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.