High Proton Conductivity by a Metal–Organic Framework Incorporating Zn8O Clusters with Aligned Imidazolium Groups Decorating the Channels

A novel metal–organic framework, [{(Zn0.25)8(O)}­Zn6(L)12(H2O)29(DMF)69(NO3)2]n (1) {H2L = 1,3-bis­(4-carboxyphenyl)­imidazolium}, has been synthesized under solvothermal conditions in good yield. It shows a Zn8O cluster that is coordinated to six ligands and forms an overall three-dimensional structure with channels along the crystallographic a and b axes. The imidazolium groups of the ligand moiety are aligned in the channels. The channels are not empty but are occupied by a large number of DMF and water molecules. Upon heating, these solvent molecules can be removed without breakdown of the overall structure of the framework as shown by variable-temperature powder X-ray diffraction patterns. Of great interest is the fact that the compound exhibits high proton conductivity with a low activation energy that is comparable to those of Nafion presently used in fuel cells.