Abstract

COSTA, J.M. Caridade  and  RODRIGUES, P.M.S.. Complex Formation Between Alkaline-Earth Cations and Anthraquinone Crown Ethers in Methanol and Acetonitrile . Port. Electrochim. Acta [online]. 2005, vol.23, n.2, pp.289-300. ISSN 0872-1904.

Formation of complexes of alkaline-earth cations with crown ethers containing an anthraquinone unit (AQ18C6 and AQ21C7) was investigated in methanol and acetonitrile solutions. Stability constants of the resulting complexes were estimated by means of potentiometric methods. The results obtained show the formation of complexes of the LM²⁺ type for all alkaline-earth ions; for the larger cations, (Sr²⁺ and Ba²⁺) and in excess of ligand, L₂M²⁺ complexes were also detected. The most stable complexes were obtained with Ba²⁺ cation and for both ligands. The binding strength for the cations towards AQ18C6 and AQ21C7 ligands and in both solvents, increased in the order, Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺. The results of this study suggest that on the 1:1 complexes, the size of the cation and its fit into the macrocyclic internal cavity of the ligand was a dominant factor on the coordination binding. The number of donor atoms in the ring of the macrocycle does not affect the binding strength and the AQ18C6 complexes have larger stability than the corresponding AQ21C7 complexes. The presence of an anthraquinone unit on the structure of the crown ether induced a decrease of the cation binding strength. The external carbonyl group of the ligand molecules was not involved on the coordination to the cation. Considering the two solvents, the complexes were more stable on acetonitrile, the solvent of weaker solvating capacity.

Keywords : alkaline-earth cations; crown ether derivatives; complexation; potentiometric methods.

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