A New Design Strategy to Access Zwitterionic Metal–Organic Frameworks\nfrom Anionic Viologen Derivates

Abstract

Two isostructural\nmicroporous zwitterionic metal–organic frameworks (ZW MOFs),\n{[M­(bdcbpy)­(OH₂)₄]·4H₂O}_<i>n</i> with M = Mn (<b>1</b>) and Ni (<b>2</b>), were synthesized by the rational design of the flexible\nanionic viologen derivate, 1,1′-bis­(3,5-dicarboxybenzyl)-4,4′-bipyridinium\ndibromide dihydrate solvate (H₄bdcbpyBr₂·2H₂O), and its self-assembly with metal­(II) acetates in an aqueous\nmedium. Single-crystal structure analyses revealed that both compounds\nexhibit three-dimensional hydrogen-bonded supramolecular frameworks\nwith one-dimensional channel pores. Significantly, the pore surfaces\nare lined with charge gradients employed by the ZW ligand bdcbpy^2– leading to the adsorption of hydrogen attributed\nto polarization effects. The thermostabilty and activation conditions\nwere systematically investigated by thermogravimetric analysis, differential\nscanning calorimetry, and powder X-ray diffraction experiments. Furthermore,\nrepeating cycles of reversible color changes are observed in air upon\nirradiation with UV light attributed to the formation of viologen\nradicals via an intermolecular electron transfer. This work also contains\nan in-depth literature analysis on ZW MOFs, which shows the need for\nthe development of alternative routes for the rational design of new\nporous ZW MOFs.