Pyridine bis(imino) iron and cobalt complexes for ethylene polymerization: influence of the aryl imino substituents

Abstract

In the present paper, the synthesis of new pyridine bis(imine) ligands modified with halogens (Cl, Br, CF₃) or alkyl groups (Heptyl, tert-butyl, Phenyl, …) is reported. When coordinated with iron or cobalt dichloride, they yielded complexes which were associated to methylaluminoxane (MAO) to achieve the polymerization of ethylene. It was shown that cobalt catalysts are generally more sensitive to the ligand substitutions than the iron ones. The addition of a chlorine atom on the ligand frame is generally unfavorable. On the contrary, the presence of a bromine atom seems more favorable. Phenyl rings lead to almost completely inactive catalysts, probably because of a too weak coordination to the metal. It was also demonstrated that a mono-substitution of the aryl groups with an electron-withdrawing group (–CF₃) is sufficient to yield polymers, whereas, considering the bulkiness of this substituent only, oligomers would have been expected.

Introduction

Since 1995, the late transition metal catalysis has raised an increasing interest. Indeed, the research activity in this field has exploded with the discovery of highly active diimine palladium- and nickel-based complexes, which yield moderately to highly branched polyethylenes [1], [2], [3], [4], [5]. In 1998, Brookhart and Gibson have described simultaneously the synthesis of new efficient pyridine bis(imine) iron- or cobalt-based catalysts for the oligomerization or the polymerization of ethylene [6], [7], [8]. When the ligands are bulky enough, the resulting complexes revealed extremely active for the synthesis of linear polyethylenes. Many theoretical and experimental mechanistic studies were conducted to identify the active species as well as the elementary polymerization processes involved during the polymerization [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. It was shown that ethylene coordinates to the metal through the axial position from the same side of the growing chain to lead to propagation. If the monomer coordinates to the metal in the trans position versus the growing chain, transfer occurs. Concerning iron complexes, it is not yet clear if the active species are based on Fe^III [17] or on Fe^II [13], [21]. For cobalt complexes, it was shown that the active species are based on Co^I [14], [15]. The copolymerization of ethylene with higher α-olefins has revealed extremely difficult [16], [22], [23].

In this paper, we report the synthesis and characterization of new pyridine bis(imino) iron and cobalt complexes (Heptyl, ditBu, triPh, CF3, …) for ethylene polymerization. The influence of the nature or the bulkiness of the ligand substituents on the catalytic activity and the PE molar masses was investigated. The efficiency of these new catalysts will be discussed and compared to already published ones (diiPr, diMe, tBu) (Scheme 1).