Monomeric Palladium Research Articles

Synthesis, Characterization, and Catalysis of Water‐Soluble Trimeric and Monomeric Palladium Complexes of 8‐Aminoquinolines

AbstractNew water soluble neutral and cationic palladium complexes were synthesized using 8‐aminoquinoline (8‐AQ) and 2‐methyl 8‐aminoquinoline (2‐Me 8‐AQ) ligands and their catalytic properties were evaluated. The neutral trimeric complexes having a Pd3N3 core were found to form when Pd(OAc)2 was reacted with 8‐AQ or 2‐Me 8‐AQ irrespective of the stoichiometry between the 2 reagents. Controlled addition of triflic acid to the neutral trimeric complex resulted in the formation of a trimeric cationic palladium complex as well as a monomeric cationic complex. A cationic palladium complex having two units of 2‐Me‐8AQ ligand was also synthesized from the cationic monomeric complex. Crystal structures of the new palladium complexes are reported in this study. The water‐soluble neutral palladium complex showed catalytic activity for the oxidation of benzyl alcohols to benzaldehydes, while the cationic palladium complexes were found to be efficient catalysts for the oxidation of styrenes to methyl ketones.

A Palladium NNC‐Pincer Complex as an Efficient Catalyst Precursor for the Mizoroki−Heck Reaction

AbstractThe Mizoroki−Heck reaction of aryl halides (iodides, bromides, or chlorides) with activated alkenes in the presence of a palladium NNC‐pincer complex at ppb to ppm loadings gave the corresponding internal alkenes in excellent yields. The total turnover number and turnover frequency reached up to 8.70×108 and 1.21×107 h−1 (3.36×103 s−1), respectively. The catalyst was applied in a ten‐gram‐scale synthesis of the UV‐B sunscreen agent octinoxate (2‐ethylhexyl 4‐methoxycinnamate). Reaction‐rate analyses, transmission electron microscopic examination of the reaction mixture, and poisoning tests suggested that a monomeric palladium species is the catalytically active species in the catalytic cycle.magnified image

Palladium Complexes of N,N'-Bis(2-aminoethyl)oxamide (H2L): Structural (PdIIL, PdII2L2, and PdIVLCl2), Electrochemical, Dynamic 1H NMR, and Cytotoxicity Studies.

The monomeric (PdL·2H2O) and dimeric (Pd2L2·7H2O) palladium(II) complexes of N,N'-bis(2-aminoethyl)oxamide (H2L) were isolated, and their structures were established by single-crystal X-ray diffraction. Both compounds display identical cis-(2Namide + 2Namine) coordination environments of the metal ion. The dimer, representing a combination of two PdL species with an open lateral chelate ring, has an "open clamshell"-like structure. The intramolecular metal-metal separation in Pd2L2 (3.215 Å) is slightly shorter than the sum of the van der Waals radii of the palladium(II) atoms. The dimeric complex is relatively stable to dissociation, and its spectral features in aqueous solutions have been compared to those of the monomeric complex. A 1H NMR spectroscopic study revealed the presence of the dynamic conformational exchange process assigned to a turning of the dimeric molecule "inside out" with an activation energy of 65 kJ/mol. Cyclic voltammetry of PdL in perchlorate-, chloride-, and sulfate-containing electrolytes revealed two-electron oxidation of the palladium center. For the dimeric complex similar, though irreversible, oxidation to the palladium(IV) state was observed in NaCl electrolyte. At the same time, in NaClO4 or Na2SO4 solutions oxidation of Pd2L2 occurs in two distinct steps. The first step is quasi-reversible and can be assigned to the formation of species in an intermediate PdIIIPdIII state. Monomeric palladium(IV) complex PdIVLCl2 was generated via chemical oxidation of PdIIL by peroxodisulfate in the presence of chloride ions and structurally characterized. The related MIIL complexes (M = Pd, Ni, Cu) showed low cytotoxicity in human cancer cell lines AGS (gastric adenocarcinoma) and HCT116 (colorectal carcinoma) with IC50 values from 204 to 525 μM, while the proligand H2L was devoid of antiproliferative activity (IC50 > 1000 μM).

Palladium complexes with 1-methyl-3-phenylindazole ligands

1-Methyl-3-phenylindazole reacts with lithium tetrachloropalladate and palladium acetate to give new coordination and (C,N)-cyclometallated dimeric and monomeric palladium complexes.

Exploring the Oxidative-Addition Pathways of Phenyl Chloride in the Presence of PdII Abnormal N-Heterocyclic Carbene Complexes: A DFT Study.

