Über Reaktionen oxygenierter Kobalt (II)-Chelate VIII. Stereochemischer Verlauf der Oxygenierung des Tris(2-aminoäthyl)amin-Kobalt(II)-Ions

Abstract

Oxygenation of tris(2-aminoethyl)aminecobalt(II) (= Co(tren)²⁺) in aqueous solution yields a binuclear complex in which, as shown by X-ray diffraction analysis, the tertiary amino groups of the two 4 dentate ligands are both in cis position regarding the peroxo bridge. This stereospecifity can be explained by the existence of an equilibrium of the kind Co(tren)(H₂O)₂²⁺ ⇄ Co(tren)(H₂O)²⁺ + H₂O in which the reactivity of the pentacoordinated species is negligible.
Kinetic results indicate that replacement of H₂O in the hexacoordinated Co(tren)(H₂O)₂²⁺ by O₂ is rate determining in the formation of the oxygen adduct. This substitution must be expected to take place predominantly in the most labile position which, because of the tendency to form a trigonal bipyramidal Co(tren)(H₂O)²⁺, is necessarily cis to the tertiary nitrogen. At pH 8 the second order rate constant for the oxygenation reaction was found to be 2.8•10² M^-1s^-1 (20°, μ = 0.2). Since the hydrolyzed species Co(tren)(OH)(H₂O)⁺ is more active, the observed rate constant increases with pH. At pH 12 it reaches the limiting value of 10³ M^-1s^-1.