Mechanisms of formation and dissociation of a cesium-calix[4]arene acetamide complex in solution: A Cs-133 dynamic NMR study

The kinetics and mechanism of the cesium cation complexation by 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(N,N-diaethylaminocarbonyl)methoxy calix[4]arene (1) were studied in a binary mixture (1:1 by volume) of deuterated acetonitrile and deuterated chloroform using H-1 and Cs-133 NMR spectroscopy. The formation of cone complexes with 1:1 and 2:1 Cs+/1 stoichiometries is reported. The complex formation of the 1:1 complex is quantitative (K-f > 10(4) M-1), and the equilibrium constant for the 2:1 complex is 3 +/- 2 M-1 at 238 K. The first complexed Cs+ is embedded in the cavity made of the four phenolic and four carbonyl oxygens, whereas it is suggested that the second Cs+ is loosely bound by the amido groups. The second Cs+ and the solvated Cs+ are in fast exchange. The embedded Cs+ is in slow exchange with solvated Cs+. The decomplexation process of the 1:1 complex follows a unimolecular dissociation mechanism. It is characterized by Delta H* = 67 +/- 2 kJ/mol and Delta S* = 40 +/- 3 J/(mol K). An intramolecular exchange between the two bound cesium cations is not observed.