4-Carboxypyridinium 3-carboxy-4-hydroxybenzenesulfonate

Cocrystallization of 4-carboxypyridine (4-CPY) and 5-sulfosalicylic acid (5-H2SSA) yields the title salt, C6H6NO2 +·C7H5O6S−. In the crystal structure, the components of the salt are linked by a combination of intermolecular O—H⋯O and N—H⋯O, and weak C—H⋯O hydrogen bonds, forming a three-dimensional framework.


Comment
As awareness of the importance of pharmaceutical molecular adducts grows, it becomes imperative to fully understand and investigate the intermolecular interactions in a binary, ternary or multi-component organic adducts (Aakeröy and Salmon, 2005). 5-sulfosalicylic acid (5-H 2 SSA), is a particularly strong acid which is capable of donating its sulfonic protons to many N-containing heterocycles, forming organic salts (Smith et al., 2006;Meng et al., 2007 and2008;Fan et al., 2005).
With the aim of gaining more insight into hydrogen-bonding interactions involving 5-H 2 SSA and pyridine derivatives, we report here the molecular and supramolecular structure of the title compound.
In the asymmetric unit of title compound (I), contains one 5-HSSAand one 4-CPY + ion. Similar to the analogous organic adducts reported (Meng et al., 2007), the H atom is transferred from the sulfonic acid group to the pyridine N atom forming an 1:1 organic salt.
In the crystal structure, the component ions are linked by a combination of O-H···O, N-H···O and C-H···O hydrogen bonds (Table 1), forming a three-dimensional network (Fig.2). An analysis using PLATON (Spek, 2003), showed that there are no other interactions (e.g. C-H···π and π-π) observed in the crystal structure.

Experimental
All reagents and solvents were used as obtained without further purification. Equivalent molar amount of 4-carboxypyridine and 5-sulfosalicylic acid dihydrate were dissolved in 95% methanol (20 ml). The mixture was stirred for 10 minutes at 330 K and then filtered. Colorless needles of (I) suitable for single-crystal X-ray diffraction analysis grew at the bottom of the vessel in one week after slow evaporation of the solution.

Refinement
Owing to the poor quality the crystal selected for diffraction, conventional least squares refinement of the structural model gave R 1 = 0.072. We attempted to select better crystals for diffraction, but none were an improvement. The title compound is racemic in solution but spontaneously resolved upon crystallization. The absolute configuration of the molecules in the crystal selected was readily determined and the configuration has no chemical significance.   (7) N1-H1A···O2 ii 0.85 (2) 2.57 (5) 3.140 (7) 125 (5) C3-H3···O3 ii 0.93 2.51 3.379 (7) 156.