Zoledronate complexes. III. Two zoledronate complexes with alkaline earth metals: [Mg(C5H9N2O7P2)2(H2O)2] and [Ca(C5H8N2O7P2)(H2O)]n

Diaquabisdihydrogen 1‐hydroxy‐2‐(imidazol‐3‐ium‐1‐yl)ethylidene‐1,1‐diphosphonato‐κ²O,O′]magnesium(II), Mg(C₅H₉N₂O₇P₂)₂(H₂O)₂], consists of isolated dimeric units built up around an inversion centre and tightly interconnected by hydrogen bonding. The Mg^II cation resides at the symmetry centre, surrounded in a rather regular octahedral geometry by two chelating zwitterionic zoledronate(1−) or dihydrogen 1‐hydroxy‐2‐(imidazol‐3‐ium‐1‐yl)ethylidene‐1,1‐diphosphonate] anions and two water molecules, in a pattern already found in a few reported isologues where the anion is bound to transition metals (Co, Zn and Ni). catena‐Polyaquacalcium(II)]‐μ₃‐hydrogen 1‐hydroxy‐2‐(imidazol‐3‐ium‐1‐yl)ethylidene‐1,1‐diphosphonato]‐κ⁵O:O,O′:O′,O′′], Ca(C₅H₈N₂O₇P₂)(H₂O)]_n, consists instead of a Ca^II cation in a general position, a zwitterionic zoledronate(2−) anion and a coordinated water molecule. The geometry around the Ca^II atom, provided by six bisphosphonate O atoms and one water ligand, is that of a pentagonal bipyramid with the Ca^II atom displaced by 0.19 Å out of the equatorial plane. These Ca^II coordination polyhedra are `threaded' by the 2₁ axis so that successive polyhedra share edges of their pentagonal basal planes. This results in a strongly coupled rhomboidal Ca₂–O₂ chain which runs along 010]. These chains are in turn linked by an apical O atom from a –PO₃ group in a neighbouring chain. This O‐atom, shared between chains, generates strong covalently bonded planar arrays parallel to (100). Finally, these sheets are linked by hydrogen bonds into a three‐dimensional structure. Owing to the extreme affinity of zoledronic acid for bone tissue, in general, and with calcium as one of the major constituents of bone, it is expected that this structure will be useful in modelling some of the biologically interesting processes in which the drug takes part.