Synthesis and characterization of ether-imine-furfural [ONO] nickel(II) complexes and their application in oligomerization of ethylene

• New class of nickel catalysts bearing ether-imine-furfural ligands.
• XPS data and DFT results indicate the formation of nickel dimeric species.
• Nickel precatalysts exhibit high activities in ethylene oligomerization.
• Activity and selectivity results are highly influenced by type of cocatalyst.

A series of new Ni(II) complexes of general formula {NiCl2(L)}2 [Ni1, L = 5-methyl-2-(C4H3O-2-CHN)C2H4OPh; Ni2, L = 5-Methyl-2-(C4H3O-2-CHN)Ph-2-OPh; Ni3, L = 2-(C4H3O-2-CHN)Ph-2-OPh; Ni4, L = 5-Methyl-2-(C4H3O-2-CHN)CH2Ph-2-OMe were prepared and characterized by IR spectroscopy, elemental analysis, high-resolution mass spectrometry (HRMS), and X-ray photoelectron spectroscopy (XPS). XPS data suggest no interaction of the pendant ether/furfural donor groups with the nickel metal center. Density functional theory studies indicate the formation of nickel dimeric species with ether-imine-furfural acting as a monodentate ligand. All nickel precatalysts, activated with polymethylaluminoxane-improved performance (PMAO-IP), exhibited moderate to good activities for ethylene oligomerization [TOF = 14.7–57.3 × 103 mol(ethylene) mol(Ni)−1 h−1] with high selectivities for production of 1-butene (63.2–83.2 wt.%). Activation of Ni2 using different types of cocatalysts (PMAO-IP, DMAO, MAO or EASC) led to formation of active oligomerization systems with a significant impact on the activity and selectivity. The use of MAO instead of PMAO-IP gave a slight improvement in the TOF [69.3 × 103 mol(ethylene) mol(Ni)−1 h−1] with lower selectivity for 1-butene. Higher activity was obtained using EASC as cocatalyst [206.1 × 103 mol(ethylene) mol(Ni)−1 h−1] along with a drastic reduction in the selectivity for 1-butene (10.4 wt.%). Under optimized conditions [[Ni] = 5 μmol, 30 °C, oligomerization time = 20 min, 20 bar ethylene, MAO as cocatalyst (500 equiv)], precatalyst Ni2 led to TOF = 63.2 × 103 mol(C2H4)·(mol(Ni)−1 h−1) and 75.2 wt.% selectivity for 1-butene.

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