Uncoordinated tetrazole ligands in metal-organic frameworks for proton-conductivity studies

Zr-based metal-organic frameworks (MOFs) were modified with pendant tetrazole ligands for proton conductivity studies. Although tetrazolate ligands coordinated to metal cations have been widely utilized to construct MOFs or porous coordination polymers, to date, the use of uncoordinated tetrazole groups in MOFs has been limited. In this study, a benzene-1,4-dicarboxylic acid with pendant tetrazole groups (BDC-N-4) was synthesized and used to prepare a Zr-based UiO-66 MOF using a mixed-ligand strategy. The tetrazole-functionalized UiO-66-N-4 was compared to MOFs containing other acidic functional groups (hydroxyl, carboxylic acid, and sulfonic acid) with respect to their proton conductivities and acidities (calculated pK(a)). Interestingly, UiO-66-N-4 showed a significant decrease in the activation energy with increase in the amount of added tetrazole groups, suggesting the Grotthuss proton conduction mechanism for uncoordinated tetrazole-containing MOFs.