Unique catalytic behavior of chromium complexes having halogenated bis(imino)pyridine ligands for ethylene polymerization

Reactions of CrCl₃(thf)₃ with bis(imino)pyridines gave a series of {bis(imino)pyridine}chromium(III) trichloride complexes, {2,6‐(RN?CMe)₂C₅H₃N}CrCl₃ [R = C₆HPr₂‐2,6 ( 1 ), C₆H₃Et₂‐2,6 ( 2 ), C₆H₃Me₂‐2,6 ( 3 ), C₆H₂Me₃‐2,4,6 ( 4 ), C₆H₃Me₂‐3,5 ( 5 ), C₆H₅ ( 6 ), cyclohexyl ( 7 ), 2‐methyl‐1‐naphthyl ( 8 ), C₆H₃F₂‐2,6 ( 9 ), C₆H₃Br₂‐2,6 ( 10 ), C₆F₅ ( 11 )]. Pseudo‐octahedral geometries of 6 , 10 , and 11 were revealed by X‐ray crystallography. The complexes having bulky substituents such as 1 – 4 showed high activity for ethylene polymerization in combination with modified methylaluminoxane (MMAO) to give linear polyethylenes. In sharp contrast, the pentafluorophenyl complex 11 /modified methylaluminoxane system was found to be moderately active for ethylene homopolymerization to give moderately branched polyethylene with only ethyl branches. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3368–3375, 2005     

Pyridine bis(imino) iron and cobalt complexes for ethylene polymerization: influence of the aryl imino substituents

In the present paper, the synthesis of new pyridine bis(imine) ligands modified with halogens (Cl, Br, CF₃) or alkyl groups (Heptyl, tert-butyl, Phenyl, …) is reported. When coordinated with iron or cobalt dichloride, they yielded complexes which were associated to methylaluminoxane (MAO) to achieve the polymerization of ethylene. It was shown that cobalt catalysts are generally more sensitive to the ligand substitutions than the iron ones. The addition of a chlorine atom on the ligand frame is generally unfavorable. On the contrary, the presence of a bromine atom seems more favorable. Phenyl rings lead to almost completely inactive catalysts, probably because of a too weak coordination to the metal. It was also demonstrated that a mono-substitution of the aryl groups with an electron-withdrawing group (-CF₃) is sufficient to yield polymers, whereas, considering the bulkiness of this substituent only, oligomers would have been expected.     

Synthesis, electronic structure, and ethylene polymerization activity of bis(imino)pyridine cobalt alkyl cations

A new spin on polymers: the title cations comprise low-spin Co(II) centers with neutral bis(imino)pyridine chelating ligands. These complexes serve as single-component ethylene polymerization catalysts and offer insight into the mechanism of chain growth and catalyst deactivation, which occurs by forming inactive cationic bis(imino)pyridine cobalt complexes with a diethyl ether ligand.     

Enantiopure C1-symmetric bis(imino)pyridine cobalt complexes for asymmetric alkene hydrogenation

Enantiopure C(1)-symmetric bis(imino)pyridine cobalt chloride, methyl, hydride, and cyclometalated complexes have been synthesized and characterized. These complexes are active as catalysts for the enantioselective hydrogenation of geminal-disubstituted olefins.     

Photolysis and thermolysis of bis(imino)pyridine cobalt azides: C-H activation from putative cobalt nitrido complexes

A series of planar aryl-substituted bis(imino)pyridine cobalt azide complexes were prepared and evaluated as synthetic precursors for the corresponding cobalt nitrido compounds. Thermolysis or photolysis of two examples resulted in intramolecular C-H activation of the benzylic positions of the aryl substituents. For the mesityl-substituted compound, C-H activation by the putative nitride resulted in formation of a neutral imine ligand and modification of the chelate by hydrogen transfer to the imine carbon.     

