Cationic Palladium Complexes Research Articles

Effect of chalcogens on CO insertion into the palladium–methyl bond of [(N^N^X)Pd(CH3)]+ (X = O, S, Se) and on CO/ethylene copolymerisation

Neutral chloromethylpalladium(II) complexes, [Pd(Cl)(CH3)(L)] (1a-5a) with ligands κ(2)-N^S-2-((3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl)-6-(phenylthiomethyl)pyridine (L1), κ(2)-N^S-2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-6-(phenylthiomethyl)pyridine (L2), κ(2)-N^Se-2-((3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl)-6-(phenylselanylmethyl)pyridine (L3), κ(2)-N^Se-2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-6-(phenylselanylmethyl)pyridine (L4), and κ(2)-N^N-2-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-6-(phenoxymethyl)pyridine (L5) have been synthesised and characterised by various spectroscopic techniques. Ligands L1-L4 exhibit Npy^S/Se bidentate coordination whereas L5 shows an Npy^Npz bidentate coordination mode in their corresponding neutral palladium complexes. Abstraction of chloride in neutral palladium complexes with NaBAr4 (BAr4 = tetrakis[3,5-bis(trifluoromethyl)-phenyl]borate) resulted in the formation of the cationic palladium complexes 1b-5b, in which L1-L4 adopt a tridentate Npz^Npy^X (X = S or Se) coordination mode in their respective cationic palladium complexes (1b-4b) whilst L5 in complex 5b adopts a Npy^Npz bidentate coordination mode and the palladium centre is stabilized by the weakly coordinating acetonitrile. Compounds 1b-5b readily undergo CO insertion into the Pd-CH3 bond to form Pd-acyl that determines their ability to catalyse CO/ethylene copolymerisation to polyketones.

電気化学的な酸化プロセスを含むカップリング反応の開発と拡張π電子系化合物合成への応用

An electrochemical method to synthesize cationic palladium complexes has been developed. Their counter anions could be changed readily by the electrolyte. The reaction could be incorporated into electro-oxidative palladium catalyzed reactions, such as Wacker-type reaction, homo-coupling of arylboronic acids, oxidative Sonogashira-type coupling, and Glaser-type coupling of terminal alkynes. These reactions exhibited a wide scope and a variety of products were obtained in high yields due to high activity of electrogenerated palladium species. We also developed site-selective sequential reactions by the integration of the electro-oxidative coupling reactions with non-electrochemical reactions. The reaction site could be controlled completely by the on/off application of electricity.

Reversible chelating in acyclic diaminocarbene palladium complex containing hydrazide fragment

Study of molar conductivity of several palladium(II) complexes has revealed that cationic acyclic diaminocarbene complexes of palladium(II) containing carbohydrazide fragment in the carbene ligand are capable of reversible cyclization to form a six-membered C,O-chelate. This process is favored by factors enhancing dissociation of the anionic ligand in the starting neutral compound. DFT simulation of cationic carbene complexes of palladium has confirmed possibility of such chelating.

Preparation of novel cationic acetylacetonate complexes of palladium with diimine ligands

Catalytic systems based on palladium and nickel complexes with α-diimine ligands are known as the Brookhart catalysts [1, 2]; they are well known in modern practice of laboratory and industrial homogeneous catalysis of oligoand polymerization of lower alkenes. The strong point of such catalysts is their stability towards polar functional groups of the monomer, allowing preparation of hyperbranched polyolefines. Significant part of palladium complexes with α-diimine ligands described in the literature are neutral (halogenide or alkyl) complexes of palladium that generally require activation with organoaluminum compounds to initiate the active cationic sites [2]. A common feature of preparations of the known cationic palladium complexes is that they are multi-stage processes. On top of that, expensive starting materials are normally used (salts of silver and thallium) as well as magnesiumortincontaining chemicals [1]. There have been no examples of preparation of cationic acetylacetonato complexes of palladium with αdiimine ligands.

Recyclable Palladium Catalysts for the Heck/Sonogashira Reaction under Microwave‐assisted Thermomorphic Conditions

