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 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 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 stoichiometries. 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 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.