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)(PMoVI9MoV3O40)] (1), [Zn0.5(H2O)(L1)][Zn(L1)1.5Cl][H2L1]0.5[PMo12O40]·1.25H2O (2), (TBA)[Cu(H2O)2L12][PMo12O40] (3), [Cu2(H2O)2(L1)3][PMVI11MoVO40] (4), (H2L2)0.5[(CuIL2)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.