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Gerald Giester, Manfred Wildner
Crystal structures of the new isotypic compounds Rb4(M2+)(Fe3+)8[SeO3]14[SeO2(OH)]2 · 2 H2O (M = Mg, Cu) with a short review on iron(III) selenites
Keywords: Iron(III) selenites, Crystal chemistry, Single crystal structure analysis, X-ray diffraction
Single crystals of the new compounds Rb4(M2+)(Fe3+)8[SeO3]14[SeO2(OH)]2 · 2 H2O (M = Mg, Cu) have been synthesized in Teflon-lined steel vessels at 495 K. Their structures were determined from X-ray single crystal diffraction data and found to be isotypic: space group P-1, Z = 1, a = 7.716(2)/7.726(2), b = 10.325(3)/10.327(3), c = 15.764(4)/15.724(4) Å, α = 77.57(1)/77.87(1), β = 89.23(1)/89.47(1), γ = 86.09(1)/86.57(1)°, V = 1223.6(6)/1224.3(6) Å3, R1 = 0.028/0.037 for the Mg/Cu compound, respectively.
The structure type is characterized by a complex polyhedral framework: the [(Fe3+)O6] octahedra are linked via all corners with typical pyramidal [SeO3]2– groups — with exception of a H2O molecule which is a ligand of one of the Fe atoms. The [(M2+)O6] polyhedron shares two corners and two edges with selenite pyramids. Further sharing corners with [FeO6] octahedra, centrosymmetric units of the type [MFe4Se4O28] may be emphasized. Voids within the framework are occupied by Rb+ cations and by the lone-pair electrons of the Se4+ cations. Three different hydrogen bonds with O···O distances in the range 2.69–2.85 Å occur.
At present 28 selenites of ferric iron are known, most of them synthesized at low-hydrothermal conditions. Commonly, framework structures are formed with ferric iron in octahedral coordination. Further building units are trigonal pyramidal [SeO3]2– and [SeO2(OH)]– groups or [Se2O5]2– dimer anions, featuring a large variety of structure arrangements. Iron(III) selenites are classified based on the presence of extra cations, on the respective anionic groups and by the type of interpolyhedral linkage. In the literature, few isotypic selenites (with Fe3+ substituted by another trivalent cation) have been reported yet.
Among 51 [FeOn] polyhedra studied all but one show ferric iron (Fe3+) in sixfold coordination with mean Fe—O bond lengths for 41 pure [FeO6] octahedra ranging from 1.989 to 2.044 Å with an overall mean value of 2.013(11) Å. Remarkably, only 14% of all [FeO6] poly hedra are centrosymmetric.
In the oxoanions investigated (61 [SeO3]2–, 9 [SeO2(OH)]– and 6 [Se2O5]2– groups) tetravalent selenium typically forms a flattened trigonal pyramid with O—Se—O bond angles of about 100° and Se—O bond lengths of 1.70 Å, completed by the lone-pair electrons E at the selenium cation to a formal [ESeO3]2– ψ1 tetrahedron. In hydrogenselenites and diselenites elongated Se—O distances occur for the protonated and the bridging oxygen atoms, respectively. With some rare exceptions all selenium atoms have the site symmetry 1.
Zeitschrift für Kristallographie, Oldenbourg Wissenschaftsverlag
Print ISSN: 0044-2968
Volume: 221, 11/2006
Pages: 722 - 731