{Mo2O2X2}2+ (X = O, S), a magic building block for the design of wheel shaped metalates

The self-condensation of the [Mo2O2E2](2+) precursors is at the origin of a family of discrete cyclic species. The condensation can be performed in the presence or not of guest species. With E = O, the resulting cycles delimit an anionic open cavity, which can be filled by neutral polar molecules such as aquo ligands, or by alkaline cations. With E = S, the resulting cycles delimit a cationic cavity, which can be filled with neutral molecules, or anions like halides or more sophisticated groups like caboxylates. The flexibility of the rings is at the origin of striking host-guest properties: the symmetry and the nuclearity of the inorganic backbone are directed by the size and shape of the encapsulated guest. The metal coordination versatility confers to the ring unusual dynamic properties. In the solid state, molecular rings arrange in striking 3-D networks in which alkaline cations organize in pillars or layers decorated with anionic rings. The control of the partial occupancy of the cationic pillars permits to synthesize very good ionic conductors.