Showing papers on "Palladium published in 2004"

Palladium Oxidase Catalysis: Selective Oxidation of Organic Chemicals by Direct Dioxygen-Coupled Turnover

Abstract: Selective aerobic oxidation of organic molecules is a fundamental and practical challenge in modern chemistry. Effective solutions to this problem must overcome the intrinsic reactivity and selectivity challenges posed by the chemistry of molecular oxygen, and they must find application in diverse classes of oxidation reactions. Palladium oxidase catalysis combines the versatility of Pd(II)-mediated oxidation of organic substrates with dioxygen-coupled oxidation of the reduced palladium catalyst to enable a broad range of selective aerobic oxidation reactions. Recent developments revealed that cocatalysts (e.g. Cu(II), polyoxometalates, and benzoquinone) are not essential for efficient oxidation of Pd(0) by molecular oxygen. Oxidatively stable ligands play an important role in these reactions by minimizing catalyst decomposition, promoting the direct reaction between palladium and dioxygen, modulating organic substrate reactivity and permitting asymmetric catalysis.

Hydroxyapatite-Supported Palladium Nanoclusters: A Highly Active Heterogeneous Catalyst for Selective Oxidation of Alcohols by Use of Molecular Oxygen

Abstract: Treatment of a stoichiometric hydroxyapatite (HAP), Ca10(PO4)6(OH)2, with PdCl2(PhCN)2 gives a new type of palladium-grafted hydroxyapatite. Analysis by means of powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX), IR, and Pd K-edge X-ray absorption fine structure (XAFS) proves that a monomeric PdCl2 species is chemisorbed on the HAP surface, which is readily transformed into Pd nanoclusters with a narrow size distribution in the presence of alcohol. Nanoclustered Pd0 species can effectively promote the alcohol oxidation under an atmospheric O2 pressure, giving a remarkably high turnover number (TON) of up to 236,000 with an excellent turnover frequency (TOF) of approximately 9800 h(-1) for a 250-mmol-scale oxidation of 1-phenylethanol under solvent-free conditions. In addition to advantages such as a simple workup procedure and the ability to recycle the catalyst, the present Pd catalyst does not require additives to complete the catalytic cycle. The diameters of the generated Pd nanoclusters can be controlled upon acting on the alcohol substrates used. Oxidation of alcohols is proposed to occur primarily on low-coordination sites within a regular arrangement of the Pd nanocluster by performing calculations on the palladium crystallites.

Highly Efficient Formation of Visible Light Tunable TiO2-xNx Photocatalysts and Their Transformation at the Nanoscale

Abstract: Using a simple nanoscale exclusive synthesis route, TiO2-xNx photocatalysts that can be tuned to absorb across the visible region are produced in seconds at room temperature. The photocatalysts are formed by employing the direct nitridation of anatase TiO2 nanostructures with alkylammonium salts. Depending on the degree of TiO2 nanoparticle agglomeration, catalytically active TiO2-xNx anatase structured particles are obtained whose absorption onset extends well into the visible region at λ ∼ 550 nm. The introduction of a small quantity of palladium in the form of the chloride or nitrate facilitates further nitrogen uptake, appears to lead to a partial phase transformation, displays a counterion effect when compared also to the acetate, and produces a material absorbing well into the near-infrared. The introduction of palladium via the chloride also facilitates the formation of small tetrahedral and octahedral palladium-based crystallites throughout the TiO2-xNx lattice. Surprisingly, no organics appear to...

Palladium-Catalyzed Oxygenation of Unactivated sp3 C−H Bonds

Abstract: This communication describes a new palladium-catalyzed method for the oxygenation of unactivated sp3 C-H bonds. A wide variety of alkane substrates containing readily available oxime and/or pyridine directing groups are oxidized with extremely high levels of chemo-, regio-, and in some cases diastereoselectivity. The substrate scope of these reactions is discussed, and the high selectivities are rationalized on the basis of the requirements of putative palladium alkyl intermediates.

Palladium‐Catalyzed CN and CO Coupling–A Practical Guide from an Industrial Vantage Point†

Abstract: The palladium-catalyzed coupling of amines with aryl halides or aryl alcohol derivatives, commonly dubbed Buchwald–Hartwig amination, has matured from a synthetic laboratory procedure to a technique that is widely used in natural product synthesis as well as in other fields of academic interest. Furthermore, due to the versatility and reliability of this reaction, researchers in industrial environments have included this methodology in their toolbox as a standard procedure for the synthesis of amine derivatives. Therefore, it is not surprising that first industrial processes up to ton-scale have been performed using this cross-coupling reaction. The authors who are involved in the application of this reaction to industrial processes on this scale give an overview of the recent developments in this field of chemistry, also including fundamental principles, with a special focus on the industrial approach and issues to be considered relevant in scaling-up this transition metal-catalyzed chemistry. This review differs from the already existing excellent academic reviews by focusing on the practical problems arising during implementing the methodology in an industrial environment as well as giving practical hints to this end.

