Colloidal Palladium Research Articles

N-Heterocyclic Olefin-Ligated Palladium(II) Complexes as Pre-Catalysts for Buchwald-Hartwig Aminations.

New N-heterocyclic olefins (NHOs) are described with functionalization on the ligand heterocyclic backbone and terminal alkylidene positions. Various PdII -NHO complexes have been formed and their use as pre-catalysts in Buchwald-Hartwig aminations was explored. The most active system for catalytic C-N bond formation between hindered arylamine and arylhalide substrates was accessed by combining a backbone methylated NHO with [Pd(cinnamyl)Cl]2 in the presence of NaOtBu as a base. In these active systems evidence suggests that catalysis is mediated by colloidal palladium metal, highlighting a different coordination ability of NHOs in comparison with commonly used N-heterocyclic carbene co-ligands.

Effects of Noncovalent Interactions on the Catalytic Activity of Unsupported Colloidal Palladium Nanoparticles Stabilized with Thiolate Ligands.

This article presents the systematic evaluation of colloidal palladium nanoparticles functionalized with well-defined small organic ligands that provide spatial control of the geometric and electronic surface properties of nanoparticle catalysts. Palladium nanoparticles stabilized with thiolate ligands of different structures and functionalities (linear alkyl vs cyclohexyl vs phenyl) are synthesized using the thiosulfate protocol in a two-phase system. The structure and composition of palladium nanoparticles are characterized using transmission electron microscopy, thermogravimetric analysis, NMR, and UV-vis spectroscopies. The catalysis studies show that the chemical and structural compositions of monolayers surrounding the nanoparticle core greatly influence the overall activity and selectivity of colloidal palladium nanoparticle catalysts for the hydrogenation, isomerization, and hydrogenolysis of allylic alcohols. Especially, noncovalent interactions between surface phenyl ligands and incoming aromatic substrates are found to have a profound influence on the selectivity of colloidal palladium nanoparticles.

Enhanced sensitivity of lateral-flow test strip immunoassays using colloidal palladium nanoparticles and horseradish peroxidase

Although lateral-flow test strip (LFTS) immunoassays are rapid and require no specialized equipment, they are less sensitive than culture and polymerase chain reaction (PCR)-based methods for detecting bacteria. This study compared the sensitivity of LFTS assays based on colloidal palladium nanoparticles (PdNPs) and colloidal gold nanoparticles (AuNPs). PdNPs demonstrated a time and concentration dependent oxidation of the horseradish peroxidase (HRP) substrate 3,3′,5,5′-tetramethylbenzidine (TMB), whereas AuNPs did not. PdNPs labeled HRP-conjugated antibody lateral flow test strip (PdNPs-HRP LFTS) assays were tested with 3,3′-diaminobenzidine (DAB), a water insoluble substrate. The sensitivity of PdNPs-HRP LFTS assays in detecting Listeria monocytogenes, Escherichia coli O157:H7 and Yersinia enterocolitica was 5–10-fold higher than that of AuNPs-based LFTS (AuNPs LFTS) assays. PdNPs-HRP LFTS assays of reduced milk inoculated with L. monocytogenes were more than 10-fold more sensitive than conventional AuNPs LFTS assays. These results suggest that the PdNPs-HRP LFTS immunoassay is a promising tool for ensuring food safety.

Design and Development of Pd‐Catalyzed Aerobic N‐Demethylation Strategies for the Synthesis of Noroxymorphone in Continuous Flow Mode

Strategies for the generation of noroxymorphone from 14‐hydroxymorphinone are presented. Noroxymorphone is the key intermediate in the synthesis of various opioid antagonists, including naloxone, naltrexone, and nalmefene, as well as mixed agonists‐antagonists such as nalbuphine. The transformation requires removal of the N‐methyl group from the naturally occurring opiates and double‐bond hydrogenation. The pivotal reaction step thereby is an N‐methyl oxidation with colloidal palladium(0) as catalyst and pure oxygen as terminal oxidant. The reaction produces a 1,3‐oxazolidine intermediate, which can be readily hydrolyzed to the corresponding secondary amine. Different reaction sequences and the use of various phenol protecting groups were explored. The most direct route consumes only H2, O2, and H2O as stoichiometric reagents and produces only H2O as a byproduct. Challenges inherent to gas/liquid reactions with oxygen as oxidant have been addressed by developing a continuous flow process.

