The ability of a cationic coordination cage to encapsulate molecular guests is enhanced by non-covalent capping of the cage portals with tripodal anions. The capped cage provides new cation binding sites at the portals, which enable accommodation of cationic substrates within the cationic cage. In addition, non-covalent capping allows neutral guests in the cage to be exchanged for cationic ones on demand.

The properties of rare‐earth‐metal ions in aqueous media often depend on their various hydration modes, which allow them to exist as monomeric or oligomeric species with different coordination numbers. Capturing rare‐earth‐metal ions in a confined cavity can fix their hydration modes and thus enable their intrinsic properties to be distinguished. However, the isolation of ionic species from bulk aqueous media is not an easy task due to competitive interaction with bulk water. Here, we report the encapsulation of hydrated rare‐earth‐metal ions in a hydrophobic cavity of a synthetic cage. Cap‐like counter anions located at the cage's portals play an important role in capturing the rare‐earth metals at a fixed position via electrostatic interactions. Preferential encapsulation of early lanthanoid (III) ions was observed even though all the rare‐earth metals have the same hydration number and geometry, as visualized by the competitive inclusion of a dye molecule. Accordingly, the early lanthanoid ion was selectively extracted from a mixture of two rare‐earth‐metal ions.

The properties of rare‐earth‐metal ions in aqueous media often depend on their various hydration modes, which allow them to exist as monomeric or oligomeric species with different coordination numbers. Capturing rare‐earth‐metal ions in a confined cavity can fix their hydration modes and thus enable their intrinsic properties to be distinguished. However, the isolation of ionic species from bulk aqueous media is not an easy task due to competitive interaction with bulk water. Here, we report the encapsulation of hydrated rare‐earth‐metal ions in a hydrophobic cavity of a synthetic cage. Cap‐like counter anions located at the cage's portals play an important role in capturing the rare‐earth metals at a fixed position via electrostatic interactions. Preferential encapsulation of early lanthanoid (III) ions was observed even though all the rare‐earth metals have the same hydration number and geometry, as visualized by the competitive inclusion of a dye molecule. Accordingly, the early lanthanoid ion was selectively extracted from a mixture of two rare‐earth‐metal ions.