Potentiometric study of the mixed-metal complex formation of tetracarboxylate-18-crown-6 with aluminum and alkali and alkaline earth cations

We give evidence for the formation of mixed-metal complexes of tetracarboxylate-18-crown-6 in the form of Al³⁺-macrocycle-M^Z+, with M^Z+ being an alkali or an alkaline earth cation. These binuclear systems are characterized by potentiometric pH] titrations and the equilibrium constants for all major complexes formed are described. The results are presented in the form of distribution diagrams revealing the concentrations of individual complex species as a function of pH]. The stability of the resulting mixed-metal complexes with Al³⁺ and alkali and alkaline earth cations vary in the order Na⁺>K⁺>Cs⁺ and Ca²⁺>Sr²⁺>Ba²⁺. The complex behavior can be rationalized in terms of electrostatic interactions and direct coordination of the cations by the carboxylate groups from the crown ether periphery.