2-(Carboxymethylsulfanyl)pyridine-3-carboxylic acid monohydrate

The title compound, C8H7NO4S·H2O, was obtained by reaction of 2-mercaptopyridine-3-carboxylic acid with chloroacetic acid. In the molecular structure, the dihedral angle between the two least-squares planes defined by the pyridine ring and the carboxy group is 8.32 (9)°. The carboxymethylsulfanyl group makes a torsion angle of 82.64 (12)° with the pyridine ring. An intramolecular O—H⋯N hydrogen bond between the acidic function of the carboxymethylsulfanyl group and the pyridine N atom stabilizes the conformation, whereas intermolecular O—H⋯O hydrogen bonding with the uncoordinated water molecules is responsible for packing of the structure, leading to chains propagating in [001].

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2333). Comment 2-Mercaptopyridine-3-carboxylic acid is an interesting ligand because of its potential versatile coordinate behavior. It may act as a deprotonated ligand through either the carboxylate or the thiolate group, such as 2-mercaptopyridine-3-carboxylate hydrate (Smith et al., 2003) or 2-mercapto-nicotinic acid (Panagiotis et al., 2003). Thus it can act as a monodentate (O or N), bidentate (O, O or O, N) or chelating (O, O or O, N) ligand interacting with metal ions, and a variety of coordination polymers have been characterised (Humphrey et al., 2006;Ma et al., 2004;Quintal et al., 2002). In this work, we report a new derivative, 2-(carboxymethylsulfanyl)pyridine-3-carboxylic acid, (I), which was obtained by reaction of 2-mercaptopyridine-3-carboxylic acid with chloroacetic acid.

Structure Reports Online
The molecular structure of (I) is presented in Fig. 1. The carboxylate group is almost parallel to the pyridine group with a dihedral angle of 8.32 (9)°, while the carboxymethylsulfanyl group makes a torsion angle of 82.64 (12)° with the pyridine ring. The carboxylic O atoms, pyridine N atom together with lattice water molecules are involved in hydrogen-bonding interactions (Fig. 2). In detail, the structure is stabilized by an intramolecular O-H···N hydrogen bond between the carboxy function of the carboxymethylsulfanyl group and the pyridine N atom. The other carboxy function acts as a donor and acceptor group for intermolecular O-H···O hydrogen bonds with the adjacent lattice water molecules which results in the formation of a chain structure running along the c direction.

Experimental
The mixture of 2-mercaptopyridine-3-carboxylic acid (1.552 g, 10.0 mmol) and chloroacetic acid (2.835 g, 30.0 mmol) was stirred and refluxed under basic condition in which sodium hydroxide solution was needed to keep the pH around 11. After reaction for 4 h at 328 K, the mixture was cooled to room temperature. By adjusting the pH around 3 with concentrated hydrochloric acid, a white precipitate appeared rapidly. The solid was filtered off and washed with water. Single crystals suitable for X-ray diffraction were obtained in the mother liquid after evaporation within a few days.

Refinement
The carbon-bound H-atoms were positioned geometrically and included in the refinement using a riding model  Fig. 1. View of the molecular structure of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level.

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 > σ(F 2 ) is used only for calculating Rfactors(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.