Proton Conduction through Water Chain in a Hydrogen-Bonded Molecular Crystal of Hexaimidazole Ruthenium(II) Complex and Dicarboxylate

The development of molecular proton conductors has garnered considerable attention owing to their potential for use as solid-state electrolytes for fuel cells. Creating hydrogen bonding networks between HIm (imidazole) and carboxylate is an attractive approach for constructing efficient proton conducting pathways. Herein, high proton conduction in a new hydrogen-bonded molecular crystal of [RuII(HIm)6](Glu). 4H2O (1.4H2O, H2Glu = glutaric acid) is demonstrated. 1.4H2O exhibited a proton conductivity of 1.33 x 1014 S cm11 at 318 K and 100% relative humidity. This is attributed to a three-dimensional (3D) hydrogen-bonding network via hydrogen-bonding between HIm and carboxylate group and water molecules, leading to produce a one-dimensional (1D) hydrogen-bonding water chain, which provides a proton conducting pathway. The calculated activation energy (Ea) value was 0.38 eV at 283-318 K, suggesting that proton transport in 1.4H2O is associated with the Grotthuss mechanism. The hydrogen-bonding networks between HIm and carboxylate may play a role to promote the proton hopping mechanism. This study will provide new molecular proton conductors using imidazole metal complexes.