Ethylene Insertion Research Articles

Synthesis of Linear α-Olefin Distributions with Flexible Mean Molecular Weight by a Ti-Al-Ni Catalyst System

The catalytic synthesis of linear α-olefins from ethylene is a key reaction in the chemical industry and one of the most important applications of homogeneous catalysis. Thus, novel protocols or concepts for the synthesis of linear α-olefins are highly desirable. Herein, we describe a trimetallic catalyst system (Ti-Al-Ni) consisting completely of earth-abundant metals. The titanium catalyst mediates coordinative chain transfer polymerization to the chain transfer agent triethylaluminum with multiple ethylene insertions prior to chain transfer. This Ti catalyst is combined with a simultaneously operating nickel catalyst. The nickel catalyst also undergoes chain transfer, displaces the grown alkyl chains via β-hydride elimination/transfer forming linear α-olefins, and recycles the chain transfer agent. The catalyst combination permits the highly efficient synthesis of various linear α-olefin distributions with flexible mean molecular weight with one catalyst system. Activities of more than 10000 kgeth mol–...

Sterically crowded dimeric diisobutylaluminum aryloxides: Synthesis, characteristics, and application as activators in homo- and copolymerization of olefins

A row of dimeric diisobutylaluminum aryloxides bearing different sterically crowded substituents in ortho-positions of the aryl ligand [(ArO)AliBu2]2 (ArO = 2-tBu-C6H4O (AlTBP), 2-Me,6-tBu-C6H3O (AlMTBP), 2,6-iPr2-C6H3O (AlDIPP), 2-Ph-C6H4O (AlPP), 2,6-Ph2-C6H3O (AlDPP), 1-C10H7O (AlN-1)) was synthesized. The Molecular structures of AlMTBP, AlDIPP, AlPP, AlDPP, and AlN-1 have been determined by X-ray crystallography. All dimeric diisobutylaluminum aryloxides were tested as activators of rac-Et (2-MeInd)2ZrMe2 in homopolymerization of ethylene, propylene, copolymerization of ethylene with propylene and terpolymerization of ethylene/propylene/5-ethylidene-2-norbornene. It was shown that only AlMTBP and AlDPP work as effective activators in all polymerization processes, with activity of AlMTBP system being significantly higher than that of AlDPP. The evaluation of dimerization energies of synthesized aryloxides by means of DFT calculations have shown AlMTBP and AlDPP to form the least stable dimers. The activation of rac-Et (2-MeInd)2ZrMe2 was modeled as bonding of diisobutylaluminum aryloxide with zirconocene followed by the first insertion of ethylene into Zr+–Me bond. As per that model, DFT calculations demonstrated the process to be the most favorable for AlMTBP and AlDPP which well corresponds to experimental observations.

NMR chemical shift analysis decodes olefin oligo- and polymerization activity of d0 group 4 metal complexes

d0 metal-alkyl complexes (M = Ti, Zr, and Hf) show specific activity and selectivity in olefin polymerization and oligomerization depending on their ligand set and charge. Here, we show by a combined experimental and computational study that the 13C NMR chemical shift tensors of the α-carbon of metal alkyls that undergo olefin insertion signal the presence of partial alkylidene character in the metal-carbon bond, which facilitates this reaction. The alkylidene character is traced back to the π-donating interaction of a filled orbital on the alkyl group with an empty low-lying metal d-orbital of appropriate symmetry. This molecular orbital picture establishes a connection between olefin insertion into a metal-alkyl bond and olefin metathesis and a close link between the Cossee-Arlmann and Green-Rooney polymerization mechanisms. The 13C NMR chemical shifts, the α-H agostic interaction, and the low activation barrier of ethylene insertion are, therefore, the results of the same orbital interactions, thus establishing chemical shift tensors as a descriptor for olefin insertion.

