Three hydrates of the bisphosphonate risedronate, consisting of one molecular and two ionic structures

Three different hydrates of risedronate were obtained by varying the pH of a solution containing the compound. At the pH values used, the N atom of the pyridine group is protonated and the compounds are zwitterionic. Crystals obtained directly from the synthesis resulted in risedronate monohydrate, or [1-hydroxy-1-phosphono-2-(pyridinium-3-yl)ethyl]phosphonate monohydrate, C₇H₁₁NO₇P₂·H₂O, (I), in which just one phosphonate group is negatively charged. Recrystallizations at pH values of 2 and 4 yielded risedronate dihydrate, or sodium [1-hydroxy-2-(pyridinium-3-yl)ethane-1,2-diyl]bis(phosphonate) dihydrate, Na⁺·C₇H₁₀NO₇ P₂^-·-2H₂O, (II). Finally, recrystallizations at pH values of 7 and 8 produced risedronate 2.5-hydrate, or sodium [1-hydroxy-2-(pyridinium-3-yl)ethane-1,2-diyl]bis(phosphonate) 2.5-hydrate, Na⁺·C₇H₁₀NO₇ P₂^-·2.5H₂O, (III). At these four pH values, both phosphonate groups in (II) and (III) are negatively charged and coordinated to an Na⁺ ion. Crystals of (II), i.e. those grown at pH values of 2 and 4, have isomorphous polymeric ion aggregate structures with geminal phosphonate and alcohol groups coordinated to the same Na⁺ ion. On the other hand, crystals of (III), i.e. those grown at pH values of 7 and 8, have isomorphism polymeric ion aggregate structures with geminal phosphonate and alcohol groups coordinated to different Na⁺ ions.