Exploring fluoride effects in sterically enhanced cobalt ethylene polymerisation catalysts; a combined experimental and DFT study

The fluoro-substituted 2,6-bis(arylimino)pyridine dichlorocobalt complexes, [2-{CMeN(2,6-(Ph₂CH)₂-3,4-F₂C₆H)}-6-(CMeNAr)C₅H₃N]CoCl₂ (Ar = 2,6-Me₂C₆H₃ Co1, 2,6-Et₂C₆H₃Co2, 2,6-iPr₂C₆H₃Co3, 2,4,6-Me₃C₆H₂Co4, 2,6-Et-4-MeC₆H₂Co5), were synthesized in good yield from the corresponding unsymmetrical N,N,N'-ligands, L1-L5. Besides characterization of Co1-Co5 by FT-IR spectroscopy, ¹⁹F NMR spectroscopy and elemental analysis, the molecular structures of Co2 and Co5 were also determined highlighting the unsymmetrical nature of the terdentate ligand and the pseudo-square pyramidal geometry about the metal center. When either MAO or MMAO were employed as activators, Co1-Co5 were able to achieve a wide range of catalytic activities for ethylene polymerisation. Co5/MAO exhibited the highest activity of the study at 60 °C (7.6 × 10⁶ g PE mol^-1 (Co) h^-1) which decreased to 3.3 × 10⁶ g PE mol^-1 (Co) h^-1 at 80 °C. In addition, it was found that the polymerisation activity increased as the steric hindrance imparted by the ortho groups was enhanced (for MMAO: Co3 > Co5 > Co2 > Co1 > Co4), a finding that was supported by DFT calculations. Furthermore, it was shown that particularly high molecular weight polyethylene could be generated (up to 483.8 kg mol^-1) when using Co5/MMAO at 30 °C, while narrow dispersities (M _w/M _n range: 1.8-4.7) and high linearity (T _m > 131.4 °C) were a feature of all polymers produced. By comparison of Co3 with its non-fluorinated analogue using experimental data and DFT calculations, the substitution of fluorides at the meta- and para-positions was demonstrated to boost catalytic activity and improve thermal stability.

Bis(imino)-6,7-dihydro-5H-quinoline-cobalt complexes as highly active catalysts for the formation of vinyl-terminated PE waxes; steps towards inhibiting deactivation pathways through targeted ligand design

A set of five related bis(imino)-6,7-dihydro-5H-quinoline-cobalt(ii) complexes, [2-(ArN = CPh)-8-(NAr)-C9H8N]CoCl2 (Ar = 2,6-Me2C6H3Co1, 2,6-Et2C6H3Co2, 2,6-i-Pr2C6H3Co3, 2,4,6-Me3C6H2Co4, 2,6-Et2-4-MeC6H2Co5), have been synthesized in reasonable yield by the template reaction of cobalt(ii) chloride hexahydrate, 2-benzoyl-6,7-dihydro-5H-quinolin-8-one and the corresponding aniline. The molecular structures of Co1 and Co4 highlight both the differences in the two imino-carbon environments (phenyl-capped chain vs. cyclic) and also the steric properties exerted by the bulky N imine-aryl groups. On pre-treatment with either modified methylaluminoxane (MMAO) or methylaluminoxane (MAO), all complexes proved productive catalysts for the polymerization of ethylene. In particular, Co1/MAO was the most active reaching a very high level of 1.62 × 107 g PE per mol (Co) per h over a 30 minute run time. Owing to the presence of the imino-phenyl substituent, Co1-Co5 were able to exhibit good thermal stability by displaying appreciable catalytic activity at temperatures between 50 and 80 °C, generating polyethylenes with narrow dispersities (M w/M n range: 1.66-3.28). In particular, the least sterically bulky precatalysts, Co1 and Co4 formed polyethylene waxes (M w range: 1.94-5.69 kg per mol) with high levels of vinyl unsaturation as confirmed by high temperature 1H/13C NMR spectroscopy and by IR spectroscopy.

Enhancing Ethylene Polymerization of NNN-Cobalt(II) Precatalysts Adorned with a Fluoro-substituent

Unsymmetrical 2-(1-(2,4-dibenzhydryl-6-fluorophenylimino)ethyl)-6-(1-alkylphenyl-imino)ethyl)pyridine compounds (Ar = 2,6-Me₂C₆H₃ in L1; 2,6-Et₂C₆H₃ in L2; 2,6- ⁱ Pr₂C₆H₃ in L3; 2,4,6-Me₃C₆H₂ in L4; 2,6-Et₂-4-Me-C₆H₂ in L5) were prepared and characterized. The treatment of CoCl₂ with the compounds L1-L5 afforded the corresponding cobalt complexes Co1-Co5 in excellent yields. The molecular structures of Co3 and Co4 were determined by single-crystal X-ray diffraction, revealing the distorted-square-pyramidal geometry with three nitrogen atoms and two chlorine atoms around the cobalt center. Compared with previous bis(imino)pyridylcobalt analogues, all of the cobalt precatalysts displayed exceptionally higher activities toward ethylene polymerization with 1.32 × 10⁷ g (PE) mol^-1 (Co) h^-1 at 60 °C in the presence of a co-catalyst MAO or MMAO. These cobalt catalysts produced highly linear polyethylene (PE) waxes with vinyl end groups and low molecular weight (M _w up to 8.23 kg mol^-1) along with a relatively lower melting point (all-round T _ms < 128 °C). The narrow dispersity of resultant polyethylenes indicated the single-site active species of the catalytic system.

