Anionic Palladium Research Articles

Na2[Pd(saccharinate)4]: A new phosphine-free water-soluble catalyst with singular structure for modification of ribose nucleosides at room temperature

The easy synthesis of the anionic palladium catalyst containing saccharinate as only ligand Na2[Pd(saccharinate)4] (I), starting from either Na2[PdCl4] or Pd(AcO)2, is reported here, together with its complete characterization. It includes high resolution powder X-ray diffraction pattern obtained on a synchrotron source, subsequent elucidation of the crystal structure, and structural refinement using the Rietveld method. The structure consists of linear chains that are linked by non-covalent interactions, and results in a porous structure with channels. This air and light water-soluble complex has shown an outstanding performance for room temperature Pd-catalyzed Suzuki functionalization of the purine base in unprotected nucleosides, improving the best results reported to date.

Triflate-Selective Suzuki Cross-Coupling of Chloro- and Bromoaryl Triflates Under Ligand-Free Conditions.

In the absence of strong ancillary ligands such as phosphines or N-heterocyclic carbenes, palladium salts are selective for C-OTf cleavage in the room-temperature Suzuki couplings of chloroaryl triflates in acetonitrile. Similar "ligand-free" conditions in DMSO also promote triflate-selective Suzuki coupling of bromoaryl triflates. This triflate selectivity complements the typical preference for reaction of bromides in prior reports of Suzuki couplings using phosphine ligands. DFT calculations and additional experimental evidence are consistent with triflate-selective oxidative addition taking place at homogeneous, possibly mononuclear, anionic palladium supported by a solvent molecule.

Methane oxidation to methyl trifluoroacetate by simple anionic palladium catalyst: Comprehensive understanding of K2S2O8-based methane oxidation in CF3CO2H

The partial oxidation of methane to methyl trifluoroacetate (MeTFA) in trifluoroacetic acid (HTFA) is one of the most selective methane conversion reactions. We report that the simple anionic form of palladium, PdCl42- can effectively convert methane to MeTFA compared to neutral PdCl2 in the K2S2O8-HTFA oxidation system. The anionic catalyst form appears to increase the Pd solubility in the polar protic HTFA solvent, thereby facilitating transformation to PdTFA42- which is supposed to be the real catalytic species in this PdCl42- - catalyzed methane oxidation. It was found that not only methane oxidation but also solvent HTFA oxidation proceeded to a substantial degree, which limited the yield of MeTFA by consuming the oxidant, K2S2O8. Furthermore, for the first time, the role of trifluoroacetic anhydride (TFAA) was identified; it removes water produced by KHSO4 and HF, which are the reduced form of K2S2O8 and the oxidation byproduct of HTFA, respectively. The reaction equations for methane to MeTFA, and HTFA to CO2 in K2S2O8-HTFA system are suggested.

One-pot Sonogashira–Hydroarylation reaction catalyzed by anionic palladium complexes in an aqueous medium

It was found that anionic Pd(II) complexes of type [CA]2[PdCl4] and [CA]2[Pd2Cl6] (CA = imidazolium or pyridinium cation) are effective catalysts for copper-free Sonogashira coupling in an aqueous medium without additional ligands or co-catalysts. Both types of substrates, containing electron donor or electron acceptor groups, formed the corresponding products in good to excellent yield. The most active complex, [bmpy]2[Pd2Cl6] (bmpy = 1-butyl-4-methylpyridinium cation), was employed to the hydroarylation of the Sonogashira product in a one-pot process, using microwave radiation, NaBPh4 as a phenyl source and water as a proton donor. In these reactions the presence of electron donor substituents facilitated a higher conversion of diarylacetylenes to triarylethenes than electron acceptor groups.

Palladium‐Catalyzed Direct C−H Arylation of Arenes Promoted by Quaternary Ammonium Salt

AbstractA new Pd‐based catalytic system for the direct C−H arylation of arenes was developed. The use of a substoichiometric amount of quaternary ammonium salt as an additive with a Pd source achieved the direct arylation of unactivated arenes under milder reaction conditions in moderate to high yields. The ammonium salt generates the anionic palladium complex, thereby facilitating arylation by a concerted metalation‐deprotonation process.

Ionic Pd/NHC Catalytic System Enables Recoverable Homogeneous Catalysis: Mechanistic Study and Application in the Mizoroki-Heck Reaction.

