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

Abstract

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 (σ) of (B₃O)MnCr and (B₂O₂)MnCr at 25 °C under 95% relative humidity were 6.2 × 10⁻⁵ and 6.7 × 10⁻⁴ S cm⁻¹, respectively, which increase to 1.7 × 10⁻⁴ and 2.0 × 10⁻³ S cm⁻¹, respectively, with increasing the temperature to 50 °C. (BO3)MnCr deliquesces under the same conditions; however, the σ 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.