Mechanochemical Synthesis of Phosphonate-Based Proton Conducting Metal-Organic Frameworks.

Water-stable metal-organic frameworks (MOFs) with proton-conducting behavior have attracted great attention as promising materials for proton-exchange membrane fuel cells. Herein, we report the mechanochemical gram-scale synthesis of three new mixed-ligand phosphonate-based MOFs, {Co(HPhDPA)(4,4'-bipy)(HO)·2HO} (), {Fe(HPhDPA)(4,4'-bipy) (HO)·2HO} (), and {Cu(HPhDPA)(dpe)(HO)·2HO} () [where HPhDPA = phenylene diphosphonate, 4,4'-bipy = 4,4'-bipyridine, and dpe = 1,2-di(4-pyridyl)ethylene]. Single-crystal X-ray diffraction measurements revealed that and are isostructural and possess a three-dimensional (3D) network structure comprising one-dimensional (1D) channels filled with guest water molecules. Instead, displays a 1D network structure extended into a 3D supramolecular structure through hydrogen-bonding and π-π interactions. In all three structures, guest water molecules are interconnected with the uncoordinated acidic hydroxyl groups of the phosphonate moieties and coordinated water molecules by means of extended hydrogen-bonding interactions. and showed a gradual increase in proton conductivity with increasing temperature and reached 4.9 × 10 and 4.4 × 10 S cm at 90 °C and 98% relative humidity (RH). The highest proton conductivity recorded for was 1.4 × 10 S cm at 50 °C and 98% RH. Upon further heating, undergoes dehydration followed by a phase transition to another crystalline form which largely affects its performance. All compounds exhibited a proton hopping (Grotthuss model) mechanism, as suggested by their low activation energy.