PdCl2 Complexes Research Articles

Pyrazole‐3‐carboxylates assisted N‐heterocyclic carbene palladium complexes: synthesis, characterization, and catalytic activities towards arylation of azoles with arylsulfonyl hydrazides

Four mononuclear and dinuclear pyrazole‐3‐carboxylates assisted NHC–Pd complexes have been synthesized and characterized. Notably, the bridge‐cleavage reactions of [Pd(μ‐Cl)(Cl)(NHC)]2 with 1H‐pyrazole‐3‐carboxylic acid afforded dinuclear complexes [(NHC)Pd(μ‐1H‐pyrazolato‐3‐carboxylate)]2, in which the 1H‐pyrazolato‐3‐carboxylate was employed as a N^N^O dianionic chelating and bridging ligand. To further explore the structural features and catalytic properties of the complexes, 1‐methyl‐1H‐pyrazole‐3‐carboxylic acid was introduced into the coordination with [Pd(μ‐Cl)(Cl)(NHC)]2 and the corresponding mononuclear complexes (NHC)PdCl(1‐methyl‐1H‐pyrazole‐3‐carboxylate) were obtained. The catalytic properties of the complexes in desulfitative arylation of azoles with arylsulfonyl hydrazides were initially investigated.

Enantioselective Synthesis of Multisubstituted Allenes by Cooperative Cu/Pd‐Catalyzed 1,4‐Arylboration of 1,3‐Enynes

AbstractA cooperative Cu/Pd‐catalyzed enantioselective synthesis of multisubstituted allenes is established. By employing chiral sulfoxide phosphine (SOP)/Cu and PdCl2(dppf) complexes as catalysts, the 1,4‐arylboration of 1,3‐enynes provides an efficient approach to trisubstituted chiral allenes with up to 92 % yield and 97:3 er. Furthermore, by using 2‐substituted 1,3‐enynes as substrates, the tetrasubstituted chiral allenes were successfully generated using this strategy. Finally, theoretical calculations indicate that the transmetallation of the allenylcopper species is the rate‐limiting step of this transformation.

Enantioselective Synthesis of Multisubstituted Allenes by Cooperative Cu/Pd-Catalyzed 1,4-Arylboration of 1,3-Enynes.

A cooperative Cu/Pd-catalyzed enantioselective synthesis of multisubstituted allenes is established. By employing chiral sulfoxide phosphine (SOP)/Cu and PdCl2 (dppf) complexes as catalysts, the 1,4-arylboration of 1,3-enynes provides an efficient approach to trisubstituted chiral allenes with up to 92 % yield and 97:3 er. Furthermore, by using 2-substituted 1,3-enynes as substrates, the tetrasubstituted chiral allenes were successfully generated using this strategy. Finally, theoretical calculations indicate that the transmetallation of the allenylcopper species is the rate-limiting step of this transformation.

Synthesis of MCM-41 Immobilized (Phenoxy)Imine Palladium(II) Complexes as Recyclable Catalysts in the Methoxycarbonylation of 1-Hexene

The immobilization of 2-phenyl-2-((3(tryethoxysicyl)propyl)imino)ethanol (HL1) and 4-methyl-2-(((3(triethoxysilyl)propyl)imino)methyl)phenol (HL2) on MCM-41 afforded the respective ligands HL1-MCM-41 (HL3) and HL2-MCM-41 (HL4). The treatment of complexes Pd(L1)2 and Pd(L2)2 with MCM-41 afforded the immobilized complexes (Pd(L1)2)-MCM-41 (1) and (Pd(L2)2)-MCM-41 (2) respectively. Separately, the reactions of HL3 and HL4 with Pd(NCMe)2Cl2 produced the immobilized complexes Pd(HL3)Cl2 (3) and Pd(HL4)Cl2 (4) respectively. The immobilized compounds were characterized by FT-IR, Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), energy-dispersive X-ray (EDX), Thermogravimetric Analysis (TGA) and X-ray Powder Diffraction (XRD). All the complexes (1–4) formed active catalysts in the methoxycarbonylation of 1-hexene to give linear and branched esters. The catalysts were recycled four times without the loss of catalytic activity. Hot filtration experiments established the absence of leaching, and the heterogeneous nature of the active species was derived from mercury drop experiments.

