Multiple hydrogen bonds in cytosinium zoledronate trihydrate.

The asymmetric unit of the title compound [systematic name: 4-amino-2-oxo-2,3-dihydropyrimidin-1-ium 1-hydroxy-2-(1H,3H-imidazol-3-ium-1-yl)ethylidenediphosphonate trihydrate], C(4)H(6)N(3)O(+)·C(5)H(9)N(2)O(7)P(2)(-)·3H(2)O, contains one cytosinium cation, one zoledronate anion and three water molecules. The zoledronate anion has a zwitterionic character, in which each phosphonate group is singly deprotonated and an imidazole N atom is protonated. Furthermore, proton transfer takes place from one of the phosphonic acid groups of the zoledronate anion to one of the N atoms of the cytosinium cation. The cytosinium cation forms a C(6) chain, while the zoledronate anion forms a rectangular-shaped centrosymmetric dimer through N-H...O hydrogen bonds. The cations and anions are held together by N-H...O and O-H...O hydrogen bonds to form a one-dimensional polymeric tape. The three water molecules play a crucial role in hydrogen bonding, resulting in a three-dimensional hydrogen-bonded network.