A Comparative Study of Proton Conduction Between a 2D Zinc(II) MOF and Its Corresponding Organic Ligand

Until now, comparative studies on proton conductivity between organic ligands and related metal-organic frameworks (MOFs) have been very limited. Herein, a stable 2D Zn(II) MOF, [Zn(L)Cl]_n (1), has been successfully synthesized by using a zwitterionic-type organic ligand, 2-(1-(carboxymethyl)-1H-benzo[d]imidazol-3-ium-3-yl)acetate (HL). It is found that there are a large amount of free carboxyl groups and hydrogen bonds in the solid-state structure of HL, and a large number of chlorine ions are aligned in the channels of 1, which is favorable to the efficient proton transfer. Correspondingly, the proton conductivity values of 1 and HL are almost of the same order of magnitude under the same conditions. Moreover, the optimal σ value of 1 (4.72 × 10^-3 S/cm at 100 °C and 98% relative humidity) is almost four times higher than that (1.36 × 10^-3 S/cm) of the HL ligand. On the basis of the crystal data, SEM images, and calculated E_a values, we discuss the differences on proton conductivities and conduction mechanisms of 1 and HL. This report provides an inspiration for the preparation of highly conductive materials with free carboxyl groups and chloride ions.