Cs8[Fe4S10] and Cs7[Fe4S8], two new sulfido ferrates with different tetrameric anions

The two new cesium sulfido ferrates Cs(8)[Fe(4)S(10)] and Cs(7)[Fe(4)S(8)] were synthesized at a maximum temperature of 1070 K in corundum crucibles from stoichiometric samples containing elemental Fe and S together with cesium disulfide, Cs(2)S(2). Their crystal structures have been determined by means of single-crystal X-ray diffraction. Cs(8)[Fe(III)(4)S(10)] crystallizes in the triclinic Cs(6)[Ga(4)Se(10)]-type structure and is thus isotypic to the corresponding rubidium salt. The structure exhibits tetramers [Fe(4)S(10)](8-) of edge-sharing tetrahedra, which represent segments of the well-known chain compounds A[FeS(2)]. The monoclinic mixed-valent iron(II/III) sulfido ferrate Cs(7)[Fe(4)S(8)], which is isotypic to the cesium tellurido ferrate, likewise contains oligomeric tetramers of four edge-sharing [FeS(4)] tetrahedra, in this case resulting in only slightly distorted tetrahedral [Fe(4)S(8)](7-) anions with a Fe(4)S(4) cubane core resembling the prominent [Fe(4)(μ(3)-S(4))](+) 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 Cs(8)[Fe(III)(4)S(10)] and a tiny energy gap of 0.1 eV resulting from a d-d transition for the mixed-valent cluster compound Cs(7)[Fe(II/III)(4)S(8)].