Palladium Coordination Research Articles

Preparation of magnetically recoverable carbon nanotube-supported Pd(II) catalyst

Multi-walled carbon nanotubes were filled or externally decorated with iron oxide nanoparticles to make them magnetically-recoverable. In the first case, the external surface was treated to be free of defects and oxygenated groups to favor internalization. A thermal decomposition route was followed in the second case. The formed nanoparticles were characterized and shown to consist in magnetite and/or maghemite. Moreover, when placed on the external surface of carbon nanotubes, they remained small and well-dispersed, leaving high proportion of free carbon surface for further functionalization. The intact sp2 carbon atoms were subsequently attacked by means of xanthate radical chemistry, followed by post-functionalization to graft a dipyridylamine ligand on the surface of both solids and pristine carbon nanotubes for comparison. Solids at each step of the syntheses were characterized by X-ray photoelectron spectroscopy (XPS). Palladium coordination onto the surface ligand was studied and the precursor [Pd(COD)Cl2] complex gave the best results to afford CNT-supported molecular Pd(II) catalysts. XPS confirmed the +2 oxidation state of palladium on the carbon surface. The so-prepared magnetically-recoverable catalysts were successfully used in Suzuki-Miyaura cross-coupling catalytic application.

A new strategy to design a graphene oxide supported palladium complex as a new heterogeneous nanocatalyst and application in carbon–carbon and carbon‐heteroatom cross‐coupling reactions

The palladium nanoparticles were successfully stabilized with an average diameter of 6–7 nm through the coordination of palladium and terpyridine‐based ligands grafted on graphene oxide surface. The graphene oxide supported palladium nanoparticles were thoroughly characterized and applied as an efficient heterogeneous catalyst in carbon–carbon (Suzuki‐Miyaura, Mizoroki‐Heck coupling reactions) and carbon–heteroatom (C‐N and C‐O) bond‐forming reactions. The catalyst was simply recycled from the reaction mixture and was reused consecutive four times with small drop in catalytic activity.

Synthesis, structure and palladium coordination of ambiphilic, pyridine- and phosphine-tethered N-boryl imine ligands.

We describe here the synthesis and structural characterization of two new classes of ambiphilic, N-boryl imine ligands, wherein boron is associated with a Lewis basic imine nitrogen. These ligands can be easily generated in two steps from the corresponding pyridinyl- and phosphinyl-tethered aldehydes. 11B NMR analysis suggests the association of the Lewis acidic boron to either the pyridine unit or via intermolecular acid/base interactions with the imine. Both of these ligands can coordinate to palladium, and their structures were confirmed by X-ray crystallography.

Control over coordination self-assembly of flexible, multidentate ligands by stepwise metal coordination of isopyrazole subunits.

Structurally flexible oligoisopyrazole molecules were self-assembled into discrete complexes with structural diversity upon palladium coordination. Structural convergence was controlled by a stoichiometry change to induce stepwise metal coordination of isopyrazole subunits. Three different types of complexes were selectively generated from a diisopyrazole ligand. Furthermore, tetraisopyrazole ligands were quantitatively assembled into a discrete complex in a predictable manner.

Reactive Palladium Carbenes: Migratory Insertion and Other Carbene–Hydrocarbyl Coupling Reactions on Well‐Defined Systems

Palladium complexes with carbene ligands are among the best known and more extensively used catalysts. Those carbenes are usually NHC or N,N‐disubstituted derivatives and their use relies on the robust nature of the carbene ligands and their role as auxiliary ligands that modify the metal's electronic and steric features. In contrast, there are other types of carbenes that, when coordinated to palladium, lead to transformations in which the carbene fragment is involved. In the last decade, palladium‐catalyzed reactions have been extended to the use of carbene precursors that can be functionalized with the formation of several C–C bonds in one reaction step. Plausible mechanisms have been proposed where a carbene–hydrocarbyl coupling occurs in the palladium coordination sphere. This microreview aims to collect and discuss the examples of organometallic reactions in well‐characterized palladium complexes where the carbene is one of the fragments involved in C–C bond formation. These examples are key in understanding and supporting the mechanisms proposed for the Pd‐catalyzed of C–C coupling reactions of carbenes.

