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.

本文讨论了结晶态氟苯类化合物1~3、7~10中存在的C−H···F−C相互作用及其本质。本研究选择该系列化合物的原因在于其仅含碳、氢、氟三种元素，这一特性对评估C−F基团的弱受体能力至关重要。所有目标化合物在室温下均为液态，其用于X射线衍射分析的单晶均通过原位法制备获得。针对所有这些晶体结构中存在的C−H···F相互作用的分析，采用了与同类C−H···O/C−H···N类似物进行对比的研究范式。
单氟苯（1）在分子间相互作用层面与氟化吡啶、1-氧化吡啶及苯甲腈具有高度相似性，表明这四种晶体结构中决定晶体排布的分子间相互作用本质完全一致。类似地，1,4-二氟苯（3）与1,4-苯醌具有结构相关性，前者中的C−H···F相互作用与后者中的C−H···O相互作用发挥着完全相同的结构调控作用。将化合物3与不对称1,4-二卤代苯（4~6）进行对比，可凸显C−H···F相互作用在该类晶体结构中的重要性。
随着分子中氟元素含量的提升，C−H键的酸性也随之增强，1,3,5-三氟苯（7）与1,2,4,5-四氟苯（8）中的C−H···F相互作用也随之变强且愈发关键。化合物7与8在晶体结构上与1,3,5-三嗪、1,2,4,5-四嗪高度相似，这一相似性进一步佐证了C−H···F相互作用与C−H···N相互作用具有相似性，并为将其归类为弱氢键提供了实验依据。1,2,3,4-四氟苯（9）存在多晶型现象，但其两种晶型中C−H···F相互作用所发挥的结构作用基本一致。
将化合物1~10中的C−H···F相互作用几何参数与剑桥晶体结构数据库（Cambridge Structural Database）中其他含碳、氢、氟的化合物进行对比后发现，1~10体系中的氢键特征更为显著。本研究得出结论：仅当碳中心的酸性提升至本研究中化合物的水平时，C−H···F相互作用的氢键本质才会得以显现。本研究同时证实，C−F基团更倾向于形成C−H···F相互作用而非F···F接触。因此，有机氟在晶体堆积中的行为与较重的卤素存在显著差异。