Diaquabis[1-hydroxy-2-(imidazol-3-ium-1-yl)-1,1′-ethylidenediphophonato-κ2 O,O′]zinc(II)

In the title complex, [Zn(C5H9NO7P2)2(H2O)2], the zinc atom is coordinated by two bidentate zoledronate [zoledronate = (2-(1-imidazole)-1-hydroxy-1,1′-ethylidenediphophonate)] ligands and two water molecules. The coordination number is 6. There is one half-molecule in the asymmetric unit with the zinc atom located on a crystallographic inversion centre. The anion exists as a zwitterion with an overall charge of −1; the protonated nitrogen in the ring has a positive charge and the two phosphonates groups each have a single negative charge. There are two intramolecular O—H⋯O hydrogen bonds. The molecules are linked into a chain by intermolecular O—H⋯O hydrogen bonds. Adjacent chains are further linked by O—H⋯O hydrogen bonds involving the aqua ligands. An N—H⋯O interaction is also observed.

In the title complex, [Zn(C 5 H 9 NO 7 P 2 ) 2 (H 2 O) 2 ], the zinc atom is coordinated by two bidentate zoledronate [zoledronate = (2-(1-imidazole)-1-hydroxy-1,1 0 -ethylidenediphophonate)] ligands and two water molecules. The coordination number is 6. There is one half-molecule in the asymmetric unit with the zinc atom located on a crystallographic inversion centre. The anion exists as a zwitterion with an overall charge of À1; the protonated nitrogen in the ring has a positive charge and the two phosphonates groups each have a single negative charge. There are two intramolecular O-HÁ Á ÁO hydrogen bonds. The molecules are linked into a chain by intermolecular O-HÁ Á ÁO hydrogen bonds. Adjacent chains are further linked by O-HÁ Á ÁO hydrogen bonds involving the aqua ligands. An N-HÁ Á ÁO interaction is also observed.

Comment
The present work is part of a project directed to the preparation and characterization of coordination complexes obtained by the interaction among metals and organic molecules of relevant pharmacological interest like bisphosphonates. Bisphosphonate compounds, which are characterized by a P-C-P backbone, are analogues of naturally occurring pyrophosphates.
The P-C-P base structure allows the bisphosphonates to bind to many metallic cations in particular divalent metal ions and as a result bisphosphonates may stick bone surfaces in vivo (Fleisch et al., 1968). Third generation bisphosphonates, like zoledronate, are characterized by having a cyclic side chain and belong to the nitrogen containing bisphosphonate group which are the most effective for medical treatment (Green et al., 1994). A large number of metal derivatives of other bisphosphonates, like Etidronate and Pamidronate are known where the bisphosphonate ligand displays a variety of coordination modes (Ferńandez, 2002;Li et al., 2008;Chen et al., 2008;Uchtman, 1972). In contrast, few metal derivatives of Zoledronic acid have been reported in CSD (Allen, 2002). The present compound is isostructural with two Co and Ni complexes (Cao et al., 2007).
So, we present herein the crystal structure of a Zinc-Zoledronate complex: monozinc dizoledronate dihydrate, (I), Zn.2(P 2 O 7 N 2 C 5 H 9 ).2(H 2 O). We also synthesized a pentacoordinated complex of zinc (II), (Freire & Vega, 2009). The zoledronate anion exists as a zwitterion with an overall charge of -1; the protonated nitrogen in the ring has a positive charge and the two phosphonates groups each have a single negative charge.
Each phosphonate has one protonated O atom, the extra electronic charge being shared by the remaining two non protonated O atoms. This fact define two distinct types of P-O bonds, as shown by the mean value in the following values of bond distances and angles: P-OH 1,572 (8) measure is in agreement with the results found for related molecules (Coiro et al., 1989;Vega et al., 1996;Vega et al., 1998).
The phosphonates groups have slighty "staggered" conformations sight in the P1···P2 direction. When this staggering is observed the non bonded torsion angle O13-P1··· P2-O23 is 4.1 °. The imidazol ring is plane, maximum deviation from the L.S. mean plane is 0.0058 Å for C5. The ring and C2 are not coplanar, the angle determined between the plane of the ring and the bond N1-C2 is 3.2 ° and C2 is 0.0861 Å far from the plane of the ring. The torsion angle C1-C2-N1-C3 is of 104.1 (8) ° and it is possible to describe it like + Anti-Clinal (+ac).

Experimental
Zoledronic Acid was obtained from Gador S. A. laboratory. The present compound was obtained as subproduct in a Zoledronate recrystallization process.

Refinement
The