A Self-Assembled Cage Binding Iodide Anions over Other Halide Ions in Water

A triangular-prism shaped cage was self-assembled by the formation of a dynamic covalent bond, namely a hydrazone, in acidic aqueous solution. The hexacationic host bears a number of relatively acidic protons pointing inside the cage cavity, which is able to accommodate an iodide anion selectively in water over other halide anions such as F-, Cl-, and Br-, which are more hydrated. As a comparison, a macrocycle analogue bearing fewer positive charges and fewer hydrogen bond donors shows no anion binding ability. As inferred from the single-crystal structure and theoretical calculations, the ability of the cage to selectively recognize iodide anions results from the combination of electrostatic forces, C-H center dot center dot center dot I- hydrogen bonds, and the hydrophobic effect.