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.