Mechanochemical Synthesis of Phosphonate-based Proton Conducting Metal Organic Frameworks

The rational design of new type of water stable metal–organic frameworks (MOFs) as promising proton-conductors has attracted great attention owing to their applications in proton-exchange membrane fuel cells (PEMFC). Herein, we report the mechanochemical gram scale synthesis of three new mixed ligand phosphonate-based MOFs, {Co(H2PhDPA)(4,4ʹ-Bipy)(H2O)·2H2O}n (BAM-1), {Fe(H2PhDPA)(4,4ʹ-Bipy)(H2O)·2H2O}n (BAM-2) and {Cu(H2PhDPA)(Dpe)2(H2O)2·2H2O}n (BAM-3) [where H2PhDPA = Phenylenediphosphonate 4,4ʹ-Bipy = 4,4ʹ-Bipyridine, and Dpe = 1,2-Di(4-pyridyl)ethylene]. Single crystal X-ray diffraction measurements revealed that BAM-1 and BAM-2 are isostructural and possess a three-dimensional (3D) network structure comprising1D channels filled with guest water molecules. Whereas BAM-3 displays a one-dimensional (1D) network structure extended to 3D supramolecular structure by hydrogen-bonding bridging and π-π interactions. In all three structures, guest water molecules are interconnected with uncoordinated acidic hydroxylgroups of the phosphonate moieties and coordinated water molecules by means of extended hydrogen-bonding interactions. BAM-1 and BAM-2 showed gradual increase of proton conductivities with increasing temperature and reached to 4.9 × 10–5 and 4.4 × 10–5 S cm–1 at 90 C and 98% RH. In case of BAM-3, the highest proton conductivity recorded was 1.4 × 10–5 S cm–1 at 50 C and 98% RH. Upon further heating, BAM-3 undergoes dehydration followed by phase transition to another crystalline form which largely effects the performance. All compounds exhibited a proton hopping (Grotthuss model) mechanism as suggested by their low activation energy.