Palladium encapsulated PAMAM dendrimers

Zhao et al (70) developed a method for the synthesis of dendrimer-encapsulated metal nanoparticles based on sorbing metal ions into (modified) PAMAM dendrimers followed by a reduction. Dendrimers encapsulating copper, palladium, and platinum nanoparticles have been prepared. Hydroxyl-terminated PAMAM dendrimers were used to prepare encapsulated palladium (PAMAM generations 4, 6, and 8) and platinum (PAMAM generations 4 and 6) nanoparticles. The dendrimer-encapsulated palladium and platinum nanocomposites catalyzed the hydrogenation reaction of allyl alcohol and N-isopropyl acrylamide in water 71). 

Using a similar approach, Chechik and Crooks (73), modified the PAMAM dendrimer-encapsulated palladium nanoparticles with perfluoropolyether tails utilizing non-covalent ion-pair interactions. The catalytic hydrogenation of six substrates under biphasic conditions (toluene/ perfluoro-2-butyltetrahydrofuran FC-75) was investigated. Allyl alcohol, methyl acrylate, vinyl isopropenyl ether, and... 

Fig. 15. Schematic representation of the formation of an inverse micelle from a PAMAM dendrimer-encapsulated palladium nanoparticle.

Polymer-stabilized palladium nanoparticles (or nanoclusters) [125-127] have recently received increasing attention in the field of synthetic organic chemistry [128, 129]. Thus, for example, the poly(iV-vinyl-2-pyrrolidone) (PVP)-supported Pd particle catalyzed the Suzuki-Miyaura coupling in water [130]. Poly(amidoamine) (PAMAM) dendrimer-encapsulated palladium nanoparticles were designed and prepared to provide highly selective catalysts for hydrogenation of olefins [131-133]. Hyperbranched aromatic amides (aramids) and PS-DVB-methacryloylethylenesulfonic acid resin have also been... 

Poly(amidoamine) (PAMAM) dendrimer-encapsulated palladium nanoparticles were designed and prepared to provide highly selective catalysts for hydrogenation of olefins [131-133]. Hyperbranched aromatic amides (aramids) and PS-DVB-methacryloylethylenesulfonic acid resin have also been... 

Dendrimer-encapstdated catalysts are another area of active research for polymer-supported catalysts. The nanoparticles are stabilized by the dendrimers preventing precipitation and a omeration. Bimetallic nanoparticles with encapsulated metals (dendrimer-encapsulated catalyst DEC) from commercially available fourth-generation PAMAM dendrimers and palladium and platinum metal salts were prepared via reduction by Crooks and co-workers [34], following previous work in this area [35], The simultaneous incorporation of Pt and Pd reflects the concentrations in solution. The bimetallic DECs are more active than the physical mixture of single-metal DEC [35, 36] in the case of the hydrogenahon of allyl alcohol in water, with a maximum TOP of 230 h compared to TOP = 190 h obtained for monometallic palladium nanoparticles (platinum TOP = 50 h ). 

While they are not strictly redox-containing dendrimers. Crooks and Zhao have encapsulated platinum and other metallic nanoparticles inside PAMAM dendrimers. The catalytic properties of these materials have been examined using cyclic voltammetry, and it was found that a gold electrode, modified with large hydroxyl-terminated PAMAM dendrimers containing clusters of 60 platinum atoms, was readily able to catalyze O2 reduction [98]. Similar results have been reported for palladium clusters encapsulated in the same hydroxyl-terminated PAMAM dendrimers [99]. 

The performance of several palladium piecatalysts, like palladium(n) acetate, palladium(0)NPs encapsulated into polyfamidoamine) (PAMAM) dendrimers (Pd DENs) and palladium(II)-PAMAM complexes, in the Stille reaction between trichloro(pheityl)stannane and iodoarenes in water was compared [26]. The reactivity of Pd DENs is sirttilar or inferior to that of palladium(II) acetate, although the presence of the derrdrimer suppresses the formation of homocouplirrg products and allows catalyst recycling. It is suggested that the reaction catalyzed by Pd DENs occtrrs via palladirrm species which are leached form the NP but which rerrrain co-ordirrated to the derrdritic macromolecule. 

Gomez MV, Giuerra J, Velders AH, Crooks RM (2009) NMR characterization of fourth-generation PAMAM dendrimers in the presence and absence of palladium dendrimer-encapsulated nanoparticles. J Am Chem Soc 131 341-350... 

Crooks group prepared monodisperse (1.7 0.2nm) palladium nanoparh-cles within the interiors of three different generations of hydroxyl-terminated PAMAM dendrimers [41]. This process involved encapsulation of the nonselec-tive catalyst (the Pd nanoparticle) within a selective nanoporous cage (the dendr-imer). These dendrimer-encapsulated palladium nanoparticles were used as catalysts to hydrogenate allyl alcohol and four R-substituted derivatives in a metha-nol/water mixture. The results showed that higher-generation dendrimer encapsulated catalysts (DECs) or larger substrates resulted in lower turnover frequencies. 

Aranishi K, Zhu QL, Xu Q. Dendrimer-encapsulated cobalt nanoparticles as high- performance catalysts for the hydrolysis of ammonia borane. Chem Cat Chem 2014 6 1375-9. Mizugaki T, et al. PAMAM dendron-stabilised palladium nanoparticles effect of generation and peripheral groups on particle size and hydrogenation activity. Chem Commun 2008 241-3. 

---