Neutral Mono(5-aryl-2-iminopyrrolyl)nickel(II) Complexes as Precatalysts for the Synthesis of Highly Branched Ethylene Oligomers: Preparation, Molecular Characterization, and Catalytic Studies

Abstract

New substituted 5-aryl-2-(N-2,6-diisopropylphenylformimino)-1H-pyrrole ligand precursors reacted with the complex trans-[Ni(o-C6H4Cl)(PPh3)2Cl] to give rise to new mono(2-iminopyrrolyl) nickel(II) complexes with general formula [Ni{κ2N,N′-5-aryl-NC4H2-2-C(H)═N-2,6-iPr2C6H3}(o-C6H4Cl)(PPh3)]. The pyrrole 5-aryl substituent is a phenyl or a bulky anthracen-9-yl ring, the first being also modified by para-substituents with electron-releasing (methoxy) or electron-withdrawing (fluorine) characteristics. The new compounds were fully characterized by NMR spectroscopy, elemental analysis, and two of them by single crystal X-ray diffraction. The complexes were tested as aluminum-free catalysts for the oligo-/polymerization of ethylene, at different reaction conditions, revealing catalytic activity even when tested as single-component catalysts. The addition of a phosphine scavenger, such as [Ni(COD)2], enhances significantly the catalytic activity. The polyethylene products obtained are hyperbranched low-molecular-weight oligomers with Mn's in the range 570–3200 g/mol (GPC-based values) and very high branching degrees (80–130 branches/1000 C atoms).