Cyclic Structural Transformations from Crystalline to Crystalline to Amorphous Phases and Magnetic Properties of a Mn(II)-Based Metal-Organic Framework

A three-dimensional Mn(II) framework, [Mn₂(H₂L)(L)_0.5(MeOH)(DEF)]·0.1MeOH·0.1DEF·1.4H₂O (1; H₄L = 2,3-dioxido-1,4-benzenedicarboxylic acid), was synthesized under solvothermal conditions in diethylformamide/methanol (DEF/MeOH), where the Mn centers adopt octahedral and unusual pentagonal bipyramidal geometries. The ligand H₄L was subject to deprotonation to create μ₄-H₂L^2- and μ₆-L^4- anionic bridges, leading to the construction of a coordination network. The MeOH exchange process of crystalline 1 allowed for another crystalline phase (1a), which reversibly returned to the original crystalline state upon resolvation in DEF/MeOH. After evacuation of 1a, the amorphous phase 1b was irreversibly formed, followed by the restoration of the original phase 1 upon resolvation in DEF/MeOH. Consequently, this framework underwent cyclic structural transformations from the crystalline (1) to crystalline (1a) to amorphous (1b) and back to crystalline (1) phase, which are unique transformations for soft coordination networks. Magnetic measurements demonstrated that antiferromagnetic interactions were operative between the Mn(II) ions and were effectively mediated by the oxygen moieties of the μ₆-L^4- bridge.