Dissociation kinetics of macrocyclic trivalent lanthanide complexes of 1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid (H(2)ODO2A)

Abstract

The dissociation rates of LnODO2A(+) complexes (ODO2A(2-) is the deprotonated dianion of the ligand H(2)ODO2A, 1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid) are all faster than those of the corresponding LnDO2A(+) complexes (DO2A(2-) is the deprotonated dianion of the ligand H(2)DO2A, 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 H(2)ODO2A is not pre-organized for Ln(3+) ion complexation but H(2)DO2A is. The dissociation kinetics of selected trivalent lanthanide (Ln(3+), Ln=La, Pr, Eu, Er, Lu) complexes of the macrocyclic ligand H(2)ODO2A (1-oxa-4,7,10-triazacyclododecane-4,10-diacetic acid), LnODO2A(+), were studied in the [H+] range (0.1-2.4) x 10(-4) M in the temperature range 15-45 degrees 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+ < > LnODO2AH(2+). The dissociation rates of LnODO2A(+) complexes are all faster than those of the corresponding LnDO2A(+) complexes (DO2A(2-) is the fully deprotonated dianion of the ligand H(2)DO2A, 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 H(2)ODO2A is not pre-organized for Ln(3+) ion complexation but H(2)DO2A is.