New Synthetic Routes to Metal-Sulfur Clusters Relevant to the Nitrogenase Metallo-Clusters

Tracing the incorporation of the "ninth sulfur" into the nitrogenase cofactor precursor with selenite and tellurite

The Mo-nitrogenase catalyzes the reduction of N₂ to NH₃ at its cofactor, an [(R-homocitrate)MoFe₇S₉C] cluster synthesized via the formation of a [Fe₈S₉C] L-cluster prior to the insertion of Mo and homocitrate. Previously, we have identified a [Fe₈S₈C] L*-cluster, which is homologous to the core structure of the L-cluster but lacks the ‘9^th sulfur’ in the belt region. However, direct evidence and mechanistic details of the L*- to L-cluster conversion upon ‘9^th sulfur’ insertion remain elusive. Here, we trace the ‘9^th sulfur’ insertion using SeO₃²⁻ and TeO₃²⁻ as ‘labeled’ SO₃²⁻. Biochemical, EPR and XAS/EXAFS studies suggest a role of the ‘9^th sulfur’ in cluster transfer during cofactor biosynthesis while revealing the incorporation of Se²⁻- and Te²⁻-like species into the L-cluster. DFT calculations further point to a plausible mechanism involving in-situ reduction of SO₃²⁻ to S²⁻, thereby suggesting the utility of this reaction to label the catalytically-important belt region for mechanistic investigations of nitrogenase.

Dual Iron Sites in Activation of N2 by Iron-Sulfur Cluster Anions Fe5S2- and Fe5S3. J Phys Chem Lett 2021;12:9269-9274. [PMID: 34533969 DOI: 10.1021/acs.jpclett.1c02683]

Inspired by the fact that the active centers of natural nitrogenases are polynuclear iron-sulfur clusters, the reactivity of isolated iron-sulfur clusters toward N₂ has received considerable attention to gain fundamental insights into the activation of the N≡N triple bond. Herein, a series of gas-phase iron-sulfur cluster anions Fe_xS_y^- (x = 1-8, y = 0-x) were prepared and their reactivities toward N₂ were investigated systematically by mass spectrometry. Among the 44 investigated clusters, only Fe₅S₂^- and Fe₅S₃^- showed superior reactivity toward N₂. Theoretical results revealed that N₂ binds molecularly to the iron sites of Fe₅S_2,3^- in a common end-on coordination mode with an unprecedented back-donation interaction from the localized d-d bonding orbitals of Fe-Fe sites to the π* antibonding orbitals of N₂. This is the first example to disclose the significant contribution of the dual metal sites rather than the single metal atom to N₂ adsorption in the prevalent end-on binding mode.

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.

Reactivity of Neutral Tantalum Sulfide Clusters Ta3Sn (n = 0-4) with N2

Nitrogen (N₂) fixation is a challenging and vital issue in chemistry. Inspired by the fact that the active sites of nitrogenases are polynuclear metal sulfide clusters, the reactivity of gas-phase metal sulfide clusters toward N₂ has received considerable attention to gain fundamental insights into nitrogen fixation. Herein, neutral tantalum sulfide clusters have been prepared and their reactivity toward N₂ has been investigated by mass spectrometry in conjunction with density functional theory (DFT) calculations. The experimental results showed that Ta₃S_n (n = 0-3) could adsorb N₂, while Ta₃S₄ was inert to N_2. The DFT calculations revealed that the complete cleavage of the N≡N bond on the trinuclear metal center in the Ta₃S_0-3/N₂ reaction systems was overall barrierless under thermal collision conditions. The sulfur ligands can facilitate the approaching of N₂ toward the metal center but weaken the electron-donating ability of the metal center. The inertness of Ta₃S₄ is ascribed to the electron-deficient state of Ta₃ in Ta₃S₄ and the least effective orbital interaction in the Ta₃S₄/N₂ couple. This study provides new insights into the ligand effect on the interaction of the metal clusters with N_2.

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.

