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

Six examples of dinuclear bis(imino)pyridine-cobalt(ii) complexes, [1,5-{2-(CMe[double bond, length as m-dash]N)-6-(CMe[double bond, length as m-dash]N(2,6-R ¹ ₂ -4-R ² -C ₆ H ₂ ))C ₅ H ₃ N} ₂ (C ₁₀ H ₆ )]Co ₂ Cl ₄ (R ¹ = Me, R ² = H Co1; R ¹ = Et, R ² = H Co2; R ¹ = ⁱ Pr, R ² = H Co3; R ¹ = Me, R ² = Me Co4; R ¹ = Et, R ² = Me Co5; R ¹ = CHPh ₂ , R ² = 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 × 10 ⁷ 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 (-CH[double bond, length as m-dash]CH ₂ ) found to rise with the reaction temperature. By modulating the steric hindrance exerted by the ortho-R ¹ substituents in the precatalyst, polyethylenes displaying a remarkably broad range of molecular weights could be obtained [from 4.52 kg mol ^-1 (R ¹ = Me) to 246.7 kg mol ^-1 (R ¹ = CHPh ₂ )].