Synthesis and Characterization of Iron-Amide and Iron-Imide-Sulfide Clusters

Abstract

The iron-molybdenum cofactor (FeMo cofactor) is the catalytic center of nitrogen fixation in molybdenum-dependent nitrognease enzymes. The resting state cofactor is a complex [MoFe7S9X] cluster, in which the central ligand X is a central hexacoordinated monoatomic light atom (2p), and the exact identity of X is uncertain. The heteroligated, nitrogen-containing core environment of the cofactor cluster may also be relevant to active states, as several mechanistic proposals for cofactor catalysis incorporate substrate-derived nitrogenous moeities into the cluster core during turnover. To this end, we have explored synthetic pathways to the dinuclear and tetranuclear nitrogen-containing iron-sufur clusters, which may mimic the heteroligated core environment of the cofactor. Dinuclear iron-amide clusters Fe2(μ-NHtBu)2[N(SiMe3)2]2 (46) and Fe2(μ-NHtBu)2(μ-S)[N(SiMe3)2]2 (47) are useful precursors for the preparation of [Fe4(NtBu)n(S)4-nCl4]z cubane complexes that span all mixed imide/sulfide core compositions between the classic [Fe4S4] and the more recently reported [Fe4(NtBu)4] homoleptic motifs. The [Fe4NS3] core of the n = 1 cluster is particularly noteworthy in being essentially isometric with the analogous [Fe4S3X] subunit of the FeMo cofactor structure. Synthetic compounds are characterized by single crystal X-ray crystallography, cyclic voltammetry, and UV-Vis, 1H NMR spectroscopies.