Aggregation of Giant Cerium-Bismuth Tungstate Clusters into a 3D Porous Framework with High Proton Conductivity

The exploration of high nuclearity molecular metal oxide clusters and their reactivity is a challenge for chemistry and materials science. Herein, we report an unprecedented giant molecular cerium-bismuth tungstate superstructure formed by self-assembly from simple metal oxide precursors in aqueous solution. The compound, {[W₁₄ Ce^IV₆ O₆₁ ]([W₃ Bi₆ Ce^III₃ (H₂ O)₃ O₁₄ ][B-α-BiW₉ O₃₃ ]₃ )₂ }^34- was identified by single-crystal X-ray diffraction and features 104 metal centers, a relative molar mass of ca. 24 000 and is ca. 3.0×2.0×1.7 nm³ in size. The cluster anion is assembled around a central {Ce₆ } octahedron which is stabilized by several molecular metal oxide shells. Six trilacunary Keggin anions ([B-α-BiW₉ O₃₃ ]^9- ) cap the superstructure and limit its growth. In the crystal lattice, water-filled channels with diameters of ca. 0.5 nm are observed, and electrochemical impedance spectroscopy shows pronounced proton conductivity even at low temperature.