Titanium-Oxide Host Clusters with Exchangeable Guests

A novel family of water-soluble, poly­oxo­cationic titanium-oxide host–guest clusters are reported herein. They exhibit an unprecedented hexagonal prismatic core structure for hosting univalent cationic guests like K+, Rb+, Cs+ and H3O+. Guest exchange has been studied using 133Cs NMR, showing the flexible pore of a host permits passage of a comparatively larger cation and giving an equilibrium constant of ca. 13 for displacing Rb+ by Cs+. Attractive ion-dipole interaction, depending on host–guest size complementarity, plays a dominant role for the preferential encapsulation of larger alkali-metal cationic guests.