Poly[piperazinediium [aqua­bis(μ-pyridine-2,5-dicarboxyl­ato)zincate] dihydrate]

The polymeric title compound, {(C(4)H(12)N(2))[Zn(C(7)H(3)NO(4))(2)(H(2)O)]·2H(2)O}(n), was obtained by the reaction of zinc(II) nitrate hexa­hydrate with the proton-transfer compound (pipzH(2))(py-2,5-dc) (where pipz is piperazine and py-2,5-dcH(2) is pyridine-2,5-dicarboxylic acid) in aqueous solution. Each Zn(II) atom is coordinated in a distorted octa­hedral geometry by four O atoms and two N atoms from one water mol­ecule and two (py-2,5-dc)(2–) ligands, which also act as bridging ligands between Zn(II) atoms. π–π Stacking inter­actions between two aromatic rings of (py-2,5-dc)(2–) fragments, with centroid–centroid distances of 3.4747 (7) and 3.7081 (7) Å are observed. The crystal structure is stabilized by O—H⋯O and N—H⋯O hydrogen bonds.