Role of secondary interactions in a series of 2:1 halogen-bonded cocrystals formed between 4-(dimethylamino)pyridine and ditopic halogen-bond donors.

The structures of a series of 2:1 cocrystals formed between 4-(dimethylamino)pyridine and each of 1,2,4,5-tetrachloro-3,6-diiodobenzene, 2CHN·CClI, 1,2,4,5-tetrabromo-3,6-diiodobenzene, 2CHN·CBrI, 1-bromo-4-iodo-2,3,5,6-tetrafluorobenzene, 2CHN·CBrFI, and 1,2-dibromo-4,5-difluoro-3,6-diiodobenzene, 2CHN·CBrFI, are reported. In all five structures, the core halogen-bonded 2:1 trimolecular units have geometrically similar parameters, with the central halogen-bond donor flanked by two pyridine halogen-bond acceptors twisted with respect to the central halogen-bond donor at angles ranging from 76 to 86°. The I...N halogen-bond separations are all short, ranging from 73.3 to 76.7% of the sum of the van der Waals radii, while the C-I...N bond angles are essentially linear. The Br...N halogen-bond separation in the cocrystal formed with 1-bromo-4-iodo-2,3,5,6-tetrafluorobenzene is 80.4% of the sum of the van der Waals radii. Subtle differences in the crystal packings are attributed to the role of secondary C-H...π and weak π-type interactions with chloro and bromo substituents. The cocrystals 2CHN·CClI and 2CHN·CBrI are isomorphous.