Synthesis, characterization and catalytic behavior toward ethylene of cobalt(II) and iron(II) complexes bearing 2-(1-aryliminoethylidene)quinolines

Abstract

A series of 2-(1-aryliminoethylidene)quinolines (L) were synthesized and used as bidentate N^N ligands in coordinating with metal (cobalt and iron) chlorides to form complexes of the type LMCl₂, cobalt(II) (Co1–Co5) and iron(II) (Fe1–Fe5). All organic compounds and metal complexes were fully characterized, and the molecular structures of the representative complexes Co3·DMF and Fe4·DMF were confirmed as distorted bipyramidal geometry at the metal by single-crystal X-ray diffraction. Upon activation with either methylaluminoxane (MAO) or modified methylaluminoxane (MMAO) under 10 atm ethylene, all complexes showed high activities in ethylene dimerization with activities of up to 1.82 × 10⁶ g mol⁻¹ (Co) h⁻¹ and 5.89 × 10⁵ g mol⁻¹ (Fe) h⁻¹, respectively.

Introduction

The discovery of bis(imino)pyridylmetal (Fe or Co) chlorides as pre-catalysts provided a milestone in ethylene oligomerization and polymerization [1], [2], [3], [4], [5], and this in turn has led to extensive investigations into modifying the substituents of the aryl groups within the bis(arylimino)pyridines [6], [7], [8], [9], [10], [11], [12]. Recent reviews have highlighted how less bulky substituents favor oligomerization processes [2], [5], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Inspired by the success of the bis(arylimino)pyridine ligand set, much effort has been devoted to developing new Fe(II) and Co(II) pre-catalysts through the design of new suitable tridentate ligands [23], [24], [25], [26], [27], [28]. Our laboratory has successfully designed ligands such as 2-benzimidazole-6-iminopyridines [29], [30], [31], 6-(quinoxalin-2-yl)-2-iminopyridines [32], 2-benzoxazolyl-6-iminopyridines [33], [34], 2-imino-1,10-phenanthrolines [35], [36], [37], [38], [39], 2-benzimidazole-1,10-phenanthrolines [40], 2-oxazoline/benzoxazole-1,10-phenanthrolines [41], N-((pyridin-2-yl)methylene)-quinolin-8-amine derivatives [42], and 2,8-bis(imino)quinolines [43], as well as 2-methyl-2,4-bis(6-iminopyridin-2-yl)-1H-1,5-benzodiazepines for bimetallic complexes [44], [45]. Upon realizing the difference of distorted square-based pyramidal geometry around cobalt complexes bearing 2,8-bis(imino)quinolines [43], further attention was paid to developing bidentate ligands for complex pre-catalysts of the 2-(1-aryliminopropylidene) quinolylcobalt dichlorides [46] and the 8-(1-aryliminoethylidene)quinaldylmetal (Fe or Co) dichlorides [47]. The latter 8-(1-aryliminoethylidene)quinaldylmetal (Fe or Co) dichlorides [47] showed high activities in ethylene dimerization, however, the 2-(1-aryliminopropylidene)quinolylcobalt pre-catalysts exhibited interesting catalytic behavior such that they were ethylene dimerization pre-catalysts at room temperature, but ethylene polymerization pre-catalysts at high temperature. The scope of N-bidentate metal (Fe or Co) complexes and their catalytic behavior has not been well explored, and as a consequence 2-acetylquinoline instead of the previous 2-propionylquinoline [46] has been employed herein to prepare 2-(1-aryliminoethylidene)quinolines (L) and their cobalt (Co1–Co5) and iron (Fe1–Fe5) complexes. In the presence of methylaluminoxane (MAO) or modified methylaluminoxane (MMAO), all metal complexes showed high activities toward ethylene dimerization, and surprisingly the cobalt pre-catalysts exhibited higher activities than did their iron analogs.