Metal ion binding by a G-2 poly(ethylene imine) dendrimer. Ion-directed self-assembling of hierarchical mono- and two-dimensional nanostructured materials

The second-generation poly(ethylene imine) dendrimer (L), based on ammonia as 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 NMe4Cl solution at 298.1 K. The interaction of L with Ni2+, Cu2+, Zn2+, Cd2+ and Pb2+ gives rise to stable complexes with 1:1 (all metal ions), 2:1 (Ni2+, Cu2+, Zn2+, Cd2+), 3:2 (Ni2+, Zn2+, Cd2+) and 3:1 (Cu2+) metal:ligand stoichiometry. The crystal structures of [Ni3L2](ClO4)6.6H2O (1) and [Cu3LCl(OH)0.5(NO3)0.5ox]Cl1.5(NO3)0.5.5.5H2O (2) were solved by X-ray diffraction. The Ni3L26+ complex cation in 1, existing in solution as a very stable species, shows two dendrimer units linked together by a bridging Ni2+ ion. In 2, the Cu3L6+ complex cation, which also exists in solution as a very stable species, gives rise, via bridging coordination of oxalate anions, to nanostructured polymeric chains which 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, 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.