Gero Frisch

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

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.

A5Fe3O6] (A = Rb, Cs), Cs[FeO2] und Cs8[Fe2O7]: Neue Oxoferrate der schweren Alkalimetalle / A5Fe3O6] (A = Rb, Cs), Cs[FeO2] and Cs8[Fe2O7]: New Oxoferrates of the Heavy Alkaline Metals

The title compounds were synthesized at a temperature of 775 K via reaction of Fe2O3 with elemental cesium and rubidium alone or in combination with their hyperoxides AO2 where required by stoichiometry. The structures of the dark-red crystals have been determined by single crystal x-ray diffraction (Cs[FeO2]: cubic, space group Fd3m, a = 839.2(2) pm, Z = 8, R1 = 0.0547 and A5[Fe3O6] (A = Cs: orthorhombic, space group P212121, a = 861.8(2), b = 870.7(2), c = 1658.7(3) pm, Z = 4, R1 = 0.0617 and A = Rb: tetragonal, space group I4̅2d, a = 862.01(8), c= 1504.7(2) pm, Z =4, R1= 0.0334). These three compounds contain three-dimensional networks [FeO2], in the case of the ideal stuffed cristobalite composed of corner-sharing tetrahedra [FeO4/2], in the case of the new mixed valence compounds A5[Fe3O6] formed by corner-sharing of tetrahedra [FeIIIO4/2] and triangles [FeIIOO2/2] in a 1:2 ratio. The crystal structure of the cation rich compound Cs8[Fe2O7] (monoclinic, space group P21/c, a = 722.32(12), b = 1789.0(3), c = 733.88(12) pm, β =118.976(3)°, Z = 4, R1=0.0287) exhibits di-ferrate anions [Fe2O7]6− composed of two cornersharing [FeIIIO4] tetrahedra with a linear Fe-O-Fe bridge.

K5[FeO4] und K17[Fe5O16]: Zwei neue Kalium-Oxoferrate(III) / K5[FeO4] and K17[Fe5O16]: Two New Potassium Oxoferrates(III)

The title compounds were synthesized from potassium rich mixtures of Fe2O3, elemental potassium and the hyperoxide KO2 by applying short reaction times, a maximum temperature of 875 K and subsequent quenching of the samples. The structures of the two new oxoferrates(III) have been determined by single crystal X-ray diffraction. The orthoferrate(III) K5[FeO4] (Na5[GaO4] structure type, space group Pbca, a = 1124,0(2), b = 667,95(9), c = 2034,8(3) pm, Z = 4, R1 = 0,0585) exhibits isolated ortho-anions [FeO4]5− with nearly ideal tetrahedral geometry and Fe-O distance in the narrow range of 189 to 192 pm. The pentaferrate K17[Fe5O16] (space group Cm, a = 671,71(5), b=3560,8(3), c=670,81(5) pm, β =119,687(5)°, Z =2, R1=0,0291) crystallizes with a new structure type. Its building units are isolated novel penta-nuclear anions composed of five corner sharing [FeO4] tetrahedra. These linear chain pieces [Fe5O16] are arranged in a hexagonal rod packing, with a stacking sequence according to |:AB:| along the large monoclinic b axis. The structure is thus related to that of the tetra-ferrate K14[Fe4O13] with a comparable packing of tetra-nuclear ferrate(III) anions.

Fehlende Glieder bekannter Reihen: Die Oxoferrate(III) Rb8[Fe2O7], Rb6[Fe2O6] und K4[Fe2O5] / Missing Links in Known Series: The Oxoferrates(III) Rb8[Fe2O7], Rb6[Fe2O6], and K4[Fe2O5]

The title compounds were synthesized at temperatures between 775 and 1175 K from (mostly stoichiometric) mixtures of Fe2O3, elemental rubidium or potassium (A) and their hyperoxides AO2. The structures have been determined by single crystal X-ray diffraction. The alkaline rich ferrate(III) Rb8[Fe2O7] (Cs8[Fe2O7] structure type, space group P21/c, a = 696.7, b = 1722.1, c = 692.0 pm, β = 119.40°, Z = 2, R1 = 0.0496) exhibits diferrate anions [Fe2O7]8- composed of two vertexsharing [FeIIIO4] tetrahedra with a linear Fe-O-Fe bridge and nearly ideal 3m symmetry. This is in marked contrast to the Na homologue, where the diferrate anions are decidedly angular. In the series A3[FeO3], the anions in the compounds of the light alkaline cations are chains 1∞[FeO2O2/2]3−, but similar to the isotypic K6[Fe2O6] and to Cs6[Fe2O6] the new ferrate Rb6[Fe2O6] (K6Fe2O6 structure type, space group C2/m, a=741.8(2), b=1148.7(2), c=680.08(12) pm, β =103.65(2)°, Z = 4, R1 = 0.0370) contains isolated binuclear anions [O2FeO2FeO2]6− composed of two edge sharing [FeO4] tetrahedra. The new potassium ferrate of the series A4[Fe2O5], K4[Fe2O5] (space group P21/c, a = 645.91(14), b = 593.69(13), c = 1003.0(2) pm, β = 103.124(4)°, Z = 4, R1 = 0.0355), constitutes a new structure type, but its structure is still closely related to the Na compound, which crystallizes in the isomorphous subgroup P21/n with a doubled a axis. Both compounds are phylloferrates with layers 2∞[Fe2O5]4− consisting of six-membered rings of [FeO4] tetrahedra. In contrast, Rb4[Fe2O5] contains chains of vertex and edge sharing tetrahedra, so that in both series, A3[FeO3] and A4[Fe2O5], the linkedness of the ferrate tetrahedra increases with the ionic radii of the A counterions.