Crystal structure and Hirshfeld surface analysis of (C7H9N4O2)[ZnCl3(H2O)]

In the title molecular salt, (C7H9N4O2)[ZnCl3(H2O)], the crystal packing exhibits O—H⋯O, O—H⋯Cl, N—H⋯O and N—H⋯Cl hydrogen bonds.

In the title molecular salt, 1,3-dimethyl-2,6-dioxo-2,3,6,7-tetrahydro-1H-purin-9ium aquatrichloridozincate(II), (C 7 H 9 N 4 O 2 )[ZnCl 3 (H 2 O)], the fused ring system of the cation is close to planar, with the largest deviation from the mean plane being 0.037 (3) Å . In the complex anion, the Zn II cation is coordinated by three chloride ions and one oxygen atom from the water ligand in a distorted tetrahedral geometry. In the crystal, inversion dimers between pairs of cations linked by pairwise N-HÁ Á ÁO hydrogen bonds generate R 2 2 (10) rings. The anions are linked into dimers by pairs of O-HÁ Á ÁCl hydrogen bonds and the respective dimers are linked by O-HÁ Á ÁO and N-HÁ Á ÁCl hydrogen bonds. Together, these generate a three-dimensional supramolecular network. Hirshfeld surfaces were generated to gain further insight into the packing.

Chemical context
Theophylline, C 7 H 8 N 4 O 2 , is an alkaloid derivative of xanthine, containing a fused pyrimidine-imidazole ring system with conjugated double bonds. It has many biological and pharmacological properties (see, for example, Rao et al., 2005;Piosik et al., 2005). Various studies have shown that theophylline can be used as a medicine for the treatment of asthmatic bronchitis and chronic obstructive bronchitis (under several brand names), and as anticancer drugs (Nafisi et al. 2003;Rao et al. 2005;Piosik et al. 2005). Furthermore, theophylline complexes with transition metals can be used in anticancer drugs (David et al., 1999).
As part of our studies in this area, we reacted theophylline with ZnCl 2 under acid conditions to give the molecular salt (C 7 H 9 N 4 O 2 )Á[ZnCl 3 (H 2 O)] and its crystal structure is described herein.

Supramolecular features
The packing is consolidated by a network of hydrogen bonds (Table 1, Fig. 2). The cations are linked into inversion dimers by pairs of N1-H1Á Á ÁO2 hydrogen bonds, which generate R 2 2 (10) rings. The anions also form inversion dimers, being linked by pairwise O3-H3AÁ Á ÁCl3 hydrogen bonds. The anions are linked to the cations via O3-H3BÁ Á ÁO1 hydrogen bonds from the water molecule to a carbonyl group of the pyrimidine ring. Finally, the cations are linked to the anions via N2-H2Á Á ÁCl2 hydrogen bonds. Taken together, these hydrogen bonds generate a three-dimensional supramolecular network ( Fig. 3), which also features short ClÁ Á Á contacts [ClÁ Á Ácentroid distances in the range of 3.533 (2)-3.620 (2) Å ].
compounds are different from that of the title compound; however, the organic-inorganic moities are linked through hydrogen bonds in all of these structures.

Hirshfeld surface analysis
In order to gain further insight into the intermolecular interactions in the title compound, we used the program Crystal Explorer (Spackman & Jayatilaka, 2009), to consider separately the (C 7 H 9 N 4 O 2 ) + organic cation and the [ZnCl 3 (H 2 O)] À inorganic anion.

Synthesis and crystallization
ZnCl 2 Á6H 2 O (0.244 g, 1 mmol) was dissolved in 5 ml of water. Then, theophylline [C 7 H 8 N 4 O 2 ] (0.180 g, 1 mmol) was dissolved in 3 ml of ethanol/water (1:1 v:v) with a few drops of conc. HCl (37%). The two solutions were mixed and after two weeks, colourless crystals of the title molecular salt were obtained.

Figure 5
Hirshfeld d norm surface of the [ZnCl 3 (H 2 O)] À anion in the title compound.
1,3-Dimethyl-2,6-dioxo-2,3,6,7-tetrahydro-1H-purin-9-ium aquatrichloridozincate(II) Crystal data (C 7  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.