2-((E)-{2-[(E)-2,3-Dihydroxybenzylideneamino]-5-methylphenyl}iminiomethyl)-6-hydroxyphenolate

The asymmetric unit of the title Schiff base compound, C21H18N2O4, consists of four independent zwitterions (A, B, C and D) with similar conformations. In each independent molecule, the methyl group is disordered over two positions; the occupancies of the two positions are 0.819 (5) and 0.181 (5) in molecule A, 0.912 (5) and 0.088 (5) in B, 0.734 (5) and 0.266 (5) in C, and 0.940 (6) and 0.060 (6) in D. The dihydroxyphenyl and the hydroxyphenolate rings in molecule A form dihedral angles of 17.36 (12) and 13.30 (12)°, respectively, with the central benzene ring, whereas the respective angles for molecules B, C and D are 30.22 (11)/7.46 (11), 35.26 (12)/11.01 (12) and 39.89 (12)/4.29 (12)°. In all independent molecules, intramolecular N—H⋯O and O—H⋯N hydrogen bonds generate S(6) ring motifs. The four independent molecules are linked into two pairs, viz. A–B and C–D, by intermolecular O—H⋯O hydrogen bonds. These pairs are linked into a two-dimensional network parallel to the ab plane by C—H⋯O hydrogen bonds. In addition, C—H⋯π and π–π [centroid–centroid distance = 3.5153 (14)–3.7810 (15) Å] interactions stabilize the crystal structure.


Experimental
A solution of 4-methyl-1,2-phenylenediamine (0.244 g, 2 mmol) in ethanol (20 ml) was added into 2,3-dihydroxybenzaldehyde (0.552 g, 4 mmol). The mixture was refluxed with stirring for 30 min. The resultant red solution was filtered. Red crystals suitable for X-ray analysis were formed after a few days of slow evaporation of the solvent at room temperature.

Refinement
All O and N bound H atoms were located in a difference Fourier map and refined freely. The rest of the H atoms were positioned geometrically and refined using a riding model, with C-H = 0.93 or 0.96 Å and U iso (H) = 1.2-1.5(methyl)U eq (C).

supplementary materials sup-2
The rotating group model was applied for the methyl groups. The methyl group in each independent molecule is disordered over two positions, with refined site-occupancies of 0.819 (5) and 0.181 (5) for molecule A, 0.912 (5) and 0.088 (5) for B, 0.734 (5) and 0.266 (5) for C, and 0.940 (6) and 0.060 (6) for D. Atoms C21F and C21H were refined isotropically. Fig. 1. Molecule A of the title compound, with atom labels and 50% probability ellipsoids for non-H atoms. The other three independent molecules are labelled similarly. Intramolecular hydrogen bonds are shown as dashed lines. Both disorder components are shown.

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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (