Poly[μ-aqua-aqua(μ-benzene-1,2,4,5-tetracarboxylato)gadolinate(III)potassium(I)]

In the title compound, [KGd(C10H2O8)(H2O)2]n, the Gd3+ ion is nine-coordinated by eight O atoms from five individual benzene-1,2,4,5-tetracarboxylate (btec) ligands and one water molecule, and the K+ ion is eight-coordinated by six O atoms from five individual btec ligands and two water molecules. In the crystal, the btec half-molecules are completed by crystallographic inversion symmetry. GdO9 and KO8 polyhedra are connected, forming layers in the ab plane, which are further interconnected by μ8- or μ12-bridging btec ligands, forming a three-dimensional structure.

In the title compound, [KGd(C 10 H 2 O 8 )(H 2 O) 2 ] n , the Gd 3+ ion is nine-coordinated by eight O atoms from five individual benzene-1,2,4,5-tetracarboxylate (btec) ligands and one water molecule, and the K + ion is eight-coordinated by six O atoms from five individual btec ligands and two water molecules. In the crystal, the btec half-molecules are completed by crystallographic inversion symmetry. GdO 9 and KO 8 polyhedra are connected, forming layers in the ab plane, which are further interconnected by 8 -or 12 -bridging btec ligands, forming a three-dimensional structure.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: SI2351).

Comment
The btec ligand is a remarkably versatile building block for the construction of supramolecular architectures due to its four rigid carboxyl groups and various coordination modes in the self-assembly reaction (Huang et al., 2009;Lu et al., 2005;Wu et al., 2001;Zhang et al., 2005). Furthermore, they can provide directional conformation of network structures via noncovalent contacts like hydrogen bonding and aromatic stacking. In this paper, we report the preparation and crystal structure of a new potassium(I)-gadolinium(III) complex, [KGd(btec)(H 2 O) 2 ] n , where btec = benzene-1,2,4,5-tetracarboxylate. The crystal structures of a Nd iii analogue has been reported recently (Dai et al., 2008).
As shown in Fig. 1, the asymmetric building unit of title compound comprises one Gd III atom, one K I atom, two coordinated water molecules and two btec ligands. The two crystallographic distinct btec ligands occupy inversion symmetry in the structure. One of them acts as a µ 12 -bridge linking six Gd III and six K I atoms, while the other one acts as a µ 8 -bridge linking four Gd III and four K I atoms. Considering the linking environment of the gadolinium(III) and potassium(I) atoms, Gd III is nine-coordinated by eight O atoms from five individual btec ligands and one water molecule, while the K I atom is eight-coordinated by six O atoms from five individual btec ligands and two water molecules, as listed in Tab. 1. As shown in Fig. 2, when C and H atoms are omitted, a two-dimensional Gd-K-O framework is emerged in the ab plane. Furthermore, these two-dimensional layers are integrated by C atoms of btec ligands into a three-dimensional framework.

Experimental
A mixture of 1,2,4,5-benzenetetracarboxylic (0.05 g), Gd 2 O 3 (0.05 g), KOH(0.05 g) and H 2 O (15 ml) was heated at 448 K for 7 d in a sealed 25 ml Teflon-lined stainless steel vessel under autogenous pressure. After cooling to room temperature at a rate of 5 C h -1 , colorless prismatic crystals were obtained in low yield.