Diaquabis(nitrato-κ2 O,O′)bis(pyrazine-2-carboxamide-κN 4)cadmium–pyrazine-2-carboxamide (1/2)

In the title compound, [Cd(NO3)2(C5H5N3O)2(H2O)2]·2C5H5N3O, the CdII cation is located on a twofold rotation axis and is coordinated by two pyrazine-2-carboxamide ligands and two water molecules and chelated by two nitrate anions in a distorted square-antiprismatic geometry. Extensive intermolecular N—H⋯O, N—H⋯N, O—H⋯O and O—H⋯N hydrogen bonds, as well as weak intermolecular C—H⋯N and C—H⋯O interactions occur in the crystal. π–π stacking between between pyrazine rings of coordinating ligands and lattice molecules [centroid–centroid distance = 3.5669 (14) Å] may further stabilize the structure.

The asymmetric unit of the title compound, (Fig. 1), contains one half of Cd II atom, one coordinated pyrazine-2carboxamide ligand, one water molecule, one nitrate anion and one none coordinated pyrazine-2-carboxamide. The Cd II atom is eight-coordinated by two N atoms from two pyrazine-2-carboxamide ligands, two O atoms from two water molecules and four O atoms from two nitrate anions. The Cd-O and Cd-N bond lengths and angles are collected in Table 1.

Experimental
A solution of pyrazine-2-carboxamide (0.50 g, 4.0 mmol) in methanol (10 ml) was added to a solution of Cd(NO 3 ) 2 .4H 2 O (0.31 g, 1.0 mmol) in methanol (10 ml) and the resulting colorless solution was stirred for 15 min at room temperature.
This solution was left to evaporate slowly at room temperature. After one week, colorless block crystals of the title compound were isolated (yield 0.56 g, 73.2%).

Figure 2
Unit-cell packing diagram for title molecule. 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.