network and stabilize reducing electrons prior to N-N bond cleavage, which may have implications when considering the lower efficiency of the vanadium-dependent nitrogenase.

A thiolate-bridged ruthenium-molybdenum complex featuring terminal nitrido and bridging amido ligands derived from N−H and N−N bond cleavage of hydrazine

Biomimetic di- or multimetallic complexes featuring NxHy species in a sulfur-rich coordination sphere have attracted considerable attention in modelling the possible scenarios of biological nitrogen fixation by nitrogenases. Although the...

Intrinsic Electron Localization of Metastable MoS2 Boosts Electrocatalytic Nitrogen Reduction to Ammonia

The advancement of efficient electrocatalysts toward the nitrogen reduction reaction (NRR) is critical in sustainable ammonia synthesis under ambient pressure and temperature. Manipulating the electronic configuration of electrocatalysts is particularly vital to form metal-nitrogen (MN) bonds during the NRR through regulating the active electronic states of sites. Here, in sharp contrast to stable 2H MoS₂ without metal chains, MoMo bonding in metastable polymorphs of MoS₂ bulk (zigzag chain in the 1T' phase and diamond chain in the 1T″' phase) is discovered to significantly increase intrinsic electron localization around the metal chains. This can enhance the charge transfer from the adsorbed nitrogen molecule to the metal chains, allowing for boosted NRR kinetics. The electrochemical experiments show that the NH₃ yield rate and the faradaic efficiency of the metastable 1T″' MoS₂ rich with abundant Mo-Mo bonds are about 9 and 12 times above average than those of 2H MoS₂ , correspondingly. Theoretical simulations reveal the high local electron density surrounding the MoMo chains and sites can promote π back-donation, which is beneficial for increasing nitrogen adsorption, strengthening the MN bonds, and reducing the cleavage barrier of the triple NN bond.

Zeolite-Stabilized Di- and Tetranuclear Molybdenum Sulfide Clusters Form Stable Catalytic Hydrogenation Sites

Supercages of faujasite (FAU)‐type zeolites serve as a robust scaffold for stabilizing dinuclear (Mo₂S₄) and tetranuclear (Mo₄S₄) molybdenum sulfide clusters. The FAU‐encaged Mo₄S₄ clusters have a distorted cubane structure similar to the FeMo‐cofactor in nitrogenase. Both clusters possess unpaired electrons on Mo atoms. Additionally, they show identical catalytic activity per sulfide cluster. Their catalytic activity is stable (>150 h) for ethene hydrogenation, while layered MoS₂ structures deactivate significantly under the same reaction conditions.

Characterization of a Mo-Nitrogenase Variant Containing a Citrate-Substituted Cofactor

Nitrogenase converts N₂ to NH₃ , and CO to hydrocarbons, at its cofactor site. Herein, we report a biochemical and spectroscopic characterization of a Mo-nitrogenase variant expressed in an Azotobacter vinelandii strain containing a deletion of nifV, the gene encoding the homocitrate synthase. Designated NifDK^Cit , the catalytic component of this Mo-nitrogenase variant contains a citrate-substituted cofactor analogue. Activity analysis of NifDK^Cit reveals a shift of CO reduction from H₂ evolution toward hydrocarbon formation and an opposite shift of N₂ reduction from NH₃ formation toward H₂ evolution. Consistent with a shift in the Mo K-edge energy of NifDK^Cit relative to that of its wild-type counterpart, EPR analysis demonstrates a broadening of the line-shape and a decrease in the intensity of the cofactor-originated S=3/2 signal, suggesting a change in the spin properties of the cofactor upon citrate substitution. These observations point to a crucial role of homocitrate in substrate reduction by nitrogenase and the possibility to tune product profiles of nitrogenase reactions via organic ligand substitution.

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.

Progress in Synthesizing Analogues of Nitrogenase Metalloclusters for Catalytic Reduction of Nitrogen to Ammonia

Ammonia (NH3) has played an essential role in meeting the increasing demand for food and the worldwide need for nitrogen (N2) fertilizer since 1913. Unfortunately, the traditional Haber–Bosch process for producing NH3 from N2 is a high energy-consumption process with approximately 1.9 metric tons of fossil CO2 being released per metric ton of NH3 produced. As a very challenging target, any ideal NH3 production process reducing fossil energy consumption and environmental pollution would be welcomed. Catalytic NH3 synthesis is an attractive and promising alternative approach. Therefore, developing efficient catalysts for synthesizing NH3 from N2 under ambient conditions would create a significant opportunity to directly provide nitrogenous fertilizers in agricultural fields as needed in a distributed manner. In this paper, the literature on alternative, available, and sustainable NH3 production processes in terms of the scientific aspects of the spatial structures of nitrogenase metalloclusters, the mechanism of reducing N2 to NH3 catalyzed by nitrogenase, the synthetic analogues of nitrogenase metalloclusters, and the opportunities for continued research are reviewed.

