In The Palladium Research Articles

Extraction properties of 4-[(hexylsulfanyl)methyl]3,5-dimethyl-1-phenyl-1<i>H</i>-pyrazole in the palladium(II) recovery from nitric acid solutions

Palladium(II) extraction from nitric acid solutions with a complex-forming reagent, 4-[(hexylsulfanyl)methyl]-3,5-dimethyl-1-phenyl-1 H -pyrazole, was studied using chloroform as a diluent. The reagent extracts Pd(II) with high efficiency from 0.5-5 M. HNO3 solutions. It has been established that palladium(II) is extracted from 2 M. HNO3 solutions by a coordination mechanism with the formation of the extracted compound [Pd(NO3)2μ-L] n ( n > 2). Palladium(II) is quantitatively stripped with a nitric acid solution of thiourea. The reagent is promising for the concentration of Pd(II) from nitric acid solutions and its highly selective separation from Fe(III), lanthanides(III), Al(III), Cu(II), and Ni(II).

Extraction Properties of 4-[(Hexylsulfanyl)methyl]-3,5-dimethyl-1-phenyl-1H-pyrazole in the Palladium(II) Recovery from Nitric Acid Solutions

Extraction Properties of 4-[(Hexylsulfanyl)methyl]-3,5-dimethyl-1-phenyl-1H-pyrazole in the Palladium(II) Recovery from Nitric Acid Solutions

An Electron-Poor Dioxa-[2.1.1]-(2,6)-pyridinophane Ligand and Its Application in Cu-Catalyzed Olefin Aziridination.

A novel macrocyclic 1,7-dioxa-[2.1.1]-(2,6)-pyridinophane ligand has been synthesized and crystallographically characterized. Two derived metal complexes, dichloropalladium(II) and chlorocopper(I), were prepared. In the palladium(II) complex LPdCl2, both in the solid state, according to its crystallographic characterization, and in CH2Cl2 solutions at -40 °C, according to 1H NMR spectroscopy, the ligand adapts a C1-symmetric κ2-N,N-coordination mode in which the metal atom binds to two nonequivalent pyridine fragments of the macrocycle. The complex is fluxional at 20 °C. In the crystalline copper(I) complex LCuCl, the macrocyclic ligand is also κ2-N,N-coordinated to the metal, but it utilizes two equivalent pyridine fragments for the binding. The copper(I) complex is fluxional in CH2Cl2 solutions in the temperature range between 20 and -70 °C and is proposed to be involved in a fast intermolecular macrocyclic ligand exchange which is slowed down below -40 °C. DFT calculations predict a lower thermodynamic stability of the dioxapyridinophane-derived complexes LPdCl2 and LCuCl, as compared to their [2.1.1]-(2,6)-pyridinophane analogs containing bridging CH2 groups instead of the oxygen atoms. The electron poor dioxapyridinophane chlorocopper(I) complex, in combination with NaBArF4 (BArF4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) in dichloromethane solutions, can serve as an efficient catalyst for aziridination of various olefins with PhINTs at 0-22 °C.

Structural and Luminescent Properties of Homoleptic Silver(I), Gold(I), and Palladium(II) Complexes with nNHC-tzNHC Heteroditopic Carbene Ligands.

Novel silver(I), gold(I), and palladium(II) complexes were synthesized with bidentate heteroditopic carbene ligands that combine an imidazol-2-ylidene (nNHC) with a 1,2,3-triazol-5-ylidene (tzNHC) connected by a propylene bridge. The silver(I) and gold(I) complexes were dinuclear species, [M2(nNHC-tzNHC)2](PF6)2 (M = Ag or Au), with the two bidentate ligands bridging the metal centers, whereas in the palladium(II) complex [Pd(nNHC-tzNHC)2](PF6)2, the two ligands were chelated on the same metal center. Because of the presence of two different carbene units, isomers were observed for the gold(I) and palladium(II) complexes. The molecular structures of the head-to-tail isomer for gold(I) complexes, with a twisted or folded-syn conformation of the bridge between the carbene units, were determined by X-ray diffraction analysis. The study was completed with a systematic structural investigation through density functional theory (DFT) calculations. For palladium(II) species, the head-to-head form was structurally characterized. The dinuclear gold(I) complexes were emissive in the solid state in the blue region (PLQY up to 8%); time-dependent density functional theory (abbreviated as TD-DFT) calculations disclosed that the absorption bands have metal-to-ligand-charge-transfer character and evidenced that the emission occurs from the T1 level (phosphorescence).

