β-diiminato titanium complexes with varying fluorine substitution patterns on the N-aryl moiety: probing the effect of ligand substitution on ethylene polymerization

A series of six mono β-diiminato titanium complexes with varying fluorine-substitution patterns on the iminophenyl moiety were designed and synthesized, and the ethylene polymerization activities were studied with modified methylaluminoxane (MMAO) as cocatalyst. The introduction of fluorine substituents significantly increased the catalytic activity of the β-diiminato titanium complexes compared with their alkyl-substituted counterparts. Furthermore, both the position and the amount of fluorine substitution greatly influenced the catalytic behavior. The activity was increased when the ortho-positions of the N-aryl were substituted by fluorine, and the more ortho-positions were substituted, the higher the activity. All of the three o-fluoro substituted complexes (2a, 2e and 2f) exhibited very high activity (up to 10⁶gmol⁻¹ h⁻¹ atm⁻¹); whereas the m- and p-fluoro derivatives (2b and 2c) were much less active. The high activity of the o-fluoro-substituted complexes is suggested to be due to the formation of non-covalent interactions between the o-fluorine atom and the ethylene monomer, which may assist in the enrichment and coordination of ethylene to the central metal, thus increase the catalytic activity.