Crystal structure of triaqua(4-cyanobenzoato-κ2 O,O′)(nicotinamide-κN 1)zinc 4-cyanobenzoate

In the title salt, [Zn(C8H4NO2)(C6H6N2O)(H2O)3](C8H4NO2), intermolecular O—H⋯O hydrogen bonds link two of the coordinating water molecules to two free 4-cyanobenzoate anions. N—H⋯O, O—H⋯O and C—H⋯O hydrogen bonds link the molecular components, enclosing (12), (8) and (9) ring motifs and forming layers parallel to (001).

The asymmetric unit of the title salt, [Zn(C 8 H 4 NO 2 )(C 6 H 6 N 2 O)(H 2 O) 3 ]-(C 8 H 4 NO 2 ), contains one complex cation and one 4-cyanobenzoate (CNB) counter-anion. The Zn II atom in the cation is coordinated by one 4cyanobenzoate ligand, one nicotinamide (NA) ligand and three water molecules, the CNB anion thereby coordinating in a bidentate O,O 0 -mode through the carboxylate group. The latter, together with one water O atom and the N atom of the NA ligand, form a distorted square-planar arrangement, while the considerably distorted octahedral coordination sphere of the Zn II atom is completed by the two O atoms of additional water molecules in the axial positions. The dihedral angles between the planar carboxylate groups and the adjacent benzene rings in the two anions are 10.25 (10) and 5.89 (14) . Intermolecular O-HÁ Á ÁO hydrogen bonds link two of the coordinating water molecules to two free CNB anions. In the crystal, further hydrogen-bonding interactions are present, namely N-HÁ Á ÁO, O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds that link the molecular components, enclosing R 2 2 (12), R 3 3 (8) and R 3 3 (9) ring motifs and forming layers parallel to (001).contacts between benzene rings [centroid-to-centroid distances = 3.791 (1) and 3.882 (1) Å ] may further stabilize the crystal structure.

Chemical context
As parts of our ongoing investigation on transition-metal complexes of nicotinamide (NA), one form of niacin (Krishnamachari, 1974), and/or the nicotinic acid derivative N,Ndiethylnicotinamide (DENA), an important respiratory stimulant (Bigoli et al., 1972), the title compound was synthesized and its crystal structure is reported herein. ISSN 2056-9890

Structural commentary
The asymmetric unit of the crystal structure of the title salt, [Zn(C 8 H 4 , is composed of one complex cation and one 4-cyanobenzoate (CNB) counter-anion. The Zn II atom is coordinated by one 4-cyanobenzoate (CNB) anion, one nicotinamide (NA) ligand and three water molecules, the CNB anion and NA ligand coordinating in bidentate and monodentate modes, respectively ( Fig. 1).
In the cation, the four coordinating atoms (O1, O2, O5 and N2) around the Zn1 atom show a distorted square-planar arrangement, while the considerably distorted octahedral coordination environment of Zn II is completed by two additional water O atoms (O4 and O6) in the axial positions (

Synthesis and crystallization
The title compound was prepared by the reaction of ZnSO 4 Á7H 2 O (1.44 g, 5 mmol) in H 2 O (30 ml) and nicotina-mide (1.22 g, 50 mmol) in H 2 O (50 ml) with sodium 4-cyanobenzoate (1.69 g, 10 mmol) in H 2 O (100 ml). The mixture was filtered and set aside to crystallize at ambient temperature for several days, giving colourless single crystals.

Refinement
The experimental details including the crystal data, data collection and refinement are summarized in Table 3. Atoms H31 and H32 (as part of the NH 2 group) and H41, H42, H51, H52, H61 and H62 (as part of the water molecules) were located in a difference Fourier map and were refined freely. The aromatic C-bound 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).   Data collection: APEX2 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012) and PLATON (Spek, 2009). 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.

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