Mol Pd Research Articles

Decomposition reaction of H 2O 2 over Pd/C catalyst in an aqueous medium at high pressure: Detailed kinetic study and modelling

Decomposition of H 2O 2 is an undesired side reaction that occurs during the direct synthesis of H 2O 2 over Pd/C catalysts. Experiments have been carried out to understand the influence of pressure (0.2–7.5 MPag), temperature (23–50 °C), pH, halide concentration, catalyst and inert gas in decomposition. A model considering these effects is proposed, achieving average deviations lower than 5%. It has been found that a pH lower than 1.8 and halide to Pd ratios over 2.2 are enough to reduce the decomposition more than 90%. Activation energies between 15.0 and 83.8 kJ/mol, with and without promoters, have been found. The influence of pressure is limited for the case of decomposition above 2.5 MPag. Decomposition increases almost linearly with the amount of the active metal catalyst. Specific decomposition rates between 6.65 and 23,295 min −1 mol Pd −1 with and without promoters have been observed in this study.

Synthesis, Structures, and Norbornene Polymerization Behavior of Aryloxide-N-Heterocyclic Carbene Ligated Palladacycles

Treatment of the o-hydroxyaryl imidazolinium pro-ligands (2-OH-3,5-tBu2C6H2)(R)(C3H3N2)+Br− [R = Me (3a), iPr (3b), tBu (3c), Ph (3d), Mes (3e)] with the palladacycle [Pd(dmba)(μ-Cl)]2 (dmba = Me2NCH2C6H5) (2) afforded the corresponding aryloxide-NHC (NHC = N-heterocyclic carbene)-ligated palladacycles (2-O-3,5-tBu2C6H2)(R)(NHC)Pd(dmba) [R = Me (4), iPr (5), Ph (6), Mes (7), tBu (8)]. Notably, without isolating 2, complexes 4−8 could also be obtained by one-pot, three-component, sequential reaction of N,N-dimethylbenzylamine, PdCl2, and the pro-ligands in refluxing acetonitrile in the presence of K2CO3. When the N-functional group on the NHCs is tert-butyl, the NHC in 8 adopts an abnormal binding (C4-bonding). All these complexes were fully characterized by 1H and 13C NMR, high-resolution mass spectrometry (HRMS), and elemental analysis. Single-crystal X-ray diffraction analysis results further confirmed the molecular structures of 4−8 and the abnormal binding of NHC in 8. With methylaluminoxane (MAO) as cocatalyst these palladacycles showed excellent catalytic activities up to 107 g of PNB (mol of Pd) −1 h−1 in the addition polymerization of norbornene.

Trans-(2 R,3 R)-2,3-Diphenylcyclopropane-1,1-dimethanol: a pivotal diol for the synthesis of novel C2-symmetric ligands for asymmetric transition metal catalysis

Various chiral ligands bearing phosphorus or nitrogen donor atoms were obtained in a straightforward manner starting from trans-(2 R,3 R)-diphenylcyclopropane-1,1-dimethanol as a key structure. These chiral ligands were tested and compared in palladium(0)-catalyzed asymmetric allylic alkylation reactions (up to 71% ee) and rhodium(I)-catalyzed asymmetric hydrogenations (up to 88% ee). Moreover, in the asymmetric allylic alkylation, we observed excellent activity with a diphosphinite ligand (TOF = 600 mol 17 × [mol Pd × h) −1].

η 5-Cyclopentadienylpalladium(II) complexes: Synthesis, characterization and use for the vinyl addition polymerization of norbornene and the copolymerization with 5-vinyl-2-norbornene or 5-ethylidene-2-norbornene

