Novel nickel catalysts based on perfluoroalkyl-β-diketone ligands for the selective dimerization of propylene to 2,3-dimethylbutenes

The propylene dimerization to 2,3-dimethylbutenes (DMB) by novel homogeneous catalysts prepared in situ by two alternative routes was described. The former synthetic procedure was based on the oxidative addition of hexafluoro-2,4-pentandione (hfacac) to bis(1,5-cyclooctadiene)nickel(0) [Ni(cod)2] in the presence of an ancillary phosphine ligand and an organoaluminium compound. The latter route consisted in the reaction of bis(perfluoroalkyl-β-diketonato)nickel(II) complexes with the organoaluminium compound in the presence of the phosphine ligand. In this context the novel bis(tetradecafluoro-4,6-nonandionato)nickel(II)·2 dimethylformamide complex [Ni(tdfnd)2(dmf)2] was synthesized and its crystal and molecular structures were determined. The catalytic systems obtained with both procedures resulted highly active in the oligomerization of propylene (TOF>20000 h−1). In particular, when Ni(hfacac)2 was combined with the bulky and basic tricyclohexylphosphine (PCy3) and MAO was adopted as co-catalyst a very high regioselectivity to DMB within the C6 cut was achieved (ca. 90%). The use of Ni(tdfnd)2(dmf)2 in the place of Ni(hfacac)2 gave similar results under the same reaction conditions. Changing the type of phosphine ligand and the nature of the organoaluminium co-catalyst it was possible to deeply modify both regio- and chemo-selectivity of the process, thus allowing to orientate the reaction towards specific target oligomeric products.