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 1 show 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 1 exhibits a high proton conductivity of 6.39 × 10^-3 S cm^-1 along one-dimensional channels, 30 times higher than that of a pellet sample at 358 K and 98% relative humidity.