Pentamethylcyclopentadienyl half-sandwich hydrazine complexes of ruthenium: preparation and reactivity

The preparation and selective oxidation of hydrazine complexes of ruthenium stabilised by a pentamethylcyclopentadienyl fragment are described. The formation of the sandwich complex [Ru(η⁵-C₅Me₅)(η⁶-C₆H₆)]BPh₄ through the oxidation of coordinate phenylhydrazine is also reported.

Cubane-Type [Mo3 S4 M] Clusters with First-Row Groups 4-10 Transition-Metal Halides Supported by C5 Me5 Ligands on Molybdenum

A synthetic protocol was developed for a series of cubane-type [Mo₃ S₄ M] clusters that incorporate halides of first-row transition metals (M) from Groups 4-10. This protocol is based on the anionic cluster platform [Cp*₃ Mo₃ S₄ ]^- ([1]^- ; Cp*=η⁵ -C₅ Me₅ ), which crystallizes when K(18-crown-6) is used as the counter cation. Treatment of in situ-generated [1]^- with such transition-metal halides led to the formation of [Mo₃ S₄ M] clusters, in which the M/halide ratio gradually changes from 1:2 to 1:1.5 and to 1:1, when moving from early to late transition metals. This trend suggests a tendency for early transition metals to tolerate higher oxidation states and adopt larger ionic radii relative to late transition metals. The properties of the [Mo₃ S₄ Fe] cluster 6 a were investigated in detail by using ⁵⁷ Fe Mössbauer spectroscopy and computational methods.

Binding small molecules and ions to [Fe4S4Cl4](2-) modulates rate of protonation of the cluster

The mechanism of the acid-catalyzed substitution reaction of the terminal chloro-ligands in [Fe4S4Cl4](2-) by PhS(-) in the presence of NHBu(n)3(+) involves rate-limiting proton transfer from NHBu(n)3(+) to the cluster (k0 = 490 ± 20 dm(3) mol(-1) s(-1)). A variety of small molecules and ions (L = substrate = Cl(-), Br(-), I(-), RNHNH2 (R = Me or Ph), Me2NNH2, HCN, NCS(-), N3(-), Bu(t)NC or pyridine) bind to [Fe4S4Cl4](2-) and this affects the rate of subsequent protonation of [Fe4S4Cl4(L)](n-). Where the kinetics allow, the equilibrium constants for the substrates binding to [Fe4S4Cl4](2-) (K(L)) and the rates of proton transfer from NHBu(n)3(+) to [Fe4S4Cl4(L)](n-) (k) have been determined. The results indicate the following general features. (i) Bound substrates increase the rate of protonation of the cluster, but the rate increase is modest (k/k0 = 1.6 to ≥72). (ii) When K(L) is small, so is k/k0. (iii) Binding substrates which are good σ-donors or good π-acceptors lead to the largest k/k0. This behaviour is discussed in terms of the recent proposal that protonation of [Fe4S4Cl4](2-) at a μ3-S, is coupled to concomitant Fe-(μ3-SH) bond elongation/cleavage.

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.

Iron-sulfur Clusters

Iron-sulfur clusters are universally distributed groups occurring in iron-sulfur proteins. They have a wide range of cellular functions which reflect the chemistry of the clusters. Some clusters are involved in electron transport and energy transduction in photosynthesis and respiration. Others can bind substrates and participate in enzyme catalysis. Regulatory functions have also been documented for clusters that respond to oxygen partial pressure and iron availability. Finally, there are some for which no function has been defined; they may act as stabilizing structures, for example, in enzymes involved in nucleic acid metabolism. The clusters are constructed intracellularly and inserted into proteins, which can then be transported to intracellular targets, in some cases, across membranes. Three different types of iron-sulfur cluster assembly machinery have evolved in prokaryotes: NIF, ISC and SUF. Each system involves a scaffold protein on which the cluster is constructed (encoded by genes nifU, iscU, sufU or sufB) and a cysteine desulfurase (encoded by nifS, iscS or sufS) which provides the sulfide sulfur. In eukaryotic cells, clusters are formed in the mitochondria for the many iron-sulfur proteins in this organelle. The mitochondrial biosynthesis pathway is linked to the cytoplasmic iron-sulfur assembly system (CIA) for the maturation of cytoplasmic and nuclear iron-sulfur proteins. In plant cells, a SUF-type system is used for cluster assembly in the plastids. Many accessory proteins are involved in cluster transfer before insertion into the appropriate sites in Fe-S proteins.

