Ultrastable Polyoxometalate-Encapsulated Supramolecular Metal–Organic Nanotubes for Single-Crystal Proton Conduction

Two unique polyoxometalate (POM)-encapsulated tubular materials with the formula K(H2O)6[M6(btp)6(H2O)22](P2W18O62)3(Hbtp)5(btp)3·52H2O [M = Mn (1) and Co (2); btp = 2,6-bis(1,2,4-triazol-1-yl)pyridine] were designed and synthesized based on the Dawson POM and V-type btp ligand, as confirmed by IR, X-ray diffraction (XRD), and element analysis. Single-crystal XRD analyses of compound 1show that two kinds of remarkable metal–organic supramolecular nanotubes, including trigonal and hexagonal nanotubes, are constructed along the c-axis direction via π···π-packing interactions between {Mn3(btp)3} rings and the btp ligands, of which [α-P2W18O62]6–anions are confined in channels, making the entire structure extraordinarily stable. Meanwhile, the coordinated [α-P2W18O62]6–anion within the hexagonal channel makes the channel highly hydrophilic and attracts a number of guest water molecules to fill in the free space, conducive to proton transport. Therefore, the single-crystal sample of 1exhibits a high proton conductivity of 6.39 × 10–3S cm–1along one-dimensional channels, 30 times higher than that of a pellet sample at 358 K and 98% relative humidity.