catena-Poly[[[triaquaeuropium(III)]-μ-(1H-benzimidazole-5,6-dicarboxylato-κ2O5:O6)-μ-(1H,3H-benzimidazol-3-ium-5,6-dicarboxylato-κ3O5:O6,O6′)] dihydrate]

In the title one-dimensional coordination polymer, {[Eu(C9H4N2O4)(C9H5N2O4)(H2O)3]·2H2O}n, one of the 1H-benzimidazole-5,6-dicarboxylate (Hbdc) ligands is protonated at the imidazole group (H2bdc). The EuIII ion is eight-coordinated by two O atoms from two Hbdc ligands, three O atoms from two H2bdc ligands and three water molecules, showing a distorted square-antiprismatic geometry. The EuIII ions are bridged by the carboxylate groups of the Hbdc and H2bdc ligands, forming a chain along [110], with an Eu...Eu separation of 5.4594 (3) Å. These chains are further connected by intermolecular O—H...O, N—H...O and N—H...N hydrogen bonds, as well as π–π interactions between the imidazole and benzene rings [centroid–centroid distances = 3.558 (3), 3.906 (2), 3.397 (3), 3.796 (2) and 3.898 (2) Å], into a three-dimensional supramolecular network.