Metal Ion Binding by a G-2 Poly(ethylene imine) Dendrimer. Ion-Directed Self-Assembling of Hierarchical Mono- and Two-Dimensional Nanostructured Materials

Abstract

The second-generation poly(ethylene imine) dendrimer (L), based on ammonia as the initiating core molecule, forms stable metal 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 0.10 M NMe(4)Cl solution at 298.1 K. The interaction of L with Ni(2+), Cu(2+), Zn(2+), Cd(2+), and Pb(2+) gives rise to stable complexes with 1:1 (all metal ions), 2:1 (Ni(2+), Cu(2+), Zn(2+), Cd(2+)), 3:2 (Ni(2+), Zn(2+), Cd(2+)), and 3:1 (Cu(2+)) metal/ligand stoichiometries. The crystal structures of [Ni(3)L(2)](ClO(4))(6)·6H(2)O (1) and [Cu(3)LCl(OH)(0.5)(NO(3))(0.5)ox]Cl(1.5)(NO(3))(0.5)·5.5H(2)O (2) were solved by X-ray diffraction. The Ni(3)L(2)(6+) complex cation in 1, existing in solution as a very stable species, shows two dendrimer units linked together by a bridging Ni(2+) ion. In 2, the Cu(3)L(6+) complex cation, which also exists in solution as a very stable species, gives rise, via bridging coordination of oxalate anions, to nanostructured polymeric chains that self-organize into two-dimensional sheets. In both structures, the hierarchical mono- and two-dimensional aggregation is triggered by the action of ionic species behaving either as functional groups on the dendrimer surface (metal ions) or as the glue (metal ions, oxalate) that sticks together dendrimer units. Two association routes, developing via coordinative forces, guide the directional aggregation of dendrimer units: (a) aggregation via metal ions shared by the surfaces of contiguous dendrimer molecules and (b) aggregation via chelating ligands bridging surface metal ions pertaining to contiguous dendrimer molecules. Such aggregation modes provide coordinative routes for the self-assembly of novel families of nanostructured functional materials.