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

In the title complex, [Zn(C5H9NO7P2)2(H2O)]·2H2O, the zinc atom is coordinated by two zoledronate anions [zoledronate = (2-(1-imidazole)-1-hydroxy-1,1′-ethylidenediphophonate)] and one water molecule. The coordination number is 5. There is one half-molecule in the asymmetric unit, the zinc atom being located on a twofold rotation axis passing through the metal centre and the coordinating water O atom. 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. Intermolecular O—H⋯O hydrogen bonds link the molecules. An N—H⋯O interaction is also present.

In the title complex, [Zn(C 5 H 9 NO 7 P 2 ) 2 (H 2 O)]Á2H 2 O, the zinc atom is coordinated by two zoledronate anions [zoledronate = (2-(1-imidazole)-1-hydroxy-1,1 0 -ethylidenediphophonate)] and one water molecule. The coordination number is 5. There is one half-molecule in the asymmetric unit, the zinc atom being located on a twofold rotation axis passing through the metal centre and the coordinating water O atom. 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. Intermolecular O-HÁ Á ÁO hydrogen bonds link the molecules. An N-HÁ Á ÁO interaction is also present.

Comment
The following 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. An informative introduction on bisphosphonates has been made in the previous paper (Freire & Vega, 2009). Although few metal derivatives of Zoledronic acid have been reported in CSD (Allen, 2002), an isostructural compound of copper has been synthesized (Cao et al., 2008).
So, we present herein the crystal structure of a Zinc-Zoledronate complex: monozinc dizoledronate trihydrate, (I), Zn.(H 2 O).2(P 2 O 7 N 2 C 5 H 9 ).2H 2 O. In (I), as in the similar hexacoordinated compound (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.
The coordination number of Zn is 5 ( Fig. 1) and the resulting coordination polyhedron is a trigonal bipiramid defined by O21, O21A, O11, O11A and O1W. Atoms O11A and O21A are generated by the symmetry operation (1 -x, y, 3/2 -z). The equatorial plane is defined by O11, O11A and O1W, the apexes are defined by and O21 and O21A. The apical Zn-O distance is 2.041 (2) Å while in the equatorial plane the mean value for the Zn-O distance is 2.004 (4) Å. The coordination angles in the equatorial plane are a little turned aside from the expected 120 ° theoretically due to the "bite" of the ligand: O11-Zn-O11A 134.08 (15) (14). The staggered conformation of PO3 groups in compound (II) is more prominent than in the hexacoordinated complex (Freire & Vega, 2009), the non bonded torsion angle O12-P1···P2 O22 is -16.1°. In (I), the imidazol ring is planar, maximum deviation from the L. S. mean plane is 0.0026 Å for C3, and it is not coplanar with C2, between the plane of the ring and the bond N1-C2 is 3.4 ° and C2 is 0.0837 Å far from the ring. The torsion angle C1-C2-N1-C3 is of -78.62 ° and it is possible to describe it like -Syn-Clinal (-sc).
Five hydrogen bonds, involving, H22, H1W, H1, H2WA and H2WB, provide intermolecular cohesion, defining a twodimensional arrangement, while the three-dimensional net completes with two more hydrogen bonds, involving H2 from the aromatic ring and H12 from the bisphosphonate group ( Fig. 2 and Table 2).

Experimental
Zoledronic Acid was obtained from Gador S. A. laboratory. Compound (II) was obtained by direct mix of a water solution of Zoledronic Acid and a water solution of ZnCl 2 . Colorless prismatic crystals were grown after a few days.

Aquabis[1-hydroxy-2-(imidazol-3-ium-1-yl)-1,1'-ethylidenediphophonatoκ 2 O,O']zinc(II) dihydrate
Crystal data [Zn(C 5   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.