Synthesis of Pd phosphine-imine complexes and their reactivity with base

Two phosphine-imine ligands were synthesized in which the heteroatoms were connected by an ethyl (1a) or a propyl (1b) linker. Both ligands were reacted with PdCl(Me)(COD), which gave the square planar complexes PdCl(Me)(κ2-P,N-1a) (2a) and PdCl(Me)(κ2-P,N-1b) (2b). Both complexes were treated with base in pyridine in an effort to prepare phosphine 1-azaallyl complexes of the type Pd(Me)(κ2-P,N-L)(py). Deprotonation of 2a gave an intractable mixture of products. In contrast, deprotonation of 2b afforded a new product 4b. However, the site of deprotonation was the methylene adjacent to the phosphine rather than Cα to the imine carbon, deprotonation of which would have afforded a 1-azaallyl moiety.

Synthesis and palladium(II) metal chemistry of thiazoline/imidazoline derived ligands: An efficient catalyst for cross-coupling reactions of arylboronic acids with acid chlorides and aryl halides

The synthesis, reactivity, spectroscopic characterization and catalytic activities of a series of Pd(II) complexes bearing the chelating ligands 2-(4,5-dihydrothiazol-2-yl)aniline (2) and 2-(4,5-dihydro-1H-imidazol-2-yl)aniline (3) are reported. The reactions of 2 with [Pd(COD)Cl2] in 1:1 and 1:2 (ligand into metal and metal into ligand) molar ratios afforded the mononuclear complexes, PdCl2{κ2−N,S − 2−(4,5-dihydrothiazol-2-yl)aniline} (4), Pd{κ2−N,N’−2−(4,5-dihydrothiazol-2-yl)aniline}2 (5) and Pd{κ12−N,N’−κ22−N,S − 2−(4,5-dihydrothiazol-2-yl)aniline}2 (6), respectively, in good yields. The neutral mononuclear palladium complexes PdCl2{κ2−N,N’−(4,5-dihydro-1H-imidazol-2-yl)aniline} (7) and Pd{κ2−N,N’−2−(4,5-dihydro-1H-imidazol-2-yl)aniline}2 (8) were synthesized using [Pd(COD)Cl2] with appropriate molar ratios. The palladium complexes 4–8 were evaluated for their catalytic activities in base assisted cross coupling reactions between acid chlorides and aryl halide with boronic acids. The complex 4 was found to be an efficient catalyst for producing unsymmetrical ketones (TOF = 19.8 min−1) while complex 7 was effective for making biaryls (TOF = 19.8 min−1).

Aqueous-phase hydrodechlorination of trichloroethylene over Pd-based swellable organically-modified silica (SOMS): Catalyst deactivation due to chloride anions

Swellable-organically modified silica (SOMS) has been demonstrated to be an efficient catalyst scaffold for catalytic treatment of water contaminated with trichloroethylene (TCE). In this study, deactivation characteristics of Pd-incorporated SOMS for aqueous-phase hydrodechlorination (HDC) of TCE were investigated. Pd/SOMS catalysts were exposed to highly-concentrated chloride solutions (up to 1 M NaCl or 0.01 M HCl) to examine the deactivation resistant behavior of Pd/SOMS. The commonly used HDC catalyst Pd/Al2O3 was also studied for comparison purposes. Pd/SOMS and Pd/Al2O3 in their pristine and treated states were tested for aqueous-phase HDC of TCE and characterized by several techniques including N2 physisorption, inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray photoelectron spectroscopy, extended X-ray absorption fine structure spectroscopy (EXAFS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO. The aqueous-phase treatments had a pronounced adverse effect on the textural properties of Pd/Al2O3, although the effect was independent of the type of the chloride precursor, NaCl or HCl. Treating Pd/Al2O3 with chloride-containing solutions lowered the catalytic activity due to formation of Pd-Cl complexes and active metal leaching. The leached Pd obtained from the treatment solution was shown to be inactive for aqueous-phase HDC of TCE. While Pd/Al2O3 underwent severe deactivation due to the chloride treatments, Pd/SOMS exhibited resistance to chloride deactivation and metal leaching. The chloride treatments did not impact the textural properties of Pd/SOMS. The achieved deactivation resistance was attributed to the novel characteristics of the SOMS support.

