Cosublimation: A Rapid Route Toward Otherwise Inaccessible Halogen-Bonded Architectures

Several synthetic techniques are available for the preparation of halogen-bonded adducts, including traditional solvent-evaporation methods and mechanochemistry. With the intention of reducing solvent usage and experimental times, we establish cosublimation as a synthetic technique for preparing halogen-bonded cocrystals, focusing on the C–I···N motif involving nitrogen-containing heterocycles. We selected four common halogen-bond donors (I 2, 1,4-diiodobenzene, 1,4-diiodotetrafluorobenzene, 1,3,5-trifluoro-2,4,6-triiodobenzene) and four halogen-bond acceptors (2,3,5,6-tetramethylpyrazine, 1,4-diazabicyclo[2.2.2]octane, 2,2′-bipyridyl, 1-adamantanecarbonitrile), by virtue of their range in volatility, and explored all 16 combinations via the following methods: solvent evaporation, overnight cosublimation in a sealed glass tube using a two-zone furnace, and expedited cosublimation in a vacuum sublimation apparatus. We show that the raw crystals obtained from overnight cosublimation are of suitable quality for single-crystal X-ray diffraction, resulting in five new crystal structures, two of which are, currently, unobtainable through other methods. Of these new structures, the first halogen bond to the nitrile group of 1-adamantanecarbonitrile is reported, resulting in the formation of a supramolecular rotor in a spinning top configuration as evidenced by 13 C and 15 N solid-state NMR spectroscopy. Strikingly, the cosublimation approach overcomes an anticooperative halogen-bonding effect to produce fully saturated cocrystals of the tritopic halogen bond donor 1,3,5-trifluoro-2,4,6-triiodobenzene with 1,4-diazabicyclo[2.2.2]octane. Finally, we show that cosublimation can be used to rapidly prepare and purify bulk samples using a vacuum sublimation apparatus, enabling the scalable preparation of halogen-bonded cocrystals within 10 to 30 min using common laboratory equipment. The methods described here enable a broad exploration of the polymorphic landscape and may be extended to the facile preparation of other classes of cocrystals.