METAL-COMPLEXES WITH MACROCYCLIC LIGANDS .36. THERMODYNAMIC AND KINETIC-STUDIES OF BIVALENT AND TRIVALENT METAL-IONS WITH 1,4,7,10-TETRAAZACYCLODODECANE-1,4,7-TRIACETIC ACID

NMR, potentiometric, and UV/VIS measurements were run to study the protonation and the In3+ and Cu2+ stability constants of 1,4,7,10-tetraazacycrododecane-1,4,7-triacetic acid (do3a, L). The protonation of do3a follows the typical scheme with two high and several low log K-H values. Between pH 11 and 13, the protonation mainly occurs at the N-atom, which is not substituted by an acetate side chain. The In3+ complex is not appreciably protonated even at low pH values (pH similar to 1.7), whereas [CuL] can add up to three protons in acidic solution to give the species [CuLH], [CuLH(2)], and [CuLH(3)], the stability of which was determined. The formation rates of the Y3+ Gd3+, Ga3+, and In3+ complexes with do3a were measured using a pH-stat technique, whereas that of Cu2+, being faster, was followed on a stopped-now spectrophotometer. In air cases, the reaction scheme implies the rapid formation of partially protonated intermediates, which rearrange themselves to the final product in the rate-determining process. ([MLH])(in), an intermediate, in which the metal ion probably is coordinated by two amino acetate groups, proved to be the reactive species for Y3+, Gd3+, and Ga3+. The formation of[Cu(do3a)] was interpreted by postulating that either ([CuLH])(in) or ([CuLH])(in), and ([CuLH(2)])(in) are the reactive complexes. The rates of dissociation of the Y3+, Gd3+, and Cu2+ complexes with do3a were studied spectrophotometrically. For Y3+ and Gd3+, arsenate III was used as a scavenger, whereas for Cu2+ the absorption associated with d-d* transition was followed. For [Y(do3a)] and [Gd(do3a)], the rate law follows the kinetic expression k(obsd) = k(0) + k(1)[H+]. The dissociation of[Cu(do3a)] goes through the proton-independent dissociation of [CuLH(3)], which is the main species at low pH.