Complexation of an indole-based α-aminoimine ligand to Pd(II)

The indole-based α-aminoimine ligand 6-Me-4-iPr-6-{(2,6-iPr2-C6H3)NCMe}-6,11-dihydro-5H-indolo[3,2-c]quinoline (6) was synthesized by condensation of (2,6-iPr2-C6H3)NC(Me)C(O)Me (5) and 2-(indol-2-yl)-6-iPr-C6H3NH2 (4). Ligand 6 coordinates in a κ2-CN,NH fashion in the square planar complex (6)PdCl2.

Redox Activity, Ligand Protonation, and Variable Coordination Modes of Diimino-Pyrrole Complexes of Palladium.

Ligand-based functionality is a prominent method of increasing the reactivity or stability of metal centers in coordination chemistry. Some of the most successful catalysts use ligand-based redox activity, pendant protons, or hemilability in order to specifically accelerate catalysis. Here we report the diimino-pyrrole ligand Tol,CyDIPyH (Tol,CyDIPy = 2,5-bis( N-cyclohexyl-1-( p-tolyl)methanimine)pyrrolide), which exhibits all three of these ligand properties. Metalation of Tol,CyDIPy to Pd gives the pseudo-square planar complex (Tol,CyDIPy)PdCl, which upon reduction forms a mixture of products, including a Pd(I)-Pd(I) dimer wherein Tol,CyDIPy bridges the dimeric unit. Upon addition of PMe3, the imine arms of (Tol,CyDIPy)PdCl are displaced to yield (Tol,CyDIPy)Pd(PMe3)2Cl, where the Tol,CyDIPy ligand binds in a monodentate fashion. This complex can be reduced to generate a ligand-based radical, as shown by EPR spectroscopy. Finally, (Tol,CyDIPy)PdCl also can be protonated at the imine arm, exhibiting a total of three different coordination modes across this series of complexes. Taken together, these studies show that Tol,CyDIPy exhibits notable flexibility in its coordination and redox chemistry.

PEPPSI-type complexes with small NHC ligands obtained according to the new method efficiently catalyzed Suzuki-Miyaura reaction

A series of 13 new PEPPSI-type complexes with small NHC ligands and differently substituted imidazoles were prepared using as substrates [Pd(μ-X)X(bmim-y)]2 dimer or imidazole complexes PdCl2(imidazole)2. Improvement of the synthetic method allowed simplification of the purification step. The obtained complexes efficiently catalyzed the Suzuki-Miyaura cross-coupling of substituted bromobenzenes and arylboronic acids in water and in iPrOH/water mixture. The observed catalytic activity was significantly better than that of Pd(OAc)2 and similar to that of Pd(IPr)Cl2(3-Cl-py) in the same conditions. TEM analysis of the post-reaction mixture evidenced formation of Pd NPs of diameter ca. 3 nm, participating in the catalytic process.

Unsymmetrical Pincer N-Heterocyclic Carbene–Nitrogen–Phosphine Chelated Palladium(II) Complexes: Synthesis, Structure, and Reactivity in Direct Csp2–H Arylation of Benzoxazoles

An unsymmetrical pincer N-heterocyclic carbene–nitrogen–phosphine (CNP) and its palladium complexes PdCl2(κ2-CP) (4) and [PdCl(κ3-CNP)]PF6 (5·PF6) were synthesized. NMR spectra disclosed that the transformation of complex 4 structure occurred in the solution. Further NMR experimental and single crystal structure analysis of complex 4 provided unequivocal and structural evidence for the formation of complex [PdCl(κ3-CNP)]Cl (5·Cl) in the solution of complex 4. The catalytic performance of palladium complexes 4 and 5·PF6 was investigated with the direct Csp2–H arylation of benzoxazoles with aryl bromides. Notably, aryl bromides could give up to 97% arylation products in the presence of 0.5% complex 4. For ortho-substituted substrates, the steric hindrance had a significant impact, but product yields could be improved remarkably by extending reaction time. This result implied the catalytic active species in this system was stable and kept in a long lifetime. The flexible backbone and unsymmetrical pincer str...

Solvent-free Buchwald–Hartwig amination with low palladium loadings

A highly efficient ‘green’ solvent-free monoarylation of primary anilines with aryl bromides mediated by the expanded-ring N-heterocyclic carbene palladium complex (THP-Dipp)Pd(cinn)Cl [THP-Dipp is 1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene; cinn is cinnamyl] can be performed at low catalyst loadings (0.1 mol%) to provide excellent yields and remarkable selectivities for various substrates.

Total chemical synthesis of histones and their analogs, assisted by native chemical ligation and palladium complexes.

