Halide coordinated homoleptic [Fe4S4X4]2- and heteroleptic [Fe4S4X2Y2]2- clusters (X, y = Cl, Br, I) - Alternative preparations, structural analogies and spectroscopic properties in solution and solid state

Abstract

New facile methods to prepare iron sulphur halide clusters [Fe4S4X4]2- from [Fe(CO)5] and elemental sulphur were elaborated. Reactions of ferrous precursors like tetrahalidoferrates(ii) or simple ferrous halides with [Fe(CO)5] and sulphur turned out to be efficient methods to prepare homoleptic [Fe4S4X4]2- (X = Cl, Br) and heteroleptic clusters [Fe4S4X4-nYn]2- (X = Cl, Br; Y = Br, I). Solid materials were obtained as salts of BTMA+ (= benzyltrimethylammonium); the new compounds containing [Fe4S4Br4]2- and [Fe4S4X2Y2]2- (X, Y = Cl, Br, I) were all isostructural to (BTMA)2[Fe4S4I4] (monoclinic, Cc) as inferred from synchrotron X-ray powder diffraction. While the solid materials contain defined heteroleptic clusters with a halide X : Y ratio of 2 : 2, dissolving these compounds leads to rapid scrambling of the halide ligands forming mixtures of all five possible [Fe4S4X4-nYn]2- clusters as could be shown by UHR-ESI MS. The variation of X and Y allowed assignment of the absorption bands in the visible and NIR; the long-wavelength bands around 1100 nm were tentatively assigned to intervalence charge transfer (IVCT) transitions.