Two different modes of halogen bonding in two 4-nitroimidazole derivatives

In the crystal structures of the two imidazole derivatives 5- chloro- 1,2- dimethyl- 4- nitro- 1H- imidazole, C5H6ClN3O2, ( I), and 2- chloro- 1- methyl- 4- nitro- 1H- imidazole, C4H4ClN3O2, ( II), C - Cl center dot center dot center dot O halogen bonds are the principal specific interactions responsible for the crystal packing. Two different halogen- bond modes are observed: in ( I), there is one very short and directional C - Cl center dot center dot center dot O contact [ Cl center dot center dot center dot O = 2.899 ( 1) A], while in ( II), the C - Cl group approaches two different O atoms from two different molecules, and the contacts are longer [ 3.285 ( 2) and 3.498 ( 2) A] and less directional. In ( I), relatively short C - H center dot center dot center dot O hydrogen bonds provide the secondary interactions for building the crystal structure; in ( II), the C - H center dot center dot center dot O contacts are longer but there is a relatively short pi-pi contact between molecules related by a centre of symmetry. The molecule of ( I) is almost planar, the plane of the nitro group making a dihedral angle of 6.97 ( 7) degrees with the mean plane of the imidazole ring. The molecule of ( II) has crystallographically imposed mirror symmetry and the nitro group lies in the mirror plane.