AbstractThe reaction of allyl palladium(II) chloride dimer and 4,4′‐bis(RfCH2OCH2)‐2,2′‐bpy, 1a–b, in the presence of AgOTf resulted in the synthesis of cationic palladium complex, [Pd(η3‐allyl)(4,4′‐bis‐(RfCH2OCH2)‐2,2′‐bpy)](OTf), 2a–b where Rf = C9F19 (a), C10F21 (b), respectively. The reaction of [PdCl2(CH3CN)] or K2PdCl4 with 1b, gave rise to the synthesis of [PdCl2(4,4′‐bis‐(C10F21CH2OCH2)‐2,2′‐bpy)], 3b. The quantitatively determined solubility curves of 2a–b and 3b in DMF indicated dramatic increase of solubility for 2a–b and 3b from −40 to 90 °C. The catalyst‐recoverable Pd‐catalyzed Heck/Sonogashira reactions were successfully achieved in DMF with microwave‐assistance. The cationic Pd‐catalyzed Heck arylation of methyl acrylate was selected to demonstrate the feasibility of recycling 2a–b using DMF as a solvent under microwave‐assisted thermomorphic conditions. At the end of each cycle, the product mixtures were cooled, and then the catalysts were recovered by decantation. The Heck arylation catalyzed by 2b under microwave‐assisted mode exhibited good recycling results favoring the trans product. To our knowledge, this is the first example of cationic Pd‐catalyzed Heck arylation under microwave‐assisted thermomorphic conditions. Additionally, recoverable 3b‐catalyzed Sonogashira reactions were also achieved successfully in DMF. The reactions under microwave‐assistance gave much better results in yield and in efficiency than that under conventional thermal heating.

Novel cationic η3-methallyl palladium complexes bearing pyridinyl-imine ligands: Synthesis, characterization and X-ray study

Ligands (pyridin-2-ylmethylene)anilines (L1–L4) and (1-(pyridin-2-yl)ethylidene)anilines (L5–L6) were obtained by condensation reactions. These ligands react with Pd(dba)2 in the presence of methallyloxytris(dimethylamino)phosphonium hexafluorophosphate [C4H7OP(NMe2)3]+PF6− to give the corresponding monometallic cationic η3-methallylpalladium complexes C1–C6 in high yields. All new complexes C1–C6 have been characterized by CHN analyses, 1H, 13C, 31P NMR and IR spectroscopy. Solid state and electronic structures of complex C5 have been determined.

The Effects of Extending of Co-planarity in a Series of Structurally Relative Polypyridyl Palladium(II) Complexes on DNA-binding and Cytotoxicity Properties.

In depth interaction studies between calf thymus deoxyribonucleic acid (CT-DNA) and a series of four structurally relative palladium(II) complexes [Pd(en)(HB)](NO3)2 (a-d), where en is ethylenediamine and heterocyclic base (HB) is 2,2'-bipyridine (bpy, a); 1,10-phenanthroline (phen, b); dipyridoquinoxaline (dpq, c) and dipyridophenazine (dppz, d) (Figure 1), were performed. These studies have been investigated by utilizing the electronic absorption spectroscopy, fluorescence spectra and ethidium bromide (EBr) displacement and gel filtration techniques. a-d complexes cooperatively bind and denature the DNA at low concentrations. Their concentration at midpoint of transition, L1/2, follows the order a >> b > c > d. Also the g, the number of binding sites per 1000 nucleotides, follows the order a >> b ~ c > d. EBr and Scatchard experiments for a-d complexes suggest efficient intercalative binding affinity to CT-DNA giving the order: d > c > b > a. Several binding and thermodynamic parameters are also described. The biological activity of these cationic and water soluble palladium complexes were tested against chronic myelogenous leukemia cell line, K562. b, c and d complexes show cytotoxic concentration (Cc50) values much lower than cisplatin.

High performance of a palladium phosphinooxazoline catalyst in the asymmetric arylation of cyclic N-sulfonyl ketimines.

A cationic palladium complex with a chiral phosphine-oxazoline ligand (iPr-phox) showed high catalytic activity and enantioselectivity in the asymmetric addition of arylboronic acids to six-membered cyclic N-sulfonyl ketimines to give high yields of the corresponding chiral cyclic sulfamidates with 96-99.9% ee. The products have tetrasubstituted stereogenic centers with an amino group and a triaryl or alkyldiaryl group as substituents.

High Performance of a Palladium Phosphinooxazoline Catalyst in the Asymmetric Arylation of Cyclic N‐Sulfonyl Ketimines

AbstractA cationic palladium complex with a chiral phosphine‐oxazoline ligand (iPr‐phox) showed high catalytic activity and enantioselectivity in the asymmetric addition of arylboronic acids to six‐membered cyclic N‐sulfonyl ketimines to give high yields of the corresponding chiral cyclic sulfamidates with 96–99.9 % ee. The products have tetrasubstituted stereogenic centers with an amino group and a triaryl or alkyldiaryl group as substituents.

