Lanthanide(III) Complexes of 2‐[4,7,10‐Tris(phosphonomethyl)‐1,4,7,10‐tetraazacyclododecan‐1‐yl]acetic Acid (H7DOA3P): Multinuclear‐NMR and Kinetic Studies

The protonation constants of 2‐4,7,10‐tris(phosphonomethyl)‐1,4,7,10‐tetraazacyclododecan‐1‐yl]acetic acid (H₇DOA3P) and of the complexes Ln(DOA3P)]^4? (Ln=Ce, Pr, Sm, Eu, and Yb) have been determined by multinuclear NMR spectroscopy in the range pD 2–13.8, without control of ionic strength. Seven out of eleven protonation steps were detected (pK =13.66, 12.11, 7.19, 6.15, 5.77, 2.99, and 1.99), and the values found compare well with the ones recently determined by potentiometry for H₇DOA3P, and for other related ligands. The overall basicity of H₇DOA3P is higher than that of H₄DOTA and trans‐H₆DO2A2P but lower than that of H₈DOTP. Based on multinuclear‐NMR spectroscopy, the protonation sequence for H₇DOA3P was also tentatively assigned. Three protonation constants (pK_MHL, pK_MH2L, and pK_MH3L) were determined for the lanthanide complexes, and the values found are relatively high, although lower than the protonation constants of the related ligand (pK , pK , and pK ), indicating that the coordinated phosphonate groups in these complexes are protonated. The acid‐assisted dissociation of Ln(DOA3P)]^4? (Ln=Ce, Eu), in the region c_H+=0.05–3.00 mol dm^?3 and at different temperatures (25–60°), indicated that they have slightly the same kinetic inertness, being the Eu(H₂O)₉]³⁺ aqua ion the final product for europium. The rates of complex formation for Ln(DOA3P)]^4? (Ln=Ce, Eu) were studied by UV/VIS spectroscopy in the pH range 5.6–6.8. The reaction intermediate Eu(DOA3P)]* as ‘out‐of‐cage’ complex contains four H₂O molecules, while the final product, Eu(DOA3P)]^4?, does not contain any H₂O molecule, as proved by steady‐state/time‐resolved luminescence spectroscopy.