Pd Center Research Articles

A Planar‐Chiral Palladium Complex Derived from a Weak Oxygen Donor N,N‐Diisopropyl Ferrocenecarboxamide Ligand

AbstractThe reaction of chiral 2‐iodoferrocenecarboxamide, palladium(II) acetate, and triphenylphosphine ligand in acetonitrile has been resulted in the formation of an air‐stable chiral (Sp)‐2‐(N,N‐diisopropyl ferrocenecarboxamide) triphenylphosphine palladium(II) iodide complex. The isolated chiral palladium complex was thoroughly characterized by multi‐nuclear (1H, 13C, 31P) NMR, IR spectroscopy, mass spectrometry, polarimetry, CHN analysis, cyclic voltammetry and single crystal X‐ray crystallography (XRD). The single‐crystal XRD analysis reveals a square planar geometry around the palladium atom with a strong amide oxygen‐Pd bond [2.095(6) Å]. The palladium complex crystallized in the C2‐chiral space group having a Flack parameter value of 0.011(14), which is close to 0 and indicative of the enantiomeric purity of the palladium complex. The electrochemical study reveals a two‐electron redox couple due to the ferrocenyl Fe(II) atom and the palladacyclic Pd(II) center. In differential pulse voltammetry (DPV), two peaks were observed at 450 mV and 590 mV vs. Ag/AgCl. Further, the synthesized chiral palladacyle has also been explored in Suzuki‐Miyaura coupling reaction as a substrate and as a catalyst for Tsuji‐Trost coupling and phosphonation reactions.

C-H bond activation via concerted metalation-deprotonation at a palladium(iii) center.

Herein we report the direct observation of C-H bond activation at an isolated mononuclear Pd(iii) center. The oxidation of the Pd(ii) complex (MeN4)PdII(neophyl)Cl (neophyl = -CH2C(CH3)2Ph; MeN4 = N,N'-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF6) yields the stable Pd(iii) complex [(MeN4)PdIII(neophyl)Cl]PF6. Upon the addition of an acetate source, [(MeN4)PdIII(neophyl)Cl]PF6 undergoes Csp2-H bond activation to yield the cyclometalated product [(MeN4)PdIII(cycloneophyl)]PF6. This metalacycle can be independently prepared, allowing for a complete characterization of both the starting and final Pd(iii) complexes. The C-H activation step can be monitored directly by EPR and UV-Vis spectroscopies, and kinetic isotope effect (KIE) studies suggest that either a pre-association step such as an agostic interaction may be rate limiting, or that the C-H activation is partially rate-limiting in conjunction with ligand rearrangement. Density functional theory calculations support that the reaction proceeds through a κ3 ligand coordination and that the flexible ligand structure is important for this transformation. Overall, this study represents the first example of discrete C-H bond activation occurring at a Pd(iii) center through a concerted metalation-deprotonation mechanism, akin to that observed for Pd(ii) and Pd(iv) centers.

Metal-directed self-assembly of constitutionally dynamic systems: control of the nuclearity of Pd(II)/Pt(II) metallacycles.

Self-assembly with the square-planar Pd(II) or Pt(II) (en)M(ONO2)2 (en = ethylenediamine) complexes of ditopic ligands incorporating both N,N'-dialkyl-4,4'-bipyridinium and N-monoalkyl-4,4'-bipyridinium or N-monoalkyl-2,7-diazapyrenium moieties leads to constitutionally dynamic systems responsive to the concentration of the components. At low concentrations, the metallamacrocyclic mononuclear [ML]5+ species is formed. In contrast, when the concentration is increased, a defined dinuclear [M2L2]10+ structure appears as a second species in equilibrium, becoming the major one at high concentrations. Besides concentration changes, the addition of an aromatic guest also allows the control of the system speciation, shifting the outcome of the metal-directed self-assembly towards the mononuclear metallacycle. This dynamic behavior enables the control of the nuclearity of the metallacycles assembled by external stimuli.

