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)MoFe3S4(PEt3)3]1+ containing the cubane-type [MoFe3(μ3-S)4]2+ reduced core undergoes facile ligand substitution reactions at the iron sites leading to an extensive set of mono- and disubstituted species [(Tp)MoFe3S4(PEt3)3–nLn]1–n with L = halide, N3–, PhS–, PhSe–, R3SiO–, and R3SiS– 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. 57Fe isomer shifts are consistent with the mean oxidation state Fe32.33+ arising from electron delocalization of the mixed-valence oxidation state description [Mo3+Fe3+Fe2+2]. Reaction of [(Tp)MoFe3S4(PEt3)2Cl] with (Me3Si)2S affords [(Tp)MoFe3S4(PEt3)2(SSiMe3)], a likely first intermediate in the formation of the tricluster compound {[(Tp)MoFe3S4(PEt)2]3S}(BPh4) from the reaction of [(Tp)MoFe3S4(PEt3)3](BPh4) and NaSSiMe3 in tetrahydrofuran (THF). The tricluster consists of three cluster units bound to a central μ3-S atom in a species of overall C3 symmetry. Relatively few clusters in the [MoFe3S4]2+ oxidation state have been prepared compared to the abundance of clusters in the oxidized [MoFe3S4]3+ state. This work is the first comprehensive study of the [MoFe3S4]2+ 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-))