Crooks et al.
some discussion of the metal-ion-containing precursors from which they are prepared (2–5,35–75). There are three general categories of metal-ion-containing dendrimers. The first is composed of dendrimers that use metal ions as an integral part of their chemical structure. This includes, for example, dendrimers having an organometallic core and dendrimers that use metal ligation to assemble branches into the complete dendrimer. The second class consists of dendrimers that have peripheral groups that are good ligands for metal ions. The third group contains internal ligands that bind to nonstructural metal ions. Only dendrimers associated with nonstructural metal ions are relevant to this chapter. Dendrimers complexed to exterior metal ions do not take full advantage of the many unique structural features of dendrimers, such as the hollow interior, the unique chemical properties of the interior microenvironment, the terminal-grouptunable solubility, or the nanofiltering capability of the dendrimer branches (vide infra). Moreover, exterior metal ions can lead to dendrimer cross-linking, agglomeration, and even precipitation. However, PAMAM and PPI dendrimers have functional groups within their interior that are also able to bind metal ions. Specifically, PAMAM dendrimer interiors contain tertiary and secondary (amide) amines, while PPI dendrimers lack amide groups (Fig. 2). We are especially interested in trapping metal ions exclusively within the interiors of unmodified, commercially available PAMAM dendrimers, because such composites are easy to prepare and retain the desirable structural properties of the uncomplexed dendrimers. It is possible to prevent metal-ion complexation to amine-terminated PAMAM dendrimers by either selective protonation of the primary amines [for PAMAM dendrimers, the surface primary amines (pKa 9.5) are more basic than the interior tertiary amines (pKa 5.5)] (47,53,57) or by functionalization with noncomplexing terminal groups. The latter approach eliminates the restrictive pH window necessitated by selective protonation and generally results in more easily interpretable results (2,76). Accordingly, most of our work has focused on hydroxyl-terminated PAMAM dendrimers (Gn-OH). Indeed, we have shown that many metal ions, including Cu2, Pd2, Pt2, Ni2, Au and Ru3, sorb into Gn-OH interiors over a broad range of pH via complexation with interior tertiary amines (57,58,77–79).
III. SYNTHESIS AND CHARACTERIZATION OF DENDRIMER-ENCAPSULATED METAL NANOPARTICLES IN PAMAM AND PPI DENDRIMERS This section briefly describes dendrimer-encapsulated metal nanoparticles, a new family of composite materials first described by us in 1998 (57), and their applications to catalysis.
Chemistry of metal nanoparticles