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Nitrogen (N 2 ) fixation by nature, which is a crucial process for the supply of bio-available forms of nitrogen, is performed by nitrogenase. This enzyme uses a unique transition-metal-sulfur-carbon cluster as its active-site co-factor ([(R-homocitrate)MoFe 7 S 9 C], FeMoco) 1,2 , and the sulfur-surrounded iron (Fe) atoms have been postulated to capture and reduce N 2 (refs. 3-6 ). Although there are a few examples of synthetic counterparts of the FeMoco, metal-sulfur cluster, which have shown binding of N 2 (refs. 7-9 ), the reduction of N 2 by any synthetic metal-sulfur cluster or by the extracted form of FeMoco 10 has remained elusive, despite nearly 50 years of research. Here we show that the Fe atoms in our synthetic [Mo 3 S 4 Fe] cubes 11,12 can capture a N 2 molecule and catalyse N 2 silylation to form N(SiMe 3 ) 3 under treatment with excess sodium and trimethylsilyl chloride. These results exemplify the catalytic silylation of N 2 by a synthetic metal-sulfur cluster and demonstrate the N 2 -reduction capability of Fe atoms in a sulfur-rich environment, which is reminiscent of the ability of FeMoco to bind and activate N 2 ., (© 2022. The Author(s), under exclusive licence to Springer Nature Limited.)

Transition metal-sulfur (M-S) compounds are an indispensable means for biological systems to convert N 2 into NH 3 (biological N 2 fixation), and these may have emerged by chemical evolution from a prebiotic N 2 fixation system. With a main focus on synthetic species, this article provides a comprehensive review of the chemistry of M-S compounds related to the conversion of N 2 and the structures/functions of the nitrogenase cofactors. Three classes of M-S compounds are highlighted here: multinuclear M-S clusters structurally or functionally relevant to the nitrogenase cofactors, mono- and dinuclear transition metal complexes supported by sulfur-containing ligands in N 2 and N 2 H x ( x = 2, 4) chemistry, and metal sulfide-based solid materials employed in the reduction of N 2 . Fair assessments on these classes of compounds revealed that our understanding is still limited in N 2 reduction and related substrate reductions. Our aims of this review are to compile a collection of studies performed at atomic to mesoscopic scales and to present potential opportunities for elucidating the roles of metal and sulfur atoms in the biological N 2 fixation that might be helpful for the development of functional materials.

The M-cluster is the [(homocitrate)MoFe 7 S 9 C] active site of nitrogenase that is derived from an 8Fe core assembled viacoupling and rearrangement of two [Fe 4 S 4 ] clusters concomitant with the insertion of an interstitial carbon and a 'ninth sulfur'. Combining synthetic [Fe 4 S 4 ] clusters with an assembly protein template, here we show that sulfite can give rise to the ninth sulfur that is incorporated in the catalytically important belt region of the cofactor after the radical S-adenosyl-L-methionine-dependent carbide insertion and the concurrent 8Fe-core rearrangement have already taken place. Based on the differential reactivity of the formed cluster species, we also propose a new [Fe 8 S 8 C] cluster intermediate, the L*-cluster, which is similar to the [Fe 8 S 9 C] L-cluster, but lacks the ninth sulfur from sulfite. This work provides a semi-synthetic tool for protein reconstitution that could be widely applicable for the functional analysis of other FeS systems.

Structural conversion of [Fe4S4] clusters into [Fe2S2] clusters has been suggested to be a fundamental process for some O2-sensing proteins. While the formation of [Fe2S2] clusters from synthetic [Fe4S4] clusters has been unprecedented, an all-ferric [Fe4S4](4+) cluster Fe4S4{N(SiMe3)2}4 (1) was found to split in the presence of pyridines, giving [Fe2S2] clusters Fe2S2{N(SiMe3)2}2(L)2 (2, L = pyridines). The structural conversion between [Fe4S4] and [Fe2S2] clusters appeared to be reversible, and the thermodynamic parameters for the equilibrium reactions between 1 + L and 2 were determined. Assembly of two [Fe2S2] clusters was also induced by chemical reductions of Fe2S2{N(SiMe3)2}2(Py)2 (Py = pyridine), and the resultant [Fe4S4] clusters [1](-) and [1](2-) were found to split into two [Fe2S2] clusters by oxidation with [Cp2Fe](+) in the presence of pyridine.

