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, 1,4-difluorobenzene, 3, and 1,4-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.