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

The reaction of TlTp‘ (Tp‘ = HB(3-mesitylpyrazolyl)3 - (TpMs), HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)- (TpMs*)) with NiCl2·6H2O affords TpMsNiCl (1) and TpMs*NiCl (2) in good yield. The compound 2 undergoes an isomerization process to form [{TpMs**}NiCl]2 (3) (TpMs**= 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) × 103 h-1. Oligomerization reaction conditions can be adjusted that lead to selectivities as high as 81% for butene-1.