3,7-Dichloroquinoline-8-carboxylic acid

The title compound (trade name: quinclorac), C10H5Cl2NO2, was crystallized from a dimethyl sulfoxide solution. Quinclorac molecules are packed mainly via π–π stacking interactions between neighbouring heterocycles (interplanar distance: 3.31 Å) and via O—H⋯N hydrogen bonding.

A quinclorac molecule, which is the asymmetric unit of the structure, is shown in Fig. 1. All the bond distances and bond angles of quinclorac are normal and call for no further comment. Two types of intermolecular interations are easily found in the structure of quinclorac (Fig. 2). There exists a π-π interaction between adjacent quinin cycles with an inversion center located halfway between the aromatic rings, thus forming stacks along the a direction. Quinclorac molecules of adjacent chains are joined through H-bonding of O1-H1···N1 i (symmetry code: (i) 1 -x, 2 -y, 1 -z) ( Table 1) into a triclinic supramolecular architecture (Fig. 2).

Experimental
Quinclorac was obtained from a commercial source and used directly without further purification. Quinclorac (0.5 mmol, 0.121 g) was dissolved in 10 mL DMSO. After ether vapor slowly diffused into the solution at room temperature for several days, colorless prismlike crystals suitable for crystallographic research were obtained.

Refinement
All the non-hydrogen atoms were located from the Fourier maps, and were refined anisotropically. The hydroxyl hydrogen, H1A, was found from the Fourier difference maps and refined isotropically with a fixed O-H bond length. All other H atoms were positioned geometrically. All isotropic vibration parameters of hydrogen atoms were related to the atoms which they are bonded to with U iso (H) = 1.2 U eq (C,O). Fig. 1. The asymmetric unit of quinclorac with atom labels and 50% probability displacement ellipsoids for non-H atoms.

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.