Assembly of Cd(ii) coordination polymers: structural variation, supramolecular isomers, and temperature/anion-induced solvent-mediated structural transformations

Abstract

The reactions of Cd(II) salts and 5-(3-(1H-imidazol-1-yl)phenyl)-1H-tetrazolate (3-HIPT) resulted in eight new coordination polymers (CPs), namely, {[Cd(3-IPT)₂(H₂O)₂]·H₂O}_n (1 and 2), {[Cd(3-IPT)₂(H₂O)₂]·2H₂O}_n (3), [Cd(3-IPT)(H₂O)Cl]_n (4), [Cd(3-IPT)Cl]_n (5), [Cd(3-HIPT)I₂]_n (6), [Cd(3-IPT)I]_n (7), and [Cd(3-HIPT)₂]_n (8). Single-crystal X-ray analysis revealed that compound 1 is a 1D beaded chain, whereas compounds 2 and 3 are made of 2D networks. Compounds 1–3 are supramolecular isomers; their synthesis can be controlled under different temperatures and concentrations. The results showed that compounds 1 and 3 are the most thermodynamically and kinetically favored products, respectively. The thermodynamic stability of compound 1 may be attributed to the formation of the smallest M₂L₂ ring in the compound. Compounds 4–7 were obtained at higher Cl⁻/I⁻ concentrations. Compound 4 is a 2D net composed of 1D [Cd(3-IPT)]_n chains and μ₂-Cl and μ₂-H₂O connectors. Compound 5 is a (3,6)-connected 3D framework with rtl topology. Compound 6 possesses a 1D chain with 3-HIPT ligands on both sides. Compound 7 is a 2D (4·8²) net. Compound 8, a 3D pcu framework based on trinuclear linear SBUs, was formed when Cd(CF₃CO₂)₂ was introduced at 170 °C. Based on a temperature-changing cycle, compounds 1 and 3 display crystal-to-amorphous-to-crystal phase transitions accompanying the dehydration–rehydration process, whereas compound 2 only displays crystal-to-amorphous phase transition when the temperature is increased and cannot go back to the crystal phase again. Interestingly, solvent-mediated structural transformations were accomplished among the selected compounds. When compounds 2, 3, 6, 7, or 8 were left in a water and NaCl solution at 170 °C, they were partly/fully transformed into compounds 1 and 5, respectively. When compound 5 was recrystallized in water at 120 and 170 °C, it was partly and fully transformed into compounds 2 and 1, respectively. Such transformations were induced by the temperature or an anion. In addition, the thermal stabilities and luminescence properties of selected compounds have also been studied in detail. The complexes exhibit intense solid-state fluorescence emission at room temperature.