Role of the Methoxy Groups in Cryptophanes for Complexation of Xenon: Conformational Selection Evidence from Xe- H NMR SPINOE Experiments.

We report the laser-polarized Xe and H NMR spectra of a series of cryptophane derivatives that differ only by the number of methoxy groups attached on their benzene rings and the syn or anti arrangement of the linkers (compounds 6 a-s, 9 a-s, 12 a-s). All these compounds bind xenon even though the characteristic signal of the gas encapsulated in the cavity of the cage-molecule cannot always be detected. Interestingly, the exchange dynamics of xenon strongly depends on the degree of substitution and is different from that of the cryptophane derivatives studied previously. In solution, the H NMR spectra of these derivatives show the presence of different conformations in a slow exchange regime that can be explained by a decrease of the flexibility of their skeleton. Thanks to Xe- H dipolar cross-relaxation (SPINOE) spectra, we demonstrate that a single conformation present in solution can bind xenon.