Evaluation of Bis(phosphine) Ligands for Ethylene Oligomerization: Discovery of Alkyl Phosphines as Effective Ligands for Ethylene Tri- and Tetramerization

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.

Chromium-Based Complexes Bearing N-Substituted Diphosphinoamine Ligands for Ethylene Oligomerization

A range of novel N-substituted diphosphinoamine (PNP) ligands Ph₂PN(R)PPh₂ [R = F₂CHCH₂ (1); R = Me₂CHCH₂ (2); R = Me₂CHCH₂CH₂ (3)] have been synthesized via one-step salt elimination reaction. The ligand-coordinated chromium carbonyls [Ph₂PN(R)PPh₂]Cr(CO)₄ (4-6) were further synthesized, and X-ray crystallography analysis of complex 6 revealed the κ²-P,P bidentate binding mode of Cr center and the molecular structure of PNP ligand 3. Then the catalytic ethylene oligomerization behaviors of PNP ligands 1-3 bridging chromium chloride complexes {[Ph₂PN(R)PPh₂]CrCl₂(μ-Cl)}₂ (7-9) were further discussed in depth. Experimental results showed that complex 7 with the strong electron-withdrawing F₂CHCH₂ group can promote the nonselective ethylene oligomerization, while both complex 8 and complex 9 with the electron-donating Me₂CHCH₂ and Me₂CHCH₂CH₂ groups can significantly enhance the selective ethylene tri/tetramerization. The good catalytic activity of 198.3 kg/(g Cr·h), the selectivity toward 1-hexene and 1-octene of 76.4%, and the low PE content of 0.2% were simultaneously achieved with the Al/Cr molar ratio of 600 using the complex 8/MMAO system at 45 °C and 45 bar. These excellent results were mainly attributed to the fact that the β-branching of bridging ligand 2 increased the steric bulk of the N-moiety for complex 8.

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...

Single- and double-bridged PNP ligands in chromium-catalysed ethylene oligomerisation

Chromium catalysts with diazaphospholane ligands have shown good activities and selectivities for ethylene tri- and tetramerisation. Oligomerisations with a doubly N-bridged cyclodiphosphazane result in a Schulz–Flory distribution of α-olefins.

Preparation of Extremely Active Ethylene Tetramerization Catalyst [iPrN(PAr2)2−CrCl2]+[B(C6F5)4]– (Ar = −C6H4-p-SiR3)

Numerous efforts have been made to achieve “on-purpose” 1-octene production since Sasol discovered a Cr-based selective ethylene tetramerization catalyst in the early 2000s. By preparing a series of bis(phosphine) ligands iPrN(PAr2)2 where the Ar contains a bulky –SiR3 substituent (Ar = −C6H4-p-Si(nBu)3 (1), −C6H4-p-Si(1-hexyl)3 (2), −C6H4-p-Si(1-octyl)3 (3), −C6H4-p-Si(2-ethylhexyl)3 (4), −C6H4-p-Si(3,7-dimethyloctyl)3 (5)), we obtained an extremely active catalyst that meets the criteria for commercial utilization. The Cr complexes [iPrN(PAr2)2−CrCl2]+[B(C6F5)4]–, obtained by reacting ligands 1–5 with [(CH3CN)4CrCl2]+[B(C6F5)4]–, showed high activity exceeding 6000 kg/g-Cr/h, when combined with the inexpensive iBu3Al, thus avoiding the use of expensive modified methylaluminoxane (MMAO). The bulky –SiR3 substituents played a key role in the success of catalysis by blocking the formation of inactive species (Cr complexes coordinated by two iPrN(PAr2)2 ligands, that is, [(iPrN(PAr2)2)2−CrCl2]+[B(C6F5)4]–). Among the complexes prepared, [3−CrCl2]+[B(C6F5)4]– exhibited the highest activity (11,100 kg/g-Cr/h, 100 kg/g-catalyst) with high 1-octene selectivity (75 wt%) and, moreover, mitigated the generation of undesired > C10 fractions (10.7 wt%). A 10-g-scale synthesis of 3 was developed, as well as a facile and low-cost synthetic method for [(CH3CN)4CrCl2]+[B(C6F5)4]–.

Highly regio- and stereoselective phosphinylphosphination of terminal alkynes with tetraphenyldiphosphine monoxide under radical conditions

The homolytic cleavage of the P^V(O)–P^III bond in tetraphenyldiphosphine monoxide simultaneously provides both pentavalent and trivalent phosphorus-centered radicals with different reactivities. The method using V-40 as an initiator is successfully investigated for the regio- and stereoselective phosphinylphosphination of terminal alkynes giving the corresponding trans-isomers of 1-diphenylphosphinyl-2-diphenylthiophosphinyl-1-alkenes in good yields. The protocol can be applied to a wide variety of terminal alkynes including both alkyl- and arylalkynes.

