Selectivity and Reactivity of Alkylamine- and Alkanethiolate-Stabilized Pd and PdAg Nanoparticles for Hydrogenation and Isomerization of Allyl Alcohol

Water-Soluble Pd Nanoparticles for the Anti-Markovnikov Oxidation of Allyl Benzene in Water

The catalytic activity and selectivity of two different water-soluble palladium nanoparticles capped with 5-(trimethylammonio)pentanethiolate and 6-(carboxylate)hexanethiolate ligands are investigated using the catalytic reaction of allyl benzene. The results show that the regioselective transformation of allyl benzene to 3-phenylpropanal occurs at room temperature and under atmospheric pressure in neat water via a Tsuji-Wacker type oxidation. Conventionally, the Tsuji-Wacker oxidation promotes the Markovnikov oxidation of terminal alkenes to their respective ketones in the presence of dioxygen. Water-soluble Pd nanoparticles, however, catalyze the anti-Markovnikov oxidation of allyl benzene to 3-phenylpropanal in up to 83% yields. Catalytic results of other aromatic alkenes suggest that the presence of benzylic hydrogen is a key to the formation of a p-allyl Pd intermediate and the anti-Markovnikov addition of H₂O. The subsequent b-H elimination and tautomerization contribute to the formation of aldehyde products. Water-soluble Pd nanoparticles are characterized using nuclear magnetic resonance (NMR), UV-vis spectroscopy, thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). Catalysis results are examined using ¹H NMR and/or GC-MS analyses of isolated reaction mixtures.

One-Pot Au@Pd Dendritic Nanoparticles as Electrocatalysts with Ethanol Oxidation Reaction

The one-pot synthesis strategy of Au@Pd dendrites nanoparticles (Au@Pd DNPs) was simply synthesized in a high-temperature aqueous solution condition where cetyltrimethylammonium chloride (CTAC) acted as a reducing and capping agent at a high temperature. The Au@Pd DNPs with highly monodisperse were shown in high yields by the Au:Pd rate. The nanostructure and optical and crystalline properties of the Au@Pd DNPs were characterized by UV–vis spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction. The Au@Pd DNPs showed an efficient electrochemical catalytic performance rate toward the ethanol oxidation reaction (EOR) due to their nanostructures and Au:Pd rate.

Redox-neutral remote amidation of alkenyl alcohols via long-range isomerization/transformation

A facile and straightforward approach for the construction of amides via redox-neutral Ru-catalyzed cross-coupling reaction of long-range alkenyl alcohols with amines to realize remote site-selective functionalization has been developed.

Synthesis and characterization of bis(amine)palladium(II) carboxylate complexes as precursors of palladium nanoparticles

The synthesis and characterization of the adducts of n-alkyl amine and palladium n-alkyl carboxylate, [Pd(R2NH2)2(R1COO)2] (R1 = 1, 7, and 11; R2 = 8, 12, and 16), as precursors for the synthesis of palladium nanoparticles (PdNPs) was carried out via differential scanning calorimetry, FT-IR, Raman and UV-Vis spectroscopy, NMR spectroscopy (1H, 13C pulsed field gradient spin-echo (PGSE), and 13C CP-MAS), and powder X-ray diffraction. Pd n-alkyl carboxylates were obtained by a ligand exchange reaction from palladium acetate and the appropriate aliphatic carboxylic acid. It is proposed that carboxyl moieties in the presence of amine ligands are bound to palladium ions via monodentate bonding as opposed to bridging bidentate coordination of pure palladium carboxylate which exists in the form of polymer aggregates. All the studied palladium carboxylate/amine complexes form bilayer lamellar structures and exhibit first-order melting transitions. The evidence presented in this study shows that the phase behavior of bivalent metal carboxylates is mainly controlled by the type of coordination of carboxylate head groups. For n-alkyl carboxylates, linear chain type aggregates replace the trimeric units of Pd acetate. In solution, in the presence of amine, palladium salt aggregates disintegrate and the Pd complex is isolated and stabilized by amine molecules. Using bis(amine) palladium carboxylate adducts as precursors, palladium nanoparticles were fabricated. During high temperature thermolysis, the bis(amine) Pd carboxylate complex decomposes to form small sized Pd nanoparticles. Combining NMR techniques with FTIR spectroscopy, it was possible to follow an original stabilization mechanism. PdNPs are stabilized by weakly interacting long chain aliphatic amide and carboxylic acid derived from the palladium precursor.

