The Cuboidal Fe₃S₄ Cluster:  Synthesis, Stability, and Geometric and Electronic Structures in a Non-Protein Environment

Of the three known low-nuclearity iron−sulfur clusters in metallobiomolecules with the core units Fe2S2, Fe4S4, and Fe3S4, the last has not been obtained in stable form outside a protein environment. We describe a direct route to such clusters in the [Fe3S4]0 oxidation state, and demonstrate an effective stereochemical and electronic structural congruence with the native cluster. The synthesis is based on iron-site-differentiated clusters. Reaction of [Fe4S4(LS3)(SEt)]2- with (Et3NH)(OTf) affords [Fe4S4(LS3)(OTf)]2-, whose unique site is activated toward terminal ligand substitution. Treatment with 1 equiv of (Et4N)2(Meida) affords [Fe4S4(LS3)(Meida)]3-, which is readily converted to [Fe3S4(LS3)]3- with 1−2 equiv of additional reactant. The trinuclear cluster is formed by abstraction of Fe2+ from a precursor cubane-type [Fe4S4]2+ core and complexation as [Fe(Meida)2]2-. An analogous procedure starting with [Fe4Se4(LS3)(SEt)]2- yields [Fe3Se4(LS3)]3-. The compound (Et4N)3[Fe3S4(LS3)]·MeCN crystallizes in orthorhombic space group P212121 with no imposed symmetry. An X-ray structure solution demonstrates the presence of the desired cuboidal [Fe3(μ3-S)(μ2-S)3]0 core in a complex of absolute configuration Δ. Property comparisons support the cuboidal structure for [Fe3Se4(LS3)]3-. A series of reactions in the systems [Fe4S4(SEt)4]2-/(Et4N)2(Meida) and [Fe3S4(LS3)]3-/NaSEt in Me2SO disclose that, while cuboidal [Fe3S4(SEt)3]3- is formed in both systems, it is one of several cluster products and tends to decay with time. [Fe3S4(LS3)]3- is completely stable in anaerobic solutions at ambient temperature. Consequently, the semirigid cavitand ligand LS3 is conspicuously superior to a simple monodentate thiolate in stabilizing the [Fe3S4]0 core. The cuboidal core is metrically very similar in structure to the cubane core of [Fe4S4(LS3)Cl]2- and to protein-bound Fe3S4 clusters. Voltammetry of [Fe3S4(LS3)]3- reveals a reversible three-membered electron transfer series which includes the core states [Fe3S4]1+,0,1-. The electronic structures of [Fe3S4(LS3)]3- and [Fe3Se4(LS3)]3- were investigated by Mössbauer and EPR spectroscopies. These studies reveal that the synthetic clusters, like the protein-bound clusters, have an electronic ground state with cluster spin S = 2 that arises from an interplay of Heisenberg and double exchange between the sites of a delocalized Fe2+Fe3+ pair and an Fe3+ site. The zero-field splittings of the S = 2 multiplet and the entire set of 57Fe hyperfine parameters of the synthetic clusters match those of the protein-bound clusters. Evidently, protein structure is not required to sustain the cuboidal geometry nor the spin-quintet ground state and its attendant electron distribution and magnetic interactions. We conclude that the clusters [Fe3Q4(LS3)]3- (Q = S, Se) are accurate structural and electronic analogues of the cuboidal sites in native and selenide-reconstituted proteins. No cluster containing a discrete cuboidal Fe3S4 core has previously been isolated in substance.