Synthesis and crystal structure of the cluster (Et4N)[(Tp*)MoFe3S3(μ3-NSiMe3)(N3)3]

This type of heterometallic and heteroleptic single cubane cluster represents a typical example within the Mo–Fe–S cluster family, which may be a good reference for understanding the structure and function of the nitrogenase FeMo cofactor.


Chemical context
Nitrogen is abundant in the atmosphere in the form of dinitrogen gas, but this type of nitrogen cannot be metabolized by organisms directly (Jia & Quadrelli, 2014;MacKay & Fryzuk, 2004).It must be fixed by nitrogenase in some selected microorganisms (Dos Santos et al., 2012).Nitrogenase can transform N 2 to NH 3 , and then the biochemical N cycle sets off (Cheng, 2008;Canfield et al., 2010).The exploration of synthetic structural analogs of nitrogenase is therefore a crucial area in modern science research.
The FeMo cofactor is believed to be one of the most important parts in nitrogenase responsible for nitrogen fixation.The FeMo cofactor contains a 2p atom in the center, which has been proven to be a carbide, resulting in the structure as [MoFe 7 S 9 C] (Spatzal et al., 2011;Lancaster et al., 2011).To mimic the structure of the FeMo cofactor, a large number of iron-sulfur clusters have been synthesized (Lee & Holm, 2004;Holm, 1977;Herskovitz et al., 1972;Liu et al., 1990;Nordlander et al., 1993).However, synthesizing heteroleptic analogs with a 2p atom in the core of the cluster is a tough challenge for researchers in this area (Sickerman et al., 2017).With the unremitting efforts of scientists, some synthetic homometallic or heterometallic iron-sulfur clusters with a 2p atom in the core have been synthesized.Lee's group have used the dinuclear precursors for the selective synthesis of the homometallic cubane clusters [Fe 4 (N t Bu) n (S) 4-n Cl 4 ] z with (n, z = 3, 1À , 2, 2À or 1, 2À ; Chen et al., 2010).Our group have developed core ligand metathesis and core ligand redox metathesis strategies and successfully synthesized versatile heterometallic iron-sulfur clusters containing a core 2p atom, including the [MFe 3 S 2 (� 2 -Q)] 1+ and [MFe 3 S 3 (� 3 -Q)] 2+ (M = W and Mo, Q = NR, OR) cubane clusters (Xu et al., 2018;He et al., 2022), and the [(Tp*) 2 W 2 Fe 6 (� 4 -N) 2 S 6 L 4 ] 2À [Tp* = tris-(3,5-dimethylpyrazol-1-yl)-hydroborate(1À ), L = Cl À or Br À ] double cubane clusters (Xu et al., 2019).Previously in our laboratory, the molybdenum-iron-sulfur cluster [(Tp*)MoFe 3 S 3 (� 3 -NSiMe 3 )Cl 3 ] À , which resembles one of the cubic subunits of the FeMo cofactor, was synthesized through a LEGO-like strategy.Based on this cluster, which has a � 3bridging N atom in the core, we explored the effects of terminal ligands on the Fe sites of heterometallic heteroleptic iron-sulfur clusters.In this work, terminal ligand substitution using NaN 3 was applied to produce the cluster [(Tp*) MoFe 3 S 3 (� 3 -NSiMe 3 )(N 3 ) 3 ] À .The synthesis and structural analysis of this compound may provide useful information for a better understanding of the structure and reactivity of the FeMo cofactor, as well as how the terminal ligand affects the physical property of the cluster (Xu et al., 2018;He et al., 2022).

Structural commentary
This title cluster crystallized as the Et 4 N + salt in the triclinic crystal system, space group P1.The different metal atoms exhibit distinct coordination models in this cluster.The Mo site coordinates three N atoms of the Tp* ligand and three � 3bridging S atoms in the core of the cluster, showing a distorted octahedral coordination sphere.Each Fe site coordinates two � 3 -bridging S atoms, one � 3 -bridging N atom from Me 3 SiN 2À , and one N atom on the terminal ligand, resulting in a distorted tetrahedral geometry.The cluster exhibits quasi-threefold symmetry in its crystal form, as a result of the steric constraint generated by the crystal packing.In the core of the cluster, the Mo-S bond lengths range from 2.3638 (13) to 2.3758 (14) A ˚, with an average value of 2.369 (2) A ˚.The Mo� � �Fe distances are between 2.7743 (12) A ˚and 2.8012 (13) A ˚, averaging 2.789 (1) A ˚.The Fe� � �Fe distances fall in the range 2.6123 (12) A ˚and 2.6368 (11) A ˚, with a mean value of 2.626 (1) A ˚.The Fe-S bond lengths range from 2.2678 (14) to 2.2923 (13) A ˚, with an average value of 2.282 (1) A ˚.The Fe-N(imide) bond lengths are in the range of 1.917 (2) A ˚to 1.9386 (19) A ˚, with an average value of 1.931 (2) A ˚.The Fe-N(azide) bond lengths are between 1.922 (2) and 1.937 (2) A ˚, with an average value of 1.930 (2) A ˚.The N-Si bond length is 1.753 (2) A ˚.The Fe-N-Fe angles range from 84.78 (7) to 86.29 (7) � with an average of 85.7 (1) � .The structure of the cluster [(Tp*)MoFe 3 S 3 (� 3 -NSiMe 3 )(N 3 ) 3 ] À is shown in Fig. 1 and some selected geometric parameters are listed in Table 1.

Supramolecular features
In the crystal, there are two sets of cluster counter-ions in each unit cell.The anionic clusters and the Et 4 N + cations are arranged in alternating layers, where electrostatic interactions might be the dominant supramolecular interactions.No Structure of the anionic cluster in the title compound with the atomnumbering scheme.Displacement ellipsoids are drawn at the 50% probability level.Hydrogen atoms are omitted for clarity.

Table 1
Selected geometric parameters (A ˚, � ).significant hydrogen-bonding or �-� stacking interactions were identified in the crystal structure.The packing of the title compound is shown in Fig. 2.

Database survey
Heteroleptic cubane-type M-Fe-S-N clusters (M = Mo or W) are very rare.In the literature, there are currently only two types of M-Fe-S-N clusters (Xu et al., 2018;He et al., 2022;Zhang et al., 2023).Thus far, cubane-type Mo-Fe-S-N clusters with azide terminal ligands have not been synthesized successfully.
residual electron density of disordered solvent molecules in the void space could not be reasonably modeled, thus the SQUEEZE (Spek, 2015) function was applied in PLATON (Spek, 2020).A total of 40 electrons in a volume of 146 A ˚3 were counted by SQUEEZE and removed per unit cell.This accounts for about one solvent molecule (probably diethyl ether) per unit cell.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.Refinement.Single-crystal X-ray diffraction data for the title compound was collected at 296 K on a Bruker APEX II CCD diffractometer operating at 50 kV and 30 mA using Mo-Kα radiation (λ = 0.71073 Å).Crystal was mounted on a loop using Parabar 10312 oil for data collection.Data was collected with a series of φ and/or ω scans.Data was integrated using SAINT and scaled with either a numerical or multiscan absorption correction using SADABS.Structure was solved using SHELXT and refined by full-matrix least-squares on F 2 using the SHELXL and OLEX2 (Dolomanov et al., 2009) programs.All non-hydrogen atoms were refined anisotropically.Geometric parameters (Å, º)

Figure 2
Figure 2Crystal packing of the title compound.Hydrogen atoms are omitted for clarity.

Table 2
Experimental details.