NMR study of the ligand interchange of 18-crown-6 complexes with La3+, Ca2+, Pb2+ and Ba2+ ions in 70% methanol solution

Abstract

The exchange kinetics of 18-crown-6 complexes with La³⁺, Ca²⁺, Pb²⁺ and Ba²⁺ ions have been investigated in 70% methanol solution by ¹H NMR line-shape analysis. In the case of La³⁺ and Ca²⁺, complexes, in the temperature range of –23 to 37 °C, the predominant mechanism for the exchange of the ligand between the free and complexed sites is a bimolecular pathway. However, in the case of Pb²⁺ and Ba²⁺ complexes, while at temperatures 7 °C the ligand exchange proceeds via a dissociative mechanism, at lower temperatures of ⩽–3 °C both the dissociative and bimolecular mechanisms contribute to the ligand exchange. The exchange reaction rates and the activation parameters, E_a, ΔH^‡, ΔS^‡ and ΔG^‡ for the exchange have been determined and the possible parameters controlling the exchange rate and mechanism are discussed.