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

The asymmetric unit of the title salt, C6H9N2 +·C7H5O6S−, contains two crystallographically independent 2-amino-5-methylpyridinium cations and two sulfosalicylate anions. In the crystal structure, the sulfonate group of each 3-carboxy-4-hydroxybenzenesulfonate anion interacts with the corresponding 2-amino-5-methylpyridinium cation via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. The ionic units are linked by N—H⋯O, O—H⋯O and C—H⋯O hydrogen bonds. Furthermore, the crystal structure is stabilized by π–π interactions between the benzene and pyridine rings [centroid–centroid distances = 3.5579 (8) and 3.8309 (8) Å]. There are also intramolecular O—H⋯O hydrogen bonds in the anions, which generate S(6) ring motifs.


Comment
Weak interactions, such as hydrogen bonding and π-π stacking, have attracted much interest as a result of their significance in chemistry and biology, especially in the field of crystal engineering (Moghimi et al., 2002;Aghabozorg et al., 2005). 5-Sulfosalicylic acid (3-carboxy-4-hydroxybenzenesulfonic acid), is a particularly strong organic acid which is capable of protonating N-containing heterocycles and other Lewis bases (Smith et al.;2004, Raj et al., 2003Muthiah et al., 2003;Wang & Wei, 2007). As part of our research programme aiming to gain further insight into hydrogen-bonding interactions involving 2-amino-5-methylpyridine and 3-carboxy-4-hydroxybenzenesulfonic acid, the present work has been undertaken.

Experimental
A hot methanol solution (20 ml) of 2-amino-5-methylpyridine (27 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.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
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.