Reactive Palladium Research Articles

Reactivity of palladium(II) complexes supported on carbon toward molecular hydrogen

The reactivity of Pd(II) complexes supported on carbon toward H2 was studied. For the carboxylate complexes Pd(RCOO)2 (R = Me, Me3C, F3C), it decreases upon decrease in the basicity of the acid RCO2H. The reactivity of Pd(II) η3-allyl complexes increases with increase in the Mulliken charges on the C atom of the allyl ligand connected to the substituent R. The results are in line with the heterocyclic mechanism of H-H bond activation in the hydrogen molecule and can be used for optimization of the composition of the initial compounds for the preparation of palladium catalysts.

Palladium nanoparticle-modified carbon nanotubes for electrochemical hydrogenolysis in ionic liquids

The electrochemical hydrogenolysis of benzyl and carboxybenzyl protecting groups is described. Optimisation of a synthetic protocol afforded well-defined palladium (Pd) nanoparticles on the surface of multiwall carbon nanotubes (CNTs). The high surface area composite was investigated, and differences observed in the voltammetry for the same composite in aqueous and ionic liquid systems are discussed. When used to reduce acidic protons in the ionic liquid (IL) 1-ethy-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) reactive palladium hydride was formed efficiently, which rapidly performed hydrogenolysis. The compounds 1-(3-(benzyloxy)propyl)-4-methoxybenzene, benzyl octyl carbonate, N,N′-bis(benzyloxycarbonyl)-L-lysine and N-benzyl-L-prolinol were investigated. Extended electrolysis of bis(trifluoromethylsulfonyl)imide (H[NTf2]) in [C2mim][NTf2] containing alcohol groups protected with benzyl and carboxybenzyl groups afforded the alcohol product in high yield. The probable hydrogenolysis of the similarly protected amine groups is also reported.

ChemInform Abstract: Synthesis, Stability and Reactivity of Palladium(0) Olefin Complexes Bearing Labile or Hemilabile Ancillary Ligands and Electron‐Poor Olefins

AbstractReview: 93 refs.

Microbial Engineering of Nanoheterostructures: Biological Synthesis of a Magnetically Recoverable Palladium Nanocatalyst

Precious metals supported on ferrimagnetic particles have a diverse range of uses in catalysis. However, fabrication using synthetic methods results in potentially high environmental and economic costs. Here we show a novel biotechnological route for the synthesis of a heterogeneous catalyst consisting of reactive palladium nanoparticles arrayed on a nanoscale biomagnetite support. The magnetic support was synthesized at ambient temperature by the Fe(III)-reducing bacterium, Geobacter sulfurreducens , and facilitated ease of recovery of the catalyst with superior performance due to reduced agglomeration (versus conventional colloidal Pd nanoparticles). Surface arrays of palladium nanoparticles were deposited on the nanomagnetite using a simple one-step method without the need to modify the biomineral surface, most likely due to an organic coating priming the surface for Pd adsorption, which was produced by the bacterial culture during the formation of the nanoparticles. A combination of EXAFS and XPS showed the Pd nanoparticles on the magnetite to be predominantly metallic in nature. The Pd(0)-biomagnetite was tested for catalytic activity in the Heck reaction coupling iodobenzene to ethyl acrylate or styrene. Rates of reaction were equal to or superior to those obtained with an equimolar amount of a commercial colloidal palladium catalyst, and near complete conversion to ethyl cinnamate or stilbene was achieved within 90 and 180 min, respectively.

Synthesis and reactivity of palladium(II) fluoride complexes containing nitrogen-donor ligands.

This article describes the synthesis, characterization, and reactivity of palladium(II) fluoride complexes containing sp(2) and sp(3) nitrogen-containing supporting ligands. Both cis and trans complexes of general structure (N)(N')Pd(II)(R)(F) (R = Ar or CH(3)) as well as cis-(N)(2)Pd(II)(F)(2) are reported. Crystallographic characterization of these molecules has allowed structural comparisons to related phosphine-ligated species. Furthermore, these studies have revealed that nitrogen-donor ligands support some of the longest and the shortest Pd-F bonds reported to date. The thermal decomposition of (N)(N')Pd(II)(R)(F) has also been examined, and no products of C-F bond-forming reductive elimination were obtained in any case.

