Iodide ion receptors: shape-persistent macrocycles of syn/anti configurations

This article reports a pair of rigid iodide ion macrocyclic receptors of syn/anti configurations. Single crystal X-ray analysis confirms the structure of the syn-isomers, and 1H Nuclear Magnetic Resonance (1H NMR) titration and ultraviolet-visible (UV-Vis) absorption are used to study the binding affinity of the isomers to halogen ions. Both the isomers show colorimetric and fluorescence recognition towards iodide anions in dichloromethane solution. By solving the association constants, it can be concluded that the interaction between the anti-isomer and iodide ions is relatively weaker than that in the syn-isomer, and the syn-isomer has much better recognition selectivity for I− in CH2Cl2 than the anti-isomer. The results of the theoretical calculations show that syn/anti configurations bring about different orientations of the alkyl chains, leading to the larger steric hindrance of the anti-isomer to hinder the combination with iodide ions, and the syn configurations with relatively smaller steric hindrance can enhance the binding between the isomers and the iodide anions. A shape persistent macrocycle with a bound conformation can be looked at as a special case of conformational selection, and the results of this study indicate a local induced fit mechanism following a conformational selection in the recognition of iodide anions, and we anticipate that this work will help to understand these two dominant binding mechanisms in molecular recognition and have a certain reference value for designing better anion receptors.