Reactive Palladium Research Articles

Metal-organic framework/polyurea composite microreactors prepared in oil-in-oil emulsions

A new technique was developed to create composite microcapsules consisting of metal–organic framework (MOF) and polyurea. This method employs oil-in-oil (O/O) emulsion as a template for forming these microcapsules. It involves emulsifying a polar solvent containing MOF reagents within a nonpolar phase containing a suitable surfactant and an isocyanate monomer. Subsequently, the addition of the amine monomer initiates two simultaneous processes: the formation of MOF crystals in the core of polar droplets and the interfacial polymerization to create polyurea shell. Optimization of the method involved adjusting various parameters affecting the composite microcapsules' structure and morphology. These parameters include the surfactant’s type and concentration, the emulsion phases’ type and concentration, and quantity of the MOF component.This method was applied to create palladium-based MOF/polyurea microreactors by encapsulating sodium tetrachloropalladate and then reducing the palladium salt with hydrogen to generate reactive palladium species. The produced catalysts were utilized in the selective hydrogenation of terminal alkynes. It was observed that the selectivity of the catalyst is influenced by the quantity of encapsulated MOF, the amount of encapsulated palladium, and the solvent type used for the reaction.

Mechanistic studies of the palladium-catalyzed S,O-ligand promoted C-H olefination of aromatic compounds.

Pd-catalyzed C-H functionalization reactions of non-directed substrates have recently emerged as an attractive alternative to the use of directing groups. Key to the success of these transformations has been the discovery of new ligands capable of increasing both the reactivity of the inert C-H bond and the selectivity of the process. Among them, a new type of S,O-ligand has been shown to be highly efficient in promoting a variety of Pd-catalyzed C-H olefination reactions of non-directed arenes. Despite the success of this type of S,O-ligand, its role in the C-H functionalization processes is unknown. Herein, we describe a detailed mechanistic study focused on elucidating the role of the S,O-ligand in the Pd-catalyzed C-H olefination of non-directed arenes. For this purpose, several mechanistic tools, including isolation and characterization of reactive intermediates, NMR and kinetic studies, isotope effects and DFT calculations have been employed. The data from these experiments suggest that the C-H activation is the rate-determining step in both cases with and without the S,O-ligand. Furthermore, the results indicate that the S,O-ligand triggers the formation of more reactive Pd cationic species, which explains the observed acceleration of the reaction. Together, these studies shed light on the role of the S,O-ligand in promoting Pd-catalyzed C-H functionalization reactions.

Temperature responsive hydrogels for biomedical applications

Hydrogel is a three-dimensional communicator for water-insoluble chemicals. The hydrogel can swell structurally with water and contain large amounts of water. Hydrogel is a chemical or physicochemical bond in different polymer chains. Hydrogels are due to their high water content, softness, flexibleness, and biocompatibility. Hydrogels have therefore become increasingly popular. Hydrogels are created by physically or chemically complex natural and human hydrophilic polymers. When an environmentally specific stimulus is specific to the hydrogel, the gel replaces with solutions. Part of the advantage of universal action by thermo-responsive hydrogels is that they show their activity without the charge of any additional external factors. These hydrogels are attractive for specific medical applications because they can swell in place under physiological conditions and facilitate administration. Due to the combination of distinctly different properties of temperature-sensitive durability through joined domains and the high chemical conversion reactivity of nonmaterial, the application of temperature-sensitive hydrogels with the ion-exchange region for nanoscale chemical conversion reactions is an emerging and enticing alternative niche. The protection and direct fabrication of reactive palladium nanoparticles in temperature-responsive hydrogel networks are defined.

Preparation of Poly(ethylene glycol)@Polyurea Microcapsules Using Oil/Oil Emulsions and Their Application as Microreactors.

