New Theoretical Insight into Fluorination and Fluorine–Fluorine Interactions as a Driving Force in Crystal Structures

Abstract

Fluorination of compounds causes an increase in the proton-donating ability and a decrease in the proton-accepting capacity of groups in their vicinity. The formation of F···F interactions is followed by the shift of the electron density in the area of F···F contact, which creates a new region with a larger surface area, a higher negative potential, and, hence, a more pronounced accepting ability. The new region also has a greater ability to form multiple (simultaneous) interactions with species from the environment, thus compensating for the reduction of the accepting capacity of the groups in the vicinity. This phenomenon explains not only the abundance of F···F interactions in crystal structures, but also a large number of structures with F···O interactions. Only C–H···F interactions are more numerous than F···F interactions in crystal structures, which indicates a high affinity of fluorinated compounds for nonpolar groups.

Synthesis, supramolecular isomerism, and photoluminescence of scandium(iii) complexes with a tetrafluoroterephthalate ligand

“…On the other hand, there is evidence for the existence of attractive interactions between fluorine atoms. 56–58 Halogen bonding interactions are characterized by the X 1 ⋯X 2 distance and two angles θ 1 = C–X 1 ⋯X 2 and θ 2 = X 1 ⋯X 2 –C and classified into three types: 1) 0° ≤ | θ 1 − θ 2 | ≤ 15° (type I); 2) | θ 1 − θ 2 | ≥ 30° (type II); 3) 15° ≤ | θ 1 − θ 2 | ≤ 30° (quasi-type I/type II). 54,55 The crystal structures of lanthanide CPs with a tetrafluoroterephthalate linker have F⋯F contacts of type I.…”

Halogen Interactions in Halogenated Oxindoles: Crystallographic and Computational Investigations of Intermolecular Interactions

“…Among their finds, it can be highlighted that complexes involving electron-withdrawing substituents obtained more favorable binding energy. This electron-withdrawing substituent character was also explored by Janjić et al [19]. In their study, the replacement of a hydrogen by a fluorine resulted in an increased interaction energy and in the number of multiple interactions of F•••F contacts.…”

“…Previous work has demonstrated that dominant partial charge concentration, either positive or negative, on specific parts of interacting molecules have important influence in the intermolecular interaction [44][45][46]. Due its high electronegativity, fluorine has the capacity of electron-withdrawing from neighbor atoms [19,47,48] and, for this reason, this atom has become an interesting substituent in many chemical systems in the search for changes the electronic properties or to enhance intermolecular interaction [47,49].…”

“…The insertion of F1/2 in place of H8 produced a change in the MEP of the entire molecule 1 compared to 2. This perturbation in the MEP is due to the high electronegativity of the fluorine atom that tends to attract electrons from atoms throughout the molecule [19,47,48]. Changes in potential on Br indicate a more electrophilic behavior in the sigma hole region (highlighted by the increase in the blue region) in 2 vs. 1.…”

“…The highly directional nature of these Type 2 contacts has been exploited to direct crystal packing through halogen-halogen interactions [11][12][13]. Halogen-halogen interactions have been studied in detail from both a crystallographic lens [14,15] as well as in theoretical investigations [16][17][18][19]. For instance, a study by Bolotin et al [20] revealed that even weak type I halogen•••halogen interactions, involving Br•••Br contacts, could be the driving forces for the crystallization of a primary organic peroxo compound.…”