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 [Fe₄S₄X_4−nY_n]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 [Fe₄S₄X_4−nY_n]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.