Kinetics of complex formation between Cobalt (II) and the Polyamine Me2Octaen

The kinetics and the equilibria of complex formation between Co(II) and the linear polyamine 1,25-dimethyl-1,4,7,10,13,16,19,22,25-nonaazapentacosane (Me(2)Octaen) have been investigated in aqueous solution under anaerobic conditions by the stopped-flow and potentiometric method. Static titrations and kinetic experiments indicate that below pH 5.6 the binding of Co(II) to the ligand gives rise to several 1: 1 complexes differing for their degree of protonation, whereas above pH 5.6 binuclear complexes are formed as well. The analysis of the kinetic data reveals that the mostly reactive form of the ligand (denoted in its fully protonated form as H9L9+) is H6L6+. The species H5L5+ and H4L4+ yield only a minor contribution to the overall process of complex formation. Measurements at different ionic strengths have shown positive salt effects, as expected for reactive processes involving particles with charges of the same sign. The analysis of the results shows that Co2+ and H6L6+ react according to the internal conjugate base mechanism.