New Open-Framework Ammonium and Amine Cadmium Zirconium Oxalates with Helical Structures

Two new open-framework cadmium zirconium oxalates containing ammonium or ethylenediamine have been prepared from precipitation methods at room pressure. The crystal
data of these oxalates are as follows:  I, 2[NH4]+[CdZr(C2O4)4]2-·3.9H2O, trigonal, space group
P6422 (no. 181), a = 9.061(5) Å, c = 23.394(5) Å, V = 1663(1) Å3, Z = 3, R1 = 0.03; II,
[C2N2H10]2+[CdZr(C2O4)4]2-·4.4H2O, trigonal, space group P3112 (no. 151), a = 9.105(5) Å, c
= 23.656(5) Å, V = 1698(1) Å3, Z = 3, R1 = 0.04. The two structures are built from MO8 (M
= Cd, Zr) distorted antiprisms connected by oxalate groups to give rise to chains based on
the sequence ...−Cd−oxalate−Zr−oxalate−... that display a helical wire arrangement. Each
helix is connected to six neighboring ones to form an anionic three-dimensional framework
[CdZr(C2O4)4]2- balanced by the cations NH4+ or [NH3−(CH2)2−NH3]2+ located in tunnels
parallel to the c axis that also contain water molecules. The apertures have hexagonal and
square cross sections of ∼5.7 Å and ellipsoidal cross sections of 5.7 × 12 Å. There is extensive
hydrogen bonding among the ammonium and amine groups, water molecules, and framework.
The thermal decomposition of the two compounds into cadmium and zirconium oxides is
described.