Dimeric Palladium Complexes Research Articles

Synthesis of a Palladium Dimer Supported by a C-Bound Trifluoroacetonate Bridge Formed by Cleavage of a Hexafluoroacetylacetonate Ligand.

Palladium(II) hexafluoroacetylacetonate (Pd(Hfacac)2) is known to form adducts of bases, such as lutidine (2,6-dimethylpyridine). When treated with approximately 3 equiv of lutidine, Pd(Hfacac)2 yields a 1:1 complex as reported in the literature, Pd(O,O-Hfacac)(C-Hfacac)(lutidine), 1. However, when the amount of excess lutidine is increased, a new complex, 2, is formed. A single-crystal X-ray structure of 2 proves it is a rare example of a dimeric palladium complex containing two Pd(Hfacac)(lutidine) fragments bridged by a dianionic trifluoroacetonate ligand, μ-CHC(O)CF3. The formation of 2 is accompanied by a white precipitate determined to be a mixture of trans-Pd(O2CCF3)2(lutidine)2 (3), confirming the fate of the missing trifluoroacetate fragment from the cleavage of the Hfacac ligand, and [lutidinium][Hfacac] (4). Subsequent experiments revealed the determinative role that water played in this reaction. The mechanism of cleavage of the Hfacac ligand was explored by DFT methods.

Synthesis, Kinetics, Reaction Mechanism, and Bioactivity Assays of a Dimeric Palladium Complex

Synthesis, Kinetics, Reaction Mechanism, and Bioactivity Assays of a Dimeric Palladium Complex

Effect of Precatalyst Oxidation State in C–N Cross-Couplings with 2-Phosphinoimidazole-Derived Bimetallic Pd(I) and Pd(II) Complexes

We report the catalytic activity of two phosphinoimidazole-derived bimetallic palladium complexes in Pd-catalyzed amination reactions. Our studies demonstrate that the starting oxidation state (Pd(I) or Pd(II)) of the dimeric complex has a significant effect on the efficiency of the catalytic reaction. The corresponding Pd(I) complex shows higher reactivity in Buchwald–Hartwig aminations, while the Pd(II) complex is much more reactive in carbonylative amination reactions. These new dimeric palladium complexes provide good to excellent reactivity and yields in the amination reactions tested.

Stereo- and Enantioselective Synthesis of Propionate-Derived Trisubstituted Alkene Motifs.

We report a new method for constructing propionate-derived trisubstituted alkene motifs in a stereoselective manner. 1-Substituted 1,1-di(pinacolatoboryl)-(E)-alk-2-enes are generated in situ from 1-substituted 1,1-di(pinacolatoboryl)alk-3-enes through ruthenium(II)-catalyzed double-bond transposition. These species undergo a chiral phosphoric acid catalyzed allylation reaction of aldehydes to produce the E isomers of anti-homoallylic alcohols. On the other hand, the corresponding Z isomers of anti-homoallylic alcohols are obtained when a dimeric palladium(I) complex is employed as the catalyst for this double-bond transposition. Thus, both E and Z isomers can be synthesized from the same starting materials. A B-C(sp2 ) bond remaining with the allylation product undergoes the Suzuki-Miyaura cross-coupling reaction to furnish a propionate-derived trisubstituted alkene motif in a stereo-defined form. The present method to construct the motifs with (E)- and (Z)-alkenes are successfully applied to the syntheses of (+)-isotrichostatic acid, (-)-isotrichostatin RK, and (+)-trichostatic acid, respectively.

4-Amino-3-pentadecyl-3H-1,2,4-triazole-3-thiones and 3-pentadecyl-1,3,4-oxadiazole-2(3H)-thione for the preparation of dimeric palladium(II) complexes and their applications in Tsuji-Trost and Mizoroki-Heck reactions

The synthesis of palladium complexes derived from 4-amino-3-pentadecyl-3H-1,2,4-triazole-3-thiones and 3-pentadecyl-1,3,4-oxadiazole-2(3H)-thiones are reported. They were obtained from palladium acetate and dipotassium tetrachloropalladate(II) and their composition was assigned by elemental analysis (solid state). The resulting metallic entities were also characterized in solution based in mass spectrometry experiments. Their application in organic synthesis as cross-coupling reaction catalysts is described. One example of both conventional Tsuji-Trost and Mizoroki-Heck reactions were efficiently carried out in very high chemical yield.

State of Palladium Complexes in the PdBr2–LiBr–CH3CN–H2O Catalytic System, Used to Obtain Succinic Anhydride

UV–Vis spectroscopy is used to study the PdBr2–LiBr–CH3CN system at different concentrations of palladium and lithium bromides. The resulting data are mathematically processed using hypotheses that include the formation of monomeric and dimeric palladium complexes. Equilibrium constants of the stages of monomer and dimer complexation are calculated, along with the extinction coefficients of palladium complexes. The spectra of individual monomeric and dimeric palladium complexes are reproduced within the model, allowing for the formation of four monomeric and three dimeric complexes. UV–Vis and IR spectroscopy are used to analyze the state of palladium complexes in the PdBr2–LiBr–CH3CN system after contact with СО, and directly during the carbonylation of acetylene into succinic anhydride.

