Ethylene Insertion Research Articles

Why could the CrOx/SiO2 and VOx/SiO2 catalysts show so different behaviors in ethylene polymerization? A theoretical approach

Phillips catalyst (CrOx/SiO2) is one of the most famous industrial catalysts for ethylene polymerization, while VOx/SiO2 catalyst is a promising catalyst capable of making UHMWPE, which was recently developed in our group. These two catalysts were known to show completely deferent behaviors in ethylene polymerization in terms of activity, H2 response and copolymerization ability. However, a molecular insight into the mechanism is not well understood. Herein, a theoretical comparison between them via the propagation through insertion and termination through chain transfer reaction pathways is performed. The results show the transition states for olefins (ethylene, 1-butene and 1-hexene) insertion are energetically more stable in VOx/SiO2. However, the binding energies of the olefins by VOx/SiO2 is obviously lower, which results in the insertion barriers for VOx/SiO2 about 2–3 kcal mol−1 higher than those for CrOx/SiO2. As to the termination, CrOx/SiO2 is more probably to terminate via the BHE (β-H elimination to Cr center) pathway, which generally has lower barriers than those for BHT (β-H elimination to monomer) in the VOx/SiO2 system. Meanwhile, VOx/SiO2 is superior to CrOx/SiO2 in hydrogenolysis, because it is more accessible by the H2, and the reaction barrier is also much lower. The energy gap between the barriers for propagation and termination is generally smaller for CrOx/SiO2, implying its tendency to produce lower MW PE when compared with VOx/SiO2. Furthermore, for CrOx/SiO2, the insertion barriers between C2H4 and α-olefin are bigger and the promotion effect of the incorporated α-olefins on termination is more obvious, which might account for the lower incorporation efficiency for comonomer by CrOx/SiO2, despite of its lower insertion barriers.

Palladium-Catalyzed Insertion of Ethylene and 1,1-Disubstituted Difunctional Olefins: An Experimental and Computational Study.

Insertion or coordination copolymerization of ethylene with di-substituted olefins is challenging and the choice of di-substituted mono-functional olefin versus di-substituted di-functional olefin (DDO) appears to be decisive. Here we show that DDO-inserted species are amenable to ethylene insertion and polymerization. DDOs such as 2-acetamidoacrylic acid (AAA), methyl 2-acetamidoacrylate (MAAA), and ethyl 2-cyanoacrylate (ECA) were treated with palladium complex [{P∧O}PdMe(L)] (P∧O=κ2 -P,O-Ar2 PC6 H4 SO2 O with Ar=2-MeOC6 H4 ; L=C2 H6 OS) and the existence of respective insertion intermediates in moderate yield (up to 37 %) was established. These intermediates were exposed to ethylene and corresponding ethylene-inserted products were isolated and characterized. A careful comparison with three model compounds confirmed ethylene insertion and polymerization. Thus, the combined experimental and computational investigations show that DDO-inserted species can undergo ethylene insertion and polymerization.

Exploring the Nature of the Energy Barriers on the Mechanism of the Zirconocene-Catalyzed Ethylene Polymerization: A Quantitative Study from Reaction Force Analysis

Ethylene polymerization mediated by methyl-bis(cyclopentadienyl)-zirconium or zirconocene catalyst, [ZrCp₂CH₃]⁺, is one of the most popular catalytic reaction for polyethylene production. Rationalizing the major effects that control the polymer growth result in a challenge for computational studies. Through quantum chemical calculations, we characterized the zirconocene ethylene polymerization reaction mechanism: chain initiation (I; first ethylene insertion) [ZrCp₂CH₂CH₂CH₃]⁺, chain propagation (P; from second (P₁) to ninth (P₉) ethylene insertion) [ZrCp₂ (CH₂)₂₀CH₃]⁺, and chain termination processes (T; β-hydrogen elimination from P₅ or P₉) [ZrHCp₂ (H₂C═CH(CH₂)₁₈CH₃]⁺ are analyzed through the potential energy surface (PES) and reaction force analysis (RFA). The RFA approach involves pulling out the portion of an activation barrier that corresponds to distorting reactants into the geometries they adopt in a transition state structure until it reaches the structural relaxation toward the equilibrium geometry of the product. Because the interactions between the zirconocene and the ethylene molecule are influenced by a combination of several kinds of steric and electronic effects, it is indispensable to understanding these interactions in order to rationalize and predict in a quantitative manner the reaction barrier heights and the concomitant polymer growth. In the present work, we employ a simple procedure within the framework of the RFA and the density functional steric energy decomposition analysis (EDA) approach to quantitatively separate the different types of interactions; steric (ΔE_s), electrostatic (ΔE_e), and quantum (ΔE_q) effects in order to predict the impact of each factor on the course of the polymerization process as well as for the polymer control and design.

