Doubly fused N,N,N-iron ethylene polymerization catalysts appended with fluoride substituents; probing catalytic performance via a combined experimental and MLR study

Abstract

Access to six examples of α,α′-bis(imino)-2,3:5,6-bis(pentamethylene)pyridine-iron(II) chloride complex, [2,3:5,6-{C₄H₈C(N(2-R¹-4-R³-6-R²C₆H₂))}₂C₅HN] (R¹ = Me, R² = R³ = CH(p-FPh)₂Fe1; R¹ = Et, R² = R³ = CH(p-FPh)₂Fe2; R¹ = iPr, R² = R³ = CH(p-FPh)₂Fe3; R¹ = F, R² = R³ = CH(p-FPh)₂Fe4; R¹ = R² = Me, R³ = CH(p-FPh)₂Fe5; R¹ = R³ = Me, R² = CH(p-FPh)₂Fe6), each appended with either one or two para-fluorinated benzhydryl substituents per N-aryl group, has been achieved via a one-pot template approach. ¹⁹F NMR spectroscopy has been used to demonstrate the restricted rotation of the ortho-CH(p-FPh)₂ groups in Fe1–Fe4 and Fe6, while the molecular structures of Fe3, Fe5 and Fe6 highlight the flexibility of the fused carbocycles and the steric protection offered to the metal center. Highly active catalysts (up to 22.5 × 10⁶ g (PE) mol⁻¹ (Fe) h⁻¹) for ethylene polymerization at 70 °C were attainable on activation of Fe1–Fe6 with either MAO or MMAO generating strictly linear polyethylene waxes with narrow polydispersity. In particular, ortho-fluoride Fe4 exhibited the highest activity of the series but delivered the lowest molecular weight polyethylene. End-group analysis of the polymers using ¹H NMR and semi-quantitative ¹³C NMR spectroscopy revealed that chain termination proceeds via two competitive pathways based on β-H elimination and chain transfer to aluminum. To complement the experimental data, multiple linear regression (MLR) analysis has been performed on Fe1–Fe6. Good agreement with the experimentally determined catalytic activities has been obtained by considering the net charge on the central metal atom and the open cone angle. Moreover, these findings highlight the dominant role played by electronic effects on catalytic performance in these systems.