Ethylene oligomerisation chromium catalysts with unsymmetrical PCNP ligands

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.

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.

The Backbone of Success of P,N-Hybrid Ligands: Some Recent Developments

Organophosphorus ligands are an invaluable family of compounds that continue to underpin important roles in disciplines such as coordination chemistry and catalysis. Their success can routinely be traced back to facile tuneability thus enabling a high degree of control over, for example, electronic and steric properties. Diphosphines, phosphorus compounds bearing two separated P^III donor atoms, are also highly valued and impart their own unique features, for example excellent chelating properties upon metal complexation. In many classical ligands of this type, the backbone connectivity has been based on all carbon spacers only but there is growing interest in embedding other donor atoms such as additional nitrogen (–NH–, –NR–) sites. This review will collate some important examples of ligands in this field, illustrate their role as ligands in coordination chemistry and highlight some of their reactivities and applications. It will be shown that incorporation of a nitrogen-based group can impart unusual reactivities and important catalytic applications.

Low-Dimensional Architectures in Isomeric cis-PtCl2{Ph2PCH2N(Ar)CH2PPh2} Complexes Using Regioselective-N(Aryl)-Group Manipulation

The solid-state behaviour of two series of isomeric, phenol-substituted, aminomethylphosphines, as the free ligands and bound to Pt^II, have been extensively studied using single crystal X-ray crystallography. In the first library, isomeric diphosphines of the type Ph₂PCH₂N(Ar)CH₂PPh₂ [1a–e; Ar = C₆H₃(Me)(OH)] and, in the second library, amide-functionalised, isomeric ligands Ph₂PCH₂N{CH₂C(O)NH(Ar)}CH₂PPh₂ [2a–e; Ar = C₆H₃(Me)(OH)], were synthesised by reaction of Ph₂PCH₂OH and the appropriate amine in CH₃OH, and isolated as colourless solids or oils in good yield. The non-methyl, substituted diphosphines Ph₂PCH₂N{CH₂C(O)NH(Ar)}CH₂PPh₂ [2f, Ar = 3-C₆H₄(OH); 2g, Ar = 4-C₆H₄(OH)] and Ph₂PCH₂N(Ar)CH₂PPh₂ [3, Ar = 3-C₆H₄(OH)] were also prepared for comparative purposes. Reactions of 1a–e, 2a–g, or 3 with PtCl₂(η⁴-cod) afforded the corresponding square-planar complexes 4a–e, 5a–g, and 6 in good to high isolated yields. All new compounds were characterised using a range of spectroscopic (¹H, ³¹P{¹H}, FT–IR) and analytical techniques. Single crystal X-ray structures have been determined for 1a, 1b∙CH₃OH, 2f∙CH₃OH, 2g, 3, 4b∙(CH₃)₂SO, 4c∙CHCl₃, 4d∙½Et₂O, 4e∙½CHCl₃∙½CH₃OH, 5a∙½Et₂O, 5b, 5c∙¼H₂O, 5d∙Et₂O, and 6∙(CH₃)₂SO. The free phenolic group in 1b∙CH₃OH, 2f∙CH₃OH_,2g, 4b∙(CH₃)₂SO, 5a∙½Et₂O, 5c∙¼H₂O, and 6∙(CH₃)₂SO exhibits various intra- or intermolecular O–H∙∙∙X (X = O, N, P, Cl) hydrogen contacts leading to different packing arrangements.