3-Nitro-5-(4-pyridinio)benzoate

The title compound, C12H8N2O4, crystallizes as a zwitterion in which the pyridyl N atom is protonated. The dihedral angle between the benzene and pyridinium rings is 27.9 (2)°. In the crystal, N—H⋯O hydrogen bonds link adjacent zwitterions into a three-dimensional structure.

The title compound, C 12 H 8 N 2 O 4 , crystallizes as a zwitterion in which the pyridyl N atom is protonated. The dihedral angle between the benzene and pyridinium rings is 27.9 (2) . In the crystal, N-HÁ Á ÁO hydrogen bonds link adjacent zwitterions into a three-dimensional structure.

Related literature
The title compound was reacted with MgCl 2 under hydrothermal conditions in an attempt to obtain a new coordination polymer as part of our investigation of pyridine caboxylate coordination polymers.   Table 1 Hydrogen-bond geometry (Å , ).

Comment
Hydrothermal synthesis has been successful in the preparation of new materials, because problems associated with ligand solubility were minimized and the reactivity of reactants was enhanced during the crystallization process in a heated sealed solution above ambient temperature and pressure (Feng et al., 2001;Tao et al., 2001). Thus, we have reacted 5-(4-pyridyl)-3-nitrobenzoic acid with MgCl 2 under hydrothermal conditions in an effort to obtain a new coordination polymer as part of further investigation of pyridine caboxylate coordination polymers (Lu et al., 2003). In fact, no complex was formed, but we report here the crystal structure of the starting organic compound.
In the title compound, C 12 H 8 N 2 O 4 , the pyridyl N atom is protonated, and the compound is formally a zwitterion.

Experimental
A mixture of MgCl 2 (0.1 mmol, 0.01g), 5-(4-pyridyl)-3-nitrobenzoic acid (0.1 mmol, 0.025 g) and 10 ml of H 2 O was loaded in a 20 ml Teflon-lined stainless steel vessel and heated at 303K for 3 days. Colourless crystals were obtained when the solution was slowly cooled to room temperature.

Refinement
H atoms were placed at calculated positions and were treated as riding on the parent C or N atoms with C-H = 0.93 Å, N-H = 0.86 Å, and with U iso (H) = 1.2 U eq (C, N). In the absence of significant anomalous scattering Friedel pairs were merged.

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.