Ni(ii) complexes bearing pyrrolide-imine ligands with pendant N-, O- and S-donor groups: synthesis, structural characterization and use in ethylene oligomerization

Abstract

A series of new Ni(II) complexes of general formula {L}NiCl [Ni1, L = 2-(C₄H₃N-2′-CHN)C₂H₄NHPh; Ni2, L = 5-tert-butyl-2-(C₄H₂N-2′-CHN)C₂H₄NHPh; Ni3, L = 2-(C₄H₃N-2′-CHN)C₂H₄OPh; Ni4, L = 2-(C₄H₃N-2′-CHN)C₆H₄-2′-OPh; Ni5, L = 2-(C₄H₃N-2′-CHN)C₆H₄-2′-SPh; Ni6, L = 2-(C₄H₃N-2′-CHN)CH₂C₆H₄-2′-OMe] were prepared and fully characterized. All nickel precatalysts, activated with methylaluminoxane (MAO), exhibited moderate to good activities for ethylene oligomerization [TOF = 6.1–71.3 × 10³ mol (C₂H₄) (mol (Ni)⁻¹ h⁻¹)] and producing high selectivities for 1-butene (68.3–94.0 wt%). The catalytic performance was substantially affected by the ligand environment regarding the pendant oxygen- and sulfur-donor groups, and the substituents on the pyrrolide group. Activation of nickel precatalyst Ni3 with ethylaluminum sesquichloride (Et₃Al₂Cl₃, EASC) instead of MAO produced a significantly more productive catalyst system than Ni2/MAO (TOF = 153 700 vs. 43 500 mol (C₂H₄) (mol (Ni)⁻¹ h⁻¹)); however, the 1-butene selectivity was drastically reduced, attaining only 53 wt% with a concomitant production of larger amounts of internal butenes (38 wt%). Under optimized conditions ([Ni] = 10 μmol, 30 °C, oligomerization time = 20 min, 20 bar ethylene, [Al]/[Ni] = 250), precatalyst Ni3 led to a TOF = 55 900 mol (C₂H₄) (mol (Ni)⁻¹ h⁻¹) and 82.8 wt% selectivity for 1-butene.