Organopalladium Complexes Research Articles

Investigating the Origin of Epimerization Attenuation during Pd-Catalyzed Cross-Coupling Reactions.

Palladium-catalyzed cross-couplings remain among the most robust methodologies to form carbon-carbon and carbon-heteroatom bonds. In particular, carbon-nitrogen (C-N) couplings (Buchwald-Hartwig aminations) find widespread use in fine chemicals industries. The use of base in these reactions is critical for catalyst activation and proton sequestration. Base selection also plays an important role in process design, as strongly basic conditions can impact sensitive stereocenters and result in erosion of stereochemical purity. Herein we investigate the role of a Pd catalyst in suppressing base-mediated epimerization of a sultam stereocenter during a C-N cross-coupling reaction to access the RORγ inhibitor GDC-0022. Online high-performance liquid chromatography-mass spectrometry (HPLC-MS) was employed to acquire reaction time course profiles and to delineate epimerization behavior, identify decomposition pathways, and monitor Pd-containing species. Our ability to monitor organopalladium complexes in real time by HPLC-MS provided strong evidence that the degree of epimerization was correlated to the Pd speciation in solution. Specifically, Pd(II) complexes were associated with mitigating epimerization of six-membered sultams. Additional studies showed that the suppression of epimerization in the presence of Pd(II) can impact Pd-catalyzed reactions of other substrates such as enolizable ketones, thus providing practical insight on the execution and optimization of such processes.

Lyotropic Chromonic Mesophases of Chiral Organopalladium(II) Complexes for Amplified Circularly Polarized Phosphorescence

AbstractHerein, it is reported that lyotropic liquid crystals that transfer carbon‐centered chirality to helical chirality and subsequently to macroscopic chirality in the mesophases. This process is accompanied by remarkable amplification of both emission quantum yield and dissymmetry factor of the circularly polarized luminescence (CPL). Cationic tridentate cyclometalated Pd(II) complexes, once functionalized with chiral hydrogen‐bonding motifs of amide, aggregated into head‐to‐tail dimers at low concentrations in acetonitrile and further self‐organized into chiral lyotropic chromonic mesophases upon concentrated to 50 wt.%. At 25 °C, the chiral chromonics display metal–metal‐to‐ligand charge‐transfer (MMLCT) phosphorescence at an emission maximum of 677 nm with quantum yields up to 38% and lifetimes up to 0.50 µs. Notably, the dissymmetry factors ranged from 0.10 to 0.76, varying with the sample thickness in the range of 200–15 µm. Comparison studies and theoretical calculations reveal that the hydrogen‐bonding amide group is pivotal for the formation of the staggered configuration in the helical superstructures. Additionally, the carbon‐centered chirality of the amide dictate the handedness of the helices and consequently influenced the properties of CPL.

Porphyrin(2.1.2.1) organopalladium complexes as efficient singlet oxygen sensitizers.

Four new non-planar and non-aromatic porphyrin organopalladium complexes were synthesized. Conformational structures and optical and electronic properties of the obtained organopalladium complexes containing meso-substituted phenyl, p-tert-butylphenyl, or pentafluorophenyl groups were fully investigated. These complexes showed potent capacity for singlet oxygen (1O2) generation under blue-light irradiation, and the 1O2 quantum yields were in the range of 41%-56%, which were comparable to that of Ru(bpy)3Cl2 (57%), and such potency made these organopalladium complexes potential 1O2 photo sensitizers for photodynamic therapy.

Palladium-Catalyzed Electrooxidative Double C-H Arylation.