The development process of catalytic core/shell microreactors, possessing a poly(ethylene glycol) (PEG) core and a polyurea (PU) shell, by implementing an emulsion-templated non-aqueous encapsulation method, is presented. The microreactors’ fabrication process begins with an emulsification process utilizing an oil-in-oil (o/o) emulsion of PEG-in-heptane, stabilized by a polymeric surfactant. Next, a reaction between a poly(ethylene imine) (PEI) and a toluene-2,4-diisocyanate (TDI) takes place at the boundary of the emulsion droplets, resulting in the creation of a PU shell through an interfacial polymerization (IFP) process. The microreactors were loaded with palladium nanoparticles (NPs) and were utilized for the hydrogenation of alkenes and alkynes. Importantly, it was found that PEG has a positive effect on the catalytic performance of the developed microreactors. Interestingly, besides being an efficient green reaction medium, PEG plays two crucial roles: first, it reduces the palladium ions to palladium NPs; thus, it avoids the unnecessary use of additional reducing agents. Second, it stabilizes the palladium NPs and prevents their aggregation, allowing the formation of highly reactive palladium NPs. Strikingly, in one sense, the suggested system affords highly reactive semi-homogeneous catalysis, whereas in another sense, it enables the facile, rapid, and inexpensive recovery of the catalytic microreactor by simple centrifugation. The durable microreactors exhibit excellent activity and were recycled nine times without any loss in their reactivity.

Synergistic effect of Thiourea and HCl on Palladium (II) recovery: An investigation on Chemical structures and thermodynamic stability via DFT

This work duly investigates the recovery of Pd (II) chlorocomplexes from industrial wastewater. Chemical structures and thermodynamic stabilities of the complex formed are evaluated via density functional theory (DFT). By applying synergistic solutions of thiourea mixed with hydrochloric acid (HCl), the stripping reaction of Pd (II) in the loaded Aliquat 336 occurs and Pd (II) chlorocomplexes coordinated thiourea ligands are formed, thus 80.19% of Pd (II) chlorocomplexes can be recovered. The aim of this study is to gain a better understanding of the stripping mechanisms and the structure of the complexes formed via the synergistic system. Such an understanding is still limited since little research has been conducted in this field. Owing to their molecular geometry, ligand coordination and donor groups play a vital role in the reactivity of palladium (II) complex. Quantum models have been developed to evaluate the chemical structure and thermodynamics stability of ((NH2)2CS·PdCl2) namely: (i) DFT with B3LYP/6-31g(d,p) and MP2/6-31g(d,p) basis set, (ii) MP2 with cc-pVTZ basis set and (iii) CCSD(T)/cc-pVTZ. Results demonstrate that the highest geometric stability exhibited is the structure of Pd-S bonding with 180° Cl-Pd-Cl. The distance (r) and angle (a) of the selected geometrical parameters for (NH2)2CS·PdCl2 complex are reported. Additionally, FTIR and UV–vis spectroscopies have been conducted to analyze the sampling solutions. Further, the calculated vibrational frequencies and experimental spectroscopic results show good agreement with the optimized geometry.

Influence of phosphine (pincer) ligands on the transition metal hydrides reactivity

• Dihydrogen bonding determines reactivity of metal hydrides. • Role of (di)hydrogen bonding and proton transfer in (de)hydrogenation processes. • Steric and electronic effects of phosphines on the reactivity. • Reactivity of palladium, nickel, iridium and rutenium pincer hydrides. Transition-metal hydride complexes are plenteous in organometallic chemistry. The reactivity of metal hydrides is determined by three possible pathways of M–H bond cleavage: homolytic cleavage yielding a hydrogen radical (H ), and heterolytic cleavage generating either a proton (H + ) or hydride (H − ). These steps, especially the proton and hydride transfer, are critical in a vast array of stoichiometric and catalytic transformations. Many late transition metal hydrides are supported by phosphine ligands, which allow tuning the electronic and steric properties at a metal center in a systematic and predictable way. In this non-exhaustive review we attempt to trace the influence of phosphines properties on the reactivity of transition metal hydrides in topical stoichiometric and catalytic processes highlighting the reactions’ mechanisms and the role of hydrogen bonding interactions in the intermediates of hydrogen transfer reactions.

Gold-Catalyzed Organometallic Cross-Coupling

Synthesis of biaryls by Suzuki-Miyaura or Hiyama coupling represents the-state-of-art in carbon-carbon bond formation, in which the coupling occurs between a nucleophile (Arδ-: arylboronates or arylsilanes) and an electrophile (Arδ+: arylhalides). The intrinsic functional group limitations in these reactions stem from the high catalytic reactivity of palladium and nickel catalysts toward halogen, boronate and base-sensitive substituents. We report a general dinuclear gold-catalyzed organometallic cross-coupling of arylboronates and arylsilanes without external base for the synthesis, with excellent functional group tolerance of asymmetric biaryls. Both coupling partners are readily available, bench-stable and non-toxic. A broad array of (pseudo)halogenated and borylated coupling partners can be successfully applied to this site-specific biaryl coupling. The biocompatibility of gold expedites the synthesis of biaryl pharmacophores avoiding the involvement of palladium residues.

