Poly[bis(μ2-pyrimidine-2-carboxylato-κ4 O,N:O′,N′)calcium]

In the crystal structure of the title polymeric complex, [Ca(C5H3N2O2)2]n, the CaII cation has site symmetry m2 and is N,O-chelated by four pyrimidine-2-carboxylate anions in a square-antiprismatic geometry. The planar pyrimidine-2-carboxylate anion is located on a crystallographic special position, three C atoms have site symmetry 2mm, while the carboxyl O atom, the pyrimidine N atom and the other C atom have site symmetry m. Each pyrimidine-2-carboxylate anion bridges two CaII cations, forming polymeric sheets extending parallel to (001). π–π stacking exists between parallel pyrimidine rings [centroid–centroid distance = 3.6436 (6) Å] of adjacent polymeric sheets. Weak C—H⋯O hydrogen bonding is also observed between these sheets.

In the crystal structure of the title polymeric complex, [Ca(C 5 H 3 N 2 O 2 ) 2 ] n , the Ca II cation has site symmetry 4m2 and is N,O-chelated by four pyrimidine-2-carboxylate anions in a square-antiprismatic geometry. The planar pyrimidine-2carboxylate anion is located on a crystallographic special position, three C atoms have site symmetry 2mm, while the carboxyl O atom, the pyrimidine N atom and the other C atom have site symmetry m. Each pyrimidine-2-carboxylate anion bridges two Ca II cations, forming polymeric sheets extending parallel to (001).stacking exists between parallel pyrimidine rings [centroid-centroid distance = 3.6436 (6) Å ] of adjacent polymeric sheets. Weak C-HÁ Á ÁO hydrogen bonding is also observed between these sheets.

S1. Comment
As π-π stacking between aromatic rings is correlated with the electron transfer process in some biological systems (Deisenhofer & Michel, 1989), a series metal complexes incorporating the aromatic compound has been prepared in our laboratory to investigate the nature of π-π stacking (Li et al., 2005;Pan & Xu, 2004). We report herein the crystal structure of the title compound of pyridinecarboxylate to show π-π stacking in the crystal structure.
A part of the polymeric structure of the title molecule is shown in Fig. 1. In the crystal structure, the Ca II cation has site symmetry -4m2 and is N,O-chelated by four pyrimidinecarboxylate anions with the square-antiprism geometry. The Ca-N and Ca-O bond distances (Table 1) agree with those found in the N,O-chelated Ca II complex (Starosta & Leciejewicz, 2004). The planar pyrimidinecarboxylate anion is located on the crystallographic special position, three C atoms have site symmetry 2 mm while the carboxyl O atom, the pirimidine N atom and the other C atom have site symmetry m. Each pyrimidinecarboxylate anion N,O-chelates two Ca II cations (Antolić et al., 2000;Zhang et al., 2008;Xu et al., 2008), forming the two-dimensional polymeric sheets, similar to those found in reported compounds (Rodríguez-Diéguez et al., 2007, 2008Zhang et al., 2008a,b;Sava et al. 2008). π-π stacking [centroid-centroid distance = 3.6436 (6) Å] exists between parallel pyrimidine rings of adjacent polymeric sheets (Fig. 2). Weak C-H···O hydrogen bonding is also observed between polymeric sheets (Table 2).

S2. Experimental
2-Cyanopyrimidine (0.2 g, 2 mmol), NaOH (1.2 g, 30 mmol) and calcium chloride (0.1 g, 1 mmol) were dissolved in water (10 ml). The solution was refluxed for 3 h. After cooling to room temperature the solution was filtered. The single crystals were obtained from the filtrate after 5 d.

S3. Refinement
H atoms were placed in calculated positions with C-H = 0.93 Å and refined in riding mode with U iso (H) = 1.2U eq (C).   A diagram showing π-π stacking between parallel pyrimidine rings of adjacent polymeric sheets.

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.