Cationic Palladium(II) Complexes of Phosphine–Sulfonamide Ligands: Synthesis, Characterization, and Catalytic Ethylene Oligomerization

A series of cationic palladium(II) complexes bearing phosphine–sulfonamide ligands, [(P,O)PdMe(lutidine)][SbF6], were synthesized and used for catalytic ethylene oligomerization. The molecular structure of the complex {[N,N-dicyclohexyl-2-(diphenylphosphanyl)benzenesulfonamide]PdMe(lutidine)}[SbF6] shows that the phosphorus atom and the oxygen atom coordinate to the palladium center. The ethylene oligomerization behavior is greatly influenced by the phosphino substituents, while the substituents on sulfonamide show only minimal effects. Complexes containing the diphenylphosphanyl group are highly selective for ethylene dimerization, affording 1-butene exclusively with moderate activity. The bulkier bis(2-methoxyphenyl)phosphanyl group leads to higher activity and gives α-olefins containing mainly 1-butene and 1-hexene, with a 1-hexene content of up to 35%. The palladium complexes bearing alkyl phosphino substituents give 1-butene and 1-hexene as the major products; a small amount of 2-butene (<5%) was observed, suggesting the occurrence of chain walking. The addition of B(C6F5)3 greatly enhances the catalytic activity. Experimental results suggest that the increase in activity is likely due to the abstraction of lutidine, not from the coordination of B(C6F5)3 to the sulfonamide oxygen atom.

New Entry to the Synthesis of α-Iminonitriles by Lewis Acid Mediated Isomerization of Cyano-Substituted Iminoisobenzofurans Prepared by Palladium-Catalyzed Three-Component Coupling of Arynes, Isocyanides, and Cyanoformates

A variety of α-iminonitriles were formed as the minor products of three-component coupling reactions of arynes, isocyanides, and cyanoformates in the presence of the cationic palladium complex [Pd(NCPh)2(dppf)](BF4)2 as the catalyst, along with cyano-substituted iminoisobenzofurans as the major products. α-Iminonitriles obtained from this process are hardly accessible by conventional methods. In addition, when the isolated iminoisobenzofurans were treated with diisobutylaluminum hydride (DIBAL-H) or AlMe3, the transformation of cyano-substituted iminoisobenzofurans into α-iminonitriles was observed.

Synthesis and Characterization of Anionic, Neutral, and Cationic PNP Pincer PdII and PtII Hydrides

The synthesis and characterization of anionic, neutral, and cationic hydride complexes of platinum and palladium are reported utilizing the PNP (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine) ligand. Comparisons by IR spectroscopy and X-ray crystallography are made across the series. Evaluation of the metal hydride stretching frequencies of the cationic through anionic complexes shows a trend of increasing M–H bond activation. The reactivity of these metal hydrides with oxygen is evaluated and compared to previously reported oxygen insertion reactions.

Electroless Plating Catalyst Performance of a Cationic Moiety Bearing Palladium Complex

Conventionally, in the electroless plating of resin, surface modification has been necessary to generate carboxylate moieties that enhances palladium catalyst adsorption. To further enhance palladium adsorption, a variety of Pd(II) amino acid complexes were synthesized, which bear a cationic pendant group with affinity for the carboxylate generated on the resin, and performance as an electroless plating catalyst evaluated. The Pd(II) complexes were characterized using UV-vis spectroscopy, revealing chelation of the amine and carboxylate groups of amino acids to Pd(II). For the Pd(II) complexes of Arginine or Lysine, affinity of the cationic pendant groups to resin surface carboxylate groups was suggested by enhanced Pd adsorption, c.a. 80 fold increase compared to complexes without cationic moieties. Furthermore, even under mild surface modification conditions with 0.5 mol/dm3 KOH, the Pd(II) complexed with Arginine or Lysine displayed performance as a electroless Ni catalyst, allowing for electolytic copper plated laminates of 1 kN/m adhesion strenghth.

A silicon-position dependent 6-endo-trig cyclization during Tsuji–Trost alkylation

Two silylated cyclohexenes products have been prepared by using a Tsuji–Trost palladium-catalyzed cyclization. It involves the generation of a cationic π-allylic palladium complex bearing a triethylsilyl group on C-3, which cyclizes via a 6-endo-trig process to afford the cyclohexene derivatives. It is also demonstrated that the position of the silyl group on the starting allylic substrate strongly influenced the reaction. It could favor either the production of the expected cyclohexenyl ring or a diene by an elimination that occurs on the silyl-substituted C-2 π-allylic palladium complex.

Water as an Activator for Palladium(II)‐Catalyzed Olefin Polymerization

By a reasonable combination of the Wacker reaction and olefin polymerization processes, water proves to be an excellent activator for the palladium(II)-catalyzed polymerization of ethylene and it provides a safe, environmental-friendly and handy initiator for olefin polymerization. The activity of the olefin polymerization is comparable to reactions catalyzed by the corresponding alkylated cationic palladium complexes.