Unlocking low-temperature and anti-SO2 poisoning performance of bimetallic PdV/TiO2 catalyst for chlorobenzene/NOx catalytic removal by antimony modification design

Controlling chlorobenzene (CB) and NOx emissions via catalytic oxidation and selective catalytic reduction necessitates high redox performance catalysts, and construction of active centers with multiple adsorption centers towards reactants is prerequisite for developing such catalysts. Herein, the electronic structure and active centers of the PdV/TiO2 bimetallic catalyst were modulated by Sb modification. The neighboring antimony (Sb) promoted the charge transfer between V and Pd centers, and suppressed the adsorption of SO2 on active sites, thus accelerating CB/NOx conversion and significantly avoiding the deposition of sulfur species on the active centers, especially the Pd species. The detailed plausible CB/NOx catalytic removal pathway (CBfree → CBads → C6H5ads + Clads + O species (Olatt and Oads) → CxHyOz + Clads + Brønsted → CO2 + H2O + HCl on SbPdV/TiO2 catalyst surface was further established. This research provided a strategy for the development of improved CB catalytic oxidation + selective catalytic reduction (CBCO + SCR) catalysts by tuning the electronic structure of active centers.

Novel hybrid phosphinite-theophylline ligands and their Pd(II) complexes. Synthesis, characterization and catalytic evaluation in Suzuki-Miyaura couplings

Two Pd(II) complexes bearing bidentated [R’2(OR)P^P(OR)R’2] (OR = 7-(2,3-dihydroxypropyl)theophylline and R’ = Ph for (1) and R’ = iPr for (2)) ligands were synthesized and fully characterized, including the unequivocal determination of their molecular structures by single crystal X-ray diffraction analysis. Thus confirming the coordination of the phosphinite-theophylline ligand to the Pd(II) center in a chelate manner. Exploration of these compounds as catalysts in Suzuki-Miyaura Couplings revealed complex (2) to be the best of the pair of complexes in the formation of CC bonds in the reactions of phenylboronic acid and a series of para-substituted bromobenzenes under mild reaction conditions (50 °C, 50 W, 5 min, and 0.05 mol % of catalyst), achieving yields of > 99 %.

Mechanism and Origins of Diastereo- and Regioselectivities of Palladium-Catalyzed Remote Diborylative Cyclization of Dienes via Chain-Walking Strategy.

Density functional theory calculations have been performed to investigate the palladium-catalyzed remote diborylative cyclization of dienes. The computations reveal that the reaction proceeds through a rarely explored Pd(II)/Pd(IV) catalytic cycle, and the formal σ-bond metathesis between the alkylpalladium intermediate and B2 pin2 occurs via the pathway of the B-B oxidative addition/C-B reductive elimination involving the high-valent Pd(IV) species. The diastereoselectivity is determined by the migratory insertion into the Pd-C bond, which is mainly due to the combination of the torsional strain effect, steric repulsion and C-H-O hydrogen-bonding interaction. The steric hindrance around the reacting carbon group in the C-B reductive elimination turns out to be a key factor to provide the driving force of the chain walking of the Pd center to the terminal primary carbon position, enabling the experimentally observed remote regioselectivity.

Surface Modification by Amino Group Inducing for Highly Efficient Catalytic Oxidation of Toluene over a Pd/KIT-6 Catalyst.

