Preparation and electrochemical properties of palladium(0) complexes coordinated by quinones and 1,5-cyclooctadiene

The complexes Pd(quinone)(COD) (COD = 1,5-cyclooctadiene) are prepared by a ligand substitution reaction of Pd₂(DBA)₃ (DBA = dibenzylideneacetone) in the presence of both quinone and COD. Palladium(0) complexes coordinated by quinones only are formed in the reaction in the absence of COD. The cyclic voltammetric behavior of Pd(quinone)(COD) has been studied. The reduction potentials for quinones shifted toward negative values on coordination to palladium(0). The oxidation potentials for the central palladium(0) in Pd(quinone)(COD) depend on the electron-withdrawing ability of the free quinones, and are in the following series: quinone = p-benzoquinone < 5,8-dihydro-1,4-naphthoquinone ～ 1,4-naphthoquinone < duroquinone. The shift of oxidation potentials for Pd(quinone)(COD) on changing the quinones as ligands is in contrast to that of Pd(quinone)(triphenylphosphine)₂.     

A new type of palladium(0) complex having both electron-withdrawing and -donating sites in a ligand: 5,8-dihydro- and 5,8,9,10-tetrahydro-1,4-naphthoquinone complexes

A new type of palladium(0) complex, (5,8-dihydro-1,4-naphthoquinone)Pd(PPh₃)₂ and (5,8,9,10-tetrahydro-1,4-naphthoquinone)Pd(PPh₃)₂, having both olefin and quinone or dihydro-quinone sites in a ligand molecule was prepared. IR and ¹H NMR spectroscopic studies of these complexes suggested that it is the quinone or dihydro-quinone CC bond which is complexed to Pd. Ligand exchange reactions showed that the stability order of the olefinic quinone complexes was as follows: (1,4-naphthoquinone)Pd(PPh₃)₂ > (5,8-dihydro-1,4-naphthoquinone) Pd(PPh₃)₂>(5,8,9,10-tetrahydro-1,4-naphthoquinone)Pd(PPh₃)₂.     

Catalytic Synthesis of 2-Methyl-1,4-Naphthoquinone (Vitamin K3) Over Silica-Supported Aminomethyl Phosphine-Ru(II), Pd(II), and Co(II) Complexes

Ru(II), Pd(II), and Co(II) complexes of the free ditertiary aminomethylphosphine ligand, N,N-bis(diphenylphosphinomethyl)aminopropyltriethoxysilane [(EtO)₃Si(CH₂)₃ N(CH₂PPh₂)₂] (DIPAPTES), and its SiO₂-DIPAPES have been synthesized under a nitrogen atmosphere using Schlenk techniques. All the complexes were used as catalysts for the oxidation of 2-methyl naphthalene (2MN) to give 2-methyl-1,4-naphthoquinone (vitamin K₃, menadione, 2MNQ) in the presence of hydrogen peroxide as a clean and cheap oxidant. The catalytic synthesis of vitamin K₃ was investigated using both homogeneous catalysis with free complexes and heterogeneous catalysis with silica-supported complexes. [(DIPAPTES)PdCl₂] and its silica-supported form showed the best catalytic activity for the selective oxidation of 2-methyl naphthalene to 2-methyl-1,4-naphtoquinone compared to the other metal complexes. 2MNQ yield reached 52.26% with the 2MN conversion of 90.52% using complex [(DIPAPTES)PdCl₂] and 58.59% with the 2MN conversion of 99.56% using the silica supported [SiO₂(DIPAPES)PdCl₂] complex for 1 h. Recycling was investigated for the silica-supported Pd(II) complex and compared with the classical production of vitamin K₃.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.     

Preparation and spectroscopic and electrochemical properties of complexes of dibenzo-and dipyrido-substituted 1,4-diazines

The spectroscopic and electrochemical properties of palladium ethylenediamine complexes [Pd(N^N)En]Cl₂ with 1,4-diazine derivatives of o-phenanthroline [(N^N) = dipyrido[a,c]phenazine (dppz), dipyrido[f,h]quinoxaline (dpq)] were studied in comparison with those of the free diimine (N^N) ligands, dibenzo-substituted 1,4-diazines [dibenzo[f,h]quinoxaline (dbq), dibenzo[a,c]phenazine (dbpz)], and cyclometallated dichloride [Pd(C^N)(μ-Cl)]₂ and ethylenediamine [Pd(C^N)En]Cl complexes derived from dibenzo-substituted 1,4-diazines.     

