The [MoFe3S4]2+ oxidation state: synthesis, substitution reactions, and structures of phosphine-ligated cubane-type clusters with the S=2 ground state.

The cluster (Tp)MoFe(3)S(4)(PEt(3))(3) containing the cubane-type MoFe(3)(μ(3)-S)(4) reduced core undergoes facile ligand substitution reactions at the iron sites leading to an extensive set of mono- and disubstituted species (Tp)MoFe(3)S(4)(PEt(3))(3-n)L(n) with L = halide, N(3)(-), PhS(-), PhSe(-), R(3)SiO(-), and R(3)SiS(-) and n = 1 and 2. Structures of 10 members of the set are reported. For two representative clusters, Curie behavior at 2-20 K indicates a spin-quintet ground state. Zero-field Mössbauer spectra consist of two doublets in a 2:1 intensity ratio. (57)Fe isomer shifts are consistent with the mean oxidation state Fe(3)(2.33+) arising from electron delocalization of the mixed-valence oxidation state description [Mo(3+)Fe(3+)Fe(2+)(2)]. Reaction of [(Tp)MoFe(3)S(4)(PEt(3))(2)Cl] with (Me(3)Si)(2)S affords [(Tp)MoFe(3)S(4)(PEt(3))(2)(SSiMe(3))], a likely first intermediate in the formation of the tricluster compound {(Tp)MoFe(3)S(4)(PEt)(2)S}(BPh(4)) from the reaction of (Tp)MoFe(3)S(4)(PEt(3))(3) and NaSSiMe(3) in tetrahydrofuran (THF). The tricluster consists of three cluster units bound to a central μ(3)-S atom in a species of overall C(3) symmetry. Relatively few clusters in the MoFe(3)S(4) oxidation state have been prepared compared to the abundance of clusters in the oxidized MoFe(3)S(4) state. This work is the first comprehensive study of the MoFe(3)S(4) state, one conspicuous feature of which is its ability to bind hard and soft σ-donors and strong to weak π-acid ligands. (Tp = tris(pyrazolyl)hydroborate(1-)).