2-Amino-6-methylpyridinium 4-methylbenzenesulfonate

In the asymmetric unit of the title salt, C6H9N2 +·C7H7O3S−, there are two independent 2-amino-6-methylpyridinium cations and two independent 4-methylbenzenesulfonate anions. Both cations are protonated at their pyridine N atoms and their geometries reveal amine–imine tautomerism. In the 4-methylbenzenesulfonate anions, the carboxylate groups are twisted out of the benzene ring planes by 88.4 (1) and 86.2 (2)°. In the crystal, the sulfonate O atoms of an anion interact with the protonated N atoms and the 2-amino groups of a cation via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. These motifs are connected via N—H⋯O hydrogen bonds, forming chains running along the a-axis direction. Within the chains there are weak C—H⋯O hydrogen bonds present. In addition, aromatic π–π stacking interactions [centroid–centroid distances = 3.771 (2), 3.599 (2), 3.599 (2) and 3.497 (2) Å] involving neighbouring chains are also observed.

In the asymmetric unit of the title salt, C 6 H 9 N 2 + ÁC 7 H 7 O 3 S À , there are two independent 2-amino-6-methylpyridinium cations and two independent 4-methylbenzenesulfonate anions. Both cations are protonated at their pyridine N atoms and their geometries reveal amine-imine tautomerism. In the 4-methylbenzenesulfonate anions, the carboxylate groups are twisted out of the benzene ring planes by 88.4 (1) and 86.2 (2) . In the crystal, the sulfonate O atoms of an anion interact with the protonated N atoms and the 2-amino groups of a cation via a pair of N-HÁ Á ÁO hydrogen bonds, forming an R 2 2 (8) ring motif. These motifs are connected via N-HÁ Á ÁO hydrogen bonds, forming chains running along the a-axis direction. Within the chains there are weak C-HÁ Á ÁO hydrogen bonds present. In addition, aromaticstacking interactions [centroid-centroid distances = 3.771 (2), 3.599 (2), 3.599 (2) and 3.497 (2) Å ] involving neighbouring chains are also observed.

Introduction
2-Aminopyridine and its derivatives play an important role in heterocyclic chemistry. Pyridine heterocycles and their derivatives are present in many large molecules having photo-chemical, electro-chemical and catalytic applications (Babu et al., 2014). Simple organic-inorganic salts containing strong intermolecular hydrogen bonds have attracted an attention as materials which display ferroelectric-paraelectric phase transitions (Sethuram, et al., 2013a,b;Huq et al., 2013;Shihabuddeen Syed et al., 2013;Showrilu et al., 2013). Hydrogen-bonding patterns involving sulfonate groups in biological systems and metal complexes are of current interest (Onoda et al., 2001). Such interactions can be utilized for designing supramolecular architectures (Baskar Raj et al., 2003). Benzenesulfonic acid, is a particularly strong organic acid which is capable of protonating N-containing heterocycles and other Lewis bases (Wang & Wei, 2007). We have recently reported the crystal structures of 2-amino-6-methylpyridinium 2,2,2-trichloroacetate (Babu et al., 2014) and 2-Amino-5-nitropyridinium hydrogen oxalate (Rajkumar et al., 2014). In continuation of our studies of pyridinium derivatives, the crystal structure determination of the title compound has been undertaken.

Comment / Result and Discussion
The asymmetric unit of title salt, Fig. 1, consists of two crystallographically independent protonated 2-amino-6-methylpyridinium cation and two crystallographically independent 4-methyl benzenesulfonate anions. The normal probability plot analyses (International Tables for X-ray Crystallography , 1974, Vol. IV, pp. 293-309) for both bond lengths and angles show that the differences between the two symmetry independent molecules are of a statistical nature. All bond lengths (Allen et al., 1987) and angles are within normal ranges and comparable with those in closely related structures (Babu et al., 2014;Rajkumar et al., 2014). A proton transfer from the carboxyl group of p-toluenesulfonic acid to atom N1 and N3 of 2-amino-6-methyl pyridine resulted in the formation of a salt. This protonation lead to the widening of the C8-N1-C12 and C21-N3-C25 angles of the pyridine rings to 124.0 (2) ° and 123.8 (2) °, compared to 115.3 (2) ° in the unprotonated aminopyridine (Anderson et al., 2005). This type of protonation is observed in various aminopyridine acid complexes (Babu et al., 2014;Rajkumar et al., 2014).
In the cation, the N2-C8 [1.325 (2)  in the amino-methylprydinium cation (Babu et al., 2014;Rajkumar et al., 2014). In contrast, in the solid state structure of amino-methylpyridinium, the N-C bond out of ring is clearly longer than that in the ring (Nahringbauer et al., 1977).
The examination of pyridinium rings shows that these units are planar with mean deviation of -0.006 (2) and 0.005 (2) Å for atoms C8 and C21, from the mean planes defined by the six constituent atoms. The dihedral angle between the 2amino-6-methylpyridinium cation and 4-methylbenzenesulfonate anion group is 88.4 (2) and 86.2 (2)° for the both molecules, respectively. In both the molecules, the protonated 2-amino-6-methylpyridinium cation is essentially planar, with maximum deviations of -0.012 (2) for atom C13 and -0.006 (2) Å for atom C25.

Hydrogen bonding interaction / Intermolecular N-H···O and C-H···O interaction
In the crystal (Fig. 2), the protonated atoms (N1 and N3) and a nitrogen atom of the 2-amino groups (N2 and N4) of the 2-amino-6-methylpyridinium cations are hydrogen bonded to the carboxylate oxygen atoms (O1, O2, O3 and O4) of the sulfonate groups of the p-toluenesulfonate anions via a pair of intermolecular N-H···O hydrogen bonds (Table 1), forming a ring motif with a graph-set notation of R 2 2 (8) [Etter, 1990;Bernstein et al., 1995]. The sulfonate group mimics the carboxylate anion's mode of association, which is more commonly seen when binding with 2-aminopyrimidines. It is well known that sulfonates imitate carboxylates in forming such bidentate motifs (Baskar Raj et al., 2003).
Furthermore, these motifs are connected via N-H···O hydrogen bonds ( Fig. 2 and Table 1), involving the 2-amino group of the 2-amino-6-methyl pyridinium cation and atoms O3 and O4 of an anion, to form a supramolecular chains along the a axis direction. Weak C-H···O hydrogen bonds, involving a pyridine group of the cation and an O atom of a sulfonate anion, within the chains are also observed ( Fig. 2 and Table 1).

Uses
The identification of such supramolecular patterns will help us design and construct preferred hydrogen bonding patterns of drug like molecules.

Synthesis and crystallization
Crystals of the title compound were obtained by slow evaporation of a 1:1 equimolar mixture of 2-amino-6-methylpyridine and benzenesulfonic acid in methanol at room temperature.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. N-bound H atoms were located in a difference Fourier map and refined with distance restraints: N-H = 0.88 (1) and 0.90 (1) Å for NH 2 and NH H atoms, respectively. The C-bound H atoms were positioned geometrically and refined using a riding model: C-H = 0.93-0.96 Å with U iso (H) = 1.5U eq (C-methyl) and = 1.2U eq (C) for other H atoms. A rotating group model was used for the methyl group.  A view of the molecular structure of the two independent benezesulfonate anions and the two independent 2-amino-6methylpyridinium cations of the title salt. Displacement ellipsoids are drawn at the 50% probability level.

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.