Dinuclear complexes of palladium(II), containing two bridging halogen (Cl or Br) ligands, [N n Bu 4] 2[(X 5C 6) 2Pd(μ-Cl) 2Pd(C 6X 5) 2] and [(X 5C 6)(L)Pd(μ-Y) 2Pd(C 6X 5)(L)] (X = F, Cl; Y = Cl, Br), readily react with cyclopentadienylthallium, C 5H 5Tl, to give the corresponding air stable half-sandwich, pseudo-trigonal η 5-cyclopentadienylpalladium complexes, [N n Bu 4][(η 5-C 5H 5)Pd(C 6X 5) 2] (X = F 1, Cl 2) and (η 5-C 5H 5)Pd(C 6X 5)(L) (X = F, L = CNBu t 3, PPh 3 4, PMe 2Ph 5, PEt 3 6, AsPh 3 7, SbPh 3 8; X = Cl, L = PMe 2Ph 9, PEt 3 10), respectively. With tetraphenylcyclopentadienylthallium, C 5Ph 4HTl or pentabenzylcyclopentadienylthallium, C 5Bn 5Tl (Bn = CH 2Ph) the air stable half-sandwich complexes (η 5-C 5Ph 4H)Pd(C 6F 5)(AsPh 3), 12 and (η 5-C 5Bn 5)Pd(C 6F 5)(AsPh 3), 13 are synthesized accordingly. The molecular structures were verified by NMR-spectroscopy, X-ray crystallography ( 7, 12, 13) and electron impact-mass spectrometry (EI-MS). The precatalysts 4 and 7 can be activated with methylalumoxane (MAO) for the homopolymerization of norbornene (NB) and 5-ethylidene-2-norbornene (ENB) and for the copolymerization of NB with 5-vinyl-2-norbornene (VNB) or ENB with activities of more than 10 6 g PNB/(mol Pd·h). The higher activity of 7/MAO over 4/MAO towards NB homopolymerization was reversed when the olefin-substituted VNB or ENB were added. Then, the more strongly bound PPh 3 ligand of 4 (versus AsPh 3 of 7) can compete with the olefin functionality of VNB or ENB and assume a directing role for the insertion of the ring double bond. As a consequence 4/MAO shows almost the same activity in NB and ENB homopolymerization.

A Microchemical System with Continuous Recovery and Recirculation of Catalyst‐Immobilized Magnetic Particles

A microchemical system for continuous flow catalytic reactions with magnetic catalyst is presented. The automatic separation of catalyst particles and recirculation by the microchemical system makes it possible to realize fully the advantages a catalytic microreactor can offer. It would be applicable to various catalytic reactions with aid of already reported magnetic catalyst or new magnetic catalyst.

Cavity‐Shaped Ligands: Calix[4]arene‐Based Monophosphanes for Fast Suzuki–Miyaura Cross‐Coupling

Five conical calix[4]arenes that have a PPh(2) group as the sole functional group anchored at their upper rim were assessed in palladium-catalysed cross-coupling reactions of phenylboronic acid with aryl halides (dioxane, 100 degrees C, NaH). With arylbromides, remarkably high activities were obtained with the catalytic systems remaining stable for several days. The performance of the ligands is comparable to a Buchwald-type triarylphosphane, namely, (2'-methyl[1,1'-biphenyl]-2-yl)diphenylphosphane, which in contrast to the calixarenyl phosphanes tested may display chelating behaviour in solution. With the fastest ligand, 5-diphenylphosphanyl-25,26,27,28-tetra(p-methoxy)benzyloxy-calix[4]arene (8), the reaction turnover frequency for the arylation of 4-bromotoluene was 321,000 versus 214,000 mol(ArBr).mol(Pd)(-1).h(-1) for the reference ligand. The calixarene ligands were also efficient in Suzuki cross-coupling reactions with aryl chlorides. Thus, by using 1 mol% of [Pd(OAc)(2)] associated with one of the phosphanes, full conversion of the deactivated arenes 4-chloroanisole and 4-chlorotoluene was observed after 16 h. The high performance of the calixarenyl-phosphanes in Suzuki-Miyaura coupling of aryl bromides possibly relies on their ability to stabilise a monoligand [Pd(0)L(ArBr)] species through supramolecular binding of the Pd-bound arene inside the calixarene cavity.

Ethylene and carbon monoxide copolymerization catalyzed by supported palladium catalyst

The kinetics of the copolymerization of ethylene with carbon monoxide catalyzed by (dppp)Pd(TsO) 2 ( I) has been investigated under homogeneous conditions and in slurry with supported catalyst I/ P ( P: polymer support polyketone (PK)), at different catalyst concentrations. The specific rate reduced to gram of Pd of the initial stage of reaction is going up with increasing catalyst concentration, and the specific rate of the second macro-kinetic stage decreases with increasing catalyst loading. The productivity of supported catalysts I/ P and I/ SiO 2 has been analyzed in terms of the PK yield per gram of supported catalyst per hour. The specific copolymer yield increases with the increase in catalyst loading in I/ P to 2 × 10 −4 mol of Pd per 1 g of the supported catalyst and attains the maximum value of 27 g PK/(g sup.cat. h) when copolymerization is carried out in the heptane–toluene mixture.

