Design of Neutral Ni[N,N] Catalysts for High-Density Polyethylene Formation: Insights into Catalyst Deactivation

Neutral Ni[N,O] and cationic Ni[N,N] complexes are two prominent classes of catalysts for the polymerization of ethylene. We report the amalgamation of these motifs and describe the synthesis and catalytic activity of two neutral Ni[N,N] catalysts [Ni(Ph)(L)(PPh3)], where L = 2-(arylamido)-5-methylcyclopent-2-en-1-arylimine (7) or 2-(arylamido)-cyclohex-2-en-1-arylimine (8); aryl is the ubiquitous 2,6-diisopropylphenyl group (Dipp). Despite the increased steric bulk, ethylene polymerization was only achieved using B(C6F5)3 as an activator, forming high-density polyethylene at 0 °C with Tm up to 140 °C, in contrast to the branched polymers characterized by lower melting points that are more typically formed using nickel catalysts. Surprisingly, both complexes undergo C–H activation over days at room temperature, eliminating benzene and forming a cyclometalated Ni(II) complex that, for the first time, was characterized by multinuclear NMR spectroscopy and single-crystal X-ray diffraction, structurally characterizing this decomposition route for Ni ethylene polymerization catalysts.