DFT calculations were performed to elucidate the oxidative addition mechanism of the dimeric palladium(II) abnormal N-heterocyclic carbene complex 2 in the presence of phenyl chloride and NaOMe base under the framework of a Suzuki-Miyaura cross-coupling reaction. Pre-catalyst 2 undergoes facile, NaOMe-assisted dissociation, which led to monomeric palladium(II) species 5, 6, and 7, each of them independently capable of initiating oxidative addition reactions with PhCl. Thereafter, three different mechanistic routes, path a, path b, and path c, which originate from the catalytic species 5, 7, and 6, were calculated at M06-L-D3(SMD)/LANL2TZ(f)(Pd)/6-311++G**//M06-L/LANL2DZ(Pd)/6-31+G* level of theory. All studied routes suggested the rather uncommon PdII /PdIV oxidative addition mechanism to be favourable under the ambient reaction conditions. Although the Pd0 /PdII routes are generally facile, the final reductive elimination step from the catalytic complexes were energetically formidable. The PdII /PdIV activation barriers were calculated to be 11.3, 9.0, 26.7 kcal mol-1 (ΔΔ≠ GLS-D3 ) more favourable than the PdII /Pd0 reductive elimination routes for path a, path b, and path c, respectively. Out of all the studied pathways, path a was the most feasible as it comprised of a PdII /PdIV activation barrier of 24.5 kcal mol-1 (ΔGLS-D3 ). To further elucidate the origin of transition-state barriers, EDA calculations were performed for some key saddle points populating the energy profiles.

Preparations of a series of coinage metal complexes with pyridine-based bis(N-heterocyclic carbene) ligands including transmetalation to palladium complexes

A silver complex bearing 2,6-bis(3-tert-butylimidazol-2-ylidene)pyridine (CNC) ligands was easily prepared from the reaction of the CNC ligand precursor with Ag2O. The CNC ligand contains relatively bulky substituents, tert-butyl, on nitrogen atoms. Use of the silver complex as a carbene transfer reagent gave rise to a copper and a gold complex, completing a series of coinage metal complexes [M2(CNC)2](BF4)2 (M = Cu, Ag, Au). In all the coinage metal complexes, the X-ray crystallographic analyses showed that the two CNC ligand strands are intertwined each other and surround the metal ions in a double helical fashion. Moreover, the carbene transfer reactions afforded a monomeric palladium complex [Pd(CNC)Cl]BF4 and a CNC-bridged dipalladium complex [{Pd(η3-C3H5)Cl}2(μ-CNC)]. Facile chloride ligand abstraction from the dipalladium complex yielded a chlorido-bridged dipalladium complex [{Pd(η3-C3H5)}2(μ-CNC)(μ-Cl)]BF4.

Dimeric and monomeric palladium(II) parent-amido (NH2) complexes: Synthesis, characterization, and reactivity

The treatment of [PdL3(NH3)]OTf (L3 = (PEt3)2(Ph) (1), (2,6-(Cy2PCH2)2C6H3) (3)) with NaNH2 in THF afforded dimeric and monomeric parent-amido palladium(II) complexes with bridging and terminal NH2, respectively, anti-[Pd(PEt3)(Ph)(μ-NH2)]2 (2) and Pd(2,6-(Cy2PCH2)2C6H3)(NH2) (4). The dimeric complex 2 crystallizes in the space group P21/n with a = 13.228(2) Å, b = 18.132(2) Å, c = 24.745(2) Å, β = 101.41(1)°, and Z = 4. It has been found that there are two crystallographically independent molecules with Pd(1)–Pd(2) and Pd(3)–Pd(4) distances of 2.9594 (10) and 2.9401(9) Å, respectively. The monomeric amido complex 4 protonates from trace amounts of water to give the cationic ammine species [Pd(2,6-(Cy2PCH2)2C6H3)(NH3)]+. Complex 4 reacts with diphenyliodonium triflate ([Ph2I]OTf) to give aniline complex [Pd(2,6-(Cy2PCH2)2C6H3)(NH2Ph)]OTf (5). Reaction of 4 with dialkyl acetylenedicarboxylate (DMAD, DEAD) yields diastereospecific palladium(II) vinyl derivative (Z)–(Pd(Cy2PCH2)2C6H3)(CR = CR(NH2)) (R = CO2Me (6a), CO2Et (6b)). Reacting complexes 6a and 6b with p-nitrophenol produces (Pd(Cy2PCH2)2C6H3)(OC6H4–p-NO2) (8) and cis-CHR = CR(NH2), exclusively.

