Synthesis,Characterization, and Catalysis of Water-Soluble Trimeric and Monomeric Palladium Complexes of 8-Aminoquinolines

New water soluble neutral and cationic palladium complexes were synthesized using 8-aminoquinoline (8-AQ) and 2-methyl 8-aminoquinoline (2-Me 8-AQ) ligands and their catalytic properties were evaluated. The neutral trimeric complexes having a Pd₃N₃ core were found to form when Pd(OAc)₂ was reacted with 8-AQ or 2-Me 8-AQ irrespective of the stoichiometry between the 2 reagents. Controlled addition of triflic acid to the neutral trimeric complex resulted in the formation of a trimeric cationic palladium complex as well as a monomeric cationic complex. A cationic palladium complex having two units of 2-Me-8AQ ligand was also synthesized from the cationic monomeric complex. Crystal structures of the new palladium complexes are reported in this study. The water-soluble neutral palladium complex showed catalytic activity for the oxidation of benzyl alcohols to benzaldehydes, while the cationic palladium complexes were found to be efficient catalysts for the oxidation of styrenes to methyl ketones.     

Palladium catalysts deposited on silicon nitride in the deep oxidation of methane

The physicochemical and catalytic properties of palladium catalysts were studied in the deep oxidation of methane. The catalysts were deposited on silicon nitride from aqueous (Pd/Si₃N₄-a) and toluene (Pd/Si₃N₄-t) solutions of palladium acetate. The use of aqueous and organic solutions of palladium acetate, all other preparation conditions being equal, resulted in the formation of palladium systems with different catalytic properties. The sample from Pd/Si₃N₄-t was characterized by high activity and stability. The systems studied had different structures and adsorption properties of palladium nanoparticles, which influenced the form of reagent adsorption, catalytic properties, and mechanism of surface reactions. The suggestion was made that the solvent played a key role in the formation of the active surface of Pd-containing catalytic systems.     

The synthesis and characterisation of immobilised palladium carbene complexes and their application to heterogeneous catalysis

Silica supported palladium NHC complexes have been prepared by two different routes: one involving the reaction of silica-supported imidazolium salts with palladium acetate and a direct immobilisation of a pre-formed complex by reacting a (trimethoxysilylpropyl)-N-aryl-imidazolylidene palladium complex with surface hydroxyl groups. A small range of catalysts of varying steric bulk were prepared in order to evaluate the effect on catalytic conversion. The activity of the palladium catalysts in Suzuki cross-coupling reactions has been established. The catalysts prepared by immobilising pre-formed palladium complexes gave superior results for the conversion of aryl bromides and aryl chlorides. In addition, use of sterically bulky NHCs (such as the N-2,6-(diisopropyl)phenyl-substituted ligand) resulted in increased catalytic activity, which is analogous to the trends noted in homogeneous catalysis.     

Immobilization of palladium(II)‐containing bis(imidazolium) ligand on ion‐exchange resins: efficient and reusable catalysts for CC coupling reactions

Suzuki reactions catalysed by a palladium(II) complex of a functionalized bis(imidazolium) ligand, Pd^II(BIM), immobilized on Dowex 50 WX8 and Amberlite IR‐120 ion‐exchange resins as heterogeneous, recyclable and active catalysts are reported. The catalysts, Pd^II(BIM)@Amberlite IR‐120 and Pd^II(BIM)@Dowex 50 WX8, were characterized using Fourier transform infrared and diffuse‐reflectance UV–visible spectroscopies and scanning electron microscopy. These heterogeneous catalysts are oxygen‐insensitive and air‐ and moisture‐stable in C? C coupling reactions, and are reusable several times without significant loss of their catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.     

Carbon-supported palladium catalysts for fuel cells

Small controlled amounts of palladium were electrochemically deposited onto various carbon supports from solutions of glycinate-chloride complexes of palladium(II) in order to obtain palladium catalysts suitable for use in fuel cells. The catalytic activity of the resulting catalytic layers was studied in reactions of reduction of atmospheric oxygen and oxidation of methanol and ethanol in acid and alkaline media by measuring cyclic voltammetric curves on a rotating disk electrode.     

Ligand effects in palladium‐catalyzed Suzuki and Heck coupling reactions

A range of sterically hindered diimine ligands and their palladium (II) complexes were synthesized. These compounds were fully characterized by elemental analysis, ¹H and ¹³C‐NMR spectroscopy. The use of the palladium complexes as catalysts for Suzuki and Heck coupling has been studied in an attempt to demonstrate the effect of side groups on catalytic activity. It was clearly seen that the location of side ? CH₃ groups which bound to benzene ring had little effect on catalytic activity. Interestingly when we changed these ? CH₃ groups with ? Cl groups the activity of the complexes increased. On the other hand, side groups which bound to imine nitrogen also had a large effect on catalytic activity. Copyright © 2008 John Wiley & Sons, Ltd.     