Chemical synthesis of histones allows precise control of the installation of post-translational modifications via the coupling of derivatized amino acids. Shortcomings of other approaches for obtaining modified histones for epigenetic studies include heterogeneity of the obtained product and difficulties in incorporating multiple modifications on the same histone. In this protocol, unprotected peptide fragments are prepared by Fmoc solid-phase synthesis and coupled in aqueous buffers via native chemical ligation (NCL; in NCL, a peptide bond is formed between a peptide with an N-terminal Cys and another peptide having a C-terminal thioester). This task is challenging, with obstacles relating to the preparation and ligation of hydrophobic peptides, as well as the requirement for multiple purification steps due to protecting-group manipulations during the polypeptide assembly process. To address this, our approach uses an easily removable solubilizing tag for the synthesis and ligation of hydrophobic peptides, as well as a more efficient and better-yielding method to remove Cys-protecting groups that uses palladium chemistry (specifically [Pd(allyl)Cl]2 and PdCl2 complexes). The utility of this approach is demonstrated in the syntheses of ubiquitinated H2B at Lys34, phosphorylated H2A at Tyr57 and unmodified H4. Each of these analogs can be prepared in milligram quantities within ∼20-30 d.

Palladium(II) complexes of aminothiazole‐based phosphines: Synthesis, structural characterization, density functional theory calculations and catalytic application in heck reaction

Three Pd(II) complexes of some hemilabile ligands, aminothiazole‐based phosphines, were synthesized to investigate the catalytic activity of them in Heck cross‐coupling reactions. The crystal structures of complexes PdCl2[(Ph2P)HN(C3H2NS)] (C1) and PdCl2[(Ph2P)HN(C7H4NS)] (C3) were determined using X‐ray crystallography, which reveals that the ligand coordinates in a chelating mode through P and N (endocyclic) atoms in square planar geometry. Experimental and theoretical studies (atoms in molecules and natural bond orbital analyses) show that the Pd(II) interacts more strongly with the P atom than the N atom in the chelated ligand, N^P. This trait can promote catalytic activity of the complexes in comparison with our previous work in which chelated ligands with two phosphorus atoms, P^P, were used. The influence of non‐covalent intermolecular interactions on the assembly of the solid‐state structures is also discussed in terms of geometrical analysis. The prepared complexes turn out to be useful pre‐catalysts in Heck cross‐coupling reactions owing to the coordinative flexibility of the hemilabile ligands. The protocol affords the corresponding products in greater yield than the same reactions with bis(phosphino)amine Pd(II) complexes, as the catalysts in our previous work.

Complexes PdCl2 with optically active hybrid ligands built of α-pinene and β-alanine molecules

Complexes Pd(LH)Cl2 (LH is the N-derivative of β-alanine based on (–)-α-pinene) and PdLEtCl2 (LEt is ethyl ester of LH) are synthesized. The X-ray diffraction analysis (СIF file ССDC no. 1471432) shows that the single crystals of the [PdLEtCl2] complex belong to the racemate. The crystal structure contains (+)- and (–)-enantiomeric molecules of the mononuclear complex. Ligand LEt is bidentate chelating. The coordination PdCl2N2 core is a distorted square. The contacts Pd…Cl and hydrogen bonds N–H…Cl between molecules of the (+)- and (–)-enantiomers result in the formation of dimeric ensembles, which are joined into chains due to the C–H…Cl hydrogen bonds. According to the X-ray powder diffraction data, the products of the synthesis of complex [PdLEtCl2] contain both the racemate and (+)-enantiomer. The (+)-enantiomer is isolated by the recrystallization of the synthesis product. Complex Pd(LH)Cl2 is an optically active right-handed compound. According to the NMR data, complex PdLEtCl2 does not dissociate in a CDCl3 solution. The results of IR and NMR spectroscopy indicate that the molecular structure of complex Pd(LH)Cl2 is similar to that of [PdLEtCl2].

