Our variations on iron and cobalt catalysts toward ethylene oligomerization and polymerization

Oxidation States: Intrinsically Ambiguous?

The oxidation state ( OS ) formalism is a much-appreciated good in chemistry, receiving wide application. However, like all formalisms, limitations are inescapable, some of which have been recently explored. Providing a broader context, we discuss the OS and its interpretation from a computational perspective for transition metal (TM) complexes. We define a broadly applicable and easy-to-use procedure to derive OS s based on quantum chemical calculations, via the use of localized orbitals, dubbed the Intrinsic OS . Applying this approach to a cobalt complex in five OS s, isolated by Hunter and co-workers (Inorg. Chem.2021, 60, 17445), we find that the calculated Intrinsic OS matches the formal OS , consistent with the experimental characterization. Through analysis of the delocalized orbitals, the ligand field of the Co(III) complex is found to be "inverted", despite every cobalt-ligand bond being classically dative from the localized perspective-a bonding scenario very similar to that of [Cu(CF3)4]-. This is not atypical but rather a natural consequence of these metals bonding in the high-valent region, and we propose a more restrictive definition of (locally) inverted bonding. Additionally, two bonding descriptors within the Intrinsic Bonding Orbital (IBO) framework (σ-gain and π-loss) are introduced, which enable facile quantification of electron-sharing covalency across a broad range of TM complexes.

Electronic Tuning of Sterically Encumbered 2-(Arylimino)Pyridine-Nickel Ethylene Polymerization Catalysts by Para-Group Modification

A collection of five related 2-(arylimino)pyridines, 2-{(2,6-(CH(C6H4-p-F)2)2-4- RC6H2)N=CMe}C5H4N, each ortho-substituted with 4,4′-difluorobenzhydryl groups but distinct in the electronic properties of the para-R substituent (R = Me L1, Et L2, i-Pr L3, F L4, OCF3 L5), were prepared and combined with (DME)NiBr2 to form their corresponding LNiBr2 complexes, Ni1–Ni5, in high yields. All the complexes were characterized by FT-IR, 19F NMR spectroscopy and elemental analysis, while Ni5 was additionally the subject of an X-ray determination, revealing a bromide-bridged dimer. The molecular structure of bis-ligated (L4)2NiBr2 (Ni4’) was also determined, the result of ligand reorganization having occurred during attempted crystallization of Ni4. On activation with either EtAlCl2 or MMAO, Ni1–Ni5 exhibited high catalytic activities (up to 4.28 × 106 g of PE (mol of Ni)−1 h−1 using EtAlCl2) and produced highly branched polyethylene exhibiting low molecular weight (Mw range: 2.50–6.18 kg·mol−1) and narrow dispersity (Mw/Mn range: 2.21–2.90). Notably, it was found that the type of para-R group impacted on catalytic performance with Ni5 > Ni4 > Ni3 > Ni1 > Ni2 for both co-catalysts, underlining the positive influence of electron withdrawing substituents. Analysis of the structural composition of the polyethylene by 1H and 13C NMR spectroscopy revealed the existence of vinyl-end groups (–CH=CH2) and high levels of internal unsaturation (–CH=CH–) (ratio of vinylene to vinyl, range: 3.1:1–10.3:1) along with various types of branch (Me, Et, Pr, Bu, 1,4-paired Me, 1,6-paired Me and LCBs). Furthermore, reaction temperature was shown to greatly affect the end group type, branching density, molecular weight and in turn the melting points of the resulting polyethylenes.

