Interpenetration control, sorption behavior, and framework flexibility in Zn(II) metal-organic frameworks

Three Zn(II) frameworks [Zn(H₂L)(bdc)]·1.4DEF·0. 6H₂O (1; H₂L = 1,4-di(1H-imidazol-4-yl)benzene, H ₂bdc = terephthalic acid), [Zn(H₂L)(bdc)]·1. 5DMF·1.2H₂O (2), and [Zn(H₂L)(L) _0.5(bdc)_0.5]·formamide·H₂O (3) were prepared under the solvothermal conditions in DEF/H₂O, DMF/H₂O, and formamide/H₂O solvent pairs, respectively. All compounds are commonly based on the adamantanoid three-dimensional networks that are mutually entangled to form a 3-fold (1) to 4-fold (2) to 5-fold interpenetrating dia structure (3). The solvent pairs used in the reactions are primarily responsible for the variation of such interpenetration degree. It is noted that the reaction time, temperature, and reactant ratio applied in the present system (2) did not lead to the interpenetration change. The activated sample (1a) shows the gas uptake of N₂, H₂, and CO ₂, characteristic of permanent porosity in the flexible framework, while the gases of N₂ and H₂ are not adsorbed on 2 and 3. The porous compound (1) also exhibits the reversible inclusion and release of I₂ in MeOH. Interestingly, 2 reveals the reversible structural transformation during the activation-resolvation process where the solid can be activated through two routes (solvent exchange/desolvation and direct desolvation). However, there is no appreciable structural flexibility upon solvent exchange in 3 with 5-fold interpenetration, indicating that this framework is more robust, compared to 1 and 2 with lower interpenetration degrees.