High-throughput kinetic study of hydrogenation over palladium nanoparticles: combination of reaction and analysis

The hydrogenation of 1-acetylcyclohexene, cyclohex-2-enone, nitrobenzene, and trans-methylpent-3-enoate catalyzed by highly active palladium nanoparticles was studied by high-throughput on-column reaction gas chromatography. In these experiments, catalysis and separation of educts and products is integrated by the use of a catalytically active gas chromatographic stationary phase, which allows reaction rate measurements to be efficiently performed by employing reactant libraries. Palladium nanoparticles embedded in a stabilizing polysiloxane matrix serve as catalyst and selective chromatographic stationary phase for these multiphase reactions (gas-liquid-solid) and are coated in fused-silica capillaries (inner diameter 250 microm) as a thin film of thickness 250 nm. The palladium nanoparticles were prepared by reduction of palladium acetate with hydridomethylsiloxane-dimethylsiloxane copolymer and self-catalyzed hydrosilylation with methylvinylsiloxane-dimethylsiloxane copolymer to obtain a stabilizing matrix. Diphenylsiloxane-dimethylsiloxane copolymer (GE SE 52) was added to improve film stability over a wide range of compositions. Herein, we show by systematic TEM investigations that the size and morphology (crystalline or amorphous) of the nanoparticles strongly depends on the ratio of the stabilizing polysiloxanes, the conditions to immobilize the stationary phase on the surface of the fused-silica capillary, and the loading of the palladium precursor. Furthermore, hydrogenations were performed with these catalytically active stationary phases between 60 and 100 degrees C at various contact times to determine the temperature-dependent reaction rate constants and to obtain activation parameters and diffusion coefficients.     

A unifying mechanism for all high-temperature Heck reactions. The role of palladium colloids and anionic species

The Heck reaction has been the subject of intense investigation in the past decade. Many new types of catalysts have been developed in addition to the existing palladium/phosphine complexes. Prominent among these are palladacycles, pincers, several types of heterogeneous palladium catalysts, colloids and ligand-free palladium, usually in the form of Pd(OAc)2. Most of the newer types function only at higher temperatures, typically between 120 and 160 degrees C. It has been shown that irrespective of the catalyst precursor, none of these catalysts are stable at these high temperatures. They all have a tendency to form soluble palladium(0) colloids or nanoparticles, certainly with less reactive substrates such as aryl bromides or chlorides. The Heck reaction takes place by attack of the arylating agent on the palladium atoms in the outer rim of the nanoparticles. This leads to formation of monomeric or dimeric anionic palladium complexes that undergo the usual steps of the Heck mechanism as described by Amatore and Jutand.     

Synthesis of small palladium nanoparticles stabilized by bisphosphine BINAP bearing an alkyl chain and their palladium nanoparticle-catalyzed carbon-carbon coupling reactions under room-temperature

A new bisphosphine ligand, C8-BINAP, and C8-BINAP-stabilized palladium nanoparticles have been prepared; C8-BINAP was found to be an effective protecting ligand for preparing and stabilizing palladium nanoparticles with very small core size and narrow size distribution and the C8-BINAP-Pd nanoparticles behave as an efficient catalyst for carbon-carbon coupling reactions at room temperature.     

Synthesis of platinum nanoparticles by reaction of filamentous cyanobacteria with platinum(IV)-chloride complex

Interaction of cyanobacteria (Plectonema boryanum UTEX 485) with aqueous platinum(IV)-chloride (PtCl(4) degrees ) has been investigated at 25-100 degrees C for up to 28 days, and 180 degrees C for 1 day. The addition of PtCl(4) degrees to the cyanobacteria culture initially promoted the precipitation of Pt(II)-organic material as amorphous spherical nanoparticles (< or =0.3 microm) in solutions and dispersed nanoparticles within bacterial cells. The spherical Pt(II)-organic nanoparticles were connected into long beadlike chains by a continuous coating of organic material derived from the cyanobacterial cells, and aged to nanoparticles of crystalline platinum metal with increase in temperature and reaction time. The stepwise reduction for the formation of platinum nanoparticles in the presence of cyanobacteria was deduced to be Pt(IV) [PtCl(4) degrees ] --> Pt(II) [Pt(II)-organics] --> Pt(0). Spherical platinum-bearing nanoparticles were not present in abiotic PtCl(4) degrees experiments conducted under similar conditions and duration.     