C-H Functionalizations by Palladium Carboxylates: The Acid Effect.

Finding optimal reaction conditions is usually complex, requires many experiments, and is therefore demanding in terms of human, financial, and environmental resources. This work provides a simple workflow for easier design of popular palladium-catalyzed C-H functionalization reactions, where the active palladium catalysts contain carboxylate ligands. The key factor for optimizing reaction conditions is to find a balance between two opposing effects of the carboxylic acid in the reaction mixture: generation of more reactive palladium catalyst versus deactivation of a substrate by its protonation.

The role of 8-quinolinyl moieties in tuning the reactivity of palladium(II) complexes: a kinetic and mechanistic study

The rate and mechanism of chloride substitution from Pd(II) complexes, chlorobis-(2-pyridylmethyl)aminepalladium(II), 1, chloro-8-[(2-pyridylmethyl)amino]quinolinepalladium(II), 2, chloro-N-(2-pyridinylmethylene)-8-quinolinaminepalladium(II), 3, and chlorobis(8-quinolinyl)aminepalladium(II), 4, are reported. The labile chloride was substituted from the complexes by thiourea nucleophiles viz, thiourea (Tu), N,N′-dimethylthiourea (Dmtu) and N,N,N′,N′-tetramethylthiourea (Tmtu). The reactions were monitored under pseudo-first-order conditions in methanol using stopped-flow spectrophotometry as a function of concentration and temperature. All the reactions obeyed the rate law kobs = k2[Nu] following the order 1 > 3 > 2 > 4 with 4 exhibiting the slowest rate of substitution due to the stronger σ-donor effect of 8-quinolyl moiety of the coordinated ligand, which makes the Pd center more electron-rich. This slows the nucleophilic attack by the nucleophiles. The values of the thermodynamic parameters (ΔH# and ΔS#) support an associative substitution mechanism. The trends in the DFT calculated data support the experimentally observed order of the reactivity of the complexes.

Synthesis and Reactivity of Palladium(II) Alkyl Complexes that Contain Phosphine-cyclopentanesulfonate Ligands

The synthesis of the phosphine-cyclopentanesulfonate pro-ligands Li/K[2-PPh2-cyclopentanesulfonate] (Li/K[2a]), Li/K[2-P(2-OMe-Ph)2-cyclopentanesulfonate] (Li/K[2b]), and H[2b], and the corresponding Pd(II) alkyl complexes (κ2-P,O-2a)PdMe(pyridine) (3a) and (κ2-P,O-2b)PdMe(pyridine) (3b) is described. The sulfonate-bridged base-free dimer {(2b)PdMe}2 (4b) was synthesized by abstraction of pyridine from 3b using B(C6F5)3. The borane-coordinated base-free dimer [{2b·B(C6F5)3}PdMe]2 (5b), in which B(C6F5)3 binds to a sulfonate oxygen, was prepared by addition of 1 equiv of B(C6F5)3 per Pd to 4b or addition of 2 equiv of B(C6F5)3 to 3b. Compounds 3b, 4b, and 5b polymerize ethylene with low activity (up to 210 kg mol–1 h–1 at 250 psi and 80 °C) to linear polyethylene (Mn = 1950–5250 Da) with predominantly internal olefin placements. 3b and 4b copolymerize ethylene with methyl acrylate to linear copolymers that contain up to 11.7 mol % methyl acrylate, which is incorporated as −CH2CH(CO2Me)CH2– (80%) in-chain u...