Active Palladium Colloids via Palladacycle Degradation as Efficient Catalysts for Oxidative Homocoupling and Cross-Coupling of Aryl Boronic Acids

Active palladium colloids formed upon degradation of a palladacyclic complex (Herrmann–Beller 1) have been isolated for the first time and thoroughly characterized with techniques such as transmission electron microscopy (TEM), high-resolution TEM, X-ray photoelectron spectroscopy (XPS), and extended X-ray absorption fine structure spectroscopy. The synthesized palladium colloids have been utilized as efficient catalysts for the oxidative homocoupling of aryl boronic acids. Cross-coupling of two different aryl boronic acids has also been made possible using these active palladium colloids. This is the first report of this kind of coupling between aryl boronic acids.

Dissimilar catalytic behavior of molecular or colloidal palladium systems with a new NHC ligand.

In this work, we describe the synthesis of a new N-heterocyclic carbene (NHC) ligand, derived from a hybrid pyrazole-imidazolium scaffold, namely 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-2-ylidene (L). This ligand has been used as a stabilizer for the organometallic synthesis of palladium(0) nanoparticles (Pd NPs). L presents a better stabilizing effect than its pre-carbenic HLCl counterpart, allowing the formation of isolated Pd NPs while HLCl yields aggregated ones. Additionally, molecular Pd(ii) coordination compounds of L and HLCl were synthesized and characterized to better understand the coordination modes of these ligands. Both molecular and colloidal Pd systems have been further tested in catalytic C-C coupling processes. Three different types of reactions have been observed depending on the catalytic system: (i) the Suzuki-Miyaura reaction takes place with Pd molecular complexes; (ii) a secondary reaction, the dehalogenation of the substrate, is always detected and (iii) the C-C homocoupling between two molecules of bromoarenes is observed with colloidal catalysts.

Toward the Synthesis of Noroxymorphone via Aerobic Palladium-Catalyzed Continuous Flow N-Demethylation Strategies

The palladium-catalyzed N-demethylation of the opioid alkaloids oxymorphone 3,14-diacetate and 14-hydroxymorphinone 3,14-diacetate to their nor-derivatives with oxygen as the terminal oxidant has been investigated. Palladium(II) acetate forms colloidal palladium(0) particles upon heating in N,N-dimethylacetamide. The palladium(0) particles are effective catalysts for the aerobic N-demethylation of these opiate alkaloids. Demethylation of 14-hydroxymorphinone 3,14-diacetate with pure oxygen as oxidant in a continuous flow reactor provided the demethylated product with excellent selectivity after residence times of only 10–20 min with 2.5–5 mol % palladium acetate as catalyst on a laboratory scale. Scale-up of the oxidation in a 100 mL flow reactor (combination of FlowPlate A6 and coiled tube to enhance the gas–liquid mass transfer), hydrogenation in a packed bed reactor, and subsequent hydrolysis afforded the desired noroxymorphone in high quality and good yield on a kg scale. The reaction sequence consume...

Palladium on Carbon-Catalyzed Silane-Reduction of Tertiary Carboxamides: Soluble Palladium Colloids are an Active Catalyst Species

The combination of commercially available palladium on carbon (Pd/C) with 1,1,3,3-tetramethyldisiloxane (TMDS) was found to be an effective catalyst system for the production of tertiary amines by deoxygenative silane-reduction of carboxamides. Facile removal of both the metallic species and silane residues can be achieved by simple extraction, and the recovered Pd/C was reusable. Other palladium complexes and salts, Pd2(dba)3(CHCl3) [dba = dibenzylideneacetone], Pd(OAc)2, and PdCl2, were also effective for the reduction of tertiary carboxamides with TMDS. Detailed mechanistic studies of the present Pd/C–TMDS system revealed that the catalytically active species is the soluble palladium colloids leached from the palladium particles on the carbon support by treating with TMDS, and carboxamides, amine products, and TMDS act as a stabilizer for the generated palladium species in solution.

