Preparation of Pincer Hafnium Complexes for Olefin Polymerization

Pincer-type [C^naphthyl, N^pyridine, N^amido]HfMe₂ complex is a flagship among the post-metallocene catalysts. In this work, various pincer-type Hf-complexes were prepared for olefin polymerization. Pincer-type [N^amido, N^pyridine, N^amido]HfMe₂ complexes were prepared by reacting in situ generated HfMe₄ with the corresponding ligand precursors, and the structure of a complex bearing 2,6-Et₂C₆H₃N^amido moieties was confirmed by X-ray crystallography. When the ligand precursors of [(CH₃)R₂Si-C₅H₃N-C(H)PhN(H)Ar (R = Me or Ph, Ar = 2,6-diisopropylphenyl) were treated with in situ generated HfMe₄, pincer-type [C^silylmethyl, N^pyridine, N^amido]HfMe₂ complexes were afforded by formation of Hf-CH₂Si bond. Pincer-type [C^naphthyl, S^thiophene, N^amido]HfMe₂ complex, where the pyridine moiety in the flagship catalyst was replaced with a thiophene unit, was not generated when the corresponding ligand precursor was treated with HfMe₄. Instead, the [S^thiophene, N^amido]HfMe₃-type complex was obtained with no formation of the Hf-C^naphthyl bond. A series of pincer-type [C^naphthyl, N^pyridine, N^alkylamido]HfMe₂ complexes was prepared where the arylamido moiety in the flagship catalyst was replaced with alkylamido moieties (alkyl = iPr, cyclohexyl, tBu, adamantyl). Structures of the complexes bearing isopropylamido and adamantylamido moieties were confirmed by X-ray crystallography. Most of the complexes cleanly generated the desired ion-pair complexes when treated with an equivalent amount of [(C₁₈H₃₇)₂N(H)Me]⁺[B(C₆F₅)₄]^-, which showed negligible activity in olefin polymerization. Some complexes bearing bulky substituents showed moderate activities, even though the desired ion-pair complexes were not cleanly afforded.