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.

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 1̄). 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.