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

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 [EuL 2 ] oc intermediate is formed under ligand excess. At room temperature, their conversion to in-cage complex is comparable but the [EuL 2 ] 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.