Palladium Species Research Articles

Palladium catalyzed divergent cycloadditions of vinylidenecyclopropane-diesters with methyleneindolinones enabled by zwitterionic π-propargyl palladium species.

A palladium-catalyzed divergent synthesis of spirooxindoles fused with a five- or a six-membered ring by a cycloaddition reaction of vinylidenecyclopropane-diesters with methyleneindolinones was disclosed. This protocol features an in situ generated unprecedented zwitterionic π-propargyl palladium species in cycloaddition reactions and a switchable process between (3+2) and (4+2) cycloadditions by changing the phosphine ligand.

Construction and evolution of active palladium species on phase-regulated reducible TiO2 for methane combustion

Non-traditional amorphous Pd2+ species on the surface of Pd/TiO2 catalysts facilitate CH4 combustion, while formed PdxTi1−xO2 would be detrimental.

Electronic Effects on the Depropargylation Process in the Reaction‐based Fluorescent Detection of Palladium Species: Benzocoumarin‐based Ratiometric Sensing Systems

The palladium catalyzed cross‐coupling reactions are indispensable in organic synthesis of pharmaceutical compounds, demanding convenient tools for the analysis of residual palladium contents. Propargyl aryl ether‐type fluorescent sensing systems that detect Pd(0)/Pd(II)/Pd(IV) species through depropargylation require no additives. We investigated a ratiometric sensing system based on a benzocoumarin dye; thus, three propargyl aryl ethers bearing electronically‐different self‐immolative linkers were synthesized and their relative response rate toward Pd(0) and Pd(II) were compared. We found that the response rate became faster when the self‐immolative linker became more electron‐deficient. The best‐performing system sensed Pd(0)/Pd(II) species selectively against various other metal species, which is also capable of fluorescent imaging of Pd(0)/Pd(II) species in cells. The work provides us a clue to accelerate the catalytic depropargylation step, which, when combined with a novel fluorophore, would enable us to develop out‐performing palladium sensing systems.

Highly efficient and enhanced sulfur resistance supported bimetallic single-atom palladium–cobalt catalysts for benzene oxidation

Catalytic oxidation is one of the effective pathways for completely eliminating volatile organic compounds (VOCs) emitted from industrial and transportation activities. Meanwhile, single-atom catalysts have excellent application prospects in numerous reactions due to their high metal atomic utilization efficiency. In this work, we adopted a novel strategy to prepare an active Pd/Co single-atom catalyst (i.e., Pd1Co1/Al2O3) for benzene oxidation. The successful formation of the atomically dispersed palladium and cobalt species on Al2O3 was verified by the aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure. By the in situ temperature-programmed techniques and in situ diffuse reflectance Fourier transform infrared spectroscopy, we observed a double effect of the palladium and cobalt oxide active sites, resulting in an enhanced performance for benzene oxidation. A benzene conversion of 90 % was achieved over the Pd1Co1/Al2O3 catalyst at 250 °C and a space velocity of 40,000 mL/(g h). Interestingly, the catalyst also possessed enhanced sulfur resistance performance. The good regeneration ability of the active sites in the catalyst was due to the single-atom dispersion of Pd and Co. In addition, we deduce that benzene oxidation might occur over Pd1Co1/Al2O3 via a pathway of benzene → cyclohexadiene → phenol → quinone → maleate → acetate → CO2 and H2O. We believe that the obtained results can provide a useful idea for rationally designing the double active site single-atom catalysts and understanding the mechanism of VOCs oxidation.

Nonclassical cooperative mechanism in Suzuki-Miyaura reaction – Is it possible?

Herein, the independent differential selectivity in the Suzuki-Miyaura reaction of two competing arylboronic acids with regard to the nature of the aryl halide (both halide and aryl moieties) was established. This pattern cannot be realized via the “textbook” mechanism of the reaction, which suggests the interaction of boron-containing species with an ArPdX intermediate (product of the aryl halide catalytic activation). The data obtained indicate the participation of two distinct palladium species in the aryl halide and arylboronic acid activations in the catalytic cycle of biaryl formation, which are in accordance with the so-called cooperative mechanism.