N-Heterocyclic carbene (NHC) ligands are ubiquitously utilized in catalysis. A common catalyst design model assumes strong M-NHC binding in this metal-ligand framework. In contrast to this common assumption, we demonstrate here that lability and controlled cleavage of the M-NHC bond (rather than its stabilization) could be more important for high-performance catalysis at low catalyst concentrations. The present study reveals a dynamic stabilization mechanism with labile metal-NHC binding and [PdX3 ]- [NHC-R]+ ion pair formation. Access to reactive anionic palladium intermediates formed by dissociation of the NHC ligands and plausible stabilization of the molecular catalyst in solution by interaction with the [NHC-R]+ azolium ion is of particular importance for an efficient and recyclable catalyst. These ionic Pd/NHC complexes allowed for the first time the recycling of the complex in a well-defined form with isolation at each cycle. Computational investigation of the reaction mechanism confirms a facile formation of NHC-free anionic Pd in polar media through either Ph-NHC coupling or reversible H-NHC coupling. The present study formulates novel ideas for M/NHC catalyst design.

Formation of Transient Anionic Metal Clusters in Palladium/Diene-Catalyzed Cross-Coupling Reactions.

Despite their considerable practical value, palladium/1,3‐diene‐catalyzed cross‐coupling reactions between Grignard reagents RMgCl and alkyl halides AlkylX remain mechanistically poorly understood. Herein, we probe the intermediates formed in these reactions by a combination of electrospray‐ionization mass spectrometry, UV/Vis spectroscopy, and NMR spectroscopy. According to our results and in line with previous hypotheses, the first step of the catalytic cycle brings about transmetalation to afford organopalladate anions. These organopalladate anions apparently undergo SN2‐type reactions with the AlkylX coupling partner. The resulting neutral complexes then release the cross‐coupling products by reductive elimination. In gas‐phase fragmentation experiments, the occurrence of reductive eliminations was observed for anionic analogues of the neutral complexes. Although the actual catalytic cycle is supposed to involve chiefly mononuclear palladium species, anionic palladium nanoclusters [PdnR(DE)n]−, (n=2, 4, 6; DE=diene) were also observed. At short reaction times, the dinuclear complexes usually predominated, whereas at longer times the tetra‐ and hexanuclear clusters became relatively more abundant. In parallel, the formation of palladium black pointed to continued aggregation processes. Thus, the present study directly shows dynamic behavior of the palladium/diene catalyst system and degradation of the active catalyst with increasing reaction time.

The role of phosphine ligands in the catalytic systems of the Heck reaction with aromatic carboxylic anhydrides

The results of a comparative study of phosphine-containing and phosphine-free catalytic systems of the Heck reaction using aromatic carboxylic anhydrides as arylating agents are presented. It was demonstrated that the patterns of diff erential selectivity of the reaction under competition of two aromatic anhydrides or two alkenes are independent of the presence of a tertiary phosphine additive in the system. It was established that palladium complexes with no phosphine ligands in their coordination sphere are catalytically active at the step of activation of aromatic carboxylic anhydride and alkene. The patterns of diff erential selectivity for regioisomers of arylated products provide the evidence of the participation of phosphine-containing anionic palladium complexes in the regioselectivity-determining step of the catalytic cycle. The data obtained are in agreement with the phosphine involvement in the catalyst transformations proceeding outside the main catalytic cycle.

Oxidative and Reductive Cross-Coupling Reactions Catalyzed by an Anionic “Ligandless” Palladium Complex

The anionic complex [NBu4][Pd(DMSO)Cl3], which can be synthesized on a gram scale in a single step starting from commercially available starting materials, has been shown to be an active catalyst in the Mizoroki–Heck reaction of aryl halides. We present two new catalytic applications of this complex: the base-free oxidative Heck reaction and the reductive homodimerization of aryl halides. This complex outperformed other palladium salts. In the latter reaction, the catalyst loading could be reduced to 0.01 mol %. The scope of the reactions has been explored, demonstrating the potential of the anionic palladium complex in these catalytic transformations.

An efficient Pd/C catalyst design based on sequential ligand exchange method for the direct synthesis of H2O2

A new catalyst preparation method involving the sequential ligand exchange of anionic palladium complexes to cationic complexes via the formation of palladium hydroxide intermediates is proposed. The novel method enables the electrostatic adsorption of an anionic palladium complex on the negatively charged carbon surface, and therefore makes it possible to obtain small and highly dispersed Pd nanoparticles. The H2O2 productivity and H2 selectivity of the ligand-exchanged catalyst were 16 and 3.6 times higher than those of the Pd/C catalyst prepared by using the same anionic palladium precursor. The strikingly different activities mainly attributed to the difference in the nanoparticle size and the oxidation state of Pd, which clearly indicates that the activity of Pd/C catalyst for the direct synthesis of H2O2 can be greatly improved by the sequential ligand exchange method.

