Halogen bonding in a series of Br(CF₂)_nBr-DABCO adducts (n = 4, 6, 8)

Halogen bonding (XB) is a highly-directional class of intermolecular interactions that has been used as a powerful tool to drive the design of crystals in the solid phase. To date, the majority of XB donors have been iodine-containing compounds, with many fewer involving brominated analogues. We report the formation of adducts in the vapour phase from a series of dibromoperfluoroalkyl compounds, BrCF₂(CF₂)_nCF₂Br (n = 2, 4, 6), and 1,4-diazabicyclo[2.2.2]octane (DABCO). Single-crystal X-ray diffraction studies of the colourless crystals identified 1,4-diazabicyclo[2.2.2]octane-1,4-dibromoperfluorobutane (1/1), C₄Br₂F₈·C₆H₁₂N₂, (I), 1,4-diazabicyclo[2.2.2]octane-1,6-dibromoperfluorohexane (1/1), C₆Br₂F₁₂·C₆H₁₂N₂, (II), and 1,4-diazabicyclo[2.2.2]octane-1,8-dibromoperfluorooctane (1/1), C₈Br₂F₁₆·C₆H₁₂N₂, (III), each of which displays a one-dimensional halogen-bonded network. All three adducts exhibit N...Br distances less than the sum of the van der Waals radii, with butane analogue (I) showing the shortest N...Br halogen-bond distances yet reported between a bromoperfluorocarbon and a nitrogen base [2.809 (3) and 2.818 (3) Å], which are 0.58 and 0.59 Å shorter than the sum of the van der Waals radii.