Dendrimer-encapsulated Pd Research Articles

In situ synthesis of dendrimer-encapsulated palladium(0) nanoparticles as catalysts for hydrogen production from the methanolysis of ammonia borane

Addressed herein is the in situ synthesis of a PAMAM dendrimer-encapsulated palladium(0) NPs (Pd(0)/Dnd) during the methanolysis of ammonia borane (AB) and the catalytic performance of the yielded Pd(0)/Dnd nanocatalysts in hydrogen production from the methanolysis of AB under ambient conditions. A two-step procedure that includes the impregnation of Pd(II) ions via their coordination to –NH2 groups of the dendrimer and then reduction of Pd(II) ions into the dendrimer-encapsulated Pd(0) NPs by AB during the methanolysis reaction was followed for the synthesis of Pd(0)/Dnd nanocatalysts. However, apart from the existing reports on the synthesis of dendrimer-encapsulated metal NPs, the present study includes for the first time the examination of effect of generation size (G4-G6), core type (ethylene diamine (E) or Jeffamine (P)) and terminal groups (-NH2, –COOH and –OH) of a PAMAM dendrimer on the stability, particle size, morphology and catalytic activity of metal NPs. After finding the optimum Pd(0)/Dnd catalysts considering all these effects, a detailed kinetic study comprising the effect of catalyst and AB concentrations as well as temperature was conducted by monitoring the hydrogen production from the methanolysis of AB. The best catalytic activity in the methanolysis of AB was obtained by using a PAMAM dendrimer with generation G6, amine terminal groups and Jeffamine core (P6.NH2) encapsulated Pd(0) NPs, providing the highest initial turnover frequency (TOF) of 55.8 mol H2.mol Pd−1.min−1 and apparent activation energy (Eaapp) of 48 ± 3 kJ.mol−1 at room temperature.

Dendrimer-Encapsulated Pd Nanoparticles, Immobilized in Silica Pores, as Catalysts for Selective Hydrogenation of Unsaturated Compounds.

Heterogeneous Pd‐containing nanocatalysts, based on poly (propylene imine) dendrimers immobilized in silica pores and networks, obtained by co‐hydrolysis in situ, have been synthesized and examined in the hydrogenation of various unsaturated compounds. The catalyst activity and selectivity were found to strongly depend on the carrier structure as well as on the substrate electron and geometric features. Thus, mesoporous catalyst, synthesized in presence of both polymeric template and tetraethoxysilane, revealed the maximum activity in the hydrogenation of various styrenes, including bulky and rigid stilbene and its isomers, reaching TOF values of about 230000 h−1. Other mesoporous catalyst, synthesized in the presence of polymeric template, but without addition of Si(OEt)4, provided the trans‐cyclooctene formation with the selectivity of 90–95 %, appearing as similar to homogeneous dendrimer‐based catalysts. Microporous catalyst, obtained only on the presence of Si(OEt)4, while dendrimer molecules acting as both anchored ligands and template, demonstrated the maximum activity in the hydrogenation of terminal linear alkynes and conjugated dienes, reaching TOF values up to 400000 h−1. Herein the total selectivity on alkene in the case of terminal alkynes and conjugated dienes reached 95–99 % even at hydrogen pressure of 30 atm. The catalysts synthesized can be easily isolated from reaction products and recycled without significant loss of activity.

Dendrimer-encapsulated Pd(0) nanoparticles immobilized on nanosilica as a highly active and recyclable catalyst for the copper- and phosphine-free Sonogashira–Hagihara coupling reactions in water

Recyclable dendrimer-encapsulated Pd(0) nanoparticles immobilized on nanosilica in the Sonogashira–Hagihara reaction under copper(i) and phosphine ligand-free conditions in water.

Dendrimer-encapsulated Pd nanoparticles as catalysts for C-C cross-couplings in flow microreactors.

The inner walls of glass microreactors were functionalized with dendrimer-encapsulated Pd nanoparticles. The catalysts were efficient for the Heck-Cassar (copper-free Sonogashira) and Suzuki-Miyaura (SMC) cross-coupling reactions. For the Heck-Cassar reaction between iodobenzene and phenylacetylene, the catalytic system exhibited a high turnover frequency (TOF) and on average required milder reaction conditions as compared to other continuous flow cross-couplings. A study of the substituent effect of para-substituted aryl halides revealed a beneficial effect of electron-withdrawing side groups for the SMC. Moreover, a reaction constant (ρ) of 1.5, determined from the Hammett plot, indicated a possible rate-determining step other than the oxidative addition.