Influence of the Degree of Chitosan Sulfoethylation on the Sorption of Palladium(II) Chloride Complexes from Multicomponent Solutions

The influence exerted by the degree of substitution of sulfoethylated chitosan cross-linked with glutaraldehyde on the sorption of Pd(II) chloride complexes from multicomponent solutions containing Pt(IV), Cu(II), Ni(II), Co(II), Cd(II), and Zn(II) was studied. The sorption of transition metal ions under the conditions of the experiment at pH 0.5–5.0 is virtually fully suppressed. The strongest interfering effect on the Pd(II) sorption is exerted by Pt(IV). Calculation of the selectivity coefficients KPd/Pt shows that the selectivity of the Pd(II) sorption relative to Pt(IV) increases with an increase in the degree of substitution of chitosan from 0.3 to 0.5. Integral kinetic curves of the Pd(II) sorption were obtained, and the dependences were subjected to mathematical processing using the models of diffusion and chemical kinetics. The equilibrium in the palladium(II) chloride solution–sorbent system is attained within 40 min. Pd and Pt are quantitatively desorbed from the sorbent surface under dynamic conditions with 3.5 M HCl solution.

The Palladium Acetate-Catalyzed Microwave-Assisted Hirao Reaction without an Added Phosphorus Ligand as a "Green" Protocol: A Quantum Chemical Study on the Mechanism.

It was proved by our experiments that on microwave irradiation, the mono‐ or bidentate phosphorus ligands generally applied in the palladium(II)‐catalyzed P–C coupling reaction of aryl bromides and dialkyl phosphites or secondary phosphine oxides may be substituted by the excess of the >P(O)H reagent that exists under a tautomeric equilibrium. Taking into account that the reduction of the palladium(II) salt and the ligation of the palladium(0) so formed requires 3 equivalents of the P‐species for the catalyst applied in a quantity of 5–10%, all together, 15–30% of the P‐reagent is necessary beyond its stoichiometric quantity. In the coupling reaction of diphenylphosphine oxide, it was possible to apply diethyl phosphite as the reducing agent and as the P‐ligand. The reactivities of the diethyl phosphite and diphenylphosphine oxide reagents were compared in a competitive reaction. The mechanism and the energetics of this new variation of the Hirao reaction of bromobenzene with Y2P(O)H reagents (Y=EtO and Ph) was explored by quantum chemical calculations. The first detailed study on simple reaction models justified our assumption that, under the conditions of the reaction, the trivalent form of the >P(O)H reagent may serve as the P‐ligand in the palladium(0) catalyst, and shed light on the fine mechanism of the reaction sequence. The existence of the earlier described bis(palladium complex) {[H(OPh2P)2PdOAc]2} was refuted by high level theoretical calculations. This kind of complex may be formed only with chloride anions instead of the acetate anion. The interaction of palladium acetate and Y2P(O)H may result in only the formation of the [(HO)Y2P]2Pd complex that is the active catalyst in the Hirao reaction. The new variation of the Hirao reaction is of a more general value, and represents the greenest protocol, as there is no need for the usual P‐ligands. Instead, the >P(O)H reagent should be used in an excess of up to 30%. Hence, the costs and environmental burdens may be decreased.

ChemInform Abstract: Design and Synthesis of 2‐Methyl‐7‐aminobenzoxazole as Auxiliary in the Palladium(II)‐Catalyzed Arylation of a β‐Positioned C(sp3)—H Bond.

