Diaquabis(1H-imidazole-κN 3)bis(4-nitrobenzoato-κO 1)cadmium

In the centrosymmetric title compound, [Cd(C7H4NO4)2(C3H4N2)2(H2O)2], the CdII atom, located on an inversion center, is coordinated by two N atoms and four O atoms in an octahedral geometry. The internal cohesion of the molecule is enhanced by an intramolecular O—H⋯O hydrogen bond. Intermolecular O—H⋯O and C—H⋯O hydrogen bonds and π–π contacts [centroid–centroid distance = 3.6549 (2) Å] define two-dimensional networks parallel to (001), which are further connected by weaker C—H⋯O interactions into a weakly connected three-dimensional supramolecular framework.

In the centrosymmetric title compound, [Cd(C 7 H 4 NO 4 ) 2 -(C 3 H 4 N 2 ) 2 (H 2 O) 2 ], the Cd II atom, located on an inversion center, is coordinated by two N atoms and four O atoms in an octahedral geometry. The internal cohesion of the molecule is enhanced by an intramolecular O-HÁ Á ÁO hydrogen bond. Intermolecular O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds andcontacts [centroid-centroid distance = 3.6549 (2) Å ] define two-dimensional networks parallel to (001), which are further connected by weaker C-HÁ Á ÁO interactions into a weakly connected three-dimensional supramolecular framework.
Data collection: RAPID-AUTO (Rigaku, 1998); cell refinement: RAPID-AUTO; data reduction: CrystalStructure (Rigaku/MSC, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97. Aromatic carboxyl acid complexes have been paid great attention these years for their potential applications in gas storage, separation, catalysis, magnetism, luminescence, and drug delivery (Kuang et al., 2007). As a N-containing aromatic carboxyl acid, nitrobenzoic acid has been widely used in dye intermediate, organic synthesis, sensitization material, functional pigment (Hsu et al., 2011). So far, to our knowledge, cadmium complexes constructed from 4-nitrobenzoato and imidazole have not been reported. In order to get new Cd II complexes with novel functions and discover their structure-property relationship, a new complex [Cd(C 7 H 4 The asymmetric unit of [Cd(C 7 H 4 consists of a Cd 2+ ion lying on an inversion centre, a 4-NBAion (4-HNBA = 4-nitrobenzoic acid), one imidazole ligand and one lattice water as illustrated in Fig. 1. The Cd 2+ cation is octahedrally coordinated by two N atoms of imidazole ligands, two O atoms from two 4-NBAions and two O atoms from two lattice water molecules; it takes a (4 + 2) octahedral geometry, with the oxygen atoms located in the equatorial plane (Cd-O1 = 2.364 (2) Å, Cd-O5 = 2.367 (2) Å, and the two nitrogen atoms occupying the axial position (Cd-N1 = 2.255 (2) Å). Table 1 presents the \p···\p contact information involving the C 3 N 2 ring (centroid, Cg1) and Table 2, the more meaningful H-bonds in the structure; the most important ones are those involving water H's. The one described in the first entry in Table 2 is intramolecular; the seocnd one, instead defines chains along a (Figure 2, vertical arrays). The weak one involving C2-H2A (Table 2, third entry) and the π-π contact (Table 1) link chains into a two-dimensional supramolecular network parallel to (001) as illustrated in Figure 2. Finaly, the remaining weak H-bonds link these 2D structures into a 3D supramolecular architecture ( Figure 3).

Experimental
Dropwise addition of 1.0 ml (1 M) of K 2 CO 3 to a stirred aqueous solution of Cd(CH 3 COO) 2 .2H 2 O (0.266 g, 1.0 mmol) in 10.0 ml of H 2 O yielded a fine white precipitate, which was separated by centrifugation and washed with water until no CH 3 COOanions were detectable in the supernatant. The fresh precipitate was then added to a stirred aqueous solution of 4-nitrobenzoic acid (0.167 g, 1.0 mmol) in C 2 H 5 OH/H 2 O (1:1, 20.0 ml), producing a white suspension, to which imidazole (0.137 g, 2.0 mmol) was added. The mixture was further stirred vigorously for about 0.5 h. After filtration, the white filtrate (pH = 6.59) was maintained at room temperature and colorless crystals were grown.

Refinement
H atoms bonded to C atoms were palced in geometrically calculated positions and were refined using a riding model.

Figure 3
The three-dimensional framework of title complex.

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.