Crystal structure of bis[4-(dimethylamino)pyridinium] bis(2-nitrobenzoate) trihydrate

The title salt crystallized with two anions and two cations in the asymmetric unit, together with three water molecules. In the crystal, the anions are linked via O—H⋯O hydrogen bonds, involving the water molecules, forming chains along [100] and the cations are linked to these chains by N—H⋯O hydrogen bonds.

The title salt, 2C 7 H 11 N 2 + Á2C 7 H 4 NO 4 À Á3H 2 O, crystallized with two anions and two cations in the asymmetric unit, together with three water molecules. Both 4-dimethylaminopyridinium cations are protonated at their pyridine N atoms with the plane of the N(CH 3 ) 2 hetero atoms inclined to the pyridine ring by 4.5 (2) and 1.4 (2) . In the 2-nitrobenzoate anions, the carboxyl and nitro groups are inclined to their respective benzene rings by 77.1 (3) and 20.0 (3) , and 75.8 (2) and 20.9 (3) . In the crystal, the anions are linked via O-HÁ Á ÁO hydrogen bonds involving the water molecules, forming chains along [100]. The cations are linked to these chains by N-HÁ Á ÁO hydrogen bonds. The chains are linked via C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á andinteractions [inter-centroid distances range from 3.617 (1) to 3.851 (1) Å ], forming a threedimensional structure.

Chemical context
Pyridine derivatives are used as calcium channel blockers and antagonists, and exhibit biological activities such as fungicidal, antibacterial, antifungal, antimycotic (Bossert et al., 1981;Lohaus & Dittmar, 1968;Wang et al.,1989). Benzene derivatives are extensively used in medicinal chemistry as important intermediates for many pharmaceutical products (Altmann et al., 2004). We herein report on the synthesis and crystal structure of the title salt prepared by the reaction of 4-dimethylaminopyridine with 2-nitrobenzoic acid in hot ethanol as solvent.

Figure 2
A view along the a axis of the crystal packing of the title salt. Hydrogen bonds are shown as dashed lines (see Table 1 for details; H atoms not involved in hydrogen bonding have been omitted for clarity).  Hosomi et al., 2000). They were studied for their potential SHG properties; only the 3,5-dintrobenzoate salt crystallized in a non-centrosymmetric space group.

Synthesis and crystallization
4-Dimethylaminopyridine (2.442 g, 1 mmol) and 2-nitrobenzoic acid (3.342 g, 1mmol) were dissolved in 50 ml of hot ethanol as a solvent. The mixture was stirred well for 8 h to give a homogeneous solution and is was then allowed to evaporate in air at room temperature. Within a few days, small colourless block-like crystals of the title salt were formed.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.