Preparation of PAMAM− and PPI−Metal (Silver, Platinum, and Palladium) Nanocomposites and Their Catalytic Activities for Reduction of 4-Nitrophenol

Abstract: Dendrimer-metal (silver, platinum, and palladium) nanocomposites are prepared in aqueous solutions containing poly(amidoamine) (PAMAM) dendrimers with surface amino groups (generations 3, 4, and 5) or poly(propyleneimine) (PPI) dendrimers with surface amino groups (generations 2, 3, and 4) The particle sizes of the metal nanoparticles obtained are almost independent of the generation as well as the concentration of the dendrimer for both the PAMAM and the PPI dendrimers; the average sizes of silver, platinum, and palladium nanoparticles are 56-75, 12-16, and 16-20 nm, respectively It is suggested that the dendrimer-metal nanocomposites are formed by adsorbing the dendrimers on the metal nanoparticles Studies of the reduction reaction of 4-nitrophenol by these nanocomposites show that the rate constants are very similar between PAMAM and PPI dendrimer-silver nanocomposites, whereas the rate constants for the PPI dendrimer-platinum and -palladium nanocomposites are greater than those for the corresponding PAMAM dendrimer nanocomposites In addition, it is found that the rate constants for the reduction of 4-nitrophenol involving all the dendrimer-metal nanocomposites decrease with an increase in the dendrimer concentrations, and the catalytic activity of dendrimer-palladium nanocomposites is highest

Structure and Reactivity of “Unusual” N-Heterocyclic Carbene (NHC) Palladium Complexes Synthesized from Imidazolium Salts

Abstract: The reaction between palladium acetate and IMES.HCl leads to the formation of a novel palladium complex. The X-ray crystal structure analysis reveals that the palladium is C(2) bound to one NHC ligand (the normal binding mode), whereas the second ligand is attached through the C(5) carbon of the second imidazolium. The metalation site on the imidazolium salt is strongly influenced by the presence of base. Furthermore, the binding mode of the NHC to Pd is shown to substantially affect the catalytic behavior of the palladium complexes in cross-coupling reactions.

General method of synthesis for metal nanoparticles

Abstract: Sugar-assisted stable monometallic nanoparticles were synthesized by wet chemical method following a general scheme. Judicious manipulation of the reducing capabilities of different sugars has shown to have a bearing on the particle size that corroborates the shift of the absorbance peak positions and TEM analysis. Evaporation of the precursor solutions on the solid surface (strong metal--support interaction), led to the formation of smaller particles. Under the experimental condition, spherical nanoparticles of approximately 1, 3, 10 and 20 nm sizes were prepared reproducibly for gold, platinum, silver and palladium, respectively. Fructose has been found to be the best suited sugar for the synthesis of smaller particles and remained stable for months together.

Nitrile-functionalized pyridinium ionic liquids: Synthesis, characterization, and their application in carbon-carbon coupling reactions

Abstract: A series of relatively low-cost ionic liquids, based on the N-butyronitrile pyridinium cation [C(3)CNpy](+), designed to improve catalyst retention, have been prepared and evaluated in Suzuki and Stille coupling reactions. Depending on the nature of the anion, these salts react with palladium chloride to form [C(3)CNpy](2)[PdCl(4)] when the anion is Cl(-) and complexes of the formula [PdCl(2)(C(3)CNpy)(2)][anion](2) when the anion is PF(6)(-), BF(4)(-), or N(SO(2)CF(3))(2)(-). The solid-state structures of [C(3)CNpy]Cl and [C(3)CNpy](2)[PdCl(4)] have been established by single-crystal X-ray diffraction. The catalytic activity of these palladium complexes following immobilization in both N-butylpyridinium and nitrile-functionalized ionic liquids has been evaluated in Suzuki and Stille coupling reactions. All of the palladium complexes show good catalytic activity, but recycling and reuse is considerably superior in the nitrile-functionalized ionic liquid. Inductive coupled plasma spectroscopy reveals that the presence of the coordinating nitrile moiety in the ionic liquid leads to a significant decrease in palladium leaching relative to simple N-alkylpyridinium ionic liquids. Palladium nanoparticles have been identified as the active catalyst in the Stille reaction and were characterized using transmission electron microscopy.

Size-selective synthesis of gold and platinum nanoparticles using novel thiol-functionalized ionic liquids.

