Coordination of alkaline-earth metal cations to a symmetrical octamethyl-substituted cucurbituril in the presence of polychlorido cadmium(ii) anions

Abstract

The interaction of the alkaline-earth metal ions Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ with the symmetrical octamethyl-substituted cucurbituril, OMeQ[6], in the presence of [CdCl₄]²⁻/[Cd₂Cl₈]⁴⁻ anions as structure-directing agent(s) in 3 M HCl has been investigated. Ca²⁺ and Sr²⁺ yielded closely related metallosupramolecular assemblies whose X-ray structures show that in the former, [Cd₂Cl₈]⁴⁻ anions are arranged to form parallelogram-like frameworks that surround linear OMeQ[6]/Ca²⁺ polymeric units. The structure of the OMeQ[6]/Sr²⁺ assembly is similar except that the [Cd₂Cl₈]⁴⁻ anions in the OMeQ[6]/Ca²⁺ assembly are replaced by [Cd₂(H₂O)₂Cl₈]⁴⁻ anions. In contrast to the above structures, interaction of the larger Ba²⁺ ion with OMeQ[6] gives rise to a different assembly type displaying a porous linear polymeric arrangement incorporating two different Ba²⁺ coordination modes; [Cd₂Cl₄]⁴⁻ anions are associated with the Ba²⁺ bridges linking adjacent OMeQ[6] units in the polymeric chain. In contrast to Ca²⁺ and Sr²⁺, the smaller Mg²⁺ ion under similar synthetic conditions failed to yield a solid product. Isothermal titration calorimetry (ITC) studies revealed that OMeQ[6] interacts with Ca²⁺, Sr²⁺ and Ba²⁺, but not Mg²⁺, in neutral aqueous solution. The respective X-ray structures reveal the presence of both channels and other voids in the three assemblies. Following application of a vacuum until constant weight was obtained, samples of the Sr²⁺ and Ba²⁺ assemblies were subjected to the saturated vapour of each of the following volatile ligands: acetonitrile, methanol, ethanol, acetone, diethylether, dichloromethane, cyclohexane and n-hexane. The respective absorption profiles show maximum absorption for methanol, with reduced uptake for the remaining larger and/or less polar guest molecules.