The electrochemical transition metal-catalyzed cross-dehydrogenative reaction has emerged as a promising platform to achieve a sustainable and atom-economic organic synthesis that avoids hazardous oxidants and minimizes undesired byproducts and circuitous functional group operations. However, a poor mechanistic understanding still prevents the widespread adoption of this strategy. In this regard, we herein present an electrochemical palladium-catalyzed oxidative coupling strategy to access biaryls in the absence of a stoichiometric chemical oxidant. The robust palladaelectrocatalysis considerably suppresses the occurrence of homocoupling and oxygenation, being compatible even with electron-deficient arenes. Late-stage functionalization and Boscalid precursor synthesis further highlighted the practical importance of our electrolysis. Remarkably, mechanistic studies including the evaluation of the reaction order of each component by variable time normalization analysis (VTNA) and initial rate analysis, H/D exchange experiment, kinetic isotope effect, and stoichiometric organometallic experiments provided strong support for the involvement of transmetalation between two organopalladium complexes in the turnover limiting step. Therefore, matching the concentrations or lifetimes of two distinct organopalladium intermediates is revealed to be a pivot to the success of electrooxidative catalysis. Moreover, the presence of cationic copper(II) seems to contribute to the stabilization of the palladium(0) catalyst instead of playing a role in the oxidation of the catalyst.

Telomerization of butadiene with ethanol mediated by palladium N-heterocyclic carbene catalysts

The telomerization reaction of dienes with alcohols mediated by organopalladium complexes represents a versatile method to convert butadiene into value-added chemicals. In this contribution, we have examined the catalytic behavior of a series of well-defined palladium N-heterocyclic carbene precatalysts (Pd-NHC) in the telomerization of butadiene with ethanol. The bulky and less sterically demanding NHC ligands greatly influence yields and selectivity towards the linear product. With more hindered NHC ligands, the branched product is obtained in unprecedented higher amounts. The telomerization of butadiene with the renewable solketal leads to selective and complete conversion to product in minutes.

Synthesis and Characterization of Post-β-Carbon-Elimination Organopalladium Complexes

Synthesis and Characterization of Post-β-Carbon-Elimination Organopalladium Complexes

Synthesis and characterization of unsymmetrical Selenoethers: Facile cleavage of the Se-C bond

Installation of in situ generated arylselenide (ArSe-) on 2‑methyl napthalene afforded the following selenoethers: 2-C10H7CH2Se(C6H5), 2-C10H7CH2Se(p-CH3C6H5), 2-C10H7CH2Se(1-C10H7) and 2-C10H7CH2Se(2,4,6-Me3C6H2), as yellow crystalline solids. These molecules can be prepared by the treatment of in situ generated arylselenide with 2-C10H7CH2Br in ethanol solvent at 0 °C in excellent yield. These molecules are characterized by 1H, 13C, 77Se NMR and elemental analysis. Among these, 2-C10H7CH2Se(C6H5) and 2-C10H7CH2Se(2,4,6-Me3C6H2) are also characterized by single-crystal X-ray studies. Phenyl mercury acetate was isolated in good yield through the treatment of 2-C10H7CH2Se(C6H5) with mercuric acetate. In this reaction cleavage of the C(aryl)–Se bond takes place. Similarly, 2-C10H7CH2Se(C6H5) and 2-C10H7CH2Se(p-CH3C6H5) also exhibited cleavage of the C(aryl)–Se bond with HgCl2 and afforded [2-C10H7CH2Se(C6H5)(C6H5HgCl)] and [2-C10H7CH2Se(p-MeC6H4)(p-MeC6H4HgCl)] complexes in good yield respectively. A organopalladium(II) complex, [3-C10H6PdCl(2-CH2SeC6H5)]2, was also developed through the reaction of 2-C10H7CH2Se(C6H5) with Na2PdCl4.

Novel graphene-supported palladium complex catalyst with flexible-bridge-chains and its recycling carbon-carbon cross-coupling activity

A novel Suzuki-Miyaura (S-M) cross-coupling catalyst (rGO-8 C-TPP-Pd) was prepared by grafting organopalladium complexes with long and flexible chains onto the surface of reduced graphene oxide (rGO) by carbon-carbon covalent bonding. The catalyst was characterized by FTIR, Raman, UV-Vis, TGA, SEM and nitrogen adsorption techniques, respectively. rGO-8 C-TPP-Pd has been used in a variety of S-M coupling reactions. The results showed that it had excellent catalytic activity for various substrates with different substituent groups or substituent positions in ethanol/water mixed solvent at room temperature. Under the optimized reaction conditions, the coupling reaction yield of 4-methyl biphenyl was more than 98 % with iodobenzene and p-tolylbonoric acid as substrates. Even if rGO-8 C-TPP-Pd was recycled for 10 times, the product yield decreased by only 10.4 %, and the chemical structure of the catalyst was basically unchanged. This novel catalytic system provides a new strategy for the design and preparation of supported catalysts.