X-ray Photoelectron Spectroscopy and the Auger Parameter As Tools for Characterization of Silica-Supported Pd Catalysts for the Suzuki−Miyaura Reaction

Palladium has been immobilized on thiol-modified mesoporous and amorphous silicates and used as a catalyst for the Suzuki−Miyaura reaction. Heterogeneous catalysts are difficult to characterize due to the difficulty of applying traditional solution state characterization methods like NMR to solid samples. As such, the catalysts are often poorly understood. In an effort to better understand the oxidation state and electronic environment of both the reactive palladium center and the thiol ligands through which it is bound in the catalyst, we characterized a wide range of both Suzuki−Miyaura catalysts and Pd compounds by both X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger spectroscopy (XAES) focusing on Pd, S, and Si. The use of XAES data allows determination of the Auger parameter for Pd and S, which provides information not only on oxidation state but also on the polarizability of the surrounding ligands in the catalyst. Changes in XPS binding energy and XAES kinetic energy are discussed pa...

More Surface, More Reactivity

CHEMISTRY To gain a better understanding of palladium's reactivity as a hydrogenation catalyst, many model studies that use well-defined single-crystal surfaces have focused on what should be the simplest substrate, ethylene. Although this reaction is facile for reactant pressures near ambient, at

Acid-base and coordination properties of some palladium(II)porphyrins

The speciation and reactions of palladium(II) complexes with meso-tetraphenylporphine (H2TPP) and meso-tetraphenyl-β-octaethylporphine (H2TetPOEP) were studied in H2SO4-H2O and H2SO4-HOAc protic solvents. H-associated species of PdTPP and PdTetPOEP were found to exist in sulfuric acid with concentrations of 16.33–17.38 and 17.48–18.22 mol/L, respectively. The kinetics of one-electron oxidation of complexes in the coordinated aromatic macrocycle were studied. A third-order rate equation was determined, and the mechanism of the oxidation reaction was substantiated with kinetically significant stages of dioxygen coordination, electron transfer from the macrocyclic aromatic system to dioxygen, and H-association equilibrium between the complex and sulfuric acid. The effects of peripheral ethyl substituents in the macrocyclic ligand on the reactivity of palladium(II)porphyrins were revealed.

Synthesis and Reactivity of Palladium and Nickel β-Diimine Complexes: Application as Catalysts for Heck, Suzuki, and Hiyama Coupling Reactions

The synthesis of a range of sterically hindered β-diimine ligands and their complexes with palladium(II) and nickel(II) of the formula Pd(κ2N,N-RNCHC(CH2)n−CHNR)(Cl2) (R = 2,6-i-Pr2Ph, 2,6-Me2Ph, Cy, t-Bu, n = 4, 5) and Ni(κ2N,N-RNCHC(CH2)4−CHNR)(Br2) (R = 2,6-i-Pr2Ph, 2,6-Me2Ph) has been investigated. Representative X-ray structures of both ligands and complexes have been determined. The use of the palladium complexes as catalysts for Suzuki coupling of aryl halides and arylboronic acids has been examined. In addition, it has been shown that the palladium complexes are also active in the Heck reaction of aryl bromides and methyl acrylate as well as in the Hiyama coupling of aryl halides and phenyltrimethoxysilane.

Influence of the phosphine arrangement on the reactivity of palladium(II) and platinum(II) polyphosphine complexes with copper(I) chloride

