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 1 H NMR, 133 Cs NMR, and 2D-EXSY 133 Cs 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 Δ H = −14 ± 1 kJ/mol and Δ S = −12 ± 4 J/(mol K), while for the alt,3 complex Δ H = −11 ± 1 kJ/mol and Δ S = −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 parameters for the complex formation between solvated Cs + and the pc conformer and for the exchange between the alt,3 and the pc complexes are Δ H * = 38 ± 2 kJ/mol and Δ S * = −66 ± 6 J/(mol K), and Δ H * = 61 ± 5 kJ/mol and Δ S * = −9 ± 15 J/(mol K), respectively. 1 H NMR spectroscopy shows that the exchange of Cs + between the two binding sites of the alt,3 complex proceeds through an interconversion to the pc complex and not through an internal tunneling process of Cs + .