Halogen⋯oxygen aggregation and disorder modes in pressure frozen XCF2CF2X : 1,4-dioxane (X = Br,I) complexes

Abstract

Formation of cocrystals and halogen⋯O interactions was studied in the structures of 1,4-dioxane with 1,2-diiodo-, 1-bromo-2-iodo- and 1,2-dibromoperfluoroethanes. Mixtures pressure-frozen in a diamond-anvil cell have been investigated. Structures of cocrystals of 1,2-diiodoperfluoroethane : 1,4-dioxane at 0.30(5) GPa/296(2) K and of 1-bromo-2-iodoperfluoroethane : 1,4-dioxane at 0.62(5) GPa/296 K were determined by single-crystal X-ray diffraction. Also the single-crystal of 1,4-dioxane separated from its mixture with 1,2-dibromoperfluoroethane, which remained liquid, was investigated at 0.42 GPa/296 K. The cocrystal of ICF₂CF₂I : C₄H₈O₂ and the C₄H₈O₂ crystals freeze into the same phases as those obtained by cooling; the BrCF₂CF₂I : C₄H₈O₂ cocrystal was not synthesized before. In all these cocrystals the structures are dominated by halogen⋯O interactions. In the structure of ICF₂CF₂I : C₄H₈O₂ the –CF₂–CF₂– moiety is disordered about the I⋯I molecular axis and the C₄H₈O₂ molecule rotates about the O⋯O axis, too; in BrCF₂CF₂I : C₄H₈O₂ the Br and I atoms are disordered at the same position with half occupancy, but the C₄H₈O₂ molecule is ordered. The pressure-freezing of 1,4-dioxane from the BrCF₂CF₂Br : C₄H₈O₂ mixture yielded ordered phase II of 1,4-dioxane previously obtained from the neat compound by cooling. Spectacular kinetic crystallization of ICF₂CF₂I : C₄H₈O₂ in the form of hexagonal snowflakes has been observed in the diamond-anvil cell.