Spectroscopic Characterization of an Eight-Iron Nitrogenase Cofactor Precursor that Lacks the "9th Sulfur"

Nitrogenases catalyze the reduction of N₂ to NH₄ ⁺ at its cofactor site. Designated the M-cluster, this [MoFe₇ S₉ C(R-homocitrate)] cofactor is synthesized via the transformation of a [Fe₄ S₄ ] cluster pair into an [Fe₈ S₉ C] precursor (designated the L-cluster) prior to insertion of Mo and homocitrate. We report the characterization of an eight-iron cofactor precursor (designated the L*-cluster), which is proposed to have the composition [Fe₈ S₈ C] and lack the "9^th sulfur" in the belt region of the L-cluster. Our X-ray absorption and electron spin echo envelope modulation (ESEEM) analyses strongly suggest that the L*-cluster represents a structural homologue to the l-cluster except for the missing belt sulfur. The absence of a belt sulfur from the L*-cluster may prove beneficial for labeling the catalytically important belt region, which could in turn facilitate investigations into the reaction mechanism of nitrogenases.

Dimensionality Expansion of a Butterfly Shaped Pd4 Framework: Constructing Edge-Sharing Pd6 Tetrahedra

The construction of well-defined transition-metal clusters has attracted substantial attention due to their unique chemical and/or physical properties. Metal clusters with 1D or 2D structures are now accessible by template-synthesis methods, in which multiple metal atoms are arranged with the aid of template molecules and their 1D or 2D structures. However, the rational synthesis of 3D clusters remains challenging, mostly due to a lack of appropriate template molecules. Herein, we report the rational synthesis of a 2D butterfly shaped Pd₄ framework (2) and 3D edge-sharing Pd₆ tetrahedra (5) by treatment of easily available organosilicon compounds with Pd(CNtBu)₂ . The diphenylsilylene moiety thereby serves as the key component to generate the butterfly structure of the Pd₄ clusters in 2. A dimensionality expansion, induced by two Cl atoms, of two butterfly shaped Pd₄ subunits supported by two diphenylsilylene moieties afforded the edge-sharing tetrahedral architecture of the Pd₆ cluster in 5.

Chemical Synthesis of an Asymmetric Mimic of the Nitrogenase Active Site

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

N2 activation on a molybdenum-titanium-sulfur cluster

The FeMo-cofactor of nitrogenase, a metal–sulfur cluster that contains eight transition metals, promotes the conversion of dinitrogen into ammonia when stored in the protein. Although various metal–sulfur clusters have been synthesized over the past decades, their use in the activation of N₂ has remained challenging, and even the FeMo-cofactor extracted from nitrogenase is not able to reduce N₂. Herein, we report the activation of N₂ by a metal–sulfur cluster that contains molybdenum and titanium. An N₂ moiety bridging two [Mo₃S₄Ti] cubes is converted into NH₃ and N₂H₄ upon treatment with Brønsted acids in the presence of a reducing agent.

Nitrogenase—whose cofactor consists of a metal–sulfur cluster—catalyzes the production of NH₃ from N₂, but designing metal–sulfur complexes capable of promoting this conversion remains challenging. Here, the authors report on the activation of N₂ by a metal–sulfur cluster containing [Mo₃S₄Ti] cubes, demonstrating NH₃ and N₂H₄ production.

Synthesis, structural characterization and conversion of dinuclear iron-sulfur clusters containing the disulfide ligand: [Cp*Fe(μ-η2:η2-bdt)(cis-μ-η1:η1-S2)FeCp*], [Cp*Fe(μ-S(C6H4S2))(cis-μ-η1:η1-S2)FeCp*], and [{Cp*Fe(bdt)}2(trans-μ-η1:η1-S2)]

The treatment of [Cp*Fe(μ-η²:η⁴-bdt)FeCp*] (1, Cp* = η⁵-C₅Me₅, bdt = benzene-1,2-dithiolate) with 1/4 equiv. of elemental sulfur (S₈) gave a dinuclear iron-sulfur cluster [Cp*Fe(μ-η²:η²-bdt)(cis-μ-η¹:η¹-S₂)FeCp*] (2), which contains a cis-1,2-disulfide ligand. When complex 2 further interacted with 1/8 equiv. of S₈, another sulfur atom inserted into an Fe-S bond to give a rare product [Cp*Fe(μ-S(C₆H₄S₂))(cis-μ-η¹:η¹-S₂)FeCp*] (3). Unexpectedly, a trans-1,2 disulfide-bridged diiron complex [{Cp*Fe(bdt)}₂(trans-μ-η¹:η¹-S₂)] (4) was isolated from the reaction of complex 1 with 1/2 equiv. of S₈, which represents a structural isomer of [2Fe-2S] ferredoxin-type clusters. In addition, cis-1,2-disulfide-bridged complex 3 can slowly convert into trans-1,2-disulfide-bridged complex 4 and the complex [Cp*Fe(μ-η²:η²-S₂)(cis-μ-η¹:η¹-S₂)FeCp*] (5) by self-assembly reaction at ambient temperature, which is evidenced by time-dependent ¹H NMR spectroscopy.

