Poly[(μ2-quinoline-3-carboxylato-κ2 N:O)(μ2-quinoline-3-carboxylato-κ3 N:O,O′)cadmium]

In the title compound, [Cd(C10H6NO2)2]n, the CdII atom is coordinated by three O atoms and two N atoms from four quinoline-3-carboxylate (L −) ligands, leading to a distorted trigonal–bipyramidal geometry. The L − ligands link the CdII atoms into a plane parallel to (100), with one ligand being tridentate, coordinating via the N atom and chelating a second Cd atom, and the other being bidentate, bridging two Cd atoms via the N and one O atom.. This two-dimensional network extends into a double-layer network by π–π interactions, with centroid–centroid distances of 3.680 (2) and 3.752 (2) Å. Another type of π–π interaction between pyridine rings [centroid–centroid distance = 3.527 (2) Å] leads to a three-dimensional supramolecular architecture.


Comment
To date, much effort has been made on the construction of cadmium coordination polymers with a wide variety of topological structures which may possess promising perspectives toward molecular luminescent materials (Chi et al., 2007;Niu et al., 2006;Song et al., 2006;Lu et al., 2007). It is well known that nicotinic acid has been proved to be effective for constructing coordination polymers due to the versatile coordination fashion (Chen et al., 2003;Song et al., 2006). Compared with nicotinic acid, the structurally similar quinoline-3-carboxylic acid (HL) have been chosen to construct a new coordination polymer. Here, we report on the crystal structure of the title compound.
There is one cadmium (II) atom and two independent Lligands in the asymmetric unit. The Cd (II) atom is five-coordinated by two N atoms [Cd1-N1 ii =2.341 (2) Å, Cd1-N2=2.319 (2) Å] and three O atoms [Cd1-O1=2.163 (2) Å, Cd-O3 i =2.386 (2) Å, Cd-O4 i =2.277 (2) Å] from four Lligands, showing a distorted trigonal bipyramidal coordination geometry (Fig. 1). The Lligand containing the N1 atom, acts as bis-monodentate mode toward cadmium centers with pyridine nitrogen atoms linking the cadmium atom and the carboxylate group linking the cadmium atom in a monodentate fashion, leading to the formation of a 1D chain structure along the the b axis. The 1D chains are linked into a 2D layer network by bis-chelating Lligand containing the N2 atom, There is in addition a 2D double-layer structure (black bond and green bond) which is connected by π-π interactions with the centroid to centroid distances of 3.680 (2) and 3.752 (2) Å, respectively (Fig. 2). The 2D double-layers are parallel to the (100) plane, and linked to each other by another type of π-π interaction between pyridine rings [centroid-to-centroid 3.527 (2) Å], resulting in a 3D supramolecular architecture.
There is a reported isostructural Zn analogue (Hu et al., 2007) which has a tetrahedral environment with Lin a bismonodentate mode, while the title compound shows a distorted trigonal bipyramidal coordination geometry with Lin bismonodentate and bis-chelating modes, respectively (Fig. 3). This comparison reveals the influence of different metal ion on the coordination mode of the ligand.

Experimental
Quinoline-3-carboxylic acid (HL) was purchased commercially and used without further purification. A mixture of CdCl 2 (18.400 mg, 0.1 mmol), and HL (17.300 mg, 0.1 mmol) was dissolved in a 10 mL of water with a pH = 6. The resulting mixture was heated in a 15 mL autoclave with Teflon-liner at 438 K for three days. Then the autoclave was slowly cooled to room temperature, and colourless block-shaped crystals were obtained with a yield of 50 %.

Refinement
All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were positioned geometrically and refined using a riding model with C-H distances of 0.93 Å and U iso (H)=1.2U eq (C). Fig. 1. View of the title compound showing displacement ellipsoids (drawn at a 30% probability level) and labeling. H atoms are drawn as a small spheres of arbitrary radius. [symmetry codes: (i) x, -y, z + 1/2; (ii) x, y -1, z.] Fig. 2. 2D double-layer structure and π-π stacking interactions between different 2D layers. All hydrogen atoms were omitted for clarity.

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.