Ru0·Ru n+/Al2O3 as a Versatile Catalyst in the Isomerization of Allyl Alcohol

This study focuses on examining the isomerization of allyl alcohol using ruthenium (Ru) supported on alumina as a heterogeneous catalyst. The synthesized Ru/Al solids were characterized by various characterization techniques. The content of Ru was estimated by the energy dispersive x-ray technique. The x-ray diffraction (XRD) confirmed the presence of phases in the support and active species in the catalysts. The surface area of the support after Ru impregnation and the pore volume were determined by nitrogen physisorption. The analysis of programmed temperature (TPR and TPO) shows different redox sites which is confirmed by XPS. The catalytic results suggest a dependence on the amount of available metallic Ru, as well as the importance of the continuous regeneration of the metal using H2 to achieve a good conversion of the allyl alcohol. For comparison purposes, the commercial Ru on alumina 5% (CAS 908142) was used. The results show up to 68% alcohol conversion and 27% yield of the isomerization product using Ru(1,5.4h)/Al catalyst in comparison with 86% conversion and 39% yield of the isomerization product using CAS 908142. In contrast, our catalysts always presented higher TOF values (149-160) in comparison with CAS 908142 (101).

Mechanistic Study of Silane Alcoholysis Reactions with Self-Assembled Monolayer-Functionalized Gold Nanoparticle Catalysts

The self-assembled monolayer (SAM)-modified metallic nanoparticles (MNPs) often exhibit improved chemoselectivity in various catalytic reactions by controlling the reactants’ orientations adsorbed in the SAM; however, there have been a few examples showing that the reaction rate, i.e., catalytic activity, is enhanced by the SAM-modification of MNP catalysts. The critical parameters that affect the catalytic activity, such as the supports, nanoparticle size, and molecular structures of the SAM components, remain uninvestigated in these sporadic literature precedents. Here, we report the mechanistic investigation on the effects of those parameters on the catalytic activity of alkanethiolate SAM-functionalized gold nanoparticles (AuNPs) toward silane alcoholysis reactions. The evaluation of the catalytic reaction over two-dimensionally arrayed dodecanethiolate SAM-functionalized AuNPs with different supports revealed the electronic interactions between AuNPs and the supports contributing to the rate enhancement. Additionally, an unprecedented size effect appeared—the AuNP with a 20 nm radius showed higher catalytic activity than those at 10 and 40 nm. Infrared reflection–absorption spectroscopy revealed that the conformational change of alkyl chains of the SAM affects the entrapment of reactants and products inside the SAM, and therefore brings about the acceleration effect. These findings provide a guideline for further applying the SAM-functionalization technique to stereoselective organic transformations with designer MNP catalysts.

Iodine activation: a general method for catalytic enhancement of thiolate monolayer-protected metal clusters

To enhance catalytic activity, the present study details a general approach for partial thiolate ligand removal from monolayer-protected clusters (MPCs) by straightforward in situ addition of iodine. Two model reactions are examined to illustrate the effects on the catalytic activity of glutathione (SG)-capped Au MPCs serving as a catalyst for the NaBH4 reduction of 4-nitrophenol to 4-aminophenol and SG-capped Pd MPCs serving as a catalyst for the hydrogenation/isomerization of allyl alcohol. Iodine addition promoted partial thiolate ligand removal from both MPCs and improved the catalytic properties, presumably due to greater surface exposure of the metal cores as a result of ligand dissociation. The rate of 4-nitrophenol reduction increased from 0.066 min-1 in the absence of I2 to 0.505 min-1 in the presence of 2.0 equivalents I2 (equivalents based on total ligated glutathione). The reaction of allyl alcohol to produce 1-propanol and propanal was similarly accelerated as indicated by the increase in turnover frequency from 131 to 230 moles products per moles catalyst per h by addition of 0.2 equivalents I2. In both reactions, as the amount of I2 added increases the catalyst recyclability decreases due to catalyst instability. Low equivalents of I2 are optimal when considering both reaction rate and catalyst recyclability.