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.

Systematic Approach for the Construction of Niobium and Tantalum Sulfide Clusters

Treatment of the imido complexes [MCl3(NR)py2] (R = (t)Bu, 2,6-Me2C6H3; M = Nb 1, 3; Ta 2, 4) (Xyl = 2,6-Me2C6H3) with (Me3Si)2S in a 1:1 ratio afforded the new cube-type sulfide clusters [MCl(NR)py(μ3-S)]4 (R = (t)Bu, 2,6-Me2C6H3; M = Nb 5, 7; Ta 6, 8) with loss of Me3SiCl. Reactions of 5 and 6 with cyclopentadienyllithium in 1:4 ratio resulted in the rupture of the coordinative M-S bonds and the replacement of a pyridine molecule and a chlorine atom by an η(5)-cyclopentadienyl group in each metal center, affording the compounds [M(η(5)-C5H5)(N(t)Bu)(μ-S)]4 (M = Nb 9, Ta 10). These processes may develop through formation of the complexes [M4(η(5)-C5H5)2(μ-Cl)(N(t)Bu)4py2(μ3-S)2(μ-S)2](C5H5) (M = Nb 11, Ta 12), also obtained by reaction of 5 and 6 with cyclopentadienyllithium in 1:3 ratio. As further evidence, 11 and 12 led to complexes 9 and 10 by treatment with one more equivalent of the lithium reagent. The structural study of these metal sulfide clusters has been also performed by X-ray crystallography.

Combining a Nitrogenase Scaffold and a Synthetic Compound into an Artificial Enzyme

Nitrogenase catalyzes substrate reduction at its cofactor center ([(Cit)MoFe7S9C](n-); designated M-cluster). Here, we report the formation of an artificial, nitrogenase-mimicking enzyme upon insertion of a synthetic model complex ([Fe6S9(SEt)2](4-); designated Fe6(RHH)) into the catalytic component of nitrogenase (designated NifDK(apo)). Two Fe6(RHH) clusters were inserted into NifDK(apo), rendering the conformation of the resultant protein (designated NifDK(Fe)) similar to the one upon insertion of native M-clusters. NifDK(Fe) can work together with the reductase component of nitrogenase to reduce C2H2 in an ATP-dependent reaction. It can also act as an enzyme on its own in the presence of Eu(II)DTPA, displaying a strong activity in C2H2 reduction while demonstrating an ability to reduce CN(-) to C1-C3 hydrocarbons in an ATP-independent manner. The successful outcome of this work provides the proof of concept and underlying principles for continued search of novel enzymatic activities based on this approach.

Heterometallic complexes with cube-type [MTi3N4] cores containing Group 10 metals in a variety of oxidation states

The metalloligand [{Ti(η⁵-C₅Me₅)(μ-NH)}₃(μ₃-N)] is capable of stabilizing Group 10 metals in the oxidation states 0,iioriv.

Protonation of bridging sulfur in cubanoid Fe4S4 clusters causes large geometric changes: the theory of geometric and electronic structure

Density functional calculations indicate that protonation of a μ₃-S atom in cubanoid clusters [Fe₄S₄X₄]²⁻ leads to a large extension of one Fe–S(H) bond such that the SH ligand is doubly-bridging, μ-SH.

Distributions of iron, phosphorus and sulfur along trichomes of the cyanobacteria Trichodesmium

Changes in the elemental composition within trichomes of the nonheterocystous cyanobacteriaTrichodesmiumare potentially related to N₂-fixation.