A Synchronous Change in Fluid Space and Encapsulated Anions in a Crystalline Polymethylene Unit Containing Metal-Organic Framework

Porous materials possessing fluid spaces are of great interest in the field of material science because they have the capability of undergoing unique, external stimuli-promoted and guest uptake/release-responsive changes in shapes and sizes. Polymethylene chains are postulated to be important components in metal-organic frameworks (MOFs) enabling them to possess fluid spaces. The three-dimensional (3D) cationic MOF, [Cu-(bib)(2.5)] (bib = 1,4-bisimidazole butane), which contains organic bridging ligands with conformationally flexible tetramethylene ((CH2)(4)) units, was designed to assess this proposal. The combined results of DSC measurements, single-crystal X-ray diffraction analysis, and impedance spectroscopy demonstrate that this MOF encapsulating bis(trifluoromethylsulfonyl)amide (NTf2-) anions contains fluid space and that it undergoes a crystal-to-crystal transition in association with a synchronous change in the conformations of both the bib ligand, caused by C-C bond rotation, and the NTf2- anion stemming from rotation about the N-S bond.