Batch- and Continuous-Flow Aerobic Oxidation of 14-Hydroxy Opioids to 1,3-Oxazolidines-A Concise Synthesis of Noroxymorphone.

14-Hydroxymorphinone is converted to noroxymorphone, the immediate precursor of important opioid antagonists, such as naltrexone and naloxone, in a three-step reaction sequence. The initial oxidation of the N-methyl group in 14-hydroxymorphinone with in situ generated colloidal palladium(0) as the catalyst and molecular oxygen as the terminal oxidant constitutes the key transformation in this new route. This oxidation results in the formation of an unexpected oxazolidine ring structure. Subsequent hydrolysis of the oxazolidine under reduced pressure followed by hydrogenation in a packed-bed flow reactor using palladium(0) as the catalyst provides noroxymorphone in high purity and good overall yield. To overcome challenges associated with gas-liquid reactions with molecular oxygen, the key oxidation reaction was translated to a continuous-flow process.

Graphite furnace atomic absorption spectrometry for determination of thallium in blood

Colloidal palladium was used as chemical modifier in the determination of blood thallium by graphite furnace atomic absorption spectrometry. Blood samples were precipitated with 5% (V/V)nitric acid, and then determined by GFAAS with colloidal palladium used as a chemical modifier. 0.2% (W/V)sodium chloride was added in the standard series to improve the matrix matching between standard solution and sample. The detection limit was 0.2 μg/L. The correlation coefficient was 0.9991. The recoveries were between 93.9% to 101.5%.The relative standard deviations were between 1.8% to 2.7%.The certified reference material of whole blood thallium was determined and the result was within the reference range Conclusion: The method is accurate, simple and sensitive, and it can meet the needs of detection thallium in blood entirely.

Investigation on Process Parameters on the Adhesion of Electroless Ni-P Film on Silane Compound Modified Si Wafer

We previously reported a novel polyvinyl alcohol-capped palladium colloids (PVA-Pd) as the catalyst for the electroless plating of nickel-phosphorous (ELP-Ni-P) film on 3-2-(2-aminoethylamino) ethylamino propyl trimethoxysilane (ETAS)-modified Si surface [1]. It was found the interaction between PVA-Pd and amino-moiety on ETAS is the root cause of the superior adhesion of ELP-Ni-P film without the need of post annealing. The adhesion of ELP-Ni-P film catalyzed by PVA-Pd on ETAS-modified Si surface is significantly improved from 4.63 MPa to 10.83MPa when compared with it made via traditional Sn/Pd process. In this study, the effect of process parameters in this method including annealing temperature, annealing time, ETAS dipping time, PVA-Pd concentration and PVA-Pd dipping time are systematically investigated by means of design of experiments (DOE). From the analysis of variance (ANOVA) table, it is found the annealing temperature, annealing time and ETAS-dipping time have main effects on the adhesion of ELP-Ni-P film. Surprisingly, SEM cross-sectional image of the ELP-Ni-P film made by this method reveals that imperfect self-assembly monolayer (SAM) formation of ETAS aids the formation of nickel-silicide intermetallic (IMC) layer, rendering an even better adhesion before annealing. On the contrary, the adhesion of ELP-Ni-P film deposited on perfect ETAS monolayer becomes worse after post annealing. We speculate that silane-compound may play as a barrier layer for the IMC formation, which may jeopardize the original function of Ni-P film as the barrier layer for the subsequent copper layer in semiconductor industry. Reference: [1] T.-C. Wei, T.-C. Pan, C.-M. Chen, K.-C. Lai, and C.-H. Wu, "Annealing-free adhesive electroless deposition of a nickel/phosphorous layer on a silane-compound-modified Si wafer," Electrochemistry Communications, vol. 54, pp. 6-9, 2015. Figure 1