Temperature-Resolved Anisotropic Displacement Parameters from Theory and Experiment: A Case Study

Anisotropic displacement parameters (ADPs) for an organopalladium complex were obtained from synchrotron diffraction data between 100 and 250 K and compared to the results from first-principles calculations at the harmonic approximation. Calculations and experiments agree with respect to the orientation of displacement ellipsoids and hence the directionality of atomic movement, but the harmonic approximation underestimates the amplitudes of motion by about 20%. This systematic but modest underestimation can only be reliably detected with a high-quality experimental benchmark at hand. Our experiments comprised diffraction data at 20 K intervals from 130–250 K on the same crystal. An additional high-resolution data set was collected at 100 K on a second crystal and underlined the robustness of our approach with respect to the individual sample, resolution, and instrumentation. In the temperature range relevant for our study and for many diffraction experiments, the discrepancy between experimentally determined and calculated displacement appears as an almost constant temperature offset. The systematic underestimation of harmonic theory can be accounted for by calculating the ADPs for a temperature 20 K higher than that of the actual diffraction. This entirely empirical “+20 K rule” lacks physical relevance but may pave the way for application in larger systems where a more reliable quasi-harmonic approximation remains computationally demanding or even entirely unaffordable.

Synthesis and characterization of organopalladium(II) complexes of N,N,S–tridentate sulfur-containing Schiff bases derived from 2-(2-pyridyl)benzothiazolines

The synthesis of mononuclear Pd(II) complexes of type [Pd(Ln)Me] (n = 1–4, 1a–4a) and dinuclear (η3–allyl)palladium(II) dichloro complexes [Pd2(Ln)(η3-C3H5)Cl2] (n = 1–3, 1b–3b) derived from 2-R-(2-pyridyl)benzothiazolines used as Schiff bases precursors (R = H, 1; R = Me, 2; R = Ph, 3; R = C5H4N-2, 4) is reported. The formation of the trinuclear complex [Pd3(L4)(η3-C3H5)2Cl3] (4c) and its behavior in solution is also described. Complexes 1a–4a, 1b–3b and 4c were obtained from the thiazole ring-opening reaction of precursors 1–4 in presence of the [Pd(tmeda)Me2] and [Pd(μ -Cl)(η3-C3H5)]2 starting materials. The {Ln}− anionic Schiff bases obtained showed a versatile behavior towards coordination to Pd(II) ion; in 1a–4a the ligand acted as tricoordinate whilst in 1b–3b and 4c, {Ln}− acted as mono- and tridentate ligand, respectively.1a–4a were stable complexes in DMSO‑d6 solution; the dinuclear 1b–3b and trinuclear 4c complexes showed the dissociation of S→Pd bond in DMSO‑d6 solution. Complexes 3a and 3b were characterized by X-ray diffraction studies and a distorted square planar geometry around the central Pd(II) ion was observed in the two five–membered chelate rings. A systematic analysis based on DFT calculation of the structural, electronic, and thermochemistry properties was performed for η3-ally complexes 1b–3b and 4c.

Selective Formation of Mononuclear Palladium and Acetonitrile-Bridged Dinuclear Palladium Complexes Containing a Chiral Tridentate Ligand.

From the chiral Schiff base tridentate ligand LPh, unusual acetonitrile-bridged dinuclear palladium complex 1 and organopalladium complex 1' were synthesized selectively by using acetonitrile and ethanol, respectively. The chiral tridentate Schiff base ligand was bound to the palladium metal center with different chelation modes ([ONO] for 1 and [CNO] for 1'). Complex 1 constitutes the first example of dinuclear metal complexes connected only by a bridging acetonitrile, in which an exceptionally short C≡N bond distance [0.945(12) Å] of bridged acetonitrile was observed. To study the influence of a phenyl group attached to an imine, the phenyl-free ligand LH was prepared and used. In that case, an acetonitrile bridge was not observed. Theoretical calculation studies supporting the formation of 1 and 1' are favored.

