2,3-Diaminopyridinium 4-hydroxybenzoate

In the title compound, C5H8N3 +·C7H5O3 −, the pyridine N atom is protonated. In the 4-hydroxybenzoate anion, the carboxylate group is twisted slightly out of the benzene ring plane by an angle of 3.77 (5)°. The protonated N atom and one of the two amino groups are hydrogen-bonded to the 4-hydroxybenzoate anion through a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. The crystal structure is further stabilized by O—H⋯O and C—H⋯O hydrogen bonds and π–π interactions involving the pyridinium rings [centroid–centroid distance of 3.6277 (5) Å], leading to the formation of a three-dimensional network.

In the title compound, C 5 H 8 N 3 + ÁC 7 H 5 O 3 À , the pyridine N atom is protonated. In the 4-hydroxybenzoate anion, the carboxylate group is twisted slightly out of the benzene ring plane by an angle of 3.77 (5) . The protonated N atom and one of the two amino groups are hydrogen-bonded to the 4hydroxybenzoate anion through a pair of N-HÁ Á ÁO hydrogen bonds, forming an R 2 2 (8) ring motif. The crystal structure is further stabilized by O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds andinteractions involving the pyridinium rings [centroid-centroid distance of 3.6277 (5) Å ], leading to the formation of a three-dimensional network.
Pyridine and its substituted derivatives are often involved in hydrogen-bond interactions (Jeffrey & Saenger, 1991;Jeffrey, 1997;Scheiner, 1997). Since our aim is to study some interesting hydrogen-bonding interactions, the crystal structure of the title compound is presented here.
In the anion, the carboxylate group is twisted slightly away from the attached ring; the dihedral angle between C1-C6 and O2/O3/C7/C6 planes is 3.77 (5)°.
The amino groups (N2 and N3) are involved in N-H···O hydrogen bonding interactions to form a R 1 2 (7) ring motif. The hydroxyl group hydrogen atom is also hydrogen-bonded to the carboxylate oxygen atom through O-H···O hydrogen bonds.

Experimental
Hot methanol solutions (20 ml) of 2,3-diaminopyridine (27 mg, Aldrich) and 4-hydroxybenzoic acid (35 mg, Merck) were mixed and warmed over a heating magnetic stirrer for 5 minutes. The resulting solution was allowed to cool slowly at room temperature. Crystals of the title compound appeared from the mother liquor 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 Cyrosystems 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 > σ(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.