Electropolymerization of palladium tetraaminephthalocyanine: Characterization and supercapacitance behavior

The formation of a supramolecular electropolymerized film of a polymeric palladium phthalocyanine on a glassy carbon electrode is reported here. The film was obtained by cycling monomeric palladium tetraaminephthalocyanine in dimethyl sulfoxide in the potential range −0.5 to +1.0 V. The functionalized polymerized film on the electrode was characterized using Raman spectroscopy, X-ray photoelectron spectroscopy and electrochemical methods. A polymeric phthalocyanine film was shown to form by the oxidation of peripheral amino groups. It uniformly and completely covered the electrode surface. The modified electrode surface slightly blocked the reversible redox behavior of a redox probe owing to the thick polymeric film on its surface. The modified electrode showed a redox capacitance of 146 F g−1 at 0.27 V in 0.5 M sulfuric acid.

A palladium(II) complex: Synthesis, structure, characterization, electrochemical behavior, thermal aspects, BVS calculation and antimicrobial activity

The reaction of equimolar proportions of Na2[PdCl4] and 1-(4-methylimidazol-5-yl) phenylhydrazonopropane-2-one oxime (LH), a 1:1 Schiff-base condensate of 1-hydrazono-1-phenyl-propan-2-one oxime (1) and 4-methylimidazole-5-carboxaldehyde, in methanol gives rise to [Pd(L)(Cl)] (2) in a satisfactory yield. The title monomeric palladium(II) complex, 2, has been characterized by C, H and N microanalyses, 1H and 13C NMR, FAB-MS, FT-IR, UV–Vis spectra, molar electric conductivity measurements and room temperature magnetic susceptibility measurements. The X-ray crystal structures of 1 and 2 have been determined. The structure of 1, a precursor of the ligand (LH), shows that the methyl and phenyl groups are in an ‘anti’ disposition. Compound 1 crystallizes in the monoclinic space group P21 with a=6.9503(5), b=6.4535(3), c=10.1631(6)Å, V=455.04(5)Å3 and Z=2. The structure of 2 reveals that it is a distorted square-planar palladium(II) compound. The palladium center is in an ‘N3Cl’ coordination chromophore. Complex 2 crystallizes in the tetragonal space group I41/a with a=22.5744(3), b=22.5744(3), c=13.2967(2)Å, V=6776.05(16)Å3 and Z=16. The thermal and electrochemical aspects of 2 have been studied. Electrochemical studies in DMF show a Pd(II) to Pd(III) oxidation at 0.573V, along with a reduction of Pd(II) to Pd(I) at −0.757V versus Ag/AgCl. A Bond-Valence Sum (BVS) model calculation was performed to assign the oxidation state of the palladium center in 2. The in vitro antimicrobial activity of 2 was tested against both Gram positive and Gram negative bacteria. Complex 2 exhibits satisfactory bacteriostatic activity.

ChemInform Abstract: Enhanced Catalyst Activity and Enantioselectivity with Chirality‐Switchable Polymer Ligand PQXphos in Pd‐Catalyzed Asymmetric Silaborative Cleavage of meso‐Methylenecyclopropanes.

AbstractDue to long‐range steric effects, rate enhancements and higher enantioselectivities are observed in the palladium‐catalyzed desymmetrization of meso‐methylenecyclopropanes (I), when polymer‐based phosphine ligands are used instead of the corresponding monomeric palladium species.

Studies on AlkylNitrogen Bond Formation via Reductive Elimination from Monomeric Palladium Complexes in High Oxidation State

AbstractOxidation of a series of defined palladium(II) complexes bearing a bidentate ligand, and a methyl and an amidato substituent was carried out with the aim to gain a better understanding of the inherent requirements for CN bond‐formation from Pd complexes in high oxidation state. This work clarified the role of the individual nitrogen sources and has important implication for alkylnitrogen bond‐forming reactions catalyzed by Pd.

Unsymmetrical phthalocyanines with cyclopalladated azo functions

In this study, a synthetic procedure for unsymmetrical metallophthalocyanines of the form M[Pc(AB3)], where A and B refer to two different types of peripheral functionality, has been developed and the new compounds have been converted to monomeric and dimeric palladium complexes. Asymmetrically substituted phthalocyanines were synthesized with the well-known statistical condensation method, by using two differently substituted precursors, namely 4-(2-ethoxyethoxy)-1-2-dicyanobenzene (1) and 4-{4-[Z/E]-phenylazo]-1-naphthyl}oxy-1,2-dicyanobenzene (2). Consequently, electron-donating 2-ethoxyethoxy groups and electron-withdrawing palladium complex are present in the same structure. Cyclopalladation was performed with [Pd(PhCN)2Cl2] to yield the bis-μ-chloro-bridged dimers and subsequently, the corresponding monomers were obtained by refluxing with three equivalents of potassium acetylacetonate. The resulting products were purified by column chromatography and characterized by several chemical and spectroscopic analysis methods. All compounds have very high solubility in organic solvents due to the presence of 2-ethoxyethoxy moiety.

