Cs8[Fe4S10] and Cs7[Fe4S8], Two New Sulfido Ferrates with Different Tetrameric anions

The two new cesium sulfido ferrates Cs8[Fe4S10] and Cs7[Fe4S8] were synthesized at a maximum temperature of 1070 K in corundum crucibles from stoichiometric samples containing elemental Fe and S together with cesium disulfide, Cs2S2. Their crystal structures have been determined by means of single-crystal X-ray diffraction. Cs8[FeIII4S10] crystallizes in the triclinic Cs6[Ga4Se10]-type structure and is thus isotypic to the corresponding rubidium salt. The structure exhibits tetramers [Fe4S10]8– of edge-sharing tetrahedra, which represent segments of the well-known chain compounds A­[FeS2]. The monoclinic mixed-valent iron­(II/III) sulfido ferrate Cs7[Fe4S8], which is isotypic to the cesium tellurido ferrate, likewise contains oligomeric tetramers of four edge-sharing [FeS4] tetrahedra, in this case resulting in only slightly distorted tetrahedral [Fe4S8]7– anions with a Fe4S4 cubane core resembling the prominent [Fe4(μ3-S4)]+ cluster, e.g., in the active site of ferredoxins. These sulfido ferrate anions are surrounded by 26 Cs cations, which are located at the 8 corners, 6 faces, and 12 edges of a cube. A dense stacking of these cubes, which ultimately results in the overall seven cesium countercations per cluster anion, describes the overall crystal structure completely. According to this arrangement of cluster-centered cubes, a relationship of the packing of Cs cations and cluster anions with the simple cubic packing (α-Po-type structure) can be established by applying the crystallographic group–subgroup formalism. FP-LAPW band-structure calculations applying antiferromagnetic spin ordering of the high-spin Fe ions in the two tetramers predict a small band gap of 1 eV associated with a L → M-CT for Cs8[FeIII4S10] and a tiny energy gap of 0.1 eV resulting from a d–d transition for the mixed-valent cluster compound Cs7[FeII/III4S8].