The distorted square-planar complexes [Pd(PNHP)Cl]Cl ( 1) (PNHP = bis[2-(diphenylphosphino)ethyl]amine), [M(P 3)Cl]Cl [P 3 = bis[2-(diphenylphosphino)ethyl]phenylphosphine; M = Pd ( 2), Pt ( 3)] and [Pt(NP 3)Cl]Cl ( 5) (NP 3 = tris[2-(diphenylphosphino)ethyl]amine), coexisting in the later case with a square-pyramidal arrangement, react with one equivalent of CuCl to give the mononuclear heteroionic systems [M(L)Cl](CuCl 2) [L = PNHP, M = Pd ( 1a); L = P 3, M = Pd ( 2a), Pt ( 3a); L = NP 3, M = Pt ( 5a)]. The crystal structure of 3a confirms that Pt(II) retains the distorted square-planar geometry of 3 in the cation with P 3 acting as tridentate chelating ligand, the central P atom being trans to one chloride. The counter anion is a nearly linear dichlorocuprate(I) ion. However, the five-coordinate complexes [Pd(NP 3)Cl]Cl ( 4), [M(PP 3)Cl]Cl (M = Pd ( 6), Pt ( 7); PP 3 = tris[2-(diphenylphosphino)ethyl] phosphine) containing three fused five-membered chelate rings undergo a ring-opening by interaction with one ( 4, 6, 7) and two ( 6, 7) equivalents of CuCl with formation of neutral MCu(L)Cl 3 [L = NP 3, M = Pd ( 4a); L = PP 3, M = Pd ( 6a), Pt ( 7a)] and ionic [MCu(PP 3)Cl 2](CuCl 2) [M = Pd ( 6b), Pt ( 7b)] compounds, respectively. The heteronuclear systems were shown by 31P NMR to have structures where the phosphines are acting as tridentate chelating ligands to M(II) and monodentate bridging to Cu(I). Further additions of CuCl to the neutral species 6a and 7a in a 1:1 ratio resulted in the achievement of the ionic complexes 6b and 7b with CuCl 2 - ions as counter anions. It was demonstrated that the formation of heterobimetallic or just mononuclear mixed salt complexes was clearly influenced by the polyphosphine arrangement with the tripodal ligands giving the former compounds. However, complexes [M(NP 3)Cl]Cl constitute one exception and the type of reaction undergone versus CuCl is a function of the d 8 metal centre.

Carbon−Oxygen Bond Formation at Metal(IV) Centers: Reactivity of Palladium(II) and Platinum(II) Complexes of the [2,6-(Dimethylaminomethyl)phenyl-N,C,N]-(Pincer) Ligand toward Iodomethane and Dibenzoyl Peroxide; Structural Studies of M(II) and M(IV) Complexes

The presence of the [2,6-(dimethylaminomethyl)phenyl-N,C,N]- (pincer) ligand (NCN) in platinum(II) complexes has been used to generate stable organoplatinum(IV) complexes that model possible intermediates and reactivity in metal-catalyzed C−O bond formation processes. The complexes M(O2CPh)(NCN) [M = Pd (1), Pt (2)] were obtained by metathesis reactions from the chloro analogues, and although 1 does not react with dibenzoyl peroxide, 2 does so to form Pt(O2CPh)3(NCN) (3) as a model intermediate for the acetoxylation of arenes by acetic acid in the presence of palladium(II) acetate and an oxidizing agent. The complex Pt(O2CPh)(NCN) (2) reacts with iodomethane in a complex manner to form PtI(NCN) (6) and cis-Pt(O2CPh)2Me(NCN) (7). Complex 7 decomposes to form Pt(O2CPh)(NCN) (2) and MeO2CPh, probably via benzoate dissociation followed by nucleophilic attack by the benzoate ion at the PtIV-Me carbon atom. The Pd(II) analogue Pd(O2CPh)(NCN) (1) reacts with MeI to give PdI(NCN) (8) and MeO2CPh, for which the potential intermediacy of Pd(IV) species could not confirmed by 1H NMR spectroscopy. The complex PtTol(NCN) (4) (Tol = 4-tolyl) reacts with (PhCO2)2 to form cis-Pt(O2CPh)2Tol(NCN) (5), but, unlike the PtIVMe analogue 7, the PtIVTol complex 5 does not undergo facile C−O bond formation. X-ray structural studies of the isostructural square-planar complexes M(O2CPh)(NCN) (1, 2) and of the octahedral Pt(IV) complexes as solvates 3·1/2Me2CO, 5·Me2CO, and 7·Me2CO·H2O are reported. Complexes 5 and 7 have cis-PtC2 and cis-Pt(O2CPh)2 configurations.

Novel Palladium Catalyst Supported on GaAs(001) Passivated by Ammonium Sulfide

Abstract A more reactive palladium catalyst than homogeneous Pd(PPh3)4 catalyst for the Heck reaction supported on a sulfur-terminated GaAs(001) plate was developed. Sulfur termination using (NH4)2Sx at 60 °C and Pd absorption in acetonitrile at 100 °C is essential for the preparation of an active and stable catalyst. The catalyst could be reused in this reaction up to ten times.

