Cation, Anion and Ion-Pair Complexes with a G-3 Poly(ethylene imine) Dendrimer in Aqueous Solution

The G-3 poly(ethylene imine) ligand L2 shows a multifaceted coordination ability, being able to bind metal cations, anions and ion-pairs. The equilibrium constants for the formation of metal (Cu²⁺, Zn²⁺), anion (SO₄^2-) and ion-pair (Cu²⁺/SO₄^2-) complexes were determined in 0.1 M Me₄NCl aqueous solution at 298.1 ± 0.1 K by means of potentiometric titrations. Thanks to its dendrimeric nature, L2 can form highly nucleated metal complexes, such as Cu₅L2¹⁰⁺ and Zn₄L2⁸⁺, in successive and well-defined complexation steps. Protonated forms of L2 give rise to relatively weak anion complexes with SO₄^2-, but the addition of Cu²⁺ significantly enhances the binding ability of the ligand toward this anion below pH 9. In more alkaline solutions, an opposite trend is observed. The coordination properties of L2 are discussed with the support of modelling calculations. According to results, L2 is a promising molecule for the preparation of solid supported materials for the recovery of cations and anions from aqueous media and/or for applications in heterogeneous catalysis.