Catalytic activity in hydrogenation processes of catalysts containing nanodimensional palladium on carbon support. Concentration effect of palladium and precursor

We showed earlier that catalysts obtained by depositing nanodimensional palladium on various nanocarbon materials: fullerenes, nanotubes, fullerene soot, and also activated coconut carbon [1, 2] possessed substantially higher catalytic activity in alkene hydrogenation in comparison with industrial catalysts, where palladium was deposited on activated carbon. At the same time, we found that the nature of nanocarbon material has no essential effect upon catalytic activity. The obtained catalysts posses a pronounced affinity towards hydrogen absorption. The dependence of the absorbed hydrogen amount on the palladium content in the catalyst is of extremal character. The absorption maximum is achieved at comparatively low palladium content in the catalyst (0.1 wt %) and equals to 30– 35 mol of Н2 per 1 mol of Pd.

Resorcin[4]arene-Derived Mono- and Diphosphines in Suzuki Cross-Coupling

Three resorcin[4]arene cavitands (1–3) having either one or two resorcinolic C-2 atoms substituted by a CH2PPh2 podand arm were assessed in the Suzuki arylation of aryl bromides with phenylboronic acid. Using P:Pd ratios of 2:1 and operating in dioxane at 100 °C with a catalyst loading of 0.001 mol% resulted in highly efficient catalytic systems. For example, TOFs up to 34570 mol(converted ArBr)⋅mol(Pd)−1⋅h−1 were obtained with the proximally-disubstituted cavitand 3 when using 4-bromotoluene as substrate. The performance was shown to vary in the following order: monophosphine 1<diphosphine 2<diphosphine 3 (where 2 is the distally disubstituted cavitand). A comparison of the catalytic properties of monophosphine-cavitand 1 with those of benzyldiphenylphosphine and o-anisylmethyldiphenyl phosphine suggests that 1 functions as a hemilabile phosphine, the oxygen atoms close to the phosphorus atom behaving as donors able to temporarily increase the electron density on the metal and/or favour the formation of mono-ligand Pd(0) species.

Pyranoside Phosphite-Oxazoline Ligand Library: Highly Efficient Modular P,N Ligands for Palladium-Catalyzed Allylic Substitution Reactions. A Study of the Key Palladium Allyl Intermediates

We have screened a library of modular phosphite-oxazoline ligands for asymmetric allylic substitution reactions. The library is efficiently prepared from the commercially available and cheap D -glucosamine. The introduction of a phosphite moiety into the ligand design is highly advantageous for the product outcome. Therefore, this ligand library affords good-to-excellent reaction rates [TOFs up to 600 mol substrate x (mol Pd x h) -1 ] and enantioselectivities (ees up to 99%) and, at the same time, shows a broad scope for mono-, di- and trisubstituted linear hindered and unhindered substrates and cyclic substrates. The NMR studies on the palladium allyl intermediates provide a deeper understanding about the effect of the ligand parameters on the origin of enantioselectivity.

Biaryl Phosphites: New Efficient Adaptative Ligands for Pd-Catalyzed Asymmetric Allylic Substitution Reactions

