Crooks et al.
at quite high temperatures (in contrast to PAMAM dendrimers, which tend to decompose rather quickly above 100°C). Finally, the successful Heck chemistry (75) conclusively proves that it is possible to catalyze synthetically useful carboncarbon bond-forming reactions between two reactants, which have significant size and mass, within a dendrimer interior. E. Homogeneous Catalysis in Supercritical CO2 Using Dendrimer-Encapsulated Metal Particles As mentioned earlier, there are good reasons to search for reaction conditions that eliminate the need for organic solvents. The use of liquid or supercritical (SC) CO2 addresses some of these issues, including catalyst recovery, reduced toxicity, and simpler product recovery (115). Until recently, however, the use of SC CO2 had been limited to organometallic Pd complexes functionalized with perfluorinated ligands (116–118), due to the limited solubility of metal colloids in CO2, and often required the use of water as a cosolvent (119). The work described here shows that dendrimers can be used to solubilize Pd nanoclusters in liquid and SC CO2. This new finding opens the door to the combined benefits of a catalyst that promotes Heck couplings but without the need for toxic ligands or solvents. As discussed earlier, perfluorinated polyether “ponytails” can be covalently grafted onto dendrimers, and DeSimone et al. recently showed that such materials are soluble in liquid CO2 (120). Preliminary results from a study of catalytic activation of the heterocoupling between arylhalides and alkenes using ponytailfunctionalized dendrimer-encapsulated Pd nanoparticles have been promising. For example, the classic Pd-catalyzed Heck coupling between arylhalides and methacrylate yields predominately (97%) the trans-cinnimaldehyde product (121). On the other hand, the CO2-soluble dendrimer nanocomposite exclusively catalyzes the production of the highly unfavored 2-phenyl-acrylic acid methyl ester isomer at 5000 psi and 75°C (122).
V. DENDRIMER-ENCAPSULATED SEMICONDUCTOR NANOPARTICLES Up to this point we have focused on the intradendrimer synthesis of metal nanoparticles, but in principle any type of particle can be prepared inside a dendrimer template if a means can be found to first sequester the components and then chemically transform them into the desired product. We recently demonstrated the versatility of the dendrimer template approach by preparing dendrimer-encapsulated semiconductor nanoparticles (123,124). Specifically, we have shown that it is possible to control the size, and thus the photoluminescent properties, of encapsulated CdS quantum dots (QDs) (125).
Chemistry of metal nanoparticles