Magic of Alpha: The Chemistry of a Remarkable Bidentate Phosphine, 1,2-Bis(di-tert-butylphosphinomethyl)benzene

The bidentate phosphine ligand 1,2-bis(di-tert-butylphosphinomethyl)benzene (1,2-DTBPMB) has been reported over the years as being one of, if not the, best ligands for achieving the alkoxycarbonylation of various unsaturated compounds. Bonded to palladium, the ligand provides the basis for the first step in the commercial (Alpha) production of methyl methacrylate as well as very high selectivity to linear esters and acids from terminal or internal double bonds. The present review is an overview covering the literature dealing with the 1,2-DTBPMB ligand: from its first reference, its catalysis, including the alkoxycarbonylation reaction and its mechanism, its isomerization abilities including the highly selective isomerizing methoxycarbonylation, other reactions such as cross-coupling, recycling approaches, and the development of improved, modified ligands, in which some tert-butyl ligands are replaced by 2-pyridyl moieties and which show exceptional rates for carbonylation reactions at low temperatures.

Ethylene oligomerisation chromium catalysts with unsymmetrical PCNP ligands

Chromium(iii) complexes of chelating diphosphines, with PNP or PCNCP backbones, are excellent catalysts for ethylene tetra- and/or trimerisations. A missing link within this ligand series are unsymmetric chelating diphosphines based on a PCNP scaffold. New bidentate PCNP ligands of the type Ph₂PCH₂N(R)PPh₂ (R = 1-naphthyl or 5-quinoline groups, 2a-d) have been synthesised and shown to be extremely effective ligands for ethylene tri-/tetramerisations. Three representative tetracarbonyl Cr⁰ complexes bearing a single PN(R)P (5), PCN(R)P (6), or PCN(R)CP (7) diphosphine (R = 1-naphthyl) have been prepared from Cr(CO)₄(η⁴-nbd) (nbd = norbornadiene). Furthermore we report a single crystal X-ray diffraction study of these compounds and discuss their structural parameters.

Computer-assisted catalyst development via automated modelling of conformationally complex molecules: application to diphosphinoamine ligands

Simulation of conformationally complicated molecules requires multiple levels of theory to obtain accurate thermodynamics, requiring significant researcher time to implement. We automate this workflow using all open-source code (XTBDFT) and apply it toward a practical challenge: diphosphinoamine (PNP) ligands used for ethylene tetramerization catalysis may isomerize (with deleterious effects) to iminobisphosphines (PPNs), and a computational method to evaluate PNP ligand candidates would save significant experimental effort. We use XTBDFT to calculate the thermodynamic stability of a wide range of conformationally complex PNP ligands against isomeriation to PPN (ΔG_PPN), and establish a strong correlation between ΔG_PPN and catalyst performance. Finally, we apply our method to screen novel PNP candidates, saving significant time by ruling out candidates with non-trivial synthetic routes and poor expected catalytic performance.

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.

Polyolefin Analyses with a 10 mm Multinuclear NMR Cryoprobe

Polyolefins are important and broadly used materials. Their molecular microstructures have direct impact on macroscopic properties and dictate end-use applications. ¹³C NMR is a powerful analytical technique used to characterize polyolefin microstructures, such as long-chain branching (LCB), but it suffers from low sensitivity. Although the ¹³C sensitivity of polyolefin samples can be increased by about 5.5 times with a cryoprobe, when compared with a conventional broadband observe (BBO) probe, further sensitivity enhancement is in high demand for studying increasingly complex polyolefin microstructures. Toward this goal, distortionless enhancement by polarization transfer (DEPT) and refocused insensitive nuclei enhanced by polarization transfer (RINEPT) are explored. The use of hard, regular, and new short adiabatic 180° ¹³C pulses in DEPT and RINEPT is investigated. It is found that RINEPTs perform better than DEPTs and a sensitivity enhancement of 3.1 can be achieved with RINEPTs. The results of RINEPTs are further analyzed with statistics software JMP and recommendations for optimal usage of RINEPTs are suggested. An example of analyzing saturated chain ends in an ethylene-octene copolymer sample with a hard 180° ¹³C RINEPT pulse is demonstrated. It is shown that the experimental time can be further reduced in half because of faster proton relaxation, where the total experimental time is about 580 times shorter when compared to using a conventional method and a 10 mm BBO probe. A naturally abundant nitrogen-containing polyolefin is analyzed using ¹H-¹⁵N HMBC and, to our knowledge, is the first ¹H-¹⁵N HMBC presented in the field of polyolefin characterization. The relative amount of similar nitrogen-containing structures is quantified by two-dimensional integration of ¹H-¹⁵N HMBC. Two pragmatic technical challenges related to using high-sensitivity NMR cryoprobes are also addressed: (1) A new ¹H decoupling sequence Bi_Waltz_65_256pl is proposed to address decoupling artifacts in ¹³C{¹H} NMR spectra which contain a strong ¹³C signal with a high signal-to-noise ratio (S/N). (2) A simple pulse sequence that affords zero-slope spectral baselines and quantitative results is presented to address acoustic ringing that is often associated with high-sensitivity cryoprobe use.