Polymerization of ethylene by SiO2-supported two-component catalytic systems containing bis(imino)pyridine and bis(imine) ligands

The polymerization of ethylene initiated by SiO₂-supported two-component catalytic systems based on 2,6-bis[1-(2,4-dimethyl-6-cyclohexylphenylimino)ethyl]pyridine iron (II) chloride (I) and 1,2-bis(2-cyclohexyl-4,6-dimethylphenylimino)acenaphthene] nickel bromide (II) was studied. Methylaluminoxane was used as a cocatalyst during support. It was shown that the activity of two-component catalytic systems and the molecular mass and short-chain branching of polyethylene samples depend on the supporting procedure: simultaneous immobilization of components I and II, separate immobilization of components on the support (first I, then II, and vice versa), and the use of a mixture of components I and II immobilized separately on SiO₂.     

Homogeneous and supported catalysts based on bis(imino)pyridyl Iron(II) complexes for ethylene polymerization

Data on ethylene polymerization on homogeneous and supported catalysts based on 2,6-bis(imino)pyridyl Fe(II) complexes activated by trialkylaluminums are considered (activity, the molecular-weight characteristics of polymers, the number of active sites, and the propagation rate constants). Unlike homogeneous systems, the supported catalysts prepared with the use of various carriers (SiO₂, Al₂O₃, and MgCl₂) exhibited high stability and activity at 70–80°C and produced high-molecular-weight polyethylene with a broad molecular-weight distribution (MWD). The molecular weights and MWDs of polymers and the propagation rate constant depended on the nature of the carrier only slightly. The reasons for an unusual effect of an increase in the activity of the supported catalysts in ethylene polymerization in the presence of hydrogen are discussed.     

Catalytic performance of bis(imino)pyridine Fe/Co complexes toward ethylene polymerization by 2D-/3D-QSPR modeling

The two-dimensional and three-dimensional quantitative structure–property relationship (2D- and 3D-QSPR) approaches are applied to investigate the catalytic performance for a total data set of 55 bis(imino)pryridine iron and cobalt complexes, including the catalytic activity, molecular weight, and melting temperature of the product. The obtained models for the catalytic performance of interest exhibit good results by both 2D- and 3D-QSPR modeling, meanwhile higher predictive and validation powers observed in the 3D type. The modeling results indicate that the bulky substituents on ortho-position of the singular side phenyl ring and positive charge on para-position of the phenyl ring within the ligand are favorable to catalytic activity, while unfavorable to the molecular weight of product. Based on the obtained QSPR models, four new complexes are designed and predicted with good catalytic activity and very high molecular weight, which are in good agreement with our recent experimental report. © 2019 Wiley Periodicals, Inc.     

Bis(pyrazolyl)pyridine vanadium(III) complexes as highly active ethylene polymerization catalysts

The synthesis, characterization and catalytic activity in ethylene polymerization of novel mononuclear vanadium complexes bearing N^∧N^∧N-tridentate (pyrazolyl-pyridine) ligands are described. With AlEtCl₂ as co-catalyst, complexes 1 and 2 produce single-site catalysts that polymerized ethylene affording high density polyethylene with fairly narrow molecular weight distribution.     

Oxidation and reduction of bis(imino)pyridine iron dicarbonyl complexes

The oxidation and reduction of a redox-active aryl-substituted bis(imino)pyridine iron dicarbonyl has been explored to determine whether electron-transfer events are ligand- or metal-based or a combination of both. A series of bis(imino)pyridine iron dicarbonyl compounds, [((iPr)PDI)Fe(CO)(2)](-), ((iPr)PDI)Fe(CO)(2), and [((iPr)PDI)Fe(CO)(2)](+) [(iPr)PDI = 2,6-(2,6-(i)Pr(2)C(6)H(3)N═CMe)(2)C(5)H(3)N], which differ by three oxidation states, were prepared and the electronic structures evaluated using a combination of spectroscopic techniques and, in two cases, [((iPr)PDI)Fe(CO)(2)](+) and [((iPr)PDI)Fe(CO)(2)], metrical parameters from X-ray diffraction. The data establish that the cationic iron dicarbonyl complex is best described as a low-spin iron(I) compound (S(Fe) = ?) with a neutral bis(imino)pyridine chelate. The anionic iron dicarbonyl, [((iPr)PDI)Fe(CO)(2)](-), is also best described as an iron(I) compound but with a two-electron-reduced bis(imino)pyridine. The covalency of the neutral compound, ((iPr)PDI)Fe(CO)(2), suggests that both the oxidative and reductive events are not ligand- or metal-localized but a result of the cooperativity of both entities.     

Dramatic effect of heteroatom backbone substituents on the ethylene polymerization behavior of bis(imino)pyridine iron catalysts

Bis(imino)pyridine iron complexes bearing ether and thioether backbone substituents have been synthesized and evaluated for the polymerization of ethylene. The methoxy derivative is inactive whereas bulky phenoxides or thioether derivatives afford activities as high as the most active systems reported to date.     

Square planar bis(imino)pyridine iron halide and alkyl complexes

Square planar iron methyl complexes containing bis(imino)pyridine (PDI) ligands have been prepared by reductive alkylation of the corresponding ferrous dichloride; dialkylation is observed upon treatment with a larger alkyl lithium.     

Influence of the para-substitution in bis(arylimino)pyridine iron complexes on the catalytic oligomerization and polymerization of ethylene

A series of nine bis(arylimino)pyridine iron complexes containing halogen or alkynyl substituents in their ligand frameworks was synthesized and characterized. After activation with methylalumoxane (MAO), these catalysts oligomerize or polymerize ethylene to give highly linear products. The introduction of halogen or alkynyl substituents in the para-position of the iminophenyl rings has a great influence on the polymerization activities of the corresponding iron complexes.     

Oligomerisation of ethylene by bis(imino)pyridyliron and -cobalt complexes

A series of bis(imino)pyridyliron and -cobalt complexes [[2,6-(CR=NAr)2C5H3N]MX2] (R=H, Me; M=Fe, Co; X=Cl, Br) 8-16 containing imino-aryl rings (Ar) with at least one small ortho substituent, as well as Ar=biphenyl and Ar=naphthyl, has been synthesised. Crystallographic analyses of complexes 9 (Ar = 2,3-dimethylphenyl), 13 and 14 (Ar= biphenyl; X= Cl or Br, respectively) reveal a distorted trigonal-bipyramidal geometry in the solid state. These complexes, in combination with methyl aluminoxane (MAO), are active catalysts for the oligomerisation of ethylene, yielding >99% linear alpha-olefin mixtures that follow a Schulz-Flory distribution. Iron ketimine (R = Me) precatalysts give the highest activities and a greater alpha-value than their aldimine (R = H) analogues. Cobalt precatalysts follow a similar trend, though their activities are almost two orders of magnitude lower than those of the corresponding iron catalysts. Ethylene pressure studies on cobalt precatalyst 15 reveal a first-order dependence on ethylene for both the rate of propagation and the rate of chain transfer, and a pressure independence of the alpha value.     

Multi‐center nature of ethylene polymerization catalysts based on 2,6‐bis(imino)pyridyl complexes of iron and cobalt

2,6‐Bis(imino)pyridyl complexes of Fe and Co in combination with methylalumoxane form very active homogeneous catalytic systems for polymerization of ethylene. GPC analysis of the polymers prepared with the complexes indicates that the Co complexes produce single‐center catalysts whereas the Fe complexes produce catalysts with numerous types of active centers. Different centers in the latter catalyst systems respond differently to reaction conditions such as the reaction duration, the [MAO]:[Fe] ratio, the ethylene concentration, etc. The article examines the effects of reaction variables on the performance of both types of catalysts and proposes an explanation for the complex behavior of the catalysts derived from the Fe complexes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6159–6170, 2006     

O2 activation by bis(imino)pyridine iron(II)-thiolate complexes

The new iron(II)-thiolate complexes [((iPr)BIP)Fe(II)(SPh)(Cl)] (1) and [((iPr)BIP)Fe(II)(SPh)(OTf)] (2) [BIP = bis(imino)pyridine] were prepared as models for cysteine dioxygenase (CDO), which converts Cys to Cys-SO(2)H at a (His)(3)Fe(II) center. Reaction of 1 and 2 with O(2) leads to Fe-oxygenation and S-oxygenation, respectively. For 1 + O(2), the spectroscopic and reactivity data, including (18)O isotope studies, are consistent with an assignment of an iron(IV)-oxo complex, [((iPr)BIP)Fe(IV)(O)(Cl)](+) (3), as the product of oxygenation. In contrast, 2 + O(2) results in direct S-oxygenation to give a sulfonato product, PhSO(3)(-). The positioning of the thiolate ligand in 1 versus 2 appears to play a critical role in determining the outcome of O(2) activation. The thiolate ligands in 1 and 2 are essential for O(2) reactivity and exhibit an important influence over the Fe(III)/Fe(II) redox potential.     

