Development of Quinoline-Amido and Hydrogenated Quinoline-Imino Hafnium Complexes and Their Olefin Polymerizations

This study presents the synthesis of quinoline-amido and hydrogenated quinoline-imino Hf complexes with a six-membered NN-chelate metallacycle and their catalytic properties toward olefin polymerization. In the presence of [Ph₃C][B(C₆F₅)₄] as the cocatalyst, quinoline-amido Hf complexes (Hf1, Hf2, Hf4, and Hf5) showed high activity (up to 32200 kg(polymer)·mol^-1(Hf)·h^-1), whereas hydrogenated quinoline-imino Hf complexes (Hf3 and Hf6) produced polymers with high-molecular weight (up to 147.9 × 10⁴ g·mol^-1). DFT calculations revealed that the aromatic quinoline ring in Hf1, Hf2, Hf4, and Hf5 induced electron delocalization and resulted in strong positive charge on Hf center, while the unexpected low energy of the important HOMO orbital in Hf3 and Hf6 led to their low reactivity of Hf-C bond and low activity. Interestingly, complex Hf1 exhibited high activity in the presence of different cocatalysts, including [Ph₃C][B(C₆F₅)₄], [HNMe₂Ph][B(C₆F₅)₄], B(C₆F₅)₃, and MAO. Particularly, the Hf1/MAO catalytic system achieved an activity as high as 35500 kg(polymer)·mol^-1(Hf)·h^-1 and produced the polyethylene wax with a low-molecular weight of 0.57 × 10⁴ g·mol^-1.