Anion and ion-pair binding by a G-2 poly(ethylene imine) dendrimer

Abstract

The second-generation poly(ethylene imine) dendrimer (L), based on ammonia as the initiating core molecule, forms anion and ion-pair complexes in aqueous solution. Speciation of the complex species formed and determination of the relevant stability constants were performed by means of potentiometric titration in a 0.10 M NMe₄Cl solution at 298.1 K. Protonated forms of L interact with NO₃⁻, SO₄²⁻, SeO₄²⁻, HPO₄²⁻ and H₂PO₄⁻ forming stable 1 : 1 anion complexes. The dendritic structure endows the molecule with a greater anion binding ability relative to analogous linear polyamines. It was previously reported that L forms stable metal complexes. We show here that protonated forms of the mononuclear complexes with Cu²⁺, Zn²⁺ and Cd²⁺ bind these anions, and Pb²⁺ complexes bind NO₃⁻. The resulting ion-pair complexes show considerable stability thanks to the cooperative effect of the oppositely charged partners. Molecular modelling calculations show that both anion–ligand and anion–metal ion interactions can participate in stabilizing such ion-pair complexes.