Nitroxide Radicals as Probes for Exploring the Binding Properties of the Cucurbit[7]uril Host

Abstract

EPR spectroscopy was used for the first time to explore the binding properties of cucurbit[7]uril (CB7), a representative member of the cucurbituril family. Evidence for the formation of a complex between nitroxide radicals and the host system in an aqueous solution was provided by large changes in the nitrogen hyperfine splitting, attributed to the different polar environments experienced by the included radical. In the presence of alkali cations, the EPR spectra of benzyl tert-butyl nitroxide were characterised by new signals attributed to the radical hosted in the CB7 cavity in which one metal cation is in close contact with the nitroxidic oxygen. The formation of the coordination complex results in a substantial increase in the electron spin density on the nitrogen in inverse order with respect to the size of the cation owing to increased localisation of negative charge on the oxygen atom from bonding to the alkali cation. The EPR spectra showed selective line-broadening effects as a result of metal exchange between bulk water and the coordination complex. Analysis of the EPR linewidth variations allowed us to measure the corresponding kinetic rate constants for the first time. NMR spectroscopy showed that this behaviour is not peculiar to nitroxides but is also exhibited by the related carbonyl compounds. These data allowed us to quantify the template effect and to reach the conclusion that, in the presence of a guest having a coordinating lone pair, the formation of ternary metal-guest-CB complexes must be taken into account when discussing the complexation behaviour of cucurbituril derivatives in the presence of salts.