Observation of a crossover in kinetic aggregation of Palladium colloids

We use field emission scanning electron microscope (FE-SEM) to investigate the growth of palladium colloids over the surface of thin films of WO3/glass. The film is prepared by Pulsed Laser Deposition (PLD) at different temperatures. A PdCl2 (aq) droplet is injected on the surface and in the presence of steam hydrogen the droplet is dried through a reduction reaction process. Two distinct aggregation regimes of palladium colloids are observed over the substrates. We argue that the change in aggregation dynamics emerges when the measured water drop Contact Angel (CA) for the WO3/glass thin films passes a certain threshold value, namely CA≈46°, where a crossover in kinetic aggregation of palladium colloids occurs. Our results suggest that the mass fractal dimension of palladium aggregates follows a power-law behavior. The fractal dimension (Df) in the fast aggregation regime, where the measured CA values vary from 27° up to 46° according to different substrate deposition temperatures, is Df=1.75(±0.02) – the value of Df is in excellent agreement with kinetic aggregation of other colloidal systems in fast aggregation regime. Whereas for the slow aggregation regime, with CA=58°, the fractal dimension changes abruptly to Df=1.92(±0.03). We have also used a modified Box-Counting method to calculate fractal dimension of gray-level images and observe that the crossover at around CA≈46° remains unchanged.

Size control and supporting of palladium nanoparticles made by laser ablation in saline solution as a facile route to heterogeneous catalysts

In the literature many investigations on colloidal stability and size control of gold nanoparticles are shown but less for ligand-free palladium nanoparticles, which can be promising materials in various applications. Palladium nanoparticles are perspective materials for a manifold of energy application like photo- and electrocatalysis or hydrogen storage. For this purpose, size-controlled nanoparticles with clean surfaces and facile immobilization on catalyst supports are wanted. Laser ablation in saline solution yields ligand-free, charged colloidal palladium nanoparticles that are supported by titania and graphene nanosheets as model systems for photo- and electrocatalysis, respectively. By adjusting the ionic strength during laser ablation in liquid, it is possible to control stability and particle size without compromising subsequent nanoparticle adsorption of supporting materials. A quantitative deposition of nearly 100% yield with up to 18wt% nanoparticle load was achieved. The average size of the laser-generated nanoparticles remains the same after immobilization on a support material, in contrast to other preparation methods of catalysts. The characterization by X-ray photoelectron spectroscopy reveals a redox reaction between the immobilized nanoparticles and the graphene support.

Unusual stability of dyads during photochemical hydrogen production.

Dyads for photochemical water splitting often suffer from instability during irradiation with visible light. However, the use of bis(bidentate) phosphines forming a five-membered ring enhances their stability. The coordination of these phosphor based chelates to soft metals like Pd(ii) prolongs the photocatalytic activity to 1000 hours. To avoid contribution to hydrogen production by colloidal metal, a small amount of Hg is added to the reaction mixture. In the course of our investigations, it turned out that colloidal palladium was not able to produce hydrogen under our irradiation conditions. As soon as metallic palladium emerged in our reaction vessels, no further hydrogen production was detected. This is confirmed by the observation that the hydrogen production depends on the kind of ancillary ligands present in the dyads. The first dyads of the type [MI(bpy)2(dppcb)MII(bpy)](4+) are presented (MI = Os, MII = Pd (1); MI = Ru, MII = Pd (2); MI = Os, MII = Pt (3); MI = Ru, MII = Pt (4)). In [Os(bpy)2(dppcb)Pd(dppm)](PF6)4 (5) the ancillary ligand is varied. Furthermore, it is also possible to produce hydrogen in an intermolecular way. Using different bidentate diphosphines instead of a bis(bidentate) tetraphosphine leads to this intermolecular approach, where the chromophore and the water reduction catalyst (WRC) belong now to two molecules. In this case the TON is sensitive to the type of diphosphine, which is only possible if intact molecules act as catalysts and no free palladium(0) is formed.

