A family of microporous mixed oxalates with isotypic-framework structures based on eight-coordinate metals

Structural relationships between mixed oxalates with crystal structures consisting of isotypic anionic frameworks [MM'(C2O4)(4)](2-) built from dodecahedrally coordinated M and M' metals, with counter cations located in tunnels, are described. The principles of a predictive structural derivation, based on the YK(C2O4)(2)(.)4H(2)O structure type, is presented. Four new phases with isotypic frameworks that conform to the classification and to a proposed empirical six-valence-sum rule are reported. The new oxalates have been synthesised from mild chemistry and their crystal structures have been determined from single-crystal or powder diffraction data. Crystal data for the new phases are: CdZrSr(C2O4)(4)(.)6H(2)O, S.G. I (4) over bar m2, a = 11.2362(4) Angstrom, c = 8.6084(3) Angstrom, Z = 2; InK(C2O4)(2)(.)4H(2)O, S.G. I4(1)/a, a = 11.0549(3) Angstrom, c = 8.9735(3) Angstrom, Z = 4; YRb(C2O4)(2)(.)4H(2)O, S.G. I4(1)/a, a = 11.4569(3) Angstrom, c = 9.1502(4) Angstrom, Z = 4; Ca2Zr(C2O4)(4)(.)5.5H(2)O, S.G. I (4) over bar m2, a = 11.2558(2) Angstrom, c = 8.4288(2) Angstrom, Z = 2. The open-framework structures of all phases can be described on the basis of building walls made of mixed dodecahedra MO8 and M'O-8, connected through bichelating oxalate groups. The third element is located in channels with an elliptic cross-section. The thermal behaviour of all phases is studied from thermogravimetry and thermodiffractometry. It is shown that the dehydration process is somewhat related to the structure features and depends on the valence of the counter cation. In all cases, zeolite-type properties involving weakly bonded water molecules are pointed out. (C) 2004 Elsevier SAS. All rights reserved.