Role of the Methoxy Groups in Cryptophanes for Complexation of Xenon. Conformational Selection Evidenced by 129 Xe-1 H NMR SPINOE Experiments

We report the laser-polarized 129 Xe and 1 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 arrangement syn or anti of the linkers (compounds 6a-s, 9a-s, 12a-s). All these compounds bind xenon even though the characteristic signal of the gas encapsulated in the cavity of the cage-molecule cannot be always 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 1 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 129 Xe-1 H dipolar cross-relaxation (SPINOE) spectra we demonstrate that a single conformation present in solution can bind xenon.