Exclusive ion recognition using host–guest sandwich complexes

Abstract

Ion recognition in porous aqueous media utilizing polyethers involves the formation of 1 : 1 and higher-order host–guest complexes. The effectiveness of these interactions relies on the optimal size of the host cavity to encapsulate the guest ions. While liquid/liquid extraction based on host–guest interactions offers higher specificity in metal ion extraction, it results in the co-extraction of unwanted coordinating solvents and counter-anions. Therefore, an improved protocol is required by which the ion can be selectively trapped within the host cavity and simultaneously decrease the guest coordination with the outside environment. This study delves into the microscopic mechanisms underpinning the exclusive ion recognition through the formation of 2 : 1 host–guest sandwich complexes, which reduce metal coordination with solvent or counter-ions, ensuring selectivity. Our analysis shows that ions with a radius larger than the host cavity, such as cesium (Cs⁺), form stable host–guest sandwich complexes at elevated host concentrations. In this study, we performed molecular dynamics simulations to investigate the microscopic details of Cs⁺ interactions with open-chain and preorganized polyethers, namely podand, crown, and cryptand in electrolyte media. Our findings reveal that the formation of stable Cs⁺–crown sandwich complexes significantly reduces Cs⁺ coordination with H₂O and NO₃⁻. This loss of solute coordination leads to exclusivity in bound metal ions, offering a potential strategy for efficient solvent extraction.