Redox behaviour of the iron–sulphur cluster [Fe₆(µ₃-S)₈(PEt₃)₆]²⁺. Synthesis and crystal structure of the new paramagnetic monopositive species [Fe₆(µ₃-S)₈(PEt₃)₆]⁺as its [PF₆]^–salt

Abstract

The electrochemical behaviour of the complex cation [Fe6(µ3-S)8(PEt3)6]2+ in non-aqueous solvents demonstrates the ability of the Fe6S8 core to undergo, through well defined steps, the one electron transfer series: [Fe6S8]4+⇄[Fe6S8]3+⇄[Fe6S8]2+⇄[Fe6S8]+⇄[Fe6S8]0. Attempts to obtain some congeners of [Fe6S8(PEt3)6]2+ either by electrochemical techniques or by common redox agents proved to be unsuccessful. However, the monopositive species [Fe6(µ3-S)8(PEt3)6]+ has been successfully isolated by reacting [Fe(H2O)6]2+ with H2S in the presence of PEt3 and [NBun4][PF6] with rigorous exclusion of oxygen and its molecular structure established by a single-crystal X-ray diffraction study [crystal data: trigonal, space group R, with a= 11.705(7)Å, α= 92.17(8)°, and Z= 1]. The structure was solved by the heavy-atom method and refined by fullmatrix least-squares methods to a conventional R value of 0.054 for 1 146 observed reflections. The monopositive species, which is paramagnetic with a magnetic moment of 7.4 µB is isostructural with the previously reported dication, its inner core consisting of an octahedron of iron atoms with all the faces symmetrically capped by sulphur ligands. The addition of one electron with respect to the dipositive species induces small changes in the metal framework.