Bis(2-amino-3-carboxypyridinium) sulfate trihydrate

In the title compound, 2C6H7N2O2 +·SO4 2−·3H2O, there are two independent cations which are connected into N—H⋯O hydrogen-bonded dimers. In the crystal, O—H⋯O hydrogen-bonded sulfate–water sheets run parallel to (001) and are linked into a three-dimensional network via intermolecular N—H⋯O and O—H⋯O hydrogen bonds through the 2-aminonicotinium dimers. Further stabilization is provided by weak intermolecular C—H⋯O hydrogen bonds. R 4 3(10) and R 2 2(8) graph-set rings are observed. The crystal studied was an inversion twin with refined components of 0.45 (6) and 0.55 (6).


Comment
Hydrogen bonds are the object of several studies, which aim to elucidate their influence on crystal construction and compounds propreties (Desiraju, 2003). Pyridine and its derivatives well known for their various chemical and biological activities, have proved their aptitude to built new edifices involving original hydrogen-bonding patterns due to their variety of potential hydrogen donors and acceptors (Athimoolam et al., 2005;Dobson & Gerkin, 1997;Giantsidis & Turnbull, 2000). The title compound was obtained from 2-aminonicotinic acid and is a part of our search for new hybrid compounds based on protonated N-heterocyclic compounds and inorganic acids (Berrah et al., 2005;2011a,b).
As shown in figure 1, the asymmetric unit includes two crystallographically independent 2-aminonicotinium cations (A and B), one sulfate anion and three water molecules. The cation geometry is similar to that reported for the structure of 2-aminonicotinic acid (Dobson & Gerkin, 1997;Pawlukojc et al., 2007) except for C-O distances in the carboxylic group.
In the 2-aminonicotinic acid structure, the two C-O distances are 1.234 (2) and 1.266 (2)Å since the carboxylic group transfers its proton to the hetero-ring nitrogen atom leading to a zwitterionic molecule.

Experimental
Colorless crystal of the title compound was obtained by slow evaporation of an aqueous solution of 2-amino-pyridine-3carboxylic acid and sulfuric acid in 2:1 stoichiometric ratio.

Refinement
The H atoms of the water molecules were located in difference Fourier maps and were refined with U iso (H) = 1.5U eq (O).