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 re-determined 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 KML 7.16; Ln3+, log KML 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 7Fo → 5Do 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 (pKh) is estimated to be 8.1 ± 0.3.

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