Xe@cryptophane Complexes with C2 Symmetry: Synthesis and Investigations by 129Xe NMR of the Consequences of the Size of the Host Cavity for Xenon Encapsulation

New cryptophanes‐223 (1), ‐233 (2), and ‐224 (3) with C2 symmetry, bearing different linkers connecting the two cyclotriveratrylene units were synthesized following a multi‐step procedure. The formation of the xenon@cryptophane complexes was investigated by 129Xe NMR spectroscopy. Cryptophanes‐223 (1) and ‐233 (2) complex xenon efficiently in 1,1,2,2‐[D2]tetrachloroethane solution with somewhat lower binding constants (K = 2810 M−1 and 810 M−1 respectively at 278 K) than that observed with cryptophane‐A (K = 3900 M−1). The free and bound guests were observed at room temperature under slow‐exchange conditions on the 129Xe NMR timescale. A linear relationship was observed between the binding constants and the internal volume of the hosts. Under the same conditions, the Xe@cryptophane‐224 [Xe3] complex underwent a fast‐exchange process on the NMR timescale. The decomplexation activation energies Ea, and the associated parameters ΔH‡ and ΔS‡, were determined for the Xe1 and Xe2 complexes from variable temperature 1D‐EXSY experiments. These investigations emphasized the importance of the size of the internal cavity of the cryptophanes for guest encapsulation, and the extreme sensitivity of the 129Xe chemical shift toward slight structural modifications of the atom’s environment. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)