Interaction of Cesium Ions with Calix[4]arene-bis(t-octylbenzo-18-crown-6): NMR and Theoretical Study

Using 1H, 13C, and 133Cs NMR spectra, it is shown that calix[4]arene-bis(t-octylbenzo-18-crown-6) (L) forms complexes with one (L·Cs+) and two (L·2Cs+) Cs+ ions offered by cesium bis(1,2-dicarbollide) cobaltate (CsDCC) in nitrobenzene-d 5. The ions interact with all six oxygen atoms in the crown-ether ring and the π electrons of the calixarene aromatic moieties. According to extraction technique, the stability constant of the first complex is log βnb(L·Cs+) = 8.8 ± 0.1. According to 133Cs NMR spectra, the value of the equilibrium constant of the second complex is log K nb (2)(L·2Cs+) = 6.3 ± 0.2, i.e., its stabilization constant is log βnb(L·2Cs+) = 15.1 ± 0.3. Self-diffusion measurements by 1H pulsed-field gradient (PFG) NMR combined with density functional theory (DFT) calculations suggest that one DCC– ion is tightly associated with L·Cs+, decreasing its positive charge and consequently stabilizing the second complex, L·2Cs+. Using a saturation-transfer 133Cs NMR technique, the correlation times τex of chemical exchange between L·Cs+ and L·2Cs+ as well as between L·2Cs+ and free Cs+ ions were determined as 33.6 and 29.2 ms, respectively.