Spin Surface Crossing between Chromium(I)/Sextet and Chromium(III)/Quartet without Deprotonation in SNS-Cr Mediated Ethylene Trimerization

Chromium Catalysts for Selective Ethylene Oligomerization Featuring Binuclear PNP Ligands

A series of novel binuclear PNP ligands based on the cyclohexyldiamine scaffold were synthesized for this study. The experimental results showed that positioning the two PNP sites at the para-positions of the cyclohexyl framework led to a significant enhancement in the catalytic activity for selective tri/tetramerization of ethylene. The PNP/Cr(acac)3/MAO(methylaluminoxane) catalytic system exhibited relatively high catalytic activity (up to 3887.7 kg·g-1·h-1) in selective ethylene oligomerization with a total selectivity of 84.5% for 1-hexene and 1-octene at 40 °C and 50 bar. The relationship between the ligand structure and ethylene oligomerization performance was further explored using density functional theory calculations.

Chromium Catalysts Based on Unsymmetrical PNP Ligands for Selective Ethylene Tri-/Tetramerization: Effect of Electron-Withdrawing/Donating Substituents on Catalytic Performance

The in situ formation and activation of Cr(III) catalysts based on unsymmetrical PNP ligands yield efficient catalytic systems for selective ethylene tri-/tetramerization. The electronic nature (electron-withdrawing or electron-donating) and position (para or meta) of the substituents over the phenyl rings of the PNP, the nature of cocatalyst (DMAO/AlEt3 and MMAO-3A), and reaction conditions have been observed to have a marked impact on catalytic performance, particularly catalytic activity. Ligand L2, bearing 4-(trifluoromethyl)phenyl substituents, yielded 33.6 kg(product).g(Cr)−1·h−1 catalytic activity with 57.7% C8 selectivity under optimal conditions. Ligand L4, having para-tolyl substituents, yielded 43.3 kg(product).g(Cr)−1·h−1 with 59.0% C8 selectivity under optimum conditions. Changing the positions of both the electron-withdrawing and electron-donating substituents from para to meta over the phenyls of the PNP may lead to both catalytic systems exhibiting poor performance.

Cr/PCCP-Catalysed Selective Ethylene Oligomerization: Analysis of Various Conformations and the Hemilabile Methoxy Group

In this work, the effect of the hemilabile methoxy group in the Cr-based catalyst bearing (C6H5)2-P(CH2)2P-(C6H5)2 (PCCP) and (o-MeOC6H4)(C6H5)-P(CH2)2P-(C6H5)(o-MeOC6H4) (PCCPOMe) ligands on ethylene tri- and tetramerization were systematically investigated by...

Effect of an additional donor on decene formation in ethylene oligomerization catalyzed by a Cr/PCCP system: a combined experimental and DFT study

Cr catalyst based on a PCCP ligand shows high activity in ethylene oligomerization, giving 1-hexene and considerable C₁₀ fraction. DFT calculation results are consistent with the experimental observations on the distribution of C₁₀ isomers.

Research progress of iron-based catalysts for selective oligomerization of ethylene

Linear α-olefins are widely used as raw materials in the chemical industry. Selective ethylene oligomerization is an important development direction of the linear α-olefin production process. Iron-based catalysts have become a research hotspot in selective ethylene oligomerization due to their advantages like high activity, high selectivity and convenience of adjusting their ligand structures. In this paper, the research progress of catalysts for selective oligomerization of ethylene was reviewed in terms of the cocatalysts, ligand structure, and immobilization of homogeneous catalysts.

Efficient chromium-based catalysts for ethylene tri-/tetramerization switched by silicon-bridged/N,P-based ancillary ligands: a structural, catalytic and DFT study

High performance catalysts switched by a series of silicon-bridged/N,P-based ancillary ligands have been explored.

Synthesis of a mononuclear, non-square-planar chromium(ii) bis(alkoxide) complex and its reactivity toward organic carbonyls and CO2

A rare non-square-planar mononuclear Cr(ii) bis(alkoxide) complex Cr(OR′)₂(THF)₂ is reported and its reactivity with organic carbonyls and CO₂ is investigated.

2D-QSPR/DFT studies of aryl-substituted PNP-Cr-based catalyst systems for highly selective ethylene oligomerization

1-Hexene and 1-octene are important comonomers for the synthesis of high performance polyolefins. Recently, various N-substituted Cr-bis(diphenylphosphino)amine (PNP-Cr) catalysts show the potential as excellent candidates for highly selective ethylene trimerization/tetramerization. In this work, a series of aryl-substituted PNP-Cr catalysts were studied by two-dimensional quantitative structure-property relationship (QSPR) method based on density functional theory (DFT) calculations. The heuristic method (HM) and best multi-linear regression (BMLR) were used to establish the best linear regression models to describe the relationship between selectivities and catalyst structures. Both Cr(I) and Cr(II) active site models for ethylene trimerization/tetramerization were considered. It was found that 1) the relativity and stability of the models were increased by using self-defined descriptors based on DFT calculations; 2) Cr(I)/Cr(III) centers were the most plausible active sites for ethylene trimerization, while Cr(II)/Cr(IV) active sites were most possibly responsible for ethylene tetramerization; and 3) the skeleton structures of the PNP-Cr system with good complanation and symmetry were crucial for achieving excellent catalytic selectivity of 1-octene, while the PNP-Cr backbone with a large steric effect on N atom would benefit ethylene trimerization. Six new PNP ligands with high selectivity toward ethylene trimerization/tetramerization were predicted based on descriptor analysis and the best linear regression models providing a good basis for further development of novel catalyst systems with better performance.

Ethylene Trimerisation with Cr-PNP Catalysts: A Theoretical Benchmarking Study and Assessment of Catalyst Oxidation State

A model for the homogeneous Cr-PNP (PNP = diphosphinoamine) ethylene trimerisation and tetramerisation catalyst system has been studied theoretically, with the aim of identifying suitable density functional theory methods for treatment of this catalyst, and evaluating the likely oxidation and spin states of the active species. Benchmarking studies involving high-level treatment reveal the difficulty of accurately calculating the thermochemistry of this system, and suggest that local density functionals, such as M06L, probably provide the best option. Density functional theory modelling of catalyst activation and the first steps of oligomerisation up until 1-hexene formation appears to favour a CrI–CrIII mechanism, involving spin surface crossing from sextet to quartet states.