Synthesis and crystal structure of new K and Rb selenido/tellurido ferrate cluster compounds

Abstract

Abstract In the course of a systematic study of alkali iron chalcogenido salts containing clusters [Fe4 Q 8] a series of new mixed-valent potassium and rubidium selenido and tellurido ferrates(II/III) was synthesized by carefully heating the pure elements enclosed in sample tubes under an argon atmosphere up to maximum temperatures of 800–900 °C. Their crystal structures have been determined by means of single crystal X-ray diffraction. The mixed-valent FeII/III tellurido ferrates A 7[Fe4Te8] form three different structure types. All structures contain tetramers of four edge sharing [FeTe4] tetrahedra, which are connected by common edges to form only slightly distorted tetrahedral [Fe4Te8]7− anions (‘stella quadrangula’) with a [Fe4Te4] cubane core. In all cases, these anions are surrounded by 26 alkali cations, which are located at the eight corners and the midpoints of the six faces and 12 edges of a cube. The three crystal structures can thus be described by three different packings of cuboid moieties: The monoclinic rubidium compound Rb7[Fe4Te8] (space group C2/c, a = 2000.16(7), b = 897.79(3), c = 1768.12(6) pm, β = 117.4995(10)°, Z = 4, R1 = 0.0296) is isotypic to the known cesium tellurido and sulfido ferrates Cs7[Fe4(S/Te)8]. Depending on the temperature, K7[Fe4Te8] forms two different but closely related new structure types: The tetragonal r.t. modification (space group P42/nmc, a = 1222.25(14), c = 872.1(2) pm, Z = 2, R1 = 0.0583) crystallizes in a supergroup of the orthorhombic l.t. (100 K) form (space group Pbcn, a = 1715.5, b = 866.76(3), c = 1715.50(7) pm, Z = 4, R1 = 0.0160). In all structures, the cluster centered cubes are stacked to form columns along the short (≈870 pm) axis. These columns are themselves densely packed with 4 (both K compounds) and 6 (A = Rb) adjacent face-sharing columns. According to these arrangements of cluster-centered cubes, a relation of the packing of K/Rb cations and cluster anions with the simple cubic packing can be established applying the crystallographic group-subgroup formalism. Attempts to synthesize the corresponding selenium compound K7[Fe4Se8] resulted in the formation of the likewise mixed-valent compound K6[Fe4Se8]. Despite the modified composition, the new orthorhombic structure (space group Pbcn, a = 1632.62(6), b = 821.10(3), c = 1592.75(6) pm, Z = 4, R1 = 0.0540) is almost isotypic to the l.t. form of K7[Fe4Te8], the only difference being a missing K site. K5Fe2Te5 crystallizes in a new structure type (cubic, space group Pa3̅, a = 1709.02(5) pm, Z = 4, R1 = 0.0594). According to K5Fe2Te5=K15[Fe3Te7]2(Te), its structure contains mixed-valent cuboidal trimers [Fe3Te7](6/7)− and isolated telluride ions, which are coordinated by cubes of K+ cations.