Competition between chalcogen and halogen bonding assessed through isostructural species.

The amino group of 2-amino-5-(4-halophenyl)-1,3,4-chalcogenadiazole has been replaced with bromo/iodo substituents to obtain a library of four compositionally related compounds. These are 2-iodo-5-(4-iodophenyl)-1,3,4-thiadiazole, C 8 H 4 I 2 N 2 S, 2-bromo-5-(4-bromophenyl)-1,3,4-selenadiazole, C 8 H 4 Br 2 N 2 Se, 2-bromo-5-(4-iodophenyl)-1,3,4-selenadiazole, C 8 H 4 BrIN 2 Se, and 2-bromo-5-(4-iodophenyl)-1,3,4-thiadiazole, C 8 H 4 BrIN 2 S. All were isostructural and contained bifurcated Ch...N (Ch is chalcogen) and X...X (X is halogen) interactions forming a zigzag packing motif. The noncovalent Ch...N interaction between the chalcogen-bond donor and the best-acceptor N atom appeared preferentially instead of a possible halogen bond to the same N atom. Hirshfeld surface analysis and energy framework calculations showed that, collectively, a bifurcated chalcogen bond was stronger than halogen bonding and this is more structurally influential in this system.