A catalytic and DFT study of selective ethylene oligomerization by nickel(II) oxime-based complexes

The reactivity of nickel(II) thiophenealdoxime complex (3) toward oligomerization of ethylene in the presence of an alkylaluminum co-catalyst has been studied. The complex was found to be a selective ethylene dimerization catalyst in the presence of co-catalysts such as methylalumoxane (MAO) and diethylaluminum chloride (DEAC). With DEAC, the productivity was considerably higher than with MAO. Under optimum conditions the productivity reaches 388 kg/mol catalyst/h/bar with DEAC whereas for MAO this value was 119 kg/mol catalyst/h/bar. Complex 3 displays good ethylene conversions of up to a maximum of 90% with exceptionally high α-selectivity for 1-butene (>99.5%) amongst C4 products. Computational studies using density functional theory (DFT) were also carried out to ascertain the decomposition pathway for 3 as well as that for Ni(II) complex of the pyridine ketoxime ligand 2. The results suggest that loss of one of the two bidentate oxime ligands attached to the metal center through reaction with DEAC is likely for both 2 and 3. Further, calculations indicate that the subsequent decomposition step was significantly more probable for 3 than for 2 thus explaining why the pyridine ketoxime ligand bound nickel complex 2 was experimentally found to be more stable than the thiophene aldoxime bound nickel complex 3. Calculations also show that the proton of the -OH group (oxime) plays a major role in the stability of the molecules. This was confirmed experimentally by synthesizing the Ni(II) dichloro complex of Pyridine-2-carbaldehyde O-methyloxime 5 and reacting it with ethylene under similar conditions. 5 was found to be highly active even at low co-catalyst concentrations.

The nickel(II)-thiophene oxime complex was found to be an excellent catalyst for selective ethylene oligomerization in the presence of alkyl aluminum co-catalysts, giving butenes and hexenes. Computational studies were also carried out to ascertain the decomposition pathway for this complex as well as that for the previously reported Ni(II) complex of the pyridine ketoxime ligand. As a result of the two studies, a new Ni(II) based catalyst system has been synthesized that gives a higher productivity and stability than both the catalyst system with nickel(II)-pyridine ketoxime and thiophene oxime complex.Figure optionsDownload high-quality image (69 K)Download as PowerPoint slideHighlights
► Complexes of nickel(II) with thiophene aldoxime and pyridine ketoxime were synthesized and characterized.
► The homogeneous complexes were found to be active for selective ethylene oligomerization in the presence of diethyl aluminum chloride co-catalyst.
► Selective formation of the ethylene dimer, 1-butene with >99% purity, was observed.
► Computational studies (DFT) on the decomposition pathway for the N–S and N–N type ligands were carried out to correlate the experimentally observed activity and the possible species responsible for the formation of products.