Prospects and crucial problems in oligomerization and polymerization with iron and cobalt complex catalysts

<正>The discovery of highly active 2,6-bis(imino)pyridyl iron and cobalt complexes provided a milestone of latetransition metal catalysts for ethylene oligomerization and polymerization with being currently investigated for the scale-up process.The crucial problems are remaining in the catalytic systems:the catalytic systems targeting ethylene polymerization produce more oligomers at elevated reaction temperatures,however,there is a recognizable amount of high-molecular-weight polyethylene remained in the modified catalytic system for the oligomerization process.Beyond the modification of bis(imino)pyridyl metal complexes,several alternative procatalysts' models have been developed in our group.This review highlighted the achievements in exploring new iron and cobalt complexes with tridentate NNN ligands as procatalysts for ethylene oligomerization and polymerization.     

Synthesis and crystal structures of the first C2-symmetric bis-aldimine NCN-pincer complexes of platinum and palladium

(S,S)-2,6-bis[(N-α-methylbenzyl)imino]phenylpalladium bromide was synthesised by oxidative addition of palladium(0) to (S,S)-1-bromo-2,6-bis[(N-α-methylbenzyl)imino]benzene. In contrast, (S,S)-2,6-bis[(N-α-methylbenzyl)imino]phenylplatinum chloride was synthesised by direct C-H activation from the reaction of potassium tetrachloroplatinate with (S,S)-1,3-bis[(N-α-methylbenzyl)imino]benzene. The X-ray crystal structures of both pincer complexes were obtained. Treatment of both complexes with silver hexafluoroanimonate gave effective but not stereoselective catalysts for a Michael reaction between methyl vinyl ketone and methyl 2-cyanopropanoate.     

Electron Effect of p-Substituent on Iron（Ⅱ） and Cobalt（Ⅱ） Pyridinebisimine Catalyst for Ethylene Polymerization

A series of 2,6-bis（imino）pyridyl iron and cobalt complexes bearing p-substituent [2,6-（ArN=CMe）2C5H3N]- MCl2 （Ar=2,6-Me2C6H3, 2,4,6-Me3C6H2, 2,6-Me2-4-BrC6H2, 2,6-Me2-4-ClC6H2, 2,4-Me2-6-BrC6H2, 2,4-Me2-6- ClC6H2, while M=Fe, Co） have been synthesized and investigated as catalysts for ethylene polymerization in the presence of modified methylaluminoxane as a cocatalyst. The electron effect and positions of the substituent of pyridinebisimine ligands were observed to affect considerably catalyst activity and polymer property.     

NCN pincer palladium complexes: their preparation via a ligand introduction route and their catalytic properties

A wide range of NCN pincer palladium complexes, [4-tert-butyl-2,6-bis(N-alkylimino)phenyl]chloropalladium (alkyl = n-butyl, benzyl, cyclohexyl, tert-butyl, adamantyl, phenyl, 4-methoxyphenyl), were readily prepared from trans-(4-tert-butyl-2,6-diformylphenyl)chlorobis(triphenylphosphine)palladium via dehydrative introduction of the corresponding alkylimino ligand groups (ligand introduction route) in excellent yields (71-98%). NMR studies on this route for forming pincer complexes revealed the intermediacy of [4-tert-butyl-2,6-bis(N-alkylimino)phenyl]chlorobis(triphenylphosphine)palladium which is in equilibrium with the corresponding NCN pincer complexes via coordination/dissociation of the intramolecular imino groups and triphenylphosphine ligands. A series of chiral NCN pincer complexes bearing pyrroloimidazolone units as the trans-chelating donor groups, [4-tert-butyl-2,6-bis{(3R,7aS)-2-phenylhexahydro-1H-pyrrolo[1,2-c]imidazol-1-on-3-yl}phenyl]chloropalladium, were also prepared from the same precursor via condensation with proline anilides in high yields. The catalytic properties of the NCN imino and the NCN pyrroloimidazolone pincer palladium complexes were examined in the Heck reaction and the asymmetric Michael reaction to demonstrate their high catalytic activity and high enantioselectivity.     