Toluene is one of the typical volatile organic compounds in industry, particularly in energy and fuels production processes, which is required to be eliminated effectively to protect the environment. Catalytic oxidation of toluene is widely studied for its high efficiency, and rational design and synthesis of metal catalysts are keys for toluene oxidation. In this study, an efficient catalyst was designed and synthesized by introducing -NH2 groups on the ordered mesoporous silica (KIT-6) surface to anchor and disperse Pd species, leading to Pd nanoparticles being highly dispersed with uniform particle size distribution. Meanwhile, it was found that the introduction of -NH2 made Pd centers present an electron-rich state, and the active Pd centers could activate O2 molecules to generate more reactive oxygen species and promote the conversion of toluene, which was verified by in situ XPS and O2-TPD characterization. Compared with the catalysts prepared by an impregnation method, the catalytic performance of the Pd/NH2-KIT-6 (0.5 wt %) catalyst was significantly improved. A conversion of 90% for toluene (2400 ppm, 24,000 mL·g-1·h-1) was achieved at 171 °C, and the toluene conversion was maintained above 90% for 900 min, displaying the excellent activity and stability of the Pd/NH2-KIT-6 catalyst.

Visualization of oxygen storage process in Pd/CeO2-ZrO2 three-way catalyst based on isotope quenching technique

Elucidation of oxygen dynamics and transport in oxygen storage component of three-way catalyst (TWC) is a crucial issue for clarifying the reaction mechanism and precise kinetic modeling. Herein, we report a direct visualization of the oxygen storage process in a Pd/CeO2-ZrO2 (CZ) via a combined method of oxygen isotope labeling and reaction quenching. 18O2 is stored in a model TWC comprising a dense CZ substrate with Pd at 600 °C, the catalyst is quenched, and the 18O distribution in the surface and bulk is visualized by secondary-ion mass spectrometry with a 100-nm-scale spatial resolution. The results show that the presence of Pd enhances both the surface 18O concentration and the 18O diffusion depth into the CZ bulk. 18O concentration on the Pd surface is higher near the Pd/CZ interface, indicating that oxygen adsorption near the Pd/CZ interface is preferred to that close to the Pd center. The promoting effect of Pd on oxygen vacancy formation in CZ is supported by density functional theory calculations. In addition, the comparison between the 18O distribution and oxygen release/storage simulation based on diffusion equation provides the local oxygen release/storage rates. These are of the same order of magnitude and qualitatively consistent with the trend of conventional oxygen storage capacity measurements. The visualization technique reported here is a promising tool for directly investigating the oxygen dynamics and transport at the metal/oxygen storage material system from both qualitative and quantitative aspects.

Isolation and Characterization of Heteroleptic Mononuclear Palladium(I) Complexes.

Catalytic transformations involving Pd(0)/Pd(II) catalytic cycles are very well known, and processes involving high-valent Pd(III) and Pd(IV) and low-valent Pd(I) intermediates have also gained interest in recent years. Although low-valent Pd(I) intermediates are proposed in these catalytic cycles, isolated and characterized mononuclear Pd(I) species are very rare. Herein, we report the isolation of two heteroleptic mononuclear Pd(I) complexes stabilized by dithiapyridinophane ligands that were fully characterized by single-crystal X-ray diffraction; EPR, IR, UV-vis spectroscopies; and computational studies. Excitingly, one of these Pd(I) complexes shows Kumada Csp3-Csp2 cross-coupling competency, and initial studies of the other shows direct evidence for Csp3-H bond activation proposed to occur at the Pd(I) center.

Shining Visible Light on Reductive Elimination: Acridine-Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids.

Despite the recent tremendous progress on transition-metal/photoredox dual catalysis in organic synthesis, single transition-metal catalysis under visible-light irradiation, which can utilize light energy more efficiently, is still underdeveloped. Herein, we report the design of photosensitizing phosphinoacridine bidentate ligands for visible-light-induced transition-metal catalysis, expecting that the electron-accepting acridine moiety would create a highly reactive electron-deficient metal center toward reductive elimination via metal-to-ligand charge transfer (MLCT). Using these ligands, we have achieved a palladium-catalyzed cross-coupling reaction of aryl halides with carboxylic acids under visible-light irradiation. Electronic tuning of the phosphinoacridine ligands not only enabled the use of a variety of aryl halides as the coupling partner, including less reactive aryl chlorides, under blue light irradiation, but also realized the employment of lower-energy green and red light for the cross-coupling. Experimental mechanistic studies have proved that the reductive elimination of aryl esters is induced by photoirradiation of phosphinoacridine-ligated arylpalladium(II) carboxylate complexes. The theoretical calculation suggests that the reductive elimination in the excited state is promoted by decreasing the electron density of the Pd center through photoinduced intramolecular electron transfer, i.e., MLCT, in the transition state owing to the electron-deficient acridine scaffold. This is a very rare example of photoinduced reductive elimination on palladium(II) complexes.