Reversion of the chain walking ability of α-diimine nickel and palladium catalysts with bulky diarylmethyl substituents

In general, α-diimine palladium species are more likely to undergo chain walking than the corresponding nickel species, resulting in more branched and topological polyethylene. Moreover, the ligand steric effects have a significant influence on the chain walking in α-diimine system. In this contribution, a series of acenaphthene-based α-diimine ligands bearing bulky diarylmethyl moieties with various electronic effects and the corresponding Ni(II) and Pd(II) complexes were synthesized and characterized. These Ni(II) complexes exhibit high activities in ethylene polymerization even at 80 °C, generating ultrahigh-molecular-weight polyethylenes with low branching density and high melting temperature. The corresponding palladium complexes display moderate activity, leading to semicrystalline polyethylene with low branching density and high melting temperature. Polar functionalized semicrystalline polyethylene with high melting temperature can also be obtained via copolymerization of ethylene with polar monomers using these palladium complexes. Moreover, the remote nonconjugated electronic substituents exert a great influence on the ethylene (co)polymerization. Most importantly, the chain walking ability of metal species can be controlled by changing the ligand steric environment, and the diarylmethyl substituents can even reverse the chain walking trend of palladium and nickel species.

Vicinal hydroxyl group-inspired selective oxidation of glycerol to glyceric acid on hydroxyapatite supported Pd catalyst

Selective oxidation of glycerol provides a feasible route towards the sustainable synthesis of high value-added chemicals. Herein, the hydroxyapatite (HAP) supported palladium (Pd) species were fabricated by impregnation and subsequent calcination. The as-obtained heterogeneous Pd catalyst afforded not only excellent selectivity to glyceric acid (GLA) up to 90% with 59% conversion of glycerol but also good recyclability by using molecular oxygen as an oxidant under mild conditions. The characterization of catalysts indicated that both the surface basicity and Pd sites on the catalyst played a crucial role in promoting glycerol oxidation. Notably, it demonstrated that the presence of the vicinal hydroxyl group of glycerol molecule can assist the oxidation reaction via forming a coordination between the vicinal hydroxyl group and Ca2+ sites on HAP-derived catalysts. In this catalytic process, the secondary hydroxyl of glycerol kept untouched and the primary hydroxyl of glycerol was converted into carboxyl group, while the Pd species acted as active centers for cooperatively promoting the subsequent oxidation to generate GLA. Additionally, this catalytic system can be extended widely for the oxidative conversion of other vicinal diols into the corresponding α-hydroxycarboxylic acids selectively. Isotope labeling experiment using H218O confirmed that H2O not only acted as solvent but also was involved in the catalytic cycles. On the basis of the results, a possible reaction mechanism has been proposed. The HAP-supported Pd catalytic system has been shown to serve as an effective approach for the upgrading of bio-derived vicinal diols to high value-added chemicals.

Palladium (II)‐catalyzed homogeneous alcohol oxidations: Disclosing the crucial contribution of palladium nanoparticles in catalysis

Versatile redox catalysts play the significant roles in alcohol oxidations, in which the mechanisms for homogeneous and heterogeneous alcohol oxidations are generally different. This work introduced a Lewis acid (LA) promoted homogeneous alcohol oxidation with Pd (OAc)2 catalyst by using oxygen balloon as the oxidant source. It was found that adding Lewis acid such as Sc (OTf)3 significantly accelerated Pd (II)‐catalyzed alcohol oxidations; notably, the time courses of oxidations monitored by GC and 1H NMR disclosed that there existed two processes including the initial sluggish oxidation followed by a rapid oxidation. The promotional effect of Lewis acid was attributed to the formation of heterobimetallic Pd (II)/LA species, which improved the oxidizing power of the palladium (II) species, thus accelerating alcohol oxidation in the induction period. Correlating the sizes of in situ generated palladium nanoparticles with the time course of alcohol oxidation further disclosed that the loosely, spherically large nanoparticles, which were composed of many tiny nanoparticles having the size less than 10 nm, were responsible for the rapid oxidation, whereas those highly dispersed, tiny nanoparticles having the size less than 10 nm were not responsible for the rapid oxidation.

