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−], TpMs*TaCl2(η2-3-hexyne) [2; TpMs* = HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)−], TpMs*TaCl2(N-2,6-iPr2C6H3) (3), and TpMsTaCl2(N-tBu) [4; TpMs = HB(3-mesitylpyrazolyl)3−], were prepared in moderate to high yields from appropriate TaCl3(R) (R; η2-3-hexyne, N-2,6-iPr2C6H3, 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 showed 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(C6F5)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 iBu3Al/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.