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

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.