Superprotonic conduction of {NBu4-n(enOH)n}[MnCr(ox)3] (Bu = n-butyl; enOH=2-hydroxyethyl; n=1-3) in successive multiplication of hydroxyl proton-carrier in the cation

Proton conduction in a new series of 2-dimensional metal-organic frameworks, {NBu4-n(enOH)n}[MnCr(ox)3] (n = 1: (B3O)MnCr; n = 2: (B2O2)MnCr; n = 3: (BO3)MnCr), was studied with respect to their molecular and network structures. Crystallographic studies revealed that (B3O)MnCr consisted of honeycomb-based oxalate-bridged bimetallic [MnCr(ox)3]- sheets with {NBu3(enOH)}+ ions intercalated in between. The enOH group was inserted into the cavity of the honeycomb sheet and the hydroxy group protruded out of the cavity to form a hydroxyl-face on the sheet, which played a pivotal role in proton conduction via the vehicle mechanism under low relative humidity. Under high relative humidity conditions, water adsorption modified the hydroxyl-face such that a hydrogen-bonding network was formed, which mediated proton transfer via the Grotthuss mechanism. The proton conductivities (sigma) of (B3O)MnCr and (B2O2)MnCr at 25 degrees C under 95% relative humidity were 6.2 x 10-5 and 6.7 x 10-4 S cm-1, respectively, which increase to 1.7 x 10-4 and 2.0 x 10-3 S cm-1, respectively, with increasing the temperature to 50 degrees C. (BO3)MnCr deliquesces under the same conditions; however, the sigma value under 70% relative humidity was higher than those of (B3O)MnCr and (B2O2)MnCr, primarily because of the further introduction of the enOH group in the cation.