Photolysis of Pincer-Ligated PdII–Me Complexes in the Presence of Molecular Oxygen

The reactions of t-BuPNP and t-BuPCP PdII–Me complexes (t-BuPNP = 2,6-bis[(di-tert-butylphosphino)methyl]pyridine and t-BuPCP = 2,6-bis[(di-tert-butylphosphino)methyl]phenyl) with O2 are described and compared with the reported O2 reactivity of related PdII–Me complexes. [(t-BuPNP)PdMe]Cl was found to react with O2 upon photolysis resulting in oxidation of the pincer ligand backbone to produce a (t-BuPNO)PdCl complex. In contrast, photolysis of (t-BuPCP)PdMe with O2 resulted in oxidation of the Pd–Me group to form (t-BuPCP)PdOCO2H. Isotopic labeling, radical initiators, and solvent studies were used to gain insight into the mechanisms of these unusual reactions of late metal alkyls with molecular oxygen.

Redox control in palladium catalyzed norbornene and alkyne polymerization

Switchable polymerization of norbornene, 5-norbornene-2-yl acetate and 1-chloro-1-octyne could be realized by using two palladium complexes (NHC)Pd(allyl)Cl (NHC = 1,3-Ar2-naphthoquinimidazolylidene, Ar = 2,6-Me2-C6H3, 2,6-iPr2-C6H3) bearing a redox-active naphthoquinone moiety.

Synthesis and thermodynamics of the reactions of [Pd(Eten)(H2O)2]2+ with ligands of biological significane with reference to the antitumour activity

Pd(Eten)Cl2 complex, where Eten = N-ethylethylenediamine , was synthesized and characterized by elemental analysis and spectroscopic techniques. The stoichiometry and stability of the complexes formed between [Pd(Eten)(H2O)2]2+ and various biologically relevant ligands as adenine, adenosine, adenosine-5’-monophosphate and some selected peptides were investigated at 25ºC and 0.1 M ionic strength. The speciation diagrams of the complexes formed in solutions are evaluated. Thermodynamic parameters for Pd(Eten)-glycylglycine complexes were estimated.

Abnormal-NHC Palladium(II) Complexes: Rational Synthesis, Structural Elucidation, and Catalytic Activity

Reaction of a C2-arylated imidazolium iodide (IPrPh)I (1) (IPrPh = 1,3-bis(2,6-diisopropylphenyl)-2-phenyl-imidazolium) with PdCl2 in the presence of Ag2O affords abnormal N-heterocyclic carbene (aNHC) palladium complexes (aIPrPh)PdCl2 (2) and (aIPrPh)2PdCl2 (3) (aIPrPh = 1,3-bis(2,6-diisopropylphenyl)-2-phenyl-imidazol-4-ylidene). Treatment of 2 with a pyridine gives Pd-PEPPSI-type complexes (aIPrPh)PdCl2(L) (L = pyridine (py), 5; L = 3-chloropyridine (3Cl-py), 6). Compounds 5 and 6 are also accessible by a one-pot reaction of 1, PdCl2, and Ag2O in a pyridine solvent. While the use of a conventional base K2CO3 leads to the formation of mixed halide complexes (aIPrPh)Pd(Cl)I(L) (7, L = py; 8, L = 3Cl-py), iodide derivatives (aIPrPh)PdI2(L) (9, L = py; 10, L = 3Cl-py) can be selectively prepared with addition of an excess of KI to the reaction mixture. Albeit in a low yield, a putative transmetalation agent {(aIPrPh)2Ag}AgI2 (4) has been isolated and characterized. Compounds 2–10 are air stable crystalline...

Streamlined Preparation and Coordination Chemistry of Hybrid Phosphine–Phosphaalkene Ligands

A rationally designed and selective synthesis of hybrid phosphine–phosphaalkene ligands E-1a (Cy2PCH2CH═PMes*, Mes* = 2,4,6-tri-tert-butylphenyl) and E-1b (Ph2PCH2CH═PMes*) was developed using phospha-Wittig methodology. The new hybrid ligands E-1a and E-1b were used to prepare the Pd and Pt dichloride complexes Pd(Cy2PCH2CH═PMes*)Cl2 (2a), Pd(Ph2PCH2CH═PMes*)Cl2 (2b), Pt(Cy2PCH2CH═PMes*)Cl2 (3a), and Pt(Ph2PCH2CH═PMes*)Cl2 (3b). The crystal structures of E-1a, E-1b, 2a·1.33CHCl3, 3a·CH3CN, and 3b were determined. DFT calculations (M06/LACV3P**) on 2a revealed that the π* orbital located on the P═C unit is low-lying and accessible. An NBO analysis concluded that the phosphaalkene ligand is a significantly poorer σ donor and a slightly better π acceptor than its tertiary phosphine counterpart, due to the presence of the P═C double bond.