Why a hexabromodiquinoline host preferentially includes small aromatic hydrocarbon guests

The preparation and properties of the new hexabromodiquinoline derivative 4 are described. This lattice inclusion host shows a strong preference for trapping small aromatic hydrocarbons. The X-ray crystal structures of the benzene, toluene, o-xylene, and p-xylene compounds are reported, and are analysed from a crystal engineering perspective. Crystallisation of 4 from the dual-nature solvent trifluoromethylbenzene yields the solvent-free material. Comparison of the parent crystal structure with those of its inclusion compounds reveals why inclusion of aromatic hydrocarbon guests is such a favoured process. The high concentration of Br...Br interactions in the structure of pure 4 is diluted and increasingly replaced by aromatic offset face-face (OFF) and aromatic edge-face (EF) interactions in the inclusion compounds, and this results in better lattice packing energies. For toluene, o-xylene, and p-xylene the host-guest ratio is 1:1. Inclusion of the smaller benzene molecule results in a change to 2:3 stoichiometry. This increase in guest content is assisted by replacement of host-host OFF and EF motifs with host-host pi-halogen dimer (PHD) interactions, which provide space for inclusion of the additional guest molecules. These changes result in the most efficient lattice packing of the series for compound (4)2.(benzene)3.