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3-Hydroxypyridinium-2-carboxylate

Comparable to many amino acids, the title compound, C6H5NO3, is a substitution product of picolinic acid. The molecule shows approximate non-crystallographic Cs symmetry. Like many amino acids, the molecule is present in its zwitterionic state. Intra- as well as intermolecular hydrogen bonds are observed, the latter connecting the molecules into zigzag chains along the crystallographic b axis. An intermolecular C—C distance of only 3.368 (2) Å exclusively involving carbon atoms of aromatic rings (centroid–centroid separation = 3.803 Å) is indicative of π–π interactions connecting the molecules into stacks along the crystallographic a axis.

Comparable to many amino acids, the title compound, C 6 H 5 NO 3 , is a substitution product of picolinic acid. The molecule shows approximate non-crystallographic C s symmetry. Like many amino acids, the molecule is present in its zwitterionic state. Intra-as well as intermolecular hydrogen bonds are observed, the latter connecting the molecules into zigzag chains along the crystallographic b axis. An intermolecular C-C distance of only 3.368 (2) Å exclusively involving carbon atoms of aromatic rings (centroid-centroid separation = 3.803 Å ) is indicative ofinteractions connecting the molecules into stacks along the crystallographic a axis.

Comment
Chelate ligands have found widespread use in coordination chemistry due to the enhanced thermodynamic stability of resultant coordination compounds in relation to coordination compounds exclusively applying comparable monodentate ligands (Gade, 1998). Combining two different donor atoms in different states of hybridization might be useful to accomodate a large variety of metal centers of variable Lewis acidity. In this aspect, 3-hyxdroxypicolinic acid seemed of interest due to its possible use as a strictly neutral or, depending on the pH value, as an anionic or cationic ligand. In addition, due to the arrangement of its functional groups, it may act as mono-or bidentate ligand offering the possibility to create five-as well as six-membered chelate rings. To enable comparative studies in terms of bond lengths and angles in envisioned coordination compounds, we determined the molecular and crystal structure of the title compound. Among a few others, the crystal structures of two mercury coordination compounds in which 3-hydroxypicolinic acid acts as mono-or bidentate ligand exist in the literature (Popović et al., 2007).
The molecule (Fig. 1) is present in its zwitterionic tautomeric form and thus resembles natural amino acids. Intracyclic angles span a range of 118.48 (14) Apart from an intramolecular hydrogen bond obvious between the hydroxyl group and the carboxylic acid group, intermolecular hydrogen bonds are observed (Fig. 2). These stem from the protonated nitrogen atom and have one of the carboxylic acid group's oxygen atoms as acceptor. In terms of graph-set analysis (Etter et al., 1990;Bernstein et al., 1995), the descriptor for this hydrogen bonding system on the unitary level is DC 1 1 (5). In total, the molecules are connected to waved zigzag chains along the crystallographic b axis. The presence of π···π interactions becomes manifest upon the presence of an intermolecular C-C distance of only 3.368 (2) Å. This interaction exclusively involves intracyclic carbon atoms and gives rise to stacks of molecules along the crystallographic a axis.
The packing of the compound is shown in Fig. 3.

Experimental
The compound was obtained commercially (Fluka). Crystals suitable for the diffraction study were obtained upon recrystallization of the compound from hot water.