2-Amino-4-methylpyridinium 6-carboxypyridine-2-carboxylate sesquihydrate

In the title compound, C6H9N2 +·C7H4NO4 −·1.5H2O, extensive O—H⋯O, O—H⋯N, N—H⋯O and C—H⋯O hydrogen bonds, as well as ion pairing, π–π stacking interactions [centroid–centroid distances = 3.4690 (8) and 3.6932 (8) Å between aromatic rings] occur in the crystal. There are hydrogen-bonding interactions between water molecules, which result in cyclic tetrameric water clusters. One of the water O molecules has half occupancy. In the anion molecules, the –CO2 and –CO2H groups make torsion angles of 1.73 (18) and −12.14 (18)° with respect to the ring.


Comment
The presence of water is important in establishing H-bonded contributions to the total lattice energy, and is significant in establishing the stability of the hydrated crystal structures (Long et al., 2004). Several water clusters found in organic or metallo-organic crystal hosts have been structurally characterized (Atwood et al., 2001). A detailed understanding of the numerous possible structures and stability of isolated water clusters in diverse surroundings can help us understand the nature of water-water interactions in bulk water or ice. In this paper, we report the synthesis and crystal structure of the title proton transfer system, (I), derived from pyridine-2,6-dicarboxylic acid (pydcH 2 ) and 2-amino-4-methylpyridine (2a4mp).
In the title compound, the asymmetric unit contains a cation, (2a4mpH) 2+ , an anion, (pydcH)and 1.5 water molecules ( Fig. 1). The bond distances and bond angles in the title compound are in agreement with the corresponding distances and angles reported in some related crystal structures (Aghabozorg et al., (2008); Tabatabaee et al., (2009). In the crystal structure, the cations and the anions are linked by hydrogen bonds (Tab. 1 and Fig. 2). In the structure, water molecules form cyclic tetrameric water clusters (Tab. 1 and Fig. 3) in the most stable pattern (Miyake & Aida, 2003). The clusters play a bridging role (Fig. 2), linking the adjacent cations and anions via hydrogen bonds and contributing to the formation of an extensive supramolecular structure.

Experimental
An aqueous solution of 2a4mp (324 mg, 3 mmol) in water (10 ml) was added to a stirring solution of pydcH 2 (501 mg, 3 mmol) in water (10 ml). The reaction mixture was stirred at 298 K for 2h. Colorless crystals of the title compound were obtained by slow concentration of the solution at room temperature.

Refinement
One of the water molecules (O2W) has 0.5 occupancy factor. The hydrogen atoms of OH, NH and NH 2 groups and water molecules were found in difference Fourier synthesis. The H-atoms of OH, NH and NH 2 groups were refined in isotropic approximation. The rest of the H-atoms were refined in riding model with C-H = 0.95 and 0.98 Å for aryl and methyl H-atoms and O-H = 0.85 Å for water molecules. The U iso (H) parameters were 1.2 U eq (C aryl /O) and 1.5 U eq (C methyl ).
supplementary materials sup-2 Figures Fig. 1. The asymmetric unit of (I), showing the atom numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.