Tantalum complexes incorporating tris(pyrazolyl)borate ligands: Syntheses, structures, and ethylene polymerization behavior

Four new Ta complexes bearing tris(pyrazolyl)borate ligands, Tp*TaCl(η²-3-hexyne)(benzyl) [1; Tp* = HB(3,5-dimethylpyrazolyl)₃^-], Tp^Ms* TaCl₂(η²-3-hexyne) [2; Tp^Ms* = HB(3-mesitylpyrazolyl)₂(5-mesitylpyrazolyl)^-], Tp ^Ms* TaCl₂(=N-2,6-ⁱ:Pr₂C ₆H₃) (3), and Tp^MsTaCl₂(=N- ^tBu) [4; Tp^Ms = HB(3-mesitylpyrazolyl)3 ], were prepared in moderate to high yields from appropriate TaCl₃(R) (R; η²-3-hexyne, =N-2,6-^tPr₂C₆H ₃, =N-^tBu) species and the potassium or thallium salt of the corresponding tris(pyrazolyl)borate ligand. The molecular structures of complexes 3 and 4 established by X-ray analyses snowed that these complexes adopted a distorted octahedral geometry in which the Ta metal is coordinated by three N atoms of the tris(pyrazolyl)borate ligand (facial coordination), two cis-located Cl atoms, and the imido group. In combination with dried methylaluminoxane or ⁱBu₃Al/Ph₃CB(C ₆F₅)₄, complexes 1-4 were all active for ethylene polymerization and produced low to very high molecular weight, highly linear polyethylenes. In particular, upon activation with ⁱBu ₃Al/Ph₃CB(C₆F₅)₄, high activities were observed for complexes 3 and 4, which possess the sterically encumbered mesityl-substituted tris(pyrazolyl)borate ligand and the imido group as an auxiliary ligand. The activities, 25.7 kg-PE/mmol-Ta-h (complex 3) and 17.1 kg-PE/ mmol-Ta . h (complex 4), represent some of the highest values reported to date for Ta-based catalysts. These highly active Ta-based catalysts were long-lived and maintained a practically steady ethylene uptake for 30 min at 80 °C.