Chloranilic acid: a redetermination at 100 K

The crystal structure of chloranilic acid, C6H2Cl2O4, was first described by Andersen in 1967 [Andersen, E. K. (1967). Acta Cryst. 22, 188–191] at room temperature using visually estimated intensities. Taking into account the importance of the title compound, we have redetermined the structure at 100 (1) K. The approximately planar molecule [the maximum deviation from the mean plane through the ring is 0.0014 (9) Å for the ring atoms and 0.029 (3) Å for the other atoms] occupies a special position, lying across the center of symmetry. In the crystal structure, a two-dimensional hydrogen-bonded network sustained by O—H⋯O interactions runs approximately parallel to [101]. The two-dimensional layers are further packed in a parallel fashion, stabilized by Cl⋯Cl interactions [Cl⋯Cl = 3.2838 (8) Å, C—Cl⋯Cl = 152.96 (6)°].

QNMHA thanks the University of Mysore for research facilities.
Very recently, a study on the formation of either salts or co-crystals by chloranilic acid with the different organic bases was published (Molčanov & Kojić-Prodić, 2010).
There is a number of structures in the Cambridge Database (Allen, 2002) that contain the chloranilic acid (2,5-dichloro-3,6-dihydroxycyclohexa-2,5-diene-1,4-dione, I -Scheme 1), either as a neutral molecule or as an anion (mono-or di-). Interestingly, the only determination of the structure of the acid itself dates back to 1967 (Andersen, 1967a; hereinafter referred to as KA67). The structure was refined based on the visually estimated intensities of the diffraction spots obtained by means of the Weissenberg equi-inclination method. The quality of this structure is excellent taking into account the technology involved, but -having in mind the importance of this small molecule -thanks to the advancement of the methodology it might be desirable to get the more accurate results. Here we report the results of the structure determination of (I) at 100 (1) K. The unit cell parameters of the accompanying room temperature experiment are in an excellent agreement with the data of KA67, but the model is much better, for instance in terms of R factors (8.9% in 1967, with 22 reflections omitted vs. 2.5% in the present determination), the only symmetry independent hydrogen atom was found in the difference Fourier map in KA67 and left in the position found, while now it was isotropicaly refined, etc. Nevertheless, the basic features of the structure are similar, and both the precision and depth of the analysis in KA67 and accompanying paper on the hydrate (Andersen, 1967b) are really remarkable.
In the crystal structure the main packing motif arises as the result of relatively strong intermolecular O-H···O hydrogen bonds, which make the antiparallel chains of molecules related by the 2 1 screw along y direction; using the graph-set notation (Bernstein et al., 1995), these first-order chains will be described as C(5). The neighboring chains are interconnected to give the centrosymmetric second-order rings R44(22) -cf. Chloranilic acid was purchased from Loba Chemie, Mumbai, India. X-ray quality crystals were obtained from methanol solution after slow evaporation.

Refinement
The position of the hydrogen atom was found in the difference Fourier map and both the positional and isotropic thermal parameters weres freely refined. Fig. 1. Anisotropic ellipsoid representation of the compound I together with atom labelling scheme. The ellipsoids are drawn at 50% probability level, hydrogen atoms are depicted as spheres with arbitrary radii.  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.