Platinum ethylene dimerization catalysts: Diphosphine vs. diimine ancillary ligand effects

Kinetic and mechanistic studies are presented for the (dfepe)Pt(Me)(NC5F5)+ (dfepe = (C2F5)2PCH2CH2P(C2F5)2) ethylene dimerization catalyst system. New labile complexes (dfepe)PtMe(L)+ (L = NC5F5, C6F5CN, C6F5NH2, C6F5NO2) have been prepared. A general extension to a variety of other chelating diphosphine analogues (PP)Pt(Me)(C2H4)+ has been accessed by methyl abstraction from donor (PP)PtMe2 precursors with Ph3C+B(C6F5)4− in the presence of ethylene to cleanly afford (PP)Pt(Me)(C2H4)+ products. Catalysis studies for these more electron-rich diphosphine systems demonstrate moderate dimerization activity which is uniformly higher than reported for (diimine)Pt(Me)(C2H4)+. In several cases allylic catalyst decomposition products (PP)Pt(η3-C3H4Me)+ have been identified. A DFT study of insertion barriers for diimine and diphosphine systems is presented which suggests that weakening of Pt-ethylene ground state binding by strong-field diphosphine ligands is a major contributing factor to the lower ethylene insertion barriers for PP systems.

Pd-Iminocarboxylate Complexes and Their Behavior in Ethylene Polymerization.

Designing co-catalyst-free late transition metal complexes for ethylene polymerization is a challenging task at the interface of organometallic and polymer chemistry. Herein, a set of new, co-catalyst-free, single-component catalytic systems for ethylene polymerization have been unraveled. Treatment of anthranilic acid with various aldehydes produced four iminocarboxylate ligands (L1-L4) in very good to excellent yield (75-92 %). The existence of 2-((2-methoxybenzylidene)amino) benzoic acid (L1) has been unambiguously demonstrated using NMR spectroscopy, MS and single-crystal X-ray diffraction. A neutral Pd-iminocarboxylate complex [{N O}PdMe(L1)] (N O=κ2 -N,O-ArCHNC6 H4 CO2 with Ar=2-MeOC6 H4 ) C1 was prepared by treating stoichiometric amount of L1.Na with palladium precursor. The identity of C1 was confirmed by 1-2D NMR spectroscopy and single-crystal X-ray diffraction studies. Along the same lines, palladium complexes C2-C4 were prepared from ligands L2-L4 respectively. In-situ high-pressure NMR investigations revealed that these Pd complexes are amenable to ethylene insertion and undergo facile β-H elimination to produce propylene. These palladium complexes were then evaluated in ethylene polymerization reaction and various reaction parameters were screened. When C1-C4 were exposed to ethylene pressures of 10-50 bar, formation of low-molecular-weight polyethylene was observed.

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.