Understanding Anionic "Ligandless" Palladium Species in the Mizoroki-Heck Reaction.

The anionic complex [NBu4][Pd(DMSO)Cl3], as a "ligandless" system, was shown to be an active catalyst in the Mizoroki-Heck coupling of aryl chlorides in the absence of strongly σ-donating ligands. To investigate the experimentally observed influence of halides and the amount of water on the catalytic activity, we employed a combination of experiments and theoretical calculations. The presence of water was shown to be critical for the formation of the active palladium(0) species by oxidation of in situ generated tributylamine. Oxidative addition to an anionic palladium(0) species was found to be the rate-determining step of the catalytic cycle. For the ensuing steps, both neutral and anionic pathways were considered. It was shown that, in the absence of strongly σ-donating neutral ligands, chloride ions stabilize the catalytic intermediates. Therefore, an anionic pathway is preferred, which explains the need for tetrabutylammonium chloride as an additive. The study of the influence of bromide ions on the catalytic activity revealed that the strongly exergonic displacement of the neutral substrates by bromide ions lowers the catalytic activity.

Correction to: Synthesis, characterization, cytotoxicity, DNA binding and computational studies of an anionic palladium(II) complex derived from 8-hydroxyquinoline and 1,1-cyclobutanedicarboxylate

Correction to: Synthesis, characterization, cytotoxicity, DNA binding and computational studies of an anionic palladium(II) complex derived from 8-hydroxyquinoline and 1,1-cyclobutanedicarboxylate

Synthesis, characterization, cytotoxicity, DNA binding and computational studies of an anionic palladium(II) complex derived from 8-hydroxyquinoline and 1,1-cyclobutanedicarboxylate

A novel 1,1-cyclobutanedicarboxylato-8-hydroxyquinolinatopalladate(II), Na[Pd(8-QO)(cbdca)] (8-QO = 8-hydroxyquinoline and cbdca = 1,1-cyclobutanedicarboxylate) has been designed and synthesized. The structure of the complex has been characterized by elemental analysis, molar conductance, FT-IR, UV–Vis, 13C NMR, 1H NMR spectroscopy. The cytotoxic activities of the complex have been tested against human foreskin fibroblast normal cell line, HFFF2 and two human breast cancer cell lines, T47D and MCF-7 by MTT assay techniques. The binding properties of the antitumor complex with calf thymus DNA (ctDNA) have been investigated under physiological condition in Tris–HCl buffer solution at pH 7.0 by absorption spectroscopy, fluorescence titration spectra, EB displacement and gel chromatography studies. All these results demonstrate that the water soluble complex can cooperatively bind to ctDNA at low concentrations through a static quenching procedure. Thermodynamic parameters were calculated according to Van’t Hoff equation which indicated that hydrogen bond and van der Waals force play predominant roles in the binding process. Finally, the results of molecular docking calculations clarify the binding mode which is in good accordance with experimental results.

Cationic–anionic palladium complexes: effect of hydrogen bond character on their stability and biological activity

The solution state of palladium cationic–anionic complexes (AmH n ) k [PdCl4] prepared for the first time, where Am is morpholine, methylmorpholine, aminoethylmorpholine, 5-aminovaleric acid, L-1-phenyl-2-methylaminopropanol, and m-xylilenediamine, has been studied by electronic absorption spectroscopy, NMR, and pH measurements. The agreement of obtained results for the state of the complexes in water and NaCl solutions with IR and X-ray diffraction data for these complexes has allowed us to substantiate the principle for designing patent formulation (C5H12NO)2[PdCl4], a new type of palladium complexes, palladium(II) cationic–anionic complexes showing high antitumor and antimetastatic activity. Crystallographic data for six obtained complexes have been presented.

Cationic versus Anionic Palladium Species in the Suzuki–Miyaura Cross‐Coupling

The cationic complex [(pym‐Im‐Me)2PdCl][PF6] (pym = pyrimidyl; Im = imidazolin‐2‐ylidene) has been identified as a highly active and reliable catalyst in the Suzuki–Miyaura cross‐coupling reaction in ethanol for a wide range of substrates without the need for an excess of boronic acid. The catalytic activity of the cationic complex is retained at very low catalyst loadings, whereas quick decomposition occurs when using the anionic complex [Bu4N][Pd(DMSO)Cl3] (DMSO = dimethyl sulfoxide). Further investigations of the induction period, the effect of catalyst poisons, and SEM analysis of the particles formed under catalytic conditions show that in both cases the catalytically active species is – at least partially – heterogeneous.