Mizoroki–Heck and Suzuki–Miyaura reactions mediated by poly(2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid)‐stabilized magnetically separable palladium catalyst

The purpose of this work was to synthesize and characterize a new magnetic polymer nanosphere‐supported palladium(II) acetate catalyst for reactions requiring harsh conditions. In this regard, an air‐stable, moisture‐stable and highly efficient heterogenized palladium was synthesized by the coordination of palladium(II) acetate with poly(2‐acrylamido‐2‐methyl‐1‐propanesulfonic acid)‐grafted modified magnetic nanoparticles with a core–shell structure. The structure of the newly developed catalyst was characterized using various techniques. The catalytic activity of the resultant nano‐organometallic catalyst was evaluated in Mizoroki–Heck and Suzuki–Miyaura reactions to afford the corresponding coupling products in good to excellent yields. High selectivity as well as outstanding turnover number (14 143, 4900) and turnover frequency (28 296, 7424) values were recorded for the catalyst in Suzuki–Miyaura and Mizoroki–Heck reactions, respectively. Magnetic separation and recycling of the catalyst for at least six runs became possible without any significant loss of efficiency or any detectable palladium leaching.

The Mechanism of Low-Temperature Oxidation of Carbon Monoxide by Oxygen over the PdCl₂-CuCl₂/γ-Al₂O₃ Nanocatalyst.

The state of palladium and copper on the surface of the PdCl2–CuCl2/γ-Al2O3 nanocatalyst for the low-temperature oxidation of CO by molecular oxygen was studied by various spectroscopic techniques. Using X-ray absorption spectroscopy (XAS), powder X-ray diffraction (XRD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), freshly prepared samples of the catalyst were studied. The same samples were also evaluated after interaction with CO, O2, and H2O vapor in various combinations. It was shown that copper exists in the form of Cu2Cl(OH)3 (paratacamite) nanophase on the surface of the catalyst. No palladium-containing crystalline phases were identified. Palladium coordination initially is comprised of four chlorine atoms. It was shown by XAS that this catalyst is not capable of oxidizing CO at room temperature in the absence of H2O and O2 over 12 h. Copper(II) and palladium(II) are reduced to Cu(I) and Pd(I,0) species, respectively, in the presence of CO and H2O vapor (without O2). It was found by DRIFTS that both linear (2114 cm−1, 1990 cm−1) and bridging (1928 cm−1) forms of coordinated CO were formed upon adsorption onto the catalyst surface. Moreover, the formation of CO2 was detected upon the interaction of the coordinated CO with oxygen. The kinetics of CO oxidation was studied at 18–38 °C at an atmospheric pressure for CO, O2, N2, and H2O (gas) mixtures in a flow reactor (steady state conditions).

Dimeric palladium 1,2,3-triazol-5-ylidene complexes – synthesis, structure, reactivity and catalytic properties in Suzuki coupling

Palladium 1,2,3-triazol-5-ylidene dimers transform to bis(carbene) species and in Suzuki coupling conditions lose carbene via reductive elimination of a 5-aryltriazolium salt.

Synthesis and coordination chemistry of enantiopure t-BuMeP(O)H.

Both enantiomers of the optically pure Secondary Phosphine Oxide (SPO) t-BuMeP(O)H (1) have been obtained by deboronation of phosphinous acid-borane t-BuMeP(O)H·BH3 (3) with HBF4 followed by hydrolysis of the intermediate adduct t-BuMeP(O)H·BF3 (1·BF3), which has been isolated and crystallographically characterised. Complexes [MCl(COD)(κP-(R)-1)] (M = Ir, Rh; 4 and 7 respectively) have been detected in solution but could not be isolated while trans-[RhCl(CO)(κP-(S)-1)2] (9) has been successfully obtained in good yield. Gold complex [AuCl(κP-(S)-1)] (10) has been prepared and its crystal structure shows the presence of aurophilic interactions. Three new ruthenium complexes ([RuCl2(η6-p-cymene)(κP-(R)-1)], 12), ([RuCl2(CO)3(κO-(S)-1)], 13) and trans-([RuCl2(CO)2(κP-(S)-1)2], 14) have been synthesised and fully characterised, including the crystal structure of 12. Four palladium coordination compounds have been prepared: trans-[PdCl2(κP-(S)-1)2] (trans-15), [Pd(μ-Cl)(κP-(S)-1)2]2 (16·OH and 16·BF2) and [Pd(μ-OAc)(κP-(S)-1)2] (17) and the crystal structure of complex 16·OH proves the pseudobidentate coordination of the two molecules of 1. Three organometallic allylpalladium complexes have been prepared namely [Pd(η3-Ph2C2H3)Cl(κP-(S)-1)] (18) and [Pd(η3-Ph2C2H3)(κP-(S)-1)2] (19·OH and 19·BF2). The crystal structure of 19·BF2 constitutes the first allylpalladium-SPO complex reported to date.

