3,4-Diaminopyridinium 4-nitrobenzoate–4-nitrobenzoic acid (1/1)

In the title compound, C5H8N3 +·C7H4NO4 −·C7H5NO4, the non-H atoms of the 3,4-diaminopyridinium cation are coplanar, with a maximum deviation of 0.022 (1) Å. The carboxylate and nitro groups of the 4-nitrobenzoate anion are twisted out of the attached ring planes by dihedral angles of 15.89 (8) and 10.20 (8)°, respectively. In the 4-nitrobenzoic acid molecule, the carboxyl and nitro groups form dihedral angles of 18.25 (8) and 6.55 (8)°, respectively, with the benzene ring. In the crystal, the constituent units form two-dimensional networks parallel to (001) by O—H⋯O, N—-H⋯O and C—H⋯O hydrogen bonds. Weak π–π interactions involving inversion-related 4-nitrobenzoic acid molecules [centroid–centroid distance = 3.7325 (8) Å] and inversion-related 4-nitrobenzoate molecules [centroid–centroid distance = 3.7124 (8) Å] are also observed.

The 4-nitrobenzoic acid hydrogen, H1O3, is hydrogen-bonded to the carboxylate oxygen atom of 4-nitrobenzoate through O-H···O bonds. The 4-nitrobenzoic acid carbon atoms (C1B & C4B) are involved in C-H···O hydrogen bonding with the carboxylic acid and nitro group O atoms O4B and O1B, to form an R 2 2 (10) ring motif (Bernstein et al., 1995). The O-H···O, N-H···O and C-H···O hydrogen bonds (Table 1) link all the constituent units to form a two-dimensional network parallel to the (001). The crystal structure is further stabilized by π-π interactions. The inversion related 4-nitrobenzoic acid molecules are stacked with a centroid-to-centroid distance of 3.7325 (8) Å. Similarly, the inversion related 4-nitrobenzoate molecules are stacked with a centroid-to-centroid distance of 3.7124 (8) Å.

supplementary materials sup-2 Experimental
Hot methanol solutions (20 ml) of 3,4-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 (except carboxyl oxygen) were located from the difference Fourier map [N-H = 0.89 (2)

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.