2-Aminoanilinium 6-carboxypicolinate monohydrate

In the title compound, C6H9N2 +·C7H4NO4 −·H2O, one amino group of diaminobenzene is protonated while one carboxy group of pyridine-2,6-dicarboxylic acid is deprotonated. In the anion, the CO2 and CO2H groups make dihedral angles of 4.0 (5) and 8.7 (4)° with the pyridine ring. In the crystal, extensive N—H⋯O, N—H⋯N and O—H⋯O hydrogen bonds occur between anions, cations and water molecules.

In the title compound, C 6 H 9 N 2 + ÁC 7 H 4 NO 4 À ÁH 2 O, one amino group of diaminobenzene is protonated while one carboxy group of pyridine-2,6-dicarboxylic acid is deprotonated. In the anion, the CO 2 and CO 2 H groups make dihedral angles of 4.0 (5) and 8.7 (4) with the pyridine ring. In the crystal, extensive N-HÁ Á ÁO, N-HÁ Á ÁN and O-HÁ Á ÁO hydrogen bonds occur between anions, cations and water molecules.

Comment
Cocrystals are most commonly thought of as structural homogeneous crystalline materials that contain two or more organic building blocks that are present in definite stoichiometric amounts. Within the development of pharmaceutical industry, molecular cocrystals are becoming increasingly important as a new drug with higher biomedical activity than the initial components. (Kapildev et al. 2011). Physicochemical properties such as the melting point, stability and solubility of an active pharmaceutical ingredient can be tuned through cocrystal formulation (Andre, et al. 2011;Blagden, et al. 2008;Smith, et al. 2000). These cocrystal forms often relie on the acid-amide H-bonds interactions. Herein, we report the crystal structure of the title compound, 2-aminoanilinium 6-carboxypicolinate monohydrate.
The asymmetric unit is composed of one 6-carboxypicolinate anion one 2-aminoanilinium cation and one water molecule

Experimental
A mixture of pyridine-2,6-dicarboxylic acid (2.0 mmol), benzene-1,2-diamine (2.0 mmol) and 40 ml water were added into a 100 ml flask and refluxed for 5 h, then cooled and filtrated. The solution was evaporated slowly in the air. Colorless block crystals suitable for X-ray analysis were obtained after one week.

Refinement
All H atoms attached to C atoms were fixed geometrically and treated as riding with C-H = 0.93 Å (aromatic) with U iso (H) = 1.2U eq (C). The H atoms bonded to N1, N2, O1W and O4 were located in a difference Fourier map, in the last stage of the refinement they were restrained with the H-N2 = 0.90, H-N1 = 0.89 and H-O = 0.82 Å. U iso (H)=1.5U eq (N1,O1W,O4) and U iso (H) = 1.2U eq (N2). As no significant anomalous scatterings, Friedel pairs were merged.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
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 > 2sigma(F 2 ) is used only for calculating R-factors(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.