4-Cyclobutylamino-3-nitrobenzoic acid

The asymmetric unit of the title compound, C11H12N2O4, contains two crystallographically independent molecules with similar geometries. Both molecules contain an intramolecular N—H⋯O hydrogen bond. The dihedral angles between the benzene ring and the mean plane of the cyclobutane ring are 38.29 (7) and 57.04 (8)° in the two molecules, and the nitro group is twisted slightly away from the plane of the benzene ring [dihedral angles = 9.15 (12) and 9.55 (12)° in the two molecules]. In the crystal, the independent molecules are linked into dimers by O—H⋯O hydrogen bonds between their carboxyl groups, and C—H⋯O and C—H⋯π interactions are formed between dimers.


Comment
Multi functionalized benzimidazole remains as an attractive scaffold to display essential binding moieties against many validated biological targets (Wright, 1951;Singh et al., 2009). This heterocycle is commonly accessed via nitro benzoic acid derivatives, which form a part of our current synthetic chemistry work (Narendra Babu et al., 2009a&b;Ishida et al., 2006). Recently, we have successfully synthesized the title compound, whose crystal structure is described here.
The asymmetric unit contains two crystallographically independent molecules (Fig. 1), A and B with similar geometries. An intramolecular N-H···O hydrogen bond is formed in both independent molecules. The dihedral angle formed by the C1A-C6A benzene ring and C7A-C10A cyclobutane is 38.29 (7)° and that between the C1B-C6B benzene ring and C7B-C10B cyclobutane is 57.04 (8)°. The nitro group in each molecule is slightly twisted away from the attached benzene ring as indicated by the torsion angle O3-N1-C1-C2, being 7.97 (15)° and 7.80 (14)° in molecules A and B, respectively.
The crystal packing (Fig. 2) is consolidated by intermolecular O-H···O and C-H···O hydrogen bonds. Molecules are linked by O-H···O hydrogen bonds between their carboxylate groups to form dimers. The crystal structure is further stabilized by C-H···π (Table 1) interactions involving the C1B-C6B benzene ring (centroid Cg1) and short O4B···O4B contacts (symmetry code: 2 -x, 1 -y, 1 -z) with distance = 2.8957 (12) Å which is shorter than the sum of van der Waals radii of the O atoms.

Experimental
The title compound was obtained by refluxing ethyl 4-(cyclobutylamino)-3-nitro-benzoate (0.2 g, 0.0007 mol) and KOH (0.08 g, 0.0015 mol) in aqueous ethanol (5 ml) for 3 h. After completion of the reaction, ethanol was distilled off and the reaction mixture was diluted with water (5 ml). The aqueous layer was washed with dichloromethane (2 × 5 ml) and acidified with concentrated hydrochloric acid to afford a yellow solid. Yellow crystals suitable for X-ray analysis were obtained after recrystallization of the crude product with hot ethyl acetate.

Refinement
H atoms bound to N and O atoms were located from difference Fourier maps. Atoms H1NA and H1NB were refined freely, while atoms H1OA and H1OB were refined as riding on the parent O atom with U iso (H) = 1.5 U eq (O). The remaining H atoms were positioned geometrically and refined using a riding model, with C-H = 0.93-0.98 Å and U iso (H) = 1.2 or 1.5 U eq (C). Fig. 1   supplementary materials sup-9