5-Hy-droxy-2-{(E)-[(3-nitro-phen-yl)iminio]meth-yl}phenolate.

The title compound, C(13)H(10)N(2)O(4), crystallized as the zwitterionic tautomer. As a result, the phenolate C-O(-) bond [1.296 (2) Å] is shorter than a normal Csp(2)-O(H) bond, and the azomethine C=N bond [1.314 (2) Å] is longer than a normal C=N double bond. The mol-ecule 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 mol-ecular conformation is stabilized by an intra-molecular N-H⋯O hydrogen bond. In the crystal, strong O-H⋯O hydrogen bonds link the mol-ecules 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 R(2) (1)(6), R(2) (1)(7) and R(2) (2)(10) ring motifs.