Tuning redox potentials of bis(imino)pyridine cobalt complexes: an experimental and theoretical study involving solvent and ligand effects

The structure and electrochemical properties of a series of bis(imino)pyridine Co(II) complexes (NNN)CoX(2) and [(NNN)(2)Co][PF(6)](2) (NNN = 2,6-bis[1-(4-R-phenylimino)ethyl]pyridine, with R = CN, CF(3), H, CH(3), OCH(3), N(CH(3))(2); NNN = 2,6-bis[1-(2,6-(iPr)(2)-phenylimino)ethyl]pyridine and X = Cl, Br) were studied using a combination of electrochemical and theoretical methods. Cyclic voltammetry measurements and DFT/B3LYP calculations suggest that in solution (NNN)CoCl(2) complexes exist in equilibrium with disproportionation products [(NNN)(2)Co](2+) [CoCl(4)](2-) with the position of the equilibrium heavily influenced by both the solvent polarity and the steric and electronic properties of the bis(imino)pyridine ligands. In strong polar solvents (e.g., CH(3)CN or H(2)O) or with electron donating substituents (R = OCH(3) or N(CH(3))(2)) the equilibrium is shifted and only oxidation of the charged products [(NNN)(2)Co](2+) and [CoCl(4)](2-) is observed. Conversely, in nonpolar organic solvents such as CH(2)Cl(2) or with electron withdrawing substituents (R = CN or CF(3)), disproportionation is suppressed and oxidation of the (NNN)CoCl(2) complexes leads to 18e(-) Co(III) complexes stabilized by coordination of a solvent moiety. In addition, the [(NNN)(2)Co][PF(6)](2) complexes exhibit reversible Co(II/III) oxidation potentials that are strongly dependent on the electron withdrawing/donating nature of the N-aryl substituents, spanning nearly 750 mV in acetonitrile. The resulting insight on the regulation of redox properties of a series of bis(imino)pyridine cobalt(II) complexes should be particularly valuable to tune suitable conditions for reactivity.     

Iron(II)–ethylene polymerization catalysts bearing 2,6-bis(imino)pyrazine ligands: Part I. Synthesis and characterization

The synthesis of a new series of 2,6-bis(imino)pyrazinyl ligands, [ArNCPyzCNAr] where the aryl groups Ar = naphtyl, 2,6-dimethylphenyl, 2,6-diisopropylphenyl, 2,4,6-trimethylphenyl, and their iron(II) complexes is described starting from monoacetylpyrazine.     

2-[1-(2,6-Dibenzhydryl-4-methylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridylcobalt(II) dichlorides: synthesis, characterization and ethylene polymerization behavior

A series of unsymmetrical 2,6-bis(imino)pyridylcobalt(II) complexes, {2-[2,6-(CH(C(6)H(5))(2))(2)-4-Me-C(6)H(2)N==C(CH(3))]-6-(2,6-R(1)(2)-4-R(2)-C(6)H(2)N==CCH(3))-C(5)H(3)NCoCl(2)} where R(1) = Me, Et or (i)Pr, R(2) = H or Me, together with the new symmetrical complex 2,6-[2,6-(CH(C(6)H(5))(2))(2)-4-Me-C(6)H(2)N==C(CH(3))](2)-C(5)H(3)NCoCl(2), were synthesized. All of the compounds were fully characterized by (1)H NMR and IR spectroscopy, as well as by elemental analysis. The molecular structures of Co1 (R(1) = Me, R(2) = H) and Co5 (R(1) = Et, R(2) = Me) were further confirmed by single crystal X-ray diffraction, which indicated that the cobalt centres were penta-coordinate with a pseudo square-pyramidal geometry. Upon treatment with MAO or MMAO, these cobalt pre-catalysts exhibited higher activities than any previously reported cobalt pre-catalysts, with values as high as 4.64 × 10(6) g PE mol(-1)(Co) h(-1) for ethylene polymerization at atmospheric pressure. The polyethylenes obtained were of high molecular weight and narrow molecular weight distribution.     

Aldimine 2,6-bis(imino)pyridine iron(II) and cobalt(II)/methyl aluminoxane catalyst systems for polymerization of <Emphasis Type="Italic">tert-</Emphasis>butylacrylate

Aldimine 2,6-bis[(imino)methyl]pyridine iron(II) (1, 4, and 6) and cobalt(II) (3 and 5) complexes bearing bulky cycloaliphatic (bornyl and myrtanyl) or aromatic (naphthyl) terminal groups have been applied successfully, after activation with methyl aluminoxane (MAO), as catalysts for the polymerization of tert-butylacrylate. For comparison reasons, complex 2 that contains the ketimine ligand, 2,6-bis[(−)-cis-myrtanylimino)ethyl]pyridine (BMEP), has also been utilized. All studied complexes showed moderate polymerization activities, and they produced high molar mass syndiorich-atactic polymers. Surprisingly, the aldimine-based catalyst systems showed comparable activities compared with the corresponding ketimine complex (2), and they produced high molar mass polymers. In addition, complexes with bulky terminal cycloaliphatic substituents on the tridentate aldimine ligands showed higher polymerization activity compared with the aromatic ones (6). Polymerization activity and polymer molar masses are dependent on the ligand framework.     