Mechanism of Insertion Polymerization of Allyl Ethers

The copolymerization of ethylene (E) with allyl ethyl ether (AEE) by [di(2-dianisyl)phosphine-2-yl]benzenesulfonato Pd(II) as a catalyst is investigated by DFT calculations and compared with the copolymerization of E with diallyl ether (DAE). For AEE, both 1,2- and 2,1-monomer insertions lead to a very stable O-Chelate product (a five-membered and a four-membered ring, respectively) that hinders any further ethylene insertion. As for DAE, a first 2,1-insertion (favored by 1.8 kcal mol–1 vs the 1,2-insertion) leads to the four-membered O-Chelate product that easily evolves to the most stable intermediate with the second DAE C═C bond coordinated to the metal promoting the following 1,2-insertion. The 2,1 + 1,2 DAE insertion product, bearing a five-membered cyclic unit, is stabilized by a β-agostic interaction that easily opens in favor of E coordination and insertion. Based on the proposed copolymerization mechanism, the stereochemistry of the E/DAE copolymer is studied and the experimental microstructure e...

Formation of a Delocalized Iridium Benzylidene with Azaquinone Methide Character via Alkoxycarbene Cleavage

Lewis base directed α,α-dehydrogenation of 2-(methoxymethyl)aniline gives a cationic iridium(III) alkoxycarbene. This alkoxycarbene is found to be subject to pyridine-promoted C–O bond cleavage in the formation of a delocalized iridium benzylidene. X-ray diffraction and DFT calculations support a structural distortion away from a classic benzylidene structure stemming from azaquinone methide character. Treatment of the iridium azaquinone methide with ethylene results in formal ethylene insertion into the vinylic C–H bond, demonstrating an unusual mechanism for net C–C bond formation via benzylic C–O cleavage.

Effect of ligands in TiCl3(OAr) catalysts for ethylene polymerization: Computational and experimental studies

Ethylene insertion into the TiC bond at the TiCl3(OAr) catalysts in ethylene polymerization was investigated with density functional theory (DFT), in order to explore the effect of different Ar- ligands, such as C6H5-, 2,6-Me2C6H3- or 2,6-iPr2C6H3-. The computational results showed the activation energy and reaction heat of the insertion of ethylene monomer into the TiC bond in the [TiClOArCH2CH3]+ active center were between those in the [TiClCpCH2CH3]+ and [TiCl2CH2CH3]+ active centers, due to that the aryloxy group could slightly change the chemical environment of the active titanium cation. Moreover, the ethylene polymerization was carried out using these TiCl3(OAr) catalysts supported by MgCl2 (TiCl3(OAr)/MgCl2), and polyethylene (PE) was obtained with higher molecular weight and relatively narrow molecular weight distribution, like the polymerization using metallocene catalysts. The experimental results were consist with the DFT computational results, indicating that the ethylene polymerization could be controlled by adjusting the Ar- ligand in the TiCl3(OAr) catalysts.

Cationic Pyridylamido Adsorbate on Brønsted Acidic Sulfated Zirconia: A Molecular Supported Organohafnium Catalyst for Olefin Homo- and Co-Polymerization

Here we report the combined application of high-resolution solid-state 13C–CPMAS-NMR and FT-IR spectroscopy, elemental analysis, kinetic poisoning/active site counting, variable dielectric constant medium, and DFT computation to characterize the surface chemistry of a pyridylamido hafnium complex (Cat1, L1-HfMe2, L1 = 2,6-diisopropyl-N-{(2-isopropylphenyl)[6-(naphthalen-1-yl)pyridin-2-yl]methyl}aniline) adsorbed on Bronsted acidic sulfated zirconia (ZrS). The spectroscopic and DFT results indicate protonolytic formation of organohafnium cations having a largely electrostatic pyridylamido-Hf-CH3+···ZrS– interaction with elongated Hf···OZrS distances of ∼2.14 A. High-molecular-weight polyethylenes and ethylene/1-octene copolymers are obtained with this supported catalyst without an activator/cocatalyst. The DFT calculations reveal that the first ethylene insertion into the Hf-methyl bond has a lower barrier than the corresponding insertion into the Hf-aryl bond of this single-site heterogeneous catalyst.