Highly Active and Stable Palladium Catalysts Supported on Surface‐modified Ceria Nanowires for Lean Methane Combustion

AbstractAn efficient strategy was presented to synthesize highly active palladium catalyst supported on ceria nanowires modified by organosilanes (abbreviated as Pd/CeO2NWs@SiO2) for lean methane combustion. It is found that such a surface‐modified strategy can significantly improve the dispersion of surface palladium species and strengthen the concentration of active surface‐adsorbed oxygen species via reconstructing the surface microenvironment, invoking an efficient performance for methane oxidation. Under the space velocity of 60,000 mLg−1h−1, 0.5 wt% Pd/CeO2NWs@SiO2 displayed extraordinary catalytic activity with 90 % conversion rate at a temperature of around 327 °C, far lower than that of pristine Pd/CeO2NWs (378 °C) under the same conditions. What's more, unexpected stability was observed under high temperature and the presence of water vapor conditions owing to the intense metal support interaction of Pd/CeO2NWs@SiO2 catalyst. The possible reaction mechanism of lean methane oxidation was probed by in situ DRIFT spectra. It is observed that the pivotal intermediate products (carbonate and carbon oxygenates) generated on Pd/CeO2NWs@SiO2 surface are more readily decomposed into CO2. Importantly, the silicon hydroxyl groups (Si−OH) formed during the reaction can efficiently restrict the generation of the stable Pd(OH)x phase and release more active sites to facilitate the catalytic performance. This study provides a convenient method to design the highly reactive and durable palladium‐based catalyst for methane combustion.

Catalysis by platinum and palladium species confined in the bulk of glass fibre materials

The results of studies on the application of silicate glass fibre materials in catalysis are summarized and analyzed. Despite the very low noble metal content, catalysts based on these materials showed exceptionally high activities and selectivities in some catalytic reactions. This is due to specificity of the glassy state, which makes it possible, first, to confine highly dispersed palladium and platinum species in the bulk of glass fibres and, second, selectively absorb polar molecules, thus excluding the undesirable reactions involving non-polar molecules. The size dependences of the complete oxidation of propane and selective hydrogenation of acetylene, the nature of the structure sensitivity of these reactions and the reaction mechanisms are discussed. Ways for improving glass fibre catalyst performance are proposed and examples of the successful application of Pt/glass fibre catalysts for purification of industrial gases from volatile organic compounds are given.The bibliography includes 175 references.

Magnetic methylene-based mesoporous organosilica composite-supported IL/Pd: a powerful and highly recoverable catalyst for oxidative coupling of phenols and naphthols

A novel magnetic methylene-based mesoporous organosilica composite-supported IL/Pd complex (Fe3O4@MePMO-IL/Pd) was synthesized and characterized, and its catalytic performance was investigated. The preparation of the Fe3O4@MePMO composite was achieved through coating of Fe3O4 nanoparticles with a mixture of tetramethoxysilane, bis(triethoxysilyl)methane, and (3-chloropropyl)-trimethoxysilane in the presence of cetyltrimethylammonium bromide surfactant. The Fe3O4@MePMO was then modified with alkyl imidazolium ionic liquid and palladium species to deliver the Fe3O4@MePMO-IL/Pd nanocatalyst. This catalyst was characterized using Fourier transform infrared, thermal gravimetric, wide-angle powder X-ray diffraction, low-angle powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, vibrating sample magnetometer, energy-dispersive X-ray, and nitrogen adsorption–desorption analyses. The Fe3O4@MePMO-IL/Pd was effectively used as a highly recoverable and durable catalyst for the selective oxidative coupling of phenols and 2-naphthols under aerobic conditions.

Palladium metallated shell layer of shell@core MOFs as an example of an efficient catalyst design strategy for effective olefin hydrogenation reaction

Facile strategies for noble metal dispersion in the shell-layer of core–shell structured metal–organic frameworks based on Zr-MOFs (UiO-66@Pd-UiO-67-BPY) was developed. The robust structure of the core-structure UiO-66 was firstly synthesized before covering up with a shell-structure of UiO-67-BPY obtaining the core–shell structure (UiO-66@UiO-67-BPY). The metallation with active palladium species (PdII) was designed on the shell-layer of core–shell MOFs via a post-synthetic method. The Pd species were coordinated explicitly in a bidentate manner to the bipyridyl linkers of the shell-structure (UiO-67-BPY). Excellent dispersion of Pd in the shell structure was obtained, resulting in UiO-66@Pd-UiO-67-BPY, as confirmed from characterizations. Taking advantage of the designed catalyst, the synthesized UiO-66@Pd-UiO-67-BPY was examined for the catalytic hydrogenation reaction. The results revealed an excellent catalytic performance of UiO-66@Pd-UiO-67-BPY compared with the traditional catalyst Pd-UiO-67-BPY. Moreover, the designed catalyst structure exhibited outstanding performance and paved the way towards a cost-effective catalyst synthesis based on MOFs combined with a noble metal.

