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 CeIV6 O61 ]([W3 Bi6 CeIII3 (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.