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

The kinetics and the equilibria of complex formation between Ni(II) and 1,2 5-dimethyl-1,4,7,10,13,16,19,22,25-nonaazapentacosane (Me(2)Octaen) have be en investigated in aqueous solution between pH 3 and 4.5 by the stopped-flo w method and UV spectrometry. Me(2)Octaen is a linear polyamine made by the union of eight en units bearing two methyl residues at the ends of the cha in. Spectrophotometric titrations and kinetic experiments indicated that be low pH 4.5 the binding of Ni(II) to the ligand gives rise to several 1:1 co mplexes differing in their degree of protonation, whereas above pH 4.5 binu clear complexes are formed as well. Concerning the mononuclear species, the ratios of the formation to the dissociation rate constants are in agreemen t with the equilibrium constants measured by static spectrometry. The analy sis 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 overal l process of complex formation. Measurements at different ionic strengths h ave shown positive salt effects, as expected for reactive processes involvi ng particles with charges of the same sign. Addition of sodium acetate resu lted 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 assumi ng that the anion of the buffer could bind to Ni2+ and/or to the ligand, th us reducing the repulsion between the positively charged reaction partners. (C) 2000 Elsevier Science B.V. All rights reserved.