A flexible Ni(ii)-based metal-organic framework exhibiting reversible breathing and enhanced proton conductivity.

Flexible metal-organic frameworks (MOFs) exhibit stimuli-responsive phase transitions that enable diverse applications. In this article we report the synthesis and characterisation of a flexible MOF, MFM-261a, [H2N(CH3)2]2[Ni3(sbpdc)4(η-H2O)2(μ-OH2)2]·solv (H2sbpdc = sulfonyl-4,4'-biphenyldicarboxylic acid), which exhibits a reversible structural phase transition. Single-crystal X-ray diffraction reveals that MFM-261a crystallises in the orthorhombic space group Pnnm. Upon activation via desolvation, MFM-261a undergoes a single-crystal-to-single-crystal (SCSC) transformation to yield MFM-261b, [H2N(CH3)2]2[Ni3(sbpdc)4(η-H2O)2(μ-OH2)2]. Crystallographic analysis confirms that MFM-261b retains the metal-ligand coordination as the parent framework but shows a different pore geometry. Notably, MFM-261b can be reversibly converted to MFM-261a by exposure to dimethylformamide, demonstrating structural breathing. Impedance spectroscopic measurements reveal a proton conductivity of 3.00 × 10-5 S cm-1 for MFM-261a at 50 °C and 3.41 × 10-6 S cm-1 for MFM-261b at 70 °C under 99% relative humidity. The calculated activation energies of 0.63 and 0.68 eV support a vehicular transport mechanism with water molecules facilitating proton transfer. This work demonstrates the potential of flexible MOFs for applications requiring tuneable proton conduction properties.