γ-Cyclodextrin cuprate sandwich-type complexes

Three structures, based on γ-cyclodextrin (γ-CD) and metal ions (Cu²⁺, Li⁺, Na⁺, and Rb⁺), have been prepared in aqueous and alkaline media and characterized structurally by single-crystal X-ray diffraction. Their dimeric assemblies adopt cylindrical channels along the c axes in the crystals. Coordinative and hydrogen bonding between the cylinders and the solvent molecules lead to the formation of two-dimensional sheets, with the identity of the alkali-metal ion strongly influencing the precise nature of the solid-state structures. In the case of the Rb⁺ complex, coordinative bonding involving the Rb⁺ ions leads to the formation of an extended two-dimensional structure. Nonbound solvent molecules can be removed, and gas isotherm analyses confirm the permanent porosity of these new complexes. Carbon dioxide (CO₂) adsorption studies show that the extended structure, obtained upon crystallization of the Rb⁺-based sandwich-type dimers, has the highest CO₂ sequestration ability of the three γ-CD complexes reported.