Synthesis and Characterization of C,C-Type Palladacycles and Their Catalytic Application in Mizoroki–Heck Coupling Reaction

Two series of ligand precursors, based on imidazo[1,2-a]pyridine and C2-phenyl substituted imidazole moieties, were developed and synthesized in high yields, featuring an N-CH2(C═O)Ar substituent on the imidazole ring. Upon reacting with palladium acetate, both series of ligands underwent double C–H bond activations at the methylene and o-aryl carbon sites on the N-CH2(C═O)Ar substituent, yielding C,C-type palladacycles bearing five-membered chelate rings. A dimeric palladium complex with bridging bromides was obtained from the ligand precursor with the bromide anion, whereas an ionic palladium complex with two “throw away” pyridine ligands was formed with the precursor of the tetrafluoroborate anion. All complexes are air-stable and were characterized by 1H and 13C NMR spectroscopy and elemental analysis. The structures of three of the new complexes were further established by single-crystal X-ray diffraction studies. These complexes have been screened for catalyzing Mizoroki–Heck coupling reaction using...

Palladium Complexes of N,N'-Bis(2-aminoethyl)oxamide (H2L): Structural (PdIIL, PdII2L2, and PdIVLCl2), Electrochemical, Dynamic 1H NMR, and Cytotoxicity Studies.

The monomeric (PdL·2H2O) and dimeric (Pd2L2·7H2O) palladium(II) complexes of N,N'-bis(2-aminoethyl)oxamide (H2L) were isolated, and their structures were established by single-crystal X-ray diffraction. Both compounds display identical cis-(2Namide + 2Namine) coordination environments of the metal ion. The dimer, representing a combination of two PdL species with an open lateral chelate ring, has an "open clamshell"-like structure. The intramolecular metal-metal separation in Pd2L2 (3.215 Å) is slightly shorter than the sum of the van der Waals radii of the palladium(II) atoms. The dimeric complex is relatively stable to dissociation, and its spectral features in aqueous solutions have been compared to those of the monomeric complex. A 1H NMR spectroscopic study revealed the presence of the dynamic conformational exchange process assigned to a turning of the dimeric molecule "inside out" with an activation energy of 65 kJ/mol. Cyclic voltammetry of PdL in perchlorate-, chloride-, and sulfate-containing electrolytes revealed two-electron oxidation of the palladium center. For the dimeric complex similar, though irreversible, oxidation to the palladium(IV) state was observed in NaCl electrolyte. At the same time, in NaClO4 or Na2SO4 solutions oxidation of Pd2L2 occurs in two distinct steps. The first step is quasi-reversible and can be assigned to the formation of species in an intermediate PdIIIPdIII state. Monomeric palladium(IV) complex PdIVLCl2 was generated via chemical oxidation of PdIIL by peroxodisulfate in the presence of chloride ions and structurally characterized. The related MIIL complexes (M = Pd, Ni, Cu) showed low cytotoxicity in human cancer cell lines AGS (gastric adenocarcinoma) and HCT116 (colorectal carcinoma) with IC50 values from 204 to 525 μM, while the proligand H2L was devoid of antiproliferative activity (IC50 > 1000 μM).

Complexation equilibria in tetrahydrofuran solutions of palladium(II) and lithium bromides

The solutions of palladium PdBr2 and lithium LiBr bromides in tetrahydrofuran (THF) at different concentrations were studied by UV–Vis spectroscopy. The data obtained were mathematically processed using various models. The best description was obtained with the model that includes the formation of four monomeric (PdBr+, PdBr2, PdBr 3 - , PdBr 4 2- ) and three dimeric (Pd2Br 2 2+ , Pd2Br4, Pd2Br 6 2- ) palladium complexes. The equilibrium constants of the monomer and dimer complexation stages and the extinction coefficients of the palladium complexes were calculated. The spectra of the individual monomer and dimer palladium complexes were calculated using this model.

Palladium complexes with 1-methyl-3-phenylindazole ligands

1-Methyl-3-phenylindazole reacts with lithium tetrachloropalladate and palladium acetate to give new coordination and (C,N)-cyclometallated dimeric and monomeric palladium complexes.

