Proton Conducting Metal‐Organic Frameworks (MOFs) via Post Synthetic Transmetallation and Water Induced Structural Transformations

Abstract

AbstractPost Synthetic Modification (PSM) of Metal‐Organic Frameworks (MOFs) is a crucial strategy for developing new MOFs with enhanced functional properties compared to their parent one. PSM can be accomplished through various methods:1) modification of organic linkers; 2) exchange of metal ions or nodes; and 3) inclusion or exchange of solvent/guest molecules. Herein, PSM of bimetallic and monometallic MOFs containing biphenyl dinitro‐tetra‐carboxylates (NCA) are demonstrated. The tetra carboxylate NCA, produces monometallic Cd‐MOF‐1 and Cu‐MOF‐1 and bimetallic CoZn‐MOF in solvothermal reactions with the corresponding metal salts. The CoZn‐MOF undergoes post‐synthetic transmetallation with Cd(NO3)2 and Cu(NO3)2 in aqueous solution to yield Cd‐MOF‐2 and Cu‐MOF‐2, respectively. Additionally, green crystals of Cu‐MOF‐1 found to undergo a single‐crystal‐to‐single‐crystal (SCSC) transformation to blue crystals of Cu‐MOF‐3 upon dipped into water at room temperature. These MOFs demonstrate notable proton conductivities ranging from 10−3 to 10−4 S cm−1 under variable temperatures and humidity levels. Among them, Cu‐MOF‐3 achieves the highest proton conductivity of 1.36×10−3 S cm−1 at 90 °C and 98 % relative humidity, attributed to its continuous and extensive hydrogen bonding network, which provides effective proton conduction pathways within the MOF. This work highlights a convenient strategy for designing proton‐conducting MOFs via post‐synthetic modification.