Polymerization of 5-norbornene-2-methyl acetate catalyzed by air-stable cationic (η-substituted allyl) palladium complexes of N-heterocyclic carbene

In situ generated cationic (η³-substituted allyl) palladium N-heterocyclic carbene (NHC) complexes are air-stable active catalysts for the olefin polymerization of (bicyclo[2.2.1]hept-5-en-2-yl)methyl acetate (5-norbornene-2-methyl acetate) (endo:exo = 7:3). Catalytic activities, polymer yields, molecular weights, and molecular weight distributions of polymers were investigated under various reaction conditions. The catalytic activity was highly dependent not only on the reaction condition such as a solvent or the reaction temperature but also on the structure of the catalyst that include substituents of the allyl group in the catalyst and the counter anion. As the bulkiness of the allyl group increased, the catalytic activity of the catalysts increased.     

Chiral phosphinooxazolidine ligands for palladium- and platinum-catalyzed asymmetric Diels-Alder reactions

Cationic palladium (Pd)- and platinum (Pt)-phosphinooxazolidine catalysts 13a-c, 15a-d, 17a-c, and 19a-c were prepared from phosphinooxazolidine ligands 1-3, MCl(2) (M = Pd and Pt), and counterions, and the activities of the catalysts in the asymmetric Diels-Alder (DA) reactions of cyclic or acyclic dienes with imide dienophiles were investigated. These catalysts demonstrated high levels of catalytic activity. The cationic Pd-POZ complex 13c provided particularly excellent enantioselectivity (98% ee) in the DA reactions of cyclopentadiene with acryloyl-, crotonyl-, and fumaroyl-1,3-oxazolidin-2-ones (20a-c).     

Addition of Tetrachloromethane to Alkenes,Catalyzed by Pt(II) Complexes

Platinum(II) complexes [dichlorobis(triphenylphosphine)platinum(II), dichlorobis(tri-m-tolylphosphine)platinum(II), dichloro(2,9-dimethyl-1,10-N, N′-phenanthroline)platinum(II), etc.] showed catalytic activity in addition of tetrachloromethane across the double bond in 1-hexene, 1-heptene, 1-octene, 1-decene, and cyclohexene. The stability of the platinum catalysts was evaluated by GLC, gas chromatography-mass spectrometry, and ³¹P NMR and IR spectroscopy; the kinetic relationships of the addition reactions were determined. A reaction mechanism involving formation of trichloromethyl radical was suggested. A correlation was revealed for the first time between the catalytic activity of platinum, palladium, and rhodium complexes and the capability of these complexes to generate hexachloroethane.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005, pp. 778–782.Original Russian Text Copyright © 2005 by Zazybin, Khusnutdinova, Osipova, Solomonov.     

Directed C−H Halogenation Reactions Catalysed by PdII Supported on Polymers under Batch and Continuous Flow Conditions

A new generation of N-heterocyclic carbene palladium(II) complexes containing vinyl groups in different positions in the backbone of the N-heterocycle have been developed. The fully characterised monomers were copolymerised with divinylbenzene to fabricate robust polymer supported NHC-Pd^II complexes and these polymers were applied as heterogeneous catalysts in directed C−H halogenation of arenes with a pyridine-type directing group. The catalysts demonstrated medium-high catalytic activity with up to 90 % conversion and 100 % selectivity in chlorination. They are heterogeneous and recyclable (at least six times) with no significant leaching of palladium in batch mode catalysis. The best catalyst was also applied under continuous flow conditions where it disclosed an exceptional activity (90 % conversion) and 100 % selectivity for the mono-halogenated product for at least six days, with no leaching of palladium, no loss of activity and an ability to maintain the original oxidation state of Pd^II.     

Density functional studies on chromium catalyzed ethylene trimerization

Mechanism of ethylene trimerization using chromium catalyst is investigated using density functional methods. Recent experimental results indicate Cr-based homogeneous catalysts to follow metallacycle pathway in ethylene tri-, teta- and oligomerization reactions. Given the importance of chlorinated Cr-based active catalysts in these reactions, we have used “bare” minimal ligands like Cl^? and considered catalytic cycles with neutral or cationic intermediates starting with [Cr(II)Cl₂(ethylene)₂] and [Cr(II)Cl(ethylene)₂]⁺, respectively. We have compared both ‘Cossee’ and the ‘metallacycle’ mechanisms on these model systems utilizing density functional computations at B3LYP/LANL2DZ(d,p) level. The metallacycle mechanism with cationic Cr(II)–Cr(IV) intermediates is found to be the most favored path, with oxidative coupling of two coordinated ethylene to form the chromacyclopentane being the rate determining step (RDS). We also found that with neutral intermediates the Cossee pathway rather than the metallacycle mechanism is followed. Thus in spite of the simplicity of using just Cl^? as ligand in the model catalytic intermediates, our computational results match remarkably well with many recent and important experimental findings.     