Catalytic descriptors and electronic properties of single-site catalysts for ethene dimerization to 1-butene

Six first-row transition metal cations (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) were evaluated as catalysts for ethene dimerization to 1-butene. This is an important reaction in the chemistry of CC bond formation and in the conversion of natural gas to higher hydrocarbons. Two related classes of transition metal cation catalysts were investigated: 1) single transition metal cations supported on zirconium oxide nodes of the metal–organic framework NU-1000 and 2) small metal hydroxide clusters with two metal atoms (M2) that could be grown by atomic layer deposition on a support exhibiting isolated hydroxyl groups. Using scaling relations, the free energies of co-adsorbed hydrogen and ethene (i.e., (H/C2H4)*) and adsorbed ethyl (i.e., C2H5*) were identified as descriptors for ethene dimerization catalysis. Using degree of rate control analysis, it was determined that the rate controlling steps are either ethene insertion (CC bond forming) or β-hydride elimination (CH bond breaking), depending on the metal. Using degree of catalyst control analysis, it was determined that activity on all the catalysts studied could be improved by tuning the free energy of C2H5*.

PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir+ Units

Several complexes of (tBu4PNP)Os (1) (tBu4PNP = C5NH3-2,6-(CH2PtBu2)2) are reported. 1 is isoelectronic with (tBu4PCP)Ir (2), which has played a leading role in homogeneous catalytic alkane dehydrogenation; the (tBu4PNP)Os complexes were investigated in this context. (tBu4PNP)OsH4 (1-H4) is analogous to (tBu4PCP)IrH4 (2-H4), but while 2-H4 has some character of a dihydrogen dihydride, 1-H4 is unambiguously a tetrahydride. Ethylene reacts with 1-H4 to afford trans-(tBu4PNP)OsH2(C2H4) (1-H2(C2H4)). At 25 °C, 1-H2(C2H4) readily undergoes reversible ethylene insertion into an Os–H bond to yield (tBu4PNP)OsH(C2H5) (1-EtH). DFT calculations indicate that alkane C–H addition to 1 is thermodynamically much more favorable than addition to 2. The favorable thermodynamics of 1-(alkyl)H, however, disfavor reductive elimination and formation of the free Os(0) fragment that is required for a catalytic cycle analogous to that reported for 2. The much greater favorability of C–H or H–H addition to 1 as compared with Ir(I...

Synthesis of Polyethylenes with Controlled Branching with α-Diimine Nickel Catalysts and Revisiting Formation of Long-Chain Branching

The synthesis of polyethylenes with precise branching, especially long-chain branching (LCB), using ethylene monomer as a single feedstock is of a significant academic and industrial interest. On the basis of the ortho-aryl effect, a series of α-diimine nickel complexes with monoaryl-substituted anilines has been designed and prepared for the synthesis of the polyethylenes with controlled branching. The introduction of the ortho-aryl on aniline moieties enhanced the branching control ability of the α-diimine nickel catalysts. A different mechanistic model was proposed to interpret the presence of methyl and LCB but absence of other short branches in the obtained polyethylenes. LCB was formed by ethylene insertion into the primary Ni-alkyl species originating from nickel migration to methyl terminal of the growing chain because of restricted ethylene insertion into secondary Ni-alkyl species with an α-ethyl or a bulkier alkyl group.

From alternating to selective distributions in chromium-catalysed ethylene oligomerisation with asymmetric BIMA ligands

The oligomerisation of ethylene with chromium-based catalysts containing asymmetric BIMA (bis(benzimidazole)methylamine) ligands produces linear alpha olefins (LAOs) that follow an alternating distribution.

