Dissociation kinetics of macrocyclic trivalent lanthanide complexes of 1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid (H₂ODO2A)

Abstract

The dissociation kinetics of selected trivalent lanthanide (Ln³⁺, Ln=La, Pr, Eu, Er, Lu) complexes of the macrocyclic ligand H₂ODO2A (1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid), LnODO2A⁺, were studied in the [H⁺] range (0.1–2.4) × 10⁻⁴ M in the temperature range 15–45 °C. Excess Cu²⁺ ions were used as the scavenger for the ligand in acetate–acetic acid buffer medium. The dissociation reactions are independent of [Cu²⁺] and follow the rate law k_obs = k_d + k_AC[Acetate] + K′k_lim[H⁺]/(1 + K′[H⁺]), where k_d, k_AC, and k_lim are the respective dissociation rate constants for the [H⁺]-independent, acetate-assisted, and the [H⁺]-dependent limiting pathways; K′ is the equilibrium constant for the protonation reaction LnODO2A⁺ + H⁺ $\leftrightarrow$ LnODO2AH²⁺. The dissociation rates of LnODO2A⁺ complexes are all faster than those of the corresponding LnDO2A⁺ complexes (DO2A²⁻ is the fully deprotonated dianion of the ligand H₂DO2A, 1,4,7,10-tetrazacyclo-dodecane-1,7-diacetic acid), consistent with the notion that LnODO2A⁺ complexes are kinetically more labile and thermodynamically less stable than the corresponding LnDO2A⁺ complexes, and H₂ODO2A is not pre-organized for Ln³⁺ ion complexation but H₂DO2A is.

Graphical Abstract

The dissociation rates of LnODO2A⁺ complexes (ODO2A²⁻ is the deprotonated dianion of the ligand H₂ODO2A, 1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid) are all faster than those of the corresponding LnDO2A⁺ complexes (DO2A²⁻ is the deprotonated dianion of the ligand H₂DO2A, 1,4,7,10-tetraazacyclododecane-1,7- diacetic acid), consistent with the fact that LnODO2A⁺ complexes are kinetically more labile and thermodynamically less stable than the corresponding LnDO2A⁺ complexes, and H₂ODO2A is not pre-organized for Ln³⁺ ion complexation but H₂DO2A is.

Keywords:

• Lanthanides
• Macrocyclic ligands
• ODO2A
• DO2A
• Dissociation kinetics

Additional information

Funding

Funding. The authors wish to thank the National Science Council and the Atomic Energy Council of the Republic of China (Taiwan) for financial support [grant number NSC-98-2113-M-010-001-MY3], [grant number NSC 96-NU-7-009-003], and [grant number NSC-100-2811-M-010-003] of this work.