Catalytic activity of Pd dithiolate complexes with large-bite-angle diphosphines in Heck coupling reactions

Palladium(II) complexes of aryl dithiolates and wide-bite-angle diphosphines Xantphos and dppf have been developed as efficient catalysts in Suzuki and Suzuki carbonylation reactions. The catalytic activity of these highly stable, discrete and charged complexes was investigated in Heck coupling reactions of styrene and a variety of aryl bromides. Under optimized reaction conditions these palladium complexes showed excellent activity with high turnover number (6 × 10⁶) and high turnover frequency (4 × 10⁵ h⁻¹). The effect of bite angle of diphosphines on the catalytic activity of the complexes [Pd₂(P^∩P)₂(SC₁₂H₈S)]₂(OTf)₄ followed the trend P^∩P = Xantphos > dppf > dppe as the order of their bite angles. The catalyst could be reused, and after three cycles the formation of significant amount of Pd nanoparticles was noticed, which were characterized using powder X-ray diffraction, energy-dispersive X-ray analysis and transmission electron microscopy. The high catalytic activity has been attributed to the Pd nanoparticles.     

Atropisomeric PˆN Ligands for the Palladium‐Catalyzed Copolymerization of Styrene with Carbon Monoxide. Preliminary Communication

Achiral and chiral cationic palladium catalysts, modified with atropisomeric P?N ligands with different steric and electronic properties, can efficiently produce poly(styrene‐alt‐CO) with essentially complete regioregularity and variable tacticity, depending on the ligand geometry; the electronic effect on catalytic activity depends on the geometry of the ligand.     

Synthesis and catalytic application of Pd complex catalysts: Atom‐efficient cross‐coupling of triarylbismuthines with haloarenes and acid chlorides under mild conditions

Palladium‐catalysed cross‐coupling reactions are some of the most frequently used synthetic tools for the construction of new carbon–carbon bonds in organic synthesis. In the work presented, Pd(II) complex catalysts were synthesized from palladium chloride and nitrogen donor ligands as the precursors. Infrared and ¹H NMR spectroscopic analyses showed that the palladium complexes were formed in the bidentate mode to the palladium centre. The resultant Pd(II) complexes were tested as catalysts for the coupling of organobismuth(III) compounds with aryl and acid halides leading to excellent yields with high turnover frequency values. The catalysts were stable under the reaction conditions and no degradation was noticed even at 150°C for one of the catalysts. The reaction proceeds via an aryl palladium complex formed by transmetallation reaction between catalyst and Ar₃Bi. The whole synthetic transformation has high atom economy as all three aryl groups attached to bismuth are efficiently transferred to the electrophilic partner.     

Catalytic activity of palladium(II) diaminocarbene complexes in the Sonogashira and Suzuki reactions

Palladium(II) complexes with chelating and non-chelating diaminocarbene ligands were assessed as catalysts in the cross-coupling reactions of haloarenes with oct-1-yne (Sonogashira reaction) and phenylboronic acid (Suzuki reaction). Both complexes exhibited a higher catalytic activity than traditional phosphine ligandbased systems in the Sonogashira reaction, and they ensured cross-coupling not only with iodoarenes but also with bromoarenes activated by electron-withdrawing substituents. The catalytic activities of the examined complexes in the Suzuki reaction were appreciably different: the palladium(II) complex with the chelating ligand turned out to be considerably less active than the complex with the non-chelating ligand.     

Defect Rich Structure Activated 3D Palladium Catalyst for Methanol Oxidation Reaction

Controlling the structure and properties of catalysts through atomic arrangement is the source of producing a new generation of advanced catalysts. A highly active and stable catalyst in catalytic reactions strongly depends on an ideal arrangement structure of metal atoms. We demonstrated that the introduction of the defect-rich structures, low coordination number (CN), and tensile strain in three-dimensional (3D) urchin-like palladium nanoparticles through chlorine bonded with sp-C in graphdiyne (Pd-UNs/Cl-GDY) can regulate the arrangement of metal atoms in the palladium nanoparticles to form a special structure. In situ Fourier infrared spectroscopy (FTIR) and theoretical calculation results show that Pd-UNs/Cl-GDY catalyst is beneficial to the oxidation and removal of CO intermediates. The Pd-UNs/Cl-GDY for methanol oxidation reaction (MOR) that display high current density (363.6 mA cm⁻²) and mass activity (3.6 A mg_Pd⁻¹), 12.0 and 10.9 times higher than Pd nanoparticles, respectively. The Pd-UNs/Cl-GDY catalyst also exhibited robust stability with still retained 95 % activity after 2000 cycles. A defects libraries of the face-centered cubic and hexagonal close-packed crystal catalysts (FH-NPs) were synthesized by introducing chlorine in graphdiyne. Such defect-rich structures, low CN, and tensile strain tailoring methods have opened up a new way for the catalytic reaction of MOR.     