Reactivity Study of Imino-N-Heterocyclic Carbene Palladium(II) Methyl Complexes

Neutral and cationic palladium(II) complexes containing three different heteroditopic bis(aryl)-substituted imino-N-heterocyclic carbene ligands have been synthesized and structurally characterized. Modifications made to the substituent at the iminic carbon allow for independent tuning of the steric and electronic environment around the metal center. The nature of this substituent profoundly affects the thermal stability of the neutral palladium complexes. While the tert-butyl derivative decomposes at 60 °C over a period of 24 h to give the corresponding 1-mesityl-2-methyl-1H-imidazole and the corresponding imidoyl chloride as the major products, the phenyl and methyl analogues show no sign of decomposition. Likewise, the corresponding cationic complexes of all three carbene derivatives are stable under the same conditions, with no evidence of decomposition. While inactive for ethylene polymerization, these palladium methyl complexes react with CO and isocyanides to form a variety of products, including structurally characterized simple adducts and single and multiple insertion products.

Synthesis and Structure of Imine–N-Heterocyclic Carbene Palladium Complexes and Their Catalytic Behavior in Norbornene Polymerization

On the basis of the steric effects of ligand, a series of imine–N-heterocyclic carbene (NHC) ligands and their corresponding five-membered palladium complexes with bulky substituents on both the imine and the NHC moieties were synthesized and characterized. Transpalladation of silver carbene complexes with (COD)PdCl2 and (COD)PdMeCl afforded the palladium dichloride and methylpalladium complexes, respectively. Bulky cationic palladium complexes were further obtained by treatment of the methylpalladium complexes with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (NaBAF) in CH3CN. Well-defined cationic palladium complexes were confirmed by X-ray crystal diffraction to have trans forms. Palladium dichloride complexes and methylpalladium complexes after activation with MMAO show high activity for norbornene polymerization, whereas cationic palladium complexes can polymerize norbornene alone without any cocatalysts and exhibit a high thermostability. Norbornene polymerization with the cationic palladium catalyst was proven to proceed through a coordination–insertion mechanism by NMR studies. Analysis of oligomers obtained by polymerizing the monomer in the presence of H2 reveals the existence of a C7 linkage in the polynorbornene (PNB) by σ-bond metathesis, which may be the reason for the insolublity of polynorbornenes obtained by palladium catalysts.

Synthesis, Structure, and Catalytic Studies of Palladium and Platinum Bis-Sulfoxide Complexes

The bis-sulfoxide ligand p-tolyl-binaso and its derivatives can be used to synthesize neutral and cationic palladium and platinum complexes. Various Pt bis-sulfoxide complexes could be made by using the precursor Pt{(M,SS,SS)-p-tolyl-binaso}Cl2 (5a) and abstracting the chlorides with silver salts. The Pd analogue of this compound (4) could not be used in the same way. In this case, the sulfoxides appear to be very weakly coordinated and different methods to make stable Pd bis-sulfoxide complexes had to be found. The nature of the bonding in the synthesized compounds was studied through analysis of their IR and NMR spectra and X-ray crystal structures. These studies revealed that the trans effect of other ligands in the complex is quite significant in determining the lability of the metal–sulfoxide bond. Some of the more stable platinum structures were then tested for their activity as catalysts in hydroboration and diboration reactions.

Spectroscopic, structure, and DFT studies of cationic palladium(II) complexes with imidazole derivative ligands

Two palladium(II) complexes with imidazole derivative ligands have been synthesized. The molecular structures of the complexes were determined by X-ray crystallography and their spectroscopic properties were studied. Based on the crystal structures, computational investigations were carried out to determine the electronic structures of the complexes. The electronic spectra were calculated with use of time-dependent DFT method, and the transitions were correlated with the molecular orbitals of the complexes. The emission of the complex with 1-methylimidazole was examined.

Kinetic isotope effects for oxidation reaction of olefins by p-quinones in water-acetonitrile solutions of cationic and chloride anionic palladium complexes

Kinetic isotope effects for oxidation reactions of ethylene and cyclohexene in solutions of cationic palladium(ii) complexes in MeCN-H2O(D2O) systems, were measured. It was established that the ratio of the initial reaction rates \({{R_0^{H_2 O} } \mathord{\left/ {\vphantom {{R_0^{H_2 O} } {R_0^{D_2 O} }}} \right. \kern-\nulldelimiterspace} {R_0^{D_2 O} }} \) is equal to 1 for both reactions with the use of cationic complexes of the type Pd(MeCN) x (H2O)4−x 2+, which differs from oxidation reactions catalyzed by chloride palladium complexes in the same solutions, where the ratio \({{R_0^{H_2 O} } \mathord{\left/ {\vphantom {{R_0^{H_2 O} } {R_0^{D_2 O} }}} \right. \kern-\nulldelimiterspace} {R_0^{D_2 O} }} \) = 5.0±0.16 and 4.73±0.14 at H+ molar fraction of 0.48 and 0.16, respectively (H+ molar fraction was calculated based on the sum of [H+] and [D+]).