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 H(2)O because of its degenerate conjugate acid–base system (H3O(+) + H(2)O ⇔ H(2)O + H(3)O(+) or OH(–) + H(2)O ⇔ H(2)O + 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 (NH(3); NH(4)(+) + NH(3) ⇔ NH(3) + NH(4)(+)) is promising as the next proton-conducting medium. In addition, there are few reports on NH(3)-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.