Structural insight into halide-coordinated [Fe4S4XnY4-n]2- clusters (X, Y = Cl, Br, I) by XRD and Mossbauer spectroscopy

Iron sulphur halide clusters [Fe4S4Br4](2-) and [Fe4S4X2Y2](2-) (X, Y = Cl, Br, I) were obtained in excellent yields (77 to 78%) and purity from [Fe(CO)(5)], elemental sulphur, I-2 and benzyltrimethylammonium (BTMA(+)) iodide, bromide and chloride. Single crystals of (BTMA)(2)[Fe4S4Br4] (1), (BTMA)(2)[Fe4S4Br2Cl2] (2), (BTMA)(2)[Fe4S4Cl2I2] (3), and (BTMA)(2)[Fe4S4Br2I2] (4) were isostructural to the previously reported (BTMA)(2)[Fe4S4I4] (5) (monoclinic, Cc). Instead of the chloride cubane cluster [Fe4S4Cl4](2-), we found the prismane-shaped cluster (BTMA)(3)[Fe6S6Cl6] (6) (P1). Fe-57 Mossbauer spectroscopy indicates complete delocalisation with Fe2.5+ oxidation states for all iron atoms. Magnetic measurements showed small chi T-M values at 298 K ranging from 1.12 to 1.54 cm(3) K mol(-1), indicating the dominant antiferromagnetic exchange interactions. With decreasing temperature, the chi T-M values decreased to reach a plateau at around 100 K. From about 20 K, the values drop significantly. Fitting the data in the Heisenberg-Dirac-van Vleck (HDvV) as well as the Heisenberg Double Exchange (HDE) formalism confirmed the delocalisation and antiferromagnetic coupling assumed from Mossbauer spectroscopy.

Automated summary

Iron sulphur halide clusters with various halide ligands were obtained in high yields and purity. The crystal structures of these compounds were found to be isostructural, and magnetic measurements confirmed the strong antiferromagnetic coupling between the iron atoms.