A Research Paper Published in Crystal Growth & Design

Three perfluorinated azobenzene ethers that are relatively large, non-planar V-shaped molecules differing only in the substituent (–H, –Cl or –CN) located on the opposite end of the molecule to the iodine atom have been prepared and then cocrystallized with a selection of nitrogen-containing halogen bond acceptors, in order to establish the effect of changing either the substituent or the acceptor on halogen bonding and the formation of cocrystals. Only 8 of the 24 possible combinations resulted in single crystals of the cocrystals, and those only for azobenzenes with the chloride (5 cocrystals) and cyanide substituents (3 cocrystals). Structural analysis has shown that the dominant supramolecular interaction in all cocrystals is the I···N halogen bond between the iodine atom of the perfluorinated fragment of the azobenzene and the nitrogen atom of the acceptor. Furthermore, in most cases discrete supramolecular trimer complexes are formed, with one exception when dimers are formed instead. Since the molecular electrostatic potential values calculated in the area around the iodine atom for the optimized geometries of azobenzene molecules expectedly feature minimal differences, reasons for the significantly different cocrystallization results should be found mostly in the stabilization of the products by secondary supramolecular interactions. This study provides insight into the cocrystallization challenges for structurally more complex halogen bond donor molecules, as well as into differences in supramolecular results compared to simpler and smaller donors such as perfluorinated iodobenzenes.

This research was funded by the Croatian Science Foundation as part of the research project New building blocks for the supramolecular design of complex multi-component molecular crystals based on halogen bonds (HaloBond IP-2019-04-1868)