Pharmaceuticals, agrochemicals, fragrances, fine chemicals, and natural product chemistry all rely on the preparation of enantiomerically enriched compounds. The palladium-catalyzed asymmetric allylic substitution, which allows for the enantioselective formation of carbon-carbon and carbon-heteroatom bonds, is a potential synthetic tool for preparing these compounds. To date, most of the successful ligands reported for the Pd-catalyzed allylic substitution reactions have used three main design strategies. The first, developed by Hayashi and co-workers, used a secondary interaction of the nucleophile with a side chain of the ligand to direct the approach of the nucleophile to one of the allylic terminal carbon atoms. The second increased the ligand's bite angle in order to create a chiral cavity in which the allyl system is perfectly embedded. To discriminate electronically between the two allylic terminal carbon atoms, the third strategy employed heterodonor ligands. Although many chiral ligands have been successfully applied in the substitution of several disubstituted substrates, problems generally remain with both substrate specificity and reaction rates using these methods. Other substrates, such as those that are monosubstituted, will require more active and more regio- and enantioselective Pd-catalysts. Overcoming these limitations requires research toward the development of new ligands. This Account discusses the application of homo- and heterodonor biaryl-containing phosphites as new, versatile, and highly effective ligands in the Pd-catalyzed asymmetric allylic substitution of several substrate types. We and others recently demonstrated that the inclusion of biarylphosphite moieties in ligand design is highly advantageous. In these systems, the catalyst's substrate specificity decreases because the chiral pocket created (the chiral cavity with the embedded allyl ligand) is flexible enough to allow the perfect coordination of hindered and unhindered substrates. Reaction rates with these ligands increase because of the larger pi-acceptor ability of these moieties. The ability of the phosphite moiety to accept pi-electrons and enhance the S(N)1 character of the nucleophilic attack increases the regioselectivity of the reactions toward the desired branched isomer in monosubstituted linear substrates. Finally, the easy synthesis of biaryl phosphites from readily available alcohols allows for simple ligand tuning as well as systematic modifications of several important ligand parameters. Taking advantage of these features, we and others have designed highly adaptative biaryl-phosphite-containing ligands for asymmetric Pd-allylic substitution reactions. In this context, several diphosphites, phosphite-oxazolines, and phosphite-phosphoroamidites have recently emerged as extremely effective ligands for this process. Using a broad range of mono- and disubstituted hindered and unhindered linear and cyclic substrates, we have obtained high activities (turnover frequencies up to 22,000 mol substrate x (mol Pd x h)(-1)) unprecedented in the literature along with excellent regio- (up to 99%) and enantioselectivitites (up to >99%) at low catalyst loadings (turnover numbers up to 10,000 mol substrate x (mol Pd x h)(-1)). Appropriate ligand tuning allows access to both enantiomers of the substitution products.

“Click” Polymer‐Supported Palladium Nanoparticles as Highly Efficient Catalysts for Olefin Hydrogenation and Suzuki Coupling Reactions under Ambient Conditions

AbstractComplexation of palladium(II) acetate [Pd(OAc)2] or dipotassium tetrachloropalladate [K2PdCl4] to “click” polymers functionalized with phenyl, ferrocenyl and sodium sulfonate groups gave polymeric palladium(II)‐triazolyl complexes that were reduced to “click” polymer‐stabilized palladium nanoparticles (PdNPs). Transmission electron microscopy (TEM) showed that reduction using sodium borohydride (NaBH4) produced PdNPs in the 1–3 nm range of diameters depending on the nature of the functional group, whereas slow reduction using methanol yielded PdNPs in the 22–25 nm range. The most active of these PdNPs (0.01% mol Pd), stabilized by poly(ferrocenyltriazolylmethyl)styrene, catalyzed the hydrogenation of styrene at 25 °C and 1 atm hydrogen, with turnover numbers (TONs) of 200,000. When stabilized by the water‐soluble poly(sodium sulfonate‐triazolylmethyl)styrene, the PdNPs (0.01% mol Pd) catalyze the Suzuki–Miyaura coupling between iodobenzene (PhI) and phenylboronic acid [PhB(OH)2] in water/ethanol (H2O/EtOH) at 25 °C with TONs of 8,200. This high catalytic activity is comparable to that obtained with “click” dendrimer‐stabilized PdNPs under ambient conditions.

Biomimetic Synthesis of Pd Nanocatalysts for the Stille Coupling Reaction

Here we report on the biomimetic synthesis of Pd nanoparticles for use as models of green catalytic systems. The nanomaterials are synthesized using peptides isolated via phage-display techniques that are specific to Pd surfaces. Using this synthetic strategy, peptide-functionalized Pd nanoparticles of 1.9 +/- 0.3 nm in diameter are produced, which are soluble and stable in aqueous solutions. Once characterized, these biobased materials were then used as catalysts to drive the formation of C-C bonds using the Stille coupling reaction. Under the conditions of an aqueous solvent at room temperature, quantitative product yields were achieved within 24.0 h employing catalyst loadings of > or = 0.005 mol % of Pd. Additionally, high TOF values of 3207 +/- 269 mol product x (mol Pd x h)(-1) have been determined for these materials. The catalytic reactivity was then examined over a set of substrates with substitutions for both functional group and halide substituents, demonstrating that the peptide-based Pd nanoparticles are reactive toward a variety of functionalities. Taken together, these bioinspired materials represent unique model systems for catalytic studies to elucidate ecologically friendly reactive species and conditions.