Synthesis and Solid-State X-ray Structure of the Mononuclear Palladium(II) Complex Based on 1,2,3-Triazole Ligand

Herein, we described the synthesis and X-ray crystal structure of the new [Pd(3)2Cl2] complex with 1,2,3-triazole-based ligand (3). In the unit cell, there are two [Pd(3)2Cl2] molecules, and the asymmetric unit comprised half of this formula due to the presence of an inversion symmetry element at the Pd(II) center. The monoclinic unit cell volume is 1327.85(6) Å3, with crystal parameters of a = 10.7712(2) Å, b = 6.8500(2) Å, and c = 18.2136(6) Å, while β = 98.851(2)°. The structure comprised two trans triazole ligand units coordinated to the Pd(II) ion via one of the N-atoms of the triazole moiety. In addition, the Pd(II) is further coordinated with two trans chloride groups, where each of the trans bonds is equidistant. The crystal structure of the [Pd(3)2Cl2] complex was compared with that for free triazole ligand 3. It was found that the coordinated ligand showed less twist around the C–N bond compared to free triazole ligand 3. The molecular packing of the latter is found controlled by short O…H, N…H, C…N, and C…C interactions in addition to the short Cl…F interhalogen and π–π interactions. H…H (23.5%), Cl…H (14.4%), N…H (14.3%), and O…H (11.2%) are the most dominant contacts. In the [Pd(3)2Cl2] complex, no significant interhalogen or π–π interactions were detected. In this case, Cl…H (31.1%), H…H (16.7%), O…H (11.6%), and F…H (9.7%) are the most dominant contacts.

Mechanistic aspects of the Pd(OAc)n (n = 1–3) catalyzed ethylene acetoxylation: A density functional theory study

Vinyl acetate is a valuable industrial material used in the production of various polymers. The homogeneous Pd(OAc)n (n = 1–3) can catalyze ethylene acetoxylation to produce vinyl acetate. However, the reaction mechanism of Pd‐catalyzed formation of vinyl acetate remains unclear, especially in recognizing the active valence states of the Pd center. The full catalytic cycle of Pd‐catalyzed vinyl acetate formation includes ethylene‐acetate coupling, β‐H elimination, and catalyst regeneration. Based on the Wacker mechanism, two pathways were proposed for the ethylene‐acetate coupling, including a six‐membered ring mechanism through the attack of carbonyl oxygen (O2) of acetate, and a four‐membered ring mechanism through the attack of acetoxy oxygen (O1) of acetate on the coordinated ethylene. Our density functional theory (DFT) calculation shows that PdOAc cannot catalyze the acetoxylation of ethylene to form vinyl acetate due to the presence of a high energetic span. Pd(OAc)2 catalyzed reaction via the six‐membered mechanism is preferred, and the β‐H elimination is the rate‐determining step. For Pd(OAc)3, the six‐membered mechanism is favored as well, and the ethylene‐acetate coupling is the rate‐determining step. Overall, Pd(OAc)2 showed high activity (path a) toward the formation of vinyl acetate with a calculated TOF of 349 h−1.

