Equilibria and Kinetics of Complex Formation between Nickel(II) and the Polyamine Me2Octaen

The kinetics and the equilibria of complex formation between Ni(II) and 1,25-dimethyl-l,4,7,10,13,16,19,22,25-nonaazapentacosane (Me2Octaen) have been investigated in aqueous solution between pH 3 and 4.5 by the stopped-flow method and UV spectrometry. Me2Octaen 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 4.5 the binding of Ni(II) to the ligand gives rise to several 1:1 complexes differing in their degree of protonation, whereas above pH 4.5 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 data reveals that the mostly reactive form of the ligand (denoted in its fully protonated form as H9L9+) is H5L5+, whereas the much more abundant species H6L6+ yields 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. Addition of sodium acetate resulted in a large enhancement of the rate of the complex formation reaction, whereas the complex dissociation rate was found to be almost insensitive to the presence of the buffer. Analogous, although more modest, effects have been found with sodium chloroacetate. This behaviour is explained by assuming that the anion of the buffer could bind to Ni2+ and/or to the ligand, thus reducing the repulsion between the positively charged reaction partners