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 kobs = kd + kAC[Acetate] + K′klim[H+]/(1 + K′[H+]), where kd, kAC, and klim 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+ 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.