Synthesis, structural characterization and catalytic property of group 4 metal complexes bearing novel salalen-type ligands

•New trans-1,2-cyclohexylene bridged salalen ligands containing halides (Cl, Br, I).•Five salalen group 4 metal complexes have been characterized by X-ray crystallography.•Titanium complexes are moderately effective catalysts for ethylene polymerization.

Reaction of novel salalen ligands trans-1-(2′-OH-3′,5′-tBu2C6H2CH2N(Me))-2-(2″-OH-3″,5″-R2-C6H2CHN)-cyclohexane (R = tert-Butyl, H2(La); R = Cl, H2(Lb); R = Br, H2(Lc); R = I, H2(Ld)) with appropriate group 4 metal precursors [MCl4(THF)2] (M = Ti, Zr, Hf) gave a family of complexes [Ti(La–d)Cl2] (1a–1d) and [M(La, c)Cl2] (M = Zr, 2a, c; M = Hf, 3a, c). X-ray single crystal analysis revealed that in the solid state these salalen group 4 metal complexes formed as single isomers with fac–mer geometry and the two chlorine groups are cis-oriented. Introducing the trans-1,2-cyclohexylene bridge and the electron-withdrawing effects of halide substituents improved the catalytic prosperity for ethylene polymerization. The titanium complexes 1a–d showed moderate activity of 37.2–110.0 kg/(molTi h atm) in the polymerization of ethylene using MAO as cocatalyst. Zirconium and hafnium complexes were almost inactive except 2a with a low activity of 4.4 kg/(molZr h atm). The high temperature 1H and 13C NMR analysis revealed that these polymers are vinyl-terminated low-molecular-weight linear polyethylene (Mn = 1800–3800 g mol−1; the vinyl selectivity is about 85–91 mol %).

Graphical abstractA series of group 4 metal complexes supported by novel trans-1,2-cyclohexylene bridged salalen ligands have been synthesized and confirmed by X-ray analysis. The salalen titanium complexes were active to catalyze ethylene polymerization with moderate activity with the MAO as cocatalyst, while salalen zirconium and hafnium complexes were almost inactive.Figure optionsDownload full-size imageDownload as PowerPoint slide