Vinyl Acetate Formation on Au/Pd(100) Alloy Surfaces

The reaction between ethylene and acetate species on oxygen-covered Au/Pd(100) alloy surfaces is explored using reflection–absorption infrared spectroscopy and temperature-programmed desorption (TPD). Distinctly different behavior is found for alloys with less than ~ 0.5 monolayers (ML) of gold, where the alloy contains Pd‒Pd bridge sites, and alloys with gold coverages above ~ 0.5 ML, where only isolated Pd sites are available. Ethylene reacts with η2 acetate species on alloys with low gold coverages where it is found that acetate removal rate increases linearly with gold coverage with a concomitant decrease in carbon monoxide formation, indicating that the selectivity also increases. A different acetate species forms on the isolated palladium sites present on the alloy with high gold coverages, which exhibits infrared peaks at ~ 1450 and 1590 cm−1, as opposed to the characteristic frequency of ~ 1414 cm−1 found for the η2 acetate. It is confirmed that the infrared features are due to adsorbed acetate species by using TPD, but this form of adsorbed acetate is found to be completely unreactive with ethylene, even in a relatively high background pressure of ~ 2 Torr, compared with the η2 acetate species that react rapidly in the presence of ethylene pressures of ~ 10−4 Torr. The structure is assigned to a unidentate acetate species with a strong Pd‒O bond, in which the C=O group can interact with an adjacent gold atom and such a structure would rationalize the lack of reactivity with ethylene because ethylene insertion into the Pd‒O bond of this species would be prohibited. This result is surprising because previous catalytic measurements on model gold–palladium alloys under realistic conditions have shown that high-gold coverage Au/Pd(100) alloys are the most active. A possible explanation for this discrepancy is that the alloys become enriched by palladium under reaction conditions until the gold coverage is reduced below ~ 0.5 ML to yield a very catalytically active surface.

Excellent electrically conductive PE/rGO nanocomposites: In situ polymerization using rGO-Supported MAO cocatalysts

Superior electrically conductive PE nanocomposites were prepared with reduced graphene oxide(rGO)-supported MAO cocatalyst via in situ ethylene polymerization. The electrical conductivity of the composites was very high at 1.2 S/m. This value is the highest among reported rGO-filled composites. The cocatalyst system was prepared by modifying with alkoxysilane compound to prevent restacking of the GO plane in the reduction process and solvothermal reduction of GO. The effect of cocatalysts on the properties of nanocomposites and distribution of nanosheets in the polymer matrix was investigated in detail. The mechanical properties and thermal stability of PE/rGO nanocomposites were significantly increased via the addition of a small amount of rGO. These properties resulted from the restacked rGO layers exfoliated in situ by the ethylene insertion between the intercalated layers.

Tritopic NHC Precursors: Unusual Nickel Reactivity and Ethylene Insertion into a C(sp3)–H Bond

AbstractThe imidazolium chloride [C3H3N(C3H6NMe2)N{C(Me)(=NDipp)}]Cl (1; Dipp=2,6‐diisopropyl phenyl), a potential precursor to a tritopic NimineCNHCNamine pincer‐type ligand, reacted with [Ni(cod)2] to give the NiI‐NiI complex 2, which contains a rare cod‐derived η3‐allyl‐type bridging ligand. The implied intermediate formation of a nickel hydride through oxidative addition of the imidazolium C−H bond did not occur with the symmetrical imidazolium chloride [C3H3N2{C(Me)(=NDipp)}2]Cl (3). Instead, a Ni−C(sp3) bond was formed, leading to the neutral NimineCHNimine pincer‐type complex Ni[C3H3N2{C(Me)(=NDipp)}2]Cl (4). Theoretical studies showed that this highly unusual feature in nickel NHC chemistry is due to steric constraints induced by the N substituents, which prevent Ni−H bond formation. Remarkably, ethylene inserted into the C(sp3)−H bond of 4 without nickel hydride formation, thus suggesting new pathways for the alkylation of non‐activated C−H bonds.

