A second triclinic polymorph of (1-ammonio-1-phosphonoethyl)phosphonate

The asymmetric unit of the second polymorph of the title compound, C2H9NO6P2, contains one molecule existing as a zwitterion. The N atom of the ammonio group is protonated and one of the phosphonic acid groups is deprotonated. Bond lengths and angles are similar in both polymorphs. Besides the differences in cell parameters, the most significant structural difference between this structure and that of the first polymorph [Dudko, Bon, Kozachkova, Tsarik & Pekhno (2008 ▶), Ukr. Khim. Zh. 74, 104–106] is the presence of strong symmetric hydrogen bonds between neighbouring phosphonate groups. H atoms involved in these hydrogen bonds are located at inversion centres and O⋯O distances are observed in the range 2.458 (5)–2.523 (5) Å. These bonds and additional O—H⋯O and N—H⋯O hydrogen bonds interlink the molecules, giving a three-dimensional supromolecular network.

The asymmetric unit of the second polymorph of the title compound, C 2 H 9 NO 6 P 2 , contains one molecule existing as a zwitterion. The N atom of the ammonio group is protonated and one of the phosphonic acid groups is deprotonated. Bond lengths and angles are similar in both polymorphs. Besides the differences in cell parameters, the most significant structural difference between this structure and that of the first polymorph [Dudko, Bon, Kozachkova, Tsarik & Pekhno (2008), Ukr. Khim. Zh. 74,[104][105][106] is the presence of strong symmetric hydrogen bonds between neighbouring phosphonate groups. H atoms involved in these hydrogen bonds are located at inversion centres and OÁ Á ÁO distances are observed in the range 2.458 (5)-2.523 (5) Å . These bonds and additional O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds interlink the molecules, giving a three-dimensional supromolecular network.

Comment
Diphosphonic acids with the P-C-P fragment, commonly named bisphosphonates, are well known since the 19 th century.
Due to the specific geometry and mutual influence of phosphonate fragments, compounds of this class possess a number of unique properties compared to other derivatives of phosphonic acids. Diphosphonic acids are structural analogues of inorganic pyrophosphate, one of the major metabolites in the cells, involved as a product in more than sixty biochemical reactions. Drugs prepared on the basis of bisphosphonates are highly efficient as a regulator of calcium metabolism and the immune response, they are used as anti-neoplastic, anti-inflammatory and antiviral agents and drugs with analgesic effect and as a component of toothpastes biphosphonates prevent the formation of tartar (Matkovskaya et al., 2001).
However, it is still not clearly understood why small structural modifications of the bisphosphonates may lead to extensive alterations in their physicochemical, biological and toxicological characteristics (Matczak-Jon & Videnova-Adrabinska, 2005). As a consequence of that determination of the structure of the bisphosphonates is very important to understand the influence of structural modifications on complex-forming abilities and physiological activities and deriving structure properties relations in general. In the present work we report a second polymorph of the title compound which crystallizes in the space group (P1) whereas the previously described polymorph modification also crystallizes in the triclinic space group P1 but with different cell parameters and cell volume (Dudko et al., 2008). The asymmetric unit of the title compound ( Fig. 1) contains one molecule in zwitterionic form with a proton transferred from one of the phosphonic groups to the amino group which is typical for all investigated 1-aminodiphosphonic acids (Fernández et al., 2003;Li et al., 2009). Bond lengths and angles are within normal ranges (Allen et al., 1987) and are comparable with the first polymorph modification of the compound. It is generally accepted that, for O-H-O interactions where O···O is about 2.50 Å, examples can be found of truly symmetric hydrogen bonds, most of which have crystallographic equivalence between donor-acceptor atoms (Meot-Ner, 2005;Catti & Ferraris, 1976). The title compound, displays such short and strong symmetric hydrogen bonds between neighboring phosphonate groups, with the H atoms located at inversion centres and O···O distances of 2.458 (5) and 2,523 (5) Å. Multiple N-H···O and O-H···O hydrogen bonds in the crystal structure form an intricate three-dimensional supramolecular network (Fig.2, Table 1).

Experimental
The title compound C 2 H 9 NO 6 P 2 , was obtained by the reaction of acetonitrile which was brought into contact with the dry hydrogen chloride and phosphorus trichloride at 278 K followed by the dropwise addition of water. The obtained solution was treated by a mixture of acetone and diethyl ether, resulting in a white precipitate of the title compound. The resulting residue was dissolved in water and was stored in a dark place for slow evaporation. After 14 d of staying, suitable crystals for X-ray data collection were obtained.
supplementary materials sup-2 Refinement H atoms bonded to O and N atoms were located in a difference Fourier map. Their positions were refined freely whereas thermal parameters were fixed to U iso (H) = 1.5U eq (N,O). Other H atoms which bonded to C were positioned geometrically and refined using a riding model with C-H = 0.96 Å for CH 3 with U iso (H) = 1.5U eq (C). Fig. 1. View of the molecular structure of the title compound. Ellipsoids are drawn at the 50% probability level. H atoms are represented as small spheres of arbitrary radii.  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.