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
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 2–3 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.
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 A–D 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 1–2
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.
References (29)
- et al.
Tetrahedron
(2011) - et al.
Polyhedron
(2006) - et al.
Polymer
(2013) - et al.
J. Organomet. Chem.
(2003) - et al.
Polym. Chem.
(2011) - et al.
Dalton Trans.
(2010) - et al.
Macromolecules
(2010) - et al.
Organometallics
(2009) - et al.
Organometallics
(2009) - et al.
J. Am. Chem. Soc.
(2008)