Comparison of isomeric meta-and para-diiodotetrafluorobenzene as halogen bond donors in crystal engineering

We provide a systematic investigation of the structures and composition of halogen-bonded cocrystals involving the bent (meta) halogen bond donor 1,3-diiodotetrafluorobenzene (1,3-tfib), in comparison to analogous systems based on its linear (para) 1,4-isomer. In particular, whereas 1,4-tfib has now been established as an archetypal, ubiquitous example of a halogen bond donor, its meta-isomer has remained almost completely unexplored. This study investigates the structures of 14 new cocrystals of 1,3-tfib and 9 new, analogous cocrystals of 1,4-tfib, with 9 monotopic and 7 ditopic nitrogen-based aliphatic and aromatic halogen bond acceptors. Combined with previously reported structures of cocrystals of 1,4-tfib, the results of our study indicate that whereas halogen bond lengths and the thermal stability of the investigated cocrystals are similar, the change in molecular shape between the two halogen bond donors brings about important consequences in supramolecular architectures and preferred stoichiometric compositions of otherwise analogous cocrystals. These preliminary results suggest that the principal factor responsible for such differences might be the different abilities of the cocrystals based on 1,3-tfib and 1,4-tfib to form close-packed structures.