Counteranion-Controlled Ag2O-Mediated Benzimidazolium Ring Opening and Its Application in the Synthesis of Palladium Pincer-Type Complexes

We report a new approach to synthesize palladium complexes through Ag2O-promoted hydrolytic ring opening followed by palladium coordination. The novelty of Ag2O-promoted hydrolysis in comparison with the commonly used base-mediated hydrolysis lies in the anion-controlled product selectivity in the synthesis of pincer-type palladium complexes. Moreover, these palladium complexes have been tested in the Suzuki reaction using aryl bromides in methanol and H2O, respectively. In comparison with palladium complexes with normal coordination, the palladium complexes generated from the hydrolytic ring-opening coordination demonstrate excellent catalytic activity.

A new molecular star is born

One thing chemists have learned over the years is that geometry is innate in molecular structures. Besides regular polygons, unexpected shapes often emerge when researchers explore new combinations of metal and ligand components. In one of the latest examples, a team led by Dillip Kumar Chand of Indian Institute of Technology Madras has built a “truncated molecular star,” a pentagram-shaped palladium coordination complex (Chem.–Eur. J. 2017, DOI: 10.1002/chem.201702264). Most self-assembled coordination complexes are built from a metal coordinated with one type of ligand (homoleptic). Complexes made using two or more types of ligand (heteroleptic) are not as common. On top of that, the ligands often are chelating, meaning the ligand binds one metal through two or more atoms. Despite the success in geometrical self-assembling coordination chemistry, Chand and coworkers discovered that their new molecular star is unique not just because of its pointless shape—a pentagon ringed by isosceles trapezoids—but also

Palladium-Catalyzed Directed Halogenation of Bipyridine N-Oxides.

The palladium-catalyzed directed C-H halogenation of bipyridine N-oxides was investigated. Using NCS or NBS (N-chloro- or N-bromosuccinimide) and 5 mol % Pd(OAc)2 in chlorobenzene (0.10 molar) at 110 °C, pyridine-directed functionalization took place and 3-chloro- or 3-bromobipyridine N-oxides were obtained in high yields. The reaction is sensitive to steric hindrance by 4- and 6'-substituents. Only in the latter case, where coordination of palladium by the pyridine is hindered, 3'-halogenation directed by the N-oxide function was observed. The halogenated products were deoxygenated by PCl3 or PBr3.

Synthesis and Characterisation of a Novel Phosphonate Ligand and Its Palladium(II) Coordination Compound

Synthesis and Characterisation of a Novel Phosphonate Ligand and Its Palladium(II) Coordination Compound

Combinatorial Synthesis of Acacen-Type Ligands and Their Coordination Compounds.

A highly modular synthetic method for the preparation of acacen-type ligands and their coordination compounds was developed. A series of 46 acacen-type ligands were synthesized by a combinatorial acid-catalyzed transamination between six primary diamines and eight enaminones. The bis-enaminone products were used as tetradentate ligands for coordination of copper(II), nickel(II), cobalt(II), and palladium(II). Dependence of the preferred E- or Z-configuration of the enaminone ligand on the α-substituent of the enaminone moiety in solution was determined by NMR and confirmed by X-ray diffraction. The copper(II) complexes were tested for their suitability as catalysts in 3 + 2 cycloaddition of azomethine imine to methyl propiolate.

Iron(III) Half‐Sandwich Complexes of the Two‐Legged Piano‐Stool [CpFe(aryl)Cl] Type from the Corresponding Aryliron(II) Precursors

Instead of reacting with the σ‐mesityliron(II) complex [Cp′′′Fe(σ‐C6H2Me3‐2,4,6)] (1; Cp′′′ = 1,2,4‐tri‐tert‐butylcyclopentadienyl) to form a cyclohexadienyl(cyclopentadienyl)iron(II) sandwich complex with palladium coordination to the ipso carbon atom of the six‐membered ring, palladium(II) chloride oxidized 1 to the mesityliron(III) complex [Cp′′′Fe(σ‐C6H2Me3‐2,4,6)Cl] (5) with a two‐legged piano‐stool geometry. The oxidation of [4CpFe(σ‐C6H2Me3‐2,4,6)] (4; 4Cp = tetraisopropylcyclopentadienyl) to [4CpFe(σ‐C6H2Me3‐2,4,6)Cl] (6) was accomplished with hexachloroethane. Two other derivatives, [4CpFe(σ‐C6H3iPr2‐2,6)Cl] (7) and [5CpFe(σ‐C6H3iPr2‐2,6)Cl] (8; 5Cp = pentaisopropylcyclopentadienyl), could also be obtained by the same method. The molecular structures of 5 and 6 are compared with the structure of the (tetraisopropylcyclopentadienyl)iron(III) dibromide [4CpFeBr2] (9), which was formed from the iron(II) analogue [4CpFe(µ‐Br)]2 during attempted nucleophilic substitution with an arylmagnesium bromide.

