Crystal structure of 4-(dimethylamino)pyridinium 4-aminobenzoate dihydrate

In the title hydrated molecular salt, C7H11N2 +·C7H6NO2 −·2H2O, the cation is protonated at the pyridine N atom and the dihedral angle between the benzene ring and the CO2 − group in the anion is 8.5 (2)°. In the crystal, the cation forms an N—H⋯O hydrogen bond to the anion and the anion forms two N—H⋯O hydrogen bonds to adjacent water molecules. Both water molecules form two O—H⋯O hydrogen bonds to carboxylate O atoms. In combination, these hydrogen bonds generate a three-dimensional network and two weak C—H⋯π interactions are also observed.


S1. Chemical context
We hereby report the synthesis and crystal structure of the title compound (I), prepared by the reaction of 4-dimethylaminopyridine with 4-aminobenzoic acid in distilled water as solvent.

S3. Supramolecular features
In the crystal, the medium-strength N-H···O and O-H···O hydrogen bonds connect the adjacent anions and cations, involving water molecules into three dimensional framework (Table 2 & Fig. 2). The crystal structure also features weak C-H···π (Table 2) interactions.

S4. Synthesis and crystallization
4-Dimethylaminopyridine (C 7 H 10 N 2 , 1.9704 g) and 4-aminobenzoic acid (C 7 H 7 NO 2 , 2.2119 g) were taken in the equimolar ratio and synthesized in distilled water and prepared solution was allowed for slow evaporation at room temperature. Colourless blocks were collected after 20 days.

S5. Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The C-bound H atoms were positioned geometrically and refined using riding model, with C-H = 0.93 and 0.97 Å for CH aromatic and CH 3 , respectively, with U iso (H) = 1.2Ueq(C) or 1.5Ueq(C). The H atoms bound to O and N atoms were found in a difference map and refined isotropically, with U iso (H) = 1.5Ueq (  The molecular structure of (I), with 30% probability displacement ellipsoids for non-H atoms.

Figure 2
The packing of (I), viewed down a axis. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.29 e Å −3 Δρ min = −0.23 e Å −3 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.