Abstract: One-phase synthesis of gold and platinum nanoparticles using new thiol-functionalized ionic liquids (TFILs) is described for the first time. TFILs as stabilizing agents for gold and platinum nanoparticles were designed to have thiol groups on either the cation or anion and symmetrical or unsymmetrical positions only in the cation. Transmission electron microscopy, electron diffraction, and NMR were used for the characterization of nanoparticles. The metal nanoparticles formed using TFILs are crystalline structures with face-centered cubic packing arrangements and have small sizes (the average diameters are 3.5, 3.1, and 2.0 nm for Au and 3.2, 2.2, and 2.0 nm for Pt, respectively) and uniform distributions (the standard deviations are 0.7, 0.5, and 0.1 nm for Au and 1.1, 0.2, and 0.1 for Pt, respectively). It is believed that the nanoparticle size and distribution depend on the number and position of thiol groups in the IL.

Metal-catalyzed reactions of diborons for synthesis of organoboron compounds

Abstract: Metal-catalyzed borylation of alkenes, alkynes, arenes, and organic halides with B-B or H-B compounds has been developed for the synthesis of organoboron compounds from simple organic substrates. The platinum(0)-catalyzed addition of bis(pinacolato)diboron to alkenes and alkynes provided a method for the stereoselective synthesis of cis-bis(boryl)alkanes or cis-bis(boryl)alkenes. The addition of diboron to 1,3-dienes with platinum(0) complexes provided a new access to cis-1,4-bis(boryl)-2-butene derivatives, which are versatile reagents for diastereoselective allylboration of carbonyl compounds. The first one-step procedure for the syntheses of aryl-, vinyl-, and allylboronates was achieved via crosscoupling reactions of diborons with aryl and 1-alkenyl halides or triflates and allyl acetates. Direct C-H borylation of arenes catalyzed by a transition metal complex was studied as an economical protocol for the synthesis of a variety of arylboron derivatives. Ir-catalyzed C-H borylation of arenes, heteroarenes, and benzylic positions of alkylarenes by bis(pinacolato)diboron or pinacolborane furnished aryl-, heteroaryl-, and benzylboron compounds. This article discusses the mechanisms of these reactions and their synthetic applications.

Bimetallic palladium-gold dendrimer-encapsulated catalysts.

Abstract: The synthesis, characterization, and catalytic properties of 1-3 nm-diameter bimetallic PdAu dendrimer-encapsulated catalysts are reported. Both alloy and core/shell PdAu nanoparticles were prepared. The catalytic hydrogenation of allyl alcohol was significantly enhanced in the presence of the alloy and core/shell PdAu nanoparticles as compared to mixtures of single-metal nanoparticles.

Palladium nanoparticles by electrospinning from poly(acrylonitrile-co-acrylic acid)-PdCl2 solutions. Relations between preparation conditions, particle size, and catalytic activity

Abstract: Catalytic palladium (Pd) nanoparticles on electrospun copolymers of acrylonitrile and acrylic acid (PAN-AA) mats were produced via reduction of PdCl2 with hydrazine. Fiber mats were electrospun from homogeneous solutions of PAN-AA and PdCl2 in dimethylformamide (DMF). Pd cations were reduced to Pd metals when fiber mats were treated in an aqueous hydrazine solution at room temperature. Pd atoms nucleate and form small crystallites whose sizes were estimated from the peak broadening of X-ray diffraction peaks. Two to four crystallites adhere together and form agglomerates. Agglomerate sizes and fiber diameters were determined by scanning and transmission electron microscopy. Spherical Pd nanoparticles were dispersed homogeneously on the electrospun nanofibers. The effects of copolymer composition and amount of PdCl2 on particle size were investigated. Pd particle size mainly depends on the amount of acrylic acid functional groups and PdCl2 concentration in the spinning solution. Increasing acrylic acid concentration on polymer chains leads to larger Pd nanoparticles. In addition, Pd particle size becomes larger with increasing PdCl2 concentration in the spinning solution. Hence, it is possible to tune the number density and the size of metal nanoparticles. The catalytic activity of the Pd nanoparticles in electrospun mats was determined by selective hydrogenation of dehydrolinalool (3,7-dimethyloct-6- ene-1-yne-3-ol, DHL) in toluene at 90 °C. Electrospun fibers with Pd particles have 4.5 times higher catalytic activity than the current Pd/Al2O3 catalyst.

