Reversible Switching of Frameworks through Single-Crystal-to-Single-Crystal Structural Transformation in Two Entangled Coordination Polymers and Their Impact on Adsorption Properties

Two coordination polymers (CPs) with a different framework entanglement, namely, a triply interpenetrated two-dimensional (2D) sheet of {[Zn­(bpe)­(pim)]­·(bpe)0.5(H2O)3} n (1) and a doubly interpenetrated three-dimensional (3D) net of {[Cd­(bpy)­(nbdc)]­·(solvent) x } n (2) [where, bpe = 4,4′-bispyridylethane, bpy = 4,4′-bipyridine, pim2– = dianion of pimelate salt, and nbdc2– = 2-nitro-1,4-benzenedicarboxylate] were synthesized at room temperature using d10 metal ions (Zn­(II)/Cd­(II)) along with two 4-dipyridyl linkers with a variation from flexible aliphatic dicarboxylates to rigid aromatic dicarboxylates. Notably, compound 1 exhibits structural dynamism through reversible solid state SC–SC structural transformation from a 3-fold interpenetrated 2D + 2D → 2D sheet to an interesting higher dimensional 2D + 2D → 3D parallel polycatenated framework (1a). This structural change occurred by heating followed by solvent exclusion. In contrast to 1, compound 2 exhibits a reversible single-crystal-to-single-crystal (SC–SC) structural transformation via the formation of a robust dehydrated framework of 2 (i.e., 2a). These structural transformations are thoroughly studied by powder X-ray diffraction analysis and infrared spectroscopy along with the single crystal X-ray diffraction. All of these structures exhibit a high degree of thermal stability as visualized by thermogravimetric analysis. The dehydrated frameworks of both the compounds were implemented for gas and vapor sorption studies.