(E)-4-Hydroxy-2-[(2-hydroxyphenyl)iminiomethyl]phenolate

The title compound, C13H11NO3, crystallizes in a zwitterionic form and has a trans configuration about the C=N bond. The molecule is almost planar, the dihedral angle between the two benzene rings being 4.32 (8)°. The two hydroxy substituents are coplanar with each of their attached benzene rings [r.m.s. deviations of 0.0053 (2) and 0.0052 (2) Å]. An intramolecular N—H⋯O hydrogen bond formed between the iminium N and the phenolate O atom generates an S(6) ring motif. In the crystal, the molecules are linked through O—H⋯O hydrogen bonds into chains along [110]. Two neighbouring chains are further connected through O—H⋯O hydrogen bonds in an antiparallel manner. π–π interactions are also observed, with centroid–centroid distances of 3.7115 (19) and 3.743 (2) Å.

The molecule of (I) (  Table 1) between the NH + with the phenolate Oatom generate an S(6) ring motif (Bernstein et al., 1995) which stabilizes the planarity of the molecule. The bond distances are in normal ranges (Allen et al., 1987) and comparable with those of related structures (Eltayeb et al., 2009;2010a,b;Tan & Liu, 2009).

Experimental
The title compound was synthesized by adding 2,5-dihydroxybenzaldehyde (0.276 g, 2 mmol) to a solution of 2-aminophenol (0.218 g, 2 mmol) in ethanol (30 ml). The mixture was refluxed with stirring for half an hour. The resultant red solution was filtered and the filtrate was evaporated to give a red powder product. Red plate-shaped single crystals of the title compound suitable for x-ray structure determination were obtained from acetone by slow evaporation in the refrigerator after a few days.

Refinement
All H atoms were located from the difference map and isotropically refined. The highest residual electron density peak is located at 0.76 Å from C8 and the deepest hole is located at 1.46 Å from H1O1. Fig. 1. The molecular structure of the title compound, with 50% probability displacement ellipsoids and the atom-numbering scheme. Hydrogen bond is shown as 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 > 2sigma(F 2 ) is used only for calculating R-factors(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.