1-Ammonio-1-phosphonopentane-1-phosphonic acid

The title compound, C5H15NO6P2, was obtained by the reaction of pentanenitrile with PCl3 followed by the dropwise addition of water. The asymmetric unit contains one molecule, which exists as a zwitterion with a positive charge on the –NH3 group and a negative charge on one of the phosphonic O atoms. The crystal structure displays N—H⋯O and O—H⋯O hydrogen bonding, which creates a three-dimensional network.

The title compound, C 5 H 15 NO 6 P 2 , was obtained by the reaction of pentanenitrile with PCl 3 followed by the dropwise addition of water. The asymmetric unit contains one molecule, which exists as a zwitterion with a positive charge on the -NH 3 group and a negative charge on one of the phosphonic O atoms. The crystal structure displays N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonding, which creates a three-dimensional network.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: EZ2151).

Comment
Organic diphosphonic acids are potentially very powerful chelating agents used in metal extractions and have been tested by the pharmaceutical industry for use as efficient drugs preventing calcification and inhibiting bone resorption (Tromelin et al., 1986, Matczak-Jon & Videnova-Adrabinska, 2005. Diphosphonic acids are used in the treatment of Paget disease, osteoporosis and tumoral osteolysis (Szabo et al., 2002).
The asymmetric unit of the title compound contains one molecule, which exists as a zwitterion with positive and negative charges on the NH 3 group and one of the phosphonic oxygen atoms, respectively. The phosphorus atoms display slightly distorted tetrahedral geometries, each provided by three oxygen atoms and one carbon atom. Bond lengths and angles have normal values (Allen et al., 1987).
There are no solvent water molecules in the asymmetric unit, which is unusual for α-aminodiphosphonic acids. This fact can be explained by the presence of the hydrophobic alkyl group. The structure is stabilized by a three-dimensional O-H···O and N-H···O hydrogen bonding network ( Fig. 1, Table 1).

Experimental
Dry hydrogen chloride at about 278 K was brought into contact with the surface of a mixture of pentanenitrile (83.13 g, 1 mol) and PCl 3 (87.4 ml, 1 mol). After an hour water (54 ml, 3 mol) was added to the mixture dropwise. After a day the solution was treated by an excess amount of water and then vacuum distilled. The obtained solution was treated by a mixture of acetone and diethyl ether, yielding colourless crystals of the title compound.