Halide ion inclusion into a dicopper(II) bistren cryptate containing ‘active’ 2,5-dimethylfuran spacers: The origin of the bright yellow colour

The inclusion of halide ions into a dicopper(II) bistren cryptate complex containing 2,5-dimethylfuran spacers has been investigated through spectrophotometric titration experiments in MeCN solution. X-ray diffraction studies on the 1:1 chloride inclusion complex have shown that the encapsulated halide ion and the furan oxygen atoms lie at an interacting distance. Such an interaction perturbs the energy of the halide-to-copper(II) charge transfer transition, which is shifted to the visible region. As a consequence, an intense yellow colour develops on halide inclusion. Such a colour change is not observed on chloride or bromide inclusion into the dicopper(II) bistren cryptate containing spacers which are not capable to interact with the encapsulated halide and do not perturb the charge transfer transition, e.g. 1,3-xylyl fragments.