4-[(2,4-Dihydroxybenzylidene)ammonio]benzenesulfonate trihydrate

The title Schiff base compound, C13H11NO5S·3H2O, formed from sulfanilic acid and 2,4-dihydroxybenzaldehyde, crystallized out as a zwitterion with the N atom protonated. The asymmetric unit consists of one 4-[(2,4-dihydroxybenzylidene)ammonio]benzenesulfonate and three water molecules. The zwitterion exists in an E configuration with respect to the central C=N double bond. The two benzene rings of the molecule are oriented at a dihedral angle of 27.33 (8)°. An intramolecular N–H⋯O hydrogen bond stabilizes the molecular structure. In the crystal, the zwitterions are linked into chains along [101] by intermolecular O—H⋯O and N—H⋯O hydrogen bonds. The three water molecules link these chains into a three-dimensional framework by additional intermolecular O—H⋯O hydrogen bonds. A π⋯π interaction [3.5485 (9) Å] further stabilizes the crystal structure.

The title Schiff base compound, C 13 H 11 NO 5 SÁ3H 2 O, formed from sulfanilic acid and 2,4-dihydroxybenzaldehyde, crystallized out as a zwitterion with the N atom protonated. The asymmetric unit consists of one 4-[(2,4-dihydroxybenzylidene)ammonio]benzenesulfonate and three water molecules. The zwitterion exists in an E configuration with respect to the central C=N double bond. The two benzene rings of the molecule are oriented at a dihedral angle of 27.33 (8) . An intramolecular N-HÁ Á ÁO hydrogen bond stabilizes the molecular structure. In the crystal, the zwitterions are linked into chains along [101] by intermolecular O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds. The three water molecules link these chains into a three-dimensional framework by additional intermolecular O-HÁ Á ÁO hydrogen bonds. A Á Á Á interaction [3.5485 (9) Å ] further stabilizes the crystal structure.

Comment
Schiff bases derived from aromatic amines and aromatic aldehydes have a wide variety of applications in many fields, e.g., biological, inorganic and analytical chemistry (Singh et al., 1975;Elmali et al., 1999;Patel et al., 1999). Schiff base compounds have been of great interest for many years. p-aminobenzenesulfonic acid is known as sulfanilic acid, which contains NH 3 + and SO 3 groups. Sulfanilic acid is a salt, but of a rather special kind, called a zwitterion. It is the product of the reaction between an acidic group and a basic group that are part of the same molecule. The hydrogen is attached to nitrogen rather than oxygen simply because the NH 2 group is a stronger base than the SO 3 substituent. A zwitterionic structure was also observed in the crystal structure of sulfanilic acid monohydrate (Rae & Maslen, 1962;Banu & Golzar, 2006). The crystal structure of the Schiff base formed from sulfanilic acid and dimethylformamide has also been reported in the literature (Hempel et al., 1999). The present work is part of a structural study of compounds of Schiff base systems and we report here the structure of the title compound, (I).  Table 2). An unusually short H2W1···H1W2 distance is also observed. A Cg1···Cg1 interaction of 3.5485 (9) Å; -x, 2-y, 1-z, further stabilizes the crystal structure [Cg1 is the centroid of the C1-C6 benzene ring].
Experimental 2,4-Dihydroxybenzaldehyde (0.069 g) and sulfanilic acid (0.861 g) in ethanol/water (40 ml) were heated under reflux for 2 h with stirring. The colour of the solution gradually changed from colourless to lemon yellow. The solution was then cooled to room temperature. After few days, slow evaporation of the solvent yielded yellow crystals of compound (I).

Refinement
The O and N bound H-atoms were located from difference Fourier map and refined freely. The H-atoms of the water molecules were located from a difference Fourier map and constrained to refine with the parent atom with U iso (H) = 1.5 U eq (O). The C-bound H-atoms were positioned geometrically with a riding model with C-H = 0.93 Å and U iso (H) = 1.2 U eq (C). Fig. 1. The molecular structure of (I) with atom labels and 50% probability ellipsoids for non-H atoms. The intramolecular hydrogen bond is shown as a dashed line.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
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.