Poly[tetraaqua-μ3-pyridine-3,5-dicarboxylato-strontium(II)]

The reaction of strontium(II) nitrate with the proton-transfer compound (pdaH2)(py-3,5-dc)·H2O (where pda = propane-1,3-diamine and py-3,5-dcH2 = pyridine-3,5-dicarboxylic acid) leads to the formation of the title polymeric compound, [Sr(C7H3NO4)(H2O)4]n. The propane-1,3-diaminium cation is not incorporated in this crystal structure. The SrII atom lies on an inversion centre and is eight-coordinated by four O atoms from three py-3,5-dc ligands and four O atoms from four coordinated water molecules. The coordination polyhedron of the SrII atom is a distorted dodecahedron. These binuclear units are connected via the carboxylate O atoms to build a one-dimensional polymeric chain. In the crystal structure, non-covalant interactions consisting of hydrogen bonds (X—H⋯O, with X = O and C) and π–π stacking interactions [3.4604 (19) Å] connect the various components to form a supramolecular structure.

The reaction of strontium(II) nitrate with the proton-transfer compound (pdaH 2 )(py-3,5-dc)ÁH 2 O (where pda = propane-1,3-diamine and py-3,5-dcH 2 = pyridine-3,5-dicarboxylic acid) leads to the formation of the title polymeric compound, [Sr(C 7 H 3 NO 4 )(H 2 O) 4 ] n . The propane-1,3-diaminium cation is not incorporated in this crystal structure. The Sr II atom lies on an inversion centre and is eight-coordinated by four O atoms from three py-3,5-dc ligands and four O atoms from four coordinated water molecules. The coordination polyhedron of the Sr II atom is a distorted dodecahedron. These binuclear units are connected via the carboxylate O atoms to build a one-dimensional polymeric chain. In the crystal structure, non-covalant interactions consisting of hydrogen bonds (X-HÁ Á ÁO, with X = O and C) andstacking interactions [3.4604 (19) Å ] connect the various components to form a supramolecular structure.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: SU2030).

S1. Comment
We have previously reported a proton transfer system, prepared using pyridine-3,5-dicarboxylic acid and propane-1,3-diamine (pdaH 2 )(py-3,5-dc).H 2 O (Aghabozorg et al. 2006;Aghabozorg et al., 2007). Pyridine-3,5-dicarboxylic acid is an interesting ligand because it is highly symmetrical, potentially multidentate and it can participate in hydrogen bonding interactions with N and O acceptors. It may also exhibit π-π interactions (Starosta et al., 2002a,b) and some polymeric Cd(II) complexes of this ligand have been published. Here we report on the crystal structure of the title polymeric compound, (I).
The compound (I) is a centrosymmetric polymer (Fig. 1). The Sr-O distances are in the range of 2.5240 (19)-2.7395 (18) Å, and the summation of the bond angles around of Sr II atom is equal to 359.24°, indicating that the Sr II atom is located in the center of the plane (O2 i ,O4W,O1 ii ,O3W,O2W). This shows that it has a distorted dodecahedral geometry ( Fig. 2).
The carboxylate groups from the py-3,5-dc ligands link two Sr II centers by two O1 atoms or two O2 atoms alternatively to form binuclear units and this results in the formation of a one-dimensional polymer chain. Each of the atoms, O1 and O2, from the py-3,5-dc ligands are connected to two Sr II atoms, but only atoms O3, O4 and N1 from these ligands build hydrogen bonds with the coordinated water molecules. There are a large number of O-H···O, N-H···O and C-H···O hydrogen bonds with distances ranging from 2.739 (4) to 3.324 (4) Å (Table 2 and Fig. 3).

S3. Refinement
All hydrogen atoms were located in difference Fourier maps. The water H-atoms were treated as riding atoms with

Poly[tetraaqua-µ 3 -pyridine-3,5-dicarboxylato-strontium(II)]
Crystal data 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq