Abstract
Complexation of Eu(III) ions with H(4)dota and its bis(phosphonate)-bearing analogs, H(8)do3aP(BP) with phosphinate spacer and H(7)dotam(BP) 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 H(4)dota. Comparison of reactivity of H(8)do3aP(BP) and H(7)dotam(BP) show important role of the spacer connecting the bis(phosphonate) group with the macrocycle.
Presence of phosphinate in H(8)do3aP(BP) leads to significantly faster (3-4 orders of magnitude) complexation than in case of the amide spacer in H(7)dotam(BP). 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.