2-Amino-5-chloropyridinium 3-carboxy-4-hydroxybenzenesulfonate

The asymmetric unit of the title salt, C5H6ClN2 +·C7H5O6S−, contains two independent 2-amino-5-chloropyridinium cations and two independent 3-carboxy-4-hydroxybenzenesulfonate anions. In both anions, the O atoms of the sulfonate group are disordered over two sets of positions, with occupancy ratios of 0.47 (5):0.53 (5) and 0.50 (8):0.50 (8). In each anion, an intramolecular O—H⋯O hydrogen bond generating an S(6) motif is observed. In the crystal structure, the cations and anions are linked via N—H⋯O, O—H⋯O and C—H⋯O hydrogen bonds, forming a two-dimensional network parallel to (110). The structure is further stabilized by π–π interactions between cations and anions [centroid–centroid distance = 3.5454 (12) Å]. The crystal studied was a non-merohedral twin, with a ratio of the twin components of 0.715 (3):0.285 (3).


Comment
Intermolecular interactions are responsible for crystal packing and gaining an understanding of them allows us to comprehend collective properties and permits the design of new crystals with specific physical and chemical properties (Lam & Mak, 2000). 5-Sulfosalicylic acid (3-carboxy-4-hydroxybenzenesulfonic acid) and its organic complexes or salts can develop well-defined non-covalent supramolecular architectures because of their ability to form multiple hydrogen bonds containing components of complementary arrays of hydrogen-bonding sites (Smith et al., 2004;Muthiah et al., 2003;Raj et al., 2003;Fan et al., 2005). The present study has been undertaken to study the hydrogen bonding patterns involving the 3-carboxy-4-hydroxybenzenesulfonate anions with the 2-amino-5-chloropyridinium cations.
In the crystal structure, (Fig. 2), the sulfonate group of each 3-carboxy-4-hydroxybenzenesulfonate anion interacts with the corresponding 2-amino-5-chloropyridinium cations via a pair of N-H···O hydrogen bonds forming an R 2 2 (8) ring motif (Bernstein et al., 1995). Here, sulfonate groups mimic the role of the carboxylate groups. The ionic units are further linked by N-H···O, O-H···O and C-H···O (Table 1) hydrogen bonds, forming a two dimensional network parallel to the (110) plane.

Experimental
A hot methanol solution (20 ml) of 2-amino-5-chloropyridine (32 mg, Aldrich) and sulfosalicylic acid (54 mg, Merck) were mixed and warmed over a heating magnetic stirrer hotplate for a few minutes. The resulting solution was allowed to cool slowly at room temperature and crystals of the title compound appeared after a few days.

Refinement
Atoms H2OA, H2OB, H1OA, H1OB, H1NA and H1NB were located in a difference Fourier map and refined freely. The

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.