Influence of the Cavity Size of Water-Soluble Cryptophanes on Their Binding Properties for Cesium and Thallium Cations

Abstract

The binding properties of water-soluble cryptophane 1 toward cesium and thallium cations, in basic solution, have recently been reported. In this Article, we show that water-soluble cryptophane-222 (2), cryptophane-223 (3), and cryptophane-233 (4), bearing zero, one, and two propylenedioxy linkers, respectively, also efficiently bind these two cations under similar experimental conditions. Their binding properties are thoroughly studied by (133)Cs and (205)Tl NMR spectroscopy, while the binding constants are determined by isothermal titration calorimetry (ITC) experiments under various experimental conditions. Complexation of cesium and thallium is also evidenced by electronic circular dichroism (ECD) using the enantiopure MM-2 compound. This study reveals that the cavity size of the cryptophane is not the main parameter to observe efficient binding. In contrast, the number of phenolate moieties surrounding the cryptophane backbone seems to be pivotal for the complexation of these two cations. These results are important in the field of detoxification.