Metal dendrimer-stabilized

Efforts to prepare composites of dendrimers and technologically important QDs [188-191] have so far yielded primarily mferdendrimer composites. Such materials have been shown to be agglomerates in which multiple dendrimers stabilize relatively large CdS QDs. Results from these important studies suggested to us that it might be feasible to prepare mfradendrimer QD nanocomposites equivalent to the dendrimer-encapsulated metals described early in this chapter. 

The bulky methylaluminoxane anion stabilizes the coordinatively unsaturated metal cation. Stabilization by noncoordinating anions such as carbosilane dendrimers is also viable.571 Aluminoxanes, however, are required to be used in large excess to be effective. Alternatively, the active catalyst can also be prepared by reacting a metal dialkyl with fluorinated boranes, borate salts or aluminate salts. 

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 ). 

Starburst dendrimers have received considerable attention in the area of heterogeneous catalyst synthesis in the last decade. Although Tomalia et al. and Bosman et al. originally discovered these hyperbranched macromolecules and their host-guest properties in the mid-1980s. Crooks and coworkers were the first to demonstrate the ability of poly(amidoamine) (PAMAM) starburst dendrimers to act as metal nanoparticle stabilizers that could potentially aid in the synthesis of supported metal catalysts. The benchmark work of Crooks et al. and subsequent literature reports have underscored the advantages of successfully utilizing PAMAM dendrimer-nanocomposite precursors over conventional catalyst preparation methods. 

PAMAM dendrimers can also be used as templating agents and nanoparticle stabilizers for the synthesis of bimetallic particles. The unique ability of dendrimers to host various metal precursors enables the simultaneous complexation of multiple metallic species at its various internal functional groups. The three primary methods of bimetallic nanoparticle synthesis through dendrimer stabilization are partial displacement, co-complexation, and sequential complexation. 

Ratheesh Kumar, V. K. and K. R. Gopidas. Synthesis and characterization of gold-nanoparticle-cored dendrimers stabilized by metal-carbon bonds. Chem. Asian. J. 5, 2010 887-896. 

A different synthetic approach, devised by the Crooks laboratory, invokes dendrimer stabilization of the nanoparticles by encapsulating them. Several different nanoparticle types 4-7o 3ggjj sequestered by poly(amidoamine) dendrimers, including Pd, Pt, Au, ferromagnetic Ni, and coreshell Pd-Au and bimetallic nanoparticles. Au and Pd nanoparticles were prepared by sequestering the metal ions into the dendrimers, and then reducing them. Size control was sufficient to observe the distinctive pattern of QDL charging. 4... 

Mixed metal catalysts can also be made using PVD [41,42] and by activation of rapidly quenched metal alloys such as Au5FeZni4 and Au5Ag Zn,4 under CO oxidation conditions at 553 K [52]. Other methods for preparing mixed metal catalysts include the use of dendrimer-stabilized bimetallic particles, and adsorption of bimetallic molecular clusters [2],... 

The use of dendrimers constitutes an attractive stabilization mode for the synthesis of metal nanoparticles for several reasons ... 

Recently, two new approaches for the preparation of metallodendritic catalysts were described. The first one involves the use of PAMAM dendrimers [11] as both template and stabilizer of metal ions. The cavities of these dendrimers can serve as host type nano-reactors for metal ion guests. This strategy is referred to as reactive encapsulation . The concept was first elegantly demonstrated by... 

However, for the dendrimer nanocomposite metallic systems this change in shape was not observed. Again, due to the high stability to intense laser pulses, the anisotropy value of the gold dendrimer nanocomposite, which can be viewed as a measure of the symmetry of the particle, did not change after several repeated cycles of measurements. It is possible that the initial optical pumping of the electron-phonon modes of the metal particles is partially absorbed by the encapsulating PAMAM dendrimer. 

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