Selective Hydrogenation by Pd Nanoparticles Embedded in Polyelectrolyte Multilayers

Abstract: Alternating adsorption of poly(acrylic acid) and a polyethylenimine−Pd(II) complex on alumina and subsequent reduction of Pd(II) by NaBH4 yield catalytic Pd nanoparticles embedded in multilayer polyelectrolyte films The polyelectrolytes limit aggregation of the particles and impart catalytic selectivity in the hydrogenation of α-substituted unsaturated alcohols by restricting access to catalytic sites Hydrogenation of allyl alcohol by encapsulated Pd(0) nanoparticles can occur as much as 24-fold faster than hydrogenation of 3-methyl-1-penten-3-ol Additionally, the nanoparticle/polyelectrolyte system suppresses unwanted substrate isomerization, when compared to a commercial palladium catalyst Selective diffusion through poly(acrylic acid)/polyethlyenimine membranes suggests that hydrogenation selectivities are due to different rates of diffusion to nanoparticle catalysts First-order kinetics are also consistent with a diffusion-limited mechanism Further exploitation of the versatility of polyelectrol

Do Quantum Size Effects Control CO Adsorption on Gold Nanoparticles

Abstract: Size effects in adsorption and reactivity of supported metal particles have been observed for at least two decades. In recent years, gold nanoparticles have received attention for their extraordinary catalytic activity for reactions such as low temperature CO oxidation. A number of different theories have been advanced to explain this reactivity in terms of special reaction sites created by the metal–support interface. Goodman's group invoked quantum effects to explain a maximum in CO oxidation activity and suggested that particle thickness, in this case two atomic layers, may be the key parameter. Herein, through a combination of scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and infrared reflection absorption spectroscopy (IRAS), we report the first experimental evidence that thin islands of gold in fact have the same CO adsorption behavior as large gold particles and extended gold surfaces. Therefore observed differences in reactivity of gold nanoparticles are proposed to arise from the presence of highly uncoordinated gold atoms. We have previously found that palladium exhibits twodimensional (2D) growth on a FeO(111) thin film, forming large monolayer islands, which display CO adsorption behavior that is different from bulk palladium. However, we have found in the case of gold that the transition from 2Dto 3D-growth occurs at a very low coverage ( 0.1 monolayers). Therefore, in the present work, we re-examine the situation at these low coverages to determine whether 2D gold structures also show deviations in CO adsorption behavior from the bulk. For coverage up to 0.1 : (effective thickness), gold forms islands of monolayer height (Figure 1a). The inset in Figure 1a shows that these monolayer islands are well shaped. At further increasing coverage the nucleation density remains fairly constant and two-layer particles form. Finally, at highest coverage studied ( 2 :), Au deposits of up to 7 nm in diameter and 4–5 layers in height are seen (Figure 1b). Note that unlike many other cases of nucleation and growth on oxide films the metal particles nucleate on regular sites of the FeO films. This implies that one may study the role of layer thickness independently of the influence of defects.

Palladium-catalyzed C-H homocoupling of thiophenes: facile construction of bithiophene structure.

Abstract: Palladium-catalyzed C−H homocoupling of thiophene derivatives takes place in the presence of silver(I) fluoride or acetate A variety of bithiophenes are obtained in good to excellent yields In particular, the reaction of 2-bromothiophene proceeds at room temperature to afford 5,5‘-dibromo-2,2‘-bithiophene, where the bromine atom is completely intact in the palladium-catalyzed reaction XRD analysis reveals that silver fluoride is reduced to silver(0) during the reaction

Synthesis, structure, theoretical studies, and Ligand exchange reactions of monomeric, T-shaped arylpalladium(II) halide complexes with an additional, weak agostic interaction

Abstract: A series of monomeric arylpalladium(II) complexes LPd(Ph)X (L = 1-AdPtBu2, PtBu3, or Ph5FcPtBu2 (Q-phos); X = Br, I, OTf) containing a single phosphine ligand have been prepared. Oxidative addition of aryl bromide or aryl iodide to bis-ligated palladium(0) complexes of bulky, trialkylphosphines or to Pd(dba)2 (dba = dibenzylidene acetone) in the presence of 1 equiv of phosphine produced the corresponding arylpalladium(II) complexes in good yields. In contrast, oxidative addition of phenyl chloride to the bis-ligated palladium(0) complexes did not produce arylpalladium(II) complexes. The oxidative addition of phenyl triflate to PdL2 (L = 1-AdPtBu2, PtBu3, or Q-phos) also did not form arylpalladium(II) complexes. The reaction of silver triflate with (1-AdPtBu2)Pd(Ph)Br furnished the corresponding arylpalladium(II) triflate in good yield. The oxidative addition of phenyl bromide and iodide to Pd(Q-phos)2 was faster than oxidative addition to Pd(1-AdPtBu2)2 or Pd(PtBu3)2. Several of the arylpalladium complexes were characterized by X-ray diffraction. All of the arylpalladium(II) complexes are T-shaped monomers. The phenyl ligand, which has the largest trans influence, is located trans to the open coordination site. The complexes appear to be stabilized by a weak agostic interaction of the metal with a ligand C-H bond positioned at the fourth-coordination site of the palladium center. The strength of the Pd.H bond, as assessed by tools of density functional theory, depended upon the donating properties of the ancillary ligands on palladium.