Mechanism of the oxidative carbonylation of terminal alkynes at the ≡C-H bond in solutions of palladium complexes

The formation mechanism of the active catalyst in the oxidative carbonylation of terminal alkynes at the ≡C-H bond has been investigated for the catalytic system Pd(OAc)₂-PPh₃-p-benzoquinone (Q)-MeOH. It has been demonstrated by NMR spectroscopy, X-ray crystallography, and kinetic measurements that the catalytically active palladium is in the oxidation state 0 and is bound into complexes stabilized by p-benzoquinone (PdL₂Q, where L = PPh₃). A mechanism is suggested for the catalytic process, which includes the formation of the complex PdL₂Q, the oxidative addition of the alkyne to this complex at the ≡C-H bond, the insertion of CO into the Pd-C bond, and steps in which hydride hydrogen is intramolecularly transferred to the p-quinone.     

Mechanism of the formation of palladium complexes serving as catalysts in hydrogenation reactions : II. Reactivity of palladium phosphine halides towards molecular hydrogen

[(PPh₃)₃(PPh₂)₂Pd₃Cl] Cl, benzene and aniline hydrochloride were isolated as products of the reactions of (PPh₃)₂PdCl₂]₂ or [(PPh₃)PdCl₂]₂ with H₂ in organic amines (Am). Similar products were obtained when (Ph₃P)₂Pd(Ph)Br was treated with H2₃ Both in amines and aromatic solvents. The reaction between H₂ and [(PBu₃)PdCl₂]₂ resulted in the formation of [(PBu₃(PBu_2)PdCl2 ·. 2 Am The kinetic data for H₂ absorption by solutions of palladium(II) complexes are consistent with the heterolytic mechanism of cleavage fo hte HH bond in the coordination sphere of palladium(II); the function of the H⁺ acceptor being performed by the bases (e.g. Am or Ph). The reaction between the palladium complexes and H₂ is autocatalytic. Reduction of the initial Pd^II complexes leads to lower oxidation state palladium complexes, which catalyse the reduction of Pd^II complexes. In the coordination sphere of the lower oxidation state palladium complexes, the oxidative addition of PR₃ to Pd takes place with formation of compounds containing a Pd-R bond. It is the reaction between these complexes and H₂ that yields palladium compounds with PR₂ ligands.     

Influence of the reaction parameters on the Pd—HCl catalysed synthesis of phenylacetic acid derivatives via reduction of mandelic acid derivatives with carbon monoxide

The catalytic system Pd/C—HCl is highly active in the reduction of mandelic acid derivatives to phenylacetic acid derivatives with carbon monoxide when the aromatic ring is para-substituted with a hydroxy group. Typical reaction conditions are: 70–110 °C, 20–100 atm of carbon monoxide, benzene—ethanol as reaction medium, substrate/Pd=10²–10⁴/1, HCl/substrate=0.3–0.8/1. [Pd] = 10⁻² −10⁻⁴ M. When the catalytic system is used in combination with PPh₃ a slightly higher activity is observed. Comparable results are observed when using a Pd(II) catalyst precursor such as PdX₂, in combination with PPh₃, or PdX₂(PPh₃)₂ (XCl, AcO). When operating at 110 °C, decomposition to metallic palladium occurs. Pd(II) complexes with diphosphine ligands, such as diphenylphosphinemethane, -ethane, -propane or -butane, do not show any catalytic activity and are recovered unchanged. These observations suggest that Pd(0) complexes play a key role in the catalytic cycle. The proposed catalytic cycle proceeds as follows: the chloride ArCHClCOOR, formed in situ upon reaction of ArCHOHCOOR with hydrochloric acid, oxidatively adds to a Pd(0) species with formation of a catalytic intermediate having a Pd—[CH(Ar)COOR] moiety, which inserts a CO molecule, yielding an acyl intermediate of the type Pd—[COCH(Ar)COOR]. The nucleophilic attack of H₂O on the carbon atom of the carbonyl ligand gives back the Pd(0) complex to the catalytic cycle and yields a phenylmalonic acid derivative, which produces the final product, ArCH₂COOR, upon CO₂ evolution. Alternatively, protonolysis of the intermediate having a Pd—[CH(Ar)COOR] moiety yields directly the final product and a Pd(II) species, which is then reduced by CO to Pd(0). Moreover, no catalytic activity is observed when the Pd/C—HCl system is used in combination with any one of the above diphosphine ligands, probably because these ligands block the sites on the catalyst able to promote the catalytic cycle or because they prevent the reduction of Pd(II) to Pd(0). The influence of the following reaction parameters has been studied: concentration of HCl, PPh₃, palladium and substrate, pressure of carbon monoxide, the temperature, reaction time and solvent. The results are compared with those obtained in the carbonylation of aromatic aldehydes to phenylacetic acid derivatives catalyzed by the same system, for which it has been proposed that the catalysis occurs via carbonylation of the aldehyde to a mandelic acid derivative as an intermediate, which is further reduced with CO to yield the final product.     

