2,3-Diaminopyridinium 4-nitrobenzoate

In the title salt, C5H8N3 +·C7H4NO4 −, the pyridine N atom of the 2,3-diaminopyridine molecule is protonated. The protonated N atom and one of the two 2-amino groups are hydrogen bonded to the 4-nitrobenzoate anion through a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. The carboxylate mean plane of the 4-nitrobenzoate anion is twisted by 3.77 (5)° from the attached ring and the nitro group is similarly twisted by 2.28 (10)°. In the crystal, the molecules are linked by N—H⋯O and C—H⋯O interactions into sheets parallel to (100).

In the title salt, C 5 H 8 N 3 + ÁC 7 H 4 NO 4 À , the pyridine N atom of the 2,3-diaminopyridine molecule is protonated. The protonated N atom and one of the two 2-amino groups are hydrogen bonded to the 4-nitrobenzoate anion through a pair of N-HÁ Á ÁO hydrogen bonds, forming an R 2 2 (8) ring motif. The carboxylate mean plane of the 4-nitrobenzoate anion is twisted by 3.77 (5) from the attached ring and the nitro group is similarly twisted by 2.28 (10) . In the crystal, the molecules are linked by N-HÁ Á ÁO and C-HÁ Á ÁO interactions into sheets parallel to (100).
In the crystal packing, Fig. 2, the protonated N1 atom and the 2-amino group (N2) is hydrogen-bonded to the carboxylate oxygen atoms (O3 and O4) via a pair of N-H···O hydrogen bonds forming a ring motif, R 2 2 (8) (Bernstein et al., 1995). The 2-amino groups (N2 and N3) are involved in N-H···O3 hydrogen bonding interactions to form a R 1 2 (7) ring motif. One of the amino group hydrogen atoms, H2N3, and the ring hydrogen atom, H10A, are connected to the 4-nitro group oxygen atoms (O1 and O2) to form an R 2 2 (8) ring motif (Table 1 and Fig. 2). These molecules are linked by these interactions into sheets parallel to (100). The crystal structure is further stabilized by a π-π stacking interactions between the aminopyridine-and carboxylate-rings with centroid-to-centroid distances of 3.8343 (10) Å.

Experimental
Hot methanol solutions (20 ml) of 2,3-diaminopyridine (27 mg, Aldrich) and 4-nitrobenzoic acid (42 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 (I) appeared from the mother liquor after a few days.

Refinement
All the H atoms were located from the difference and allowed to refine freely. In the absence of significant anomalous scattering effects, 2144 Friedel pairs were merged.

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.  (3) 159 (2)