Zirconium and Hafnium Complexes Bearing Tridentate ONN-Ligands: Extremely High Activity toward Ethylene (Co)Polymerization

The pursuit of high-performance catalysts in the realm of polyolefins is a constant goal. In this study, a range of zirconium (<b>1-ZrCl</b>_<b>3</b>, <b>2-ZrCl</b>_<b>3</b>, <b>3-ZrCl</b>_<b>4</b>, <b>1</b>_<b>2</b><b>-Zr</b>) and hafnium (<b>1-HfCl</b>_<b>3</b>, <b>1</b>_<b>2</b><b>-Hf</b>) complexes featuring phenoxy-imine-amine ONN-ligands (2,6-R₂-C₆H₃-NH-C₆H₄-N═CH-C₆H₂-3,5-<i>^t</i>Bu₂-OH; <b>1-L</b>: R = H; <b>2-L</b>: R = F; <b>3-L</b>: R = <i>ⁱ</i>Pr) were synthesized and characterized using NMR spectroscopy, as well as single-crystal X-ray diffraction for <b>2-ZrCl</b>_<b>3</b>, <b>3-ZrCl</b>_<b>4</b>, and <b>1</b>_<b>2</b><b>-Zr</b>. These Zr and Hf complexes exhibited remarkable efficiency for ethylene homopolymerization and copolymerization with 1-octene when paired with MAO as the cocatalyst. Notably, the Zr complexes outperformed the Hf complexes with the same ligand, underscoring the substantial impact of the metal center on catalytic performance. The substituents and coordination modes of the ligands also exerted significant influence on the catalytic behavior, affecting both the activity and properties of the resulting polymers. Particularly noteworthy was the exceptional activity of <b>1-ZrCl</b>_<b>3</b>, achieving activity as high as 6.30 × 10⁸ g(PE)·mol^-1(Zr)·h^-1 for ethylene homopolymerization and generating bi- or multimodal distribution polyethylene. The activation of <b>1-ZrCl</b>_<b>3</b> by 5 or 20 equiv of <i>d</i>-MAO afforded a dinuclear Zr complex bridged by two chlorides (<b>μ-Cl</b>_<b>2</b><b>-(1-ZrCl</b>_<b>2</b><b>)</b>_<b>2</b>), which was analyzed and confirmed by <i>in situ</i> ¹H NMR spectroscopy and single-crystal X-ray diffraction.