Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

Solution properties of complexes of a new H₄dota-like ligand containing three acetate and one methylphosphonate pendant arms (H₅do3ap, H₅L) were studied. The ligand exhibits a high last dissociation constant (pK_A = 13.83) as a consequence of the presence of phosphonate moiety. In solution, successive attachment of protons leads to several reorganizations of protonation sites and the neutral zwitterionic species H₅do3ap has the same solution structure as in the solid state, where the nitrogen atom binding methylphosphonate and the opposite nitrogen atoms are protonated. Stability constants with Na⁺ and trivalent lanthanide ions (La³⁺, Ce³⁺, Eu³⁺, Gd³⁺, Lu³⁺) and Y³⁺ have been determined. The constants are comparable or higher than those of H₄dota due to the higher overall basicity of H₅do3ap. Formation of the stable protonated complexes, as well as complexes with the L:M = 1:2 stoichiometry, was proved. Formation and decomplexation kinetics of the Ce³⁺ and Gd³⁺ complexes were investigated. The mechanism of formation of the H₅do3ap complexes is similar to that observed for H₄dota complexes and the complex species with mono- or diprotonated ligand on the cyclen ring are considered as the reaction intermediates. Acid-assisted decomplexation of H₅do3ap complexes is faster in comparison with those of H₄dota. This is caused by higher basicity of the phosphonate pendant arm and the ring nitrogen atoms, which facilitates the proton transfer from the bulk solution to the nitrogen atoms of cyclen ring.

Keywords: Azacrown compounds; Phosphonate complexes; Macrocyclic ligand; Cyclen derivative; Tetraazacyclododecane; DOTA; Thermodynamics; Kinetics; Crystal structure determination; Potentiometry; Cerium; Gadolinium; Lanthanide cations.

References: 94 live references.