Hydrogen bond-directed coordination of phosphine-amino-alcohol (P,N,OH) ligands: Stereochemical considerations and catalytic studies

Novel phosphine-aminoalcohol type chiral ligands of the chemical formula Ph2PCH(CH3)CH2CH(CH3)NHCH(R)CH2OH (1a: (R,R)-(S), R = CH3, 1b: (S,S)-(S), R = CH3, 1c: (S,S), R = H) have been synthesized in two simple steps using cyclic sulfates. The coordination behavior of 1a-c having stereochemically labile nitrogen donor to square planar Pd(II) center was investigated by X-ray crystallography, 1D and 2D NMR methods and by DFT calculations. In the solid state of complex [Pd(1a)Cl2] an intramolecular hydrogen bond could be observed between the OH-moiety and one of the Cl co-ligands, while intermolecular hydrogen bonds were detected in the case of [Pd(1b)Cl2] between the same functionalities. In the dichloromethane solution of the complexes the hydrogen bond was identified as a crucial factor in determining ring conformation and nitrogen configuration. Ligand 1a coordinated stereoselectively to the metal in [Pd(1a)Cl2] leading to a complex having a single conformationally rigid six-membered chelate and a configurationally fixed N-donor. In contrast, coordination of ligands 1b-c resulted in the formation of a mixture of isomers with different chelate conformation and nitrogen configuration. The ligands were utilized in Pd-catalyzed asymmetric allylic alkylation where high enantioselectivities (ees up to 96 %) and activities could be obtained.

Formic Acid Generation from CO2 Reduction by MOF-253 Coordinated Transition Metal Complexes: A Computational Chemistry Perspective

The inclusion of transition metal elements within metal–organic frameworks (MOFs) is considered one of the most promising approaches for enhancing the catalytic capability of MOFs. In this study, MOF-253 containing bipyridine coordination sites is investigated for possible transition metal chelation, and a consequent possible CO2 reduction mechanism in the formation of formic acid. All transition metal elements of the third, fourth and fifth periods except hafnium and the lanthanide series are considered using density functional theory calculations. Two distinct types of CO2 reduction mechanisms are identified: (1) the five-coordination Pd center, which promotes formic acid generation via an intramolecular proton transfer pathway; (2) several four-coordination metal centers, including Mn, Pd, and Pt, which generate formic acid by means of heterolytic hydrogen activation. The MOF-253 environment is found to promote beneficial steric hindrance, and to constrain metal–ligand orientation, which consequently facilitates the formation of formic acid, particularly with the tetrahedral Mn center at high-spin electronic state.

Substrate-Specific Activation and Long-Range Olefin Migration Catalysis at the Pd Centers in a Porous Metal-Macrocycle Framework

Abstract Developing catalysts comparable to natural enzymes that achieve only specific reactions has long been a key challenge in the field of molecular synthesis. Long-range olefin migration reactions have attracted great interest as molecular transformation reactions that can realize remote re-functionalization of a molecule with high atom economy. Herein we report that linear alkenyl alcohols of a certain chain length can chemically activate Pd centers precisely arranged in a porous metal-macrocycle framework (MMF), thereby promoting substrate-specific, catalytic long-range olefin migration reactions that produce aldehydes or ketones. Furthermore, MMF can be chemically activated by highly reactive alkenyl alcohol substrates, converting another olefinic substrate that normally would not react with nonactivated MMF into an olefin-migrated product at a high conversion. These reaction specificities are discussed in relation to the arrangement mode of the Pd active centers in the confined space.

A Distinctive Pattern for Substituent Effects on Transition Metal Centers: Enhanced Electron-Donating Capacity of Cationic Palladium Species

A Distinctive Pattern for Substituent Effects on Transition Metal Centers: Enhanced Electron-Donating Capacity of Cationic Palladium Species

Vinyl-addition polymerizations of norbornene and methyl methacrylate by the palladium(II) complexes ligated by 2-iminomethylquinoline and 2-iminomethylpyridine derivatives