The effect of phenylalanine ligands on the chiral-selective oxidation of glucose on Au(111)

As typical glucose oxidase nanozymes, gold nanoparticles (Au NPs) have attracted much attention due to their wide-ranging applications. Ligand caps, as the "cure-all solution" for NPs, not only play important roles in the size and shape control of Au NPs but also influence their catalytic activity and selectivity. A deep understanding of the catalytic mechanism and precise description of the important role of ligands can provide possible ways to design functional Au NPs. Here, with the specific example of Au(111) capped with chiral phenylalanine (Phe), the chiral selective oxidation mechanism of glucose and the important role of the ligands were studied via first-principles calculations. All results show that the dehydrogenation of glucose to form glucono delta-lactone (GDL) is favored on clean Au(111), while the subsequent hydrolysis of GDL is the rate-limiting step for glucose oxidation. The flat and nonchiral Au(111) surface shows negligible selectivity in relation to the oxidation of d- and l-glucose, while chiral Phe-Au(111) shows selective adsorption towards d- and l-glucose. l-Phe-capped Au(111) prefers to adsorb d-glucose, while d-Phe-capped Au(111) prefers to adsorb l-glucose. Considering the three steps in the capped ligand catalysis (adsorption, replacement and reaction), we propose that the ligands play key roles in selectively adsorbing reactants before the subsequent exchange and reaction steps.

Synthesis of Alkanethiolate-Capped Metal Nanoparticles Using Alkyl Thiosulfate Ligand Precursors: A Method to Generate Promising Reagents for Selective Catalysis

Evaluation of metal nanoparticle catalysts functionalized with well-defined thiolate ligands can be potentially important because such systems can provide a spatial control in the reactivity and selectivity of catalysts. A synthetic method utilizing Bunte salts (sodium S-alkylthiosulfates) allows the formation of metal nanoparticles (Au, Ag, Pd, Pt, and Ir) capped with alkanethiolate ligands. The catalysis studies on Pd nanoparticles show a strong correlation between the surface ligand structure/composition and the catalytic activity and selectivity for the hydrogenation/isomerization of alkenes, dienes, trienes, and allylic alcohols. The high selectivity of Pd nanoparticles is driven by the controlled electronic properties of the Pd surface limiting the formation of Pd⁻alkene adducts (or intermediates) necessary for (additional) hydrogenation. The synthesis of water soluble Pd nanoparticles using ω-carboxylate-S-alkanethiosulfate salts is successfully achieved and these Pd nanoparticles are examined for the hydrogenation of various unsaturated compounds in both homogeneous and heterogeneous environments. Alkanethiolate-capped Pt nanoparticles are also successfully synthesized and further investigated for the hydrogenation of various alkynes to understand their geometric and electronic surface properties. The high catalytic activity of activated terminal alkynes, but the significantly low activity of internal alkynes and unactivated terminal alkynes, are observed for Pt nanoparticles.

Encapsulation of Crabtree's Catalyst in Sulfonated MIL-101(Cr): Enhancement of Stability and Selectivity between Competing Reaction Pathways by the MOF Chemical Microenvironment

Crabtree's catalyst was encapsulated inside the pores of the sulfonated MIL-101(Cr) metal-organic framework (MOF) by cation exchange. This hybrid catalyst is active for the heterogeneous hydrogenation of non-functionalized alkenes either in solution or in the gas phase. Moreover, encapsulation inside a well-defined hydrophilic microenvironment enhances catalyst stability and selectivity to hydrogenation over isomerization for substrates bearing ligating functionalities. Accordingly, the encapsulated catalyst significantly outperforms its homogeneous counterpart in the hydrogenation of olefinic alcohols in terms of overall conversion and selectivity, with the chemical microenvironment of the MOF host favouring one out of two competing reaction pathways.

Photocatalytic Transfer Hydrogenolysis of Allylic Alcohols on Pd/TiO2 : A Shortcut to (S)-(+)-Lavandulol

We report herein a regio- and stereoselective photocatalytic hydrogenolysis of allylic alcohols to form unsaturated hydrocarbons employing a palladium(II)-loaded titanium oxide; the reaction proceeds at room temperature under light irradiation without stoichiometric generation of salt wastes. Olefin and saturated alcohol moieties tolerated the reaction conditions. Hydrogen atoms were selectively incorporated into less sterically congested carbons of the allylic functionalities. This protocol allowed a short-step synthesis of (S)-(+)-lavandulol from (R)-(-)-carvone by avoiding otherwise necessary protection/deprotection steps.