Vanadium(III) thiolate complexes carrying a phenoxide-based tridentate ligand were prepared from the reactions of V(NMe2)4 with the protonated forms of tridentate ligands (H2(O,P,O) = bis(3,5-di-tert-butyl-2-hydroxyphenyl)phenylphosphine or H2(O,O,O) = bis(3,5-di-tert-butyl-2-hydroxyphenyl)phenylphosphineoxide) and thiols (HSR; R = mesityl (Mes), 2,4,6-(i)Pr3C6H2 (Tip)). The vanadium-thiolate complexes were subjected to the V/Fe/S cluster synthesis via treatment with an Fe(II) thiolate complex [(TipS)Fe]2(μ-SDmp)2 (4, Dmp = 2,6-(mesityl)2C6H3) and elemental sulfur in toluene, leading to the formation of two new V/Fe/S clusters. One is an edge-bridged double-cubane-type [VFe3S4]-[VFe3S4] cluster [(O,P,O)VFe3S4(SDmp)(HNMe2)]2 (5) having face-capping tridentate (O,P,O) ligands on vanadium atoms. The other is a [VFe3S4-Fe] cluster [(μ-O,O,O)VFe3S4(SDmp)(STip)Fe(μ-SDmp)] (6), the core of which consists of a cubane-type [VFe3S4] unit and an external iron atom. The external iron is bound to an SDmp ligand and two oxygen atoms of the tridentate (O,O,O) ligand. Cluster 6 is structurally relevant to the active site of nickel-dependent CO dehydrogenase, and their common structural features include a cubane-type unit with a heterometal, one more iron atom besides the cubane unit, and a bridging ligand between the external iron and the heterometal of the cubane unit.

An amide-bound [Fe4S4](3+) cluster, [Fe4S4{N(SiMe3)2}4](-) (1), was found to serve as a convenient precursor for synthetic analogues of the oxidized form of high-potential iron-sulfur proteins. Treatment of 1 with 4 equiv of bulky thiols led to replacement of the amide ligands with thiolates, giving rise to a series of [Fe4S4(SR)4](-) clusters (R = Dmp (2a), Tbt (2b), Eind (2c), Dxp (2d), Dpp (2e); Dmp = 2,6-di(mesityl)phenyl, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl, Dxp = 2,6-di(m-xylyl)phenyl, Dpp = 2,6-diphenylphenyl). These clusters were characterized by the mass spectrum, the EPR spectrum, and X-ray crystallography. The redox potentials of the [Fe4S4](3+/2+) couple, -0.82 V (2a), -0.86 V (2b), -0.84 V (2c), -0.74 V (2d), and -0.63 V (2e) vs Ag/Ag(+) in THF, are significantly more negative than that of [Fe4S4(SPh)4](-/2-) (-0.21 V).