Palladium‐Catalyzed Intermolecular C(sp3)H Amidation

Dual capacity: A new palladium-catalyzed intermolecular sequence consisting of the CH activation and amidation of methyl groups relies on N-fluorobis(phenylsulfonyl)imide (NFSI) as both the oxidant and the nitrogen source. The reaction provides the corresponding arylamines as bissulfonimides along with HF as the only by-product. Both experimental and computational results suggest the involvement of a monomeric palladium(IV) intermediate.

Cyclopalladation of thiophene-substituted thiosemicarbazones

The thiosemicarbazones obtained from the condensation of thiosemicarbazide or 4,4′-dimethylthiosemicarbazide with 2-acetylthiophene react with K 2PdCl 4 in the presence of a base to give dark, poorly soluble materials in high yields. Treatment of these oligomeric compounds with Ph 3P, dppe or dppf gives monomeric palladium(II) phosphine complexes in which the deprotonated thiosemicarbazones coordinate to the metal in a tridentate fashion through the C3-cyclometallated thiophene ring, the imine nitrogen atom and the sulphur atom. This coordination mode was confirmed by X-ray structure analysis of several derivatives.

Butylene linked palladium N-heterocyclic carbene complexes: Synthesis and catalytic properties

New bis(NHC)–Pd complexes were synthesized and characterized by elemental analysis, 1H NMR, 13C NMR, and IR spectroscopy. The reaction of Pd(OAc) 2 and bis(benzimidazolium) salts in DMSO gave the monomeric palladium complex in which the N-heterocyclic carbene was bound to the metal centre. The crystal and molecular structure of the cis-dibromo{1,1′-di[2,3,4,5,6-pentamethylbenzyl]-3,3′-butylenedibenzimidazol-2,2′-diylidene}–palladium(II) complex was determined by single-crystal X-ray diffraction. The activity of the Pd(II) complexes in the direct arylation of benzothiazole with arylbromides was investigated. A preliminary catalytic study showed that these bis(NHC)–Pd complexes were highly active in the direct arylation of benzothiazole with arylbromides.

Suzuki–Miyaura and Hiyama reactions catalyzed by orthopalladated triarylphosphite complexes

Orthometallated, dimeric, and monomeric palladium complexes with triphenylphoshito ligands and square-planar complexes of the type PdCl2[P(OR)3]2 (where R=Ph, m-MeC6H4, o-MeC6H4, C6H3-2,4-tBu2) were applied in the Suzuki–Miyaura and the Hiyama reactions leading to the same product, 2-Mebiphenyl. The desired product was obtained in high yield in reactions performed in ethane-1,2-diol with Cs2CO3 as a base. The optimized procedure was also applied to the synthesis of other biphenyl derivatives, and in most cases the Suzuki–Miyaura procedure led to higher yields of the product.

Mechanism of the Cobalt Oxazoline Palladacycle (COP)-Catalyzed Asymmetric Synthesis of Allylic Esters

The catalytic enantioselective S(N)2' displacement of (Z)-allylic trichloroacetimidates catalyzed by the palladium(II) complex [COP-OAc](2) is a broadly useful method for the asymmetric synthesis of chiral branched allylic esters. A variety of experiments aimed at elucidating the nature of the catalytic mechanism and its rate- and enantiodetermining steps are reported. Key findings include the following: (a) the demonstration that a variety of bridged-dipalladium complexes are present and constitute resting states of the COP catalyst (however, monomeric palladium(II) complexes are likely involved in the catalytic cycle); (b) labeling experiments establishing that the reaction proceeds in an overall antarafacial fashion; (c) secondary deuterium kinetic isotope effects that suggest substantial rehybridization at both C1 and C3 in the rate-limiting step; and (d) DFT computational studies (B3-LYP/def2-TZVP) that provide evidence for bidentate substrate-bound intermediates and an anti-oxypalladation/syn-deoxypalladation pathway. These results are consistent with a novel mechanism in which chelation of the imidate nitrogen to form a cationic palladium(II) intermediate activates the alkene for attack by external carboxylate in the enantiodetermining step. Computational modeling of the transition-state structure for the acyloxy palladation step provides a model for enantioinduction.

