Investigating C═S···I Halogen Bonding for Cocrystallization with Primary Thioamides

Cocrystallization utilizing halogen bonding involving the thiocarbonyl functional group of a series of primary aromatic thioamides has been investigated. The well-known organoiodide 1,4-diiodotetrafluorobenzene was utilized as the halogen bond donor and the C═S···I halogen bond was established as a robust supramolecular synthon in these systems. Weak N–H···S hydrogen bonding involving the thioamides influences the overall supramolecular architectures, meaning that there is a diverse range of structural motifs and cocrystal stoichiometries observed. The majority (60%) of the cocrystals obtained have a 2:1 ratio of thioamide/organiodide with the latter present over an inversion center. The higher ratio of organoiodide seen in the other cocrystals is achieved by additional I···I and I···π halogen bonding. The C═S···I halogen bond is replaced by N···I halogen bonding in the one cocrystal containing a pyridyl-substituted thioamide. The ability of the thioamides to form cocrystals and the strength of the halogen bond were influenced by the nature of the substituents on the aromatic ring, with derivatives containing electron donating groups most likely to form cocrystals. Calculated molecular electrostatic potential values on the sulfur atom in the thioamides corroborate these experimental results.