trans-Tetraaquabis(isonicotinamide-κN 1)zinc bis(3-hydroxybenzoate) tetrahydrate

The asymmetric unit of the title compound, [Zn(C6H6N2O)2(H2O)4](C7H5O3)2·4H2O, contains half of the complex cation with the ZnII ion located on an inversion center, a 3-hydroxybenzoate counter-anion and two uncoordinating water molecules. Four water O atoms in the equatorial plane around the ZnII ion [Zn—O = 2.089 (2) and 2.128 (2) Å] form a slightly distorted square-planar arrangement and the distorted octahedral geometry is completed by the two N atoms [Zn—N = 2.117 (2) Å] from two isonicotinamide ligands. In the anion, the carboxylate group is twisted from the attached benzene ring at 9.0 (2)°. In the crystal, a three-dimensional hydrogen-bonding network, formed by classical O—H⋯O and N—H⋯O and weak C—H⋯O hydrogen bonds, consolidates the crystal packing, which exhibits π–π stacking between the benzene and pyridine rings, with centroid–centroid distances of 3.458 (2) and 3.609 (2) Å. One of the two H atoms of each uncoordinating water molecule is disordered over two orientations with an occupancy ratio of 0.60:0.40.

The asymmetric unit of the title compound, [Zn(C 6 H 6 N 2 O) 2 -(H 2 O) 4 ](C 7 H 5 O 3 ) 2 Á4H 2 O, contains half of the complex cation with the Zn II ion located on an inversion center, a 3-hydroxybenzoate counter-anion and two uncoordinating water molecules. Four water O atoms in the equatorial plane around the Zn II ion [Zn-O = 2.089 (2) and 2.128 (2) Å ] form a slightly distorted square-planar arrangement and the distorted octahedral geometry is completed by the two N atoms [Zn-N = 2.117 (2) Å ] from two isonicotinamide ligands. In the anion, the carboxylate group is twisted from the attached benzene ring at 9.0 (2) . In the crystal, a threedimensional hydrogen-bonding network, formed by classical O-HÁ Á ÁO and N-HÁ Á ÁO and weak C-HÁ Á ÁO hydrogen bonds, consolidates the crystal packing, which exhibitsstacking between the benzene and pyridine rings, with centroid-centroid distances of 3.458 (2) and 3.609 (2) Å . One of the two H atoms of each uncoordinating water molecule is disordered over two orientations with an occupancy ratio of 0.60:0.40.
The title compound (I) is isostructural with the related Ni (Zaman et al., 2012a) and Co (Zaman et al., 2012b) complexes. In (I) (Fig. 1), four O atoms (O5, O6, and the symmetry-related atoms, O5′, O6′) in the equatorial plane around the Zn atom form a slightly distorted square-planar arrangement, while the slightly distorted octahedral coordination is completed by the two pyridine N atoms (N1, N1′) of the INA ligands at 2.117 (2) Å from the Zn atom in the axial positions (Fig. 1). The average Zn-O bond length is 2.108 (2) Å. The intramolecular O-H···O hydrogen bonds (Table 1)link the uncoordinated water molecules to the HB anion. The dihedral angle between the planar carboxylate group (O1/O2/C1) and the benzene ring A (C2-C7) is 9.0 (2)°, while that between rings A and B (N1/C8-C12) is In the crystal structure, intermolecular O-H···O, N-H···O and weak C-H···O hydrogen bonds (Table 1) link the molecules into a three-dimensional network, in which they may be effective in the stabilization of the structure. π···π Contacts between the benzene and phenyl rings, Cg1-Cg2 and Cg2-Cg1 i , [symmetry code: (i) 1 + x, y, z, where Cg1 and Cg2 are centroids of the rings A (C2-C7) and B (N1/C8-C12), respectively] may further stabilize the structure, with centroid-centroid distances of 3.609 (2) and 3.458 (2) Å, respectively.

Experimental
The title compound was prepared by the reaction of ZnSO 4 .H 2 O (0.89 g, 5 mmol) in H 2 O (100 ml) and INA (1.220 g, 10 mmol) in H 2 O (50 ml) with sodium 3-hydroxybenzoate (1.601 g, 10 mmol) in H 2 O (100 ml). The mixture was filtered and set aside to crystallize at ambient temperature for four weeks, giving colorless single crystals.

Refinement
Atoms H51, H52, H61, H62, H71, H72, H81 and H82 (for H 2 O), H21 and H22 (for NH 2 ) and H3A (for OH) were located in a difference Fourier map and refined isotropically. The C-bound H-atoms were positioned geometrically with C-H = 0.93 Å, for aromatic H-atoms, and constrained to ride on their parent atoms, with U iso (H) = 1.2U eq (C). During the refinement process the disordered H72A, H82A and H72B, H82B atoms were refined with occupancies ratios of 0.60:0.40. The highest residual electron density was found 1.38 Å from O6 and the deepest hole 0.83 Å from H61.

Figure 1
The molecular structure of the title molecule with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Primed atoms are generated by the symmetry operator: (′) -x, -y, -z. Only one of the disordered hydrogen atoms for each of the two uncoordinated water molecules is shown for clarity. Hydrogen bonds are shown as dashed lines.

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