Lanthanide(III)–1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid complexes in acidic medium: significant decrease in water exchange rate†

Abstract

UV-Vis and lanthanide-induced ¹⁷O shift measurements on the complex [Eu(DOTA)]^– (H₄DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) have shown that the inner co-ordination sphere of the Ln³⁺ ion is not affected on protonation which suggests that the proton is attached to a non-co-ordinated oxygen atom of a carboxylate group. Proton NMR measurements performed as a function of the H⁺ concentration revealed that the protonation slightly accelerates the intramolecular dynamic processes: the enantiomerization for [La(DOTA)]^– and the enantiomerization and interconversion between the major and minor isomer for [Eu(DOTA)]^–. Contrary to first glance expectations, the water exchange rate on [Gd(DOTA)(H₂O)]^– decreases significantly with increasing extent of protonation, and at 1.0 M H⁺ concentration is about ten times lower than in neutral media. In 1.0 M acidic solution the proton relaxivities were found to be higher than expected solely on the basis of the water exchange rates. This finding is interpreted with a faster proton exchange in acidic solutions which is the consequence of the catalytic effect of H⁺ ions.