α,α'‐Bis (imino)‐2,3:5,6‐bis (pentamethylene)pyridines appended with benzhydryl and cycloalkyl substituents: Probing their effectiveness as tunable N,N,N‐supports for cobalt ethylene polymerization catalysts

Abstract

A single‐pot method has been utilized to prepare the bis(cycloheptyl)fused N,N,N‐cobalt (II) chloride complexes, [2,3:5,6‐{C4H8C(NAr)}2C5H3N]CoCl2 (Ar = 2,6‐(C5H9)2–4‐(CHPh2)C6H2 Co1, 2‐(C5H9)‐4,6‐(CHPh2)2C6H2 Co2, 2‐(C6H11)‐4,6‐(CHPh2)2C6H2 Co3, 2‐(C8H15)‐4,6‐(CHPh2)2C6H2 Co4, 2‐(C12H23)‐4,6‐(CHPh2)2C6H2 Co5) in reasonable yields. The molecular structure of Co1 highlights not only the steric shielding of the metal center provided by the N‐2,6‐dicyclopentyl‐4‐benzhydrylphenyl groups but also the trans‐configuration of the puckered sections of the two fused seven‐membered rings. Besides this structural characterization, all complexes have been characterized by elemental analysis and Fourier transform infrared spectroscopy (FT‐IR) spectroscopy. In the presence of modified methylaluminoxane (MMAO) or methylaluminoxane (MAO), Co1–Co5 afforded highly linear polyethylenes (Tm′s > 126°C) with dispersities that were influenced by the type of aluminoxane activator [Mw/Mn range: 1.53–1.81 (MMAO) vs. 8.96–15.5 (MAO)]. In common to both co‐catalysts, the catalytic activity of the precatalysts fell in the order: Co1 > Co2 > Co5 > Co4 ~ Co3, reflecting the differences in steric/electronic properties of the ortho‐cycloalkyl substituents. In terms of thermostability of the catalyst, Co1/MMAO attained optimal performance at 30°C (2.04 × 106 g PE mol−1[Co] h−1), while Co1/MAO reached it at 60°C albeit with lower productivity (0.70 × 106 g PE mol−1[Co] h−1). In general, the polyethylenes were of reasonably high molecular weight (e.g., between 39.9 and 65.8 kg mol−1 using MMAO) which can be linked to the steric hindrance imposed on chain transfer by the cycloalkyl and benzhydryl ortho‐substituents.