Cs6[Fe2O6] and Rb4[Fe2O5]: New oxoferrates(III) with edge sharing FeO4 tetrahedra

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Abstract The two new oxoferrates(III) Cs6[Fe2O6] and Rb4[Fe2O5] were synthesized at temperatures of 500 and 600 °C via reaction of Fe2O3 with the elemental alkali metals (A) and their hyperoxides AO2 in the form of dark-red crystals. Their crystal structures have both been determined by single crystal X-ray diffraction. The structure of Cs6[Fe2O6] (monoclinic, P21/c, a = 724.6(2), b = 1212.1(5), c = 767.6(3) pm, β = 105.03(4)°, Z = 2, R1 = 0.0272, K6[Mn2O6] structure type) contains isolated bi nuclear anions [O2FeO2FeO2]6- composed of two edge sharing FeO4 tetrahedra, which are also known from the potassium and rubidium compounds A6[Fe2O6] that crystallize in the klassengleiche super group C2/m. Comparable building blocks are connected via common O vertices to form chains 1 ∞[O1/2OFeO2FeOO1/2]4– in the rubidium ferrate(III) Rb4[Fe2O5] (monoclinic, C2/c, a = 711.43(14), b = 1750.0(5), c = 725.3(2) pm, β = 104.72(2)°, Z = 4, R1 = 0.0611). As a consequence of the edge sharing of FeO4 tetrahedra, both compounds show short Fe—Fe distances of 273.3(2) pm in Cs6[Fe2O6] and 265.6(3) pm in Rb4[Fe2O5] respectively. Based on similarities in the packing of the (FeIII/AI) cations, a structural relationship between the two compounds can be derived. The influence of the different counterions on the structure of the oxoferrate units in the phases of general composition A6[Fe2O6] and A4[Fe2O5] is discussed.