Two imidazole multicarboxylate-based MOFs: syntheses, structures and proton conductive properties

Abstract

Considering the coordination advantage of N-heterocyclic carboxylate ligands in the construction of metal–organic frameworks (MOFs), we hope to adopt them to build up more structurally stable MOFs for proton conduction research. Therefore, the multifunctional ligand of 2-(2-carboxyphenyl)-4,5-imidazole dicarboxylic acid (o-CPH₄IDC) was selected by us. Consequently, two highly water-stable Ba(II) and Cd(II) MOFs, {[Cd(o-CPH₂IDC)(4,4′-bipy)_0.5(H₂O)₂]·3H₂O}_n (1) (4,4′-bipy = 4,4′-bipyridine) and [Ba(o-CPH₂IDC)]_n (2) were made. The further study of water-assisted proton conduction for 1 and 2 demonstrated that both were temperature- and humidity-dependent proton conductors, and their best proton conductivities could reach 10⁻⁴ S·cm⁻¹ at 100 °C and 98% RH. The vehicle mechanism can be proposed for 1 and 2 by calculating the activation energy and by analysing their crystal structures. Apparently, the adsorbed water molecules and carboxylate groups in the MOFs played an important role in the proton conduction process. Our work gives more choice for the design of novel proton conductive MOF materials.