Pd-Catalyzed Approach for Assembling 9-Arylacridines via a Cascade Tandem Reaction of 2-(Arylamino)benzonitrile with Arylboronic Acids in Water.

A novel palladium-catalyzed protocol for the synthesis of 9-arylacridines via tandem reaction of 2-(arylamino)benzonitrile with arylboronic acids in water has been developed with good functional group tolerance. The present synthetic route could be readily scaled up to gram quantity without difficulty. This methodology was further extended to the synthesis of a 4'-OH derivative, which showed estrogenic biological activity. Preliminary mechanistic experiments showed that this transformation involves a nucleophilic addition of aryl palladium species to the nitrile to generate an aryl ketone intermediate followed by an intramolecular Friedel-Crafts acylation and dehydration to acridines.

Influence of oxygen and water content on the formation of polychlorinated organic by-products from catalytic degradation of 1,2-dichlorobenzene over a Pd/ZSM-5 catalyst

Understanding the generation and influence mechanism of polychlorinated organic by-products during the catalytic degradation of chlorinated volatile organic compounds (CVOCs) is essential to the safe and environmentally friendly treatment of those pollutants. In this study, a systematic investigation of the catalytic oxidation of 1,2-dichlorobenzene (1,2-DCB) was conducted using various oxygen and water contents over a Pd/ZSM-5(25) catalyst. It was found that decreasing the oxygen content and increasing the water content resulted in the improvement of the 1,2-DCB catalytic activity, while the amount and variety of polychlorinated organic by-products decreased. More importantly, when water was the sole oxidant, the Pd/ZSM-5(25) catalyst also demonstrated high activity towards 1,2-DCB catalytic degradation. Only chlorobenzene and 1,3-dichlorobenzene were detected as by-products. X-ray photoelectron spectra (XPS) and UV–vis DRS spectra results indicated that the polychlorinated organic by-products were suppressed mainly due to inhibition of the chlorination of the palladium species by regulating the oxygen and water content in the reaction atmosphere. Similar surface species were formed under aerobic and anaerobic atmospheres via the study of the in situ FTIR spectra. We therefore proposed that 1,2-DCB undergoes similar catalytic degradation reaction mechanisms under both aerobic and anaerobic conditions.

Preparation of chitosan/poly(methacrylic acid) supported palladium nanofibers as an efficient and stable catalyst for Heck reaction

Homogeneous chitosan/poly(methacrylic acid) solution containing Pd2+ cations was successfully prepared by addition of pyridine-2-carboxaldehyde. Then, this homogeneous solution was smoothly electrospun into uniform composite nanofibers with mean diameter of 442 ± 163 nm, followed by annealing at elevated temperature to improve the solvent resistance. Scanning electron microscopy (SEM) analysis shows that the solvent resistance of the composite nanofibers was improved with the increment of annealing temperature. FT-IR spectra demonstrate that the chemical structure of the composite nanofibers was not oxidized after annealing process. Moreover, the palladium species were homogeneously incorporated inside the composite nanofibers. The catalytic performance of these novel composite nanofibers was evaluated by Heck reaction. The catalysis result shows that these composite nanofibers were very efficient to catalyze the Heck reaction of aromatic iodides with acrylates with yields over 86%. Moreover, these composite nanofibers can be reused for 10 times with little decrement of yield. A novel palladium embedded chitosan composite nanofiber mat has been prepared by electrospinning and thermal techniques. This novel fiber palladium catalyst exhibited high catalytic activity and stability for Heck reaction.

