Poly[di-μ3-hydroxy[μ4-5-(4-carboxyphenyl)pyridine-2-carboxylato-κ5 N,O 2:O 2′:O 4:O 4′]dicadmium]

The asymmetric unit of the title polymeric complex, [Cd2(C13H7NO4)(OH)2]n, consists of two independent CdII atoms, one 5-(4-carboxyphenyl)pyridine-2-carboxylate ligand and two hydroxy groups. One CdII atom is six-coordinated by two O atoms from two ligand molecules and by four μ3-OH groups in a distorted trigonal–prismatic geometry. The other is five-coordinated by one N and two O atoms from two ligands and by two μ3-OH groups, forming a distorted square-pyramidal geometry. The two independent CdII atoms are connected by the ligand molecules and the OH groups into a three-dimensional framework. O—H⋯O hydrogen bonds between the OH groups and the carboxylate O atoms are observed.

The asymmetric unit of the title polymeric complex, [Cd 2 (C 13 H 7 NO 4 )(OH) 2 ] n , consists of two independent Cd II atoms, one 5-(4-carboxyphenyl)pyridine-2-carboxylate ligand and two hydroxy groups. One Cd II atom is six-coordinated by two O atoms from two ligand molecules and by four 3 -OH groups in a distorted trigonal-prismatic geometry. The other is five-coordinated by one N and two O atoms from two ligands and by two 3 -OH groups, forming a distorted squarepyramidal geometry. The two independent Cd II atoms are connected by the ligand molecules and the OH groups into a three-dimensional framework. O-HÁ Á ÁO hydrogen bonds between the OH groups and the carboxylate O atoms are observed.

Fan-Jin Meng, Heng-Qing Jia, Ning-Hai Hu and Hua Zhou Comment
The rational design and construction of coordination polymers based on metal ions and N-heterocyclic multicarboxylate ligands have attracted considerable attention for their intriguing structural topologies along with potential applications (Li et al., 2008;Mahata & Natarajan, 2005;Sun et al., 2001;Wang et al., 2009). In this paper, we report a cadmium complex with a three-dimensional framework based on 5-(4-carboxyphenyl)pyridine-2-carboxylic acid (H 2 L).
The asymmetric unit of the title compound contains two crystallographically independent Cd II ions with different coordination geometries (Fig. 1). The Cd1 atom is six-coordinated by two O atoms from two L ligands and four µ 3 -OH groups in a distorted trigonal prismatic geometry. The Cd1-O bond lengths are in a range of 2.176 (4)-2.587 (4) Å. The Cd2 atom is five-coordinated by one N and two O atoms from two L ligands and two µ 3 -OH groups in a distorted squarepyramidal geometry. The Cd2-N bond length is 2.354 (4) Å and the Cd2-O bond lengths are in a range of 2.196 (4)-2.368 (4) Å. Interestingly, the carboxylate groups and hydroxy groups connect the Cd II ions into a layer parallel to (011) and adjacent layers are further linked by the L ligands as pillars along the a-axis, generating a three-dimensional framework (Fig. 2). O-H···O hydrogen bonds between the hydroxy groups and carboxylate O atoms stabilize the structure (Table 1).

Experimental
The H 2 L ligand was prepared by a similar method described by Ben & Gordon (1951) and Liu et al. (2005). A precursor ligand, 2-methyl-5-p-tolylpyridine, was prepared through the Suzuki reaction between 4-methylphenylboronic acid (2.039 g, 15 mmol) and 5-bromo-2-methylpyridine (1.730 g, 10 mmol). The H 2 L ligand was obtained by the oxidation of potassium permanganate. The title compound was synthesized under hydrothermal conditions. A mixture of Cd(CH 3 CO 2 ) 2 .2H 2 O (0.053 g, 0.2mmol) and H 2 L (0.024 g, 0.1 mmol) in methanol (2 ml) and distilled water (5 ml) was stirred for 20 min in air, and the pH value was adjusted to about 8.5 with 0.1M CH 3 COOH and 0.1M NaOH solutions.
Then the mixture was sealed in a 23 ml Teflon-lined stainless steel autoclave, which was heated to 433 K for 72 h. After cooling to room temperature, colorless block crystals of the title compound suitable for X-ray diffraction were obtained.

Refinement
C-bound H atoms were positioned geometrically and refined as riding atoms, with C-H = 0.93 Å and with U iso (H) = 1.2U eq (C). H atoms of hydroxy groups were located in a difference Fourier map and their positions were refined with bond-length restraints of 0.82 (1) Å, and with U iso (H) = 1.5U eq (O). The highest residual electron density was found at 0.96 Å from Cd1 atom and the deepest hole at 0.70 Å from Cd1 atom.   The packing diagram of the title compound, showing the three-dimensional framework.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.