Catalytic and stoichiometric multielectron reduction of hydrazine to ammonia and acetylene to ethylene with clusters that contain the MFe3S4 cores (MMo, V). Relevance to the function of 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.

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.

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.

Catalytic hydrazine disproportionation mediated by a thiolate-bridged VFe complex

A heterobimetallic VFe complex is demonstrated to catalyse hydrazine disproportionation with yields of up to 1073 equivalents of NH₃ per catalyst, comparable to the highest turnover known for any molecular catalyst.

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.

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.

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.

On the accuracy of density functional theory for iron—sulfur clusters

A simple, yet powerful wave function manipulation method was introduced utilizing a generalized ionic fragment approach that allows for systematic mapping of the wave function space for multispin systems with antiferromagnetic coupling. The use of this method was demonstrated for developing ground state electronic wave function for [2Fe-2S] and [Mo-3Fe-4S] clusters. Using well-defined ionic wave functions for ferrous and ferric irons, sulfide, and thiolate fragments, the accuracy of various density functionals and basis sets including effective core potentials were evaluated on a [4Fe-4S] cluster by comparing the calculated geometric and electronic structures with crystallographic data and experimental atomic spin densities from X-ray absorption spectroscopy, respectively. We found that the most reasonable agreement for both geometry and atomic spin densities is obtained by a hybrid functional with 5% HF exchange and 95% density functional exchange supplemented with Perdew's 1986 correlation functional. The basis set seems to saturate only at the triple-zeta level with polarization and diffuse functions. Reasonably preoptimized structures can be obtained by employing computationally less expensive effective core potentials, such as the Stuttgart-Dresden potential with a triple-zeta valence basis set. The extension of the described calibration methodology to other biologically important and more complex iron-sulfur clusters, such as hydrogenase H-cluster and nitrogenase FeMo-co will follow.

Cleavage of Hydrazine N−N Bonds by RuMo3S4 Cubane-Type Clusters

The mixed-metal cubane-type clusters [(Cp*Mo)3(mu3-S)4RuH2(PR3)][PF(6)] [Cp* = eta5-C5Me5; R = Ph (2), Cy (5)] were effective for the N-N bond cleavage of hydrazine and phenylhydrazine via a disproportionation reaction. The ammonia cluster [(C*Mo)3(mu3-S)4Ru(NH3)(PPh3)][PF6] (3) and/or the unprecedented double-cubane-type cluster with bridging nitrogenous ligands [{(Cp*Mo)3(mu3-S)4Ru}2(mu2-NH2)(mu2-NHNH2)][PF6]2 (4) were isolated from the reaction mixtures, and their structures were determined by X-ray diffraction studies.

Cubane-type heterometallic sulfido clusters: incorporation of two metal fragments into a dinuclear ReS(mu-S)2ReS core affording bimetallic M2Re2(mu 3-S)4 clusters (M = Ru, Pt, Cu) or trimetallic MM'Re2(mu 3-S)4 clusters via incomplete cubane-type MRe2(mu 3-S)(mu 2-S)3 intermediates (M = Ru, Rh, Ir; M' = Mo, W, Pd, Ru, Rh)

Reactions of a dirhenium tetra(sulfido) complex [PPh(4)](2)[ReS(L)(mu-S)(2)ReS(L)] (L = S(2)C(2)(SiMe(3))(2)) with a series of group 8-11 metal complexes in MeCN at room temperature afforded either the cubane-type clusters [M(2)(ReL)(2)(mu(3)-S)(4)] (M = CpRu (2), PtMe(3), Cu(PPh(3)) (4); Cp = eta(5)-C(5)Me(5)) or the incomplete cubane-type clusters [M(ReL)(2)(mu(3)-S)(mu(2)-S)(3)] (M = (eta(6)-C(6)HMe(5))Ru (5), CpRh (6), CpIr (7)), depending on the nature of the metal complexes added. It has also been disclosed that the latter incomplete cubane-type clusters can serve as the good precursors to the trimetallic cubane-type clusters still poorly precedented. Thus, treatment of 5-7 with a range of metal complexes in THF at room temperature resulted in the formation of novel trimetallic cubane-type clusters, including the neutral clusters [[(eta(6)-C(6)HMe(5))Ru][W(CO)(3)](ReL)(2)(mu(3)-S)(4)], [(CpM)[W(CO)(3)](ReL)(2)(mu(3)-S)(4)] (M = Rh, Ir), [(Cp*Ir)[Mo(CO)(3)](ReL)(2)(mu(3)-S)(4)], [[(eta(6)-C(6)HMe(5))Ru][Pd(PPh(3))](ReL)(2)(mu(3)-S)(4)], and [(Cp*Ir)[Pd(PPh(3))](ReL)(2)(mu(3)-S)(4)] (13) along with the cationic clusters [(Cp*Ir)(CpRu)(ReL)(2)(mu(3)-S)(4)][PF(6)] (14) and [(Cp*Ir)[Rh(cod)](ReL)(2)(mu(3)-S)(4)][PF(6)] (cod = 1,5-cyclooctadiene). The X-ray analyses have been carried out for 2, 4, 7, 13, and the SbF(6) analogue of 14 (14') to confirm their bimetallic cubane-type, bimetallic incomplete cubane-type, or trimetallic cubane-type structures. Fluxional behavior of the incomplete cubane-type and trimetallic cubane-type clusters in solutions has been demonstrated by the variable-temperature (1)H NMR studies, which is ascribable to both the metal-metal bond migration in the cluster cores and the pseudorotation of the dithiolene ligand bonded to the square pyramidal Re centers, where the temperatures at which these processes proceed have been found to depend upon the nature of the metal centers included in the cluster cores.

A new dinuclear vanadium(V)-citrate complex from aqueous solutions. Synthetic, structural, spectroscopic, and pH-dependent studies in relevance to aqueous vanadium(V)-citrate speciation

Vanadium interactions with low molecular mass binders in biological fluids entail the existence of vanadium species with variable chemical and biological properties. In the course of efforts to elucidate the chemistry related to such interactions, we have explored the oxidative chemistry of vanadium(III) with the physiologically relevant tricarboxylic citric acid. Aqueous reactions involving VCl(3) and anhydrous citric acid, at pH approximately 7, resulted in blue solutions. Investigation into the nature of the species arising in those solutions revealed, through UV/visible and EPR spectroscopies, oxidation of vanadium(III) to vanadium(IV). Further addition of H(2)O(2) resulted in the oxidation of vanadium(IV) to vanadium(V), and the isolation of a new vanadium(V)-citrate complex in the form of its potassium salt. Analogous reactions with K(4)[V(2)O(2)(C(6)H(4)O(7))(2)].6H(2)O and H(2)O(2) or V(2)O(5) and citrate at pH approximately 5.5 afforded the same material. Elemental analysis pointed to the molecular formulation K(4)[V(2)O(4)(C(6)H(5)O(7))(2)].5.6H(2)O (1). Complex 1 was further characterized by FT-IR and X-ray crystallography. 1 crystallizes in the triclinic space group P(-)1, with a = 11.093(4) A, b = 9.186(3) A, c = 15.503(5) A, alpha = 78.60(1) degrees, beta = 86.16(1) degrees, gamma = 69.87(1) degrees, V = 1454.0(8) A(3), and Z = 2. The X-ray structure of 1 reveals the presence of a dinuclear vanadium(V)-citrate complex containing a V(V)(2)O(2) core. The citrate ligands are triply deprotonated, and as such they bind to vanadium(V) ions, thus generating a distorted trigonal bipyramidal geometry. Binding occurs through the central alkoxide and carboxylate groups, with the remaining two terminal carboxylates being uncoordinated. One of those carboxylates is protonated and contributes to hydrogen bond formation with the deprotonated terminal carboxylate of an adjacent molecule. Therefore, an extended network of hydrogen-bonded V(V)(2)O(2)-core-containing dimers is created in the lattice of 1. pH-dependent transformations of 1 in aqueous media suggest its involvement in a web of vanadium(V)-citrate dinuclear species, consistent with past solution speciation studies investigating biologically relevant forms of vanadium.

Metabolic pathway of anaerobic ammonium oxidation on the basis of 15N studies in a fluidized bed reactor

Summary: A novel metabolic pathway for anaerobic ammonium oxidation with nitrite as the electron acceptor has been elucidated using 15N-Iabelled nitrogen compounds. These experiments showed that ammonium was biologically oxidized with hydroxylamine as the most probable electron acceptor. The hydroxylamine itself is most likely derived from nitrite. Batch experiments in which ammonium was oxidized with hydroxylamine transiently accumulated hydrazine. The conversion of hydrazine to dinitrogen gas is postulated as the reaction generating electron equivalents for the reduction of nitrite to hydroxylamine. During the conversion of ammonium, a small amount of nitrate was formed from some of the nitrite. The addition of NH2OH to an operating fluidized bed system caused a stoichiometric increase in the ammonium conversion rate (1 mmol I−1 h−1) and a decrease in the nitrate production rate (0.5 mmol I−1 h−1). Addition of hydrazine also caused a decrease in nitrate production. On the basis of these findings, it is postulated that the oxidation of nitrite to nitrate could provide the anaerobic ammonium-oxidizing bacteria with the reducing equivalents necessary for CO2 fixation.