Tritopic NHC Precursors: Unusual Nickel Reactivity and Ethylene Insertion into a C(sp3 )-H Bond.

The imidazolium chloride [C3 H3 N(C3 H6 NMe2 )N{C(Me)(=NDipp)}]Cl (1; Dipp=2,6-diisopropyl phenyl), a potential precursor to a tritopic Nimine CNHC Namine pincer-type ligand, reacted with [Ni(cod)2 ] to give the NiI -NiI complex 2, which contains a rare cod-derived η3 -allyl-type bridging ligand. The implied intermediate formation of a nickel hydride through oxidative addition of the imidazolium C-H bond did not occur with the symmetrical imidazolium chloride [C3 H3 N2 {C(Me)(=NDipp)}2 ]Cl (3). Instead, a Ni-C(sp3 ) bond was formed, leading to the neutral Nimine CHNimine pincer-type complex Ni[C3 H3 N2 {C(Me)(=NDipp)}2 ]Cl (4). Theoretical studies showed that this highly unusual feature in nickel NHC chemistry is due to steric constraints induced by the N substituents, which prevent Ni-H bond formation. Remarkably, ethylene inserted into the C(sp3 )-H bond of 4 without nickel hydride formation, thus suggesting new pathways for the alkylation of non-activated C-H bonds.

Selective Insertion in Copolymerization of Ethylene and Styrene Catalyzed by Half‐Titanocene System Bearing Ketimide Ligand: A Theoretical Study

The copolymerization of ethylene and styrene can be efficiently carried out by using Cp*TiCl2(N = CtBu2)/MAO (Cp*=η5‐C5Me5) system, yielding the poly(ethylene‐co‐styrene)s with isolated styrene units. In order to investigate the reasons for formation of the structure, the mechanism of copolymerization, especially the selective insertion of ethylene and styrene, is studied in detail by density functional theory (DFT) method. At the initiation stage, insertion of ethylene is kinetically more favorable than insertion of styrene, and insertion of styrene kinetically and thermodynamically prefers 2,1‐insertion. That is different from the conventional half‐titanocene system, in which the 1,2‐insertion is favorable. At chain propagation stage, the computational results suggest that the continuous insertion of styrene is hard to occur at room temperature due to the high free energy barriers (28.90 and 35.04 kcal/mol for 1,2‐insertion, and 29.15 and 34.00 kcal/mol for 2,1‐insertion) and thermodynamically unfavorable factors in two different conditions. That is mainly attributed to the steric hindrance between the coming styrene and chain‐end styrene or ketimide ligand. The computational results are in good agreement with the experimental data.

Comparison of PdII vs RhI-catalyzed catalytic cycle for single step styrene production

Styrene production by a (FlDAB)PdII(TFA)(η2-C2H4) complex was modeled using density functional theory (DFT). Benzene CH activation by this complex was studied via five mechanisms, which were oxidative addition/reductive elimination, sigma-bond metathesis, and concerted metalation deprotonation (CMD). The final two mechanisms were calculated without ligated ethylene, CMD activation of ethylene, and benzene substitution of ethylene followed by CMD of the ligated benzene. Calculations provided evidence that conversion of benzene and ethylene to styrene was initiated by the fifth pathway, arylation via CMD of coordinated benzene, followed by ethylene insertion into the RuPh bond, and then β-hydrogen elimination. The results obtained from present study were compared with that of a recently reported RhI complex to help identify more suitable catalysts for the direct production of styrene from ethylene and benzene.