Rhodium Complexes in P−H Bond Activation Reactions

The feasibility of oxidative addition of the P-H bond of PHPh2 to a series of rhodium complexes to give mononuclear hydrido-phosphanido complexes has been analyzed. Three main scenarios have been found depending on the nature of the L ligand added to [Rh(Tp)(C2 H4 )(PHPh2 )] (Tp= hydridotris(pyrazolyl)borate): i) clean and quantitative reactions to terminal hydrido-phosphanido complexes [RhTp(H)(PPh2 )(L)] (L=PMe3 , PMe2 Ph and PHPh2 ), ii) equilibria between RhI and RhIII species: [RhTp(H)(PPh2 )(L)]⇄[RhTp(PHPh2 )(L)] (L=PMePh2 , PPh3 ) and iii) a simple ethylene replacement to give the rhodium(I) complexes [Rh(κ2 -Tp)(L)(PHPh2 )] (L=NHCs-type ligands). The position of the P-H oxidative addition-reductive elimination equilibrium is mainly determined by sterics influencing the entropy contribution of the reaction. When ethylene was used as a ligand, the unique rhodaphosphacyclobutane complex [Rh(Tp)(η1 -Et)(κC,P -CH2 CH2 PPh2 )] was obtained. DFT calculations revealed that the reaction proceeds through the rate limiting oxidative addition of the P-H bond, followed by a low-barrier sequence of reaction steps involving ethylene insertion into the Rh-H and Rh-P bonds. In addition, oxidative addition of the P-H bond in OPHPh2 to [Rh(Tp)(C2 H4 )(PHPh2 )] gave the related hydride complex [RhTp(H)(PHPh2 )(POPh2 )], but ethyl complexes resulted from hydride insertion into the Rh-ethylene bond in the reaction with [Rh(Tp)(C2 H4 )2 ].

Diamido Complexes of Titanium and Zirconium as Catalyst Precursors for Ethylene Polymerization

A series of 8 new complexes of titanium and zirconium with diamido ligands bearing an ethylene and propylene bridge between the two amido groups were synthesized and tested for ethylene polymerization. Titanium complexes bearing an ethylene bridge between the two amido groups showed higher activities than the derivatives with a propylene bridge. In the case of the zirconium complexes, the propylene bridged complexes were more active than the corresponding ethylene bridged. The introduction of bulky groups on the ligand structure resulted in an increase in the activity. DFT calculations were performed to determine the activation energy barriers for different reaction steps. The calculated activation energy for the insertion of ethylene into an M-CH3 bond is in the range of 12.2-16.8 kcal/mol and the activation energy for the chain termination via β-H transfer reaction is 12.5-14.4 kcal/mol.

Two-Carbon Ring Expansion of 1-Indanones via Insertion of Ethylene into Carbon-Carbon Bonds.

A rhodium-catalyzed direct insertion of ethylene into a relatively unstrained carbon-carbon bond in 1-indanones is reported, which provides a two-carbon ring expansion strategy for preparing seven-membered cyclic ketones. As many 1-indanones are commercially available and ethylene is inexpensive, this strategy simplifies synthesis of benzocycloheptenones that are valuable synthetic intermediates for bioactive compounds but challenging to prepare otherwise. In addition, the reaction is byproduct-free, redox neutral, and tolerant of a wide range of functional groups, which may have implications on unconventional strategic bond disconnections for preparing complex cyclic molecules.

Rh-Catalyzed Annulative Insertion of Terminal Olefin onto Pyridines via a C–H Activation Strategy Using Ethenesulfonyl Fluoride as Ethylene Provider

A Rh(III)-catalyzed annulative insertion of ethylene onto picolinamides was achieved, providing a portal to a class of unique pyridine-containing molecules bearing a terminal olefin moiety for diversification. Application of this method for modification of Sorafenib was also accomplished.

The Hafnium-Pyridyl Amido-Catalyzed Copolymerization of Ethene and 1-Octene: How Small Amounts of Ethene Impact Catalysis

Copolymerization of ethene and 1-octene is catalyzed by the Hf-pyridyl amido complex used in chain-shuttling polymerization. Active site counts and the molar mass distribution of catalyst-bound polymeryls are determined by chromophore quench-labeling. Catalytic data for copolymerization (1-octene consumption, active site counts, molar mass distributions of polymers, and I2-labeling of Zn-polymeryls) are compared with similar data for the homopolymerization of 1-octene. Such comparisons reveal that small amounts of ethene have a significant impact on catalysis. The rate of 1-octene consumption increases in a copolymerization ∼3-fold compared to a homopolymerization; this likely results from in situ ligand modification. The first insertion of alkene occurs into the Hf–naphthyl bond of the Hf-pyridyl amido catalyst; in copolymerization, competition between ethene and 1-octene for this insertion creates multiple active species. We propose that ethene insertion into the Hf–naphthyl bond leads to a faster polym...

