Proton transfer in hydrogen-bonded degenerate systems of water and ammonia in metal-organic frameworks

Porous crystalline metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) are emerging as a new class of proton conductors with numerous investigations. Some of the MOFs exhibit an excellent proton-conducting performance (higher than 10⁻² S cm⁻¹) originating from the interesting hydrogen(H)-bonding networks with guest molecules, where the conducting medium plays a crucial role. In the overwhelming majority of MOFs, the conducting medium is H₂O because of its degenerate conjugate acid-base system (H3O⁺ + H₂O ⇔ H₂O + H₃O⁺ or OH⁻ + H₂O ⇔ H₂O + OH⁻) and the efficient H-bonding ability through two proton donor and two acceptor sites with a tetrahedral geometry. Considering the systematic molecular similarity to water, ammonia (NH₃; NH₄⁺ + NH₃ ⇔ NH₃ + NH₄⁺) is promising as the next proton-conducting medium. In addition, there are few reports on NH₃-mediated proton conductivity in MOFs. In this perspective, we provide overviews of the degenerate water (hydronium or hydroxide)- or ammonia (ammonium)-mediated proton conduction system, the design strategies for proton-conductive MOFs, and the conduction mechanisms.