Palladium-Catalyzed Intermolecular Trans-Selective Carbofunctionalization of Internal Alkynes to Highly Functionalized Alkenes

A palladium/DPEphos-catalyzed intermolecular trans-selective carbofunctionalization of internal alkynes has been established herein. This method proceeds through a formal anti-carbopalladation, forming trans-alkenyl palladium species, which was trapped by aryl boronic acids to provide all-carbon tetrasubstituted alkenes in 32–92% yields. The trans-selective arylsilylation/remote C–H silylation and hydroarylation/remote C–H borylation of internal alkynes were also achieved using hexamethyldisilane and bis(pinacolato)diboron as trapping reagents, respectively. The reaction features good regio- and stereoselectivity and high functional group tolerance. A preliminary mechanistic study and DFT calculations show that a cis to trans isomerization of cis-alkenyl palladium species was involved in this transformation.

Solvent switchable Pd/DNA catalyst in carbonylative Sonogashira coupling

Palladium catalysts immobilized on DNA (Pd/DNA) were applied for the first time in the carbonylative Sonogashira coupling, at 1 atm of CO. Different alkynones were obtained with a high yield in DMF. In the presence of water, the selectivity completely changed and diaryl alkynes, typical Sonogashira products, were selectively formed. In addition to Pd/DNA, soluble palladium species also participated in the catalytic process. The stability of the catalytic system increased after the addition of DNA excess.

Naphthalimide‐Benzothiazole Conjugate: A Dosimetric Probe for Colorimetric and Fluorometric Detection of Palladium

AbstractA naphthalimide‐benzothiazole conjugate 1 as a dosimetric probe has been designed and synthesized. The probe is sensitive to palladium species over a series of other metals in CH3CN‐HEPES buffer [4 : 1 v/v, pH 7.2, 10 mM, NaBH4 (10 mM), PPh3 (10 mM)] and reports the detection both by color and emission changes. Upon addition of increasing concentrations of palladium salts (c=1 x 10−3 M), compound 1 displays distinct absorption at 512 nm with a color change from light yellow to reddish pink. This was in sharp contrast to other metal ions where there was no color change of the solution of 1 as well as no absorption at 512 nm. In fluorescence, emission of the probe is reduced in presence of Pd(0) species. The probe acts as a dosimeter involving Pd‐induced deallylation reaction.

Selective Hydrogenation of Biomass-Derived Furfural: Enhanced Catalytic Performance of Pd-Cu Alloy Nanoparticles in Porous Polymer.

Here, the development of a new catalyst is reported for the selective furfural (FF) hydrogenation to furfuryl alcohol (FA) based on about 7 nm sized Pd-Cu alloy nanoparticles (NPs) formed in inexpensive, commercially available micro/mesoporous hypercrosslinked polystyrene (HPS). A comparison of the catalytic properties of as-synthesized and reduced (denoted "r") catalysts as well as Pd-Cu alloy and monometallic palladium NPs showed a considerable enhancement of the catalytic performance of Pd-Cu/HPS-r compared to other catalysts studied, resulting in about 100 % FF conversion, 95.2 % selectivity for FA and a TOF of 1209 h-1 . This was attributed to the enrichment of the NP surface with copper atoms, disrupting the furan ring adsorption, and to the presence of both zerovalent and cationic palladium and copper species, resulting in optimal hydrogen and FF adsorption. These factors along with exceptional stability of the catalyst in ten consecutive catalytic cycles make it highly promising in practical applications.

Mechanism of Palladium‐Catalyzed Spiroannulation of Naphthols with Alkynes: A Density Functional Theory Study

The Cover Feature shows ortho-bromophenylnaphthalenols and acetylene substrates that, just like the fish in the sea, jump through an energy barrier to achieve the spiroannulation product. In their Full Paper, X. Luo et al. performed DFT calculations to investigate the mechanism and enantioselectivity of Pd-catalyzed spiroannulation of ortho-bromophenylnaphthalenols with acetylenes to achieve asymmetric dearomatization. They propose that the catalytic cycle is composed of an oxidative addition of the aryl bromide to Pd(0) to afford an aryl palladium species, followed by acetylene insertion into the Pd–C(aryl) bond, Pd-assisted intramolecular deprotonation of the naphthol moiety and reductive elimination to give the spiroannulation product. They found that both the rate- and enantioselectivity-determining steps are acetylene insertion, and explained that the enantioselectivity is controlled by the steric repulsion between the naphthol moiety and the ligand. More information can be found in the Full Paper by X. Luo et al.