AbstractThe direct Pd‐catalyzed site‐selective arylation of methylene C(sp3)—H bonds in aliphatic carboxylic amides is developed using 2‐methyl‐7‐aminobenzoxazole (Oxa) as an effective auxiliary.

Design and Synthesis of 2‐Methyl‐7‐aminobenzoxazole as Auxiliary in the Palladium(II)‐Catalyzed Arylation of a beta‐Positioned C(sp3)H Bond

AbstractA palladium(II)‐catalyzed direct arylation of methylene C(sp3)H bonds by 2‐methyl‐7‐aminobenzoxazole as an effective auxiliary is reported. This process exhibited high beta‐site selectivity, broad substrate scope, and compatibility with different functional groups with moderate to high yields up to 89%.magnified image

Kinetic Studies on the Palladium(II)-Catalyzed Oxidative Cross-Coupling of Thiophenes with Arylboron Compounds and Their Mechanistic Implications.

Reaction orders for the key components in the palladium(II)-catalyzed oxidative cross-coupling between phenylboronic acid and ethyl thiophen-3-yl acetate were obtained by the method of initial rates. It turned out that the reaction rate not only depended on the concentration of palladium trifluoroacetate (reaction order: 0.97) and phenylboronic acid (reaction order: 1.26), but also on the concentration of the thiophene (reaction order: 0.55) and silver oxide (reaction order: -1.27). NMR spectroscopy titration studies established the existence of 1:1 complexes between the silver salt and both phenylboronic acid and ethyl thiophen-3-yl acetate. A low inverse kinetic isotope effect (kH /kD =0.93) was determined upon employing the 4-deuterated isotopomer of ethyl thiophen-3-yl acetate and monitoring its reaction to the 4-phenyl-substituted product. A Hammett analysis performed with para-substituted 2-phenylthiophenes gave a negative ρ value for oxidative cross-coupling with phenylboronic acid. Based on the kinetic data and additional evidence, a mechanism is suggested that invokes transfer of the phenyl group from phenylboronic acid to a 1:1 complex of palladium trifluoroacetate and thiophene as the rate-determining step. Proposals for the structure of relevant intermediates are made and discussed.

Palladium(II)-Catalysed Oxidation of Alkenes

This review provides a summary of recent developments in the palladium(II)-catalysed oxidation of alkenes, focusing largely on reactions which lead to the formation of new carbon–oxygen or carbon–nitrogen bonds. Three classes of reaction are covered: i) oxidations proceeding via allylic C–H bond cleavage and formation of a π-allyl complex; ii) Wacker-type oxidations proceeding via nucleopalladation followed by β-hydride elimination; and iii) 1,2-difunctionalisation of alkenes proceeding via nucleopalladation followed by functionalisation of the resulting σ-alkylpalladium(II) intermediate. The mechanisms are discussed alongside the scope and limitations of each reaction. 1 Introduction 1.1 Background 1.2 Oxidation Pathways 1.3 Observation of Reaction Intermediates 2 Allylic Oxidation 2.1 Background 2.2 Allylic Oxygenation 2.3 Allylic Amination 2.4 Allylic Functionalisation with Other Nucleophiles 3 The Wacker Oxidation 3.1 Background 3.2 Variation of the Co-Oxidant 3.3 Direct Oxygen-Coupled Wacker Oxidations 3.4 Aldehyde-Selective Wacker Oxidations 3.5 Wacker Oxidation of Internal Alkenes 3.6 Aza-Wacker Oxidations 4 Intermolecular 1,2-Difunctionalisation of Alkenes 4.1 Introduction 4.2 Oxyhalogenation Reactions 4.3 Dioxygenation Reactions 4.4 Oxycarbonylation Reactions 4.5 Aminohalogenation Reactions 4.6 Diamination Reactions 4.7 Aminooxygenation Reactions 4.8 Aminocarbonylation Reactions 5 Summary and Conclusions

Switching the Z/E selectivity in the palladium(II)-catalyzed decarboxylative Heck arylations of trans-cinnamaldehydes by solvent.

