Bis-cycloheptyl-fused bis(imino)pyridine-cobalt catalysts for PE wax formation: positive effects of fluoride substitution on catalytic performance and thermal stability

Abstract

The α,α′-bis(imino)-2,3:5,6-bis(pentamethylene)pyridyl-cobalt(II) chlorides, [2,3:5,6-{C₄H₈C(N(2-R¹-4-R³-6-R²C₆H₂))}₂C₅HN] CoCl₂ (R¹ = Me, R² = R³ = CH(p-FPh)₂Co1; R¹ = Et, R² = R³ = CH(p-FPh)₂Co2; R¹ = i-Pr, R² = R³ = CH(p-FPh)₂Co3; R¹ = Cl, R² = R³ = CH(p-FPh)₂Co4; R¹ = F, R² = R³ = CH(p-FPh)₂Co5; R¹ = F, R² = R³ = CHPh₂Co5′′, R¹ = R² = Me, R³ = CH(p-FPh)₂Co6; R¹ = R³ = Me, R² = CH(p-FPh)₂Co7), have been synthesized by a one-pot template reaction of α,α′-dioxo-2,3:5,6-bis(pentamethylene)pyridine, cobalt(II) chloride and the respective aniline in n-butanol. By contrast, the mixed cobalt(II) chloride/acetate complex, [2,3:5,6-{C₄H₈C(N(2-F-4,6-(CH(p-FPh)₂)₂C₆H₂))}₂C₅HN]CoCl(OAc) (Co5′), was isolated when the corresponding template reaction was carried out in acetic acid. Structural characterization of Co4, Co5 and Co5′′ revealed distorted square pyramidal geometries while six-coordinate Co5′, incorporating a chelating acetate ligand, exhibited a distorted octahedral geometry. On activation with either MAO or MMAO, 2-fluoride-4,6-bis{di(p-fluorophenyl)methyl}-substituted Co5 showed maximum catalytic activity for ethylene polymerization at a high operating temperature of 60 °C (up to 2.1 × 10⁷ g (PE) mol⁻¹ (Co) h⁻¹), producing highly linear (T_ms > 121 °C), low molecular weight polyethylene waxes (M_w range: 1.5–5.0 kg mol⁻¹) with narrow dispersity (M_w/M_n range: 1.7–2.9). End-group analysis of the waxes reveals β-H elimination as the dominant chain transfer process.