Synthesis of iron nanoparticles via chemical reduction with palladium ion seeds

We report on the synthesis of highly monodisperse iron nanoparticles, using a chemical reduction method. Iron nanoparticles with an average diameter of 6 nm and a geometric standard deviation of 1.3 were synthesized at a pH of 9.50 from ferric chloride precursor with sodium borohydride as the reducing agent, polyacrylic acid as the dispersing agent, and palladium ions as seeds for iron nanoparticle nucleation. The resulting nanoparticles were ferromagnetic at 5 K and superparamagnetic at 350 K. The dispersing agent polyacrylic acid (PAA) was shown to prevent iron nanoparticles and possibly palladium clusters from aggregating; in the absence of PAA, only aggregated iron nanoparticles were obtained. The addition of palladium ions decreased the diameter of iron nanoparticles presumably by providing sites for heterogeneous nucleation onto palladium clusters. In the absence of palladium ions, the mean diameter of iron nanoparticles was approximately 110 nm and the standard deviation increased to 2.0. The pH of the solution also was found to have a significant effect on the particle diameter, likely by affecting PAA ionization and altering the conformation of the polymer chains. At lower pH (8.75), the PAA is less ionized and its ability to disperse palladium clusters is reduced, so the number of palladium seeds decreases. Therefore, the resulting iron nanoparticles were larger, 59 nm in diameter, versus 6 nm for nanoparticles formed at a pH of 9.50.     

High catalytic activity and stability of palladium nanoparticles prepared by the laser electrodispersion method in chlorobenzene hydrodechlorination

Palladium nanoparticles deposited on thermally oxidized silicon and on the carbon support Sibunit by the laser electrodispersion method are extremely active in the gas-phase hydrodechlorination of chlorobenzene at 100–200°C. High conversion of chlorobenzene (above 90%) has been achieved with catalysts with an unusually low metal content (from 10^?4 to 10^?3 wt %). The cyclohexane-to-benzene ratio in the reaction products depends on the process duration, palladium content, and support nature. According to X-ray photoelectron spectroscopy (XPS) data, palladium in the catalysts retains its metallic state over a long time under the reaction conditions. Possible causes of the high catalytic activity (10⁵ mol (mol Pd)^?1 h^?1) of the palladium nanoparticles and their stability to chlorination are discussed.     

Amphiphilic ionic liquid stabilizing palladium nanoparticles for highly efficient catalytic hydrogenation

The highly water-soluble palladium nanoparticles (NPs) were synthesized by using the amphiphilic poly(ethylene glycol)-functionalized dicationic imidazolium-based ionic liquid (C(12)Im-PEG IL) as a stabilizing agent. The aqueous dispersed palladium NPs in the range of 1.9 ± 0.3 nm were observed by transmission electron microscopy (TEM). The physicochemical properties of C(12)Im-PEG IL in aqueous phase have been characterized by electrical conductivity, surface tension and dynamic light scattering (DLS) measurements. It was demonstrated that the amphiphilic ionic liquid can form micelles above its critical micelle concentration (CMC) in aqueous solution and the micelles played a crucial role in stabilizing the palladium NPs and thus promoted catalytic hydrogenation. Furthermore, the dicationic ionic liquid can also act as a gemini surfactant and generated emulsion between hydrophobic substrates and the catalytic aqueous phase during the reaction. The aqueous dispersed palladium NPs showed efficient activity for the catalytic hydrogenation of various substrates under very mild conditions and the stabilizing Pd(0) nanoparticles (NPs) can be reused at least eight times with complete conservation of activity.     

Efficient Heck reactions catalyzed by a highly recyclable palladium(II) complex of a pyridyl-functionalized imidazolium-based ionic liquid

The reactions of 2-(2-pyridyl)imidazole with alkyl iodides at 25 degrees C in the presence of base gave rise to 1-alkyl-2-(2-pyridyl)imidazole. Subsequent neat reactions with alkyl or polyfluoroalkyl halides at 100 degrees C, followed by anion exchange with LiN(SO(2)CF(3))(2), generated the mono-quaternary ionic liquids. All of them have excellent thermal stability and wide liquid range. Most of the salts with asymmetric N-substituents are liquid at room temperature. The effect of N-substituent variation and symmetry on NMR, TGA and DSC is discussed. Reaction of with palladium(II) chloride produced a mononuclear palladium ionic liquid complex, the structure of which was confirmed by single-crystal X-ray diffraction analysis. The Heck cross-coupling reactions using in ionic liquid demonstrated excellent stability and recyclability.     

