Capillary electrophoresis, potentiometric and laser excited luminescence studies of lanthanide(III) complexes of 1,7-dicarboxymethyl-1,4,7,10-tetraazacyclododecane (DO2A)

The protonation constants of the ligand 1,7-dicarboxymethyl-1,4,7,10-tetraazacyclododecane (DO2A) were redetermined by the potentiometric pH titration method (log K 10.94, 9.55, 3.85, 2.55) and the macrocycle ring protonation sites have also been confirmed by NMR techniques to be the secondary amine nitrogen atoms. The stability constants of calcium(II) and all trivalent lanthanide (Ln³⁺) metal complexes of DO2A (Ca²⁺, log K_ML 7.16; Ln³⁺, log K_ML 10.94-13.31) were determined by the potentiometric pH titration and capillary electrophoresis methods, respectively. In general, the stabilities of the Ln(DO2A)⁺ complexes increase with increasing atomic number for the lighter lanthanides (La³⁺-Sm³⁺) and remain relatively unchanged for the heavier lanthanides (Eu³⁺-Lu³⁺). Laser-excited spectroscopy of the ⁷F_o → ⁵D_o transition of Eu³⁺ is used to study the aqueous Eu³⁺-DO2A complex system. At low pH (e.g. pH 5-6) Eu³⁺ forms a 1:1 species with the ligand DO2A, presumably Eu(DO2A)(H₂O)_q⁺, where q is the number of inner-sphere coordinated water molecules. As the solution pH increases the hydrolysis product, Eu(DO2A)(OH)(H₂O)_{q - 1}, is formed. Lifetime measurements of each species in H₂O and D₂O allow the determination of the corresponding number of inner-sphere coordinated water molecules to be 3.0 and 2.6, consistent with the proposed structures (i.e. q = 3). The first hydrolysis constant (pK_h) is estimated to be 8.1 ± 0.3.