Synthesis, characterization and ethylene (Co)polymerization of binuclear Vanadium(IV) catalysts based on xanthene-bridged bis-salicylaldiminato ligands

Abstract

Bimetallic complexes have attracted much attention due to their excellent catalytic performances in many fields. Herein six xanthene-bridged bis-salicylaldiminato [ON] bidentate binuclear vanadium(IV) complexes V2L1-V2L6 were synthesized and characterized by MALDI-TOF-MS, FTIR spectroscopy and elemental analysis, and the crystal structures of V2L1 and V2L2 were further characterized by XRD, in which each vanadium atom was found coordinated with three O, one Cl and one N atoms to form a twisted trigonal bipyramidal configuration. Under Et2AlCl and ethyl trichloroacetate (ETA) activation, all the binuclear vanadium(IV) complexes exhibited high activity over 106 g/mol(V)·h·atm towards ethylene polymerization and copolymerization with 1-hexene. The activity of binuclear V2L4 for ethylene polymerization was much higher than that of its mononuclear counterpart VL, indicating that strong bimetallic synergetic effect existed in these binuclear vanadium complexes. The substituents on the ligands have a great influence on the catalytic performances of the vanadium complexes. The introduction of bulky isopropyl group into the ligand had little effect on the activity, but the introduction of electron-withdrawing fluorine atoms could significantly increase the activity, suggesting that the electronic, rather than the steric factors had a greater impact. As the amount of fluorine substituent on the iminophenyl moiety of the complex increased, the catalytic activity increased appreciably. With five fluorine atoms on the benzene ring of the imine moiety, the complex V2L6 showed the highest activity of 1.10 × 107 g/mol(V)·h·atm for ethylene polymerization to produce the highest molecular weight of polyethylene. Furthermore, the introduction of fluorine atoms into ligands could also increase the catalytic activity of vanadium complexes, comonomer incorporation and molecular weight of obtained copolymer towards ethylene/1-hexene copolymerization. The highest copolymerization activity of V2L6 was 4.13 × 106 g/molV·h·atm, which was more than three time that of the fluorine-free complex V2L1. The insertion rate of 1-hexene for the fluorine-containing complex V2L3-V2L6 was also higher than that for V2L1.