C-H•••F interactions in the crystal structures of some fluorobenzenes

The existence and nature of C-H ... F-C interactions in crystalline fluorobenzenes 1-3 and 7-10 are discussed. These compounds were chosen because they contain only C, H, and F atoms; this is necessary in the evaluation of the weak acceptor capabilities of the C-F group. All of these compounds are liquids at room temperature, and single crystals for X-ray diffraction were grown in situ. The analysis of the C-H F interactions that are found in all of these crystal structures takes the form of comparisons with related C-H ... O/C-H ... N analogues. Fluorobenzene, 1, bears a close relationship to pyridinium fluoride, pyridine 1-oxide, and benzonitrile at the level of individual interactions, showing that the character of the structure-determining intermolecular interactions in these four crystal structures are the same. Similarly, 1,1,4-difluoro-; benzene, 3, and 14-benzoquinone are related, the C-H ... F interactions in the former playing the same structural role as the C-H ... O interactions in the latter. A comparison of 3 with the unsymmetrical 1,4-dihalogenated benzenes, 4-6 indicates the importance of C-H ... F interactions in these structures. With an increase in the F content of the molecules, the C-H acidity also increases and the C-H ... F interactions in 1,3,5-trifluorobenzene, 7, and 1,2,4,5-tetrafluorobenzene, 8, become stronger and more important. Compounds 7 and 8 are structurally very similar to 1,3,5-triazine and 1,2,4,5-tetrazine, and this similarity further strengthens the argument that C-H ... F interactions resemble C-H ... N interactions and provides evidence for their description as weak hydrogen bonds. 1,2,3,4-Tetrafluorobenzene, 9, is polymorphic but the role of the C-H ... F interactions in the two forms is similar. A comparison of the C-H ... F geometries in compounds 1-10 with other C-, H-, and F-containing compounds in the Cambridge Structural Database reveals that the hydrogen bond properties are more pronounced in 1-10. It is concluded that only when the carbon acidity is enhanced to the levels of the compounds in the present study, is the hydrogen-bond nature of the C-H ... F interaction even revealed. This study also demonstrates that the C-F group prefers to form C-H ... F interactions rather than F F contacts. The behavior of organic fluorine in crystal packing is therefore quite different from the heavier halogens.