A Cryptophane Core Optimized for Xenon Encapsulation

Abstract

The smallest cryptophane ever synthesized, cryptophane-1.1.1 (1), whose internal volume is estimated to be 81 Å3, exhibits the highest binding constant (10000 M-1 at 293 K) in organic solution for xenon encapsulation determined to date. The very slow kinetics of decomplexation on the NMR time scale results in an extremely sharp 129Xe NMR signal for the Xe@1 complex, which shows an unexpected low-frequency shift. The free energy of activation for decomplexation of xenon at 293 K is ΔG⧧out = 64.9 ± 3 kJ mol-1 (ΔH⧧out = 43.2 ± 1.6 kJ mol-1; ΔS⧧out = −74 ± 3 J mol-1 K-1). These properties suggest that the cryptophane core of 1 is optimized for use in sensing applications in combination with laser-polarized xenon.