Kinetics and Mechanisms of Complexation of the Cesium Cation by 5,11,17,23-Tetra-p-tert-butyl-25,26,27,28-tetramethoxycalix[4]arene in Solution

The kinetics and mechanism of the cesium cation complexation by 5,11,17,23-tetra-p-tert-butyl- 25,26,27,28-tetramethoxycalix[4]arene (1) were studied in a 1:1 (v/v) mixture of deuterated chloroform and deuterated acetonitrile using 1H NMR, 133Cs NMR, and 2D-EXSY 133Cs NMR spectroscopy. The results show the formation of 1:1 complexes with the partial cone (pc) and 1,3-alternate (alt,3) conformer of 1. In the latter case, Cs+ is bound asymmetrically to one of the two metal binding sites, composed of two phenolic oxygen and two phenyl groups. The equilibrium association constants are respectively 194 and 33 M-1 at 236 K. In the case of the pc complex ΔH = −14 ± 1 kJ/mol and ΔS = −12 ± 4 J/(mol K), while for the alt,3 complex ΔH = −11 ± 1 kJ/mol and ΔS = −11 ± 7 J/(mol K). The preferred complexation pathway of Cs+ by 1 follows the complexation by the pc conformer, which can undergo a conformational exchange to the alt,3 conformer, from which Cs+ cannot escape directly with a measurable rate. The activation pa...