Elsevier

Journal of Organometallic Chemistry

Volume 822, 1 November 2016, Pages 241-249
Journal of Organometallic Chemistry

2-iminopyridine nickel(II) complexes bearing electron-withdrawing groups in the ligand core: Synthesis, characterization, ethylene oligo- and polymerization behavior

https://doi.org/10.1016/j.jorganchem.2016.08.031Get rights and content

Highlights

  • 2-iminopyridine nickel complexes containing electron-withdrawing groups.

  • Activation with methylaluminoxane (MAO) and Et2AlCl.

  • Catalytic ethylene polymerization and oligomerization.

  • Highly branched low-molecular-weight polyethylenes and ethylene oligomers.

Abstract

A series of novel 2-[(arylimino)alkyl]pyridine derivatives, containing electron-withdrawing groups (F, Cl, Br, CF3, NO2, CN) in the aryl rings, have been synthesized and characterized by 1H, 13C and 19F NMR spectroscopy. The corresponding nickel(II) complexes (24 complexes) have been prepared in good yields. The nature of the Ni complexes has been established by elemental analysis, paramagnetic 1H NMR and X-ray diffraction studies. Those complexes, activated with MAO, have been found to be highly active in the oligomerization and polymerization of ethylene (up to 9600 kg (mol of Ni)−1 h−1 bar−1), affording highly branched low-MW PE or ethylene oligomers, predominantly butenes and hexenes. The dependence of content and ratio of ethylene oligomers as well as of the polymer characteristics (molecular weight, degree of branching) on the nature of the substituents in the aryl(imino) and pyridine rings has been analyzed systematically. (E)-N-(1-(pyridin-2-yl)ethylidene)-2-(trifluoromethyl)aniline Ni complex supported on silica-alumina affords polyethylene with higher molecular weights, broader molecular weight distribution, and lower level of branching than the homogeneous catalyst.

Introduction

The history of post-metallocene Ni-based olefin polymerization catalysts dates back to the Brookhart's milestone discovery of nickel(II) α-diimine complexes; those complexes, activated with methylaluminoxane (MAO), displayed very high activity in the polymerization of ethylene and α-olefins [1]. An important catalytic feature of the Brookhart's catalysts has been the formation of branched polyethylene via the «chain-walking» mechanism [2], [3]. In recent years, complexes of nickel(II) with bi- and tridentate N-donor ligands have attracted great academic and practical attention as highly active and versatile catalysts of olefin polymerization and oligomerization [4], [5], [6], [7], [8], [9], [10], [11], [12], providing the access to novel polymeric functional materials [13]. Lately, close attention has been paid to Ni-based catalysts, capable of affording highly- or even hyperbranched polyolefins during polymerization [14], [15], [16], [17]. Such polymers represent a challenging class of polyethylene materials with unprecedented chain architectures and unique physical properties. Possible application areas of such materials have been delineated, including e.g. lubricant additives or drug delivery [4], [18].

Nickel(II) complexes with bidentate iminopyridine ligands are known to carry out ethylene polymerization, affording branched low-MW polyethylene. For the first time such catalysts (containing 2,6-dialkylphenylimino moieties) were reported by Leskelä and co-workers [19], [20], [21]. Later, catalytic behaviors of various homogeneous single-site catalysts based on 2-iminopyridine nickel complexes with electron-donating substituents in ethylene polymerization were described [22], [23], [24], [25], [26], [27], [28], [29], [30]. In addition, a few examples of bridged dinuclear [31], [32], [33], [34], [35], dendritic [36], [37], [38] and MOF-supported [39] Ni(II) complexes containing 2-iminopyridine moieties were synthesized and tested towards ethylene reactivity. Structurally similar 2-iminoquinolyl nickel complexes displayed high activity towards ethylene oligomerization [40], [41].

So far, researchers have mostly focused on 2-iminopyridine nickel catalysts bearing electron-donating groups (EDG) in the aryl rings. It would thus be intriguing to examine the effect of electron-withdrawing groups (EWG) on the catalytic behavior of 2-iminopyridine nickel complexes, keeping in mind that the introduction of the EWGs had been reported to enhance the catalytic activity of analogous iron, cobalt [42], [43], [44], [45], [46], [47], [48], [49] and nickel [50] complexes for ethylene oligo- and polymerization, and for polymerization of norbornene.

Herein, the synthesis and characterization of a series of 2-iminopyridine ligands bearing various electron-withdrawing groups (F, Cl, Br, CF3, NO2, CN), and of the corresponding nickel(II) complexes, are reported. The influence of both electronic and steric properties of the ligand on the catalytic behavior of those complexes in ethylene polymerization and oligomerization is discussed, using methylaluminoxane (MAO) as cocatalyst. Furthermore, supported nickel catalysts have demonstrated good activity in ethylene polymerization also with Et2AlCl, modified MAO (MMAO), and Al(i-Bu)3 as cocatalysts.

Section snippets

General experimental details

All manipulations with air- and/or moisture-sensitive compounds were performed under an atmosphere of argon using glove box, break-sealed or standard Schlenk techniques. Toluene was dried over molecular sieves (4 Å) and distilled over sodium under argon. THF was stored over NaOH and distilled over sodium under argon before use. CH3CN was stored over molecular sieves (4 Å) under an atmosphere of argon. Dichloromethane used for the synthesis of Ni(II) complexes was stored over Na2SO4 and

Synthesis and characterization of 2-(imino)pyridine ligands and of nickel(II) complexes

2-Iminopyridine ligands were synthesized according to modified literature procedure [58] via Schiff-base condensation of the corresponding substituted 2-pyridinecarboxaldehyde (or 2-quinolinecarboxaldehyde, 2-acetylpyridine, 2-benzoylpyridine) with aniline derivatives containing EWG in the presence of catalysts (Brønsted or Lewis acids). The general method for the preparation of the 2-iminopyridine ligands and of the corresponding nickel(II) complexes is illustrated in Scheme 1.

It is well-known

Conclusions

In this work, a series of novel 2-(imino)pyridine ligands containing electron-withdrawing groups and their nickel(II) dihalide complexes (24 complexes) have been prepared and characterized. Depending on the ligand structure, all nickel complexes in the presence of MAO displayed moderate to high ethylene oligomerization or polymerization activities (up to 6600 kg (mol of Ni)−1 h−1 bar−1, which is the highest reported for ethylene polymerization on 2-(imino)pyridine nickel complexes), affording

Acknowledgements

The GPC and NMR measurements were supported by the Russian Academy of Sciences and Federal Agency of Scientific Organizations (project V.44.2.1). The ligands and complexes were synthesized with the financial support from the Russian Foundation for Basic Research (grant 16-53-53014).

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