The Z/E selectivity of Pd(II)-catalyzed decarboxylative Heck-type arylations of trans-cinnamaldehydes can be controlled readily by switching the reaction solvent. Depending on the type of solvent used, each of the two isomeric products can be obtained with good to excellent Z/E ratio. In THF, Z-isomers were formed preferentially, whereas DMF provided the E-isomers predominantly.

Synthesis, characterization, and electrocatalytic properties of 3D aggregates of palladium nanocrystallites

Three-dimensional (3D) aggregates composed of 5·0±0·4 nm palladium nanocrystallites (PdNCs) were prepared using ultrasonics in the palladium (II) chloride (PdCl2) H2O/EtOH (5∶1, V/V) solution by the addition of a quantity of Poly(vinyl pyrrolidone) (PVP) and hexadecyl trimethyl ammonium bromide (CTAB). The morphologies, crystal structures and the optical properties of 3D Pd nanostructures were characterised by X-ay diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, selected area electron diffraction and UV-visible absorption spectroscopy, respectively. The mechanism of sonochemical reduction of Pd (II) ions was also discussed. The results showed that the formation of aggregates of PdNCs depended on the interaction between PVP and CTAB. Moreover, the electrocatalytic properties of 3D Pd nanostructures modified glassy carbon electrode for ethanol oxidation had been also investigated by cyclic voltammetry. The material exhibited remarkable electrocatalytic behaviour for ethanol oxidation in 1M KOH and appeared as a promising candidate for use in direct ethanol fuel cells.

Sonochemical synthesis of three-dimensional aggregates of pd nanoparticles with high electrocatalytic activity towards ethanol oxidation

Flower-like aggregates composed of (4.0±0.8) nm palladium(Pd) nanoparticles were prepared via ultrasonics in the palladium(II) chloride(PdCl2) H2O/EtOH(5/1, volume ratio) solution with the addition of a quantity of poly(vinyl pyrrolidone)(PVP) and sodium dodecyl sulfonate(SDS). The morphologies, crystal structures and the optical properties of the flower-like Pd nanostructures were characterized by X-ray diffraction(XRD). X-ray photoelectron spectroscopy(XPS). scanning electron microscopy(SEM). transmission electron microscopy(TEM). selected area electron diffraction(SAED) and UV-visible absorption spectroscopy, respectively. The mechanism of sonochemical reduction of Pd(II) ions was also investigated. The results show that the molar ratio of PVP to SDS affected the formation of the flower-like aggregates of Pd nanoparticles. Moreover, the electrocatalytic properties of Pd aggregates modified glassy carbon electrode for ethanol oxidation were also investigated by cyclic voltammetry(CV). This material exhibits remarkable electrocatalytic activity for ethanol oxidation in 1 mol/L KOH and appears as a promising candidate to be applied in direct ethanol fuel cells.

Modular Palladium Bipyrazoles for the Isomerization of Allylbenzenes – Mechanistic Considerations and Insights into Catalyst Design and Activity, Role of Solvent, and Additive Effects

AbstractThe catalytic activity of novel bidentate N,N‐chelated palladium complexes derived from electron excessive, backbone fused 3,3′‐bipyrazoles in the selective isomerization of terminal arylpropenoids and 1‐alkenes is described. The catalysts are easily modified by appropriate wing tip substitution, while maintaining the same bulky, rigid unreactive aliphatic backbone. Eleven novel palladium complexes with different electronic and steric properties were investigated. Their performance in the palladium(II)‐catalyzed isomerization of a series of substituted allylbenzenes was evaluated in terms of electronic as well as steric effects. Besides the clear finding of a general trend towards higher catalyst activity with more electron‐donating properties of the coordinated N,N‐bidentate ligands, we found that the catalytic process strongly depends on the choice of solvents and additives. Extensive solvent screening revealed that reactions run best in a 2:1 toluene‐methanol mixture, with the alcohol employed being a crucial factor in terms of electronic and steric factors. A reaction mechanism involving a hydride addition–elimination mechanism starting with a palladium hydride species generated in situ in alcoholic solutions, as corroborated by experiments using deuterium labeled allylbenzene, seems to be most likely. The proposed mechanism is also supported by the observed reaction rate orders of κobs[cat.]≈1 (0.94), κobs [substrate]=0.20→1.0 (t→∞) and κobs [methanol]=−0.51 for the isomerization of allylbenzene. Furthermore, the influence of acid and base, as well as the role of the halide coordinated to the catalyst, are discussed. The system catalyzes the isomerization of allylbenzenes very efficiently yielding high E:Z selectivities under very mild conditions (room temperature) and at low catalyst loadings of 1 mol% palladium even in unpurified solvents. The integrity and stability of the catalyst system were confirmed by multiple addition reaction cycles, successive filtration and isolation experiments, and the lack of palladium black formation.