Mechanism of the solvent effect on the rate of the cyclohexene hydroxymethoxycarbonylation catalyzed by bis(triphenylphosphine)palladium

Kinetic data of the cyclohexene hydroxymethoxycarbonylation catalyzed by bis(triphenylphosphine) palladium Pd(PPh₃)₂ were processed and considered on the basis of the quantum-chemical calculations. By the method of density functional DFT PBE/3z we found that among the possible catalyst moleculs based on the tetrakis(triphenylphosphine)palladium the most stable is Pd(PPh₃)₂ with the coordination number of palladium equal 2. The interaction energy of Pd(PPh₃)₂ with acetone, acetonitrile, dichloroethane, 1,4-dioxane, nitromethane, and tetrahydrofuran calculated by PM3 method was found to correlate linearly with the reaction rate logarithm. The mechanism of the solvent effect on the reaction rate consists in a specific complexation with the catalyst depending on the molecule rigidity and the creation of energetic and steric constraints for the substrate to access the catalyst.     

Synthesis and structural characterization of binuclear palladium(II) complexes of salicylaldimine dithiosemicarbazones

A series of binuclear palladium(II) salicylaldiminato dithiosemicarbazone complexes have been synthesized and characterized. The palladium complexes were obtained by the reaction of various ethylene- and phenylene-bridged dithiosemicarbazones with Pd(PPh₃)₂Cl₂. The free salicylaldimine ligands and their palladium complexes were characterized by NMR and IR spectroscopies, ESI-mass spectrometry, elemental analyses and for two representative complexes also by X-ray diffraction. In both metal complexes, the solid-state structures show the two palladium centers to be coordinated in a slightly distorted square-planar geometry, which gives rise in each case to five- and six-membered chelate rings. The salicylaldimine thiosemicarbazone ligands coordinate to palladium in a tridentate manner, through the phenolic oxygen, imine nitrogen and thiolate sulfur atoms.     

Synthesis of regioregular poly(3‐octylthiophene)s via Suzuki polycondensation and end‐group analysis by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry

Regioregular poly(3‐octylthiophene)s were synthesized through a palladium‐catalyzed Suzuki polycondensation of 2‐(5‐iodo‐4‐octyl‐2‐thienyl)‐4,4,5,5‐tetramethyl‐1,3,2‐dioxaborolane. The effects of the palladium catalyst {tetrakis(triphenylphosphine)palladium(0) [Pd(PPh₃)₄], palladium(II) acetate [Pd(OAc)₂], [1, 1′‐bis(diphenylphosphino)ferrocene]dichloropalladium(II) [Pd(dppf)Cl₂], tris(dibenzylideneacetone)dipalladium(0), or bis(triphenylphosphine)palladium(II) dichloride [Pd(PPh₃)₂Cl₂]} and the reaction conditions (bases and solvents) were investigated. NMR spectroscopy revealed that poly(3‐octylthiophene)s prepared via this route were essentially regioregular. According to size exclusion chromatography, the highest molecular weights were obtained with in situ generated Pd(PPh₃)₄ and tetrakis(tri‐o‐tolylphosphine]palladium(0) {Pd[P(o‐Tol)₃]₄} catalysts or more reactive, phosphine‐free Pd(OAc)₂. Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry was used to analyze end groups and allowed the determination of some mechanistic aspects of the Suzuki polycondensation. The polymers were commonly terminated with hydrogen or iodine as a result of deboronation and some deiodination. Pd(PPh₃)₄, Pd(PPh₃)₂Cl₂, and Pd[P(o‐Tol)₃]₄ induced aryl–aryl exchange reactions with the palladium center and resulted in some chains having phenyl‐ and o‐tolyl‐capped chain ends. Pd(dppf)Cl₂ yielded only one type of chain, and it had hydrogen end groups. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1454–1462, 2005     