Anionic Palladium(0) and Palladium(II) Ate Complexes.

Palladium ate complexes are frequently invoked as important intermediates in Heck and cross-coupling reactions, but so far have largely eluded characterization at the molecular level. Here, we use electrospray-ionization mass spectrometry, electrical conductivity measurements, and NMR spectroscopy to show that the electron-poor catalyst [L3 Pd] (L=tris[3,5-bis(trifluoromethyl)phenyl]phosphine) readily reacts with Br- ions to afford the anionic, zero-valent ate complex [L3 PdBr]- . In contrast, more-electron-rich Pd catalysts display lower tendencies toward the formation of ate complexes. Combining [L3 Pd] with LiI and an aryl iodide substrate (ArI) results in the observation of the PdII ate complex [L2 Pd(Ar)I2 ]- .

Direct Kinetic Evidence for the Active Anionic Palladium(0) and Palladium(II) Intermediates in the Ligand-Free Heck Reaction with Aromatic Carboxylic Anhydrides

Differential selectivity in the competitive Heck reaction with aromatic carboxylic anhydrides has been found to depend on the nature of both the cation and anion of a salt used as an additive for the catalytic system. This fact indicates that the anions present in the system are involved in active anionic species.

Cross-Coupling Catalysis by an Anionic Palladium Complex

Recent studies have shown that anionic palladium complexes are viable catalysts for a range of catalytic cross-coupling reactions. We present a one-step synthesis of the anionic “ligandless” palladium complex [NBu4][Pd(DMSO)Cl3] together with its crystal structure. This compound has been shown to be an active precatalyst in the Mizoroki–Heck reaction. Under Jeffery conditions, activated aryl chlorides can be coupled in yields of up to 94% without the need of an additional ligand. The presence of a small amount of water was necessary for product formation. An Amatore–Jutand-type catalytic cycle is consistent with the results presented herein. For comparison with the known mixed complex [(pym-Im-Me)2PdCl][Pd(DMSO)Cl3], the cationic complex [(pym-Im-Me)2PdCl]PF6 (pym = pyrimidyl, Im = imidazolin-2-ylidene) has been synthesized and characterized using standard techniques.

Effect of imidazolinium salts bearing hydroxy substituents on palladium-catalysed Suzuki–Miyaura and Heck coupling reactions

Palladium anionic complexes with imidazolinium cations containing hydroxyl substituents were used as catalyst precursors in the Suzuki–Miyaura and the Heck cross-coupling. High activity of anionic complexes was noted in the Suzuki–Miyaura reaction of 2-bromotoluene with phenylboronic acid at 40 oC in 2-propanol and a 2-propanol/water mixture using conventional heating or microwaves. Similarly, bromonaphtalene and iodonaphtalene reacted efficiently with naphtylboronic acid and 4-methylnaphtylboronic acid. A remarkably lower activity was noted when anionic palladium complexes were employed in the Heck coupling of 2,3-dihydrofuran with iodobenzene. An increase in conversion in the Heck reaction was achieved when Pd(OAc)2 was used as a catalyst precursor together with imidazolinium salts as co-catalysts.

Synthesis, crystal structure, and π–π stacking magnetism of the mononuclear radical complex [N-hydrogenpyridinium][Pd(mnt)2

The mononuclear radical anionic complex [N-hydrogenpyridinium][Pd(mnt)2] (mnt = malonitrile-2,3-dithiolate) with a new countercation has been prepared and its crystal structure determined by X-ray crystallography. In the mononuclear radical anionic complex the palladium ion assumes a slightly distorted square-planar geometry and is bridged by four S atoms of two mnt anionic ligands. There are two kinds of slipped π–π stacking interactions between adjacent mononuclear radical anionic complexes in the crystal (i.e., PS-1 and PS-2). The ESR spectra, the variable-temperature susceptibilities, and the theoretical calculations reveal a very strong antiferromagnetic interaction between the adjacent mononuclear radical anionic complexes, leading to diamagnetism of the present complex. Further, the theoretical calculations indicate that the strong antiferromagnetic strength should be attributed to the PS-1 slipped π–π stacking interaction; the large spin densities on the associated short contact atoms should be a major factor leading to the strong magnetic coupling strength. This study is the first to reveal the mechanism of the strong antiferromagnetic interaction of a mononuclear radical anionic palladium complex with mnt as ligand.