Binary and ternary complexes of copper(II) containing some potentially tridentate ligands

Abstract

The multiple equilibria involved in some new ternary systems of general formula Cu^II-A-B [A=DL-2,3-diaminopropionic acid (dapa) or DL-2,4-diaminobutyric acid (daba); B=daba, DL-ornithine (Orn), DL-3-aminobutyric acid (3-aba), or DL-4-amino-3-hydroxybutyric acid (ahba) for the systems with A=dapa, and B=Orn, DL-2-aminobutyric acid (2-aba), 3-aba, or ahba for the systems with A=daba] have been studied pH-metrically at 37°C and I=0.15 mol dm⁻³(Na[ClO₄]). Auxiliary data, such as the acid dissociation constants of dapa, daba, Orn, and ahba and their binary stability constants with Cu^II, have also been obtained under identical conditions. The results suggest that dapa, daba, and ahba bind in a tridentate manner in their 1:1 binary complexes. However, Orn appears to co-ordinate in a glycine-like mode both in the binary and ternary systems. The probable structures for the various binary and ternary complexes in this study are discussed in terms of their stability-constant data. In general, the results suggest that Cu^II prefers to have one five- and one six-membered chelate ring rather than two rings of the same size in its co-ordination sphere.