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Unexpected Formation of Neutral Alkyl-Ni(II) Complexes Bearing Unsymmetrical C,N,N’-Pincer Iminopyrrolyl Ligands via Intramolecular C–H Bond Activation
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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.