Methyl 3-carboxy-5-nitrobenzoate

The structure of the title compound, C9H7NO6, is essentially planar [maximum deviation 0.284 (2)Å] except for the methyl H atoms. The crystal structure is stabilized by asymmetric O—H⋯O hydrogen bonds linking the hydrogen carboxylates into pairs around the inversion centres. There is also π–π stacking of the benzene rings [centroid–centroid distance 3.6912 (12) Å].


Comment
The molecule of the title complex ( Fig.1) is useful as an important intermediate for the preparation of iodinated X-ray contrast media, such as iotalamic acid, ioxitalamic acid, and Ioxilan, which are used clinically all over the world (Morin et al., 1987;Singh et al., 1980;Stacul et al., 2001). We report here the crystal structure of title compound. The crystal data show that the bond lengths and angles are within expected ranges. TThe molecule is essentially planar: the maximum deviation from the weighted least-squares plane calculated through all the non-H atoms is 0.284 (2)Å for O2. The molecules are stacked via π-π interactions, with the centroid-centroid distance of 3.6912 (12) The O-H···O hydrogen bonds bind the hydrogencarboxylates into pairs.

Experimental
Dimethyl 5-nitroisophthalic acid (956 mg, 4 mmol) was dissolved in hot methanol (6 ml), then sodium hydroxide (152 mg, 3.8 mmol) in methanol (2 ml) was added and refluxed for 30 min. Methanol was distilled off. The solid residue was extracted by warm water and the undissolved diester was filtered off. The filtrate was acidified with 1 mol/l hydrochloric acid (4 ml). The precipitate was filtered and washed with cold water. The crude product was purified by recrystallization.
Single crystals were grown by slow evaporation of a ethanol/water (v/v 1:1) solution: colourless block-shaped crystals were formed after several days.

Refinement
All the H atoms could have been discerned in the difference electron density maps. With exception of the hydrogen belonging to the hydroxyl group of the hydrogencarboxylate the hydrogens were situated into the idealized positions and refined in riding motion approximation. The hydroxyl hydrogen was refined freely. The used constraints: C aryl -H = 0.93 Å, U iso (H) = 1.2U eq (C aryl ); C methyl -H = 0.96 Å, U iso (H) = 1.5U eq (methyl). Fig. 1. A view of the title compound with the atomic numbering scheme. Displacement ellipsoids were drawn at the 30% probability level.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > 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.