2-Amino-5-chloropyridinium 4-hydroxybenzoate

In the title salt, C5H6ClN2 +·C7H5O3 −, the carboxylate mean plane of the 4-hydroxybenzoate anion is twisted by 7.16 (9)° from the attached ring. In the crystal structure, the cations and anions are linked via O—H⋯O and N—H⋯O hydrogen bonds, as well as C—H⋯O contacts, forming a three-dimensional network. In addition, weak π–π interactions involving the benzene and pyridinium rings, with centroid-to-centroid distances of 3.8941 (9) Å, are observed.

In the crystal packing ( Fig. 2), the protonated N1 atom and the N2-amino group is hydrogen-bonded to the carboxylate oxygen atoms (O2 and O3) via a pair of N-H···O hydrogen bonds forming a R 2 2 (8) ring motif (Bernstein et al., 1995). The hydroxyl hydrogen atom is also hydrogen-bonded to the carboxylate oxygen atom through an O-H···O hydrogen bond.

Experimental
A hot methanol solution (20 ml) of 2-amino-5-chloropyridine (65 mg, Aldrich) and 4-hydroxybenzoic acid (69 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
All the H atoms were located in a difference Fourier map and allowed to refine freely

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.