Silica–Dendrimer Core–Shell Microspheres with Encapsulated Ultrasmall Palladium Nanoparticles: Efficient and Easily Recyclable Heterogeneous Nanocatalysts

We report the synthesis, characterization, and catalytic properties of novel monodisperse SiO(2)@Pd-PAMAM core-shell microspheres containing SiO(2) microsphere cores and PAMAM dendrimer-encapsulated Pd nanoparticle (Pd-PAMAM) shells. First, SiO(2) microspheres, which were prepared by the Stöber method, were functionalized with vinyl groups by grafting their surfaces with vinyltriethoxysilane (VTS). The vinyl groups were then converted into epoxides by using m-chloroperoxybenzoic acid. Upon treatment with amine-terminated G4 poly(amidoamine) (PAMAM) dendrimers, the SiO(2)-supported epoxides underwent ring-opening and gave SiO(2)@PAMAM core-shell microspheres. Pd nanoparticles within the cores of the SiO(2)-supported PAMAM dendrimers were synthesized by letting Pd(II) ions complex with the amine groups in the cores of the dendrimers and then reducing them into Pd(0) with NaBH(4). This produced the SiO(2)@Pd-PAMAM core-shell microspheres. The presence of the different functional groups on the materials was monitored by following the changes in FTIR spectra, elemental analyses, and weight losses on thermogravimetric traces. Transmission electron microscopy (TEM) images showed the presence of Pd nanoparticles with average size of 1.56 ± 0.67 nm on the surface of the monodisperse SiO(2)@Pd-PAMAM core-shell microspheres. The SiO(2)@Pd-PAMAM core-shell microspheres were successfully used as an easily recyclable catalyst for hydrogenation of various olefins, alkynes, keto, and nitro groups, giving ~100% conversion and high turnover numbers (TONs) under 10 bar H(2) pressure, at room temperature and in times ranging from 10 min to 3 h. In addition, the SiO(2)@Pd-PAMAM core-shell microspheres were proven to be recyclable catalysts up to five times with barely any leaching of palladium into the reaction mixture.

Subnanoscale Size Effect of Dendrimer-encapsulated Pd Clusters on Catalytic Hydrogenation of Olefin

Abstract Dendrimer-encapsulated subnano Pd clusters catalyze selective hydrogenation of 1,3-cyclooctadiene to cyclooctene. The catalytic activity increases with the size of the subnano Pd clusters. The activity of the threefold hollow sites of the subnano Pd clusters plays an important role in the hydrogenation reaction.

Dendrimer-encapsulated Pd nanoparticles anchored on carbon nanotubes for electro-catalytic hydrazine oxidation

This study describes a facile method for preparing amine-terminated poly(amidoamine) (G4-NH 2)-encapsulated palladium nanocatalysts with carbon nanotubes as a support. The Pd 2+/G4-NH 2 complexes are synthesized by a template approach in which PdCl 4 2 - ions are extracted into the interior of G4-NH 2 dendrimers, and then bonded on multi-walled carbon nanotubes (MWNTs) support through dendrimer covalent linkage. Conversion of Pd 2+/G4-NH 2 into dendrimer-encapsulated nanoparticles was achieved by the addition of NaBH 4. Transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–vis spectroscopy confirmed that the size-monodisperse Pd nanoparticles anchored on MWNTs was about 2 nm in diameter. The obtained Pd/G4-NH 2/MWNT nanocomposites exhibit high electro-catalytic activity for the hydrazine oxidation by cyclic voltammetry.

Dendrimer-Encapsulated Pd Nanoparticles versus Palladium Acetate as Catalytic Precursors in the Stille Reaction in Water

The performance of several palladium precatalysts, namely, palladium(II) acetate, palladium(0) nanoparticles encapsulated into poly(amidoamine) (PAMAM) dendrimers (Pd DENs), and palladium(II)-PAMAM complexes, in the Stille reaction between trichloro(phenyl)stannane and iodoarenes in water is compared. The reactivity of Pd DENs is similar or inferior to that of palladium(II) acetate, although the presence of the dendrimer suppresses the formation of homocoupling products and allows catalyst recycling. It is suggested that the reaction catalyzed by Pd DENs occurs via palladium species which are leached from the nanoparticle but which remain coordinated to the dendritic macromolecule.

