4-[3-(Pyridin-4-yl)propyl]pyridinium 2-carboxybenzoate

In the title molecular salt, C13H15N2 +·C8H5O4 −, the 2-carboxybenzoate anions are joined into a chain along [010] by strong O—H⋯O hydrogen bonds, with the H atoms disordered about the intervening centres of inversion. The presence of N—H⋯O hydrogen bonds between cations generates an additional chain along [010] and parallel to that of the anions. The chains are assembled into a three-dimensional framework via weak C—H⋯O interchain interactions. In the cation, thee dihedral angle between the pyridine rings is 48.91 (4)°.

In the title molecular salt, C 13 H 15 N 2 + ÁC 8 H 5 O 4 À , the 2carboxybenzoate anions are joined into a chain along [010] by strong O-HÁ Á ÁO hydrogen bonds, with the H atoms disordered about the intervening centres of inversion. The presence of N-HÁ Á ÁO hydrogen bonds between cations generates an additional chain along [010] and parallel to that of the anions. The chains are assembled into a threedimensional framework via weak C-HÁ Á ÁO interchain interactions. In the cation, thee dihedral angle between the pyridine rings is 48.91 (4) .

Experimental
Data collection: RAPID-AUTO (Rigaku, 1998); cell refinement: RAPID-AUTO; data reduction: CrystalStructure (Rigaku/MSC, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97. Co-crystals have been proven particularly successful as functional materials with applications in pharmaceuticals, molecular electronics, optical applications, and synthetic organic chemistry (Schultheiss & Newman 2009;Sarma et al., 2011). For any given two chemical partners it is always possible to obtain more than one crystalline solid due to the differences in stoichiometries or supramolecular synthons (Callear et al., 2010). The idea of engineering co-crystals serves the purpose of building large solid-state structures without the hassles of covalent synthesis. The synthon that is formed between carboxylic acids and pyridine moieties is one of the most exploited synthon for designing co-crystals (Braga et al., 2011). In this contribution, we present the crystal structure of the phthalic acid and 1,3-bis(4-pyridyl)propane (bpp) co-crystal.
In the title compound the bppH + cation lies on a mirror plane while the 2-carboxybenzoate anion lies on a two-fold axis ( Fig. 1). The anions are linked into chains parallel to the [010] direction by strong O-H···O hydrogen bonds with an O···O distance of 2.378 (2) Å and with the H atom disordered about the intervening inversion centre. The bppH + cations engage in N-H···O hydrogen bonds to forms chains extending along the b axis (Fig. 2). Weak C-H···O hydrogen bond interactions between the cationic and anionic chains are responsible for the three-dimensional framework assembly (Table 1).

Experimental
1:1 molar quantities of phthalic acid (0.166 g, 1 mmol) and 1,3-bis(4-pyridyl)propane (0.198 g, 1 mmol) were dissolved in a water-methanol (1:1) mixture (15 mL) and the solution stirred for 10 min. After slow evaporation of the solution for one week at room temperature, colorless block crystals suitable for X-ray diffraction were obtained.

Refinement
All H atoms were located in a difference map. Those attached to C and N were adjusted to give C-H = 0.97 -0.98 Å and N-H = 0.89 Å and included as riding contributions with U iso (H) = 1.2U eq (C, N). Careful inspection of a difference map in the region between the oxygen atoms of the anions flanking the inversion centre indicated a significant elongation of the density along the line joining the two oxygen atoms suggesting a disorder of this hydrogen about the centre. This atom was placed in the best location indicated by the difference map (O-H = 0.86 Å) and included as a riding contribution with U iso (H) = 1.2U eq (O).

Computing details
Data collection: RAPID-AUTO (Rigaku, 1998); cell refinement: RAPID-AUTO (Rigaku, 1998); data reduction: CrystalStructure (Rigaku/MSC, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).    where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.25 e Å −3 Δρ min = −0.23 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.009 (2) 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 > 2s˘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.