Tetraaquabis[(1-ammonio-1-phosphonoethyl)phosphonato]zinc(II) tetrahydrate

The title compound, [Zn(C2H8NO6P2)2(H2O)4]·4H2O, was synthesized by the reaction of ZnCl2 with 1-aminoethane-1,1-diyldiphosphonic acid in aqueous solution. The asymmetric unit contains one-half of the complex and two water molecules of solvation. The Zn atom occupies a special position on an inversion centre. This results in a slightly distorted octahedral coordination environment, which consists of the O atoms from two phosphonic acids and four water molecules. The crystal structure displays N—H⋯O and O—H⋯O hydrogen bonding, which creates a three-dimensional network.

The title compound, [Zn(C 2 H 8 NO 6 P 2 ) 2 (H 2 O) 4 ]Á4H 2 O, was synthesized by the reaction of ZnCl 2 with 1-aminoethane-1,1diyldiphosphonic acid in aqueous solution. The asymmetric unit contains one-half of the complex and two water molecules of solvation. The Zn atom occupies a special position on an inversion centre. This results in a slightly distorted octahedral coordination environment, which consists of the O atoms from two phosphonic acids and four water molecules. The crystal structure displays N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonding, which creates a three-dimensional network.

Related literature
Diphosphonic acids are efficient drugs for the prevention of calcification and the inhibition of bone resorption, see: Matczak-Jon & Videnova-Adrabinska (2005). Diphosphonic acids and their metal complexes are used in the treatment of Pagets disease, osteoporosis and tumoral osteolysis, see: Szabo et al. (2002). For related structures, see: Li et al. (2006Li et al. ( , 2007; Lin et al. (2007).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: FJ2201).

Comment
Organic diphosphonic acids are potentially very powerful chelating agents used in metal extractions and are tested by the pharmaceutical industry for use as efficient drugs preventing calcification and inhibiting bone resorption (Matczak-Jon et al., 2005). Diphosphonic acids and their metal complexes are used in the treatment of Pagets disease, osteoporosis and tumoral osteolysis (Szabo et al., 2002). The asymmetric unit of title compound contains one-half of the formula unit ( Refinement H atoms bonded to N and O were located in a difference map and were freely refined with U iso (H) = 1.2 U eq of the carrier atom. Other H atoms which bonded to C were positioned geometrically and refined using a riding model with C-H = 0.98 Å for CH 3 [U iso (H) = 1.5Ueq(C)].

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.