Unexpected Formation of Neutral Alkyl-Ni(II) Complexes Bearing Unsymmetrical <i>C</i> , <i>N</i> , <i>N</i> ’-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-Me 2 C 6 H 3 )-2-( N -2,6-R 2 C 6 H 3 )formiminopyrrole (R = Me I, i Pr II, 3,5-(CF 3 ) 2 C 6 H 3 III ) with trans -[Ni(Ar)(PPh 3 ) 2 Cl] (Ar = o -C 6 H 4 Cl or C 6 H 5 ) afforded unsymmetrical tridentate C, N, N’ -pincer iminopyrrolyl nickel(II) complexes [Ni{κ 3 C,N,N ’-5-(2′-CH 2 –6-MeC 6 H 3 )-NC 4 H 2 -2-C(H)═N-2,6-R 2 C 6 H 3 }(PPh 3 )] (R = Me 1, i Pr 2, 3,5-(CF 3 ) 2 C 6 H 3 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.