Iron–Amide–Sulfide and Iron–Imide–Sulfide Clusters: Heteroligated Core Environments Relevant to the Nitrogenase FeMo Cofactor

Heteroligated cluster cores consisting of weak-field iron, strongly basic nitrogen anions, and sulfide are of interest with respect to observed and conjectured environments in the FeMo cofactor of nitrogenase. Selective syntheses have been developed to achieve such environments with tert-butyl-substituted amide and imide core ligands. A number of different routes were employed, including nominal ligand substitution and oxidative addition reactions, as well as novel transformations involving the combination of different cluster precursors. New cluster products include precursor Fe2(μ-NHtBu)2[N­(SiMe3)2]2 (6), Fe2(μ-NHtBu)2(μ-S)­[N­(SiMe3)2]2 (7), which has a rare confacial bitetrahedral geometry previously unknown in iron chemistry, [Fe2(μ-NtBu)­(μ-S)­Cl4]2– (2), and cuboidal [Fe43-NtBu)33-S)­Cl4] (8), [Fe43-NtBu)23-S)2Cl4]2– (9), and [Fe43-NtBu)­(μ3-S)3Cl4]2– (10), as well as selenide-substituted derivatives Fe2(μ-NHtBu)2(μ-Se)­[N­(SiMe3)2]2 (7-Se) and [Fe43-NtBu)­(μ3-Se)3Cl4]2– (10-Se). The imide–sulfide clusters complete the compositional sets [Fe2(μ-NtBu)n(μ-S)2–nCl4]2– (n = 0–2) and [Fe43-NtBu)n3-S)4–nCl4]z (n = 0–4), represented previously only by the all-imide and all-sulfide core congeners, and they share chemical and physical properties with the parent homoleptic core species. All imide–sulfide cores are compositionally stable and show no evidence of core ligand exchange over days in solution. Beyond structural differences, the impact of mixed core ligation is most evident in redox potentials, which show progressive decreases of −435 (for z = 1–/2−) or −385 mV (for z = 2–/3−) for each replacement of sulfide by the more potent imide donor, and a corresponding effect may be expected for the interstitial heteroligand in the FeMo cofactor. Cluster 10 presents an [Fe4NS3] core framework virtually isometric with the isostructural [Fe4S3X] subunit of the FeMo cofactor, thus providing a synthetic structural representation for this portion of the cofactor core.