Highly Selective Nickel Ethylene Oligomerization Catalysts Based on Sterically Hindered Tris(pyrazolyl)borate Ligands

The reaction of TlTp‘ (Tp‘ = HB(3-mesitylpyrazolyl) 3 - (Tp Ms ), HB(3-mesitylpyrazolyl) 2 (5-mesitylpyrazolyl) - (Tp Ms* )) with NiCl 2 ·6H 2 O affords Tp Ms NiCl ( 1 ) and Tp Ms* NiCl ( 2 ) in good yield. The compound 2 undergoes an isomerization process to form [{Tp Ms** }NiCl] 2 ( 3 ) (Tp Ms** = HB(5-mesitylpyrazolyl) 2 (3-mesitylpyrazolyl) - ) in 68% yield. Treatment of the tris(pyrazolyl)borate nickel compounds 1 and 2 with alkylaluminum cocatalysts such as methylalumoxane (MAO) and trimethylaluminum (TMA) in toluene generates active catalysts for ethylene oligomerization. The compound 1 shows turnover frequencies in the range of (2.2−43.1) × 10 3 h - 1 . Oligomerization reaction conditions can be adjusted that lead to selectivities as high as 81% for butene-1.