Halide-ion encapsulation by a flexible dicopper(II) bis-tren cryptate

The appearance of a bright yellow color signals the inclusion of a halide ion (X=Cl, Br, I) into the dicopper(II) cryptate 1 in aqueous solution at pH 5.2. The X− ion bridges the two metal centers, as is shown by the crystal structure of the bromide inclusion complex. At pH 8, X− is replaced by the smaller OH− ion, and the solution turns emerald green. The cage can adjust its intermetallic cavity to include polyatomic anions as large as N3− and NCS−.