COMPLEXATION STUDIES OF ALKALINE EARTH METAL IONS WITH MACROCYCLIC LIGANDS OF THE ANTHRAQUINONE-CROWN ETHER TYPE BY ELECTROCHEMICAL METHODS

Abstract

The electrochemical and complexation behaviour of synthetic ligands of the crown ether type attached to an anthraquinone unit has been studied using cyclic voltammetry. In the presence of alkaline earth metal ions the positions and type of the ligand redox waves were changed and the potentials of the respective redox couples were shifted to more positive values. These shifts in the potentials translate as an increase in the thermodynamic stability of complexation to the anionic when compared to the unreduced species, as well as a dependence on the complexed cation. The magnitude of the binding enhancement is larger for cations with higher ionic potential excluding Mg2+ and is also favoured by an increased number of donor sites in the macrocyclic cavity of the ligand. The following trends have been observed, Ca2+ > Sr2+ > Ba2+ > Mg2+, with the reduced ligands studied showing a remarkably high degree of cation selectivity for Ca2+ over Mg2+. The reduced carbonyl oxygen positioned within the crown ring makes a significant contribution to coordination; the reduced carbonyl group outside the crown ring participates weakly if at all in coordination. The results are discussed in terms of electronic properties, number of binding sites and macrocyclic ring structural flexibility in the ligands, as well as of the size and solvation of the metal cations.