Proton-conducting metal–organic frameworks with linkers containing anthracenyl and sulfonate groups

Abstract

Two new MOFs, namely, Co(dia)_1.5(Hsip)(H₂O)·H₂O (1) and Zn₂(μ-OH)(dia)₂(sip)·2H₂O (2), were synthesized with the same set of ligand precursors under hydrothermal conditions. The use of dia linker with a large anthracene ring successfully prevented framework interpenetration in these MOFs. The three functional groups in the sulfonate-based ligand enabled the presence of a deprotonated or undeprotonated carboxylate group for hydrogen bonding interactions. MOF 1 exhibited a 5-connected 3D framework with a bnn topology, whereas the underlying net in 2 showed a 4-connected dia topology. The proton conductivities of these two MOFs were tested at different temperatures and relative humidities (RH). MOF 1 was more proton-conducting than 2. The former MOF gave a proton conductivity value of 3.461 × 10⁻⁵ S cm⁻¹ at 85 °C under 98% RH, whereas the value for 2 was 1.527 × 10⁻⁵ S cm⁻¹ under the same conditions. The presence of undeprotonated carboxylic acid groups and aqua ligands, along with a strong and uninterrupted hydrogen bonding network in 1, endues it with higher proton conductivity.