Poly[[diaqua(μ8-benzene-1,2,4,5-tetracarboxylato)calciumzinc] monohydrate]

In the title complex, {[CaZn(C10H2O8)(H2O)2]·H2O}n, the ZnII ion is coordinated by four O atoms from four benzene-1,2,4,5-tetracarboxylate anions in a distorted tetrahedral geometry. The CaII ion is eight-coordinated by six O atoms from four benzene-1,2,4,5-tetracarboxylate anions and by two water molecules in a distorted square-antiprismatic geometry. The CaII and ZnII ions and the lattice water molecule are located on twofold rotation axes; the centroid of the benzene-1,2,4,5-tetracarboxylate anion is located on a centre of inversion. The μ8-bridging mode of the anion results in the formation of a three-dimensional structure with channels extending along [100] in which lattice water molecules are situated. Intermolecular O—H⋯O hydrogen bonds involving the coordinating and lattice water molecules as donors and the carboxylate O atoms and lattice water molecules as acceptors are present in the structure.

In the title complex, {[CaZn(C 10 H 2 O 8 )(H 2 O) 2 ]ÁH 2 O} n , the Zn II ion is coordinated by four O atoms from four benzene-1,2,4,5-tetracarboxylate anions in a distorted tetrahedral geometry. The Ca II ion is eight-coordinated by six O atoms from four benzene-1,2,4,5-tetracarboxylate anions and by two water molecules in a distorted square-antiprismatic geometry. The Ca II and Zn II ions and the lattice water molecule are located on twofold rotation axes; the centroid of the benzene-1,2,4,5-tetracarboxylate anion is located on a centre of inversion. The 8 -bridging mode of the anion results in the formation of a three-dimensional structure with channels extending along [100] in which lattice water molecules are situated. Intermolecular O-HÁ Á ÁO hydrogen bonds involving the coordinating and lattice water molecules as donors and the carboxylate O atoms and lattice water molecules as acceptors are present in the structure.
Data collection: CrystalClear (Rigaku/MSC, 2004); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: publCIF (Westrip, 2010 A large number of complexes constructed from the multidentate aromatic ligand benzene-1,2,4,5-tetracarboxylic acid and its corresponding anions have been extensively studied due to the diversity of the coordination modes and sensitivity to pH values of the carboxylate anions. Some of the final products exhibit useful functional properties (Prajapati et al., 2009;Xie et al., 2008). In order to further explore complexes with new structures, we selected benzene-1,2,4,5-tetracarboxylic acid as precursor to self-assembly with ZnCl 2 and CaCl 2 simultaneously in solution and obtained the title complex, , the crystal structure of which is reported herein.
As shown in Figure 1, the Zn II ion displays a distorted tetrahedral coordination geometry defined by four oxygen atoms from four symmetry-related benzene-1,2,4,5-tetracarboxylate groups (O1, O1A, O4B and O4C). The Ca II ion is bound to six oxygen atoms from four benzene-1,2,4,5-tetracarboxylate groups (O1E, O1D, O2, O2F, O3D, O3E) and two water molecules (O5, O5F) leading to a distorted square-antiprismatic geometry. The base plane of the square antiprism consists of atoms O1E, O2F, O3E, and O5F with a mean deviation of 0.1065 Å from the least-squares plane. The top plane of the square antiprism consists of symmetry-related atoms O1D, O2, O3D, and O5. The dihedral angle between the two planes is 3.9 °. The three-dimensional set-up of the structure leaves space for channels extending along [100] where the lattice water molecules are located.
Intermolecular O-H···O hydrogen bonds between coordinating water molecules and lattice water molecules, between coordinating water molecules and carboxylate groups, and between solvent water molecules and carboxylate groups consolidate the crystal packing (Table 1, Fig. 2).

Refinement
The H atom bound to C5 was positioned geometrically and refined as riding, with C-H = 0.93 Å. H atoms bound to water O atoms were found from difference maps and refined with distance restraints of O-H = 0.85 Å. All H atoms were refined with U iso (H) = 1.2 U eq (C,O).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.