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

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).