Parallelogram 3d-4f-5d heterometallic clusters based on trilacunary tungstoantimonates with excellent proton conductivity

Two 3d-4f-5d heterometallic cluster-containing polyoxometalates, formulated as Na22{(SbW9O33)4[La3W6MO18(H2O)8(CH3COO)4]2}·nH2O (abbreviated as La6M2, M = Co/Mn) were synthesized and structurally characterized. Single-crystal X-ray diffraction analyses reveal that the polyanions of La6Co2 and La6Mn2 consist of the uncommon 3d-4f-5d clusters {La6W12Co2} and {La6W12Mn2}, which are encapsulated by four trilacunary Keggin tungstoantimonates to form the parallelogram-shaped title compounds. Additionally, the polyanions can be extended into a two-dimensional (2D) frame by the linkage of peripheral Na+ ions. The inner space of the 2D layer was filled with water molecules and thus an H-bonded network was formed, which is expected to exhibit a fascinating proton conductivity. The study of water-assisted proton conduction demonstrated that La6Co2 and La6Mn2 were temperature- and humidity-dependent proton conductors, respectively, and the proton conductivities could reach 1.3 × 10−2 and 2.3 × 10−2 S/cm at 65 °C and 90% RH conditions.