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

• Published in: Chemistry : a European journal 2024-12, Vol.30 (69), p.e202402896-n/a
• Main Authors: Pramanik, Bikram, Sahoo, Rupam, Yoshida, Yukihiro, Manna, Arun K., Kitagawa, Hiroshi, Das, Madhab C.
• Format: Article
• Language: English
• Subjects: Acidity; Ammonia; Carbon dioxide; Cations; Cobalt; Comparative studies; Conduction; Conductivity; Dicarboxylic acids; Fuel cells; Hydrogen atoms; Metal-organic frameworks; Metalo hydrogen-bonded organic frameworks; Metals; Proton conduction; Proton exchange membrane fuel cells; Pyrazine; Quantum chemistry; Solvents; Water chemistry
• ISSN: 0947-6539; 1521-3765; 1521-3765
• DOI: 10.1002/chem.202402896

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 pKa 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. Chemically robust proton‐conducting materials, In‐H2O‐MOF and MHOF‐4, having different solvent‐coordinated metal cationic species (H2O and NH3) were rationally designed and synthesized to demonstrate their proton conduction achieved through coordinated polar protic solvents molecules.