N-Heterocyclic Carbene Palladium Complexes Bearing Carboxylate Ligands and Their Catalytic Activity in the Hydroarylation of Alkynes

The synthesis and reactivity of palladium acetate/trifluoroacetate complexes stabilized by the presence of N-heterocyclic carbene (NHC) ligands are described. The structures and coordination characteristics of both (IPr)Pd(OAc)2 (1) and (IPr)Pd(OOCCF3)2(H2O) (2) were elucidated by spectroscopic and X-ray diffraction studies. The structure of 1 shows a novel coordination of the anions in a monomeric complex, with one acetate anion acting as a monodentate ligand while the second one coordinates through both oxygens. The NHC ligands in 1 and 2 are stable under acidic conditions. The complexes were used as precatalysts in the hydroarylation of alkynes. Using this simple protocol, a number of arenes react with various alkynes to produce stilbenes.

Synthesis, structure and reactivity of palladium(ii) complexes of chiral N-heterocyclic carbene–imine and –amine hybrid ligands

The synthesis and structures of chiral N-heterocyclic carbene (NHC)-N-donor complexes of silver(I) and palladium(II) are reported. The X-ray structure of an NHC-imine silver(I) complex [((nPr)CN(CHPh))AgBr](2) exhibits an Ag(2)Br(2) dimer motif where the imine group is not coordinated to the silver atom. Reaction between 2 and [PdCl(2)(MeCN)(2)] gives the palladium(II) complex [(kappa(2)-(nPr)CN(CHPh))PdCl(2)](3) that contains a chelating NHC-imine ligand as shown by single-crystal X-ray diffraction. Slow hydrolysis of related complexes [(kappa(2)-(nPr)CN(CHPh))PdCl(2)](3) and [(kappa(2)-((Ph)(2)CH)CN(CHPh))PdCl(2)](4) using triethylammonium chloride and water lead to the precipitation of single crystals of insoluble NHC-amino palladium(II) complexes [(kappa(2)-(nPr)CN(H(2)))PdCl(2)](6) and [(kappa(2)-((Ph)(2)CH)CN(H(2)))PdCl(2)](7), respectively. In the solid state, complexes 6 and 7 both exhibit intermolecular hydrogen bonding between chlorine and an amino-hydrogen atom resulting in an infinite chain structure. Substitution of an amino hydrogen for an ethyl group gives the soluble complex [(kappa(2)-(iPr)CN((H)Et))PdCl(2)](12). Reaction between two equivalents of 2 and [PdCl(2)(MeCN)(2)] gives the di-NHC complex [(kappa(1)-(nPr)CN(CHPh))(2)PdCl(2)](5) that does not contain a coordinated imine as shown by single crystal X-ray diffraction. Conproportionation between 5 and an equivalent of [PdCl(2)(MeCN)(2)] to does not occur at temperatures up to 100 degrees C in CD(3)CN.

Reactivity of palladium(II) methyl complexes towards CO2: formation of carbonate complexes

Treatment of a benzene solution of (tmeda)PdMe2 or (dppe)PdMe2 with carbon dioxide gives the corresponding methyl bicarbonate complex, (L–L)PdMe(O2COH). These were characterised by NMR spectroscopy and elemental analysis. Under strictly dry conditions no reaction was observed. Recrystallisation of the tmeda bicarbonate complex from acetone yields the corresponding η2-carbonate complex, which was characterised by X-ray crystallography. The reaction probably proceeds through attack by free carbonic acid on the dimethyl complex.

Dechlorination of chlorinated ethenes and acetylenes by palladized iron

The degradation of chlorinated ethenes and acetylenes by palladium coated iron was studied in batch experiments. The reactivity of palladium coated iron with tetrachloroethene and its less chlorinated ethene and acetylene derivatives showed that dechlorination rates increased as the number of chlorines decreased. Thus, vinyl chloride, the most toxic compound in the chlorinated ethenes, was most rapidly degraded by the palladium coated iron. This reactivity order is opposite to the results previously observed with plain zero valent metals. The major dechlorination products of trichloroethene were ethane, ethene, and acetylene and the carbon mass balance was found to be about 81–91%. No chlorinated products were detected indicating that multiple or concerted dechlorinations rather than sequential reactions were occurring on palladium coated iron. The proposed reaction pathway is that once the chlorinated ethenes have adsorbed on the metal surface, all the chlorines are removed before desorption without release of less chlorinated products such as dichloroethenes and vinyl chloride. As the palladium content increased from 0.025% to 0.77%, the reaction rate was increased. The reaction rate increased linearly up to 0.098% of palladium, however, the increasing rate diminished with higher amounts of Pd. Results of Energy Dispersive X‐ray Spectroscopy showed that uneven palladium coatings were formed on the iron at high contents of palladium.

