2-Amino-5-methylpyridinium 4-nitrobenzoate

In the title compound, C6H9N2 +·C7H4NO4 −, the nitro group of the 4-nitrobenzoate anion is twisted by 6.2 (2)° from the attached ring. In the crystal structure, the cations and anions are linked via strong N—H⋯O and weak C—H⋯O hydrogen bonds, forming a three-dimensional network.

In the title compound, C 6 H 9 N 2 + ÁC 7 H 4 NO 4 À , the nitro group of the 4-nitrobenzoate anion is twisted by 6.2 (2) from the attached ring. In the crystal structure, the cations and anions are linked via strong N-HÁ Á ÁO and weak C-HÁ Á ÁO hydrogen bonds, forming a three-dimensional network.
They are often involved in hydrogen-bond interactions (Jeffrey & Saenger, 1991;Jeffrey, 1997;Scheiner, 1997). We have recently reported the crystal structure of 2-amino-4-methylpyridinium 4-nitrobenzoate (Hemamalini & Fun, 2010). In a continuation of our studies of pyridinium derivatives, the crystal structure of title compound is presented here.
Bifurcated hydrogen bonds are observed between the carboxylate oxygen atoms (O3 & O4) and the protonated N atom to form a four-membered R 1 2 (4) hydrogen-bonded ring. The crystal structure is further stabilized by weak C-H···O (Table   1) hydrogen bonds to form a three-dimensional network.

Experimental
A hot methanol solution (20 ml) of 2-amino-5-methylpyridine (27 mg, Aldrich) and 4-nitrobenzoic acid (42 mg, Merck) were mixed and warmed over a heating magnetic stirrer 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
The methyl H atoms were positioned geometrically and were refined using a riding model, with U iso (H) = 1.5U eq (C). A  Fig. 1. The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

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 s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The 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 > 2σ(F 2 ) is used only for calculating Rfactors(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.