Metallocenyl Dendrimers and Their Applications in Molecular Electronics, Sensing, and Catalysis

We have investigated the movement of electrons around the peripheries of dendrimers and between their redox termini and electrodes through studies of the electrochemistry of dendrimers presenting metallocenes (and other transition metal sandwich complexes) as terminal groups. Because these compounds can be stabilized in both their oxidized and their reduced forms, their electrochemical and chemical redox processes proceed without decomposition (chemical reversibility). Most interestingly, electrochemical studies reveal that electron transfer within the dendrimers and between the dendrimers and electrodes are both very fast processes when the branches are flexible (electrochemical reversibility). When the dendrimer branches are sufficiently long, the redox events at the many termini of the metallodendrimer are independent, appearing as a single wave in the cyclic voltammogram, because of very weak electrostatic effects. As a result, these metallodendrimers have applications in the molecular recognition, sensing, and titration of anions (e.g., ATP(2-)) and cations (e.g., transition metal complexes). When the recognition properties are coupled with catalysis, the metallodendrimers function in an enzyme-like manner. For example, Pd(II) can be recognized and titrated using the dendrimer's terminal redox centers and internal coordinate ligands. Redox control over the number of Pd(II) species located within a dendrimer allows us to predetermine the number of metal atoms that end up in the form of a dendrimer-encapsulated Pd nanoparticle (PdNP). For hydrogenation of olefins, the efficiency (turnover frequency, TOF) and stability (turnover number, TON) depend on the size of the dendrimer-encapsulated PdNP catalysts, similar to the behavior of polymer-supported PdNP catalysts, suggesting a classic mechanism in which all of the steps proceed on the PdNP surface. On the other hand, Miyaura-Suzuki carbon-carbon bond-forming reactions catalyzed by dendrimer-encapsulated PdNPs proceed with TOFs and TONs that do not depend on the size of the PdNPs. Moreover these catalysts are more efficient when employed in lower (down to "homeopathic") amounts, presumably because of a leaching mechanism whereby Pd atoms escape from the PdNP surface subsequent to oxidative addition of the aryl halide. Under these conditions, the "mother" PdNPs have greater difficulty quenching the extremely active leached Pd atoms because of their low concentration. Although dendrimers presenting catalysts at their branch termini can be recovered and reused readily, their inner-sphere components can lead to steric inhibition of substrate approach. In contrast, star-shaped catalysts do not suffer from such steric problems, as has been demonstrated for water-soluble dendrimers bearing cationic iron-sandwich termini, which are redox catalysts of cathodic nitrate and nitrite reduction in water.

Enhanced stability of charged dendrimer-encapsulated Pd nanoparticles in ionic liquids

Highly stable dendrimer-encapsulated Pd nanoparticles in ionic liquids were prepared for the first time by using charged PAMAM dendrimers as templates, which could maintain hydrogenation efficiency for up to at least 12 recycles.

Dendrimer-Encapsulated Pd Nanoparticles as Aqueous, Room-Temperature Catalysts for the Stille Reaction

We report that dendrimer-encapsulated Pd nanoparticles having a diameter of approximately 1.7 nm are effective and general catalysts for coupling aryl halides to organostannanes (the Stille reaction) under mild conditions. The significant results of this study are that the Stille reaction is catalyzed by dendrimer-encapsulated Pd nanoparticles in very good yield, in aqueous solution at 23 degrees C, and using only 0.100 atom % of Pd as catalyst.

Supramolecular Catalysts by Encapsulating Palladium Complexes within Dendrimers.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Supramolecular catalysts by encapsulating palladium complexes within dendrimers.

Dendrimers are well-defined and highly branched macromolecules. By utilizing their capsular architectures, dendrimers encapsulating various catalytically active species can be prepared, which often bring about unique catalysis. Treatment of the alkylated PPI dendrimer with 4-diphenylphosphinobenzoic acid and [PdCl(C3H5)]2 afforded the dendrimer-encapsulated Pd complex using ionic interactions. The dendrimers encapsulating Pd complexes acted as unique supramolecular catalysts for the Heck reaction and allylic amination. The specific nanoenvironment created by the dense amino groups inside the dendrimers can provide high catalytic activity and stability for the Pd complexes. Facile recovery of the dendritic catalysts could be achieved by thermomorphic systems.

