Fluorinated 2,6-bis(arylimino)pyridyl iron complexes targeting bimodal dispersive polyethylenes: probing chain termination pathways via a combined experimental and DFT study

Abstract

In this work, fluorinated 2,6-bis(arylimino)pyridyl iron(II) complexes, [2-[CMeN{2,4-{(4-FC₆H₄)₂CH}₂-6-F}]-6-(CMeNAr)C₅H₃N]FeCl₂ (Ar = 2,6-Me₂C₆H₃Fe1, 2,6-Et₂C₆H₃Fe2, 2,6-ⁱPr₂C₆H₃Fe3, 2,4,6-Me₃C₆H₂Fe4, and 2,6-Et₂-4-MeC₆H₂Fe5) and [2-[CMeN{2-{(4-FC₆H₄)₂CH}-4-{(C₆H₅)CHAr′}-6-F}]-6-(CMeN(2,6-ⁱPr₂C₆H₃))C₅H₃N]FeCl₂ (Ar′ = 3-{(4-FC₆H₄)₂CH}₂-4-NH₂-5-FC₆H₂Fe6), verified with different steric substituents, were synthesized and characterized. The molecular structures of Fe2 and Fe3 were determined by X-ray diffraction, revealing a pseudo-square-pyramidal geometry. High activities were achieved toward ethylene polymerization in each iron complex case. The sterically least demanding ligand enhanced the activity of its complex Fe1 with the highest activity up to 16.8 × 10⁶ g of PE (mol of Fe)⁻¹ h⁻¹at 70 °C, while the bulkiest ligand led to the formation of the highest molecular weight of the resulting polyethylene using Fe6. Generally, the resulting polyethylenes are highly linear and most of them have a tendency to display bimodal distributions by virtue of the presence of multiple sites or competing chain transfer reactions. End-group analysis of polyethylenes confirms that the end groups include both unsaturated vinyl-end groups and saturated n-propyl or i-butyl, revealing the co-existence of two chain termination pathways including primary chain transfer to aluminium and secondary β-H transfer. The chain termination processes were interpreted with the 1D sequence inverse-gated decoupled ¹³C NMR measurement of the resulting polyethylenes and DFT calculations along with the relevant polymerization mechanism.