catena-Poly[[[aqua­cadmium(II)]bis­(μ-4-hydroxy­pyridine-2,6-dicarboxyl­ato)[aqua­cadmium(II)]di-μ-aqua] tetra­hydrate]

The title polymeric compound, {[Cd(2)(C(7)H(3)NO(5))(2)(H(2)O)(4)]·4H(2)O}(n) or {[Cd(2)(hypydc)(2)(H(2)O)(4)]·4H(2)O}(n) (where hypydcH(2) is 4-hydroxy­pyridine-2,6-dicarboxylic acid), was synthesized by the reaction of cadmium(II) nitrate hexa­hydrate with 4-hydroxy­pyridine-2,6-dicarboxylic acid and propane-1,3-diamine, in a 1:2:2 molar ratio in aqueous solution. The compound is a seven-coordinate binuclear polymeric complex with distorted penta­gonal bipyramidal geometry around Cd(II) [Cd—O = 2.247 (4)–2.474 (3) Å]. In the binuclear monomeric units, the central atoms join together by O atoms of two bridging tridentate (hypydc)(2−) ligands, and the polymer propagates via two bridging water mol­ecules that link each Cd(II )centre of one monomer to the adjacent neighbour. Propane-1,3-diamine (pn) does not appear in the product but plays a role as a base. Inter­molecular O—H⋯O and C—H⋯O hydrogen bonds, and π–π stacking inter­actions, with distances of 3.725 (3) and 3.766 (3) Å, connect the various components.