Halogen Bonding in Halothiophene Building Blocks

Thiophenes bearing monotopic and symmetric ditopic halogen substituents and asymmetric ditopic halogen and pyridine substituents that act as halogen bond acceptors were evaluated for their halogen bonding interactions with 1,4-diiodotetrafluorobenzene and N-iodosuccinimide. The combinations resulted in C/N-I···X′ (X′ = Cl, Br, I) and C/N-I···N_Py (Py = pyridine) interactions, the former characterized by an interaction energy (ΔE_int) ranging from −4.4 to −18.7 kJ mol^-1 and the latter from −26.3 to −56.0 kJ mol^-1. X-ray crystallography studies show that the ditopic asymmetric systems consisting of both halogens and pyridine self-associate through C-X···N_Py (X = Cl, Br, I) halogen bonding, and in their optimized structures the energies range from −5.4 to −19.6 kJ mol^-1 depending on the type of halogen atom present. The ¹H NMR association constants of N-I···N_Py halogen bonds range between 1405 and 6397 M^-1. The σ-hole strengths of halogens have been useful in describing the interaction energies and solution models.