An ultra-light and high electromagnetic shielding effectiveness material based on melamine foam with its skeleton metallized

In this paper, an ultra-light and high electromagnetic shielding effectiveness (SE) composite material based on melamine foam with its skeleton metallized was prepared via electroless silver plating techniques. Salt-based colloid palladium was applied as the activator to modify the foam skeleton. X-ray photoelectron spectroscopy was used for tracking the Pd activated process. The effects of silver plating time, dynamic and static electroless plating conditions on the surface morphology, electrical and EMI shielding properties were investigated. The silver content in the foams subjected to electroless plating for different time was measured by thermo gravimetric analysis. Crystal structure of the silver nanoparticles was examined by the X-ray diffraction (XRD). Results show that a dense, uniform, consecutive silver layer forms on the surface of foam skeleton with the electroless silver plating time increases. A relatively dynamic electroless plating conditions do not help to construct a complete silver layer without cracks. The conductivity and SE of the foams is enhanced with the extension of silver plating time. The foam just containing 63.5 wt% silver achieves a minimum resistivity of 0.06 Ω cm and a SE of above 90 dB at the frequency range of 0.05–18 GHz. The foam prepared under a dynamic electroless plating condition exhibits lower SE than that under a static electroless plating condition. The XRD pattern shows the deposited silver coating exists as a typical face-centered cubic crystalline structure and the average crystal size is 14.4 nm. All results imply that such conductive melamine foam exhibits excellent electrical and EMI shielding properties, which can be used in a broad application range.

Detection of digoxin in urine samples by surface-assisted laser desorption/ionization mass spectrometry with dispersive liquid–liquid microextraction

A novel method for the detection of digoxin using dispersive liquid–liquid microextraction (DLLME) coupled to the surface-assisted laser desorption/ionization mass spectrometric detection (SALDI/MS) was developed. Acetone and chloroform were used as the disperser solvent and extraction solvents, respectively. After the extraction, digoxin was detected using SALDI/MS with colloidal palladium as the matrix. Under optimal extraction and detection conditions, the calibration curve, which ranged from 0.01 to 0.50μM, was observed to be linear. The limit of detection (LOD) at a signal-to-noise ratio of 3 was 2nM for digoxin. With a sample-to-extract volume ratio of 400, the enrichment factor for digoxin was calculated to be 252. This novel method was successfully applied for the determination of digoxin in human urine samples.

ChemInform Abstract: Distinguishing Between the Homogeneous and Heterogeneous Mechanisms of Catalysis in the Mizoroki—Heck and Suzuki—Miyaura Reactions: Problems and Prospects

AbstractReview: 124 refs.

Influence of a Surfactant and Reducing Agent on Preparation of Palladium

We investigated the influence of a surfactant and reducing agent on the preparation of palladium nanoparticles, characterizing the prepared palladium nanoparticles by Transmission Electron Microscopy (TEM), and X-ray diffractometer (XRD). Colloidal palladium nanoparticles are prepared using PdCl2 as a precursor and polyvinylpyrrolidone (PVP) as a surfactant. In addition, hydrogen and sodium borohydride are used as reducing agents are used to characterize the prepared palladium nanoparticles. The concentrations of a precursor, a surfactant and its molecular weight affect the size of palladium nanoparticles, while the reducing agents influence the morphologies and structures of palladium nanoparticles.

A Simple Method for Producing Colloidal Palladium Nanocrystals: Alternating Voltage‐Induced Electrochemical Synthesis

Alternating voltage-induced electrochemical synthesis (AVIES) produces well-dispersed, size-controlled, single-crystalline, colloidal palladium nanocrystals (Pd-NCs). An alternating voltage is applied to two Pd wires inserted in an electrolyte solution containing capping ligands. Pd-NCs are directly ejected from the Pd electrodes through cathodic reduction of the PdO intermediates. The obtained Pd-NCs are soluble in either polar or non-polar solvents, depending on the selected capping ligands.

Preparation of Nano Pd/C Catalyst for Purification of Terephthalic Acid by Sol-Gel Method

The traditional method of preparing 0.5 wt.% nano Pd/C catalysts used for pure terephthalic acid(PTA) refining is the precipitate-reduction method, and this paper proposed the sol-gel method, in which the palladium colloid was firstly prepared, and followed by the load process onto the activated carbon. The activity and microstructure of catalysts were investigated by means of SEM, TEM, EDS，XRD and HPLC. It was found that the activity of catalyst with the sol-gel method was much higher than that with precipitate-reduction method. The preparation technology has less effects on the micostructure of Pd/C catalysts, while the palladium particles of catalyst obtained with sol-gel method are much finer and the relatively palladium content is much higher.These two factors contribute to the higher activity of Pd/C catalyst.