Kinetics and mechanism of the dissociation of a sodium-calix[4]arene ester complex in nonaqueous solution

Abstract

The kinetics and mechanism for the dissociation of sodium ion complexes of a calix[4]arene ester 1 were studied in nonaqueous solution by a dynamic ¹H NMR. Life times τ_c of the Na⁺-1 complexes and activation parameters Δ_dH^‡ and Δ_dS^‡ for the dissociation process were determined in five organic solvents. In methanol, the life time (8.8×10⁻³ s) of the sodium complex at 25°C was 440 times larger than that of crown ether (18C6) and was ca. 40 times less than that of cryptands (C211 and C222). The activation parameters for the dissociation process, Δ_dH^‡ of 67, 64, 57, 57, and 46 kJ mol⁻¹, and Δ_dS^‡ of −22, −29, −7.7, −13, and −33 J mol⁻¹ K⁻¹ were determined in deuteriated nitromethane, acetonitrile, acetone, methanol, and dimethylformamide, respectively. It was observed that the activation enthalpies tend to decrease with increasing the electron-donating ability of solvents as indicated by the Gutmann donor number, while the activation enthalpies do not correlate to the donor number and all the values are negative. These results suggest that in the activated state of the Na⁺-1 complex, additional solvent molecules bind to the sodium ion encapsulated by ethoxycarbonylmethoxy groups in 1, and the disruption of the bonding between a sodium ion and the oxygens in the OCH₂CO moieties plays a major contribution in the dissociation process. In acetonitrile, the life times of the Na⁺-1 complexes were not affected by the concentration of the free ligand of 1, suggesting that the dissociation proceeds via a unimolecular dissociation not a bimolecular exchange between free and complexed 1.