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
Hz. Cai et Ta. Kaden, 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, Helvetica Chimica Acta, 77(1), 1994, pp. 383-398
NMR, potentiometric, and UV/VIS measurements were run to study the pro
tonation and the In3+ and Cu2+ stability constants of 1,4,7,10-tetraaz
acycrododecane-1,4,7-triacetic acid (do3a, L). The protonation of do3a
follows the typical scheme with two high and several low log K-H valu
es. 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 i
s 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+ co
mplexes with do3a were measured using a pH-stat technique, whereas tha
t of Cu2+, being faster, was followed on a stopped-now spectrophotomet
er. In air cases, the reaction scheme implies the rapid formation of p
artially protonated intermediates, which rearrange themselves to the f
inal product in the rate-determining process. ([MLH])(in), an intermed
iate, in which the metal ion probably is coordinated by two amino acet
ate 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 co
mplexes. The rates of dissociation of the Y3+, Gd3+, and Cu2+ complexe
s with do3a were studied spectrophotometrically. For Y3+ and Gd3+, ars
enate III was used as a scavenger, whereas for Cu2+ the absorption ass
ociated 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.