Incorporation of Radicals during Ni(II)-Catalyzed Ethylene Insertion Polymerization

A possible involvement of radicals in catalytic insertion chain growth remains elusive, albeit it could account for thus far inconclusive experimental observations and enable yet unachieved synthetic reaction schemes. This work shows that radicals present, deliberately generated for this purpose, are incorporated as end groups in ethylene insertion polymerizations. Azo initiators (R-N = N-R′ with R = C(CH3)2COOMe, Ph, 3,5-F2C6H3) added to pressure reactor Ni(II)salicylaldiminato-catalyzed ethylene polymerizations afforded initiator-derived aryl or alkyl end groups, respectively, as revealed by in-depth NMR studies of the polymer formed. The observation of additional solvent- derived aryl end groups, originating from radical transfer to toluene, underlines that end groups are indeed formed from radicals. Evidence for incorporation as a chain-initiating as well as -terminating end group is discussed.

Diphosphazane-monoxide and Phosphine-sulfonate Palladium Catalyzed Ethylene Copolymerization with Polar Monomers: A Computational Study

Density functional theory (DFT) calculations have been comparatively carried out to disclose the origin of different catalytic performance of diphosphazane-monoxide and phosphine-sulfonate palladium complexes toward copolymerization of ethylene and vinyl polar monomers. A theoretical comparison of the two catalytic systems indicates that the rigid five-membered backbone and cationic nature of the diphosphazane-monoxide palladium complex are beneficial for its copolymerization activity. Having achieved agreement between theory and experiment, it is found that the favorable 2,1-selective insertion of methyl methacrylate (MMA) into the diphosphazane-monoxide palladium complex originates from less geometrical deformation and the MeO···H interaction between the ancillary ligand and the methyl of MMA. In this catalyst system, it is kinetically difficult for the MMA pre-enchained species to undergo subsequent ethylene insertion due to the steric repulsion between the methyl of MMA and the inserting ethylene moie...

Facile insertion of ethylene into a group 14 element-carbon bond: effects of the HOMO-LUMO energy gap on reactivity.

The diarylstannylenes, Sn(AriPr4)2 and Sn(AriPr6)2, (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2, AriPr6 = C6H3-2,6-(C6H2-2,4,6-iPr3)2), undergo a facile migratory insertion reaction with ethylene at 60 °C to afford the alkyl aryl stannylenes AriPr4SnCH2CH2AriPr4 and AriPr6SnCH2CH2AriPr6 which were characterized via1H, 13C and 119Sn NMR, UV-vis and IR spectroscopy, as well as by X-ray crystallography. Quantum mechanical calculations were performed, and two potential mechanisms were identified, with a migratory insertion reaction pathyway being energetically preferred.

Interconversion and reactivity of manganese silyl, silylene, and silene complexes.

