Bis(1-ammonioethane-1,1-diyldiphosphonato-κ2 O,O′)diaquacobalt(II) nonahydrate

In the title compound, [Co(C2H8NO6P2)2(H2O)2]·9H2O, the CoII atom has a slightly distorted octahedral coordination environment consisting of four deprotonated phosphonate O atoms of two independent 1-aminoethylidendiphosphonate anions and complemented by the O atoms of two water molecules in cis positions. The anions exists in the zwitterionic form (protonated amino group and two deprotonated phosphonate O atoms) and constitute two six-membered chelate rings. The crystal structure also contains nine partly disordered uncoordinated water molecules, which create an extensive three-dimensional network of strong O—H⋯O and N—H⋯O hydrogen bonds.

In the title compound, [Co(C 2 H 8 NO 6 P 2 ) 2 (H 2 O) 2 ]Á9H 2 O, the Co II atom has a slightly distorted octahedral coordination environment consisting of four deprotonated phosphonate O atoms of two independent 1-aminoethylidendiphosphonate anions and complemented by the O atoms of two water molecules in cis positions. The anions exists in the zwitterionic form (protonated amino group and two deprotonated phosphonate O atoms) and constitute two six-membered chelate rings. The crystal structure also contains nine partly disordered uncoordinated water molecules, which create an extensive three-dimensional network of strong O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds.

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). There is evidence that application of transition metal bisphosphonates can improve fixation of cementless metal implants by enhancing the extent of osseointegration (Eberhardt et al., 2005). In this respect, a detailed structure-correlated study of the individual properties and the complex-forming driving factors is desired in order to sufficiently understand bisphosphonate physiological activities.
Several structures of Co II aminoethylidenediphosphonates have been reported previously (Xiang et al. 2007;Yin et al. 2005). The main difference between these structures and the title compound is the presence of two water molecules instead of a 1,10-phenanthroline ligand in the coordination environment of the transition metal ion (Xiang et al. 2007), leading also to a different symmetry.
The asymmetric unit of the title compound contains one molecule of the complex (Fig.1). Two 1-aminoethylidendiphosphonate anions chelate the central metal ion via two oxygen atoms from phosphonate groups forming six-membered non-planar metalla rings. Two water molecules complement the slightly distorted octahedral coordination environment of Co in cis-position. The Co-O bond lengths have expected values and conform with the previously reported related structures (Xiang et al., 2007). The values of the O-Co-O angles are in the range from 89.23 (7)° to 91.54 (5)°. The Co1-O1-P1-C1-P2-O4 and Co1-O7-P3-C3-P4-O10 metalla cycles have an envelope conformation with the C1 and C3 atoms out of plane by 0.850 (2) Å and 0.795 (2) Å, respectively. The dihedral angle between the planar fragments Co1-O1-P1-P2-O4 and Co1-O7-P3-P4-O10 is 84.20 (3)°. The coordinated ligand molecules exists in the zwitterionic form with a proton transfer from one of the phosphonic groups to the amino group which is representative for all 1-aminodiphosphonic acids. In addition, the amino group does also not participate in coordination (Dudko et al. 2009).
In the crystal structure of the title compound, nine solvent water molecules are present. Such an amount of solvent molecules could be explained by the presence of two coordinated water molecules in addition to the more hydrophilic phosphonate groups. As a result, a 3-D network of mostly strong O-H···O and N-H···O hydrogen bonds is observed in the structure ( Fig. 2; Table 1). Several H-bonds can not be unambiguously derived from the model because some of the crystal lattice water molecules are disordered.

Experimental
Light pink crystals of the title compound were obtained from the mixture of 10 ml (10 -2 mol/l) water solution of Co(NO 3 ) 2 with 20 ml (10 -2 mol/l) solution of 1-aminoethylidendiphosphonic acid. The combined solution was stored in a dark place for slow evaporation. After 20 days of staying, suitable crystals for X-ray data collection were obtained.

Refinement
In the crystal structure of the title compound, O atoms O18 and O20 are disordered over two sites with occupancies 0.87/ 0.13. Disordered O atoms O22 and O23 were treated with occupancies 0.88/0.12 and 0.71/0.29, respectively. The major component of the disordered site was refined anisotropically, the corresponding minor occupied sites were refined isotropically. Hydrogen atoms bonded to the disordered oxygen atoms could not be located from difference Fourier maps and were eventually omitted from refinement. Other H atoms bonded to N and O atoms were located in a difference map and refined freely with Uiso(H) = 1.5Ueq(N) and Uiso(H) = 1.2Ueq(O), respectively. Methyl hydrogens atoms were positioned geometrically and were refined using a riding model with C-H = 0.98 Å for CH 3 [Uiso(H) = 1.5Ueq(C)]. Fig. 1. The molecular structure of title compound showing 50% probability displacement ellipsoids for the non-hydrogen atoms. Solvent water molecules are omitted for clarity.

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