1,5-Naphthyl-linked bis(imino)pyridines as binucleating scaffolds for dicobalt ethylene oligo-/polymerization catalysts: exploring temperature and steric effects

Abstract

Six examples of dinuclear bis(imino)pyridine-cobalt(II) complexes, [1,5-{2-(CMeN)-6-(CMeN(2,6-R¹₂-4-R²-C₆H₂))C₅H₃N}₂(C₁₀H₆)]Co₂Cl₄ (R¹ = Me, R² = H Co1; R¹ = Et, R² = H Co2; R¹ = ⁱPr, R² = H Co3; R¹ = Me, R² = Me Co4; R¹ = Et, R² = Me Co5; R¹ = CHPh₂, R² = Me Co6), have been prepared from the corresponding bis(tridentate) compartmental ligands (L1–L6) in reasonable yields. The molecular structures of Co3 and Co5 revealed two N,N,N-cobalt dichloride units to adopt anti-positions about the 1,5-naphthyl linking unit, with each cobalt center exhibiting a distorted trigonal bipyramidal geometry. On activation with either MAO or MMAO, Co1–Co6 were shown to promote both polymerization and oligomerization of ethylene with high overall activities (up to 1.03 × 10⁷ gPE per·mol(Co) per·h for Co1/MAO at 70 °C). Curiously, on increasing the reaction temperature a larger proportion of polymer was noted, while at lower temperature an enhanced selectivity for oligomer was seen. In general, the oligomeric products displayed Schulz–Flory distributions with high selectivities for α-olefins (>99%). On the other hand, the highly linear polymers displayed narrow dispersities and comprised both fully saturated and unsaturated chain ends with the vinyl content (–CHCH₂) found to rise with the reaction temperature. By modulating the steric hindrance exerted by the ortho-R¹ substituents in the precatalyst, polyethylenes displaying a remarkably broad range of molecular weights could be obtained [from 4.52 kg mol⁻¹ (R¹ = Me) to 246.7 kg mol⁻¹ (R¹ = CHPh₂)].