Improving the electrochemical performances of organo-palladium (II) complex as promising anode material for Li-ion batteries: Effect of double emulsion preparation

The influence of the particle morphology on electrochemical performance of organo-palladium (II) complex as anode material for Li-ion batteries is studied in this work. The double emulsion-evaporation technique was used to control the particle morphology of the [IPd(4-AcOC6H4)(PPh3)2] complex obtained after synthesis. SEM and TEM images of the particles obtained after double emulsion show spherical hollow and solid microparticles with a particle size lower than 15 μm, composed of an aggregation of nanoparticles with a particle size lower than 15 nm. The NMR analysis confirms that the molecular structure of the iodine complex is preserved after double emulsion. The effect of electrode slurry homogenization duration was explored and investigated. The comparison of the CV curves and the cycling performance demonstrates that the material prepared by double emulsion and with one week of slurry homogenization presents the best electrochemical performance. This shaped electrode exhibits the highest specific discharge capacity of 740 mAh g−1 at 50 mA g−1 compared to 170 mAh g−1 for the complex initially synthesized. These results indicate that the particle morphology and duration of slurry homogenization have a great influence on the improvement of specific capacity, cyclability, and rate capability of [IPd(4-AcOC6H4)(PPh3)2] as anode material for Li-ion batteries.

Exploring organo-palladium(II) complexes as novel organometallic materials for Li-ion batteries

Three novel trans-halogeno(4-acetoxyphenyl)bis(triphenylphosphine)palladium(II) complexes (XPd(4-AcOC6H4)(PPh3)2, with X = I (3), Br (4) and Cl (5)) were synthesized and characterized by spectroscopic techniques and by X-ray diffraction analysis. The SEM images of the particles obtained after synthesis and microwave treatment show uniform morphology and particle size lower than 1 μm for the iodo-palladium complex (3). The electrochemical performance of the palladium-based anode materials was evaluated by cyclic voltammetry and galvanostatic cycling. The results show that the nature of the halogen ligand (X = I, Br and Cl) has a high impact on the reaction mechanism during the first discharge and cycling performance. Thus, the three materials IPd(4-AcOC6H4)(PPh3)2 (3), BrPd(4-AcOC6H4)(PPh3)2 (4) and ClPd(4-AcOC6H4)(PPh3)2 (5) deliver high initial irreversible capacities of 1089, 444 and 684 mAh g−1, respectively, during the first discharge at 20 mA g−1 at a voltage window between 0.01 and 3.0 V. In addition, the iodo-palladium-based anode material shows the highest specific capacity and a high capacity retention after 200 cycles at a current density of 50 mA g−1 with an average discharge capacity of about 170 mAh g−1.

Antiaromatic Carbaporphyrinoids: Fluorene as a Fused Motif toward the Synthesis of meso-Fused Heterobenziporphyrins.

meso-Tetraphenyl meta-benziporphyrins are nonaromatic macrocycles which can be changed to antiaromatic by fusing the core benzene ring with one of the adjacent meso-phenyl groups, as demonstrated here by adopting a premodification method. We used a fluorene moiety in place of the m-phenylene ring as a premodified fused aromatic motif to synthesize fused heterobenziporphyrins. Spectral and X-ray data indicated that the macrocycles are antiaromatic, which was supported by DFT, ACID, and NICS calculations. These macrocycles formed organopalladium complexes.

Living Cyclocopolymerization through Alternating Insertion of Isocyanide and Allene via Controlling the Reactivity of the Propagation Species: Detailed Mechanistic Investigation.