Preparation and characterization of 4-dimethylaminopyridine-stabilized palladium nanoparticles

4-Dimethylaminopyridine (DMAP)-stabilized palladium nanoparticles with a mean diameter of 3.4 +/- 0.5 nm are prepared from the aqueous phase reduction of Na2PdCl4 using NaBH4 in the presence of DMAP. TEM and UV-vis spectroscopy characterization of the nanoparticle dispersion shows no obvious change in the nanoparticles several months after preparation. 1H NMR spectroscopy of the nanoparticles shows that the nanoparticle dispersion also contains a boron/DMAP complex and two palladium/DMAP complexes. One of the palladium complexes crystallizes out of the dispersion and is identified as Pd(DMAP)4(OH)2 by X-ray crystallography. Following extensive analysis, it is believed that the palladium/DMAP complexes are formed following the oxidation of the palladium nanoparticles. The prepared nanoparticle dispersion promotes selective hydrogen/deuterium (H/D) exchange on the carbon atoms alpha to the endocyclic nitrogen atom on the DMAP-stabilizing ligands through reaction with D2O. This activity is attributed to the presence of the nanoparticles rather than to the presence of the oxidized palladium/DMAP complexes.     

Macroporous polystyrene-supported palladium catalyst containing a bulky N-heterocyclic carbene ligand for Suzuki reaction of aryl chlorides

Macroporous polystyrene (MPS)-supported 1-mesitylimidazolium chloride resin was prepared by reacting macroporous chloromethyl polystyrene with 1-mesitylimidazole as a supported N-heterocyclic carbene (NHC) precursor for the immobilization of a palladium catalyst. This MPS-supported NHC precursor readily formed a stable complex with Pd(OAc)2, which effectively catalyzed the Suzuki reaction of aryl iodide and bromides at room temperature and even aryl chlorides at elevated temperatures (100 degrees C). This catalyst showed reusability in the Suzuki reaction of aryl bromide.     

Structure and catalytic properties of nanosized alumina supported platinum and palladium particles synthesized by reaction in microemulsion

Mixtures of nanosized platinum and palladium particles have been prepared by reduction of salt-containing microemulsion droplets using hydrazine as the reducing agent. To avoid possible negative effects of the presence of sulfur compounds during the preparation the microemulsion was made using the sulfur-free nonionic polyoxyethylene 4 lauryl ether surfactant. Transmission electron microscopy showed that the as-prepared mixtures contained crystalline platinum particles of fairly homogeneous size (20 to 40 nm) with adsorbed amorphous palladium particles 2 to 5 nm in size. Catalyst samples were prepared by depositing the nanoparticles on a gamma-Al(2)O(3) support followed by heating in air at 600 degrees C. Alloyed particles of platinum and palladium with sizes ranging from 5 to 80 nm were obtained during the heating. The majority of the particles had the fcc structure and their compositional range was dependent upon the Pt:Pd molar ratio of the microemulsion. A catalyst prepared from a microemulsion with a 20:80 Pt:Pd molar ratio showed the highest catalytic activity for CO oxidation, while pure platinum and palladium catalysts showed higher sulfur resistance. These results differ from the performance of conventional wet-impregnated catalysts, where a 50:50 Pt:Pd molar ratio resulted in the highest catalytic activity as well as the highest sulfur resistance.     

A one-pot efficient and fast Hiyama coupling using palladium nanoparticles in water under fluoride-free conditions

An efficient Hiyama cross-coupling of a wide variety of substituted aryl bromides and iodides with arylsilanes has been achieved at 100 °C in water through a simple one-pot operation using in situ generated palladium nanoparticles. This reaction is very fast (5 min) and has been performed in air under fluoride-free conditions, giving excellent chemoselectivity and high yields.     

