A unique 3D metal–organic framework based on a 12-connected pentanuclear Cd(ii) cluster exhibiting proton conduction

Abstract

Solvothermal reaction of a TCA (TCA = 4,4′,4′′-tricarboxytriphenylamine) ligand with Cd(NO₃)₂ yielded a unique 3D 12-connected metal–organic framework of 1, which can be simplified as a (3,3,12)-connected 3-nodal net with a Schläfli symbol of {4¹⁶·6³⁶·8¹⁴}{4²·6}₂{4³}₂ based on a 12-connected node of a pentanuclear Cd(II) cluster and a three-connected organic linker of TCA. Compound 1 exhibits both photoluminescence and proton conductivity. The luminescence properties of 1 originate from the triphenylamine chromophore of the TCA ligand. The proton conductivity of 1 is supposed to be relevant to the structure attributes of 1. On the one hand, the curved narrow channels lined with a hydrophilic pentanuclear Cd(II) cluster in 1 can improve the water affinity of framework and facilitate the water absorption under humid conditions. On the other hand, the coordinated water molecules within the pentanuclear Cd(II) cluster can produce a mobile H⁺ proton due to coordination activation. At 80 °C and 85% RH (relative humidity), compound 1 shows the proton conductivity of 1.45 × 10⁻⁶ S cm⁻¹.