Proton Conduction via Water and Ammonia Coordinated Metal Cationic Species in MOF and MHOF Platforms

Although metal-organic frameworks (MOFs) and metalo hydrogen-bonded organic frameworks (MHOFs) are designed as promising solid-state proton conductors by incorporating various protonic species intrinsically or extrinsically, design and development of such materials by employing the concept of proton conduction through coordinated polar protic solvent is largely unexplored. Herein, we have constructed two proton-conducting materials having different solvent coordinated metal cationic species: In-H₂O-MOF, ({[In(H₂O)₆][In₃(Pzdc)₆] ⋅ 15H₂O}_n; H₂Pzdc: pyrazine-2,3-dicarboxylic acid) with coordinated water molecules from hexaaquaindium cationic species, and MHOF-4, ([{Co(NH₃)₆}₂(2,6-NDS)₂(H₂O)₂]_n; 2,6-H₂NDS: 2,6-naphthalenedisulfonic acid) with coordinated ammonia from hexaammoniacobalt cationic species. Interestingly, higher proton conductivity was achieved for In-H₂O-MOF (1.5×10^-5 S cm^-1) than MHOF-4 (6.3×10^-6 S cm^-1) under the extreme conditions (80 °C and 95 % RH), which could be attributed to enhanced acidity of coordinated water molecules having much lower pK_a value than that of coordinated ammonia. Greater charge polarization on hydrogen atoms of In³⁺-coordinated water molecules than that of Co²⁺-coordinated ammonia led to the high conductivity of In-H₂O-MOF, as evident by quantum chemical studies. Such a comparative study on metal-coordinated protic polar solvents in achieving proton conduction in crystalline solids is yet to be made.