A coordinatively unsaturated dinuclear iron(II) complex of bulky thiolates, [(TipS)Fe(micro-SDmp)]2 (1; Tip = 2,4,6-(i)Pr(3)C(6)H(2), Dmp = 2,6-(mesityl)(2)C(6)H(3)), was synthesized from stepwise reactions of Fe{N(SiMe(3))2}2 with 1 equiv of HSDmp and then with 1 equiv of HSTip. Complex 1 was found to react with elemental sulfur (S8) in toluene to generate a new class of [8Fe-7S] cluster, [(DmpS)Fe(4)S(3)]2(micro-SDmp)2(micro-STip)(micro(6)-S) (2). The cluster 2 was also produced from one-pot reactions of Fe{N(SiMe(3))2}2 + HSDmp + HSTip + S8 (8:6:10:7/8) and Fe3{N(SiMe(3))2}2(micro-STip)4 + HSDmp + S8 (8/3:16/3:7/8), where another [8Fe-7S] cluster, [(TipS)Fe(4)S(3)]2(micro-SDmp)2{micro-N(SiMe(3))2}(micro(6)-S) (3), was also found as a minor byproduct. In either of the clusters, two Fe(4)S(3) incomplete cubane units are connected by three anionic ligands, namely three thiolate S atoms for 2 or two thiolate S atoms and one amide N atom for 3, and one hexa-coordinate S atom resides at the center of the [8Fe-7S] core. They have a common Fe(II)(5)Fe(III)3 oxidation states, and an S = 1/2 ground spin state was indicated by rhombic EPR signals at 10 K with g = 2.19, 2.07, and 1.96 for 2 and g = 2.13, 2.06, and 1.93 for 3. The structural relevance of clusters 2 and 3 to P-cluster, FeMo-co, and FeFe-co of nitrogenases is discussed.

The [8Fe-7S] core of the P-clusters in nitrogenases is unique among the known [Fe-S] clusters which are essential to electron-transfer processes in nature. The [8Fe-7S] cluster has been thought unstable and to exist only in protein environments. We found that this unusual [8Fe-7S] structure can be self-assembled from the reaction of Fe(II) bis-amide, tetramethylthiourea, 2,4,6-triisopropylbenzenethiol, and elemental sulfur in a specific mole ratio. The structure of the complex isolated therefrom closely resembles that of the reduced form (PN) of the P-clusters, while the 6Fe(II)2Fe(III) oxidation state was manifested by the Mössbauer study.

A synthetic protocol was developed for a series of cubane‐type [Mo3S4M] clusters that incorporate halides of first‐row transition metals (M) from Groups 4–10. This protocol is based on the anionic cluster platform [Cp*3Mo3S4]− ([1]−; Cp*=η5‐C5Me5), 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 [Mo3S4M] 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 [Mo3S4Fe] cluster 6 a were investigated in detail by using 57Fe Mössbauer spectroscopy and computational methods. Cubic clusters: A synthetic protocol was developed for a series of cubane‐type [Mo3S4M] clusters, based on the anionic cluster platform [Cp*3Mo3S4]−, which incorporate halides of first‐row transition metals (M) from Group 4–10. The M/halide ratio gradually increases from 1:2 to 1:1.5 and to 1:1 upon moving from early to late transition metals. [ABSTRACT FROM AUTHOR]

Iron-sulfur clusters are inorganic centers that are widely distributed in nature, mediating electron-transfer processes and transformation reactions. Their properties and functions arise from the different amino acid residues on the iron centers, some weak interactions within the protein matrices, and the extent of exposure to the medium. Studies on biomimetic model clusters have elucidated the molecular basis of the effects of iron-bound ligands and solvents on the synthesis and properties of iron-sulfur clusters. This review summarizes the effects of ligands and media on the properties and structure of both biological and biomimetic iron-sulfur clusters. [ABSTRACT FROM AUTHOR]

The nitrogenase metallo-clusters, namely P-cluster and FeMo-cofactor, have been attracting considerable interests because they are indispensable for biological reduction of N2 to ammonia. These clusters contain eight transition metals and several sulfur atoms in the core, and represent the most complex class of the active sites of metallo-enzymes. The complexity of these metallo-clusters should be associated with their roles in the nitrogenase reaction, whereas the relationship between their structures and functions still remains unclear. Mutagenesis studies and spectroscopic techniques are useful to provide insights into properties of the P-cluster and the FeMo-cofactor. A complementary approach to gain better understanding of these clusters would be the chemistry of their molecular models based on inorganic synthesis. This review summarizes the recent progresses in the synthesis and properties of metal-sulfur clusters relevant to the P-cluster or the FeMo-cofactor of nitrogenase. Although synthesis of reliable models of FeMo-cofactor is still a matter of substantial challenges, the synthetic analogues of P-cluster are approaching the stage that enables the comparison of the structures and properties with the native cluster. [ABSTRACT FROM AUTHOR]