Methylpalladium Complexes Research Articles

Comparative study of CO2 insertion into pincer supported palladium alkyl and aryl complexes.

The insertion of CO2 into metal alkyl bonds is a crucial elementary step in transition metal-catalyzed processes for CO2 utilization. Here, we synthesize pincer-supported palladium complexes of the type (tBuPBP)Pd(alkyl) (tBuPBP = B(NCH2PtBu2)2C6H4-; alkyl = CH2CH3, CH2CH2CH3, CH2C6H5, and CH2-4-OMe-C6H4) and (tBuPBP)Pd(C6H5) and compare the rates of CO2 insertion into the palladium alkyl bonds to form metal carboxylate complexes. Although, the rate constant for CO2 insertion into (tBuPBP)Pd(CH2CH3) is more than double the rate constant we previously measured for insertion into the palladium methyl complex (tBuPBP)Pd(CH3), insertion into (tBuPBP)Pd(CH2CH2CH3) occurs approximately one order of magnitude slower than (tBuPBP)Pd(CH3). CO2 insertion into the benzyl complexes (tBuPBP)Pd(CH2C6H5) and (tBuPBP)Pd(CH2-4-OMe-C6H4) is significantly slower than any of the n-alkyl complexes, and CO2 does not insert into the palladium phenyl bond of (tBuPBP)Pd(C6H5). While (tBuPBP)Pd(CH2CH3) and (tBuPBP)Pd(CH2CH2CH3) are resistant to β-hydride elimination, we were unable to synthesize complexes with n-butyl, iso-propyl, and tert-butyl ligands due to β-hydride elimination and an unusual reductive coupling, which involves the formation of new C-B bonds. This reductive process also occurred for (tBuPBP)Pd(CH2C6H5) at elevated temperature and a related process involving the formation of a new H-B bond prevented the isolation of (tBuPBP)PdH. DFT calculations provide insight into the relative rates of CO2 insertion and indicate that steric factors are critical. Overall, this work is one of the first comparative studies of the rates of CO2 insertion into different metal alkyl bonds and provides fundamental information that may be important for the development of new catalysts for CO2 utilization.

Palladium (II) complexes chelated by 1‐substituted‐4‐pyridyl‐1H‐1,2,3‐triazole ligands as catalyst precursors for selective ethylene dimerization

A series of neutral as well as cationic palladium methyl complexes bearing 1‐substituted‐4‐pyridyl‐1H‐1,2,3‐triazole ligands were prepared and fully characterized by a range of analytical techniques. Conventional and 2D NMR spectroscopy as well as single‐crystal X‐ray diffraction analysis unambiguously determined the molecular structure of the complexes. The neutral complexes activated by methylaluminoxane were found to be effective catalysts in the ethylene dimerization reaction. The catalyst performance of the in‐situ‐generated active species was compared with the discrete cationic complexes of the same ligand scaffold. Activities and selectivities for the two systems were remarkably similar, pointing to similarities in the nature of the active species. Both catalytic systems showed a strong correlation of activity and selectivity with the nature of the ligand scaffold. Highest activities were attained when electron‐withdrawing groups were incorporated into the triazole ring, while increasing steric bulk in the ortho‐position on the pyridyl ring of the ligand led to the almost exclusive dimerization of ethylene with selectivities up to 94% observed toward 1‐butene.

Air and Moisture Tolerant Synthesis of a Chelated bis(NHC) Methylpalladium(II) Complex Relevant to Alkyl Migration Processes in Catalysis

An air- and moisture-tolerant alternate synthetic pathway to the preparation of a cationic chelated bis(NHC) methylpalladium(ii) complex, [{(MesIm)2CH2}Pd(Me)(NCMe)][PF6], is described. The pathway involves the isolation of a bis(NHC) AgI complex, [{(MesIm)2CH2}2Ag2][PF6]2, via metallation of the corresponding diimidazolium salt with Ag2O followed by carbene transfer to [(COD)PdBrMe]. This new method avoids a previously reported unstable intermediate that displayed rapid decomposition at room temperature, attaining the targeted cationic methylpalladium(ii) complex in high yield. CO/ethylene copolymerisation catalysis trials are reported showing solvent dependent catalyst lifetime and copolymer yields. Preliminary ethylene insertion studies are also outlined revealing possible pathways leading towards catalyst deactivation.

