Zoledronic acid: monoclinic and triclinic polymorphs from powder diffraction data

The crystal structures of the monoclinic and triclinic poly­morphs of zoledronic acid, C₅H₁₀N₂O₇P₂, have been established from laboratory powder X-ray diffraction data. The mol­ecules in both polymorphs are described as zwitterions, namely 1-(2-hy­droxy-2-phosphon­ato-2-phosphono­ethyl)-1H-imidazol-3-ium. Strong inter­molecular hydrogen bonds (with donor–acceptor distances of 2.60 Å or less) link the mol­ecules into layers, parallel to the (100) plane in the monoclinic polymorph and to the (10) plane in the triclinic polymorph. The phospho­nic acid groups form the inner side of each layer, while the imidazolium groups lie to the outside of the layer, protruding in opposite directions. In both polymorphs, layers related by translation along [100] inter­act through weak hydrogen bonds (with donor–acceptor distances greater than 2.70 Å), forming three-dimensional layered structures. In the monoclinic polymorph, there are hydrogen-bonded centrosymmetric dimers linked by four strong O—H...O hydrogen bonds, which are not present in the triclinic polymorph.