NNN-type iron(II) complexes consisting sterically hindered dibenzocycloheptyl group: Synthesis and catalytic activity towards ethylene polymerization

Abstract

Unsymmetrical Fe(II) complexes bearing tridentate NNN-bis(arylimino)pyridine ligand of the general formula [2-(2,4-bis(C15H13)2-6-CH3C6H2N)-6-(ArN)C9H9N]FeCl2 {Ar = 2,6-(CH3)2C6H3 (Fe1), 2,6-(C2H5)2C6H3 (Fe2), 2,6-(C3H7)2C6H3 (Fe3), 2,4,6-(CH3)3C6H2 (Fe4) and 2,6-(C2H7)2-4-CH3C6H2 (Fe5)} have been prepared in good yields. All the complexes were characterized by FT-IR spectroscopy, elemental analysis, and single-crystal X-ray diffraction analysis for Fe2 and Fe3. Upon activation with MAO or modified-MAO, all the complexes displayed high activity at elevated temperature with peak performance up to 2.13 × 107 g PE (mol of Fe)−1 h-1 using Fe4 at 70 °C in combination with modified-MAO. The most sterically hindered precatalyst Fe3 produced high molecular-weight polyethylene up to 307.12 kg mol-1 at 70 °C with MAO as co-catalyst. Particularly, polyethylene of vinyl end functionality was obtained with MAO. Significantly, this developed catalytic system maintained the good catalytic performance of 9.67–7.59 × 106 g PE (mol of Fe)−1 h-1 and 1.51–1.09 × 107 g PE (mol of Fe)−1 h-1 using Fe1 with MAO or modified-MAO at 80 °C–90 °C, respectively. Notably, the activity of 3.12 or 2.76 × 106 g PE (mol of Fe)−1 h-1 was achieved even at 100 °C; indicating high thermal stability of the active species. Indeed, the thermal stability and molecular weight of the resultant polyethylene reached the highest values to be reported for this class of unsymmetrical 2,6-bis(arylimino)pyridine-ferrous precatalysts.