Palladium‐Catalyzed Divergent Arylation with Triazolopyridines: One‐Pot Synthesis of 6‐Aryl‐2‐α‐styrylpyridines

AbstractWe have developed a new strategy for palladium‐catalyzed arylation reactions with triazolopyridines, wherein two different chemical transformations (C‐3 vs. C‐7) are observed by differentiating the substrates using different bases. The reactive palladium carbenoids were directly generated from triazolopyridines and underwent denitrogenative arylations with aryl bromides. Intriguingly, when potassium carbonate was replaced with potassium tert‐butoxide, direct CH arylation occurred at the most acidic position (C‐7). Moreover, two different catalytic arylation events were successfully performed in a one‐pot sequence, providing a convenient access to 6‐aryl‐2‐α‐styrylpyridines.magnified image

The Direct Non‐Perturbing Leaching Test in the Phosphine‐Free Suzuki–Miyaura Reaction Catalyzed by Palladium Nanoparticles

AbstractThe aryl halides normally unreactive in the reaction with phenylboronic acid in the presence of a palladium nanoparticle catalyst were shown to become involved, if a more reactive aryl iodide is simultaneously participating in the process. The involvement phenomenon can be interpreted as a direct evidence of the formation of smaller and more reactive palladium nanoparticles through leaching from the initial nanocatalyst. The involvement test was applied both to free unsupported Pd nanoparticles and supported species, thus exhibiting essentially the same character of evolutions of various nanomaterials during the catalytic transformation associated with leaching of reactive species from the initial particles. The system allows for concurrent processing of less reactive aryl bromides and even chlorides together with much more reactive aryl iodides. The interpretation of the observed phenomena and implications for practical realization of nanoparticle catalysts are discussed.

ChemInform Abstract: Synthesis, Characterization, and Reactivity of Palladium(II) Complexes Containing Piperidoimidazolin‐2‐ylidene.

ChemInform Abstract: Synthesis, Characterization, and Reactivity of Palladium(II) Complexes Containing Piperidoimidazolin‐2‐ylidene.

Synthesis, characterization, and reactivity of palladium(II) complexes containing piperidoimidazolin‐2‐ylidene

A series of Pd–N‐heterocyclic carbene (Pd‐NHC) complexes with pyrazine (1) or pyridine (2) and NHC (3) were synthesized and characterized by elemental analysis and spectroscopic methods. In addition, the molecular structure of 3 was determined by X‐ray diffraction studies. The effects of these ligands on catalyst activation and the performance of complexes 1, 2, 3 were studied on Suzuki–Miyaura reactions of phenylboronic acid with aryl chlorides. Finally, we demonstrated that complex 1 is very adept at re‐forming the Kumada–Tamao–Corriu cross‐coupling reaction. Copyright © 2013 John Wiley & Sons, Ltd.

Synthesis, Electronic Structure, and Reactivity of Palladium(I) Dimers with Bridging Allyl, Cyclopentadienyl, and Indenyl Ligands

The synthesis of three new Pd(I) dimers, (μ-All)(μ-Cp){Pd(PEt3)}2 (All = C3H5, Cp = C5H5), (μ-All)(μ-Ind){Pd(PEt3)}2 (Ind = C7H9), and (μ-Cp)(μ-Ind){Pd(PEt3)}2, which contain a combination of bridging allyl, Cp, or indenyl ligands and are all supported by triethylphosphine as the ancillary ligand, is reported. The solid-state geometries, electronic structures, and reactivity of these new compounds have been compared with those of the dimers (μ-All)2{Pd(PEt3)}2 and (μ-Cp)2{Pd(PEt3)}2, which have previously been reported. This work establishes that there are many similarities in the solid-state and electronic structures of complexes containing bridging allyl, Cp, or indenyl ligands. For example, in all cases the bridging ligands bind through three carbon atoms to the two Pd atoms, with only the central carbon atom of the bridging group bound to both metal centers. However, there are also important differences based on the identity of the bridging ligand. As a result of different overlap between the metal ce...

Pd and S binding energies and Auger parameters on a model silica-supported Suzuki–Miyaura catalyst: Insights into catalyst activation

Model Suzuki–Miyaura reaction catalysts have been developed by immobilizing palladium on a mercaptopropyltrimethoxysilane (MPTMS) functionalized Si substrate. Two types of Pd species were found on the fresh catalysts that may be attributed to a S-bound Pd (II) species and Pd nanoparticles. The binding energy of the nanoparticles is strongly size dependent, and is higher than that of metallic Pd. A sulfur species that has not been previously reported on this class of catalysts has also been observed. A systematic investigation of various palladium/sulfur complexes using XPS was carried out to identify this species, which may be assigned to high oxidation state sulfur formed by oxidation of thiol during the reduction of the Pd(OAc)2 used to load the catalyst with Pd. Shifts in binding energy observed for both Pd and S spectra of the used catalysts were examined in order to probe the change of electronic environment of reactive palladium center and the thiol ligand during the reaction. Electron and atomic force microscopic imaging of the surfaces demonstrates the formation of Pd nanoparticles on fresh catalysts and subsequent size reduction of the Pd nano-particles following reaction.

