Fusing Carbocycles of Inequivalent Ring Size to a Bis(imino)pyridine-Iron Ethylene Polymerization Catalyst: Distinctive Effects on Activity, PE Molecular Weight, and Dispersity

The 4,6-bis(arylimino)-1,2,3,7,8,9,10-heptahydrocyclohepta[b]quinoline-iron(II) chlorides (aryl = 2,6-Me₂C₆H₃ Fe1; 2,6-Et₂C₆H₃ Fe2; 2,6-i-Pr₂C₆H₃ Fe3; 2,4,6-Me₃C₆H₂ Fe4; and 2,6-Et₂-4-Me₂C₆H₂ Fe5) have been prepared in good yield by a straightforward one-pot reaction of 2,3,7,8,9,10-hexahydro-1H-cyclohepta[b]quinoline-4,6-dione, FeCl₂·4H₂O, and the appropriate aniline in acetic acid. All ferrous complexes have been characterized by elemental analysis and FT-IR spectroscopy. In addition, the structure of Fe3 has been determined by single crystal X-ray diffraction, which showed the iron center to adopt a distorted square pyramidal geometry with the saturated sections of the fused six- and seven-membered carbocycles to be cis-configured. In combination with either MAO or MMAO, Fe1-Fe5 exhibited exceptionally high activities for ethylene polymerization (up to 15.86 × 10⁶ g(PE) mol^-1 (Fe) h^-1 at 40°C (MMAO) and 9.60 × 10⁶ g(PE) mol^-1 (Fe) h^-1 at 60°C (MAO)) and produced highly linear polyethylene (HLPE, T _m ≥ 128°C) with a wide range in molecular weights; in general, the MMAO-promoted polymerizations were more active. Irrespective of the cocatalyst employed, the 2,6-Me₂-substituted Fe1 and Fe4 proved the most active while the more sterically hindered 2,6-i-Pr₂ Fe3 the least but afforded the highest molecular weight polyethylene (M _{w :} 65.6-72.6 kg mol^-1). Multinuclear NMR spectroscopic analysis of the polymer formed using Fe4/MMAO at 40°C showed a preference for fully saturated chain ends with a broad bimodal distribution a feature of the GPC trace (M _w/M _n = 13.4). By contrast, using Fe4/MAO at 60°C a vinyl-terminated polymer of lower molecular weight (M _w = 14.2 kg mol^-1) was identified that exhibited a unimodal distribution (M _w/M _n = 3.8). Moreover, the amount of aluminoxane cocatalyst employed, temperature, and run time were also found to be influential on the modality of the polymer.