Poly[[hexaaquabis(μ3-pyrimidine-4,6-dicarboxylato)dicalcium] dihydrate]

The polymeric structure of the title compound, {[Ca2(C6H2N2O4)2(H2O)6]·2H2O}n, is built up of molecular layers composed of CaII ions bridged by both ligand N and O atoms with one of the O atoms being bis-monodentate. Two adjacent CaII ions are bridged by these O atoms, forming a centrosymmetric dimer which is the building unit of the structure. The dimers are nodes of a cross-linked molecular layer parallel to (101). The CaII ion is coordinated by two bidentate ligands, one monodentate ligand and three water molecules in the form of a distorted polyhedron with a coordination number of eight. Solvate water molecules located between adjacent layers participate as donors and acceptors in a system of hydrogen bonds in which coordinating water molecules also act as donors and non-coordinating carboxylate O atoms act as acceptors.

The polymeric structure of the title compound, {[Ca 2 (C 6 H 2 -N 2 O 4 ) 2 (H 2 O) 6 ]Á2H 2 O} n , is built up of molecular layers composed of Ca II ions bridged by both ligand N and O atoms with one of the O atoms being bis-monodentate. Two adjacent Ca II ions are bridged by these O atoms, forming a centrosymmetric dimer which is the building unit of the structure. The dimers are nodes of a cross-linked molecular layer parallel to (101). The Ca II ion is coordinated by two bidentate ligands, one monodentate ligand and three water molecules in the form of a distorted polyhedron with a coordination number of eight. Solvate water molecules located between adjacent layers participate as donors and acceptors in a system of hydrogen bonds in which coordinating water molecules also act as donors and non-coordinating carboxylate O atoms act as acceptors.

Table 2
Hydrogen-bond geometry (Å , ).  2). Solvation water molecules are located between adjacent layers. The Ca II ion has a distorted eight coordinate geometry. The observed Ca-N and Ca-O bond distances are typical (Table 1). The pyrimidine ring is planar with an r.m.s. of 0.0079 (2) Å; carboxylate groups C7/O1/O2 and C8/O3/O4 make dihedral angles with the plane of 9.7 (1)° and 8.9 (1)°, respectively. Bond distances and bond angles within the pyrimidine ring do not differ from those reported for the parent acid (Beobide, et al., 2007). In a system of hydrogen bonds, which is responsible for structure stability, solvation and coordinated water molecules act as donors and as acceptors and coordination inactive carboxylato O atoms act as acceptors (Table 2). Centro-symmetric dimeric units in which two Ca II ions are bridged by ligand bidentate carboxylato O atoms have been also observed in the structures of complexes with pyrazine-2,6-dicarboxylate and water ligands. In all, dimeric units bridged by pairs of coordinated aqua O atoms form "ladder" type molecular ribbons (Starosta, et al., 2003(Starosta, et al., , 2004.

Experimental
An aqueous solution containing 1 mmol of calcium acetate hydrate and 1 mmol of pyrimidine-4,6-diarboxylic acid dihydrate was refluxed with constant stirring for 6 h. After cooling to room temperature, the solution was left to evaporate. Well formed single-crystal blocks appeared overnight at the bottom of the reaction pot. They were separated from the mother liquid, washed with cold water and dried in air.

Refinement
Hydrogen atoms attached to water molecules were located in a difference map and refined isotropically, while two H atoms attached to pyrimidine C atoms were located at a calculated positions and treated as riding on the parent atoms with C-H=0.93 Å and

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.