Living Cyclocopolymerization through Alternating Insertion of Isocyanide and Allene via Controlling the Reactivity of the Propagation Species: Detailed Mechanistic Investigation.

Living cyclocopolymerization through the alternating insertion of an isocyanide and allene into palladium-carbon bond was developed based on the controlling the reactivity of the propagation species using bidentate ligands. We revealed that the rate of the presented cyclocopolymerization was depended on the ligands of Pd-initiator. When the palladium-methyl complexes having appropriate cis-chelating ligand, such as 1,3-bis(diphenylphosphino)propane (dppp), were used as initiator, the cyclocopolymerization of bifunctional aryl isocyanides (1) that contain both isocyano and allenyl moieties polymerized to afford poly(quinolylene-2,3-methylene)s with controlled molecular weight and narrow molecular weight distributions. The resulting polymer was characterized by 1H and 13C NMR analyses, which clearly showed that the terminal moiety of the polymer formed well-defined organopalladium complex as the resting state for the polymerization, which could undergo further polymerization; not only cyclocopolymerization with 1 but also homopolymerization of simple aryl isocyanide. In the analysis of the cyclocopolymerization mechanism, we conclusively demonstrated that the insertion reaction of isocyanide is the rate-determination step in the cyclocopolymerization, which proceeds via a five-coordinate intermediate with a geometrical change. The cis-chelating ligand controls the site interchange reaction, which dominates the reactivity of propagation species.

Synthesis of methyl palladium complexes on silica as single site catalysts activating C[sbnd]Cl bonds in heck reactions

Supported catalysts with molecularly dispersed, uniform palladium species were synthesized by the method of grafting. New specific synthesis protocols were developed in order to exclude self-reduction and agglomeration of palladium during immobilization. The successful synthesis of isolated mononuclear palladium(II) surface complexes of uniform structure was confirmed by MAS NMR, infrared spectroscopy and elemental analysis. The catalytic potential of these single-site catalysts for the Heck reaction was directly compared to well-characterized supported palladium oxide materials known and reported for their extraordinarily high activity in the conversion of aryl bromides. In contrast to those, the immobilized palladium surface complexes were able to quantitatively convert demanding non-activated aryl chlorides as substrates with conversions up to 62% and high selectivity. Although believed to be only pre-catalysts, the ligand sphere of the surface complexes directly affects their activity. The time-resolved reaction of chlorobenzene with styrene revealed that catalysts with supported amine complexes are able to constantly convert chlorobenzene during several hours of reaction time. It is proposed that the catalysts continuously provide active, dissolved Pd(0) species during the whole course of reaction.

Ethylene Oligomerization and Polymerization by Palladium(II) Methyl Complexes Supported by Phosphines Bearing a Perchlorinated 10-Vertex closo-Carborane Anion Substituent

The synthesis and ethylene reactivity of the zwitterionic Pd methyl complexes (κ2-P,Cl-PR2CB9Cl9)PdMe(THF) (7, R = iPr; 8, R = Ph) and (κ2-P,O-P(o-OMe-Ph)2CB9Cl9)PdMe(THF) (9), which contain the first phosphines appended with anionic 10-vertex perchlorinated closo-carboranes, are described. Complexes 7 and 8 oligomerize ethylene (23 °C, 2 atm) to a Schulz–Flory distribution of C4–C10 olefins with TOFs of ca. 8000 and 1800 t.o./h, respectively. 8 is ca. 4 times more active than the analogous (κ2-P,F-ortho-PPh2C6H4BF3)PdMe(L) (L = pyridine or collidine) system reported by Jordan and Piers, which produces butenes. Complex 9 reacts with ethylene to yield polyethylene wax (Mn ca. 1000, Đ ca. 1.5) that is similar to commercial Fischer–Tropsch waxes. The activities of 7–9 are independent of ethylene pressure and the presence of B(C6F5)3, suggesting that the catalyst resting state is the corresponding (PR2CB9Cl9)PdR(H2C═CH2) adduct. The molecular weights of the oligomer/polymer products are independent of ethylen...

