5-Hy­droxy-2-{(E)-[(3-nitro­phen­yl)iminio]meth­yl}phenolate

The title compound, C₁₃H₁₀N₂O₄, crystallized as the zwitterionic tautomer. As a result, the phenolate C—O⁻ bond [1.296 (2) Å] is shorter than a normal Csp²—O(H) bond, and the azomethine C=N bond [1.314 (2) Å] is longer than a normal C=N double bond. The mol­ecule is nearly planar, the mean plane of the nitro-substituted benzene ring forming dihedral angles of 9.83 (7) and 8.45 (9)° with the other benzene ring and with the nitro group, respectively. The mol­ecular conformation is stabilized by an intra­molecular N—H...O hydrogen bond. In the crystal, strong O—H...O hydrogen bonds link the mol­ecules into double-stranded chains along the b-axis direction. Within the chains there are π–π interactions involving the benzene rings of adjacent molecules [centroid–centroid distance = 3.669 (1) Å]. The chains are linked via C—H...O hydrogen bonds, forming R₂¹(6), R₂¹(7) and R₂²(10) ring motifs.