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

Nobbs, James D.; Tomov, Atanas K.; Young, Craig T.; White, Andrew J. P.; Britovsek, George J. P.

doi:10.1039/c7cy01896g

Cited by 12 publications

(8 citation statements)

References 23 publications

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“…The identity of these complexes was established on the basis of combustion elemental analysis, IR spectroscopy and high‐resolution mass spectrometry (HRMS). Upon reaction with CrCl 3 (THF) 3 , either [CrCl 3 (N,O)(THF)] or the dimer [Cr(N,O)Cl 3 ] 2 were expected, both of which would maintain the favored hexacoordination of chromium (III) [ 14,24,27,28,45,46 ] ; however, the elemental analyses of complexes Cr1‐Cr3 confirmed the formation of dimeric species of general formula [CrCl 2 (N,O)(μ‐Cl)] 2 . Furthermore, the ESI‐HRMS results indicated the formation of [M‐Cl] + ions for all chromium complexes (Figures S1‐S3).…”

Section: Resultsmentioning

confidence: 99%

“…[ 13–16 ] In this way, significant efforts have been dedicated to the synthesis of new families of ligands based on a wide variety of donor‐group combinations aiming to generate more efficient chromium catalyst systems that are capable of selectively forming α‐olefins with high productivities. [ 17–81 ] In particular, chromium catalysts stabilized by pyrazolyl‐based tridentate ligands have been successfully synthetized and their catalytic application in ethylene oligomerization explored in details. [ 51,81–91 ] Selected examples are presented in Chart 1.…”

Section: Introductionmentioning

confidence: 99%

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Chromium Complexes Supported by Phenyl Ether‐Pyrazolyl [N,O] Ligands as Catalysts for the Oligo‐ and Polymerization of Ethylene

Milani

Casagrande

2020

Applied Organom Chemis

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A new series of Cr (III) complexes [Cr{1‐(3‐phenoxypropyl)‐1H‐pyrazole}Cl3]2 (Cr1), [Cr{1‐(3‐phenoxypropyl)‐3,5‐dimethyl‐1H‐pyrazole}Cl3]2 (Cr2), and [Cr{1‐(3‐phenoxypropyl)‐3‐phenyl‐1H‐pyrazole}Cl3]2 (Cr3) have been synthesized and characterized by elemental analysis, high‐resolution mass spectrometry (HRMS) and IR spectroscopy. Upon activation with methylaluminoxane (MAO), chromium precatalysts Cr2 and Cr3 showed moderate activity in ethylene oligomerization [TOF = 17,900–29,200 mol (ethylene)·mol (Cr)−1·h−1 at 80 °C] with Schultz‐Flory distribution of oligomers (K = 0.54–0.66) and production of polymer varying from 2.8 to 6.7 wt.%. On the other hand, under identical oligomerization conditions, Cr1/MAO behaved as a polymerization catalyst generating predominantly polyethylene (63.7 wt%). The amount of 1‐butene is the largest component in the liquid fraction suggesting that these precatalysts operate via a Cossee‐Arlman mechanism. The catalytic activities, selectivity and product distribution are quite sensitive to the R‐group at the 3‐ and 5‐position of the pyrazolyl ring. Based on the electronic and steric effects of R‐ substituents, it is possible to stablish a trend of activity: Cr2(PzMe2) > Cr3(PzPh) > Cr1(Pz). Moreover, the effect of oligomerization parameters (cocatalyst, temperature, [Al]/[Cr] molar ratio, time) on the activity and on the product distribution were examined.

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Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Chromium Complexes Supported by Phenyl Ether‐Pyrazolyl [N,O] Ligands as Catalysts for the Oligo‐ and Polymerization of Ethylene

Milani

Casagrande

2020

Applied Organom Chemis

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“…Some support for β-H elimination has been provided by the observation of methylcyclopentane and methylenecyclopentane as well as cyclopentyl end groups in the polymer chains, which highlights the involvement of alkenyl hydride intermediate [156]. The mechanism of tetramerization has also been the subject of a number of studies [5,45,48,137,149,150,[157][158][159]. The conclusion of many of these works is that a modified metallacyclic mechanism is operational in which chromacycloheptane species is now stable relative to 1-hexene elimination that occurs in the trimerization cycle, with the result that the larger 9-membered ring is now obtainable (Scheme 3).…”