DFT Studies on Isoprene/Ethylene Copolymerization Catalyzed by Cationic Scandium Complexes Bearing Different Ancillary Ligands

AbstractThe isoprene/ethylene copolymerization catalyzed by cationic rare earth metal complexes [(η5‐C5Me5)Sc(CH2SiMe3)]+ (A) had afforded alternating isoprene‐ethylene copolymer with rich 3,4‐polyisoprene microstructures, whereas no isoprene‐ethylene copolymer was observed by using analogous [(PNPPh)Sc(CH2SiMe3)]+ (B) under the same conditions. Theoretical calculations in this work have revealed that, in the case of A, successive 3,4‐insertion of isoprene resulted in a noncovalent interaction between the C = C double bond of penultimate unit and the metal center, suppressing the further insertion of monomers due to higher energy barrier and endergonic character. On the other hand, the ethylene pre‐inserted species with alkyl active site is more suitable for the subsequent kinetically and thermodynamically favorable isoprene insertion and copolymerization is therefore realized. In the case of B, the experimentally observed cis‐1,4‐specific homopolymerization of isoprene was the outcome of both kinetic and thermodynamic control. And, the unfavorable ethylene insertion into the isoprene pre‐inserted species with allyl active site could account for the experimental finding that no isoprene‐ethylene copolymer was obtained. These computational results are expected to provide some hints for the design of rare earth copolymerization catalysts.

Studies of the decomposition of the ethylene hydrophenylation catalyst TpRu(CO)(NCMe)Ph

TpRu(CO)(NCMe)Ph is a catalyst for the conversion of benzene and ethylene to ethylbenzene. Previously, the formation of ethylbenzene has been shown to occur through a pathway that involves ethylene coordination to Ru, insertion of ethylene into the Ru–phenyl bond and Ru–mediated benzene C–H activation. The effect of ethylene pressure and catalyst concentration (between 0.2 and 0.01 mol % based on benzene) on the decomposition of TpRu(CO)(NCMe)Ph was examined. Studies have shown that there are two competing catalyst deactivation pathways. At higher concentrations of TpRu(CO)(NCMe)Ph, the dominant deactivation pathway is likely initiated by a binuclear reaction of two Ru complexes that leads to formation of unidentified paramagnetic species. Kinetic studies reveal that this pathway for catalyst decomposition occurs with a second-order rate of 0.007 (1) M−1 s−1. At lower Ru concentrations, ethylene C–H activation to form the allyl complex TpRu(CO)(η3-C4H7) is the predominant deactivation pathway. The effect of ethylene pressure on catalyst decomposition was also examined. At higher ethylene pressure nearly quantitative formation of TpRu(CO)(η3-C4H7) was observed.

Ethylene insertion in the presence of new alkoxysilane electron donors for Ziegler-Natta catalyzed polyethylene

Density functional theory (DFT) calculations have been carried out to investigate the ethylene insertion pathway using a Ziegler-Natta catalyst in the absence and presence of electron donor (ED) systems on the (110) MgCl2 surface. The coadsorptions of four different EDs i.e. Si(OEt)mCln (m+n=4) were investigated. The presence of Si(OEt)4 on the (110) MgCl2 surface with the preferential bidentate mode was found to have the strongest adsorption energy (ΔEads). The potential energy surface (PES) map indicated that the reaction mechanism of the ethylene insertion into the TiC bond on the (110) Mg13Cl26.TiCl3-CH2CH3 surface is pseudo-concerted. As the differences in the intrinsic activation energies (ΔEa) obtained from all systems are so small, this energy cannot be used to fully explain the significant changes in the rates of the ethylene insertion reaction observed when an ED is employed. Here, the apparent activation energy (ΔΔEaa) was calculated using the PBE functional and the 6-31G(d, p) basis set for C, H, O, Mg and Cl, and the LANL2DZ basis set with an ECP function for the Ti atom. All EDs presented in this work in the ethylene insertion reaction can significantly reduce the apparent energy barrier when compared to an absence of any ED system. The obtained ΔΔEaa for the four ED complexes were found to decrease in the following order: SiOEtCl3>Si(OEt)2Cl2>Si(OEt)3Cl>Si(OEt)4. The obtained data lead to the conclusion that Si(OEt)4 is the most suitable ED to increase the productivity of PE in the presence of alkoxysilane.