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

✍ Scribed by Hui-Zhi Cai; Thomas A. Kaden

• Publisher: John Wiley and Sons
• Year: 1994
• Tongue: German
• Weight: 766 KB
• Volume: 77
• Category: Article
• ISSN: 0018-019X
• DOI: 10.1002/hlca.19940770136

✦ Synopsis

Abstract

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.