Halogen Bonding in 3-Iodopyridine·Organoiodine Cocrystals: A Study of Diverse Crystal Growth Approaches

Five cocrystals of 3-iodopyridine with perfluorinated di- and tri-iodobenzenes were structurally characterized. All five cocrystals exhibited strong I···N interactions where an iodine atom of the perfluorinated organoiodine was the halogen bond donor. This led to 1:1 or 2:1 cocrystal stoichiometries of 3-iodopyridine:organoiodine via discrete I···N subunits. The iodine atom of iodopyridine often acted as a halogen bond donor for I···I interactions (occurring in three cocrystals), and in one of these cases also as a halogen bond acceptor for I···I interactions originating from the perfluorinated organoiodine. These I···I interactions extended the I···N halogen-bonded units into one-dimensional and two-dimensional motifs. The use of mechanochemistry and slow cooling in crystal growth complemented solution-based cocrystallization to provide a more complete picture of the phase space. In the 3-iodopyridine:135-F3I3B system, mechanochemistry revealed an additional 1:1 cocrystal stoichiometry that was not apparent from solution-based reactions that produced a 2:1 cocrystal. In the 3-iodopyridine:o-F4DIB system, thermal analysis of the liquid product from solution and mechanochemical reactions revealed an accessible pathway for crystal growth by slow cooling. This enabled the structural characterization of this 1:1 cocrystal that exists as a liquid at room temperature.