Chelate bis(imino)pyridine cobalt complexes: synthesis, reduction, and evidence for the generation of ethene polymerization catalysts by Li+ cation activation

Treatment of the bis(iminobenzyl)pyridine chelate Schiff-base ligand 8 (ligPh) with FeCl2 or CoCl2 yielded the corresponding (ligPh)MCl2 complexes 9 (Fe) and 10 (Co). The reaction of 10 with methyllithium or "butadiene-magnesium" resulted in reduction to give the corresponding (ligPh)Co(I)Cl product 11. Similarly, the bis(aryliminoethyl)pyridine ligand (ligMe) was reacted with CoCl2 to yield (ligMe)CoCl2 (12). Reduction to (ligMe)CoCl (13) was effected by treatment with "butadiene-magnesium". Complex 13 reacted with Li[B(C6F5)4] in toluene followed by treatment with pyridine to yield [(ligMe)Co+-pyridine] (15). The reaction of the Co(II) complexes 10 or 12 with ca. 3 molar equiv of methyllithium gave the cobalt(I) complexes 16 and 17, respectively. Treatment of the (ligMe)CoCH3 (17) with Li[B(C6F5)4] gave a low activity ethene polymerization catalyst. Likewise, complex 16 produced polyethylene (activity = 33 g(PE) mmol(cat)(-1) h(-1) bar(-1) at room temperature) upon treatment with a stoichiometric amount of Li[B(C6F5)4]. A third ligand (lig(OMe)) was synthesized featuring methoxy groups in the ligand backbone (22). Coordination to FeCl2 and CoCl2 yielded the desired compounds 23 and 24. Reaction with MeLi gave (ligOMe)CoMe (25/26). Treatment of 25/26 with excess B(C6F5)3 gave the eta6-arene cation complex 27, where one Co-N linkage was cleaved. Activation of 25/26 with Li[B(C6F5)4] again gave a catalytically active species.     

Specific features of ethylene polymerization on self-immobilizing catalytic systems based on titanium bis(phenoxy imine) complexes

The kinetics of ethylene polymerization on six methylalumoxane-activated self-immobilizing bis(phenoxy imine) complexes of titanium chloride with allyloxy groups in the m- and p-positions of the N-phenyl ring and with various substituents in the salicylaldehyde fragment was studied. The activity of the complexes in the temperature range 20–60°C and ethylene pressure of 0.4 MPa was evaluated.     

Effect of alkylaluminum on ethylene polymerization catalyzed by 2,6‐bis(imino)pyridyl complexes of Fe(II)

In the presence of tetraethylaluminoxane (TEAO), iron complexes were used to catalyze ethylene polymerizations with extremely high activities and generally produced polyethylene with a bimodal molecular weight distribution (MWD). This bimodal MWD of polyethylene was mainly derived from residual triethylaluminum in TEAO and was produced through a mechanism of chain transfer to aluminum. Ethylaluminoxane and tetraisobutylaluminoxane also were used to polymerize ethylene with high activities in the presence of iron complexes, and only polyethylene with a unimodal MWD was produced. The ratio of the rate constant of chain transfer to aluminum (k_trA) to the rate constant of chain propagation (k_p) was determined to be 0.12 for {[ArN?C(Me)]₂C₅H₃N}FeCl₂ when Ar was 2,6‐diisopropylphenyl ( 1 ) and 2.48 for {[ArN?C(Me)]₂C₅H₃N}FeCl₂ when Ar was 2,6‐dimethylphenyl ( 2 ); these values are far larger than those for metallocene‐based catalysts. This explains why an iron complex usually produces polyethylene with a broader MWD than metallocene‐based catalysts. Additionally, it can be concluded from the great difference between 1 and 2 with respect to k_trA/k_p that an iron complex with less congested aryl substituents is subjected to chain transfer to aluminum. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1599–1606, 2005