Self-Assembly of Four Coordination Polymers in Three-Dimensional Entangled Architecture Showing Reversible Dynamic Solid-State Structural Transformation and Color-Changing Behavior upon Thermal Dehydration and Rehydration

Abstract

A unique three-dimensional (3D) supramolecular compound, [Co(dpe)(BTC)(H2O)][Co(dpe)(BTC)(H2O)3][Co(dpe)(HBTC)(H2O)][Co(dpe)2(H2O)3.5(EtOH)0.5]·1.5H2O (1; dpe = 1,2-bis(4-pyridyl)ethane and H3BTC = benzenetricarboxylic acid), has been synthesized and structurally characterized by the single-crystal X-ray diffraction method. Compound 1 consists of four coordination polymers (CPs), two are two-dimensional (2D) layered metal–organic frameworks (MOFs) with (4,4) topology of [Co(dpe)(BTC)(H2O)]− and [Co(dpe)(HBTC)(H2O)], whereas the other two are one-dimensional (1D) polymeric chains of [Co(dpe)(BTC)(H2O)3]− and [Co(dpe)(H2O)3.5(EtOH)0.5]2+. The 3D supramolecular architecture of 1 is constructed via the penetration of interdigitated double-layered 2D rectangular-grid frameworks by two 1D coordination polymeric chains and entangled tightly by the subtle combination of intermolecular hydrogen bonding and π–π interactions among the four CPs. Controlled heating of the as-synthesized crystal 1 at ∼160 °C produces a desolvated 1 and accompanying color-changing behavior from pink to deep-blue, and the deep-blue desolvated 1 regenerates the pink rehydrated crystal with the chemical formula of [Co(dpe)(BTC)(H2O)][Co(dpe)(BTC)(H2O)3][Co(dpe)(HBTC)(H2O)][Co(dpe)2(H2O)4]·3H2O (2) upon exposure to water vapor. The structural determination of 2 shows almost the same structural characteristics as that of 1 with the only difference being the replacement of disordered coordinated solvent (half H2O and half EtOH molecules) by H2O and the numbers of solvated water molecules. The cyclic thermogravimetric analysis and powder X-ray diffraction measurements of desolvated 1 demonstrate a reversible rehydration/dehydration property, which is associated with solid-state structural transformation and thermally induced UV–vis absorption properties.