Highly selective nickel catalysts for ethylene oligomerization based on tridentate pyrazolyl ligands

A series of new nickel complexes NiCl2(L) based on tridentate NZN ligands (1, L = bis[2-(3,5-dimethyl-pyrazolyl)methyl]benzylamine; 2, L = bis[2-(3,5-dimethyl-l-pyrazolyl)methyl]butylamine; 3, L = bis[2-(3,5-dimethyl-l-pyrazolyl)methyl]sulfide; 4, L = bis[2-(3-phenyl-l-pyrazolyl)methyl]sulfide)) has been prepared and characterized by elemental analysis. Upon activation with methylaluminoxane (MAO) or diethylaluminum chloride (DEAC), these new pre-catalysts show high activity in ethylene oligomerization (TOF = 1.7–104.5 × 103 mol(ethylene) (mol(Ni))−1 h−1), giving predominantly 1-butene (70–94% yield). The catalytic performance is substantially affected by the co-catalyst type and ligand environment, especially the bridge donor atom. Under optimized conditions ([Ni] = 10 μmol, 30 °C, 20 bar ethylene, MAO-to-Ni = 250), pre-catalyst 3 leads to TOF = 104.5 × 103 h−1 and 70% selectivity for 1-butene.

Graphical abstractDimerization of ethylene in the presence of Ni(II) 5-membered chelate complexes based on tridentate nitrogen- or sulfur-bridged bis(pyrazolyl) ligands (NZN) (Z = N, S) activated with alkylaluminum (MAO or DEAC) proceeds with high activity and selectivity under moderate conditions. Figure optionsDownload full-size imageDownload as PowerPoint slide