1,2-Bis(dimethylsilyl)phenylidene bridged zirconocene and hafnocene dichloride complexes as precatalysts for ethylene polymerization
Graphical abstract
Introduction
A wide range of bridged bis(indenyl) metallocene complexes has been synthesized and tested as catalyst precursors for olefin polymerization [1], [2], [3], [4], [5], [6], [7]. These complexes include different bridging-type moieties. However, the most commonly used bridges are the dimethylsilylene, ethylidene, isopropylidene, and methylene units [8], [9], [10], [11], [12], [13], [14]. The structure of the bridging unit in ansa metallocene complexes has a strong influence on the catalyst activity and the molecular weight of the generated polymers [15]. The activity can be improved by enlarging the reaction space around the metal center, mainly by increasing the metal-centroid distance and the dihedral angle between the two indenyl based π-ligands. In contrast to silylene and alkylidene bridged metallocene complexes, few examples of ansa-metallocene complexes are known containing bulky aromatic groups as bridges. For instance, o-xylidene- and naphthylidene-bridged catalysts have recently been synthesized and evaluated as catalysts for ethylene and propylene polymerization after activation with MAO [16], [17]. In this context ansa bis(indenyl) complexes of Ti, Zr and Hf with dimethylsilylene bridges and sterically demanding 4-aryl substituents proved as highly isoselective catalyst precursors for the polymerization of propylene [18], [19], [20]. Herein, a synthesis of new ansa-complexes of zirconium and hafnium is described, with two indenyl ligands linked through a 1,2-bis(dimethylsilyl)phenylidene bridge. The catalytic behavior for ethylene polymerization after activation with methylaluminoxane (MAO) in homogeneous solution is described (see Scheme 1).
Section snippets
Synthesis of the ligand precursor
1,2-Bis(inden-1-yldimethylsilyl)benzene 3 was prepared via a three step reaction that comprises Grignard coupling of 1,2-dibromobenzene with chlorodimethylsilane to give 1,2-bis(dimethylsilyl)benzene (1), followed by chlorination with carbon tetrachloride in the presence of a catalytic amount of palladium dichloride to obtain 1,2-bis(chlorodimethylsilyl)benzene (2) [21]. The desired compound was readily prepared by the reaction of compound 2 with two equivalents of indenyllithium (prepared
General
All reactions were carried out under an inert gas atmosphere of pure oxygen-free argon using standard Schlenk techniques. n-Pentane, n-hexane, diethyl ether, toluene, and tetrahydrofuran were purified by distillation over Na/K alloy. Diethyl ether was additionally distilled over lithium aluminum hydride. Toluene was additionally distilled over phosphorus pentoxide. Methylene chloride and carbon tetrachloride were dried over phosphorus pentoxide. Deuterated organic solvents (CDCl3, CD2Cl2, and C6
Conclusions
This contribution confirms the statement for ansa metallocene catalysts: the bridge makes the difference. On the first look, this is hard to understand because the bridge of ansa metallocene complexes is far away from the active center. However, the bridge has an influence on the dihedral angle the aromatic ligands are forming and hence on the kinetics of the incoming monomer and the produced polymer chain. In addition, the bridge seems to interact with the bulky MAO counteranion that is formed
Acknowledgements
The authors thank the Deutsche Akademischer Austauschdienst, DAAD, for financial support and Dr. W.P. Kretschmer for the GPC measurements.
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