The halogen bond has been widely used as an important supramolecular tool in various research areas. However, there are relatively few studies on halogen bonding related to molecular chirality. 3-(2-Halophenyl)quinazoline-4-thione derivatives have stable atropisomeric structures due to the rotational restriction around an N-C single bond. In X-ray single crystal structures of the racemic and optically pure N-C axially chiral quinazoline-4-thiones, we found that different types of intermolecular halogen bonds (C=S⋯X) are formed. That is, in the racemic crystals, the intermolecular halogen bond between the ortho-halogen atom and sulfur atom was found to be oriented in a periplanar conformation toward the thiocarbonyl plane, leading to a syndiotactic zig-zag array. On the other hand, the halogen bond in the enantiomerically pure crystals was oriented orthogonally toward the thiocarbonyl plane, resulting in the formation of a homochiral dimer. These results indicate that the corresponding racemic and optically pure forms in chiral molecules are expected to display different halogen bonding properties, respectively, and should be separately studied as different chemical entities.
Keywords: atropisomers; axial chirality; halogen bonds; quinazolinones; thiones.