Propane-1,3-diaminium–2-carboxy­pyridine-6-carboxyl­ate–pyridine-2,6-dicarboxylic acid–water (1/2/2/8)

The title proton-transfer compound, C(3)H(12)N(2) (2+)·2C(7)H(4)NO(4) (−)·2C(7)H(5)NO(4)·8H(2)O or (pnH(2))(pydcH)(2).2(pydcH(2))·8H(2)O, was obtained by the reaction of pyridine-2,6-dicarboxylic acid (pydcH(2)) and propane-1,3-diamine (pn) in aqueous solution. Both neutral and monoanionic forms of the diacid are observed in the crystal structure. The negative charge of two monoanions is balanced by the dicationic propane-1,3-diaminium species. In addition, considerable π–π stacking inter­actions between the aromatic rings of the (pydcH)(−) and (pydcH(2)) fragments [with centroid–centroid distances of 3.5108 (11)–3.5949 (11) Å] are observed. The most important feature of this crystal structure is the presence of a large number of O—H⋯O, O—H⋯N, N—H⋯O, N—H⋯N, C—H⋯O and C—H⋯N hydrogen bonds, with D⋯A ranging from 2.445 (2) to 3.485 (3) Å. These inter­actions as well as ion pairing and π–π stacking connect the various fragments into a supra­molecular structure.