Studies of the oxidation of palladium complexes by the advanced oxidation process Pretreatment of model catalysts for precious metal analysis

The advanced oxidation process (AOP) for the pretreatment of model palladium catalysts has been studied. Most standard metal analysis techniques are for metal ions free of organic ligands. Spent palladium catalysts contain organic ligands that need to be removed prior to analysis. AOP uses a combination of hydrogen peroxide and UV light to generate radicals that decompose such ligands, freeing up metals for further analysis. Palladium acetate Pd(OAc)₂, palladium acetylacetonate Pd(acac)₂, and tris(dibenzylideneacetone)dipalladium (Pd₂(dba)₃) were chosen as model precious metal catalysts for investigation. AOP was found to decompose ligands in Pd(OAc)₂, Pd(acac)₂ and give accurate Pd(II) quantification, while ligand decomposition and oxidation of Pd(0) to Pd(II) were demonstrated in treatments involving Pd₂(dba)₃. The effects of solubility of the palladium complexes, continuous addition of H₂O₂ during AOP treatments, sample pH, concentration of H₂O₂, and length of UV irradiation are reported.     

Study of equilibria in solutions of buckminsterfullerene exohedral derivative, (η2-C60)Pd(PPh3)2, using electrochemical methods and electronic spectroscopy

Polarization curves of electrochemical oxidation and reduction, as well as electronic absorption spectra of the palladium complex of fullerene, C₆₀Pd(PPh₃)₂, and its mixtures with Pd(PPh₃)₄ have been studied in toluene-acetonitrile (9:1) solutions. The experimental data can be explained by the assumption that equilibrium takes place between the initial complex, polymetallated compounds, C₆₀[Pd(PPh₃)₂]_n (n=1–4), and free fullerene, C₆₀.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2153–2158, December, 1994.This work was supported by the Russian Foundation for Basic Research, Project No. 93-03-18725.     

Bis[N-(2,6-di-t-butyl-1-hydroxyphenyl)salicylideneaminato]palladium(II) complexes and their radical intermediates generated by PbO2 and PPh3

New bis[N-(2,6-di-t-butyl-1-hydroxyphenyl)salicylideneminato]palladium(II) [Pd(L ^x )₂] complexes bearing HO and MeO substituents on the salicyaldehyde moiety were prepared, and their spectroscopic properties, as well as redox reactivity towards PbO₂ and PPh₃, examined by e.s.r. and u.v. spectroscopy. The complexes display charge-transfer bands in the 670–692 nm range in polar solvents, which are assigned to the d(Pd) * (chelated quinoid) transition. One-electron oxidation of Pd(L ^x )₂ produces Pd^II-stabilized radicals in which the unpaired electrons are localized on the phenoxy fragments and do not couple with the two radical centers. The complexes are easily reduced with PPh₃ via intramolecular electron-transfer from ligand to metal to give various radical intermediates and Pd. All detected radical species have been characterized by e.s.r. spectroscopy.     

Photoreactions of p-quinones with dimethyl sulfide and dimethyl sulfoxide in aqueous acetonitrile. goerner@mpi-muelheim.mpg.de

The effects of dimethyl sulfide (DMS) and dimethyl sulfoxide (DMSO) on the photoreactions of 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), 9,10-anthraquinone (AQ) and several derivatives in acetonitrile/water were studied. The observed triplet state of the quinones is quenched and the rate constant is close to the diffusion-controlled limit for reactions of most quinones with DMS and lower with DMSO. Semiquinone radical anions (Q*-) produced by electron transfer from sulfur to the triplet quinone were detected. For both DMS and DMSO the yield of Q*- is similar, being generally low for BQ and NQ, substantial for AQ and largest for chloranil. The specific quencher concentrations and the effects of quinone structure and redox potentials on the time-resolved photochemical properties are discussed.     

Palladium(0) catalyzed reaction of 1,3-diene 1,4-epiperoxides

Reaction of 1,3-diene 1,4-epiperoxides with Pd(PPh₃)₄ catalyst forms the corresponding 4-hydroxy enones, syn diepoxides, and 1,4-diols as the major products. The results are interpreted as being due to competing Pd(0)/Pd(II) and Pd(0)/Pd(I) exchange mechanisms.     