A silica-supported palladium-bis(oxazoline) complex efficiently catalyzes the synthesis of cinnamic acid derivatives via Mizoroki–Heck coupling reactions

A palladium(II)-bis(oxazoline) complex supported on silica (Pd-BOX-Si) was prepared, characterized and applied as a catalyst in Mizoroki–Heck cross-coupling reactions. The bis(oxazoline) (BOX) ligand has a hydroxyl group that can be anchored to 4-benzyl chloride-functionalized silica gel, followed by the coordination of palladium(II) chloride. The catalytic activity and the recyclability of Pd-BOX-Si have been investigated in the production of cinnamic acid derivatives via Mizoroki–Heck coupling reactions of acrylates with aryl halides; The Pd-BOX-Si catalyst demonstrated excellent catalytic activity. Characterization of the recycled Pd-BOX-Si catalyst revealed its good stability under the reaction conditions employed.

Self‐Assembled Palladium and Platinum Coordination Cages: Photophysical Studies and Anticancer Activity

AbstractInvited for the cover of this issue is the group of Fritz Kühn from Technische Universität München, Germany. The cover image shows a palladium cage encapsulating cisplatin, represented by a bowling ball, knocking down the pins, which symbolize cancer cells.

Self‐Assembled Palladium and Platinum Coordination Cages: Photophysical Studies and Anticancer Activity

Self‐assembled coordination cages are interesting as drug‐delivery systems. Therefore, the synthesis of new M2L4 (M = Pd, Pt) molecular cages derived from highly fluorescent, rigid polyaromatic ligands is reported herein, and the first Pt2L4 cage with a ligand consisting of three pyridine moieties is described. The photophysical properties were examined, and they showed high quantum yields Φ of up to 48 % for the methoxy‐functionalized ligands. Coordination of the ligands to palladium and platinum ions did, however, reduce the fluorescence of the metallocages. The host–guest chemistry of the palladium cages with cisplatin was investigated, which confirmed the encapsulation. The cages encapsulating cisplatin show significantly increased cytotoxicity towards A549 (human lung adenocarcinoma) cells relative to that shown by cisplatin and, thus, appear to be promising delivery vectors for the anticancer drug cisplatin.

Mechanism and Selectivity in the Pd-Catalyzed Difunctionalization of Isoprene.

The three-component coupling of isoprene, an alkenyl triflate, and styrenylboronic acid catalyzed by a palladium pyrox complex affords access to skipped dienes from simple chemical feedstocks. Unfortunately, the transformation proceeds with only moderate selectivity and yields. The reaction mechanism and factors responsible for the resulting regioselectivity were elucidated using M06/SDD/6-311++G(d,p) + SMD calculations. Distortion of the palladium coordination sphere in the transition structure of the migratory insertion step is found to control the 4,1- vs 1,x-selectivity. The calculated ΔΔG⧧ of 1.0 kcal/mol for this step is in excellent agreement with the experimentally observed selectivity of 1:9.9 disfavoring the 4,1-product. The transmetalation was found to be the regioselectivity determining step for the formation of the 1,2- vs 1,4-addition products. Systematic conformational searches for the transmetalation transition structure revealed a series of steric interactions between the t-Bu substituent on the ligand and the substrates in the model system that are balanced by additional repulsive interactions between the substrates and the pyridyl portion of the ligand. The combination of these effects leads to the low to moderate 1,2- vs 1,4-selectivity in the experimentally studied system.

Synthesis, structure, and chromogenic properties of polymethylene-vaulted trans-bis(salicylaldiminato)palladium(II) complexes

The synthesis, structure, and chromogenic properties of polymethylene-vaulted trans-bis(salicylaldiminato)palladium(II) complexes are described. A series of the Pd complexes incorporating nona-, deca-, undeca- and dodecamethylene bridges (1a: n=9; 1b: n=10; 1c: n=11; 1d: n=12) was prepared by the treatment of Pd(OAc)2 with the corresponding N,N′-bis(salicylidene)-1,ω-alkanediamines. The trans-coordination, vaulted structures, and crystal packing of 1a–d have been unequivocally established from X-ray diffraction studies. Complexes 1a–d exhibit linker-dependent chromatic properties in 2-methyl-2,3,4,5-tetrahydrofuran, where the absorption wavelengths at around 400nm are shifted hypsochromically with a decrease in the linker length, which is in contrast to the structure-independence of the absorption properties for the corresponding polymethylene-vaulted Pt analogues 2a–d observed under the same conditions. Density functional theory and time-dependent DFT calculations revealed that the present specific linker dependence in the chromism of 1 is ascribed to specific alteration of the d–π conjugation on the flexible trans-bis(salicylaldiminato)palladium coordination platforms by variation of the coordination planarity.