Cocrystals of 1,2-Diiodotetrafluorobenzene with Pyridine Derivatives: Pyridine Nitrogen as a Bifurcated Acceptor with Ortho-Diiodo Halogen Bond Donors

The behavior of 1,2-diiodotetrafluorobenzene (<b>12tfib</b>) as a halogen bond donor was studied by cocrystallizing it with a series of aromatic nitrogen bases covering a wide range of p<i>K</i> _a values (2.10 ≤ p<i>K</i> _a ≤ 9.60) and comparing the results with those reported for other perfluorinated iodobenzenes. The cocrystal screening was performed by grinding <b>12tfib</b> and each of the selected bases in a 1:2 donor:acceptor stoichiometric ratio as well as crystallization from solution in a small excess of the acceptor (ratio 1:2.5). Of the 14 bases used in this study, three weakest bases (p<i>K</i> _a below ca. 4) have failed to produce cocrystals, nine intermediate bases (4.85 ≤ p<i>K</i> _a ≤ 6.72) have yielded four new phases, which were only obtainable in grinding experiments and not as pure samples, four 1:1 cocrystals and one 1:2 cocrystal, while the two strongest bases (p<i>K</i> _a above ca. 7.5) have yielded 1:2 cocrystals. A total of eight new solids were studied by single-crystal X-ray diffraction. The three 1:2 cocrystals comprise discrete halogen-bonded trimers with <b>12tfib</b> acting as a ditopic halogen bond donor to two base molecules. Among the five 1:1 cocrystals, in two, <b>12tfib</b> was found to act as a monotopic donor; however, in other three, <b>12tfib</b> was found to be ditopic, with the nitrogen atom of the base being a bifurcated acceptor of a pair of halogen bonds. This highly unusual binding motif has been further investigated by quantum chemical calculations and a detailed CSD survey. The computational study has found that a binding site comprising two converging iodine atoms possesses a wide, single minimum potential, which allows for a more favorable binding of weakly and intermediately basic pyridine (aromatic sp²) nitrogen atoms than a single iodine atom. The CSD survey has shown that the aromatic sp² nitrogen atom acting as a bifurcated halogen acceptor is indeed an extremely rare occurrence (appearing in only ca. 2.2% of the total) but considerably more likely to occur in the presence of <i>ortho</i>-diiodo halogen bond donors.