Tuning the Catalytic Activity and Selectivity of Pd Nanoparticles Using Ligand-Modified Supports and Surfaces

The organic moiety plays an essential role in the design of homogeneous catalysts, where the ligands are used to tune the catalytic activity, selectivity, and stability of the transition metal centers. The impact of ligands on the catalytic performance of metal nanoparticle catalysts is still less understood. Here, we prepared supported nanoparticle (NP) catalysts by the immobilization of preformed Pd NPs on the ligand-modified silica surfaces bearing amine, ethylenediamine, and diethylenetriamine groups. After excluding any size effect, we were able to study the influence of the ligands grafted on the support surface on the catalytic activity of the supported nanoparticles. Higher activity was observed for the Pd NPs supported on propylamine-functionalized support, whereas the presence of ethylenediamine and diethylenetriamine groups was detrimental to the activity. Upon the addition of excess of these amine ligands as surface modifiers, the hydrogenation of alkene to alkane was fully suppressed and, therefore, we were able to tune Pd selectivity. The selective hydrogenation of alkynes into alkenes, although a considerable challenge on the traditional palladium catalysts, was achieved here for a range of alkynes by combining Pd NPs and amine ligands.

Facile Fabrication of Ordered Component-Tunable Heterobimetallic Self-Assembly Nanosheet for Catalyzing "Click" Reaction

How to maximize the number of desirable active sites on the surface of the catalyst and minimize the number of sites promoting undesirable side reactions is currently an important research topic. In this study, a new way based on the synergism to achieve the successful fabrication of an ordered heterobimetallic self-assembled monolayer (denoted as BMSAM) with a controlled composition and an excellent orientation of metals in the monolayer was developed. BMSAM consisting of phenanthroline and Schiff-base groups was prepared, and its novel heterobimetallic (Cu and Pd) self-assembled monolayer anchored in silicon (denoted as Si-Fmp-Cu-Pd BMSAM) with a controlled composition and a fixed position was fabricated and characterized by UV, cyclic voltammetry, Raman, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and water-drop contact angle (WDCA) analyses. The effects of Si-Fmp-Cu-Pd BMSAM on its catalytic properties were also systematically investigated using "click" reaction as a template by WDCA, XPS, SEM, XRD, ICP-AES and in situ Fourier transform infrared analyses in a heterogeneous system. The results showed that the excellent catalytic characteristic could be attributed to the partial (ordered or proper distance) isolation of active sites displaying high densities of specific atomic ensembles. The catalytic reaction mechanism of the click reaction interpreted that the catalytic process mainly occurred on the surface of the monolayer, internal active site (Pd) and rationalized that the Cu(I) species and Pd(0) reduced from the Cu(II) and Pd(II) catalyst were active species, which had a proper distance between two different metals. The cuprate-triazole intermediate and the palladium intermediate, whose production is the key step, should lie in a proper position between the copper and active palladium sites, with which the reaction rate of transmetalation would be improved to increase the amount of the undesired Sonogashira coupling product.

Preparation of Partially Poisoned Alkanethiolate-Capped Platinum Nanoparticles for Hydrogenation of Activated Terminal Alkynes

Stable and isolable alkanethiolate-stabilized Pt nanoparticles (PtNP) were synthesized using the two-phase thiosulfate method with sodium S-alkylthiosulfate as ligand precursor. The mechanistic formation of octanethiolate-capped PtNP (Pt-SC₈) from both sodium S-octylthiosulfate and 1-octanethiol ligands was investigated by using ¹H NMR and UV-vis spectroscopies, which revealed the formation of different Pt complexes as the reaction intermediates. The synthesis using S-octylthiosulfate ligand precursor produced Pt-SC₈ in higher yields than that using 1-octanethiol ligand. The obtained nanoparticles were characterized by ¹H NMR, UV-vis spectroscopy, infrared spectroscopy (IR), thermogravimetric analysis, and transmission electron microscopy (TEM). The results obtained from ¹H NMR, IR, and UV-vis spectroscopy were consistent with the formation of stable and pure alkanethiolate-capped PtNP. TEM images of PtNP confirmed their small average core size (∼1.5 nm) and high monodispersity. The partially poisoned PtNP with thiolate monolayer ligands were further investigated for the hydrogenation of various alkynes to understand the organic ligands-induced geometric and electronic surface properties of colloidal Pt nanoparticle catalysts. The high catalytic activity of activated terminal alkynes, but the significantly low activity of internal alkynes and unactivated terminal alkynes, were observed under the mild reaction conditions (room temperature and atmospheric pressure). These results indicated that the presence of alkanethiolate ligands could decrease the coordination activity of PtNP surface especially for the bulkier and unactivated substrates.

