Straightforward Loading of Imidazole Molecules into Metal–Organic Framework for High Proton Conduction

A one-step straightforward strategy has been developed to incorporate free imidazole molecules into a highly stable metal-organic framework (NENU-3, ([Cu₁₂(BTC)₈(H₂O)₁₂][HPW₁₂O₄₀])·Guest). The resulting material Im@(NENU-3) exhibits a very high proton conductivity of 1.82 × 10⁻² S cm⁻¹ at 90% RH and 70 °C, which is significantly higher than 3.16 × 10⁻⁴ S cm⁻¹ for Im-Cu@(NENU-3a) synthesized through a two-step approach with mainly terminal bound imidazole molecules inside pores. Single crystal structure reveals that imidazole molecules in Im-Cu@(NENU-3a) isolate lattice water molecules and then block proton transport pathway, whereas high concentration of free imidazole molecules within Im@(NENU-3) significantly facilitate successive proton-hopping pathways through formation of hydrogen bonded networks.