High proton conductivity by a metal-organic framework incorporating Zn ₈O clusters with aligned imidazolium groups decorating the channels

A novel metal-organic framework, [{(Zn_0.25)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.