Mixed-valence dinitrogen-bridged Fe(0)/Fe(II) complex

The reactions of a dinitrogen-bridged Fe(II)/Fe(II) complex [(FeH(PP(3)))(2)(μ-N(2))](2+) (3) (PP(3) = P(CH(2)CH(2)PMe(2))(3)) with base were investigated using (15)N labeling techniques to enhance characterization. In the presence of base, 3 is initially deprotonated to the Fe(II)/Fe(0) dinitrogen-bridged complex [(FeH(PP(3)))(μ-N(2))(Fe(PP(3)))](+) (4) and then to the symmetrical Fe(0)/Fe(0) dinitrogen-bridged complex (Fe(PP(3)))(2)(μ-N(2)) (5). [(FeH(PP(3)))(μ-N(2))(Fe(PP(3)))](+) (4) exhibits unusual long-range (31)P-(31)P NMR coupling through the bridging dinitrogen ligand from the phosphines at the Fe(0) center and those at the Fe(II) center. Reaction of 4 with base under an atmosphere of argon resulted in the known dinitrogen Fe(0) complex Fe(N(2))(PP(3)) (6) and a solvent C-H activation product. Complexes 3, 4, and 5 were fully characterized by multinuclear NMR spectroscopy, and complexes 3 and 4 by X-ray crystallography.

Nitrogenase structure and function relationships by density functional theory

Modern density functional theory has tremendous potential with matching popularity in metalloenzymology to reveal the unseen atomic and molecular details of structural data, spectroscopic measurements, and biochemical experiments by providing insights into unobservable structures and states, while also offering theoretical justifications for observed trends and differences. An often untapped potential of this theoretical approach is to bring together diverse experimental structural and reactivity information and allow for these to be critically evaluated at the same level. This is particularly applicable for the tantalizingly complex problem of the structure and molecular mechanism of biological nitrogen fixation. In this chapter we provide a review with extensive practical details of the compilation and evaluation of experimental data for an unbiased and systematic density functional theory analysis that can lead to remarkable new insights about the structure-function relationships of the iron-sulfur clusters of nitrogenase.

C-H bond activation of decamethylcobaltocene mediated by a nitrogenase Fe(8)S(7) P-cluster model

A C-H bond of Cp*(2)Co was found to be cleaved by a [Fe(8)S(7)] cluster model of the nitrogenase P-cluster. This is the first example of C-H bond activation mediated by a biologically relevant Fe/S cluster. The reaction mechanism probably consists of electron transfer from Cp*(2)Co to the [Fe(8)S(7)] cluster and subsequent proton abstraction by the reduced form of the cluster.

Involvement of thiolate ligands in binding substrates to Fe-S clusters

Kinetic studies on reactions between [Fe4S4(SR)4]2- (R = Et or But) and 4-YC6H4COCl (Y = MeO, H or Cl) to form [Fe4S4Cl4]2- and 4-YC6H4COSR indicate that the terminal thiolate ligand is involved in the initial binding of the acid chloride to the cluster.

Mechanistic insight into N=N cleavage by a low-coordinate iron(II) hydride complex

The reaction pathways of high-spin iron hydride complexes are relevant to the mechanism of N2 reduction by nitrogenase, which has been postulated to involve paramagnetic iron-hydride species. However, almost all known iron hydrides are low-spin, diamagnetic Fe(II) compounds. We have demonstrated that the first high-spin iron hydride complex, LtBuFeH (LtBu = bulky beta-diketiminate), reacts with PhN=NPh to completely cleave the N-N double bond, giving LtBuFeNHPh. Here, we disclose a series of experiments that elucidate the mechanism of this reaction. Crossover and kinetic experiments rule out common nonradical mechanisms, and support a radical chain mechanism mediated by iron(I) species including a rare eta2-azobenzene complex. Therefore, this high-spin iron(II) hydride can break N-N bonds through both nonradical and radical insertion mechanisms, a special feature that enables novel reactivity.