Synthesis and Reactivity of Methylpalladium Complexes Bearing a Partially Saturated IzQO Ligand

A saturated N-heterocyclic carbene–phenolate bidentate ligand, 3,3a,4,5-tetrahydroimidazo[1,5-a]quinolin-9-olate-1-ylidene (SIzQO), was synthesized and characterized. The SIzQO ligand was then treated with PdClMe(pyridine)2 and [Pd(μ-Cl)Me(2,6-lutidine)]2, which afforded C,C-cis-(SIzQO)PdMe(pyridine) and the thermodynamically unstable trans isomer C,C-trans-(SIzQO)PdMe(2,6-lutidine), respectively. The latter isomerizes at 40 °C into the corresponding cis isomer via a dissociative mechanism. These palladium/SIzQO complexes catalyze the polymerization of ethylene at 100–120 °C, although the catalytic activity is lower than that of a previously reported palladium/imidazo[1,5-a]quinolin-9-olate-1-ylidene (IzQO) system.

Kinetics, Mechanism and Theoretical Studies of Norbornene-Ethylene Alternating Copolymerization Catalyzed by Organopalladium(II) Complexes Bearing Hemilabile α-Amino-pyridine.

Cationic methylpalladium complexes bearing hemilabile bidentate α-amino–pyridines can serve as effective precursors for catalytic alternating copolymerization of norbornene (N) and ethylene (E), under mild conditions. The norbornyl palladium complexes in the formula of {[RHNCH2(o-C6H4N)]Pd(C7H10Me)(NCMe)}(BF4) (R = iPr (2a), tBu (2b), Ph (2c), 2,6-Me2C6H3 (2d), 2,6-iPr2C6H3 (2e)) were synthesized via single insertion of norbornene into the corresponding methylpalladium complexes 1a–1e, respectively. Both square planar methyl and norbornyl palladium complexes exhibit facile equilibria of geometrical isomerization, via sterically-controlled amino decoordination–recoordination of amino–pyridine. Kinetic studies of E-insertion, N-insertion of complexes 1 and 2, and the geometric isomerization reactions have been examined by means of VT-NMR, and found in excellent agreement with the results estimated by DFT calculations. The more facile N-insertion in the cis-isomers, and ready geometric isomerization, cooperatively lead to a new mechanism that accounts for the novel catalytic formation of alternating COC.

Application of Methyl Bisphosphine‐Ligated Palladium Complexes for Low Pressure N‐11C‐Acetylation of Peptides

AbstractA mild and effective method is described for 11C‐labeling of peptides selectively at the N‐terminal nitrogen or at internal lysine positions. The presented method relies on the use of specific biphosphine palladium–methyl complexes and their high reactivity towards amino‐carbonylation of amine groups in the presence [11C]carbon monoxide. The protocol facilitates the production of native N‐11C‐acetylated peptides, without any structural modifications and has been applied to a selection of bioactive peptides.

Application of Methyl Bisphosphine-Ligated Palladium Complexes for Low Pressure N-11 C-Acetylation of Peptides.

A mild and effective method is described for 11 C-labeling of peptides selectively at the N-terminal nitrogen or at internal lysine positions. The presented method relies on the use of specific biphosphine palladium-methyl complexes and their high reactivity towards amino-carbonylation of amine groups in the presence [11 C]carbon monoxide. The protocol facilitates the production of native N-11 C-acetylated peptides, without any structural modifications and has been applied to a selection of bioactive peptides.

Methylpalladium complexes with pyrimidine-functionalized N-heterocyclic carbene ligands.

A series of methylpalladium(II) complexes with pyrimidine-NHC ligands carrying different aryl- and alkyl substituents R ([((pym)^(NHC-R))PdII(CH3)X] with X = Cl, CF3COO, CH3) has been prepared by transmetalation reactions from the corresponding silver complexes and chloro(methyl)(cyclooctadiene)palladium(II). The dimethyl(1-(2-pyrimidyl)-3-(2,6-diisopropylphenyl)imidazolin-2-ylidene)palladium(II) complex was synthesized via the free carbene route. All complexes were fully characterized by standard methods and in three cases also by a solid state structure.

