Encapsulated Pd nanoparticles

Table 9.3 Hydrogenation of alkenes using dendrimer-encapsulated Pd nanoparticles.a) (Reprinted with permission of the American Chemical Society [59])...

Kaneda et al. reported substrate-specific hydrogenation of olefins using the tri-ethoxybenzamide-terminated polypropylene imine) dendrimers (PPI) as nanoreactors encapsulating Pd nanoparticles (mean diameter 2-3 nm) [59]. The catalytic tests were performed in toluene at 30 °C under dihydrogen at atmospheric pressure (Table 9.3). The hydrogenation rates were seen to decrease with increasing generation of dendrimers, from G3 to G5. 

Fig. 19. Turn-over frequencies for hydrogenation of allyl alcohol and N-isopropyl acrylamide obtained in water using dendrimer-encapsulated Pd nanoparticles of constant average size. The hydroxyl-terminated PAMAM dendrimer generation varies from G4 to G8...

Figure 23 shows a biphasic toluene/perfluoro-2-butyltetrahydrofuran (Fluoro-inert FC-75) mixture containing dendrimer-encapsulated Pd nanoparticles complexed with poly(hexafluoropropylene oxide-co-difluoromethylene oxide) monocarboxylic acid prepared via the type of electrostatic modification de-... 

Fig. 23. This photograph shows a two-phase system consisting of (top phase) toluene and (bottom phase) perfluoro-2-butyltetrahydrofuran (Fluoroinert FC-75).The dark color (brown) in the bottom phase indicates that the dendrimer-encapsulated Pd nanoparticles, complexed with poly (hexafluoropropylene oxide-co-difluoromethylene oxide) monocarboxylic acid, are selectively extracted into the fluorous phase. No detectable color was observed in the organic phase. Reprinted with permission from Ref. 103 Copyright 2000 American Chemical Society...

Table 2. Substrate, structures, and turn-over frequencies obtained for hydrogenation reactions in fluorous biphasic systems employing dendrimer-encapsulated Pd nanoparticles ...

Preliminary results from a study of catalytic activation of the heterocoupling between arylhalides and alkenes using pony-tail-functionalized dendrimer-encapsulated Pd nanoparticles have shown promise. For example, the classic Pd-catalyzed Heck coupling between arylhalides and methacrylate yields predominately (> 97%) the trans-cinnimaldehyde product [176]. On the other hand, the C02-soluble dendrimer nanocomposite exclusively catalyzes the production of the highly unfavored 2-phenyl-acrylic acid methyl ester isomer at 5000 psi and 75 °C (Fig. 27] [177]. 

Chen L, Chen H, Luque R, Li Y. Metalorganic framework encapsulated Pd nanoparticles towards advanced heterogeneous catalysts. ChemSci 2014 5 3708-14. 

Bernechea M, de Jesus E, Lopez-Mardomingo C, Terreros P (2009) Dendrimer-encapsulated Pd nanoparticles versus palladium acetate as catalytic precursors in the Stille reaction in water. Inorg Chem 48(10) 4491-4496... 

Garcia-Martinez JC, Lezutekong R, Crooks RM (2005) Dendrimer-encapsulated Pd nanoparticles as aqueous, room-temperature catalysts for the Stille reaction. J Am Chem Soc 127 5097-5103... 

IshidaT, Onumab Y, KinjoK, etal. Preparation ofmicroporous polymer-encapsulated Pd nanoparticles and their catalytic performance for hydrogenation and oxidation. Tetrahedron. 2014 70 6150-6155. 

Rhee and coworkers published the synthesis of bimetallic Pt-Pd nanoparticles [57] or Pd-Rh nanoparticles [58] within dendrimers as nanoreactors. These nanocatalysts showed a promising catalytic activity in the partial hydrogenation of 1,3-cyclooctadiene. The reaction was carried out in an ethanol/water mixture at 20 °C under dihydrogen at atmospheric pressure. The dendrimer-encapsulated nanoclusters could be reused, without significant loss of activity. 

Y. M. Chung and H. K. Rhee, Partial hydrogenation of 1,3-cyclooctadiene using dendrimer-encapsulated Pd-Rh bimetallic nanoparticles, J. Mol. Catal. A—Chem. 206, 291-298 (2003). 

R. W. J. Scott, O. M. Wilson, and R. M. Crooks, Titania-supported Au and Pd composites synthesized from dendrimer-encapsulated metal nanoparticle precursors, Chem. Mater. 16, 5682-5688 (2004). 

Two classes of catalysts account for most contemporary research. The first class includes transition-metal nanoparticles (e.g., Pd, Pt), their oxides (e.g., RUO2), and bimetallic materials (e.g., Pt/Ni, Pt/Ru) [104,132-134]. The second class, usually referred to as molecular catalysts, includes all transition-metal complexes, such as metalloporphyrins, in which the metal centers can assume multiple oxidation states [ 135 -137]. Previous studies have not only yielded a wealth of information about the preparation and catalytic properties of these materials, but they have also revealed shortcomings where further research is needed. Here we summarize the main barriers to progress in the field of metal-particle-based catalysis and discuss how dendrimer-encapsulated metal nanoparticles might provide a means for addressing some of the problems. 

Using an approach similar to that discussed previously, Pd nanoparticles were prepared within amine-terminated PAMAM dendrimers. To prevent coordination of Pd + to the primary amine groups of the dendrimers, the solution pH was adjusted to around 2, which preferentially protonates the exterior amines to a greater extent than the interior tertiary amines. Accordingly, Pd + binds preferentially to the interior tertiary amines and upon reduction Pd particles form only within the dendrimer interior. G4-NH2 dendrimer-encapsulated nanoparticles can then be quantitatively transported from an aqueous phase into toluene by addition of 10-20% of dodecanoic acid to the organic phase (Fig. 21) [19]. This transition is readily visualized by the color change the brown aqueous solution of Pd nanoparticles becomes clear after addition of the acid, while the toluene layer turns brown. Our studies have shown that this is a consequence of... 

Toluene solutions of Pd encapsulated within dendrimer-templated inverted micelles have been tested for catalytic activity by examining their effectiveness towards hydrogenation of allyl alcohol in organic solvents [19]. The reaction product was confirmed to be n-propanol by H NMR spectroscopy, and the turnover frequency, calculated from the rate of hydrogen uptake, was 760 mol H2 (mol Pd) h at 20 °C. This value compares favorably with the value of 218 mol H2 (mol Pd) h obtained for the same reaction carried out in water using Pd nanoparticles encapsulated in hydroxy-terminated dendrimers. 

Zhao, M., and Crooks, R. M., Homogeneous hydrogenation catalysis using monodisperse, dendrimer-encapsulated Pd and Pt nanoparticles, Angew. Chem. Int. Ed. 38, 364 (1999b). 

Fig. 11 Schematic representation of a preparation of cell targeted ferritin with the binding RGD-4C peptide on the exterior surface of apo-Fr including Fe O, b preparation of a Pd nanoparticle in apo-Fr and olefin hydrogenation, c hydrogen production reaction using Pt apo-Hsp, and d encapsulation of horseradish peroxidase (HRP) in the cavity of CCMV and an enzymatic reaction...

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... 

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