Extractive recovery of palladium(II) from hydrochloric acid solutions with Cyphos®IL 104

Extractive recovery of palladium(II) from a hydrochloric acid solution with quaternary phosphonium salt: trihexyl(tetradecyl)phosphonium bis-(2,4,4-trimethylpentyl)phosphinate (Cyphos®IL 104) as a novel reagent in the presence of toluene has been investigated. Extraction data indicate that Cyphos®IL 104 is a very efficient and fast extractant. The increase in HCl concentration has negative influence on the extraction and 96 and 52% of palladium(II) ions can be effectively extracted with Cyphos®IL 104 from 0.1 and 3.0M HCl, respectively. The equilibrium of palladium(II) extraction from aqueous 0.1 and 3.0M HCl with this phosphonium ionic liquid is achieved after 5min. Extraction of palladium(II) with Cyphos®IL 104 proceeds at the interface according to the anion-exchange mechanism. The best stripping solution from among the studied ones is 0.5M NH4OH. The feasibility of reuse Cyphos®IL 104 in several cycles of extraction-stripping process was also examined. Increase in temperature from 20 to 70°C causes a negligible decrease in the palladium(II) extraction. The calculated values of ΔH°, ΔS° and ΔG° evidence that the extraction of palladium(II) with Cyphos®IL 104/toluene mixture is an exothermic reaction both from 0.1 and 3.0M HCl solutions.

Direct Hiyama Cross-Coupling of Enaminones With Triethoxy(aryl)silanes and Dimethylphenylsilanol

2,3-Dihydropyridin-4(1H)-ones undergo direct C-H functionalization at C5 in the palladium(II)-catalyzed Hiyama reaction, using triethoxy(aryl)silanes and dimethylphenylsilanol. The reagent CuF(2) has a dual role in the reactions with triethoxy(aryl)silanes. It is a source of fluoride to activate the silane in the Hiyama reaction and also serves as the reoxidant to convert Pd(0) to Pd(II) in the catalytic cycle.

Confirmation of the absolute configuration of (−)-aurantioclavine

We confirm our previous assignment of the absolute configuration of (−)-aurantioclavine as 7 R by crystallographically characterizing an advanced 3-bromoindole intermediate reported in our previous synthesis. This analysis also provides additional support for our model of enantioinduction in the palladium(II)-catalyzed oxidative kinetic resolution of secondary alcohols.

Chelate-Size Effects on the Structures, Chemical Behavior, Properties, and Catalytic Activity of the New Palladium(II)−Allyl Complexes [Pd(η3-1-R1-C3H4){FcCH═N-CH2-(CH2)n-NMe2}][PF6] {Fc = (η5-C5H5)Fe(η5-C5H4), n = 2 or 1, and R1 = H or Ph}

