The stability constants (log Aul) of lanthanide complexes of macrocyclic polyamino carboxylates, LnL (where Ln is Ce3+, Gd3+, and Lu3+ and L is D03A = l,4,7,10-tetraazacyclododecane-l,4,7-triacetic acid and HP-D03A = 10-(hydroxypropyl)-l,4,7,10-tetraazacyclododecane-l,4,7-triacetic acid), have been determined at 25.0 ± 0.1 °C and µ -0.1 ((CH3)4NC1). The stability constants of lanthanide complexes of D03A increase with the decreasing ionic radii or increasing charge density of Ln3+. For L = HP-D03A, the stability constant of GdL is greater than that of CeL; however the stability constants of GdL and LuL are similar. The protonation constants, KH (M-1), of the LnL complexes have been determined at 25.0 ±0.1 °C and µ = 1.0 (NaCl), and the values are 18 ± 3 for Ce(D03A), 115 ± 8 for Gd(DG3A), 11 ± 1 for Ce(HP-DG3A), and 240 ± 25 for Gd(HP-D03A). The rates of acid-assisted dissociation of the protonated complexes, ML(H), have been measured at 25.0 ± 0.1 ®C and µ = 1.0 (NaCl). Direct, k¿ (s-1), and acid-assisted, k\ (M-1 s-1), dissociation rate constants are k¿ = (1.8 ± 0.8) X 10-3 and k, = (1.12 ± 0.04) X 10-1 for Ce(D03A) and /fcd = (1.4 ± 0.2) X 1(H and k\ = (2.00 ± 0.04) X 10-3 for Ce(HP-D03A). The rates of dissociation of the other complexes are first order in acid at low [H+], but the rates saturate at high [H+]. The calculated equilibrium constants, K\ (M-1), for the formation of the diprotonated species, LnL(H2), and their dissociation rate constants, k2 (s™1), are 3.6 ± 0.4 and (7.4 ± 0.4) X 10-3 for Gd(D03A), 7.7 ± 1.8 and (3.6 ± 0.1) X 10-3 for Lu(D03A), and 1.0 ± 0.1 and (6.4 ± 0.3) X 1(H for Gd(HP-D03A), respectively. Evidence of a direct dissociation pathway for Gd(D03A)(H) is also observed with a rate constant, k¿ (s-1), of (4.4 ± 0.1) X 1 Or4. A linear correlation of log k0b$d with charge density for dissociation of Ln(DG3 A) complexes in 0.1 M HC1 was observed.
