Tantalum Complexes Incorporating Tris(pyrazolyl)Borate Ligands: Syntheses, Structures, and Ethylene Polymerization Behavior

Abstract

Four new Ta complexes bearing tris(pyrazolyl)borate ligands, Tp*TaCl(η²-3-hexyne)(benzyl) [<b>1</b>; Tp* = HB(3,5-dimethylpyrazolyl)₃⁻], Tp^{Ms^*}TaCl₂(η²-3-hexyne) [<b>2</b>; Tp^{Ms^*} = HB(3-mesitylpyrazolyl)₂(5-mesitylpyrazolyl)⁻], Tp^{Ms^*}TaCl₂(N-2,6-^<i>i</i>Pr₂C₆H₃) (<b>3</b>), and Tp^MsTaCl₂(N-^<i>t</i>Bu) [<b>4</b>; Tp^Ms = HB(3-mesitylpyrazolyl)₃⁻], were prepared in moderate to high yields from appropriate TaCl₃(R) (R; η²-3-hexyne, N-2,6-^<i>i</i>Pr₂C₆H₃, N-^<i>t</i>Bu) species and the potassium or thallium salt of the corresponding tris(pyrazolyl)borate ligand. The molecular structures of complexes <b>3</b> and <b>4</b> 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 <i>cis</i>-located Cl atoms, and the imido group. In combination with dried methylaluminoxane or ^<i>i</i>Bu₃Al/Ph₃CB(C₆F₅)₄, complexes <b>1</b>−<b>4</b> were all active for ethylene polymerization and produced low to very high molecular weight, highly linear polyethylenes. In particular, upon activation with ^<i>i</i>Bu₃Al/Ph₃CB(C₆F₅)₄, high activities were observed for complexes <b>3</b> and <b>4</b>, 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 <b>3</b>) and 17.1 kg-PE/mmol-Ta·h (complex <b>4</b>), 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.