Anion-induced structural transformation of a sulfate-incorporated 2D Cd(II)-organic framework

A Cd(II)-organic framework {[Cd₂(tpim)₄(SO₄)(H₂O)₂]·(SO₄)·21H₂O}_n (1) was synthesized by reacting CdSO₄·8/3H₂O and 2,4,5-tri(4-pyridyl)imidazole (tpim) under hydrothermal conditions. A structural analysis showed that compound 1 adopts a layered structure in which the [Cd(tpim)₂]_n chains are linked by sulfate anions. These 2D layers are further packed into a 3D supramolecular framework via π-π interactions. The structure contains two types of SO₄^2- anions, i.e., bridging SO₄^2- and free SO₄^2- anions, the latter of which are included in the large channels of the framework. Compound 1 exhibits interesting anion exchange behavior. In the presence of SCN^- anions, both the bridging and free SO₄^2- anions in 1 were completely exchanged by SCN^- ligands to form a 1D species [Cd(tpim)₂(SCN)₂] (1A), in which the SCN^- moieties function as a monodentate ligand. On the other hand, when compound 1 was ion exchanged with N₃^- anions in aqueous solution, the bridging SO₄^2- moieties remained intact, and only the free guest SO₄^2- were replaced by N₃^- anions. The gas adsorption behavior of the activated compound 1 was also investigated.