Synthesis, Properties, and Reactivity of Palladium and Nickel NHC Complexes Supported by Combinations of Allyl, Cyclopentadienyl, and Indenyl Ligands

The synthesis of a series of Pd and Ni complexes containing combinations of 2-methylallyl (C4H7), cyclopentadienyl (C5H5, Cp), and indenyl (C7H9, Ind) ligands is reported. In all cases these complexes are supported by the electron-donating N-heterocyclic carbene ligand 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IPr). The mixed Cp/2-methylallyl complexes (η1-Cp)(η3-2-methylallyl)Pd(IPr) (CpAllPd) and (η5-Cp)(η1-2-methylallyl)Ni(IPr) (CpAllNi) were synthesized through the reaction of IPr with (Cp)(2-methylallyl)M (M = Ni, Pd). The binding mode of the ligands is different in the two complexes, and as a result the total valence electron count around the metal is 18 for the Ni complex and only 16 for the Pd species. In the case of Pd, an analogue of CpAllPd containing an indenyl ligand, (η1-Ind)(η3-2-methylallyl)Pd(IPr) (IndAllPd), was synthesized through the reaction of (η3-Ind)Pd(IPr)Cl (IndClPd) with (2-methylallyl)magnesium chloride. The corresponding Ni complex (η5-Ind)(η1-2-methylallyl)Ni(IPr) (IndAllNi) could not be isolated. The binding modes of the ligands in the mixed indenyl/Cp complexes (η1-Ind)(η5-Cp)M(IPr) (M = Ni (IndCpNi), Pd (IndCpPd)) were the same for both Ni and Pd. IndCpPd was prepared through the reaction of IndClPd with NaCp, while IndCpNi was synthesized through the reaction of (η5-Cp)Ni(IPr)Cl (CpClNi) with lithium indenyl. Similarly, the structures of the bis(Cp) complexes (η5-Cp)(η1-Cp)Ni(IPr) (CpCpNi) and (η5-Cp)(η1-Cp)Pd(IPr) (CpCpPd) were identical for the two different metals. In contrast to CpCpPd, which is an 18-electron complex, the related bis(indenyl) Pd complex (η3-Ind)(η1-Ind)Pd(IPr) (IndIndPd) is a 16-electron species, while no Ni analogue of IndIndPd was characterized. Preliminary reactivity studies with electrophiles indicate that, in all systems with mixed ligands, the η1-ligand is nucleophilic and reacts selectively. The complexes CpAllPd, CpAllNi, CpCpPd, CpCpNi, IndClPd, IndAllPd, and IndIndPd were characterized by X-ray crystallography.

Synthesis and Reactivity of Palladium- and Platinum-Bridged Heterobimetallic [3]Trochrocenophanes

Heterobimetallic [3]trochrocenophanes with PdCl2 and PtCl2 bridges were prepared in reasonable yields. The PdCl2 species possess, like their Pt analogues, poor solubility but can be made more soluble by ligand substitution. By reaction of Pd-bridged [3]trochrocenophanes with MeLi, both Cl atoms can be substituted by a Me group. Likewise, reaction of Pt-bridged complexes with LiC≡CPh leads to the expected disubstituted compound. Herein we present the synthesis of MCl2- and MR2-bridged (M = Pd, Pt; R = Me, C≡CPh) [3]trochrocenophanes, as well as some solid-state structures of these [3]trochrocenophanes.

ChemInform Abstract: Carbon—Heteroatom Bond‐Forming Reductive Elimination from Palladium(IV) Complexes

AbstractReview: 78 refs.

ChemInform Abstract: Recent Advances in Palladium‐Catalyzed Cascade Cyclizations

AbstractReview: 140 refs.