Amphiphilic polymer supported N-heterocyclic carbene palladium complex for Suzuki cross-coupling reaction in water

We synthesized amphiphilic polymer-supported N-heterocyclic carbene (NHC) precursor resins by loading polyethylene glycol (PEG) containing imidazolium groups on Merrifield resin. These PS-PEG-NHC precursor resins were compatible with water and readily formed a stable complex with palladium. These PS-PEG-NHC-Pd catalysts showed excellent catalytic activity for Suzuki cross-coupling reactions of various aryl iodides and bromides with phenylboronic acid in water than the previously described polystyrene based catalysts. In addition, the PS-PEG-NHC-Pd catalysts continued to provide excellent catalytic activity in Suzuki cross-coupling reactions after five consecutive recycles.     

The orthopalladation dinuclear [Pd(L1)(μ-OAc)]2, [Pd(L2)(μ-OAc)]2 and mononuclear [Pd(L3)2] complexes with [N, C, O] or [N, O] containing ligands: Synthesis, spectral characterization, electrochemistry and catalytic properties

Treatment of the salicylaldimine ligands (L₁H, L₂H, L₃H, L₄H and L₅H) with palladium(II) acetate in absolute ethanol gave the orthopalladation dinuclear [Pd(L₁)(μ-OAc)]₂, [Pd(L₂)(μ-OAc)]₂ and mononuclear [Pd(L₃)₂] with the tetradentate ligands [N, C, O] or [N, O] moiety. The ligands L₁H and L₂H are coordinated through the imine nitrogen and aromatic ortho carbon atoms, whereas the ligand L₃H coordinated through the imine nitrogen and phenolic oxygens atoms. The Pd(II) complexes have a square-planar structure and were found to be effective catalysts for the hydrogenation of both nitrobenzene and cyclohexene. These metal complexes were also tested as catalysts in Suzuki-Miyaura coupling of aryl bromide in the presence of K₂CO₃. The catalytic studies showed that the introduction of different groups on the salicyl ring of the molecules effected the catalytic activity towards hydrogenation of nitrobenzene and cyclohexene in DMF at 25 and 45 °C. The Pd(II) complexes easily prepared from cheap materials could be used as versatile and efficient catalysts for different C-C coupling reactions (Suzuki-Miyaura reactions). The structure of ligands and their complexes was characterized by UV-Vis, FT-IR, ¹H and ¹³C NMR, elemental analysis, molar conductivity, as well as by electrochemical techniques.     

Novel cyclodextrin‐modified h‐BN@Pd(II) nanomaterial: An efficient and recoverable catalyst for ligand‐free C‐C cross‐coupling reactions in water

An environmentally friendly palladium(II) catalyst supported on cyclodextrin‐modified h‐BN was successfully prepared. The catalyst was characterized by FT‐IR, SEM, TG, XRD and XPS, and the loading level of Pd in h‐BN@β‐CD@Pd(II) was measured to be 0.088 mmol g⁻¹ by ICP. It exhibits excellent catalytic activity for the Suzuki and Heck reactions in water, and can be easily separated and consecutively reused for at least nine times. In addition, a series of pharmacologically interesting products were successfully synthesized using this catalyst to demonstrate its potential applications in pharmaceutical industries. Above all, this work opens up an interesting and attractive avenue for the use of cyclodextrin‐functionalized h‐BN as an efficient support for hydrophilic heterogeneous catalysts.     

Palladium(II) 3-iminophosphine (3IP) complexes: Active precatalysts for the intermolecular hydroamination of 1,2-dienes (allenes) and 1,3-dienes with aliphatic amines under mild conditions

The synthesis of alicyclic 3-iminophosphine ligands is extended to include a new framework incorporating a cyclohexenyl backbone with an N-aryl imino functionality (3IP^Ar). Accordingly, a series of palladium(II) complexes employing this new ligand have been synthesized and utilized in the intermolecular hydroamination of 3-methyl-1,2-butadiene (1,1-dimethylallene) and 2,3-dimethyl-1,3-butadiene with secondary amines. The complex [(3IP^Ar)Pd(allyl)]OTf displays excellent catalytic activity in these reactions, selectively producing allylic amine products in high conversion under mild conditions, with an improved rate relative to that observed for our previously reported catalysts. Further, the reactivity trends for the (3IP)Pd triflate systems prove to be complimentary to other known late transition metal based catalytic systems.