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.