Equilibria and kinetics of complex formation between copper(II) and the polyamine Me 2octaen

Abstract

The kinetics and the equilibria of complex formation between Cu(II) and 1,25-dimethyl-1,4,7,10,13,16,19,22,25-nonaazapentacosane (Me 2octaen) have been investigated in acidic aqueous solution, at an ionic strength of 0.1 mol dm −3 and 25°C, by the stopped-flow method and UV spectrometry. Me 2octaen is a linear polyamine made by the union of eight en units bearing two methyl residues at the ends of the chain. Spectrophotometric titrations and kinetic experiments indicated that below pH 2.4 the binding of Cu(II) to the ligand gives rise to several 1:1 complexes differing for their degree of protonation, whereas above pH 2.4 binuclear complexes are formed as well. Concerning the mononuclear species, the ratios of the formation to the dissociation rate constants are in agreement with the equilibrium constants measured by static spectrometry. The analysis of the kinetic and spectrophotometric data reveals that, in the range of pH between 1.7 and 2.4, the most reactive forms of the ligand (denoted in its fully protonated form as H 9L 9+) are H 7L 7+, H 6L 6+, and H 5L 5+. The analysis of the results shows that Cu 2+ and H 6L 6+ may react according to the internal conjugate base mechanism (ICB), although the observed internal conjugate base effect is more modest than that displayed by the Ni 2+–Me 2octaen system. The small magnitude of the ICB effect is explained in terms of the enhanced reactivity of copper(II) owing to the Jahn–Teller distortion occurring for Cu 2+ complexes.