Exploration, Synthesis, and Characterization of Bioinspired Iron–Imide and Iron–Amide Clusters

Abstract

Iron-sulfur clusters with high-spin irons play a crucial role in various biological processes. These clusters are found in enzymes such as ferredoxins, aconitase, and nitrogenase, where they function as redox cofactors or active sites for catalysis. One particularly significant transformation is the reduction of atmospheric dinitrogen to ammonia, which occurs at a complex iron-sulfur cluster with core composition [MFe7S9C] where M = Mo, V or Fe. Notably, this cluster features a μ6-carbide, whose function in the cluster remains unclear. In synthetic iron-sulfur chemistry, Fe4S4 clusters have been extensively studied with various ligands and core compositions. To explore the effects of light 2p-element ligation, nitrogen anions in amide or imide motifs can be employed. Research in the Lee group has led to the synthesis of a series of iron-imide-sulfide clusters [Fe4(NtBu)nS4-nCl4]z (n = 1-4). This class of compounds extends from [Fe4S4] to [Fe4(NtBu)4] cores, with intermediate species in the series containing a mixture of imide and sulfide ligands. The [Fe4(NtBu)4] core is synthesized via the reaction of FeCl3 with two equivalents of LiNHtBu, yielding [Fe4(NtBu)4Cl4]1–, Fe4(NtBu)4Cl3(NtBu) and FeCl2(NH2tBu)2 as the primary iron-containing products, with an approximate combined in-situ yield of 50% based on starting iron content. The [Fe4(NtBu)4Cl4]1– species can be isolated in 24% yield and undergoes both chemical oxidation and reduction to form [Fe4(NtBu)4Cl4] and [Fe4(NtBu)4Cl4]2–, respectively. The [Fe4(NtBu)4Cl4]z series has been characterized using a range of spectroscopic and structural techniques to elucidate its solid-state and solution phase properties. LiNHtBu is synthesized via the lithiation of tBuNH2 with one equivalent of n-BuLi. Upon workup, this reaction affords white crystals which display an octameric ladder structure with eight molecules of LiNHtBu in the solid state. When excess tBuNH2 (ca. 1.1 equivalents) is used, a white colloidal solution forms, yielding an infinite polymeric ladder structure in the solid state. The cyclic ladder structure was determined to be the metastable polymorph and the infinite polymer ladder structure was determined to be the thermodynamic polymorph using DSC analysis and synthetic procedures. The cyclic ladder structure can be converted to the infinite polymer structure by heat or by addition of a donor ligand to catalyze the transformation The [Fe4(NtBu)S3Cl4]2– cluster is synthesized via a controlled synthetic protocol involving the formation of an iron-amide dinuclear intermediate, Fe2(μ-NHtBu)2[N(SiMe3)2]2. This intermediate arises from the protonolysis reaction of Fe[N(SiMe3)2]2 with tBuNH2. Notably, this transformation is unusual, as analogous reactions with Fe[N(SiMe3)2]2 typically proceed with ligands that are more acidic than HN(SiMe3)2. To explore the scope of this reactivity, a series of amines with varying acidity, steric hindrance and nitrogen substitution patterns were examined. The products that can form from reactivity of Fe[N(SiMe3)2]2 with amines include a mononuclear amine adduct, di– and tri–substituted dinuclear complexes and homo– and heteroleptic trinuclear complexes. The type of complex formed depended on the stoichiometry of amine to Fe[N(SiMe3)2]2 and the acidity, nitrogen substitution and steric hindrance around the nitrogen. Finally, the reduction of [Fe4(NtBu)S3(SPh)4]2– was attempted. Although reduction to the z = 3– cluster was achieved, the resulting product proved unstable in CD3CN, decomposing into a new species accompanied by thiolate release. Upon oxidation of the decomposition product, [Fe4(NtBu)S3(SPh)4]2– was regenerated, indicating that the Fe4(NtBu)S3 core likely remains intact. Further ligand tuning revealed that the use of p-methylbenzene thiolate allowed for the isolation of the reduced cluster; however, purification was hindered by its limited solubility. The synthesis of the oxidized, [Et4N][Fe4(NtBu)S3(SMes)4] cluster was achieved by oxidization of the z = 2– cluster by ferrocenium. Surprisingly, the oxidized cluster displayed a ground spin state of S = 3/2.