Nickel and palladium complexes bearing ortho-phenoxy modified anilido-imine ligands: Drastic steric effect on coordinated geometry, and catalytic property toward olefin polymerization

A series of new nickel complexes and palladium complexes bearing ortho-phenoxy modified anilido-imine ligands have been synthesized and characterized. X-ray diffraction analyses of the single crystal structures reveal that there are no direct metal–O interactions in all of the complexes. The steric hindrance of complexes has an importance influence on their coordinated geometries. The bulky complexes with 2,6-diisopropylphenyl substituent exist as a dimers with bromine-bridged structure while those with 2,6-dimethylphenyl or phenyl substituents adopt a distorted tetrahedral geometry with four nitrogen atoms of two anilido-imine ligands. The nickel complexes exhibited high activity up to 7.33 × 10 6 g/(mol of Ni · h) and palladium complexes showed very high activity up to 2.63 × 10 8 g/(mol of Pd · h) for norbornene polymerization with methylaluminoxane as cocatalyst. The nickel catalysts were attempted to polymerize ethylene at atmosphere pressure, however, only oligomers were produced.

Nickel and Palladium Complexes of Pyridine−Phosphine Ligands as Ethene Oligomerization Catalysts

Pyridine−phosphine ligands 1−5 have been used to prepare neutral nickel dichloride complexes, neutral methylpalladium chloride complexes, and cationic methylpalladium complexes. The ligands consist of a diphenylphosphine and a pyridine moiety and differ in the backbone connecting those donor groups. Nickel complexes 9−13 are paramagnetic complexes, and they were characterized by elemental analysis, high-resolution mass spectrometry, and, for 10 and 12, single-crystal X-ray diffraction. Neutral palladium complexes 14−18 were fully characterized. Single-crystal X-ray diffraction was performed on complexes 15 and 16, and variable-temperature NMR demonstrated that 16 exhibits slow inversion of the metallacycle. Cationic palladium species 19−23 were obtained from the neutral complexes after chloride abstraction. Like its neutral precursor, 21 showed slow ring inversion. The nickel species were evaluated as ethene oligomerization catalysts after activation with MAO. All complexes were highly active, with TOFs between 24 × 103 and 85 × 103 (mol C2H4)·(mol Ni·h)−1. Butenes were the major product in all cases, forming 76 to 96 mol % of the product. Selectivities for 1-butene were between 10% and 40%. The cationic palladium species showed a very low productivity for ethene oligomerization, with TOFs ≤16 (mol C2H4)·(mol Pd·h)−1 and 38 to 88 mol % butenes as the main product.

Synthesis and Characterization of NiII and PdII Complexes Bearing N,N,S Tridentate Ligands and Their Catalytic Properties for Norbornene Polymerization

AbstractA series of new N,N,S tridentate nickel(II) complexes [(ArN = CHC6H4NPh‐2‐SPh)NiBr] (Ar = 2,6‐diisopropylphenyl, 1; Ar = 2,6‐dimethylphenyl, 2; Ar = Ph, 3) and palladium(II) complexes [(ArN = CHC6H4NPh‐2‐SPh)PdCH3] (Ar = 2,6‐diisopropylphenyl, 4; Ar = 2,6‐dimethylphenyl, 5; Ar = Ph, 6) were synthesized and characterized. X‐ray diffraction analyses of the single‐crystal structures revealed that tridentate complexes 1, 2, 4, 5, and 6 featured a distorted square‐planar coordination of the central metal. Compared to the bidentate metal complexes bearing anilido–imine ligands, the N,N,S tridentate metal complexes showed better stability. In the presence of methylaluminoxane (MAO), nickel complexes 1–3 showed moderate activity toward ethylene oligomerization at atmosphere pressure, but the palladium complexes were inactive. However, the nickel complexes exhibited high activity up to 8.20 × 106 g/(mol of Ni) h and palladium complexes showed very high activity up to 2.68 × 108 g/(mol of Pd) h toward norbornene polymerization with MAO as cocatalyst. The obtained polynorbornenes are vinylic addition polymers with high molecular weights. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

