Lanthanide(III) Complexes of 2‐[4,7,10‐Tris(phosphonomethyl)‐1,4,7,10‐tetraazacyclododecan‐1‐yl]acetic Acid (H 7 DOA3P): Multinuclear‐NMR and Kinetic Studies

The protonation constants of 2‐[4,7,10‐tris(phosphonomethyl)‐1,4,7,10‐tetraazacyclododecan‐1‐yl]acetic acid (H 7 DOA3P) 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 (p K $\rm{{_{i}^{H}}}$ =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 H 7 DOA3P, and for other related ligands. The overall basicity of H 7 DOA3P is higher than that of H 4 DOTA and trans‐ H 6 DO2A2P but lower than that of H 8 DOTP. Based on multinuclear‐NMR spectroscopy, the protonation sequence for H 7 DOA3P was also tentatively assigned. Three protonation constants (p K MHL , p K MH2L , and p K MH3L ) were determined for the lanthanide complexes, and the values found are relatively high, although lower than the protonation constants of the related ligand (p K $\rm{{_{3}^{H}}}$ , p K $\rm{{_{4}^{H}}}$ , and p K $\rm{{_{5}^{H}}}$ ), 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(H 2 O) 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 H 2 O molecules, while the final product, [Eu(DOA3P)] 4− , does not contain any H 2 O molecule, as proved by steady‐state/time‐resolved luminescence spectroscopy.