Zoledronate complexes. II. catena-Poly[[tetraaquabis[hemihydrogen μ3-1-hydroxy-2-(imidazol-3-ium-1-yl)ethylidene-1,1-diphosphonato-κ3O:O′:O"]bis[μ3-1-hydroxy-2-(imidazol-3-ium-1-yl)ethylidene-1,1-diphosphonato-κ4O:O,O′:O"]trisodium] dihydrate]

The title compound, {[Na-3(C5H9N2O7P2)(2)(C5H9.5N2O7P2)(2)(H2O)(4)]center dot 2H(2)O}(n), (II), is polymeric and consists of undulating chains parallel to [011] interconnected by hydrogen-bonding and pi-pi interactions. There are two independent Na+ cations in the asymmetric unit (one lying on an inversion centre), two zoledronate anions and three water molecules, two of which are coordinated and one of which is a free solvate. Each cation is surrounded in an octahedral fashion by O atoms from four different zoledronate units and two/one coordinated water molecules. The zoledronate groups present their usual zwitterionic character, with negative charges in the protonated phosphonates and a positive charge at the protonated imidazole N atom. Two symmetry-related phosphonate groups share (in the form of a very strong linear hydrogen bond) an H atom lying on a symmetry centre, midway between the O atoms involved. The zoledronate binding modes present in (II) are both unreported for bisphosphonate anions. Intra-and inter-chain interactions are enhanced by a variety of hydrogen bonds where all the available O-H and N-H donors are involved, in addition to a strong imidazole-phosphonate C-H center dot center dot center dot O interaction, typical in these kinds of structures.