Unexpected Formation of Neutral Alkyl-Ni(II) Complexes Bearing Unsymmetrical C , N , N ’-Pincer Iminopyrrolyl Ligands via Intramolecular C–H Bond Activation: A Distinct Type of Single-Component Ethylene Polymerization Catalysts

During the development of neutral aryl-nickel(II) triphenylphosphine complexes bearing bidentate 5-aryl-2-(N-aryl)formiminopyrrolyl ligands as single-component ethylene polymerization catalysts, an unexpected intramolecular C-H activation was observed. Reaction of sodium salts of ligand precursors 5-(2,6-Me₂C₆H₃)-2-(N-2,6-R₂C₆H₃)formiminopyrrole (R = Me I, iPr II, 3,5-(CF₃)₂C₆H₃ III) with trans-[Ni(Ar)(PPh₃)₂Cl] (Ar = o-C₆H₄Cl or C₆H₅) afforded unsymmetrical tridentate C,N,N'-pincer iminopyrrolyl nickel(II) complexes [Ni{κ³C,N,N'-5-(2'-CH₂-6-MeC₆H₃)-NC₄H₂-2-C(H)═N-2,6-R₂C₆H₃}(PPh₃)] (R = Me 1, iPr 2, 3,5-(CF₃)₂C₆H₃ 3). Complexes 1-3 and their ligand precursors were characterized using multinuclear NMR, X-ray diffraction, and elemental analysis. NMR revealed initial coexistence of tridentate C,N,N'- and bidentate N,N'-iminopyrrolyl Ni(II) complexes; at higher temperature, only the tridentate remain. DFT calculations indicated that cyclometalation proceeds via an agostic C-H interaction with the Ni center, enabling C-H activation. Thermally stable 1-3 were evaluated as single-component catalysts for ethylene polymerization (25-50 °C, 3-15 bar), both with and without Ni(COD)₂ as phosphine scavenger. The resulting monomodal hyperbranched polyethylenes (HBPEs) displayed higher molecular weights and branching than those from parent bidentate 2-iminopyrrolyl phenyl-Ni(II) catalysts. These novel tridentate C,N,N'-pincer systems exhibited significantly enhanced activity and robustness. The HBPEs produced are promising as synthetic base oils for high-performance lubricants and pour-point depressants.