A new design strategy to access zwitterionic metal-organic frameworks from anionic viologen derivates

Two isostructural microporous zwitterionic metal-organic frameworks (ZW MOFs), {[M(bdcbpy)(OH2)4]·4H2O}n with M = Mn (1) and Ni (2), were synthesized by the rational design of the flexible anionic viologen derivate, 1,1'-bis(3,5-dicarboxybenzyl)-4,4'-bipyridinium dibromide dihydrate solvate (H4bdcbpyBr2·2H2O), and its self-assembly with metal(II) acetates in an aqueous medium. Single-crystal structure analyses revealed that both compounds exhibit three-dimensional hydrogen-bonded supramolecular frameworks with one-dimensional channel pores. Significantly, the pore surfaces are lined with charge gradients employed by the ZW ligand bdcbpy(2-) leading to the adsorption of hydrogen attributed to polarization effects. The thermostabilty and activation conditions were systematically investigated by thermogravimetric analysis, differential scanning calorimetry, and powder X-ray diffraction experiments. Furthermore, repeating cycles of reversible color changes are observed in air upon irradiation with UV light attributed to the formation of viologen radicals via an intermolecular electron transfer. This work also contains an in-depth literature analysis on ZW MOFs, which shows the need for the development of alternative routes for the rational design of new porous ZW MOFs.