Exploring fluoride effects in sterically enhanced cobalt ethylene polymerisation catalysts; a combined experimental and DFT study.

The fluoro-substituted 2,6-bis(arylimino)pyridine dichlorocobalt complexes, [2-{CMeN(2,6-(PhCH)-3,4-FCH)}-6-(CMeNAr)CHN]CoCl (Ar = 2,6-MeCH Co1, 2,6-EtCHCo2, 2,6-iPrCHCo3, 2,4,6-MeCHCo4, 2,6-Et-4-MeCHCo5), were synthesized in good yield from the corresponding unsymmetrical ,,'-ligands, L1-L5. Besides characterization of Co1-Co5 by FT-IR spectroscopy, F NMR spectroscopy and elemental analysis, the molecular structures of Co2 and Co5 were also determined highlighting the unsymmetrical nature of the terdentate ligand and the -square pyramidal geometry about the metal center. When either MAO or MMAO were employed as activators, Co1-Co5 were able to achieve a wide range of catalytic activities for ethylene polymerisation. Co5/MAO exhibited the highest activity of the study at 60 °C (7.6 × 10 g PE mol (Co) h) which decreased to 3.3 × 10 g PE mol (Co) h at 80 °C. In addition, it was found that the polymerisation activity increased as the steric hindrance imparted by the groups was enhanced (for MMAO: Co3 > Co5 > Co2 > Co1 > Co4), a finding that was supported by DFT calculations. Furthermore, it was shown that particularly high molecular weight polyethylene could be generated (up to 483.8 kg mol) when using Co5/MMAO at 30 °C, while narrow dispersities ( / range: 1.8-4.7) and high linearity ( > 131.4 °C) were a feature of all polymers produced. By comparison of Co3 with its non-fluorinated analogue using experimental data and DFT calculations, the substitution of fluorides at the - and -positions was demonstrated to boost catalytic activity and improve thermal stability.