2-(Arylimino)benzylidene-8-arylimino-5,6,7-trihydroquinoline Cobalt(II) Dichloride Polymerization Catalysts for Polyethylenes with Narrow Polydispersity

A series of 2-(arylimino)benzylidene-8-arylimino-5,6,7-trihydroquinoline cobalt(II) chlorides (Co1–Co6) containing a fused ring and a more inert phenyl group as the substituent at the imino-C atom has been synthesized using a one-pot synthesis method and fully characterized by FT-IR and elemental analysis. The molecular structures of Co2 and Co5 have been confirmed by X-ray diffraction as having a distorted square pyramidal geometry around a cobalt core with a tridentate N,N,N-chelating ligand and two chlorides. On activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), Co1–Co6 exhibited high activities for ethylene polymerization. The least sterically hindered Co2 showed a maximum activity of 16.51 × 106 g (PE) mol−1 (Co) h−1 at a moderate temperature 50 °C. Additionally, ortho-fluoride Co6 was able to maintain a high activity not only at 70 °C but also after 60 min at 50 °C, highlighting its excellent thermal-stability and long catalytic lifetime. The resultant polyethylene showed clearly narrower molecular weight distribution (PDI: 1.3–3.1) than those produced by structurally related cobalt counterparts, indicating the positive influence of benzhydryl substitution on the catalysis. Moreover, the molecular weight (1.7–386.6 kg mol−1) of vinyl- or n-propyl-terminated polyethylene can be finely regulated by controlling polymerization parameters.

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.

2-(N,N-Diethylaminomethyl)-6,7-trihydroquinolinyl-8-ylideneamine-Ni(ii) chlorides: application in ethylene dimerization and trimerization

The unusual influence of co-catalysts on the oligomerization of ethylene is observed with the title nickel complexes, indicating major dimerization and major trimerization with the co-catalysts MAO and MMAO, respectively.

Plastomeric-like polyethylenes achievable using thermally robust N,N'-nickel catalysts appended with electron withdrawing difluorobenzhydryl and nitro groups

A new set of five unsymmetrical N,N'-diiminoacenaphthenes, 1-[2,6-{(4-FC₆H₄)₂CH}₂-4-NO₂C₆H₄N]-2-(ArN)C₂C₁₀H₆ (Ar = 2,6-Me₂C₆H₃L1, 2,6-Et₂C₆H₃L2, 2,6-ⁱPr₂C₆H₃L3, 2,4,6-Me₃C₆H₂L4, 2,6-Et₂-4-MeC₆H₂L5), have been synthesized and used to prepare their corresponding nickel(ii) halide complexes, LNiBr₂ (Ni1-Ni5) and LNiCl₂ (Ni6-Ni10). The molecular structures of Ni3(OH₂) and Ni4 reveal distorted square pyramidal and tetrahedral geometries, respectively, while the ¹H NMR spectra of all the nickel(ii) (S = 1) complexes show broad paramagnetically shifted peaks. Upon activation with either methylaluminoxane (MAO) or ethylaluminum sesquichloride (Et₃Al₂Cl₂, EASC), Ni1-Ni10 displayed very high activities for ethylene polymerization with the optimal performance being observed using 2,6-dimethyl-containing Ni1 in combination with EASC (1.66 × 10⁷ g PE mol^-1 (Ni) h^-1 at 50 °C) which produced high molecular weight plastomeric polyethylene (M_w = 3.93 × 10⁵ g mol^-1, T_m = 70.6 °C) with narrow dispersity (M_w/M_n = 2.97). Moreover, Ni1/EASC showed good thermal stability by operating effectively at an industrially relevant 80 °C with a level of activity (6.01 × 10⁶ g of PE mol^-1 (Ni) h^-1) that exceeds previously disclosed N,N'-nickel catalysts under comparable reaction conditions. This improved thermal stability and activity has been ascribed to the combined effects imparted by the para-nitro and fluoride-substituted benzhydryl ortho-substituents.

