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-(Ph(2)CH)(2)-3,4-F(2)C(6)H)}-6-(CMeNAr)C(5)H(3)N]CoCl(2) (Ar = 2,6-Me(2)C(6)H(3) Co1, 2,6-Et(2)C(6)H(3)Co2, 2,6-iPr(2)C(6)H(3)Co3, 2,4,6-Me(3)C(6)H(2)Co4, 2,6-Et-4-MeC(6)H(2)Co5), were synthesized in good yield from the corresponding unsymmetrical N,N,N′-ligands, L1–L5. Besides characterization of Co1–Co5 by FT-IR spectroscopy, (19)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 pseudo-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(6) g PE mol(−1) (Co) h(−1)) which decreased to 3.3 × 10(6) g PE mol(−1) (Co) h(−1) at 80 °C. In addition, it was found that the polymerisation activity increased as the steric hindrance imparted by the ortho 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(−1)) when using Co5/MMAO at 30 °C, while narrow dispersities (M(w)/M(n) range: 1.8–4.7) and high linearity (T(m) > 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 meta- and para-positions was demonstrated to boost catalytic activity and improve thermal stability.