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(mu(3)-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-n)L-n](1-n) with L = halide, N-3(-), 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 Mossbauer spectra consist of two doublets in a 2:1 intensity ratio. Fe-57 isomer shifts are consistent with the mean oxidation state Fe-3(2.33+) arising from electron delocalization of the mixed-valence oxidation state description [Mo3+Fe3+Fe22+]. Reaction of [(Tp)MoFe3S4(PEt3)(2)Cl] with (Me3Si)(2)S affords [(Tp)MoFe3S4(PEt3)(2)(SSiMe3)], a likely first intermediate in the formation of the tricluster compound {[(Tp)MoFe3S4(PEt)(2)](3)S}(BPh4) from the reaction of [(Tp)MoFe3S4(PEt3)(3)](BPh4) and NaSSiMe3 in tetrahydrofuran (THF). The tricluster consists of three duster units bound to a central mu(3)-S atom in a species of overall C-3 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 sigma-donors and strong to weak pi-acid ligands. (Tp = tris(pyrazolyOhydroborate(1-))