Weak intermolecular interactions and molecular aggregation in isostructural dihaloperfluoroethanes,

Abstract

A series of 1,2-dihalotetrafluoroethanes X(CF₂)₂X (X = Br, I) has been in-situ pressure crystallized in a diamond-anvil cell (DAC), their structures determined by single crystal X-ray diffraction and rationalized by weak intermolecular interactions. For isoelectronic and isostructural 1,2-dibromotetrafluoroethane (BrCF₂CF₂Br), 1,2-diiodotetrafluoroethane (ICF₂CF₂I) and 1-bromo-2-iodotetrafluoroethane (BrCF₂CF₂I), the crystals are monoclinic, space groupP2₁/n, with the midpoint of the C–C bond located at the centre of inversion. The freezing pressures of these compounds have been determined to be 0.80 GPa, 0.30(5) GPa and 0.10(5) GPa for BrCF₂CF₂Br, BrCF₂CF₂I and CF₂ICF₂I, respectively. In the structure of ICF₂CF₂I, the –CF₂–CF₂– moiety is orientationally disordered at 0.16 GPa, but it becomes ordered at 0.86 GPa; in the BrCF₂CF₂I crystal the Br and I atoms are substitutionally disordered; and the BrCF₂CF₂Br structure is completely ordered. The formation of isostructural crystals by these compounds and different types of molecular disorder can be rationalized by the intermolecular interactions at varied thermodynamical conditions. The phenomenon of “self-healing” has been observed, in which crystal defects, formed during initial crystallization in the DAC, repaired themselves during the annealing process, which greatly facilitates the formation of single crystals. Pressure considerably affects the crystal morphology of ICF₂CF₂I, where the elimination of –CF₂–CF₂– disorder coincides with the appearance of dendritic forms of the crystal phases.