High-temperature stable chromium complexes bearing fluorinated PNP ligands for selective ethylene tetramerization toward 1-octene

Abstract

A series of new fluorinated PNP ligands of bis(ortho-fluorophenyl)-type (L1-L2) and ortho-fluorophenyl-type (L3-L4) have been synthesized and assessed for the selectivity of ethylene oligomerization. Compared to the non-fluorinated analogue ligand L6, the fluorinated ligand L1 exhibited approximately three times high catalytic activity. The bis(ortho-fluorophenyl)-type ligand L2 with a diethylphosphine substituent offered the highest catalytic performance of 3548 kg∙g−1∙h−1 and the 1-octene selectivity is 67.6 %. Both the L1 and L2 ligands exhibited excellent the activity of catalyst at a high temperature of 100 ℃, with ligand L2 providing the catalytic performance of 1587 kg∙g−1∙h−1 and good selectivity of 47.5 % for 1-C8=. The ortho-fluorophenyl substituent enhances the catalytic activity, while the para-fluorophenyl substituent reduces the catalytic activity. In addition, density functional theory (DFT) calculations and UV–vis spectroscopy have been used to rationalize the role of the fluorine effect in promoting catalytic performance. Our results indicated that the strong inductive effect of fluorine atoms and the non-covalent interaction between fluorine atoms and active centers jointly enhance the stability of the ortho-fluorophenyl-based catalyst and promote its overall activity.