Nickel alendronate

The title compound {systematic name: bis(μ2-dihydrogen 4-azaniumyl-1-hydroxybutane-1,1-diphosphonato)bis[aqua(dihydrogen 4-azaniumyl-1-hydroxybutane-1,1-diphosphonato)nickel(II)] dihydrate}, [Ni2(C4H12NO7P2)4(H2O)2]·2H2O, was synthesiized under hydrothermal conditions. Its structure is isotypic with the CoII analogue. The crystal structure is built up from centrosymmetric dinuclear complex molecules and the structure is reinforced by a net of intermolecular O—H⋯O and N—H⋯O hydrogen bonds. One water molecule is bound to the NiII atom in the octahedral coordination sphere, while the second is part of the intermolecular hydrogen-bond system.


Comment
Bisphosphonates are organic analogues of pyrophosphates with the P-O-P bridge replaced with a hydrolytically resistant P-C-P moiety. Their structure and therapeutic properties have been of vivid scientific interest for over 40 years (Russell, 2011). Bisphosphonates play essential role in modification of biomineralization in bones. Apart from the most important calcium salts, transition metal complexes are also being studied in respect to complex formation constants and X-ray structures e.g. to estimate and elucidate potential side effects of bisphosphonate drugs against osteoporosis. It was noticed (Man et al. 2006) that the length of side alkyl chain is crucial for determination of aggregation of metal bisphosphonates.
For instance, in the case of Co bisphosphonates with six-carbon chain the mononuclear product was found, while the four-carbon hydrocarbon chain facilitated formation of the dinuclear complexes while shorter hydrocarbon side chains led to more or less complicated polymeric structures. Magnetic properties of the cobalt compounds have risen some interest and were examined in details.
Consequently divalent metals give neutral complexes (usually chelates) with metal to ligand ratio of 1:2.
The title compound was obtained from sodium salt of 4-amino-1-hydroxy-1,1-butylidenebisphosphonic acid and nickel(II) chloride in acidic aqueous solution. Both acidification and rising the temperature to ca 130 °C were necessary to obtain single crystals of X-ray quality. The product is insoluble in water and common organic solvents. The afforded crystals were investigated by single-crystal X-ray diffraction and additionally by microanalysis and powder diffraction in order to test the purity and composition of the whole batch.
The structure of the obtained compound, C 16 H 52 N 4 Ni 2 O 30 P 8 *2(H 2 O), turned out to be isomorphic with structure of cobalt derivative which was determined by Man et al. 2006. Structure composed of dinuclear complexes (though not isomorphic with the described above) was also found for zinc alendronate (Dufau et al. 1995). The text below recapitulates the main structural features of the determined structure.
Crystals are build up from centrosymmetric dinuclear complex molecules. Each metal atom coordination is close to octahedral, with one terminal water molecule, one terminal and two bridging bisphosphonate anions. All bisphosphonato ligands are chelating and contain one NH 3 + and two -P(O)(O -)(OH) groups. The terminal ligands are bidentate, while the bridging ones are tridentate: one PO 3 H group is monodentate 1κ-O and the other is bridging bidentate 1κ-O′,2κ-O′′, using both nagatively charged O atoms and one oxygen atom from P=O group. Bond lengths allow only for general identification of P=O and P-O -(ca 1.50 Å) or P-OH groups (ca 1.57 Å).
The system of hydrogen bonds is rather complex, see the relevant table. Packing of molecules is reinforced by O-H···O and by charge assisted (+)N-H···O hydrogen bonds. However, all internal hydrogen bonds can be easily recognized by the symmetry code of the acceptor atom being [-x + 1, -y + 1, -z + 1] (intramolecular inversion) or none. The alkylammonium chain N1 extends away from the core and forms only intermolecular hydrogen bonds with ligating and non supplementary materials ligating phosphonate O-atoms. Interesting R 2 2 (16) centrosymmetric motif is formed by N1 ··· O5 bond around the b axis (see Figure 2.) The other alkylamonium chain is bent towards the central dinuclear core to facilitate intramolecular hydrogen bonding between the ammonium terminus and the O atoms. In fact, N2 ammonium groups form intramolecular as well as intermolecular hydrogen bonds. Hydroxyl group bound to carbon forms internal hydrogen bond O13-H ···O12[1 -x,1 -y,1 -z] and O14-H···O4[1 -x,1 -y,1 -z] and O14-H···O7. Water molecule (O15) bound to nickel atom forms hydrogen bonds with the next water molecule (O16) in the second coordination sphere. Apart from that extended intermolecular hydrogen bond network is present.
Microanalysis and powder diffraction pattern confirm the expected composition. Some discrepances between monocrystalic simulated intensities and experimental powder XRD intensities stem most likely from not uniform distribution of orientation of microcrystalites in the "powder" sample. Nevertheless, positions of all recorded peaks are correct.

Refinement
Structure was solved with all heavy atoms treated as anisotropic and H-atoms as isotropic. All C-H atoms were refined as riding on their bonded counterpart atoms with the usual constrains. Hydrogen atoms belonging to water molecules were refined with constrained O-H bond length to 0.84 Å. Two reflections (040) and (011)    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 > 2σ(F 2 ) is used only for calculating R-factors(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.