Catalytic Systems Based on Chromium(III) Silylated-Diphosphinoamines for Selective Ethylene Tri-/Tetramerization

Chromium Ethylene Tri-/Tetramerization Catalysts Supported by Iminophosphine Ligands

A new class of highly active ethylene tri-/tetramerization chromium catalysts supported by iminophosphine ligands has been studied. The impact of electronic and steric changes of these ligands on selectivity and activity has been investigated by varying P- and/or N-substituents. Upon activation with MMAO, the ligand bearing a P-cyclohexyl group displayed a high activity of 307 kg/(g Cr/h) with a high trimerization selectivity of 92.6%. Decreasing the steric hindrance of N-aryl group led to a decrease in 1-hexene selectivity (74.5%), producing more 1-octene (10.3%). X-ray diffraction analysis verifies that the ligands coordinate with the chromium center in a κ2-P,N mode.

Unveiling meta-Alkyloxy/-Silyloxy-Substituted N-Aryl PNP Ligands for Efficient Cr-Catalyzed Ethylene Tetramerization

Novel N-aryl-functionalized PNP ligands (1-4) bearing m-alkyloxy/-silyloxy substituents were prepared and evaluated for chromium-catalyzed ethylene oligomerization using MMAO-3A as an activator. The selected Cr/PNP system under optimized condition exhibited high 1-octene-selective (up to 70 wt %) ethylene tetramerization at a remarkable rate (over 3000 kg gCr^-1 h^-1). More importantly, the undesirable polyethylene selectivity was restricted to a minimum level of ∼1-2 wt % for pre-catalysts derived with ligands 1 and 2. Employing chlorobenzene as a reaction medium yielded best productivity in conjunction to the total α-olefin (1-C₆ + 1-C₈) selectivity (∼88 wt %). N-aryl PNP ligands (3 and 4) incorporating m-silyloxy substituents in the phenyl ring exhibited relatively poorer tetramerization performance while yielding higher PE fraction as compared to their m-alkyloxy derivatives. A detailed molecular structure of the best-performing pre-catalyst 1-Cr was established by single-crystal X-ray diffraction analysis. The stability of 1/Cr-based catalyst system was investigated for a reaction time of up to 2 h under optimized condition.

Chromium-Based Complexes Bearing Aminophosphine and Phosphine-Imine-Pyrryl Ligands for Selective Ethylene Tri/Tetramerization

A series of Cr-based complexes 6-10 bearing aminophosphine (P,N) ligands Ph₂P-L-NH₂ [L = CH₂CH₂ (1), L = CH₂CH₂CH₂ (2), and L = C₆H₄CH₂ (3)] and phosphine-imine-pyrryl (P,N,N) ligands 2-(Ph₂P-L-N=CH)C₄H₃NH [L = CH₂CH₂CH₂ (4) and L = C₆H₄CH₂ (5)] were prepared, and their catalytic properties were examined for ethylene tri/tetramerization. X-ray crystallographic analysis of complex 8 indicated the κ²-P,N bidentate coordination mode at the Cr(III) center and the distorted octahedral geometry of monomeric P,N-CrCl₃. Upon activation by methylaluminoxane (MAO), complexes 7-8 bearing P,N (PC₃N backbone) ligands 2-3 showed good catalytic reactivity for ethylene tri/tetramerization. On the other hand, complex 6 bearing the P,N (PC₂N backbone) ligand 1 was found active for non-selective ethylene oligomerization, while complexes 9-10 bearing P,N,N ligands 4-5 only produced polymerization products. In particular, the high catalytic activity of 458.2 kg/(g·Cr·h), excellent selectivity of 90.9% (1-hexene and 1-octene combined), and extremely low PE content of 0.1% were obtained with complex 7 in toluene at 45 °C and 45 bar. These results suggest that rational control of P,N and P,N,N ligand backbones, including a carbon spacer and rigidity of a carbon bridge, can lead to the high-performance catalyst for the ethylene tri/tetramerization process.

Nickel-Catalyzed Ethylene Dimerization Based on PNP(NR2)2 Ligands

Nickel (II) complexes stabilized by PNP(NR2)2 (L1: R = Methyl, L2: R = ethyl, L3: R = isopropyl) ligands were synthesized and characterized. A narrow range of products was observed for catalytic systems containing nickel complexes and ethyl aluminum dichloride (EADC). All exhibit considerable activity in the ethylene dimerization to produce 1-butene. Precatalyst 1 is the most conducive for ethylene dimerization, producing 83.4% C4 (1-C4 36.8%) and 103.0 × 105 g/(molNi·h) in terms of its activity under the appropriate conditions. By adjusting the conditions of the catalytic system for precatalyst 2, high C4 selectivity (88.1%) with reasonable activity (76.9 × 105 g/(molNi·h)) can be obtained. The X-ray single-crystal analysis of complexes presents mononuclear bidentate coordination at the Ni center, and the relationship between certain bite angles may also imply catalytic performance.