Synthesis, Characterization, and Stability of Dendrimer-Encapsulated Palladium Nanoparticles

Here we report on the synthesis, physical and chemical properties, and stability of Pd nanoparticles encapsulated within poly(amidoamine) (PAMAM) dendrimers. Specifically, amine- and hydroxyl-terminated PAMAM dendrimers ranging in generation from 4 to 8 were studied. Under appropriate conditions, addition of K2PdCl4 results in covalent attachment of the PdCl3- hydrolysis product of this complex to tertiary amines within the dendrimers. Reduction with NaBH4 results in conversion of dendrimer-encapsulated PdCl3- to nearly size monodisperse, encapsulated, zerovalent Pd nanoparticles. Details regarding the Pd species present in solution and within the dendrimer prior to reduction are reported, as is the maximum Pd2+ loading of the dendrimers. Dendrimer-encapsulated Pd nanoparticles undergo oxidation in air, but this process is slowed significantly when coordinating ions are removed from solution. In the absence of O2, dendrimer-encapsulated Pd nanoparticles are stable indefinitely. The oxidation product is not PdO, but rather Pd ions coordinated to the dendrimer interior. Dendrimer generation does not affect the rate of Pd oxidation. The dendrimer itself undergoes irreversible oxidation in the presence of O2. Finally, the oxidation of dendrimer-encapsulated Pd nanoparticles is reversible. Specifically, H2 gas can be used to re-reduce partially oxidized Pd nanoparticles without changing their average size.

Extraction of Monodisperse Palladium Nanoparticles from Dendrimer Templates

We describe a strategy for extracting nearly size-monodisperse, 1.7-nm-diameter Pd nanoparticles from within dendrimer hosts. The approach involves two steps. First, an aqueous solution of dendrimer-encapsulated Pd nanoparticles is prepared, followed by the addition of a toluene solution containing an n-alkanethiol. When the two-phase system is stirred, n-alkanethiols presumably self-assemble onto the surface of the Pd nanoparticles, extract them from the dendrimer, and transport them to toluene phase. UV-vis spectroscopy indicates that the Pd nanoparticles, in the form of monolayer-protected clusters, are transported into the toluene phase. FT-IR spectroscopy shows that the dendrimers remain in the aqueous phase. High-resolution transmission electron microscopy shows that the average size and size distribution of the particles do not change during the extraction process.

Partial hydrogenation of 1,3-cyclooctadiene using dendrimer-encapsulated Pd–Rh bimetallic nanoparticles

Abstract This article reports the first effort for the preparation of Pd–Rh bimetallic nanoparticles in the presence of poly(amidoamine) dendrimers with surface hydroxyl groups (fourth generation, PAMAM-OH). The resulting dendrimer-encapsulated Pd–Rh bimetallic nanoparticles show a promising catalytic activity in partial hydrogenation of 1,3-cyclooctadiene.

Dendritic Nanoreactors Encapsulating Pd Particles for Substrate-Specific Hydrogenation of Olefins

Dendrimer-encapsulated Pd(0) nanoparticles inside poly(propylene imine) (PPI) dendrimers functionalized with triethoxybenzamide groups have been prepared by extraction of Pd2+ and subsequent chemical reduction. The resulting dendrimer−Pd nanocomposites are unique catalysts for substrate-specific hydrogenation of polar olefins, due to the strong interaction between polar substrates and tertiary amino groups within the dendrimers.

Dendrimer-Encapsulated Pd Nanoparticles as Fluorous Phase-Soluble Catalysts

Dendrimer-Encapsulated Pd Nanoparticles as Fluorous Phase-Soluble Catalysts

Homogeneous Hydrogenation Catalysis with Monodisperse, Dendrimer-Encapsulated Pd and Pt Nanoparticles.

Extraordinarily stable, monodisperse noble metal nanoparticles can be prepared by using dendrimers as both templates and stabilizers. Dendrimer-encapsulated Pd nanoparticles (see the schematic representation) exhibit high catalytic activity for the hydrogenation of alkenes in water. The catalytic activity and selectivity of these materials can be controlled by adjusting the dendrimer generation.

Homogeneous Hydrogenation Catalysis with Monodisperse, Dendrimer-Encapsulated Pd and Pt Nanoparticles

Homogeneous Hydrogenation Catalysis with Monodisperse, Dendrimer-Encapsulated Pd and Pt Nanoparticles