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Metal complexes with macrocyclic ligands. Part XXXVI . Thermodynamic and kinetic studies of bivalent and trivalent metal ions with 1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid
Author(s) -
Cai HuiZhi,
Kaden Thomas A.
Publication year - 1994
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19940770136
Subject(s) - protonation , chemistry , potentiometric titration , dissociation (chemistry) , metal , dota , ion , chemical stability , medicinal chemistry , metal ions in aqueous solution , stability constants of complexes , inorganic chemistry , stereochemistry , crystallography , chelation , organic chemistry
NMR, potentiometric, and UV/VIS measurements were run to study the protonation and the In 3+ and Cu 2+ stability constants of 1,4,7,10‐tetraazacyclododecane‐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 In 3+ complex is not appreciably protonated even at low pH values (pH ⋐ 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 Y 3+ , Gd 3+ , Ga 3+ , and In 3+ complexes with do3a were measured using a pH‐stat technique, whereas that of Cu 2+ , being faster, was followed on a stopped‐flow spectrophotometer. In all 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 Y 3+ , Gd 3+ , and Ga 3+ . 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 Y 3+ , Gd 3+ , and Cu 2+ complexes with do3a were studied spectrophotometrically. For Y 3+ and Gd 3+ , arsenazo III was used as a scavenger, whereas for Cu 2+ 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.