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 (<b>7</b>) or 2-(arylamido)-cyclohex-2-en-1-arylimine (<b>8</b>); 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 <i>T</i> _m 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.