The imide tautomer of sulfasalazine.

The title compound, 5-[4-[(2-pyridylideneamino)sulfonyl]phenyldiazenyl]salicylic acid, C(18)H(14)N(4)O(5)S, crystallizes as the imide tautomer in the monoclinic space group P2(1)/c. In addition to an intramolecular O-H.O hydrogen bond, intermolecular O-H.O interactions link adjacent molecules into helices, which are connected by pairwise N-H.N interactions into two-dimensional hydrogen-bonded layers. Both the molecular conformation and the packing differ from those seen in the triclinic amide form [Filip et al. (2001). Acta Cryst. C57, 435-436].