The synthesis, X-ray crystal structures, and the study of the solution behavior of the palladium(II) allyl complexes [Pd(η3-1R1-C3H4){FcCH═N-CH2-(CH2)n-NMe2}][PF6] {with Fc = (η5-C5H5)Fe(η5-C5H4), R1 = H, and n = 2 (4) or 1 (5) or R1 = Ph and n = 2 (6) or 1 (7)} are described. The structural studies of 4, 5, 6, and 7 showed that the allyl ligands were bound to palladium(II) in an η3-fashion and the ferrocenyl-Schiff bases behaved as neutral (N,N′) bidentate ligands. As observed for other Pd complexes of ligands 1 and 2, the imine conformation of the ligand in the complex strongly depends on the length of the hydrocarbon chain between the two nitrogen donors: compounds 4 and 6 exhibited an anti-(E) conformation, whereas the syn-(Z) form is observed in 5 and 7. 57Fe-Mössbauer spectra (in the solid state) and electrochemical studies based on cyclic voltammetry of 4, 5, 6, and 7 are reported. These studies reveal that the proclivity of the iron(II) toward oxidation is affected by the nature of the substituent R1 of the allyl group {H (in 4 and 5) or Ph (in 6 and 7)}. Compounds 4 and 5 have been tested in the palladium(II) allylic alkylation of (E)-3-phenyl-2-propenyl (cinnamyl) acetate using sodium diethyl 2-methylmalonate as the nucleophile. In all cases the formation of the trans-(E) isomer of the linear product (9) was strongly favored versus that of the branched derivative (10). The stoichiometric reactions of the palladium(II) allyl intermediates [Pd(η3-1-Ph-C3H4){FcCH═N-CH2-(CH2)n-NMe2}][PF6] {with n = 2 (6) or 1 (7)} with the nucleophile have been studied. Density functional theory (DFT) calculations of these systems have allowed us to rationalize the structural differences as well as the regioselectivity of the catalytic and stoichiometric reactions.

DFT STUDIES ON THE STABILITY OF THE TRANS AND CIS ISOMERS IN THE SQUARE PLANAR PALLADIUM(II) COMPLEXES Pd(I)(PPh3)(η3-XCHC(Ph)CHR) (X = CMe3, CO2Me, P(O)(OMe)2, AND SO2H; R = H, Me)

The relative stability of the trans and cis isomers in the square planar palladium(II) complexes Pd ( I )( PPh 3)(η3- XCHC ( Ph ) CHR ) ( X = H , Me , CMe 3, CO 2 Me , P ( O )( OMe )2, and SO 2 H ; R = H , Me ) was investigated with the aid of the B3LYP density functional theory calculations. We examined how the substituents X, with different electronic properties, of the η3-allyl ligands affect the relative stability of the trans and cis isomers. Through the investigation, we were able to explain the trans/cis relative stability derived from the experimentally measured trans/cis isomer ratios in the palladium(II) complexes.

Enantio‐ and Diastereotopos Differentiation in the Palladium(II)‐Catalyzed Hydrosilylation of Bicyclo[2.2.1]alkene Scaffolds with Silicon‐Stereogenic Silanes

AbstractThe palladium(II)‐catalyzed hydrosilylation of meso‐configured bicyclo[2.2.1]alkene scaffolds proved to be an invaluable model reaction for the development of reagent‐controlled asymmetric transformations based on silicon‐stereogenic silanes as stereoinducers. In the present investigation, the subtle structural requirements of the silane substitution pattern in enantiotopos‐differentiating single hydrosilylations of a norbornene‐type substrate are disclosed. Extension of this chemistry to a double hydrosilylation of norbornadiene entails a significant increase in stereochemical complexity. Although differentiation of enantiotopic positions by the chiral reagent is demanded in the first hydrosilylation, the same reagent must then differentiate diastereotopic positions in the second. Remarkably high stereocontrol was found in this double hydrosilylation with several silanes first used in the hydrosilylations of the norbornene‐type system. Depending on the enantiomeric purity of the silane, C2‐ and Cs‐symmetric adducts, respectively, were obtained. The identity of the key quaternary silanes was revealed by crystallographic analysis. By this method, the relative and absolute configurations were also assigned, which, in turn, imply that all enantiospecific substitutions at silicon proceed with stereoretention. On the basis of these solid‐state structures, we also discuss the structural implications of silane substitution for the diastereoselectivity‐determining step of this palladium(II)‐catalyzed hydrosilylation reaction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)