4-Chloro-N-(2,6-dichlorophenyl)benzamide

The title compound, C13H8Cl3NO, crystallizes with four molecules in the asymmetric unit. In the molecular structure, the conformations of the central amide –CONH group show a wide range of dihedral angles with respect to the attached aromatic rings (benzoyl and anilino). The dihedral angles between the amide group and the benzoyl ring are 8.1 (3), 4.3 (3), 27.8 (1) and 32.7 (2)° in the four molecules. The amide group is twisted out of the plane of the anilino ring, as shown by the dihedral angles of 85.4 (1), 74.3 (1), 88.1 (1) and 77.6 (1)° in the four molecules. The aromatic rings are oriented at dihedral angles of 86.6 (1), 78.0 (1), 60.3 (1) and 69.8 (1)° in the four molecules. The crystal structure is stabilized via intermolecular N—H⋯O hydrogen bonds, aromatic aromatic interactions, short Cl⋯Cl contacts and C—H⋯Cl hydrogen bonds. Intermolecular hydrogen bonds connect the molecules into two distinct chains running along the c axis of the crystal. One molecule forms an inversion dimer in which the main interactions are π–π stacking [centroid–centroid distances = 3.749 (1) and 3.760 (1) Å] and a short Cl⋯Cl contact of 3.408 (1) Å.

The title compound, C 13 H 8 Cl 3 NO, (Fig.1), has four unique molecules in the asymmetric unit (further marked as A, B, C and D). In the molecular structure, the conformations of the central amide group CONH with respect to the attached aromatic rings (benzoyl C2/C7, anilino C8/C13) show a wide range of dihedral angle values, which is an indication that the energy of intermolecular interactions is comparable to the barriers of internal rotations. The dihedral angle between the amide group and the benzoyl ring is 8.1 (3), 4.3 (3), 27.8 (1) and 32.7 (2)° in the molecules A, B, C and D, respectively. The amide group is heavily twisted out of the plane of the anilino ring, with dihedral angles of 85.4 (1), 74.3 (1), 88.1 (1), 77.6 (1)° for the molecules A, B, C and D, respectively. This conformation can be attributed to the steric effect of the bulky chloro groups in ortho positions. We recall that the corresponding dihedral angle in the parent molecule benzanilide is ca 31° (Bowes, et al., 2003). The aromatic rings (benzoyl and anilino) are oriented at dihedral angles of 86.6 (1), 78.0 (1), 60.3 (1) and 69.8 (1)°i n the molecules A, B, C and D, respectively. In benzanilide, the aromatic rings make a dihedral angle of ca 61°. The bond lengths and bond angles lie within the ranges expected for similar compounds. The endocyclic bond angles in both aromatic rings are sligtly distorted from the ideal value of 120°, reflecting the chloro-substitution effect on the benzene rings. The orientation of an amide group with respect to aromatic rings strongly depends on the local crystal field, which is manifested by significant differences in the torsion angles describing the conformations in the molecules A, B, C and D.
The crystal structure is stabilized by intermolecular N-H···O hydrogen bonds, aromatic aromatic interactions, short Cl···Cl contacts and weak C-H···Cl hydrogen bonds. Intermolecular N-H···O hydrogen bonds connect the molecules into two distinct chains running along the c axis of the crystal (Fig. 2). The first chain is linked by hydrogen bonds arising between amidic N, O atoms of the molecules A and D. The second chain is linked by hydrogen bonds arising between amidic N, O atoms of the molecules B and C. The chains are coupled via stacking interactions. The two most important π -π stacking formations, which we found using the PLATON software (Spek, 2009), are: The stacking between the benzoyl rings of the molecule C at the positions (x,y,z) and (1 -x,1 -y,1 -z). The interplanar distance is 3.538 Å, offset 1.241 Å and ring-centroids separation 3.749 Å. The second stacking is between the anilino rings of the molecule C at the positions (x, y, z) and (1 -x, -y, 1 -z). The interplanar distance is 3.411 Å, offset 1.582 Å and ring-centroids separation 3.760 Å.
The molecule C forms an interesting inversion dimer (Fig. 3), with a head-to-tail arrangement. The dimer is stabilized by π -π interaction, a short Cl···Cl contact of 3.408 (1) Å, and possibly dipolar interaction between carbonyl group dipoles. A supplementary materials sup-2 non-conventional C-H···Cl hydrogen bond (C10c-H10c···Cl2b) adds to the mosaic of interactions in the crystal structure of the title compound. The H10c···Cl2b distance of 2.82 Å is 0.13 Å shorter than the the sum of van der Waals radii for H and Cl.

Experimental
The title compound was prepared according to the method of Gowda et al. (2003). Single crystals used in X-ray diffraction studies were obtained by slow evaporation of its ethanolic solution at room temperature. The purity of the compound was checked by determining its melting point (172-173 °C). The compound was characterized via IR and NMR spectroscopy. (C-9,13), 128.8 (C-10,12), 128.6 (C-11).

Refinement
Most of hydrogen atoms were placed in calculated positions with C-H distances in the range 0.93-0.96 Å and constrained to ride on their parent atoms. Amide H atoms were seen in difference maps and were refined with the N-H distances restrained to 0.85 (2) Å. Hydrogen atoms H10, H11 and H12 in the molecule D were refined with C-H distance restrained to 0.94 (3) Å with the aim to remove a Hirschfeld test alert. The U values of 8 carbon atoms and 2 chlorine atoms were subject to a rigid bond restraint (DELU command), i.e. the components of the displacement parameters in the direction of the bond were restrained to be equal within an effective standard deviation (e.s.d. = 0.007 for atoms C2a, C3a, C8c, C9c, C13c, Cl3c, C10d, C11d and e.s.d. = 0.004 for atoms C13d and Cl3d). The U iso (H) values were set at 1.2U eq (C-aromatic,N).
H atoms not involved in hydrogen bonding are omitted.  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.