Achieving polydispersive HDPE by N,N,N-Co precatalysts appended with N-2,4-bis(di(4-methoxyphenyl)methyl)-6-methylphenyl

A family of unsymmetrical 2-(2,4-bis(di(4-methoxyphenyl)methyl)-6-MeC₆H₂N)-6-(1-(arylimino)ethyl)pyridine-cobalt dichloride complexes has been synthesized and characterized by NMR spectroscopy, FT-IR spectroscopy and elemental analysis as well as single crystal X-ray diffraction for Co2 and Co4. Activated with either MAO or MMAO, all the cobalt precatalysts displayed high activities toward ethylene polymerization and produced highly linear polyethylenes with high molecular weights as well as wide polydispersities; for example, the performance using Co1/MAO at 50 °C reached 9.17 × 10⁶ g PE (mol of Co)⁻¹ h⁻¹ with the production polyethylene of molecular weight as high as M_w = 3.14 × 10⁵ g mol⁻¹, T_m = 134.3 °C besides its wide polydispersity of M_w/M_n of 54.6. Besides the terminal vinyl group of the resultant polyethylenes, it is rare for a late-transition metal catalyst to achieve highly linear polyethylenes with not only wide polydispersity but also high molecular weights, being similar to high-density polyethylenes produced using Phillips catalyst.

Introducing a practical application of N,N,N-Co precatalysts for highly linear polyethylenes with wide polydispersity and high molecular weights, targeting HDPE using Phillips catalyst.

6-Arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)-iminopyridylmetal (Fe and Co) Complexes as Highly Active Precatalysts for Ethylene Polymerization: Influence of Metal and/or Substituents on the Active, Thermostable Performance of Their Complexes and Resultant Polyethylenes

A series of 6-arylimino-2-(2-(1-phenylethyl)naphthalen-1-yl)iminopyridines and their iron(II) and cobalt(II) complexes (Fe1–Fe5, Co1–Co5) were synthesized and routinely characterized as were Co3 and Co5 complexes, studied by single crystal X-ray crystallography, which individually displayed a distorted square pyramidal or trigonal bipyramid around a cobalt center. Upon treatment with either methyluminoxane (MAO) or modified methyluminoxane (MMAO), all complexes displayed high activities regarding ethylene polymerization even at an elevated temperature, enhancing the thermostability of the active species. In general, iron precatalysts showed higher activities than their cobalt analogs; for example, 10.9 × 10⁶ g(PE) mol⁻¹ (Co) h⁻¹ by Co4 and 17.0 × 10⁶ g(PE) mol⁻¹ (Fe) h⁻¹ by Fe4. Bulkier substituents are favored for increasing the molecular weights of the resultant polyethylenes, such as 25.6 kg mol⁻¹ obtained by Co3 and 297 kg mol⁻¹ obtained by Fe3. A narrow polydispersity of polyethylenes was observed by iron precatalysts activated by MMAO, indicating a single-site active species formed.

High molecular weight polyethylenes of narrow dispersity promoted using bis(arylimino)cyclohepta[b]pyridine-cobalt catalysts ortho-substituted with benzhydryl & cycloalkyl groups

A one-pot template strategy has been utilized to synthesize sterically enhanced bis(imino)cyclohepta[b]pyridine-cobalt(ii) chlorides, [2-{(Ar)N[double bond, length as m-dash]CMe}-9-{N(Ar)}C10H10N]CoCl2 (Ar = 2-(C5H9)-4,6-(CHPh2)2C6H2Co1, 2-(C6H11)-4,6-(CHPh2)2C6H2Co2, 2-(C8H15)-4,6-(CHPh2)2C6H2Co3, 2-(C12H23)-4,6-(CHPh2)2C6H2Co4, 2,6-(C5H9)2-4-(CHPh2)C6H2Co5). All five complexes have been characterized by a combination of FT-IR spectroscopy, elemental analysis and single crystal X-ray diffraction. The molecular structures of Co1, Co3 and Co5 highlight the substantial steric hindrance imparted by the 2-cycloalkyl-6-benzhydryl or 2,6-dicyclopentyl ortho-substitution pattern; distorted square pyramidal geometries are exhibited in each case. On activation with methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all the complexes (apart from Co4/MAO) were active ethylene polymerization catalysts (up to 3.70 × 106 g PE per mol (Co) per h for Co5/MMAO), operating effectively at temperatures between 50 °C and 60 °C, producing polyethylenes with high molecular weights (up to 589.5 kg mol-1 for Co3/MAO). Furthermore, all polymers were highly linear (Tm > 130 °C) with narrow dispersities (Mw/Mn range: 2.0-3.0). The coexistence of two chain termination pathways, β-H elimination and transfer to aluminum, has been demonstrated using 13C/1H NMR spectroscopy.