Manganese disilyl hydride complexes [(dmpe)2MnH(SiH2R)2] (4Ph : R = Ph, 4Bu : R = n Bu) reacted with ethylene to form silene hydride complexes [(dmpe)2MnH(RHSi[double bond, length as m-dash]CHMe)] (6Ph,H : R = Ph, 6Bu,H : R = n Bu). Compounds 6R,H reacted with a second equivalent of ethylene to generate [(dmpe)2MnH(REtSi[double bond, length as m-dash]CHMe)] (6Ph,Et : R = Ph, 6Bu,Et : R = n Bu), resulting from apparent ethylene insertion into the silene Si-H bond. Furthermore, in the absence of ethylene, silene complex 6Bu,H slowly isomerized to the silylene hydride complex [(dmpe)2MnH([double bond, length as m-dash]SiEt n Bu)] (3Bu,Et ). Reactions of 4R with ethylene likely proceed via low-coordinate silyl {[(dmpe)2Mn(SiH2R)] (2Ph : R = Ph, 2Bu : R = n Bu)} or silylene hydride {[(dmpe)2MnH([double bond, length as m-dash]SiHR)] (3Ph,H : R = Ph, 3Bu,H : R = n Bu)} intermediates accessed from 4R by H3SiR elimination. DFT calculations and high temperature NMR spectra support the accessibility of these intermediates, and reactions of 4R with isonitriles or N-heterocyclic carbenes yielded the silyl isonitrile complexes [(dmpe)2Mn(SiH2R)(CNR')] (7a-d: R = Ph or n Bu; R' = o-xylyl or t Bu), and NHC-stabilized silylene hydride complexes [(dmpe)2MnH{[double bond, length as m-dash]SiHR(NHC)}] (8a-d: R = Ph or n Bu; NHC = 1,3-diisopropylimidazolin-2-ylidene or 1,3,4,5-tetramethyl-4-imidazolin-2-ylidene), respectively, all of which were crystallographically characterized. Silyl, silylene and silene complexes in this work were accessed via reactions of [(dmpe)2MnH(C2H4)] (1) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C2H4 and C2D4) hydrosilylation was investigated using [(dmpe)2MnH(C2H4)] (1) as a pre-catalyst, resulting in stepwise conversion of primary to secondary to tertiary hydrosilanes. Various catalytically active manganese-containing species were observed during catalysis, including silylene and silene complexes, and a catalytic cycle is proposed.

Olefin Polymerization Reactivity of Group 4 Post‐Metallocene Catalysts Bearing a Four‐Membered C(sp3)‐Donor Chelate Ring

AbstractThe synthesis, spectroscopic and structural characterization, and olefin polymerization behavior of a family of pyridine‐2‐phenolate‐6‐arylmethine [O,N,CH(Ar)] Group 4 catalysts bearing a four‐membered N,C(sp3)‐donor metallacycle are described. The racemic [O,N,CH(Ar)] complexes adopt C1 symmetry, as confirmed by NMR spectroscopy and X‐ray crystallography. Advanced NMR experiments have been conducted to probe for possible C−H⋅⋅⋅F−C interactions within fluorinated derivatives. Use of a bis‐pyridyl ligand (Ar=py) resulted in the formation of C2v‐symmetric [O,NCHN] complexes containing a six‐membered partially delocalized N,N‐donor chelate. All complexes have been evaluated as ethylene polymerization catalysts. Notably, the Ti derivatives in conjunction with [Ph3C][B(C6F5)4]/iBu3Al display excellent catalytic efficiencies (TOF over 4×105 h−1 [atm C2H4]−1) at 22 °C, and are considerably more active than previously reported Ti−[O,N,C(σ‐aryl)] relatives. DFT calculations have been performed to gain insights into catalytic behavior. These studies indicate that although the ethylene assimilation process (comprising initial ethylene insertion into the Ti−C(methine) bond of the four‐membered chelate) is accessible, there exists a kinetic preference for normal chain propagation for some catalysts. The DFT results for the Ti−[O,N,CH(Ar)] (Ar=Ph) catalyst are consistent with the narrow molecular weight distributions of the polymers produced (Mw/Mn down to 2.2), suggesting close to single‐site character.

Theoretical Study of Solvent Effect on the Kinetics and Thermochemistry of the Reaction of a (NHC)Cu(boryl) Complex with Ethylene

In this investigation, we describe solvent effect on the ethylene insertion into Cu–B bond in a (NHC)Cu(boryl) complex using MPW1PW91 quantum chemical methods. Solvent effects were explored by using the self-consistent reaction field (SCRF) method based on polarizable continuum model (PCM) in cyclopentanol, acetone, methanol, nitromethane, and dimethylsulfoxide. The progress of the reaction was quantitatively studied using the bond orders of the C–C bond of ethylene, and the percentages of bond cleavage (BCCC) of C–C bond at the transition state were described. The linear correlations between barrier energy, bond distances variations and thermodynamic parameters with dielectric constants of solvents were investigated. Rate constant of the title reaction over the temperature range of 200–1200 K was calculated in gas phase.