Cationic Two-Coordinate Complexes of Pd(I) and Pt(I) Have Longer Metal-Ligand Bonds Than Their Neutral Counterparts

Summary One-electron oxidation of known ( t Bu 3 P) 2 M (1, M = Pd; 2, M = Pt) with [Ph 3 C][HCB 11 Cl 11 ] leads to two-coordinate, monovalent cations of the formula [( t Bu 3 P) 2 M][HCB 11 Cl 11 ] (3, M = Pd; 4, M = Pt), which also possess linear geometry but with elongated M–P bonds. Spectroscopic and computational studies consistently show that the unpaired electron of the d 9 configuration of 3 and 4 belongs to largely non-bonding orbitals: the s/d z2 hybrid for Pd and the degenerate d x2-y2 /d xy pair for Pt. We show that molecular-orbital-based arguments alone are incapable of predicting or rationalizing the observed M–P bond lengthening on oxidation; correct prediction and rationalization are achieved only by inclusion of electrostatic and Pauli effects. This emphasizes the dangers of interpreting any perturbative changes in bond metrics solely on the basis of energies and occupancies of molecular orbitals; the inclusion of electrostatic and Pauli components is essential to providing a more complete picture.

Oxidative Cleavage of Styrenes Catalyzed by a PdII Complex of an 8‐Hydroxyquinolinonate Ligand

Synthesis and catalytic activity of palladium complexes of 8‐(pyridin‐2‐ylmethoxy)quinolinone (3) were investigated. Complexation of 3 with [Pd(CH3CN)2Cl2] gave the dimeric palladium complex [{N,N‐(3)PdCl}2] (4). The structure of 4 was characterized by 1H/13C NMR spectroscopy and mass spectrometry, and further confirmed by X‐ray crystallography. Complex 4 acts as a catalyst for oxidative cleavage of C=C bonds, and it catalyzes the hydration of styrenes in an anti‐Markovnikov Wacker manner to give terminal aldehydes as the initial product, which then undergo C–C cleavage to yield the corresponding benzaldehydes. Systematic studies of this catalysis were performed.

ChemInform Abstract: Mechanochemically Activated Oxidative Coupling of Indoles with Acrylates Through C—H Activation: Synthesis of 3‐Vinylindoles and β,β‐Diindolyl Propionates and Study of the Mechanism.

Construction of 3-vinylindoles (3) and β,β-diindolyl propionates (4) through solvent-free C-H functionalization has been explored under high-speed ball-milling conditions. The reaction selectivity is influenced by the catalyst dramatically: Pd(OAc)2 provides 3 in moderate to good yields, whereas PdX2 (X = Cl, I) affords 4 as the major products. The reaction mechanism has been further studied by using electrospray ionization mass spectrometry, implicating the dimeric palladium complex A as the key intermediate in an explanation of the selectivity.

Mechanochemically Activated Oxidative Coupling of Indoles with Acrylates through C-H Activation: Synthesis of 3-Vinylindoles and β,β-Diindolyl Propionates and Study of the Mechanism.

Construction of 3-vinylindoles (3) and β,β-diindolyl propionates (4) through solvent-free C-H functionalization has been explored under high-speed ball-milling conditions. The reaction selectivity is influenced by the catalyst dramatically: Pd(OAc)2 provides 3 in moderate to good yields, whereas PdX2 (X = Cl, I) affords 4 as the major products. The reaction mechanism has been further studied by using electrospray ionization mass spectrometry, implicating the dimeric palladium complex A as the key intermediate in an explanation of the selectivity.

Recyclable Pd(0)-Pd(II) composites formed from Pd(II) dimers with NHC ligands under Suzuki–Miyaura conditions

Dimeric complexes of the type [Pd(μ-X)X(NHC)]2 were employed in the Suzuki–Miyaura cross-coupling leading to 2-methylbiphenyl. The excellent activity of the studied dimers is perfectly illustrated by a TOF of up to 106 h−1. Mechanistic investigations with the application of TEM, XPS, and mercury poisoning tests provided an insight into the nature of the catalytic process. Accordingly, the reduction of the dimeric palladium complex [Pd(μ-X)X(NHC)]2 resulted in the formation of a composite containing Pd(0) nanoparticles protected by a layer of a Pd(II) species such as anions [PdBr4]2− or [PdBr3(NHC)]−. The in situ formation of Pd(0)–Pd(II) composites resulted in the high stability of the catalytic system, which was active in ten subsequent recycles without any additional protection.