Unsupported Micellar Palladium Nanoparticles for Biphasic Hydrogenation and Isomerization of Hydrophobic Allylic Alcohols in Water

This article presents the evaluation of water-soluble palladium nanoparticles with hydrophobic active sites that are ideal for the biphasic colloidal catalysis of water-insoluble organic substrates in aqueous solution. Palladium nanoparticles stabilized with ω-carboxylate-functionalized alkanethiolate are first synthesized using ω-carboxylate-S-alkylthiosulfate as their ligand precursor. The biphasic catalysis is carried out for the reaction of hydrophobic allylic alcohols without using any additional mixing solvent or surfactant, which results in the complete consumption of substrates under the atmospheric pressure of H₂ gas and at room temperature in less than 24 h. Systematic investigations on the influence of pH and substrate size are also performed to examine the utility of these thiolate-capped palladium nanoparticles as structurally stable and water-soluble micellar catalysts for the biphasic reaction.

Multicatalytic colloids with highly scalable, adjustable, and stable functionalities in organic and aqueous media

Despite a large number of developments of noble metal (or metal oxide) NP-based catalysts, it has been a great challenge to prepare high-performance recyclable catalysts with integrated functionalities that can be used in various solvent media. Here, we report on layer-by-layer (LbL) assembled multicatalysts with high catalytic performance, showing high dispersion and recycling stability in organic and aqueous media. The remarkable advantages of our approach are as follows. (i) Various metal or metal oxide NPs with desired catalytic performance can be easily incorporated into multilayered shells, forming densely packed arrays that allow one colloid to be used as a multicatalyst with highly integrated and controllable catalytic properties. (ii) Additionally, the dispersion stability of catalytic colloids in a desired solvent can be determined by the type of ultrathin outermost layer coating each colloid. (iii) Lastly, the covalent bonding between inorganic NPs and dendrimers within multilayer shells enhances the recycling stability of multicatalytic colloids. The resulting core-shell colloids including OA-Fe3O4 NPs, TOABr-Pd NPs, and OA-TiO2 NPs exhibited excellent performance in the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) and photocatalysis in aqueous media and in the Sonogashira coupling reaction (99% yield) in organic media. Given that the catalytic properties of recyclable colloids reported to date have entirely depended on the functionality of a single catalytic NP layer deposited onto colloids in selective solvent media, our approach provides a basis for the design and exploitation of high-performance recyclable colloids with integrated multicatalytic properties and high dispersion stability in a variety of solvents.

Pd/Al2O3-catalysed redox isomerisation of allyl alcohol: application in aldol condensation and oxidative heterocyclization reactions

The application of the Pd/Al₂O₃ catalyst in allyl alcohol isomerization and subsequent aldol-condensation and heterocyclization reactions is described.

Catalytic Properties of Unsupported Palladium Nanoparticle Surfaces Capped with Small Organic Ligands

This Minireview summarizes a variety of intriguing catalytic studies accomplished by employing unsupported, either solubilized or freely mobilized, and small organic ligand-capped palladium nanoparticles as catalysts. Small organic ligands are gaining more attention as nanoparticle stabilizers and alternates to larger organic supports, such as polymers and dendrimers, owing to their tremendous potential for a well-defined system with spatial control in surrounding environments of reactive surfaces. The nanoparticle catalysts are grouped depending on the type of surface stabilizers with reactive head groups, which include thiolate, phosphine, amine, and alkyl azide. Applications for the reactions such as hydrogenation, alkene isomerization, oxidation, and carbon-carbon cross coupling reactions are extensively discussed. The systems defined as "ligandless" Pd nanoparticle catalysts and solvent (e.g. ionic liquid)-stabilized Pd nanoparticle catalysts are not discussed in this review.

Nanosilver as a new generation of silver catalysts in organic transformations for efficient synthesis of fine chemicals

Silver nanoparticles catalysis has been of great interest in organic synthesis and has expanded rapidly in the past ten years because of nanosilver catalysts' unique reactivity and selectivity, stability, as well as recyclability in catalytic reactions.

Efficient catalytic activation of Suzuki–Miyaura C–C coupling reactions with recyclable palladium nanoparticles tailored with sterically demanding di-n-alkyl sulfides

The catalytic and other properties of didocosyl sulfide protected NPs depend on Pd compounds (best: Na₂PdCl₄) used for their synthesis.

Hydrogenation of allyl alcohols catalyzed by aqueous palladium and platinum nanoparticles

Hydrogenations of allyl alcohols in aqueous media, at room temperature and pressure, were performed in presence of Pd or Pt lignin nanoparticles as catalysts to obtain saturated alcohols.