4-[2-(Hydrogen phosphonato)-2-hydroxy-2-phosphonoethyl]pyridinium

The title compound, C7H11NO7P2, exists as a zwitterion in which the positive charge resides on the protonated pyridyl N atom and the negative charge on one of the two phosphate groups. In the crystal, adjacent molcules are linked by O—H⋯O and N—H⋯O hydrogen bonds into a three-dimensional network.

The title compound, C 7 H 11 NO 7 P 2 , exists as a zwitterion in which the positive charge resides on the protonated pyridyl N atom and the negative charge on one of the two phosphate groups. In the crystal, adjacent molcules are linked by O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds into a three-dimensional network.

Comment
Metal diphosphonates have been extensively studied for the diversity structures. Monomeric, dimeric, polymeric, two-dimensional layers to three-dimensional frameworks are all featured among these complexs Ma et al., 2008;Ma et al., 2009).
Because diphosphonates have capabilities to stabilize transition metals in a wide range of oxidation states. The title compound was synthesized and characterized by X-ray crystal structure analysis.

Experimental
The title compound was synthesized by reaction of 4-pyridine acetic acid hydrochloride (0.520 g, 3 mmol) and phosphite (0.711 g, 8.7 mmol) in chlorobenzene (15 mL). The solution was stirred while phosphorus trichloride (0.64 g, 4.7 mmol) was added drop by drop and the temperature should control between 110-120 o C with vigorous stirring for 4 hours. The crude product was concentrated in vacuo. White block crystals formed in high yield by recrystallization.

Refinement
Carbon-bond H atoms were positioned geometrically (C-H = 0.93 Å), and were included in the refinement in the riding mode approximation, with U iso (H) = 1.2U eq (C). H atoms bound to O and N atoms were located in a difference Fourier map and refined with restraints [N-H and O-H = 0.82 (1) Å, with U iso (H) values fixed at 1.5U eq (N) and 1.5U eq (O)]. Fig. 1

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 > 2sigma(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.