Section: Tri-/tetramerization Of Ethylenementioning

confidence: 98%

Recent advances in homogeneous chromium catalyst design for ethylene tri-, tetra-, oligo- and polymerization

Bariashir

Huang

Solan

et al. 2019

Coordination Chemistry Reviews

112

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This review focuses on recent progress made using well-defined molecular chromium complexes that, upon suitable activation, can catalyze the tri-, tetra, oligo-and/or polymerization of ethylene. In particular, emphasis will be placed on the tuning of the performance characteristics of these homogeneous catalysts through structural modifications made to the multidentate ligand manifold (e.g., donor atoms, charge, backbone and strain) and the effects these changes have on the resulting ethylene derivatives. While the ability of these catalysts to mediate the formation of high molecular weight linear polyethylene continues to see many developments, their capacity to form polyethylene waxes and oligomers has witnessed some major advances. Moreover, the impressive selectivity of some chromium systems to generate commercially important 1-hexene and more recently 1-octene has seen the implementation of this technology at the industrial level. The types of precatalysts to be discussed will be divided broadly on the basis of their ability to generate either polymers/oligomers or short chain αolefins; the effects of co-catalyst and reaction conditions (e.g., temperature, pressure, solvent) on catalytic activity and selectivity, will be also developed. In addition, current proposals as to the mechanistic details displayed by these versatile chromium catalysts will be highlighted.

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“…Herein, we report another type of MAO‐free and extremely active catalytic system, whose activity is more than 10 times that of the original MAO‐based Sasol system. Catalyst development for ethylene oligomerization is still an active research topic in both academia and industry …”

Section: Introductionmentioning

confidence: 99%

MAO‐free and extremely active catalytic system for ethylene tetramerization

Kim

Lee

Park

et al. 2019

Applied Organom Chemis

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The original Sasol catalytic system for ethylene tetramerization is composed of a Cr source, a PNP ligand, and MAO (methylaluminoxane). The use of expensive MAO in excess has been a critical concern in commercial operation. Many efforts have been made to replace MAO with non‐coordinating anions (e.g., [B(C6F5)4]−); however, most of such attempts were unsuccessful. Herein, an extremely active catalytic system that avoids the use of MAO is presented. The successive addition of two equivalent [H(OEt2)2]+[B(C6F5)4]− and one equivalent CrCl3(THF)3 to (acac)AlEt2 and subsequent treatment with a PNP ligand [CH3(CH2)16]2C(H)N(PPh2)2 (1) yielded a complex presumably formulated as [1‐CrAl (acac)Cl3(THF)]2+[B(C6F5)4]−2, which exhibited high activity when combined with iBu3Al (1120 kg/g‐Cr/h; ~4 times that of the original Sasol system composed of Cr (acac)3, iPrN(PPh2)2, and MAO). Via the introduction of bulky trialkylsilyl substituents such as –SiMe3, –Si(nBu)3, or –SiMe2(CH2)7CH3 at the para‐position of phenyl groups in 1 (i.e., by using [CH3(CH2)16]2C(H)N[P(C6H4‐p‐SiR3)2]2 instead of 1), the activities were dramatically improved, i.e., tripled (2960–3340 kg/g‐Cr/h; more than 10 times that of the original Sasol system). The generation of significantly less PE (<0.2 wt%) even at a high temperature is another advantage achieved by the introduction of bulky trialkylsilyl substituents. NMR studies and DFT calculations suggest that increase of the steric bulkiness on the alkyl‐N and P‐aryl moieties restrict the free rotation around (alkyl)N–P (aryl) bonds, which may cause the generation of more robust active species in higher proportion, leading to extremely high activity along with the generation of a smaller amount of PE.

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

Abstract: 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.

Cited by 12 publications

References 23 publications

Chromium Complexes Supported by Phenyl Ether‐Pyrazolyl [N,O] Ligands as Catalysts for the Oligo‐ and Polymerization of Ethylene

Chromium Complexes Supported by Phenyl Ether‐Pyrazolyl [N,O] Ligands as Catalysts for the Oligo‐ and Polymerization of Ethylene

Recent advances in homogeneous chromium catalyst design for ethylene tri-, tetra-, oligo- and polymerization

MAO‐free and extremely active catalytic system for ethylene tetramerization

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