Crystal structure of bis(μ-3-nitrobenzoato)-κ3 O,O′:O;κ3 O:O,O′-bis[bis(3-cyanopyridine-κN 1)(3-nitrobenzoato-κ2 O,O′)cadmium]

In the title cadmium complex of 3-nitrobenzoate and 3-cyanopyridine, binuclear centrosymmetric molecules are present, with cadmium being surrounded in an N2O5 coordinaton set in a distorted pentagonal–bipyramidal shape.

The asymmetric unit of the title compound, [Cd 2 (C 7 H 4 NO 4 ) 4 (C 6 H 4 N 2 ) 4 ], contains one Cd II atom, two 3-nitrobenzoate (NB) anions and two 3cyanopyridine (CPy) ligands. The two CPy ligands act as monodentate N(pyridine)-bonding ligands, while the two NB anions act as bidentate ligands through the carboxylate O atoms. The centrosymmetric dinuclear complex is generated by application of inversion symmetry, whereby the Cd II atoms are bridged by the carboxylate O atoms of two symmetry-related NB anions, thus completing the distorted N 2 O 5 pentagonal-bipyramidal coordination sphere of each Cd II atom. The benzene and pyridine rings are oriented at dihedral angles of 10.02 (7) and 5.76 (9) , respectively. In the crystal, C-HÁ Á ÁN hydrogen bonds link the molecules, enclosing R 2 2 (26) ring motifs, in which they are further linked via C-HÁ Á ÁO hydrogen bonds, resulting in a three-dimensional network. In addition,stacking interactions between parallel benzene rings and between parallel pyridine rings of adjacent molecules [shortest centroid-to-centroid distances = 3.885 (1) and 3.712 (1) Å , respectively], as well as a weak C-HÁ Á Á interaction, may further stabilize the crystal structure.

Chemical context
In the last two decades, research on metal-organic frameworks (MOFs) has received considerable attention due to their extensive structural chemistry (Li et al., 2016) and their potential applications, including gas storage, nonlinear optics and ion exchange (Carlucci et al., 2003). In the syntheses of compounds having MOF structures, various carboxylate ligands have been used (Li et al., 2004). ISSN 2056-9890 On the other hand, transition-metal complexes with biochemically active molecules show interesting physical and/ or chemical properties, through which they may find applications in biological systems (Antolini et al., 1982). Some benzoic acid derivatives, such as 4-aminobenzoic acid, have been extensively reported in coordination chemistry, as bifunctional organic ligands, due to the varieties of their coordination modes (Chen & Chen, 2002;Amiraslanov et al., 1979;Hauptmann et al., 2000).
The structure-function-coordination relationships of arylcarboxylate ions in Zn II complexes of benzoic acid derivatives change depending on the nature and position of the substituted groups on the benzene ring, the nature of the additional ligand molecule or solvent, and the pH range and temperature of the synthesis Nadzhafov et al., 1981;Antsyshkina et al., 1980;Adiwidjaja et al., 1978). When pyridine and its derivatives are used instead of water molecules, the structure is completely different (Catterick et al., 1974).
The structures of some dinuclear complexes obtained from the reactions of transition metal(II) ions with nicotinamide (NA; C 6 H 6 N 2 O) and some benzoic acid derivatives as ligands, e.g.

Structural commentary
The asymmetric unit of the title complex contains one Cd II atom, two 3-nitrobenzoate (NB) anions and two 3-cyanopyridine (CPy) ligands. The two CPy ligands are monodentate (through the pyridine N atoms), while both NB anions act as bidentate ligands through their carboxylate O atoms (Fig. 1). The centrosymmetric dinuclear molecule is completed by application of inversion symmerty. The Cd II atoms are bridged by the carboxylate O atoms of one NB anions (O6 and O5) and its symmetry-related counterpart [symmetry code: (i) Àx, Ày + 1, Àz + 1]. Hence, this carboxylate group not only chelates to one Cd II atom but also bridges two Cd II atoms (Fig. 2). Thus, each Cd II atom is surrounded by three NB anions and two CPy ligands. The overall coordination sphere of the Cd II atom is defined by the bridging/chelating NB anions (O5, O5 i and O6), one chelating NB anion (O1 and O2) and two 3-cyanopyridine (CPy) ligands (N3 and N5), resulting in a distorted pentagonal-bipyramidal environment.  The molecular structure of the binuclear title molecule. Symmetry-related atoms are related by the symmetry code (Àx, Ày + 1, Àz + 1). H atoms have been omitted for clarity. Symmetry code: (i) Àx; Ày þ 1; Àz þ 1.

Figure 1
The asymmetric unit of the title molecule, showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.

Refinement
The experimental details including the crystal data, data collection and refinement are summarized in Table 3. Aromatic H atoms were positioned geometrically, with C-H = 0.93 Å , and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C). The maximum and minimum electron densities were found at 1.43 and 0.80 Å from atoms O2 and Cd1, respectively.  Table 2 Hydrogen-bond geometry (Å , ).

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.