Palladium(II) complexes of 2-, 3-, and 4-quinolinyl(diphenyl)phosphane and di-(3-quinolinyl)phenylphosphane: Synthesis, characterization, and catalytic screening

One previously known and three novel quinolinyl phosphanes were synthesized by either a reaction between a lithiated diphenylphosphane and the appropriate chloroquinoline or by a reaction between a lithiated haloquinoline and an arylchlorophosphane. The reaction of the quinolinyl phenylphosphane ligands with PdCl 2(cod) produced monomeric palladium complexes in diethyl ether and dimeric, chlorine-bridged complexes in dichloromethane. Crystal structures of the palladium complexes confirm that the quinolinyl phenylphosphanes do not form chelated structures while bonded to the metal centre. 2-Quinolinyl(diphenyl)phosphane has a tendency to form a cis-isomer while bonded to the metal centre in the mononuclear complex due to attractive interactions between two ligands. A catalytic study showed that the quinolinyl phenylphosphane ligands are moderately active in the Suzuki–Miyaura coupling of various aryl halides in air.

Experimental and Theoretical Investigations of New Dinuclear Palladium Complexes as Precatalysts for the Amination of Aryl Chlorides

A series of new palladium dinuclear species with general formula [Pd2X(mu-X)[mu-P(t)Bu2(Bph-R)]] (X = Cl, Br; Bph = biphenyl; R = H, Me, NMe2) have been prepared. The two palladium centers in these species are bridged by one of the aromatic rings of the biphenyl group present in the corresponding phosphine. The X-ray crystal structure of one of these complexes has been obtained, providing a clear picture of the bonding pattern. The stability of these dimers in solution is shown to be highly dependent on the nature of the phosphine R group and also on the bridging halide. When R = NMe2, the dimers dissociate, yielding the palladium(II) compounds PdX2[P(t)Bu2(BPh-NMe2)] (X = Cl, Br), and the X-ray crystal structure of one of them (X = Br) has shown that the biphenyl group from the phosphine interacts directly with the metal center. This interaction seems to play an important role in stabilizing the otherwise coordinatively unsaturated palladium(II) complex. In contrast, when R = H or Me, the analogous monomeric palladium(II) complexes are unstable and undergo cyclometalation to generate a palladium(II) dinuclear species in which each of the two phosphines cyclometalates with the palladium centers forming a strained four-membered ring. In addition to their unusual structures, these aryl-bridged dimers have also proven to be excellent precatalysts for the amination of aryl chlorides. To rationalize some of the experimental results, a detailed DFT computational study has been carried out and is presented herein.

Palladium(II) chloride complexation: Spectrophotometric investigation in aqueous solutions from 5 to 125°C and theoretical insight into Pd-Cl and Pd-OH 2 interactions

Monomeric palladium(II) chloro aqua complexes of the form PdCl r(H 2O) 4-r 2-r ( r = [0,4]) were studied both experimentally and theoretically to gain insight on both the stabilities and the nature of palladium-chloride interactions. The thermodynamic stabilities of these complexes were studied in aqueous solutions from 5 to 125°C with UV-vis spectrophotometry using a quartz flow-through cell. Tentative measurements up to 200°C were also carried out in pressurised titanium and gold-lined optical cells but revealed important losses in soluble palladium. The strong ligand-to-metal charge transfer bands of the palladium complexes below 350 nm were used to constrain the stepwise thermodynamic formation constants at each temperature, using results of singular value decompositions of the spectra over a broad range of palladium:chloride ratios and wavelengths. The temperature-dependent constants were used to obtain changes in enthalpy and in entropy for each reaction. The thermodynamic stabilities of PdCl(H 2O) 3 +, PdCl 2(H 2O) 2 0, and PdCl 3(H 2O) - are larger at higher temperatures, whilst the one of PdCl 4 2- is smaller. All changes in entropies are positive for the former three species, but negative for the latter, presumably due to a larger solvent reorganisation around the doubly charged PdCl 4 2- species. The room temperature thermodynamic values derived from this study are also in agreement with previously published calorimetric data. Theoretical calculations on the intramolecular distributions of electrons in the different palladium(II) chloro aqua complexes, using the methods of atoms in molecules and of the electron localisation function, showed Pd-Cl and Pd-OH 2 interactions to be of largely closed-shell/ionic nature. These interactions induce an important distortion of the outer core shell electrons of Pd, as well as stable accumulations of electrons between adjacent Pd-Cl and Pd-OH 2 bonds known as ligand opposed core charge concentrations.