Palladium(II) Complexes Bearing an Indazole-Derived N-Heterocyclic Carbene and Phosphine Coligands as Catalysts for the Sonogashira Coupling and the Hydroamination of Alkynes

Indazolin-3-ylidenes (indy) are among the most strongly donating N-heterocyclic carbenes, but the structural diversity of their complexes is still limited. Two dimeric palladium(II) complexes, [PdBr2(indy-5)]2 (2a) and [PdBr2(indy-6)]2 (2b) (indy-5 = 2,3-dihydro-1H-pyrazolo[1,2-a]indazolin-3-ylidene, indy-6 = 6,7,8,9-tetrahydropyridazino[1,2-a]indazolin-3-ylidene], bearing indazolin-3-ylidene ligands with different sizes of the fused aliphatic ring can be obtained by silver carbene transfer. The reaction of these dimers with pyridine yielded trans-[PdBr2(indy)(pyridine)] complexes (3a,b), while the poorly soluble monophosphine complexes cis-[PdBr2(indy)(PPh3)] (4a,b) were obtained by reaction with triphenylphosphine. Ligand substitution of the latter with silver trifluoroacetate afforded cis-[Pd(O2CCF3)2(indy)(PPh3)] complexes (5a,b) with improved solubilities, allowing for their detailed characterizations. In the presence of sodium tetrafluoroborate, cationic bis(phosphine) complexes trans-[PdBr(PPh3)2][...

N-Heterocyclic carbene (NHC) palladium(II) complexes bearing chiral oxazoline ligands and their catalytic activities in allylic alkylation reactions

NHC--Pd(II) complexes bearing a chiral oxazoline ligand (IIIa--f) were synthesized by cleavage of dimeric palladium complexes with the oxazoline ligand. They were fully characterized by 1H and 13C NMR and elemental analyses. The catalytic activity of the complexes (IIIa--f) in allylic alkylation reactions was evaluated. The complex IIIa was found to be the most active catalyst. Very low ee values indicate that the oxazoline is dissociated from the active catalytic species.

Dimethylaminoalkyl chalcogenolate palladium(II) complexes as an efficient copper- and phosphine-free catalyst for Sonogashira reaction

N,N-Dimethylaminoalkyl chalcogenolate Pd(II) complexes [PdCl(E∩NMe2)]n has been investigated as a moisture/air-stable and robust catalyst for Sonogashira cross-coupling reaction in the absence of copper and phosphine ligand. The dimeric palladium(II) complex of selenium containing ligand shows the best catalytic activity as compared with monomeric and trimeric complexes. The variety of functional groups are tolerated under optimized catalytic systems and provide excellent yields of the products.

Isomerizing Olefin Metathesis as a Strategy To Access Defined Distributions of Unsaturated Compounds from Fatty Acids

The dimeric palladium(I) complex [Pd(μ-Br)(t)Bu(3)P](2) was found to possess unique activity for the catalytic double-bond migration within unsaturated compounds. This isomerization catalyst is fully compatible with state-of-the-art olefin metathesis catalysts. In the presence of bifunctional catalyst systems consisting of [Pd(μ-Br)(t)Bu(3)P](2) and NHC-indylidene ruthenium complexes, unsaturated compounds are continuously converted into equilibrium mixtures of double-bond isomers, which concurrently undergo catalytic olefin metathesis. Using such highly active catalyst systems, the isomerizing olefin metathesis becomes an efficient way to access defined distributions of unsaturated compounds from olefinic substrates. Computational models were designed to predict the outcome of such reactions. The synthetic utility of isomerizing metatheses is demonstrated by various new applications. Thus, the isomerizing self-metathesis of oleic and other fatty acids and esters provides olefins along with unsaturated mono- and dicarboxylates in distributions with adjustable widths. The cross-metathesis of two olefins with different chain lengths leads to regular distributions with a mean chain length that depends on the chain length of both starting materials and their ratio. The cross-metathesis of oleic acid with ethylene serves to access olefin blends with mean chain lengths below 18 carbons, while its analogous reaction with hex-3-enedioic acid gives unsaturated dicarboxylic acids with adjustable mean chain lengths as major products. Overall, the concept of isomerizing metatheses promises to open up new synthetic opportunities for the incorporation of oleochemicals as renewable feedstocks into the chemical value chain.

Unsymmetrical phthalocyanines with cyclopalladated azo functions

In this study, a synthetic procedure for unsymmetrical metallophthalocyanines of the form M[Pc(AB3)], where A and B refer to two different types of peripheral functionality, has been developed and the new compounds have been converted to monomeric and dimeric palladium complexes. Asymmetrically substituted phthalocyanines were synthesized with the well-known statistical condensation method, by using two differently substituted precursors, namely 4-(2-ethoxyethoxy)-1-2-dicyanobenzene (1) and 4-{4-[Z/E]-phenylazo]-1-naphthyl}oxy-1,2-dicyanobenzene (2). Consequently, electron-donating 2-ethoxyethoxy groups and electron-withdrawing palladium complex are present in the same structure. Cyclopalladation was performed with [Pd(PhCN)2Cl2] to yield the bis-μ-chloro-bridged dimers and subsequently, the corresponding monomers were obtained by refluxing with three equivalents of potassium acetylacetonate. The resulting products were purified by column chromatography and characterized by several chemical and spectroscopic analysis methods. All compounds have very high solubility in organic solvents due to the presence of 2-ethoxyethoxy moiety.