Novel Pd(II) complexes, namely, [LnPdCl2] (Ln = LA and LB), where LA is (E)-3-methoxy-N-(quinolin-2-ylmethylene)propan-1-amine and LB is (E)-N-(pyridin-2-ylmethylene)hexan-1-amine, were synthesized and characterized. X-ray diffraction studies revealed distorted square planar geometries around the Pd(II) center in [LnPdCl2] (Ln = LA and LB). The catalytic capabilities of the synthesized complexes were tested in vinyl addition polymerization of norbornene (NB) in the presence of the cocatalyst modified methylaluminoxane (MMAO), displaying high activities with conversions ≥ 94% in chlorobenzene at 25 °C. The complex [LBPdCl2] displayed the highest activity of up to 4.40 × 106 gPNBmolPd−1h−1 at 25 °C, generating insoluble polynorbornene (PNB) at an NB/Al/Pd ratio of 8,000:500:1. In addition, preliminary methyl methacrylate (MMA) polymerization studies of the [LnPdCl2]/MMAO system demonstrated that the activities toward polar vinyl monomer production were in the range of 8.00 × 104 to 8.20 × 104gPMMAmolPd−1h−1 at 60 °C, yielding a high-molecular-weight syndiotactic poly(methyl methacrylate) (PMMA, Mw = 1.89 × 106gmol−1).

Interplay between the Directing Group and Multifunctional Acetate Ligand in Pd-Catalyzed anti-Acetoxylation of Unsymmetrical Dialkyl-Substituted Alkynes.

The cooperative action of the acetate ligand, the 2-pyridyl sulfonyl (SO2Py) directing group on the alkyne substrate, and the palladium catalyst has been shown to be crucial for controlling reactivity, regioselectivity, and stereoselectivity in the acetoxylation of unsymmetrical internal alkynes under mild reaction conditions. The corresponding alkenyl acetates were obtained in good yields with complete levels of β-regioselectivity and anti-acetoxypalladation stereocontrol. Experimental and computational analyses provide insight into the reasons behind this delicate interplay between the ligand, directing group, and the metal in the reaction mechanism. In fact, these studies unveil the multiple important roles of the acetate ligand in the coordination sphere at the Pd center: (i) it brings the acetic acid reagent into close proximity to the metal to allow the simultaneous activation of the alkyne and the acetic acid, (ii) it serves as an inner-sphere base while enhancing the nucleophilicity of the acid, and (iii) it acts as an intramolecular acid to facilitate protodemetalation and regeneration of the catalyst. Further insight into the origin of the observed regiocontrol is provided by the mapping of potential energy profiles and distortion–interaction analysis.

Palladium-Catalyzed One-Step Synthesis of Stereodefined Difunctionalized Glycals

Palladium-Catalyzed One-Step Synthesis of Stereodefined Difunctionalized Glycals

Salt-Enhanced Oxidative Addition of Iodobenzene to Pd: An Interplay Between Cation, Anion, and Pd–Pd Cooperative Effects

Halide salts facilitate the oxidative addition of organic halides to Pd(0). This phenomenon originates from a combination of anionic, cationic, and Pd-Pd cooperative effects. Exhaustive computational exploration at the density functional theory level of the complexes obtained from [Pd0(PPh3)2] and a salt (NMe4Cl or LiCl) showed that chlorides promote phosphine release, leading to a mixture of mononuclear and dinuclear Pd(0) complexes. Anionic Pd(0) dinuclear complexes exhibit a cooperativity between Pd(0) centers, which favors the oxidative addition of iodobenzene. The higher activity of Pd(0) dimers toward oxidative addition rationalizes the previously reported kinetic laws. In the presence of Li+, the oxidative addition to mononuclear [Pd0L(Li2Cl2)] is estimated barrierless. LiCl coordination polarizes Pd(0), enlarging both the electrophilicity and the nucleophilicity of the complex, which promotes both coordination of the substrate and the subsequent insertion into the C-I bond. These conclusions are paving the way to the rational use of the salt effects in catalysis for the activation of more challenging bonds.