Living cyclocopolymerization through the alternating insertion of an isocyanide and allene into palladium-carbon bond was developed based on the controlling the reactivity of the propagation species using bidentate ligands. We revealed that the rate of the presented cyclocopolymerization was depended on the ligands of Pd-initiator. When the palladium-methyl complexes having appropriate cis-chelating ligand, such as 1,3-bis(diphenylphosphino)propane (dppp), were used as initiator, the cyclocopolymerization of bifunctional aryl isocyanides (1) that contain both isocyano and allenyl moieties polymerized to afford poly(quinolylene-2,3-methylene)s with controlled molecular weight and narrow molecular weight distributions. The resulting polymer was characterized by 1H and 13C NMR analyses, which clearly showed that the terminal moiety of the polymer formed well-defined organopalladium complex as the resting state for the polymerization, which could undergo further polymerization; not only cyclocopolymerization with 1 but also homopolymerization of simple aryl isocyanide. In the analysis of the cyclocopolymerization mechanism, we conclusively demonstrated that the insertion reaction of isocyanide is the rate-determination step in the cyclocopolymerization, which proceeds via a five-coordinate intermediate with a geometrical change. The cis-chelating ligand controls the site interchange reaction, which dominates the reactivity of propagation species.

Tetranuclear Palladacycles of 3-Acetyl-7-methoxy-2 H-chromen-2-one Derived Schiff Bases: Efficient Catalysts for Suzuki-Miyaura Coupling in an Aqueous Medium.

Tetranuclear organopalladium(II) complexes 1-3 and mononuclear complex 4 have been synthesized by the complexation of 3-acetyl-7-methoxy-2 H-chromen-2-one derived Schiff bases with potassium tetrachloropalladate K2[PdCl4]. Structural confirmation for the complexes (1-3) has been achieved by single-crystal X-ray diffraction analysis. The ligands are found to bind with the palladium ion through its azomethine nitrogen, thiolate sulfur, and C4 carbon atom of the coumarin moiety subsequent to C-H activation. The monomeric nature of complex 4 was confirmed from its mass spectroscopic data. In complex 4, coordination occurred via the lactone oxygen, azomethine nitrogen, and thiolate sulfur atoms. Computational study has been used to determine the optimized molecular structures of the complexes. An explanation on the energies of their highest occupied and lowest unoccupied molecular orbital levels and their electronic spectra has also been provided on the basis of the theoretical calculations. A systematic study of the application of these complexes as catalysts in Suzuki-Miyaura coupling (SMC) has been done with different aryl halides and phenyl boronic acid in an aqueous medium. Optimization of the reaction indicated that complex 2 exhibits greater efficiency than other complexes. An appreciable yield of the coupled products was observed with the minimum use of catalyst (μmol), and the C-C coupling has been confirmed by GC/GC-MS. An interesting result of our catalyst is the coupling of four different chloroquinolines with phenyl boronic acid to afford the coupled products in good yields.

Synthesis, solution behaviour and potential anticancer activity of new trinuclear organometallic palladium(II) complex of {S}-1-phenylethyl dithiooxamide: Comparison with the trinuclear heterobimetallic platinum(II) analogue

Addition of H2R2DTO (R = {S}-1-phenylethyl and DTO = dithiooxamide) to the bis(benzonitrile)palladium(II) chloride complex in chloroform afforded the mononuclear Pd(DTO)2·2HCl complex. The complex treated with NaHCO3 for removing of HCl and then reacted with [Pd(ƞ3-allyl)(µ-Cl)]2 for preparation of a new trimetallic organopalladium(II) complex. The molecular structure of the trimetallic complex was determined by X-ray diffraction indicating a planar geometry around each palladium center. Also, variable temperature spectroscopy for this complex was performed in CDCl3 in the range 298–390 K, and simulations of the dynamic spectra were performed using the gNMR program. A comparison between the target-based activity of the Pd(II) complex and its trinuclear heterobimetallic platinum(II) analogue as potential anticancer agents was also conducted. All the biological tests showed that both Pd(II) and Pt(II) complexes can fairly inhibit cathepsin B (Cat-B) and cathepsin L (Cat-L) as well as two out of three activities of 20S proteasome.