Heterogeneous catalysts for gas-phase conversion of ethylene to higher olefins

The reduced availability of propylene and C4 products from steam crackers continues to provoke on-purpose technologies for light olefins such that almost 30% of propylene in 2025 is predicted to be supplied from unconventional sources. Furthermore, the recent discoveries of natural gas reservoirs have urged interest in the conversion of surplus alkanes and alkenes, especially ethane and ethylene. The direct conversion of ethylene to propylene or a combination of value-added chemicals, including butylenes and oligomers in the range of gasoline and diesel fuel, provides the capability of responding to the fluctuations in the balance between supply and demand of the main petrochemicals. A comprehensive review of heterogeneous catalysts for the gas-phase conversion pathways is presented here in terms of catalytic performances (ethylene conversion and product selectivities), productivities, lifetimes, active sites, physicochemical properties, mechanisms, influence of operating conditions, deactivation and some unresolved/less-advanced aspects of the field. The addressed catalysts cover both zeolitic materials and transition metals, such as tungsten, molybdenum, rhenium and nickel. Efforts in both experimental and theoretical studies are taken into account. Aside from the potential fields of progress, the review reveals very promising performances for the emerging technologies to produce propylene, a mixture of propylene and butenes, or a liquid fuel from ethylene.

From polyethylene waxes to HDPE using an α,α′-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridyl-chromium(iii) chloride pre-catalyst in ethylene polymerisation

Depending on the aluminium-alkyl co-catalyst employed, the title Cr(iii) chlorides can exhibit high catalytic activities in ethylene polymerization, producing either high molecular weight PE or linear PE waxes.

Highly branched unsaturated polyethylenes achievable using strained imino-cyclopenta[b]pyridyl-nickel precatalysts

Highly branched and unsaturated PEs with narrow PDIs have been obtained using nickel catalysts that display high activities, rapid regeneration of active species and high rates of chain isomerization.

Controlling the molecular weights of polyethylene waxes using the highly active precatalysts of 2-(1-aryliminoethyl)-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridylcobalt chlorides: synthesis, characterization, and catalytic behavior

2-(1-aryliminoethyl)-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridylcobalt chlorides polymerized ethylene in high activities, producing useful waxes.

A practical ethylene polymerization for vinyl-polyethylenes: synthesis, characterization and catalytic behavior of α,α′-bisimino-2,3:5,6-bis(pentamethylene)pyridyliron chlorides

A series of α,α′-bis(arylimino)-2,3:5,6-bis(pentamethylene)pyridyliron chlorides exhibits high activities toward ethylene polymerization, yielding highly linear vinyl-polyethylenes.

Thermally stable and highly active cobalt precatalysts for vinyl-polyethylenes with narrow polydispersities: integrating fused-ring and imino-carbon protection into ligand design

Finely tuned cobalt complexes, upon activation with either MAO or MMAO, exhibited high activities up to 80 °C and produced highly linear vinyl-polyethylenes.

Ethylene polymerization by the thermally unique 1-[2-(bis(4-fluoro phenyl)methyl)-4,6-dimethylphenylimino]-2-aryliminoacenaphthylnickel precursors

Finely tuned substituents within ligands control the catalytic performance of the corresponding complex precursors, tailoring resulting polyethylenes.

Nickel-based solid catalysts for ethylene oligomerization – a review

The properties of Ni-based inorganic porous materials and their catalytic performance in ethylene oligomerization are reviewed.

2-(1-Aryliminoethyl)-9-arylimino-5,6,7,8-tetrahydrocycloheptapyridyl iron(ii) dichloride: synthesis, characterization, and the highly active and tunable active species in ethylene polymerization

The title complexes showed high activities towards ethylene polymerization, producing linear polyethylenes.

Tailoring iron complexes for ethylene oligomerization and/or polymerization

Recent progress in the use of iron-based complex pre-catalysts for ethylene reactivity is reviewed, illustrating the current state-of-the-art and the potential usefulness of such systems for delivering solely ethylene oligomerization or polymerization products. The problems associated with the industrial use of late transition metal complex pre-catalysts are generally regarded as catalyst deactivation and the formation of more products of lower molecular weight at elevated temperature. These problems have been addressed for iron-based complex pre-catalysts via the fine tuning of substituents of existing ligands and/or the design of new ligand sets. Results revealed that modified bis(imino)pyridyliron dichlorides were capable of operating at elevated temperatures, and were capable of delivering highly linear polyethylene. Other new models of iron complexes have achieved high activity for ethylene oligomerization and/or polymerization. Particularly successful has been the use of the 2-iminophenanthrolyliron pre-catalyst, which have now been utilized in a 500 tonne pilot plant.