The reaction of formyl fluoride with transition metal complexes

Formyl fluoride reacts with metal carbonyl anions in a manner similar to acetic formic anhydride. Although formyl complexes may have been formed as unstable intermediates, no neutral formyl complexes could be isolated but rather the expected decomposition products, the metal carbonyl hydrides or ?imers, were produced. The attempted oxidative addition of formyl fluoride to various coordinately unsaturated metal complexes also did not result in the formation of formyl derivatives. HF adducts were obtained from the reaction ?fIr(CO)Cl(PR₃)₂ or M(PPh₃)₄ (M Pt or Pd) with formyl fluoride whereas Ru(NO)Cl(PPh₃)₂ and Rh(PPh₃)₃ Cl give the CO complexes Ru(NO)(CO)Cl(PPh₃)₂ and Rh(CO)Cl(PPh₃)₂, respectively.     

Reaction of Palladium Bis(acetylacetonate) with Diphenylphosphine: Spectral Studies

Interaction of palladium bis(acetylacetonate) with diphenylphosphine is studied by NMR, IR, and UV methods. Reaction between reagents taken in equimolar amounts gives binuclear and trinuclear palladium complexes with bridging diphenylphosphide and the chelate acetylacetonate [Pd(Acac)PPh₂]₂ and [Pd₃(Acac)₂(PPh₂)₄] ligands. With excess PPh₂H, the trinuclear palladium complex, whose composition is supposed to be [Pd₃(PPh₂)₄(PPh₂–PPh₂) · C₆H₆], is isolated and characterized on the basis of the spectral data.     

Palladium catalyzed reductive decarboxylation of allyl α-alkenyl-β-ketoesters. A new synthesis of (E)-3-alkenones

The reductive decarboxylation of α-alkenyl derivatives of allyl-β-ketoesters was achieved by use of palladium(0) catalyst generated in situ from Pd(OAc)₂ and PPh₃, with triethylammonium formate as the hydride source, in THF. The reaction proceeds smoothly and cleanly, with linear alkenyl derivatives of allyl-β-ketoesters, to afford (E)-3-alkenones in good to excellent yields (73-92%) and high stereoselectivity (>98%).     

Synthesis and crystal structures of new palladium catalysts for the hydromethoxycarbonylation of alkenes

The preparation and structural characterization of dimeric Pd(I)-Pd(I) complex [Pd₂{(PPh₃)(OSO₂CF₃)}₂].CH₂Cl₂ (1) and three palladium center [Pd₃{(PPh₃)(OSO₂CF₃)}₂] (2) and [Pd₃(PPh₃)₄](SO3CF₃)₂ (3) complexes are reported. The complexes exhibit coordination in which the phosphine phenyl ring is used to stabilize Pd(I) centers in (1) and, Pd(I) and Pd(0) centers in (2) and (3) by acting as π electron donors. The complexes were characterized by single crystal X-ray crystallography.     

Amidophosphine‐stabilized palladium complexes catalyse Suzuki–Miyaura cross‐couplings in aqueous media

We report a simple and efficient procedure for Suzuki–Miyaura reactions in aqueous media catalysed by amidophosphine‐stabilized palladium complexes trans‐{L³PPh₂}₂PdCl₂ ( 3 ), trans‐{L³PPhtBu}₂PdCl₂ ( 4 ), [Pd(η³‐C₃H₅)(L³PPh₂)Cl] ( 5 ) and {Pd[2‐(Me₂NCH₂)C₆H₄](L³PPh₂)Cl} ( 6 ). The acidity of the NH proton in complexes 3 , 4 , 5 , 6 plays an important role in their catalytic activity. In addition, the palladium complexes cis‐{L¹PPh₂}PdCl₂ ( 1 ) and trans‐{L²PPh₂}₂PdCl₂ ( 2 ) stabilized by phosphines containing Y,C,Y‐chelating ligands L^1,2 have also been found to be useful catalysts for Suzuki–Miyaura reactions in aqueous media. The method can be effectively applied to both activated and deactivated aryl bromides yielding high or moderate conversions. The catalytic activity of couplings performed in pure water increases when utilizing a Pd complex with more acidic NH protons. A decrease of palladium concentration from 1.0 to 0.5 mol% does not lead to a substantial loss of conversion. In addition, Pd complex 1 can be efficiently recovered using two‐phase system extraction. Copyright © 2016 John Wiley & Sons, Ltd.