Highly phosphorescent organopalladium(ii) complexes with metal-metal-to-ligand charge-transfer excited states in fluid solutions.

Dinuclear pincer-type cyclometalated Pd(ii) complexes with foldable diacetylide ligands show crystallographically determined intramolecular PdPd contacts of 3.203-3.380 Å. In deoxygenated fluid solutions, these Pd(ii) complexes are highly phosphorescent in the red region with emission quantum yields up to 48%, which has been ascribed to metal-metal-to-ligand charge-transfer (MMLCT) excited states in nature.

Desulfination versus decarboxylation as a means of generating three- and five-coordinate organopalladium complexes [(phen)nPd(C6H5)]+ (n = 1 and 2) to study their fundamental bimolecular reactivity

Routes to the formation of the 1,10-phenanthroline (phen) ligated organopalladium complexes [(phen)Pd(C6H5)]+ and [(phen)2Pd(C6H5)]+ via thermal extrusion of CO2 or SO2 from mono-nuclear, mono-carboxylate or sulfinate complexes [(phen)nPd(O2XC6H5)]+ (X = C or S; n = 1 and 2) are examined using a combination of low energy collision induced dissociation experiments in an ion trap mass spectrometer and DFT calculations. [(phen)Pd(C6H5)]+ is formed from both [(phen)Pd(O2CC6H5)]+ and [(phen)Pd(O2SC6H5)]+, but only [(phen)2Pd(O2SC6H5)]+ fragments to form [(phen)2Pd(C6H5)]+. In contrast, [(phen)2Pd(O2CC6H5)]+ fragments via loss of a phen ligand to form [(phen)Pd(O2CC6H5)]+. The experimental results are consistent with DFT calculations, which show that the barriers associated with the desulfination reactions are lower than those for the decarboxylation reactions. Of the organopalladium cations [(phen)Pd(C6H5)]+ and [(phen)2Pd(C6H5)]+, only the three-coordinate complex reacts with pyridine via a ligand coordination reaction to yield [(phen)Pd(C6H5) (NC5H5)]+ and with formic acid via an acid-base reaction to form [(phen)Pd(O2CH)]+. DFT calculations highlight that the former reaction energy is −48 kcal/mol while the later reaction proceeds via a favourable six-centered transition structure.

Aerosol-Assisted Rapid Fabrication of a Heterogeneous Organopalladium Catalyst with Hierarchical Bimodal Pores.

Heterogeneous organometallic catalysts with well-defined active sites and hierarchical pores hold tremendous promise for efficient and eco-friendly chemical processes. However, the simple and scalable preparation of these materials remains difficult to date, which has hampered a more broad application scope. Herein, we reported a low-cost, rapid, and scalable aerosol-assisted assembly approach for the synthesis of a well-defined PdDPP (PdCl2(PPh2(CH2)2))_ complex-containing benzene-bridged organosilica-based catalyst with a hierarchical bimodal micro-macroporous structure. This novel material was realized by using Pd(II) organometallic silane (Pd[PPh2(CH2)2Si(OEt)3]2Cl2) as the active species, organosilane 1,4-bis(triethoxysilyl)benzene (Ph[Si(OEt)3]2) as the silicate scaffold and the surfactant cetyltrimethylammonium bromide and the inorganic salt NaCl as the dual templates on a home-built aerosol spraying-instrument. Multiple techniques including X-ray photoelectron spectroscopy and solid-state NMR spectra revealed that the organopalladium complex with a well-defined molecular configuration of major trans model and minor cis model existed in the phenyl-functionalized silica material. As expected, it efficiently promoted a variety of important carbon-carbon cross-coupling transformations including Tsuji-Trost, Sonogashira, and Suzuki reactions in pure water without the assistance of any additives. In comparison with the homogeneous catalyst PdCl2(PPh2CH3)2, it even exhibited enhanced activity and selectivity in some cases owing to the unique confinement effect and the shape selectivity generated from the hierarchical porous structure. Meanwhile, it was easily recycled and reused eight times without the loss of its activity.