A rational approach towards selective ethylene oligomerization via PNP-ligand design with an N-triptycene functionality

Novel PNP ligands bearing an N-triptycene backbone were developed and evaluated for selective ethylene oligomerization. Upon activation with MMAO-3A, the pre-catalyst mixture containing Cr(acac)₃/ligand efficiently promotes ethylene tetramerization with remarkably high productivities (up to 1733 kg g_Cr^-1 h^-1) and C₈ olefin selectivities (up to 74.1 wt%). More importantly, ligands with a PNP moiety connecting at the 1- or 1,4-position of the triptycene molecule could achieve exceptionally high alpha (1-C₆ + 1-C₈) selectivities, exceeding 90 wt%, as a result of high 1-C₆ purity (>90 wt%) in the C₆ fraction. Based on comparative catalytic studies employing various PNP ligands with or without an N-triptycene backbone, we illustrate the fact that a rational design of PNP ligands with an optimum degree of steric profile around the N-center could provide C₆ cyclics controlled highly α-selective ethylene oligomerization.

Highly Efficient Ethylene Tetramerization Using Cr Catalysts Constructed with Trifluoromethyl-Substituted N-Aryl PNP Ligands

Tetramerization of ethylene by chromium catalysts stabilized with functionalized N-aryl phosphineamine ligands C₆H₄(m-CF₃)N(PPh₂)₂ (1), C₆H₄(p-CF₃)N(PPh₂)₂ (2), C₆H₄(o-CF₃)N=PPh₂-PPh₂ (3), and C₆H₃(3,5-bis(CF₃))N(PPh₂)₂ (4) was evaluated. The parameter optimization includes temperature, co-catalyst, and solvent. Upon activation with MMAO-3A, the new catalyst system especially with m-functional PNP ligand (1) exhibited high 1-octene selectivity and productivity while giving minimum undesirable polyethylene and C₁₀ ⁺ olefin by-products. Using PhCl as a solvent at 75 °C led to a remarkable α-olefin (1-C₆ + 1-C₈) selectivity (>90 wt %) at a reaction rate of 2000 kg·g_Cr ^-1·h^-1. Under identical conditions, analogous PNP ligands bearing -CH₃, -Et, and -Cl functional moieties at the meta position of the N-phenyl ring displayed significantly lower reactivity. The catalyst with p-functional ligand (2) exhibited lower activity and comparable selectivities, while the Cr/PPN (with ligand 3) system gave no noticeable reactivity. The molecular structure of the precatalyst (1-Cr), exhibiting a monomeric structural feature, was elucidated with the aid of single-crystal X-ray diffraction study.

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...

Replacement of the Common Chromium Source CrCl3(thf)3 with Well-Defined [CrCl2(μ-Cl)(thf)2]2

CrCl₃(thf)₃ is a common starting material in the synthesis of organometallic and coordination compounds of Cr. Deposited as an irregular solid with no possibility of recrystallization, it is not a purity guaranteed chemical, causing problems in some cases. In this work, we disclose a well-defined form of the THF adduct of CrCl₃ ([CrCl₂(μ-Cl)(thf)₂]₂), a crystalline solid, that enables structure determination by X-ray crystallography. The EA data and XRD pattern of the bulk agreed with the revealed structure. Moreover, its preparation procedure is facile: evacuation of CrCl₃·6H₂O at 100 °C, treatment with 6 equivalents of Me₃SiCl in a minimal amount of THF, and crystallization from CH₂Cl₂. The ethylene tetramerization catalyst [iPrN{P(C₆H₄-p-Si(nBu)₃)₂}₂CrCl₂]⁺[B(C₆F₅)₄]⁻ prepared using well-defined [CrCl₂(μ-Cl)(thf)₂]₂ as a starting material exhibited a reliably high activity (6600 kg/g-Cr/h; 1-octene selectivity at 40 °C, 75%), while that of the one prepared using the impure CrCl₃(thf)₃ was inconsistent and relatively low (~3000 kg/g-Cr/h). By using well-defined [CrCl₂(μ-Cl)(thf)₂]₂ as a Cr source, single crystals of [(CH₃CN)₄CrCl₂]⁺[B(C₆F₅)₄]⁻ and [{Et(Cl)Al(N(iPr)₂)₂}Cr(μ-Cl)]₂ were obtained, allowing structure determination by X-ray crystallography, which had been unsuccessful when the previously known CrCl₃(thf)₃ was used as the Cr source.

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.

Chromium complexes supported by bidentate thioether-imine [N,S] ligands: synthesis and ethylene oligomerization studies

Chromium complexes bearing thioether-imine ligands were synthesized and their catalytic behavior in ethylene oligomerization has been investigated, evaluating the effect of the ligand and the experimental parameters on the activity, selectivity, and product distribution.