The [MoFe3S4]2+ Oxidation State: Synthesis, Substitution Reactions, and Structures of Phosphine-Ligated Cubane-Type Clusters with the S = 2 Ground State

Abstract

The cluster [(Tp)MoFe(3)S(4)(PEt(3))(3)](1+) containing the cubane-type [MoFe(3)(μ(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)MoFe(3)S(4)(PEt(3))(3-n)L(n)](1-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)](3)S}(BPh(4)) from the reaction of [(Tp)MoFe(3)S(4)(PEt(3))(3)](BPh(4)) 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)](2+) oxidation state have been prepared compared to the abundance of clusters in the oxidized [MoFe(3)S(4)](3+) state. This work is the first comprehensive study of the [MoFe(3)S(4)](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-)).