Synthesis and reactivity of palladium and platinum diimine complexes containing boronate esters

Condensation of α-diketones (2,3-butanedione, benzil, and acenaphthenequinone) with 3-H2NC6H4Bpin (pin = 1,2-O2C2Me4) gave the corresponding boron-containing α-diimines. The addition of these ligands to [MCl2(coe)]2 (coe = cis-cyclooctene, M = Pd or Pt) gave complexes of the type cis-MCl2(α-diimine) in moderate to high yields. The platinum complexes have been examined for their ability to bind to DNA using enzyme digest studies. These complexes were found to bind to single-stranded DNA as well as, or better than, the non-boron containing controls, and showed binding similar to that of cisplatin (cis-PtCl2(NH3)2).Key words: boronate esters, diazabutadienes, DNA-binding, platinum.

Synthesis, characterization and reactivity of palladium(II) compounds containing terdentate [Csp 2, N, S] − or [Csp 3, N, S] − ligands

The study of the reactivity of RCHN(C 6H 4-2-SMe) with R=C 6H 5 or 2,4,6-Me 3-C 6H 2 with palladium(II) salts is reported. These studies have allowed us to prepare and characterize the coordination complexes: cis-[Pd{RCHN(C 6H 4-2-SMe)}Cl 2] {R=C 6H 5 or 2,4,6-Me 3-C 6H 2} and the cyclopalladated compounds [Pd{C 6H 4CHN(C 6H 4-2-SMe)}Cl] and [Pd{(2-CH 2-4,6-Me 2-C 6H 2)CHN(C 6H 4-2-SMe)}Cl]. The X-ray crystal structures of the latter complexes reveal that the thioimines act as a [Csp 2, phenyl,N,S] − and as a [Csp 3, N,S] − terdentate group, respectively. The study of the reactions of the cyclopalladated compounds with PPh 3 is also reported.

Reactivity of palladium(0) complexes in the oxidative addition of allylic acetates

The oxidative addition of the allyl acetate CH 2CHCH 2OAc to the Pd 0 complex generated from [Pd 0(dba) 2] and 2 equiv. PPh 3 (monodentate ligand) or 1 equiv. dppb (bidentate ligand) gives a cationic (η 3-allyl)palladium(II) complex with AcO − as the counter-anion. This reaction is reversible and proceeds from SPd 0(PPh 3) 2 or from SPd 0(dppb) through at least two successive equilibria. The overall equilibrium constants have been determined. The overall equilibrium lies more in favor of the cationic (η 3-allyl)palladium(II) complex when dppp is considered, compared to PPh 3. The reaction proceeds via a neutral intermediate complex Pd 0(η 2-CH 2CHCH 2OAc)(dppb)] whose formation has been kinetically established. The rate constants of the successive steps have been determined in DMF by UV spectroscopy and conductivity measurements, for the dppb ligand. The overall complexation step of the Pd 0 by the allyl acetate CC bond is faster than the oxidative addition/ionization step which gives the cationic (η 3-allyl)palladium(II) complex.

Palladium(II) allyl complexes with potentially terdentate ancillary ligands. Mechanism of allyl amination by piperidine

The reactivity of palladium(II) allyl complexes containing potentially terdentate ligands toward allyl amination was studied in CHCl 3 in the presence of the activated olefin fumaronitrile. The influence of different L–L′–L terdentate ligands on the fluxionality in solution and on the reactivity was discussed. Quite surprisingly the behavior of S–N–S, N–S–N and N–N–N ligands is very similar to that of the corresponding N–S and N–N bidentate ligands. In these cases the conventional stepwise mechanism is observed which involves a fast pre-equilibrium in which the terdentate ligand is displaced by the entering amine and the concomitant rate-determining bimolecular attack of the amine itself to give the final allylamine and the olefin-stabilized Pd(0) complex. At variance, the P–N–N ligand imparts to the allyl complex a reactivity similar to that of the corresponding complexes containing a strongly hindered bidentate P–N species, from which the ligand is not displaced thanks to the strongly bound phosphine group.