Synthesis, structures, and norbornene polymerization behavior of palladium methyl complexes bearing N-heterocyclic carbene-sulfonate ligands

A series of N-heterocyclic carbene-sulfonate (NHC-sulfonate) palladium methyl complexes 2a−d were synthesized from the reaction of NHC-sulfonate ligands with Ag2O and Pd2(μ-Cl)2Me2(2,6-lutidine)2 in good yields. All these complexes were fully characterized by 1H and 13C NMR, elemental analysis, and high-resolution mass spectrometry (HRMS). The molecular structure of the representative complex 2a was also determined by single-crystal X-ray diffraction analysis. Upon activation with either methylaluminoxane (MAO) or [Ph3C]+[B(C6F5)4]−, all the palladium methyl complexes showed excellent catalytic activities [up to 108 g of polynorbornene (PNB) (mol of Pd)−1 h−1] in the vinyl polymerization of norbornene.

Synthesis and Reactivity of Neutral Palladium(II) Alkyl Complexes that Contain Phosphinimine-Arenesulfonate Ligands

The synthesis and characterization of ortho-phosphiniminium-arenesulfonate zwitterions 1a–1d, the corresponding sodium salts 3a–3d, methyl(pyridine)palladium(II) complex 4a, and methyl(4-tert-butylpyridine)palladium(II) 5a–5d complexes are reported. Complex 4a suffers from limited solubility in aromatic hydrocarbons, while 5a–5d are soluble in benzene and toluene. Complex 5a forms a tridentate orthopalladated species 6a upon heating to 80 °C in toluene. Introduction of alkyl substituents on the phosphinimine phosphorus atom retards orthopalladation, and the (nBu)3P functionalized complex 5d does not undergo orthopalladation after heating at 80 °C for 24 h. The Pd–C bond in the orthopalladated complex 6a is inert toward ethylene insertion. Complexes 5a–5d do not react with ethylene at 80 °C and decompose to Pd0 and zwitterions 1a–1d.

Living Vinyl Addition Polymerization of Substituted Norbornenes by a t-Bu3P-Ligated Methylpalladium Complex

The vinyl addition polymerization of substituted norbornene (NB) monomers, via (t-Bu3P)PdMeCl activated by [Li(OEt2)2.5]B(C6F5)4, is investigated. NB monomers bearing alkyl, aryl, fluoroaryl, and even hexafluoroisopropanol substituents yield polymers exhibiting monomodal and narrow molecular weight distributions, with molecular weight controlled by reaction time and monomer to initiator ratio, demonstrating the living nature of these polymerizations. These polymers are soluble in common organic solvents and possess excellent thermal stability. Block copolymers are also prepared via sequential monomer addition; these are the first examples of well-defined block copolymers of substituted NB monomers enchained by vinyl addition polymerization.

Differentiation between Chelate Ring Inversion and Aryl Rotation in a CF3-Substituted Phosphine-Sulfonate Palladium Methyl Complex

The solution conformations and dynamic properties of the CF3-sbustituted (ortho-phosphinoarenesulfonate)Pd complexes (PO-CF3)PdMe(L) ([PO-CF3]− = 2-{(o-CF3-Ph)2P}-4-Me-benzenesulfonate, L = 2,6-lutidine (3), pyridine (4)) were studied by NMR spectroscopy, taking particular advantage of 31P–19F through-space couplings and 1H–1H and 1H–19F nuclear Overhauser effects. In CD2Cl2 solution in the temperature range of −80 to 20 °C, 3 adopts an exo2 conformation. One o-CF3-Ph ring is positioned such that the CF3 group points toward Pd (exo) and exhibits through-space 4JPF coupling. The other o-CF3-Ph ring is positioned such that the CF3 group points away from Pd (endo) and does not exhibit through-space 4JPF coupling, and the o-H lies in the deshielding region near an axial site of the Pd square plane and exhibits a low-field chemical shift (δ > 9). Complex 4 exists as a 2:1 mixture of exo2 and exo3 isomers in CD2Cl2 solution at −90 °C. In exo2-4, one CF3 group is exo and exhibits through-space 4JPF coupling, while the other CF3 group is endo and does not exhibit through-space 4JPF coupling. In exo3-4, both CF3 groups are exo and exhibit through-space 4JPF couplings. Complex 4 undergoes two dynamic processes: rotation of the axial o-CF3-Ph ring (AaR), which interconverts exo2-4 and exo3-4 (ΔG⧧ = 9.9(5) kcal/mol), and chelate ring inversion (RI), which permutes the axial and equatorial o-CF3-Ph rings (ΔG⧧ = 21(1) kcal/mol).

