Formation kinetics of europium(III) complexes of DOTA and its bis(phosphonate) bearing analogs

Abstract

Complexation of Eu(III) ions with H4dota and its bis(phosphonate)-bearing analogs, H8do3aPBP with phosphinate spacer and H7dotamBP with carboxamide spacer, was studied by UV–Vis spectrophotometry and luminescence life-time measurements. The results show formation of two types of out-of-cage intermediates. The [EuL]oc intermediate is formed under metal ion excess, whereas [EuL2]oc intermediate is formed under ligand excess. At room temperature, their conversion to in-cage complex is comparable but the [EuL2]oc transformation into the in-cage complex is significantly faster at elevated temperatures. Both bis(phosphonate)-bearing ligands show slower complexation rate than H4dota. Comparison of reactivity of H8do3aPBP and H7dotamBP show important role of the spacer connecting the bis(phosphonate) group with the macrocycle. Presence of phosphinate in H8do3aPBP leads to significantly faster (3–4 orders of magnitude) complexation than in case of the amide spacer in H7dotamBP. Both bis(phosphonate)-bearing ligands show nonlinear dependence of the complexation rate on concentration of hydroxide anions due to changes in protonation state of the bis(phosphonate) moiety.