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

The asymmetric unit of the title Schiff base compound, C(21)H(18)N(2)O(4), 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.