Lactam synthesis via the intramolecular hydroamidation of alkynes catalyzed by palladium complexes

The palladium complexes catalyzed intramolecular hydroamidation reaction of amidoalkynes gives the corresponding lactams in good to high yields. For example, in the presence of 10 mol % of Pd(PPh3)4 and 20 mol % of PhCOOH, the reaction of the amidoalkyne 3a in 1,4-dioxane at 100 degrees C proceeded smoothly to give the corresponding lactam 4a in 92% yield.     

Stabilized spherical aggregate of palladium nanoparticles prepared by reduction of palladium acetate in octa(3-aminopropyl)octasilsesquioxane as a rigid template

Spherical aggregates of palladium nanoparticles were produced by stirring palladium(II) acetate with dendritic molecules (i.e., octa(3-aminopropyl)silsesquioxane octahydrochloride (POSS-NH3+) or the amine-terminated G1.0 poly(amidoamine) dendrimer (G1-NH2)) in methanol at room temperature via self-organized spherical templates of Pd(OAc)2 and the dendritic molecule. The mixing ratio of the terminal amino groups of the dendritic molecule and palladium ions (Z=[Pd2+]/[-NH2]) affected the formation of the spherical aggregates of palladium nanoparticles. Maximum Z values with no reduction of palladium ions (the solution remained yellow) were 1.0 for POSS-NH3+ and 1.6 for G1-NH2, respectively. TEM observations suggested that the density of the palladium nanoparticles in the aggregates using POSS-NH3+ as a template was higher than that using G1-NH2. From tapping mode atomic force microscopy, shapes of the aggregates using POSS-NH3+ and G1-NH2 were a spherical form and an oval form on plates, respectively. Increasing the rigidity of the silsesquioxane core of the dendritic molecules increased the stability of the spherical form in the dry state.     

Synthesis of palladium nanoparticles by sonochemical reduction of palladium(II) nitrate in aqueous solution

The sonochemical synthesis of stable palladium nanoparticles has been achieved by ultrasonic irradiation of palladium(II) nitrate solution. The starting solutions were prepared by the addition of different concentrations of palladium(II) nitrate in ethylene glycol and poly(vinylpyrrolidone) (PVP). The resulting mixtures were irradiated with ultrasonic 50 kHz waves in a glass vessel for 180 min. The UV-visible absorption spectroscopy and pH measurements revealed that the reduction of Pd(II) to metallic Pd has been successfully achieved and that the obtained suspensions have a long shelf life. The protective effect of PVP was studied using Fourier transform infrared (FT-IR) spectroscopy. It has been found that, in the presence of ethylene glycol, the stabilization of the nanoparticles results from the adsorption of the PVP chain on the palladium particle surface via the coordination of the PVP carbonyl group to the palladium atoms. The effect of the initial Pd(II) concentration on the Pd nanoparticle morphology has been investigated by transmission electron microscopy. It has been shown that the increase of the Pd(II)/PVP molar ratio from 0.13 x 10(-3) to 0.53 x 10(-3) decreases the number of palladium nanoparticles with a slight increase in particle size. For the highest Pd(II)/PVP value, 0.53 x 10(-3), the reduction reaction leads to the unexpected smallest nanoparticles in the form of aggregates.     

Hydrogen storage properties of isocyanide-stabilized palladium nanoparticles

Monodispersed palladium nanoparticles protected with n-octyl isocyanide were prepared, and their hydrogen absorption behavior was evaluated. The formation of the nanoparticles has been confirmed by means of 1H NMR and elemental analysis. Fourier transform infrared (FT-IR) showed that three distinct bands (2156, 1964, and 1611 cm(-1)) assigned to mono-, double-, and triple-bridged isocyanide ligands on the palladium surface. The average diameter of the particles was estimated to be 2.1 +/- 0.7 nm from observation by transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) analysis revealed that the particles contained Pd(0) with little amounts of Pd(II) or Pd(IV), in sharp contrast to the thiol- or phosphine-stabilized palladium nanoparticles. The absorption and desorption of hydrogen were reversible, and the reactions were much faster for the nanoparticles than for the bulk palladium metal, whereas the storage capacity was almost the same, 0.6 wt %.     

Rationally designed pincer-type heck catalysts bearing aminophosphine substituents: Pd IV intermediates and palladium nanoparticles

The aminophosphine-based pincer complexes [C6H3-2,6-(XP(piperidinyl)2)2Pd(Cl)] (X=NH 1; X=O 2) are readily prepared from cheap starting materials by sequential addition of 1,1',1'-phosphinetriyltripiperidine and 1,3-diaminobenzene or resorcinol to solutions of [Pd(cod)(Cl)2] (cod=cyclooctadiene) in toluene under N2 in "one pot". Compounds 1 and 2 proved to be excellent Heck catalysts and allow the quantitative coupling of several electronically deactivated and sterically hindered aryl bromides with various olefins as coupling partners at 140 degrees C within very short reaction times and low catalyst loadings. Increased reaction temperatures also enable the efficient coupling of olefins with electronically deactivated and sterically hindered aryl chlorides in the presence of only 0.01 mol % of catalyst. The mechanistic studies performed rule out that homogeneous Pd 0 complexes are the catalytically active forms of 1 and 2. On the other hand, the involvement of palladium nanoparticles in the catalytic cycle received strong experimental support. Even though pincer-type Pd IV intermediates derived from 1 (and 2) are not involved in the catalytic cycle of the Heck reaction, their general existence as reactive intermediates (for example, in other reactions) cannot be excluded. On the contrary, they were shown to be thermally accessible. Compounds 1 and 2 show a smooth halide exchange with bromobenzene to yield their bromo derivatives in DMF at 100 degrees C. Experimental observations revealed that the halide exchange most probably proceeded via pincer-type Pd IV intermediates. DFT calculations support this hypothesis and indicated that aminophosphine-based pincer-type Pd IV intermediates are generally to be considered as reactive intermediates in reactions with aryl halides performed at elevated temperatures.     

The electroanalytical detection of hydrazine: a comparison of the use of palladium nanoparticles supported on boron-doped diamond and palladium plated BDD microdisc array

We show that both a random distribution of palladium nanoparticles supported on a BDD electrode or a palladium plated BDD microelectrode array can each provide a sensing platform for the electrocatalytic detection of hydrazine. The palladium nanoparticle modified electrode displays a sensitivity and limit of detection of 60 mA mol(-1) L and 2.6 microM respectively while the array has a sensitivity of 8 mA mol(-1) L with a detection limit of 1.8 microM. The beneficial cost implications of using palladium nano- or micro-particles in sensors compared to a palladium macroelectrode are evident. Interestingly the array of the nanoparticles shows similar sensitivity and limit of detection to the microelectrode array which probably indicates that the random distribution of the former leads to 'clumps' of nanoparticles that effectively act as microelectrodes.     

Highly active thermomorphic fluorous palladacycle catalyst precursors for the Heck reaction; evidence for a palladium nanoparticle pathway

[reaction: see text] The fluorous Schiff base p-Rf8(CH2)3C6H4C(=N(CH2)3Rf8)(CH2)2Rf8 (Rf8 = n-C8F17) is prepared in six steps from p-iodobenzaldehyde and then cyclopalladated (Pd(OAc)2) to give highly effective catalyst precursors for Heck reactions, conducted under homogeneous conditions (DMF, 80-140 degrees C, turnover numbers >10(6)) in the absence of fluorous solvents. Rate, recycling, and other data suggest that the palladacycles serve as sources of palladium nanoparticles, which are the dominant active catalysts.     

Gelatin as a bioorganic reductant, ligand and support for palladium nanoparticles. Application as a catalyst for ligand- and amine-free Sonogashira-Hagihara reaction

Palladium nanoparticles were deposited and reduced by gelatin as a safe edible, naturally occurring and cheap support. No extra reducing agents were used for the generation of Pd(0) nanoparticles from the Pd(II) salt. The nanoparticles of Pd supported on gelatin were characterized by SEM, TEM and AFM images, UV-Vis and XRD spectra and the amount of palladium entrapped in the gelatin was measured by ICP and atomic absorption analysis. The nanoparticles showed high catalytic activity for the Sonogashira-Hagihara coupling reaction of various aryl iodides, bromides and chlorides as well as heteroaryl halides and also β-bromo styrene with phenylacetylene under copper-, ligand- and amine-free conditions. The reactions were carried out at 100 °C in molten tetrabutylammonium bromide (TBAB) or polyethylene glycol (PEG400) in the presence of potassium acetate as a base in argon atmosphere. Dimerization of phenylacetylene applying similar conditions in air is also described.