Reactivity of Bis(pyridyl)-N-alkylaminato Methylpalladium Complexes toward Ethylene: Insights from Experiment and Theory

A series of novel neutral and cationic methylpalladium complexes bearing N-alkyl-2,2′-dipyridylaldiminato ligands were prepared and characterized. In the presence of ethylene, the cationic complexes were active as dimerization catalysts, producing a mixture of 1- and 2-butenes. A Pd–ethyl π-ethylene species was identified as the catalyst resting state by low-temperature spectroscopic and DFT studies, which provided insights into the effect of both steric and electronic factors on the observed reactivity.

Interaction of alkynes with palladium POCOP-pincer hydride complexes and its unexpected relation to palladium-catalyzed hydrogenation of alkynes

Palladium POCOP-pincer hydride complexes [2,6-(R2PO)2C6H3]PdH (R = tBu, 2a; R = iPr, 2b; R = cPe, 2c, cPe = cyclopentyl) have been synthesized from [2,6-(R2PO)2C6H3]PdCl (1a–c) and LiAlH4 or LiBEt3H. These hydride complexes react with phenylacetylene to afford H2, [2,6-(R2PO)2C6H3]PdCCPh (3a–c) and a small amount of styrene. When the R groups are isopropyl groups, a second palladium species is generated, and it has been identified as an alkenyl complex (E)-[2,6-(iPr2PO)2C6H3]PdCHCHPh (4b). Mechanistic studies have shown that decomposition of these palladium pincer complexes and related palladium methyl complexes [2,6-(R2PO)2C6H3]PdCH3 (5a–c) occurs at room temperature in the presence of H2 (1 atm or lower), resulting in the leaching of palladium particles. These particles have been shown to catalyze the hydrogenation of phenylacetylene and diphenylacetylene to their alkene and alkane products. A mechanism for the formation of palladium particles has been proposed. The structures of 1a, 1c, 2a, 2c, 3a, 4b and 5b have been studied by X-ray crystallography.

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.

On the Configurational Stability of Chiral Heteroatom-Substituted [D1]Methylpalladium Complexes as Intermediates of Stille and Suzuki-Miyaura Cross-Coupling Reactions.

Enantiomerically pure (S)-tributylstannyl[D1]methanol and (R)- and (S)-tributylstannyl[D1]methyl benzoates were Stille-coupled with bromobenzene and benzoyl chloride in 1,4-dioxane and toluene using [(Ph3P)4Pd] or [(Ph3P)2PdCl2] either alone or in combination with CuCN as cocatalyst at temperatures up to 80 °C. The products were found to be enantiomerically pure. (R)- and (S)-N-(tributylstannyl[D1]methyl)phthalimides gave enantiomerically pure products with benzoyl chloride, but with bromobenzene protected phenyl[D1]methylamines gave products of only 52–69 % ee depending on the solvent used. Tributyl(thio[D1]methyl)stannanes could not be Stille-coupled with benzoyl chloride or with bromobenzene. Similarly, dimethyl phenyl[D1]methylboronate underwent a Suzuki–Miyaura coupling with bromobenzene to give phenyl[D1]methylsilane with 99 % ee. All couplings followed a retentive course and, except in one case, the intermediate [XCHDPdLn] complexes were found to be microscopically configurationally stable.