Elsevier

Journal of Organometallic Chemistry

Volume 793, 15 September 2015, Pages 155-159
Journal of Organometallic Chemistry

Coordination compounds of titanium and zirconium with 6,6′-(piperazine-1,4-diyl)-bis-(2,4-di-tert-butylphenol) and study of their catalytic activity towards ethylene and 1-hexene polymerizations

This paper is dedicated in memory of Professor A.E. Shilov with recognition of his contribution to the field of olefin polymerization processes, chemical kinetics and catalysis.
https://doi.org/10.1016/j.jorganchem.2014.11.014Get rights and content

Highlights

  • New Ti(IV) and Zr(IV) complexes based on [ONNO]2− ligand were synthesized.

  • The molecular structure of the complex 2 was determined by X-ray crystallography.

  • Complexes 23 are active in olefin polymerization only in the presence of Et2AlCl/Bu2Mg.

  • Ability of Mg derivatives to improve the catalytic ability was determined.

Abstract

Novel titanium and zirconium alkoxo complexes bearing 6,6′-(piperazine-1,4-diyl)-bis-(2,4-di-tert-butylphenolate) ligand (L), LTi2(OiPr)6 (2) and L2Zr2(OBu)4 (3), has been synthesized in acceptable yields by the stoichiometric reaction of L with the respective M(OR)4. All complexes were fully characterized by elemental analysis and NMR spectroscopy, and the molecular structures of the representative complex 2 was confirmed as distorted trigonal bipyramid at titanium (coordination number 5) by single-crystal X-ray diffraction. Pre-catalysts 23 are active in ethylene and 1-hexene polymerization in the presence of Et2AlCl/Bu2Mg binary co-catalyst; but there were completely inactive in presence of MAO or Me3Al. Obtained polyethylene and polyhexene samples were of high molecular weight. The results of this study show the unique ability of Mg derivatives to improve the catalytic ability of post-metallocene pre-catalysts.

Graphical abstract

Newly titanium(IV) and zirconium(IV) alkoxo complexes bearing 6,6′-(piperazine-1,4-diyl)-bis-(2,4-di-tert-butylphenolate) ligand were synthesized. The crystal structure of titanium complex 2 was determined by X-ray crystallography. Pre-catalysts 23 were completely inactive in olefin polymerization in the presence of MAO or Me3Al. Catalytic systems formed only in the presence of Et2AlCl/Bu2Mg binary co-catalyst.

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Introduction

Recently, ligands with two phenol fragments connected through the linker containing additional donor atoms were successfully used in coordination chemistry. Systems based on OSSO [1] and ONNO-type ligands are well described. Examples of ONNO-type ligand bearing complexes that show catalytic activity in polymerization of ethylene [2], propylene [3], styrene and 1-hexene [4], [5], [6], [7], vinylcyclohexane [8] and l-lactide [9], [10] are presented in Fig. 1.

The widely used ligands similar to AD are the products of phenols aminomethylation or salenes reduction. They all tend to form six-membered chelate rings. Ligands generating a more strained five-membered chelate rings with metal core, such as E (Fig. 1), are less common. Such conformational rigidity of the structure undoubtedly influences the catalytic activity. The ligand 1 (Fig. 2) suitable for these structural requirements can be obtained by the reaction of 3,5-di-tert-butyl-ortho-benzoquinone with piperazine [11], [12].

Herein, we describe the synthesis of a titanium and zirconium complexes with ligand 1, their structural characterization, and their catalytic capacity in ethylene and 1-hexene polymerization reactions.

Section snippets

Synthesis and structure of titanium and zirconium complexes 12

Reaction of 6,6-(piperazine-1,4-diyl)-bis-(2,4-di-tert-butylphenol) (1) with Ti(OiPr)4 led to binuclear complex 2 (Fig. 2). The structure was determined by single crystal X-ray diffraction (Fig. 3). Changes in reaction protocol, such as variation of amount, mol ratio (L:Ti = 1:0.5–2) and reagents addition order did not yield the mononuclear product. At the same time using piperazine-containing bisphenolate ligand, capable of forming a less strained six-membered metallocycle [9], allows to

General methods

All manipulations were carried out according to the standard Schlenk techniques. Solvents for air- and moisture-sensitive reactions were dried over sodium benzophenone ketyl and stored over the 3 or 4 Å molecular sieves.

Ligand 1, 6.6′-(piperazine-1,4-diyl)-bis-(2,4-di-tert-butylphenol) was synthesized as described in Refs. [11], [12]; its physicochemical characteristics and 1H and 13C NMR spectra were found to match the published data. The structure of 1:HCl was also characterized using single

Acknowledgements

We are grateful to to Russian Science Foundation (project nos. 14-13-00682).

Authors thank Dr. I.V. Fedyanin (X-ray structural center, INEOS RAS) for X-ray diffraction study of the hydrochloride of the ligand 1.

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