Komplexe mit makrocyclischen Liganden,II. Zweischrittmechanismus bei der Komplexbildung von Cu2+ mit meso-5,7,7,12,14,14-Hexamethyl-1,4,8,11-tetraazacyclotetradecan

The complex formation between Cu^II and the title compound (tet a) is studied by spectrophotometry and pH-stat techniques. Between pH 4 and 5,5 the reaction proceeds in two steps, the first giving a blue intermediate, Cu(teta)²⁺ (blue), exhibiting a broad absorption band at 620 nm. Titration with NaOH and the absorption spectrum suggest that, in the intermediate, Cu^II is coordinated to only two amino groups of the ligand. Both steps are slow compared to other complex formation reactions of Cu^II. The rate of the first step, in which Cu(tet a)²⁺ (blue) is formed, is given by v₁ = k₁ · Cu²⁺] (tet a) H]/H⁺] with k₁ = 2,7 · 10^?6 s^?1 at 40° and I = 0,1. In the second step the last two nitrogens of the quadridentate ligand are bound to Cu^II, giving the mauve end product. The rate of this step is given by v₂ = k₂ · Cu(tet a)OH⁺ (blue)] OH^?] with k₂ = 1,2 · 10³ M^?1 s^?1 at 50° and I = 0,5.