Ionothermal Synthesis of Five Keggin-Type Polyoxometalate-Based Metal–Organic Frameworks

Five unique Keggin-type polyoxometalate (POM)-based metal-organic frameworks (POMFs), namely [CH₂L1]₂[(CuL1₂)(PMo^VI₉Mo^V₃O₄₀)] (1), [Zn_0.5(H₂O)(L1)][Zn(L1)_1.5Cl][H₂L1]_0.5[PMo₁₂O₄₀]·1.25H₂O (2), (TBA)[Cu(H₂O)₂L1₂][PMo₁₂O₄₀] (3), [Cu₂(H₂O)₂(L1)₃][PM^VI₁₁Mo^VO₄₀] (4), (H₂L2)_0.5[(Cu^IL2)₂(PMo₁₂O₄₀)]·H₂O (5), L1 = 4,4'-bis((1H-1,2,4-triazol-1-yl)methyl)-biphenyl, L2 = 1,4-bis((1H-1,2,4-triazol-1-yl)methyl)-benzene, have been synthesized under ionothermal conditions. According to single-crystal data, 1 exhibits a novel mechanically interlocked molecular architecture (MIMA) constructed by two-dimensional (2D) interpenetrating polyrotaxane layers with unique cyclophanes (tetra-cationic viologen macrocycle cyclobis(paraquat- p-phenylene) (CBPQT⁴⁺ system)), resulting in an H-bonded 3D supermolecule, and is the first synthesis of self-assembled cyclophane-PMOFs under ionothermal conditions. 2 shows a novel 2D three-fold interpenetrating polyrotaxane host and guest network. 1 and 2 are presented as the first MIMA polyrotaxane structures to have been synthesized under ionothermal conditions. Electrochemical impedance spectroscopy (EIS) reveals that 2, 4, and 5 show high proton conductivity owing to their unique structural properties. Solid-state diffuse reflection UV-vis-NIR measurements show the title compounds are potentially semiconductor materials. Photocatalytic investigations indicate that 1-5 possess high and stable photocatalytic H₂ evolution and rhodamine B (RhB) degradation under visible-light irradiation.