Design and Synthesis of Bis-β-ketoimine Binuclear Titanium Complex Isomers to Probe Bimetallic Synergistic Effect for Ethylene Polymerization

Binuclear complexes have attracted extensive attention in fields such as catalysis because of their likely bimetallic synergistic effect; however, the mechanism and factors influencing this synergism remain unclear. In this work, six bis-β-ketoimine binuclear titanium complexes 4a–4f containing different alkylthio sidearms and configurations were synthesized and characterized by nuclear magnetic resonance hydrogen spectrum (¹H-NMR), nuclear magnetic resonance carbon spectrum (¹³C-NMR), Fourier transform infrared spectrum (FTIR), and elemental analysis. The intermetallic distances of isomeric complexes 4a, 4d, 4e and 4f determined through density functional theory (DFT) optimization were in the order 4a<4d<4e<4f and were found to significantly influence the catalytic performance for ethylene (co)polymerization. These complexes could efficiently catalyze ethylene polymerization and ethylene/1-hexene or ethylene/1-octene copolymerization with high activity to produce high-molecular-weight ethylene homo- and co-polymers. Among the three binuclear titanium complexes 4a–4c with similar structures but different lengths of alkylthio sidearms, complex 4a, which contained the shortest methylthio sidearm, exhibited the highest activity for ethylene polymerization and copolymerization with 1-hexene or 1-octene. Additionally, for ethylene/1-hexene or ethylene/1-octene copolymerization, it showed the highest comonomer incorporation compared with propylthio (4b) and octylthio (4c) derivatives because of the smaller steric hindrance of the methyl group in 4a and the more open coordination space for vinyl monomers. Furthermore, among the isomeric complexes 4a, 4d, 4e and 4f, complex 4a with the shortest bimetallic distance also exhibited the highest activity towards ethylene (co)polymerization, and the highest 1-hexene or 1-octene incorporation in comparison with its regioisomeric counterparts 4d, 4e and p-phenyl-bridged analog 4f, owing to a more appropriate bimetallic distance that is conducive to a synergistic effect.