Fluorine···Fluorine Interactions in a High-Pressure Layered Phase of Perfluorobenzene

Michalina Rusek, Karolina Kwaśna, Armand Budzianowski, and Andrzej Katrusiak

DOI: 10.1021/acs.jpcc.9b09428

Synopsis

Several structures of hexafluorobenzene at low temperature at atmospheric pressure or high pressure at room temperature have been published.

There are also structures of the new high-pressure phase.

Abstract

Short contacts F···F and their triangular motifs F3 are the eminent features in the new high-pressure perfluorobenzene (C6F6) phase II formed above 0.89 GPa. Fluorine atoms are strongly electronegative and weakly polarizable; however, their close location in many crystal structures of molecular crystals suggests the electron-pair···σ-hole attraction. In C6F6 phase II the molecules are arranged into unprecedented planar pseudohexagonal sheets with multiple F···F contacts, much more frequent compared to the ambient-pressure low-temperature C6F6 phase I, where contacts F···F combine molecules into linear trimers. The short F···F contacts in C6F6 phase II are compatible with the angular requirements of halogen bonds, and they remarkably resemble the characteristic motif X3 in heavier halobenzenes C6X6 (X = Cl, Br, I). However, in isostructural C6X6 crystals the molecules are strongly inclined to their neighbors, and they are X···X halogen-bonded into a three-dimensional network. The structure of F···F-bonded sheets in C6F6 polymorph II was rationalized, besides the favored electron-pair···σ-hole contact, corresponding to type II halogen bonds, by enhanced electron-density features arising in the resonance mesomeric states of the fluorinated molecules embedded in the electronegative environments.

Download: