Palladium Metal Center Research Articles

Study of an interesting interplay between thiourea and 4,6-dimethylpyrimidine-2-thiol with palladium(II) phosphine substrate

Treatment of [PdCl2(dppe)] with one equivalent of thiourea yielded a partially substituted complex [PdCl(dppe){SC(NH2)2}]Cl (1a) whereas the compound [Pd(dppe){SC(NH2)2}2]2+ (1b) was obtained on performing the same reaction with two equivalents of thiourea. The reaction between [PdCl2(dppe)] and two equivalents of the sodium salt of 4,6-dimethyl pyrimidine-2-thiol resulted a cis configured complex [Pd{2-SC4H(4,6-Me)2N2}2(dppe)] (2). In the same pattern the reaction of [PdCl2(dppe)] with 4,6-dimethylpyrimidine-2-thiol, triethyl amine and thiourea in 1:1:1 mol ratio afforded a mononuclear heteroleptic complex [Pd{2-SC4HN2(4,6-Me)2}(dppe)]{SC(NH2)2}]Cl (3). All the reported complexes were characterized by elemental analyses, NMR (1H, 13C{1H},31P{1H}), IR, UV-vis spectroscopy. The molecular structure of complex [Pd{2-SC4HN2(4,6-Me)2}(dppe)]{SC(NH2)2}]Cl (3) was established through single crystal X-ray diffraction and showed the distorted square planar geometry around the palladium metal centre.

Chemistry of azo-imine based palladium complexes: a brief review

Abstract This paper provides a general overview of palladium complexes incorporating numerous multidentate azo-imine ligands, their structural diversity, synthetic protocol, interesting properties such as redox properties, biological activity and their further application on C – N coupling reactions, C – C cross-coupling reactions (e.g., Suzuki and Heck) and catalytic activity on different organic transformations. A detailed literature survey revealed that no brief review has yet been published on the azo-azomethine-based palladium complex. Therefore, the present review incorporates the research undertaken on the synthetic strategies and coordinating behaviour of various azo-imine ligands with palladium metal centre and a few important properties of metal complexes.

Novel palladium(II) complex derived from mixed ligands of dithizone and triphenylphosphine synthesis, characterization, crystal structure, and DFT study

ABSTRACT. A novel distorted square-planar palladium(II) complex of the type [Pd(Hdz)(PPh3)Cl], where (Hdz = dithizone mono deprotonate and PPh3 = triphenylphosphine), was synthesized in dichloromethane reactions between PdCl2 and a mixture of Hdz and PPh3. The new Pd(II) complex has been identified by FT-IR, electronic spectra, DFT calculations, molar conductivity, and single-crystal X-ray diffraction. An X-ray diffraction study revealed the structure of this complex, indicating distorted square planar coordination geometry around the Pd(II) ion by N, S, P, and Cl donor atoms. XRD analysis has also shown that the Pd(II) complex contains one five-membered ring formed by the coordination of the Hdz ligand through the nitrogen and sulfur atoms to the palladium metal center. To comprehend the strength of nucleophilic and electrophilic attack between the ligands and metal ions, the natural bond orbital (NBO) was used. Finally, density functional theory (DFT) was used to show the molecular reactivity and stability of the ligands and palladium complex.
 
 KEY WORDS: Palladium(II), Dithizone mono deprotonated, Distorted square planar geometry, NBO analysis, DFT calculations
 Bull. Chem. Soc. Ethiop. 2022, 36(3), 617-626. \
 DOI: https://dx.doi.org/10.4314/bcse.v36i3.11

Selective Formation of Mononuclear Palladium and Acetonitrile-Bridged Dinuclear Palladium Complexes Containing a Chiral Tridentate Ligand.

From the chiral Schiff base tridentate ligand LPh, unusual acetonitrile-bridged dinuclear palladium complex 1 and organopalladium complex 1' were synthesized selectively by using acetonitrile and ethanol, respectively. The chiral tridentate Schiff base ligand was bound to the palladium metal center with different chelation modes ([ONO] for 1 and [CNO] for 1'). Complex 1 constitutes the first example of dinuclear metal complexes connected only by a bridging acetonitrile, in which an exceptionally short C≡N bond distance [0.945(12) Å] of bridged acetonitrile was observed. To study the influence of a phenyl group attached to an imine, the phenyl-free ligand LH was prepared and used. In that case, an acetonitrile bridge was not observed. Theoretical calculation studies supporting the formation of 1 and 1' are favored.

A new and unexpected coordination mode of a bis-phosphine monoxide (BPMO) ligand in a palladacycle complex

A new bis-phosphine monoxide ligand of the formula [P(O)Ph2CH2PPh2C(H)C(O)C6H4-p-NO2] (1) was synthesized and its coordination chemistry explored. The reaction of this ligand with the dinuclear complex [Pd(bzq)(μ-Cl)]2 yields a mononuclear complex of formula [Pd(bzq)(P(O)Ph2C(H)PPh2C(H)C(O)C6H4-p-NO2)] (2) by means of bridge splitting reactions. The synthesized compounds have been characterized by elemental analysis, spectroscopic and crystallographic methods. A novel coordination mode for phosphine monoxide ligand (1) was found in cyclopalladated benzo[h]quinolinate complex, the ligand is coordinating to the palladium metal center through the carbon atom of methine group lying between the two phosphorus atoms and the oxygen atom of carbonyl group, acting as a chelating C,O-donor ligand. The 31P{1H} NMR spectrum of (1) contains two doublets centered at 11.45 and 29.54 ppm, due to the phosphonium and phosphine oxide groups, respectively, while in (2) the corresponding peaks are centered at 27.01 and 42.58 ppm, resulting in a 14 ppm downfield shift on complexation of the ligand to the palladium center. Overall, these types of ligands display interesting reactivity due to their diverse coordination modes and ability to attract considerable attention as an active research field.

Well-Defined Palladium N-Heterocyclic Carbene Complexes: Direct C-H Bond Arylation of Heteroarenes.

A series of palladium N-heterocyclic carbene (NHC) complexes of type trans-{(NHC)PdCl2L} (L = C5H5N, 3-ClC5H4N, and PPh3) (3-5) have been developed as efficient precatalysts for direct C-H bond arylation of various heteroarenes. In particular, an in situ generated new NHC ligand derived from {1,3-di-(2,6-diethylphenyl)acenaphtho[1,2-d] imidazolium} chloride (2) is used for the stabilization of the palladium metal center. Among the screened palladium precatalysts (3-5), the most active PEPPSI themed complex (3) was successfully employed toward direct C-H bond arylation of various heteroarenes and aryl bromides. A range of functional groups on aryl bromides as well as on heteroarenes sustained throughout the standard reaction conditions for easy access of various arylated heterocyclic compounds. Significantly, the utility of the protocol was demonstrated by the effective synthesis of a precursor of raloxifene, a selective estrogen receptor modulator.

Synthesis of palladium tellurolate complexes derived from hemi-labile tellurolate ligands and studies their reactivity as gas sensing materials

The reaction of [PdCl2(dppf)] with the sodium salt of pyridyl/pyrimidyl tellurolate resulted a mononuclear cis configured complex [Pd(TeAr)2(dppf)] (Ar = C5H4N, C4H3N2). These complexes were characterized by NMR (1H, 31P) spectroscopy and elemental analysis. In addition, the thin film of this complex was fabricated on the glass using solution process and it was found to be sensitive towards detection of H2S gas. The chemi-resistive response of the film was ascribed to the fact that an auxiliary tellurolate ligand attached to palladium metal centre substituted reversibly by thiolate (HS−) species of hydrogen sulphide gas.

Palladium-catalyzed phosphination and amination through C[sbnd]H bond functionalization on biphenyl: Amido-substituent as directing group

The formation of N-(2'-(diphenylphosphoryl)-[1,1′-biphenyl]-2-yl)-2,3,4,5,6-pentafluorobenzamide (3_eaaa) has been achieved through the palladium-catalyzed and Ag(I)-assisted CH functionalization of N-([1,1′-biphenyl]-2-yl)-2,3,4,5,6-pentafluorobenzamide (1_eaa) via phosphination and in the presence of diphenylphosphine oxide (2_a). The reaction was accompanied with small amount of carbazole derivative, (9H-carbazol-9-yl)(perfluorophenyl)methanone (4_eaa). Crystal structures of 3_eaaa and 4_eaa were both determined by X-ray crystal diffraction methods. Indeed, phosphination took place at the ortho-position of inter-annular ring of 1_eaa. Substituents with various electron-withdrawing/donating capacities on the amido-fragment of 1 were found to greatly affect the ratio distribution of products 3 and 4. By contrast, substituents on various locations of the biphenyl in 1 do not influence much on the outcome of the products distribution. A reaction mechanism is proposed to account for these experimental observations. Computational studies employing Density Functional Theory methods (DFT) on this proposed mechanism found that the electron density property of the palladium metal center in the intermediate II is critical to the formation of either 3 or 4. A relatively electron-poor palladium metal center favors the pathway of forming 3. On the contrary, the pathway of producing 4 will be prevailed while the palladium metal center is having fair amount of electron-density in the proposed intermediate II.

Phosphine‐Free Bis(Pyrrolyl)pyridine Based NNN‐Pincer Palladium(II) Complexes as Efficient Catalysts for Suzuki‐Miyaura Cross‐Coupling Reactions of Aryl Bromides in Aqueous Medium

AbstractPalladium complexes of phosphine free pincer based ligands are currently growing credence as efficient catalysts for C–C cross‐coupling reaction under environmentally benign and mild reaction conditions. A series of new palladium complexes 2 a–2 c supported by 2,6‐bis(pyrrolyl)pyridine based [NNN]‐pincer ligands have been synthesized, and their catalytic activities for Suzuki‐Miyaura cross‐coupling reaction were examined. The palladium complexes (2 a–2 c) were synthesized by the reaction of 2,6‐bis(pyrrolyl)pyridine ligands (LH2) with Pd(OAc)2 in acetonitrile as a solvent. Electrochemical studies of complexes 2 a–2 c showed two prominent oxidation peaks, which were ascribed to the dianionic pyrrolide ligand and an oxidation peak that can be ascribed to palladium metal center. There is an exhibition of well‐coordinated metal directed oxidation of the complexes which indicates a well‐established electronic network. Computational studies on complexes 2 a–2 c further supported the experimental observations. These palladium complexes 2 a–2 c can efficiently catalyze Suzuki‐Miyaura cross‐coupling reaction of aryl bromides in an aqueous medium.

Palladium(II) Picoline Thiourea Complex as Homogenous Catalyst in Heck Cross-Coupling Reaction in the Formation of C=C Bond

Palladium (II) complex containing picoline thiourea (Pd-PTu) was prepared by reacting bis (benzonitrile) palladium (II) chloride (Pd-PhCN) withN’-2-(5-picolyl)-N-4-bromobenzoylthiourea (PTu). All synthesized compounds have been characterized via several selected typical spectroscopic and analytical techniques namely elemental analysis, Infrared (IR) spectroscopy, Ultraviolet-visible (UV-Vis) spectroscopy, Nuclear Magnetic Resonance (1H and13C NMR) and gas chromatography-flame ionization detector (GC-FID). For Pd-PTu, IR spectrum showed significant shift in the ν (C=N) on the pyridine ring and ν (C=S) stretching vibration at around 30cm-1compared to the free ligand. The finding indicated that the expected coordination of ligand to the palladium metal centre took place via N and S atoms. The catalytic reaction was monitored using GC-FID with conversion of 83.81 %, proved that Pd-PTu can be used as an ideal homogenous catalyst in any Heck cross-coupling reactions in the near future.

Cyclopalladation of azobenzenes and their reactivity towards N-substituted imidazolidine-2-thiones and allied ligands: Synthesis, structures, spectroscopy and ESI-mass studies

Reactions of palladium(II) chloride with azobenzene (azbH), p-methoxyazobenzene (mazbH) and 4, 4′-diethoxyazobenzene (deazbH) in methanol yielded halogen bridged dinuclear precursors [Pd2(κ2:C,N-L)2(μ-Cl)2] (here L = azb−, mazb− and deazb−). These precursors were further reacted with a series of N, S- donor thio-ligands, namely, 1,3-imidazolidine-2-thiones (imdzSH-NR; R = H, Me, Et, Prn, Bun, Ph), N-methyl-imidazoline-2-thione (imzSH-NMe), 1,3-benzimidazoline-2-thione (bzimSH-NH) and 1, 3- thiazolidine-2-thione (tzdSH). Nineteen dinuclear organometallic compounds synthesized are listed as follows: (a) [Pd2(κ2:C, N-azb)2(μ-N, S-imdzS-NR)2] (R = H, 1; Me, 2; Et 3; Prn4; Bun5; Ph 6), (b) [Pd2(κ2:C, N-mazb)2(N, S-imdzS-NR)2] (R = Me, 7; Et, 8; Prn9; Bun10), (c) [Pd2(κ2:C, N-deazb)2(N, S-imdzS-NR)2] (R = Et, 11; Prn12; Bun13; Ph 14), (d) [Pd2(κ2:C, N-azb)2(μ-N, S-L)2] (L = bzimS-NH 15; imzS-NMe 16; tzdS 17), and (e) [Pd2(κ2:C, N-deazb)2(N, S-L)2] (L = bzimS-NH 18; imzS-NMe 19). All these compounds have been characterized with the help of analytical data, electron absorption, NMR and fluoresecence spectroscopy, ESI-mass spectrometry and single crystal X-ray crystallography. The loss of one C-H proton of an azobenzene and one N-H proton of a thio-ligand occurred per palladium metal center in dinuclear complexes synthesized, which have C, N-chelating azobenzenes and N, S-bridging heterocyclic thiolates.

Synthesis of N,N′,X‐tridentate 2‐iminomethylpyrrole‐coordinated palladium(II) complexes via N─H bond activation of pyrrole moiety

The reaction of [Pd(CH3CN)2Cl2] with N‐functional group‐substituted 2‐iminomethylpyrrole‐based ligands, namely N1‐((1H‐pyrrol‐2‐yl)methylene)‐N3,N3‐dimethylpropane‐1,3‐diamine (LA), N1‐((1H‐pyrrol‐2‐yl)methylene)‐N3‐methyl‐N3‐phenylpropane‐1,3‐diamine (LB), N‐((1H‐pyrrol‐2‐yl)methylene)‐3‐(methylthio)propan‐1‐amine (LC) and N‐((1H‐pyrrol‐2‐yl)methylene)‐3‐methoxypropan‐1‐amine (LD), resulted in [LnPdCl] (Ln = LA–LD) complexes in high yield via N─H bond activation of pyrrole moiety without use of base. [LnPdCl] existed as monomeric four‐coordinated complexes with slightly distorted square planar geometries around the palladium metal center. The ligands show N,N′,X‐tridentate binding mode to the palladium metal center to give two fused ring metallacycles. [LBPdCl] gave the highest activity (3.29 × 105 g PMMA (mol Pd)−1 h−1) for a methyl methacrylate (MMA) polymerization in the presence of modified methylaluminoxane at 60 °C compared to the other Pd(II) analogues, and resulted in PMMA with higher molecular weight (Mw = 7.16 × 105 g mol−1) and narrower polydispersity index. Syndiotactic‐enriched PMMA resulted in all cases.

Geometry Constrained N-(5,6,7-Trihydroquinolin-8-ylidene)arylaminopalladium Dichloride Complexes: Catalytic Behavior toward Methyl Acrylate (MA), Methyl Acrylate-co-Norbornene (MA-co-NB) Polymerization and Heck Coupling.

A new pair of plladium complexes (Pd4 and Pd5) ligated with constrained N-(5,6,7-trihydroquinolin-8-ylidene)arylamine ligands have been prepared and well characterized by 1H-, 13C-NMR and FTIR spectroscopies as well as elemental analysis. The molecular structure of Pd4 and Pd5 in solid state have also been determined by X-ray diffraction, showing slightly distorted square planar geometry around the palladium metal center. All complexes Pd1–Pd5 are revealed highly efficient catalyst in methyl acrylate (MA) polymerization as well as methyl acrylate/norbornene (MA/NB) copolymerization. In the case of MA polymerization, as high as 98.4% conversion with high molecular weight up to 6282 kg·mol−1 was achieved. Likewise, Pd3 complex has good capability to incorporate about 18% NB content into MA polymer chains. Furthermore, low catalyst loadings (0.002 mol %) of Pd4 or Pd5 are able to efficiently mediate the coupling of haloarenes with styrene affording up to 98% conversion.

Synthesis and Structural Characterisation of Palladium(II) Complexes with N,N′,N-Tridentate N′-Substituted N,N-Di(2-picolyl)amines and their Application to Methyl Methacrylate Polymerisation

The reaction of [Pd(CH3CN)2Cl2] with N′-substituted N,N-di(2-picolyl)amine-based ancillary ligands, for example N,N-di(2-picolyl)cyclohexylmethylamine (L1), N,N-di(2-picolyl)benzylamine (L2), N,N-di(2-picolyl)aniline (L3), and 1,4-bis[bis(2-pyridylmethyl)aminomethyl]benzene (L4), in the presence of NaClO4 in ethanol yields a new series of [(NN′N)PdCl]X (X = ClO4, Cl) complexes, i.e. mononuclear [LnPdCl]ClO4 (Ln = L1, L2, L3) and binuclear [L4Pd2Cl2]Cl2. X-Ray crystallographic analysis determined that the Pd atom in complexes [(NN′N)PdCl]X showed a slightly distorted square-planar geometry involving three nitrogen atoms and a chlorido ligand. Moreover, the unit cell included a ClO4– or Cl– anion as the counterion. The complex [L1PdCl]ClO4 showed the highest catalytic activity for the polymerisation of methyl methacrylate in the presence of modified methylaluminoxane at 60°C among the mononuclear PdII complexes. Specifically, the activity of binuclear [L4Pd2Cl2]Cl2 was 2-fold higher than the corresponding mononuclear [L2PdCl]ClO4 per active palladium metal centre.

Conducting Metallopolymers as Precursors to Fabricate Palladium Nanoparticle/Polymer Hybrids for Oxygen Reduction

The novel conducting metallopolymer [N,N'-((2,2'-dimethyl)propyl)bis(5-(2,2'-bithiophene-5-yl)salcylideniminato-palladium(II)](n) prepared by electropolymerization provides a polymer matrix in which the palladium metal centers are evenly distributed. The metal centers embedded directly in the conducting metallopolymer backbone serve as seed points for size-controlled palladium nanoparticle (NP) growth of 3.8 to 4.9 nm NPs within the conducting metallopolymer film. The palladium NP/conducting metallopolymer hybrid material has demonstrated electrocatalytic behavior toward oxygen reduction with peak current densities around 400 μA/cm(2) in acidic aqueous conditions. These results demonstrate a promising new method for the production of electrocatalytically active hybrid materials.

Synthesis, Coordination Properties and Application of New N,N‐Ligands Based on Bornyl and Binaphthylazepine Chiral Backbones in Palladium‐Catalyzed Allylic Substitution Reactions

AbstractThe synthesis of new imine‐amine and diamine ligands, based on both the atropisomeric (Sa)‐ or (Ra)‐1,1′‐binaphthyl and (R)‐bornyl backbones, and incorporating an ethylenediamino spacer, are reported. In addition, analogue ligands in which one chiral arm is replaced by the achiral NMe2 group were synthesized. These N,N‐ligands when coordinated to a palladium metal centre form highly enantioselective catalysts for the asymmetric allylic alkylation of rac‐2‐acetoxy‐1,3‐diphenylpropene by dimethyl malonate. In one case, the synergic effect of the chirality elements in the palladium catalyst afforded the (S) substitution product with an enantiomeric excess (ee) of >99 %. Based on NMR studies of the active species in solution, a reliable explanation for the origin of the enantioselectivity of these palladium catalysts is also provided.

Synthesis and structural characterization of Pd(II) complexes containing 2,6-bis[(dimethylamino)methyl]-4-methylphenolate ligand

Treatment of 2,6-bis[(dimethylamino)methyl]-4-methylphenol ( 1) with [Pd(PhCN) 2Cl 2] in a 1:1 molar ratio gives the mononuclear Pd(II) complex [PdCl 2(OC 6H 2(CH 2NMe 2)-2-Me-4-(CH 2NHMe 2)-6)] ( 2) containing one ligand with an ammonium hydrogen atom, which forms a bifurcated hydrogen bonding to the phenoxy oxygen and the chlorine atoms, as shown by the single crystal X-ray diffraction study. The reaction between the lithium salt of 1 and [Pd(COD)Cl 2] gives the mononuclear Pd(II) complex [Pd(OC 6H 2(CH 2NMe 2) 2-2,6-Me-4) 2] ( 3). The X-ray structure of 3 showed the presence of two ligands coordinated to one palladium metal center in a trans fashion with two dangling dimethylamine groups. The yield of the complex 3 was improved by carrying out the reaction between [Pd(OAc) 2] and 1 in acetone. The solid state structures of the complexes 2 and 3 were confirmed by 1H, 13C, HETCOR NMR, IR and elemental analysis methods. The 1H NMR spectra of 2 and 3 showed two different chemical shifts corresponding to the coordinated and uncoordinated amine groups of the ligand. No decoalescence of signals for the chelate ring puckering process was observed in variable-temperature NMR spectra.

Molecular Structure of an Unexpected Binuclear Salicylaldimine Semicarbazone Palladium(II) Complex

Reaction of a salicylaldehyde semicarbazone ligand with PdCl2(PPh3)2 yields an unexpected binuclear Pd(II) complex, 2. The complex crystallizes in a triclinic system with a P-1 space group, with unit cell parameters a = 14.0283(3) A, b = 15.2603(3) A, c = 18.7142(3) A, α = 66.662(10)°, β = 86.162(10)°, γ = 76.265(10)°, Z = 2. The semicarbazone ligand acts as a tridentate chelating donor to one palladium metal centre, forming a five- and six- membered ring, and a monodentate donor to the second palladium centre. There are four molecules of solvent per complex molecule in the asymmetric unit. Each metal centre is contained within a slightly distorted square-planar environment. The reaction of a salicylaldehyde–semicarbazone with PdCl2(PPh3)2 yields a binuclear Pd(II) complex (2) where the semicarbazone acts as a tridentate donor to one metal centre and as a monodentate donor to the second metal centre.

Ethylenediamine and 1,10-phenanthroline biscyclometallated complexes of Pd(II) and Pt(II) based on 4,6-diphenylpyrimidine

Biscyclometallated [(M(N∧N))2(μ-dphpm)](ClO4)2 and [(N∧N)Pd(μ-dphpm)Pt(N∧N)]Cl2 complexes [M = Pd(II), Pt(II); (N∧N) ethylenediamine (En), 1,10-phenanthroline (phen); dphpm2 — bisdeprotonated form of 4,6-diphenylpyrimidine)] have been characterized by the 1H NMR, electronic absorption and emission spectroscopy, and also cyclic voltammetry methods. The lowest unoccupied molecular orbital (LUMO) of biscyclometallated complexes with ethylenediamine, responsible for low-energy photo- and electro-stimulated processes irrespective of the metal nature, is assigned to the π* orbital mainly localized on the pyrimidine part of the bridging ligand. In the case of complexes with phenanthroline chelating ligands, the replacement of one or two palladium metal centers [{Pd(phen)}2(μ-dphpm)]2+ by platinum centers changes the LUMO nature of the complexes for the π* orbital mainly localized on the peripheral metal-complex fragment {Pt(phen)}.

Palladium complexes based on tridentate pyrazolyl-ligands: Synthesis, structures and use in Suzuki cross-coupling reactions

The reaction of [PdCl(Me)(cod)] (cod = 1,5-cyclooctadiene) with tridentate bis(pyrazolyl) ligands affords the alkyl-palladium(II) complexes [Pd(CH 3)(NZN)][BF 4] ( 1, NZN = 1,1′-(2,2′-oxybis(ethane-2,1-diyl)bis(3,5-dimethyl-1H-pyrazole); 2, NZN = bis[2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyl]sulfane) in good yield. Compounds 1– 2 were characterized by elemental analysis, multinuclear NMR spectroscopy, and X-ray structural analysis. Single crystal X-ray structural analyses indicate that the complexes are monomeric and the palladium center resides in a slightly distorted square planar environment. Furthermore, despite their intrinsic similarity, the NON and NSN ligands adopt different coordination modes around the palladium metal center. Additionally, the [PdCl 2(NON)] ( 3) was shown to be an efficient catalyst precursor for the coupling of aryl bromides and iodides with arylboronic acids, esters and borate salts.