2-Amino-5-methylpyridinium 2-carboxybenzoate

In the title salt, C6H9N2 +·C8H5O4 −, the hydrogen phthalate anion is essentially planar, with a maximum deviation of 0.011 (2) Å. In the crystal structure, the protonated N atom of the pyridine ring and the 2-amino group of the cation are hydrogen bonded to the carboxylate O atoms of the anion via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. In the hydrogen phthalate anion, there is a very strong, almost symmetric, intramolecular O—H⋯O hydrogen bond, generating an S(7) motif [O⋯O = 2.382 (3) Å]. Furthermore, these two molecular motif rings are connected by a bifurcated N—H⋯(O,O) hydrogen-bonded motif R 1 2(4), forming a supramolecular ribbon along the b axis. The crystal structure is further stabilized by π–π interactions between the cations and anions [centroid–centroid distance = 3.6999 (10) Å].

In the title salt, C 6 H 9 N 2 + ÁC 8 H 5 O 4 À , the hydrogen phthalate anion is essentially planar, with a maximum deviation of 0.011 (2) Å . In the crystal structure, the protonated N atom of the pyridine ring and the 2-amino group of the cation are hydrogen bonded to the carboxylate O atoms of the anion via a pair of N-HÁ Á ÁO hydrogen bonds, forming an R 2 2 (8) ring motif. In the hydrogen phthalate anion, there is a very strong, almost symmetric, intramolecular O-HÁ Á ÁO hydrogen bond, generating an S(7) motif [OÁ Á ÁO = 2.382 (3) Å ]. Furthermore, these two molecular motif rings are connected by a bifurcated N-HÁ Á Á(O,O) hydrogen-bonded motif R 1 2 (4), forming a supramolecular ribbon along the b axis. The crystal structure is further stabilized byinteractions between the cations and anions [centroid-centroid distance = 3.6999 (10) Å ].

Comment
The crystal structure of phthalic acid (Nowacki & Jaggi, 1957;Küppers, 1981;Ermer, 1981) has been reported several times. Analysis of the structures archived in the Cambridge Structural Database (Version 5.28; Allen, 2002) shows that the hydrogen phthalate ions of phthalate salts occur in two different forms: (i) non-planar, where both the carboxyl (COOH) and the carboxylate (COO -) groups are twisted out of the plane of the benzene ring (Jessen, 1990;Jin et al., 2003), and (ii) planar, in which both the COOH and the COOgroups are coplanar with the benzene plane (Küppers, 1978). Since our aim is to study some interesting hydrogen-bonding interactions, the crystal structure of the title compound is presented here.

Experimental
A hot methanol solution (20 ml) of 2-amino-5-methylpyridine (27 mg, Aldrich) and phthalic acid (41 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 H1N1, H2N2, H1N2 and H1O1 were located from a difference Fourier map and were refined freely [N-H= 0.90

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.