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

✍ Scribed by Maria Paula Campello; Marina Balbina; Isabel Santos; Přemysl Lubal; Radek Ševčík; Romana Ševčíková

• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: German
• Weight: 449 KB
• Volume: 92
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.200900176

✦ Synopsis

Abstract

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.