New functionalized acenaphthene-1,2-diimines and derived zinc and copper complexes

Russian Chemical Bulletin - The new compound 1,2-bis[(2,6-diisopropyl-4-iodophenyl)imino]acenaphthene (I2-dppbian, 1) was synthesized by the condensation of acenaphthenequinone with...     

Synthesis and Spin State of the Cobalt(II) Complexes with Substituted 2,6-Bis(pyrazol-3-yl)pyridine Ligands

Russian Journal of Coordination Chemistry - The direct template reactions afford the cobalt(II) complexes (I–III) with the hydroxy- and acetyl-substituted 2,6-bis(pyrazol-3-yl)pyridine...     

Synthesis and reactivity of the complexes [(dpp-bian)SiCl2] and [(dpp-bian)Si{FeCp(CO)}2(μ-CO)] (dpp-bian is 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene)

Russian Chemical Bulletin - The complex [(dpp-bian)SiCl2] (1) was synthesized by the reaction of the free 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) ligand with Si2Cl6. Despite...     

Study of the nature and mechanism of the formation of paramagnetic species in nickel-based Brookhart-type catalytic systems

The catalytic systems NiBr₂(DPP-DAB) + 20 MAO and NiBr₂(DPP-BIAN) + 20 MAO (DPP-BIAN = bis(2,6-diisopropylphenyl)-bis(imino)-acenaphtene, MAO = methylaluminoxane), as well as a number of model systems, are studied under conditions of their activation and functioning. There are paramagnetic nickel complexes and radical-anion aluminum complexes in the systems under real conditions of activation and functioning. The highest activity is observed when the Ni(I) signal intensity in the ESR spectrum is maximal. A mechanism of paramagnetic species formation is proposed.     

Synthesis of some imidazole- and pyrazole- derived chelating agents

Procedures involving condensation of o-phenylenediamines with carboxylic acids, and reaction of bifunctional alkyl halides with bifunctional nucleophiles are described. Syntheses are reported of 2,6-bis-(2-benzimidazyl)-pyridine, 1,3-bis(2-benzimidazyl)-2-thiapropane, 1,7-bis(2-benzimidazyl)-2,6-dithiaheptane, 2-hydroxymethyl-5,6-dimethylbenzimidazole, 2-chloromethyl-5,6-dimethylbenzimidazole hydrochloride, 1,7-bis(5,6-dimethyl-2-benzimidazyl)-2,6-dithiaheptane, 3,6-bis(1-pyrazolyl)pyridazine, 2-(2-hydroxy-3-methyl–phenyl)benzimidazole, 2-(2-hydroxyphenyl)benzimidazole, 5-(2-hydroxphenyl)-3-methyl-1-phenylpyrazole, 3(5)-(2-hydroxyphenyl)-5(3)-methylpyrazole, 3(5)-(2-hydroxyphenyl)-5(3)-phenylpyrazole, and 1,3-bis((5-methylpyridyl)imino)isoindoline.     

Polyethylene with bimodal molecular weight distribution synthesized by 2,6-bis(imino)pyridyl complexes of Fe(II) activated with various activators

Polyethylenes with bimodal molecular weight distribution (MWD) were synthesized by 2,6-bis(imino)pyridyl complexes of Fe(II) combined with different activators, which were prepared from alkylaluminium. It is found that the molecular weight (MW) and MWD was influenced by not only iron complexes but activators as well. The activator plays key important role in determination of the MW and MWD of final polymer and the MWD shape could be regulated by selection of various activators and polymerization conditions. The study on the variation of the MWD with the polymerization time and fitting of bimodal MWD with two Flory distributions suggests that bimodal MWD is caused by chain transfer reaction to activator or two active sites.     

T-型氢键液晶复合物的制备与液晶行为

以不具有液晶行为的2,6-二[N,N′-二-(4-烷基苯甲酰基)]氨基吡啶(A系列)和4-正烷氧基苯甲酸(D系列)作为氢键液晶复合物的单体,组装成T-型氢键液晶系列复合物(AmDn)。用红外光谱对其结构进行了表征,用DSC及偏光显微方法对其液晶行为进行了研究。结果表明:所合成的21种复合物分子间存在氢键且都具有向列相。通过调整2,6-二[N,N′-二-(4-烷基苯甲酰基)]氨基吡啶分子上柔性烷基的长度和极性,可以有效地调节它与4-烷氧基苯甲酸形成的氢键复合物的液晶相变温度以及液晶态的稳定性;增加2,6-二[N,N′-二-(4-烷基苯甲酰基)]氨基吡啶分子上柔性烷基的长度,其复合物AmDn的液晶相温度范围趋于变窄,清亮点逐渐下降,其液晶态稳定性也逐渐下降;以2,6-二[N,N′-二-(4-烷基苯甲酰基)]氨基吡啶分子替代2,6-二[N,N′-二-(4-烷氧基苯甲酰基)]氨基吡啶分子,可以降低分子的极性,使其单体的熔点及其氢键复合物AmDn的相变温度下降。     

Stannylenes based on neutral,anionic, and dianionic 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene

Russian Chemical Bulletin - The reaction of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian (1)) with one equivalent of SnCl2 gave the complex [(dpp-bian)0SnCl2] (2) containing the...     