High catalytic activity and stability of palladium nanoparticles prepared by the laser electrodispersion method in chlorobenzene hydrodechlorination

Palladium nanoparticles deposited on thermally oxidized silicon and on the carbon support Sibunit by the laser electrodispersion method are extremely active in the gas-phase hydrodechlorination of chlorobenzene at 100–200°C. High conversion of chlorobenzene (above 90%) has been achieved with catalysts with an unusually low metal content (from 10−4 to 10−3 wt %). The cyclohexane-to-benzene ratio in the reaction products depends on the process duration, palladium content, and support nature. According to X-ray photoelectron spectroscopy (XPS) data, palladium in the catalysts retains its metallic state over a long time under the reaction conditions. Possible causes of the high catalytic activity (105 mol (mol Pd)−1 h−1) of the palladium nanoparticles and their stability to chlorination are discussed.

The Pd2(CH CC6H5)2(C5H7O2)3(BF3)2BF4 catalyst for the polymerization of norbornene

Pd2(CHCC6H5)2(C5H7O2)3(BF3)2BF4 and Pd(acac)2 + 2PhA + 4BF3OEt2 are active catalysts for the polymerization of norbornene. For the Pd2(CHCC6H5)2(C5H7O2)3(BF3)2BF4 a “particular” catalyst activity up to 17,600 kg polynorbornene per mol Pd for an hour and viscosity average molar masses up to 520,000 g/mol were observed. IR and NMR spectroscopy study of the polymer showed the microstructure presented by the rr and mr triads with low diisotacticity.

Studies on the copolymerization of norbornene with CO catalyzed by a new catalytic system PdCl2/phen/M(CF3SO3)n

A new three-component catalytic system, PdCl2/phen/M(CF3SO3)n where M = La, Y, Yb, Zn, and Cu, was studied for the copolymerization of norbornene (NBE) with CO to prepare polyketone (PK). It was found that the CF3SO3H catalytic system gave a low catalytic activity for the copolymerization of norbornene with CO, but when M(CF3SO3)n was introduced instead of CF3SO3H, the PdCl2/phen/M(CF3SO3)n catalytic system exhibited much higher activity. The effects of ligands, M(CF3SO3)n, solvents, and temperatures on the copolymerization have been discussed in detail. The results showed that with 1,10-phenanthroline (phen) and Cu(CF3SO3)2 used as cocatalysts, the corresponding reaction rate reached 82 000 g PK (mol Pd)−1h−1 when the reaction was carried out in methanol at 90°C and 3.0 MPa of CO, and the weight average molecular weight (M w) of the resultant copolymer is 1090 g/mol. The copolymer was characterized with various techniques such as FT-IR, 1HNMR, 13CNMR, TGA, and DSC. The infrared spectrum of the product includes two features at 1697 and 1732 cm−1 for the NBE/CO copolymer in CH3OH that are attributed to carbonyl groups in ketones (repeating unit) and esters (end group), respectively. Due to the tension of the ring of norbornene, the degree of copolymerization is not high.

A Trinuclear Silver(I) Functionalized N-Heterocyclic Carbene Complex and Its Use in Transmetalation: Structure and Catalytic Activity for Olefin Polymerization

Deprotonation of an imidazolium salt with silver oxide forms the silver carbene complex {[Ag(C∧N)2]3I}I2 (2; C∧N = 3-methyl-1-picolylimidazolin-2-ylidene). The Ag−carbene complex undergoes a facile reaction with Pd(PhCN)2Cl2 to yield the carbene complex Pd(C∧N)Cl2 (3). Complexes 2 and 3 have been characterized by IR, 1H and 13C NMR spectra, and elemental analyses. The molecular structures of complexes 2 and 3 have been confirmed by X-ray single-crystal analyses. Complex 2 shows moderate catalytic activities of up to 105 g of PE (mol of Ag)-1 h-1 for ethylene polymerization, and complex 3 shows very high catalytic activities of up to 108 g of PNB (mol of Pd)-1 h-1 for the addition polymerization of norbornene in the presence of methylaluminoxane (MAO) as cocatalyst.