Solvent-Assisted Modification to Enhance Proton Conductivity and Water Stability in Metal Phosphonates

Although proton-conductive metal phosphonates with well-defined structure offer a favorable platform for exploring their structure-property relationship, investigating of the synergic effect of phosphonate groups and functional moieties on proton conduction is rare. In this work, we have synthesized two new copper phosphonates, [Cu(4-cppH)(4,4'-bipy)(H₂O)₃] (FJU-80) and [Cu(4-cppH)(4,4'-bipy)]·H₂O·DMF (FJU-81), by the method of solvent-assisted modification, giving a 1D metal coordination polymer and a 3D metal open framework, respectively. Single-crystal X-ray diffraction shows that FJU-80 is full of hydrogen-bonding sites contributed from the improved synergic effect of phosphonate groups, carboxylate groups, and coordinated water molecules, thereby facilitating continuous hydrogen-bonding networks, whereas FJU-81 only has discrete hydrogen-bonding fragments. Powder X-ray diffraction and impedance analyses confirm that FJU-80 possesses higher water stability as well as improved proton conductivity, indicating that solvent-assisted modification is effective in increasing the hydrogen-bonding sites from phosphonate groups and functional moieties and then realizing facile proton transfer.