Rhodium catalyzed hydroformylation of olefins.

DFT and CCSD(T) methods were used to examine 61 different rhodium catalysts for the hydroformylation of ethylene. The carbon monoxide (CO) stretching frequency was a key electronic parameter to understand the π-accepting nature of the ligand. Normally, π-accepting ligands lead to increased CO stretching frequencies and a reduction in CO dissociation energy. There was no relationship between CO dissociation energy and CO stretching frequency. However, a clear relationship exists between the ethylene insertion barrier (from the rhodium dicarbonyl hydride resting state) and the CO stretching frequency as stronger π-accepting ligands systematically led to a reduction in the barrier. Due to the multistep nature of the rate-limiting step, the overall barrier can be divided into the CO/ethylene equilibrium and an intrinsic ethylene insertion barrier and both are systematically reduced as the π-accepting nature of the ligand is increased. A comparison of the carbonylation transition state (TS) to the ethylene insertion TS allowed us to understand reversibility of olefin insertion. While the ethylene insertion TS systematically decreases with increasing CO stretching frequency, the carbonylation TS is relatively flat. The lines cross at 2156 cm-1 implying a change in the rate-limiting step in this region given a standard set of process conditions. © 2018 Wiley Periodicals, Inc.

Iron-Mediated Coupling of Carbon Dioxide and Ethylene: Macrocyclic Metallalactones Enable Access to Various Carboxylates.

Treatment of (iPrPDI)Fe(N2)2 (iPrPDI, 2,6-(2,6-iPr2C6H3N═CMe)2C5H3N) with CO2 and ethylene resulted in the formation of a homologous series of saturated and unsaturated iron carboxylate products, (iPrPDI)Fe(O2CR), the distribution of which depends on the ratio of the reagents. The solid-state and electronic structures of a saturated product, (iPrPDI)Fe(O2CC2H5), were elucidated. Product distributions, deuterium labeling studies, and stoichiometric experiments support initial formation of a five-membered metallalactone intermediate, which undergoes subsequent ethylene insertions to generate macrocyclic metallalactones. Competitive β-hydride elimination, CO2 insertion, or reaction with H2 determines the fate of the metallalactone, the latter accounting for formation of iron complexes with saturated carboxylates. Similar reactivity was observed upon addition of propiolactone and ethylene to (iPrPDI)Fe(N2)2, supporting C-O oxidative addition and C-C bond formation through metallacycle intermediates.

Introducing electron-donating substituents on ligand backbone of α-diimine nickel complex and the effects on catalyst thermal stability in ethylene polymerization

A novel α-diimine nickel pre-catalyst Cat.1 with two electron-donating methyl substituents on the ligand backbone was synthesized and investigated for ethylene polymerization using methylaluminoxane as cocatalyst. Cat.1 exhibited higher polymerization activity and produced highly branched polyethylene with slightly higher molecular weight than that of the typical Brookhart catalyst (B-Cat) at elevated temperature. By tracing the changes of active center number in the polymerization process, Cat.1 was found to have higher ratio of activated nickel ([C∗]/[Ni]), longer catalyst lifetime and larger chain propagation rate constant (kp) as compared with B-Cat. The computational studies demonstrated the electron-donating methyl substituents can make a more electron-rich ligand to influence the catalytic properties of Cat.1. Stronger interactions between the electron-rich ligand and the nickel center should be the main reasons for longer catalyst lifetime and enhanced thermal stability of Cat.1. The production of polyethylene with higher molecular weight by Cat.1 can be explained by its larger kp value and more stable transition state for ethylene insertion than for chain transfer reaction due to the electron-donating ligand. Overall, the results of this work clearly support the importance of ligand electronic effects on the thermal stability and intrinsic reactivity of active centers in α-diimine Ni(II) catalysts.