Stability and kinetics of decomposition of binuclear Cu(II) complexes with a symmetrical hexaaza macrocycle:: the effect of SCN- as ancillary ligand

The effect of added KBr and KSCN on the stability constants of the mono and binuclear Cu(II) complexes with a symmetrical hexaazamacrocycle L has been examined in 0.1 M KNO3. The presence of these salts does not cause any change in the ligand protonation constants, which indicates that, in the presence of 0.1 M KNO3, there is not preferential interaction of the Br- and SCN- anions with the highly protonated forms of the ligand. No ternary Cu-L-Br complexes are detected in the potentiometric study of the equilibrium, but several mono and binuclear Cu-L-SCN complexes are formed at significant amounts and their stabilities are reported. The kinetics of decomposition of the binuclear Cu-L and Cu-L-SCN complexes upon addition of an excess of acid has been also measured. The results obtained for the Cu-L complexes agree well with those previously reported in 1.0 M KNO3, and they indicate that the release of both Cu(II) ions is statistically controlled. The existence of some differences between the kinetic data corresponding to decomposition of solutions at different starting pH is interpreted in terms of parallel decomposition of the binuclear Cu2L4+, Cu2L(OH)(3+) and Cu2L(OH)(2)(2+) complexes, the kinetic parameters for the three complexes being slightly different. This interpretation is also supported by the kinetics of decomposition of the Cu-L-SCN- complexes that also reveals differences between the several complexes in solution. If the present data are interpreted in terms of the classical mechanism for decomposition of Cu(II)-polyamine complexes, they suggest that the nature of the ancillary ligands does not cause large changes in the lability of the Cu-N bonds but it largely affects to the relative rates of attack by Hf and water. (C) 2001 Elsevier Science B.V. All rights reserved.