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

Abstract

Diaquabis[dihydrogen 1-hydroxy-2-(imidazol-3-ium-1-yl)ethylidene-1,1-diphosphonato-2O,O0]magnesium(II), [Mg(C5H9-N2O7P2)2(H2O)2], consists of isolated dimeric units built uparound an inversion centre and tightly interconnected byhydrogen bonding. The MgII cation resides at the symmetrycentre, surrounded in a rather regular octahedral geometry bytwo chelating zwitterionic zoledronate(1) [or dihydrogen1-hydroxy-2-(imidazol-3-ium-1-yl)ethylidene-1,1-diphosphonate]anions and two water molecules, in a pattern alreadyfound in a few reported isologues where the anion is bound totransition metals (Co, Zn and Ni). catena-Poly[[aquacalcium(II)]-3-[hydrogen 1-hydroxy-2-(imidazol-3-ium-1-yl)-ethylidene-1,1-diphosphonato]-5O:O,O0:O0,O00], [Ca(C5H8-N2O7P2)(H2O)]n, consists instead of a CaII cation in a generalposition, a zwitterionic zoledronate(2) anion and a coordinatedwater molecule. The geometry around the CaII atom,provided by six bisphosphonate O atoms and one waterligand, is that of a pentagonal bipyramid with the CaII atomdisplaced by 0.19 A ˚ out of the equatorial plane. These CaIIcoordination polyhedra are ?threaded? by the 21 axis so thatsuccessive polyhedra share edges of their pentagonal basalplanes. This results in a strongly coupled rhomboidal Ca2?O2chain which runs along [010]. These chains are in turn linkedby an apical O atom from a ?PO3 group in a neighbouringchain. This O-atom, shared between chains, generates strongcovalently bonded planar arrays parallel to (100). Finally,these sheets are linked by hydrogen bonds into a threedimensionalstructure. Owing to the extreme affinity ofzoledronic acid for bone tissue, in general, and with calciumas one of the major constituents of bone, it is expected that thisstructure will be useful in modelling some of the biologicallyinteresting processes in which the drug takes part.