Transition metal complexes bearing 2,6-bis(imino)pyridyl:Synthesis, structure, ethylene polymerization/oligomerization studies

A series of new complexes {2,6-bis[1-((2-methyl-4-methoxyphenyl)imino)ethyl]pyri-dine}MCI2 [M=Fe(Ⅱ) (2), Co(Ⅱ) (3), Ni(Ⅱ) (4), Cu(Ⅱ) (5), Zn(Ⅱ) (6)] have been synthesized. At 25℃, using 500 equiv of methylaluminoxane (MAO), the activities of Fe(Ⅱ), Co(Ⅱ) catalysts can reach 4.02×106 g/mol-Fehatm for ethylene polymerization and 3.98×105 g/mol-Cohatm for ethylene oligomerization. The effects of polymerization conditions such as reaction temperature, Al/M molar ratio and time on the activity of catalyst have been explored.     

Crystal Structure and Ethylene Oligomerization Catalytic Activity of {2-Carbethoxy-6-[1-[（2,6-diethylphenyl）imino]ethyl]pyridine}CoCl2

The unsymmetric precursor ethyl 6-acetylpyridine-2-carboxylate （4） was synthesized from 2,6-dimethylpyridine （1）. On the basis of this precursor, a new mono（imino）pyridine ligand （5） and the corresponding Co（Ⅱ） complex {2-carbethoxy-6-[1-[（2,6-diethylphenyl）imino]ethyl]pyridine}CoCl2 （6） were prepared. The crystal structure of complex indicates that the 2-carbethoxy-6-iminopyridine is coordinated to the cobalt as a tridentate ligand using [N, N, O] atoms, and the coordination geometry of the central cobalt is a distorted trigonal bipyramid, with the pyridyl nitrogen atom and the two chlorine atoms forming the equatorial plane. Being applied to the ethylene oligomedzation, this cobalt complex shows catalytic activity of 1.820× 10^4 g/mol-Cooh at 101325 Pa of ethylene at 15.5℃ for 1 h, when 1000 equiv, of methylaluminoxane （MAO） is employed as the cocatalyst.     

Polymerization of vinyl monomers via MAO activated iron(II) dichloro complexes bearing bis(imino)pyridine-, quinolinaldimine- and thiophenaldimine-based tridentate nitrogen ligands

A series of iron(II) complexes (4a-d, 10, and 11) bearing 2,6-bis[(imino)ethyl]pyridine-(3a-d), quinolinaldimine-(8) and thiophenaldimine-(9) based ligands were disclosed as active complexes for the polymerization of tert-butylacrylate (t-BA). After activation with methyl aluminoxane (MAO), the complexes showed moderate to high polymerization activities and produced high molar mass polymers. In addition, the catalyst system 4d/MAO was examined for the polymerization of methyl methacrylate (MMA) and n-vinylcarbazole (NVC). The influence of MAO/Cat. molar ratio, polymerization time, and monomer concentration on the polymerization reaction of methyl methacrylate was explored.In the polymerization of tert-butylacrylate with 2,6-bis[(imino)ethyl]pyridine iron(II)-based catalysts, bulky terminal aliphatic substituents have a favorable influence on the polymerization activity compared to the aromatic ones. This catalyst system was also more active than the quinolinaldimine-, (10) and thiophenaldimine-, (11) based catalysts.     

Stereoselectivity in reactions of metal complexes IX Stereoselective formation of cobalt(III) complexes with new linear pentadentate ligands

The synthesis of cobalt(III) complexes with the new linear pentadentate ligands meso- and racemic 2,6-bis(3S)-3-carboxy-4-methyl-2-azapentylpyridine ( 2 ) are described. Only one of the different possible isomers is obtained from each ligand. The structure of the complexes has been assigned on the basis of their ¹H-NMR and CD spectra. The structure of the aqua-cobalt(III)- 1a and the aqua-cobalt(III)- 1b has been confirmed by X-ray analysis. Partial resolution of optical antipodes of the aquacobalt(III)- 1b was achieved by column chromatography and a tentative assignment of their absolute configuration is made.