4,4'-Difluorobenzhydryl-modified bis(imino)-pyridyliron(ii) chlorides as thermally stable precatalysts for strictly linear polyethylenes with narrow dispersities

The 4,4'-difluorobenzhydryl-modified bis(imino)pyridylferrous chlorides, [2-{CMeN(2,6-(4-FC6H4)2CH2-4-t-BuC6H2)}-6-(CMeNAr)C5H3N] FeCl2 (Ar = 2,6-Me2C6H3Fe1, 2,6-Et2C6H3Fe2, 2,6-i-Pr2C6H3Fe3, 2,4,6-Me3C6H2Fe4, 2,6-Et-4-MeC6H2Fe5 and 2,6-(4-FC6H4)2CH2-4-t-BuC6H2Fe6), were synthesized in good yields. All iron complexes were characterized by 1H/19F NMR and elemental analysis, and the molecular structures of representative complexes Fe1 and Fe6 were determined by single crystal X-ray diffraction, which revealed a slightly distorted square pyramid around the iron center. Activated with either MAO or MMAO, Fe1-Fe5 exhibited very high activities (up to 17.2 × 106 g (PE) mol-1 (Fe) h-1 for Fe1/MMAO) toward ethylene polymerization, producing highly linear polyethylenes with narrow dispersities as required by industry and value added PEs. Importantly, the Fe1/MAO maintained an activity of 9.5 × 106 g (PE) mol-1 (Fe) h-1 at 100 °C of operating temperature, making the catalytic system suitable for practical application. Simulation quantitatively revealed the mechanism of the enhanced catalytic performance from the electronic and steric point of view.

Synthesis of characteristic polyisoprenes using rationally designed iminopyridyl metal (Fe and Co) precatalysts: investigation of co-catalysts and steric influence on their catalytic activity

A series of iminopyridyl ligand-based iron and cobalt complexes are utilized as catalysts in isoprene polymerization, in which the microstructure of resultant polyisoprenes was investigated.

Probing the effect of ortho-cycloalkyl ring size on activity and thermostability in cycloheptyl-fused N,N,N-iron ethylene polymerization catalysts

The depicted iron(ii) precatalysts displayed exceptionally high activities for ethylene polymerization at temperatures of up 100 °C producing linear polyethylene with a wide range of molecular weights.

Activity and Thermal Stability of Cobalt(II)-Based Olefin Polymerization Catalysts Adorned with Sterically Hindered Dibenzocycloheptyl Groups

Five examples of unsymmetrical 2-(2,4-bis(dibenzocycloheptyl)-6-methylphenyl- imino)ethyl)-6-(1-(arylyimino)ethyl)pyridine derivatives (aryl = 2,6-Me₂C₆H₃ in L1; 2,6-Et₂C₆H₃ in L2; 2,6-i-Pr₂C₆H₃ in L3; 2,4,6-Me₃C₆H₂ in L4 and 2,6-Et₂-4-MeC₆H₂ in L5) were prepared and characterized. Treatment with CoCl₂ offered the corresponding cobalt precatalysts Co1–Co5, which were characterized by FT-IR and NMR spectroscopy as well as elemental analysis. The molecular structures of Co3 and Co4 determined by single crystal X-ray diffraction revealed distorted square pyramidal geometries with τ₅ values of 0.052–0.215. Activated with either MAO or MMAO, the precatalysts displayed high activities in ethylene polymerization, where Co1 with the least bulky substituents exhibited a peak activity of 1.00 × 10⁷ g PE mol⁻¹ (Co) h⁻¹ at 60 °C. With MAO as a cocatalyst, the activity was reduced only by one order of magnitude at 90 °C, which implies thermally stable active sites. The polymerization product was highly linear polyethylene with vinyl end groups. Co3 with the most sterically hindered active sites was capable of generating polyethylene of high molecular weight, reaching 6.46 × 10⁵ g mol⁻¹. Furthermore, high melting point and unimodal molecular weight distribution were observed in the resulting polyethylene. It must be stressed that the thermal stability of the catalyst and the molecular weight of the obtained polyethylene attain the highest values reported for the unsymmetrical 2,6-bis(imino)pyridylcobalt (II) chloride precatalysts.