Structural insight into halide-coordinated [Fe$_4$S$_4$X$_n$ Y$_{4− n}$]$^{2−}$ clusters (X, Y = Cl, Br, I) by XRD and Mössbauer spectroscopy

Abstract

Iron sulphur halide clusters [Fe$_4$S$_4$Br$_4$]$^{2−}$ and [Fe$_4$S$_4$X$_2$Y$_2$]$^{2−}$ (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)$_2$[Fe$_4$S$_4$Br$_4$] (1), (BTMA)$_2$[Fe$_4$S$_4$Br$_2$Cl$_2$] (2), (BTMA)$_2$[Fe$_4$S$_4$Cl$_2$I$_2$] (3), and (BTMA)$_2$[Fe$_4$S$_4$Br$_2$I$_2$] (4) were isostructural to the previously reported (BTMA)$_2$[Fe$_4$S$_4$I$_4$] (5) (monoclinic, Cc). Instead of the chloride cubane cluster [Fe$_4$S$_4$Cl$_4$]$^{2−}$, we found the prismane-shaped cluster (BTMA)$_3$[Fe$_6$S$_6$Cl$_6$] (6) (P$\bar{1}$). $^{57}$Fe Mössbauer spectroscopy indicates complete delocalisation with Fe$^{2.5+}$ oxidation states for all iron atoms. Magnetic measurements showed small χ$_M$T 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 χ$_M$T 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 Mössbauer spectroscopy