Bis(2-amino-4-methylpyridinium) terephthalate tetrahydrate

In the crystal structure of the title salt, 2C6H9N2 +·C8H4O4 2−·4H2O, the terephthalate carboxylate groups interacts with the 2-amino-4-methylpyridinium cations via a pair of N—H⋯O hydrogen bonds, forming an R 2 2(8) ring motif. The water molecules form an R 6 6(12) ring motif through O—H⋯O hydrogen bonds and these motifs are fused, forming a supramolecular chain along the c axis. The R 2 2(8) and R 6 6(12) ring motifs are connected via O—H⋯O hydrogen bonds. In addition, π–π stacking interactions are observed between the pyridinium rings [centroid–centroid distance = 3.522 (12) Å].

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and PLATON (Spek, 2009 Comment Supramolecular architectures assembled via various delicate noncovalent interactions such as hydrogen bonds, π-π stacking and electrostatic interactions, etc., have attracted intense interest in recent years because of their fascinating structural diversity and potential applications for functional materials (Desiraju, 2007;Corna et al., 2004). Especially, the application of intermolecular hydrogen bonds is a well known and efficient tool in the field of organic crystal design owing to its strength and directional properties (Aakeröy & Seddon, 1993). Pyridine and its derivatives play an important role in heterocyclic chemistry (Pozharski et al., 1997;Katritzky et al., 1996). They are often involved in hydrogen-bond interactions (Jeffrey & Saenger, 1991;Jeffrey, 1997;Scheiner, 1997). Terephthalic acid (H 2 TPA), a rod-like aromatic diacid, has often been used in the synthesis of metal-organic frameworks as a linker molecule (Serre et al., 2007;Mukherjee et al., 2004;Sun et al., 2000).
Recently, with the increase in interest in controlling the crystalline structures of organic-based solid-state materials, H 2 TPA is being increasingly employed in constructing supramolecular structures (Lynch & Jones, 2004;Spencer et al., 2004;Devi & Muthiah, 2007). Since our aim is to study some interesting hydrogen-bonding interactions, the crystal structure of the title compound is presented here.

Experimental
A hot methanol solution (20 ml) of 2-amino-4-methylpyridine (54 mg, Aldrich) and terephthalic acid (83 mg, Merck) were mixed and warmed over a heating magnetic stirrer hotplate for a few minutes. The resulting solution was allowed to cool slowly at room temperature and crystals of the title compound appeared after a few days. and were refined using a riding model, with U iso (H) = 1.2 or 1.5 U eq (C). A rotating group model was used for the methyl groups. In the absence of significant anomalous dispersion, 2680 Friedel pairs were merged before the final refinement. Fig. 1. The asymmetric unit of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems 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 > 2σ(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.