Boosting the proton conduction in a magnetic dysprosium-organic framework by introducing conjugate NH4+-NH3 pairs

Metal-organic frameworks (MOFs) with inherent porosity and suspended acidic groups are promising proton conducting materials in water or aqua-ammonia media. Herein we report a new lanthanide phosphonate, namely, Dy2(amp2H2)2(mal)(H2O)2∙5H2O (MDAF-6). It possesses a 3D open-framework structure, and shows a high NH3 adsorption capacity of 142.4 cm3/g at P/P0 = 0.98 at 298 K due to acid-base interaction. Interestingly, the proton conductivity of MDAF-6-NH3 is enhanced by five orders of magnitude compared to MDAF-6 after 8.5 h exposure in saturated NH3-H2O vapor, indicating the importance of coexistent conjugate acid-base pairs of H3O+-H2O and NH4+-NH3 in promoting proton conduction. Magnetic studies of MDAF-6 revealed slow magnetization relaxation under zero dc field, characteristic of single-molecule magnet behavior. This work provides not only a new multifunctional MOF material, but also a new strategy to improve proton conduction in aqua-ammonia medium.