KINETICS AND MECHANISM OF THE SODIUM-CATION COMPLEXATION BY 5,11,17,21-TETRA-P-TERT-BUTYL-25,26,27,28-TETRAMETHOXY-CALIX[4]ARENE IN SOLUTION

The kinetics and the mechanism of complexation of the sodium cation by the title compound (1) were studied in a 1:1 (v/v) mixture of chloroform and acetonitrile using Na-23 and 2D-EXSY H-1 NMR. The results show the formation of a 1:1 complex where the calixarene is fixed in the cone conformation, (Na,c)(+), with an equilibrium constant of formation of (9.3 +/- 0.7) x 10(2) at 320 K with Delta H = -22 +/- 2 kJ mol(-1) and Delta S = -7 +/- 5 J mol(-1) K-1. The exchange of the sodium cation between the solvated and the complexed sites follows a dissociation/recombination mechanism, with a dissociation rate constant of 37.3 +/- 0.4 s(-1) at 300 K. The kinetic parameters for the dissociation are Delta H double dagger = 47 +/- 2 kJ mol(-1) and Delta S double dagger = -61 +/- 6 J mol(-1) K-1. Calixarene 1 shows an intermediate behavior between crown ethers and cryptands for the complexation processes of the sodium cation in solution, dissociating with rates in the typical orders of magnitude found for crown ethers, but with rates of formation in the range of those typically found for cryptands.