Achiral Cs‑Symmetric Half-Sandwich Scandium(III) Complexes with Imine–Cyclopentadienyl Ligands Catalyze Isotactic Polymerization of 1‑Hexene

Unchelated scandium­(III) trichloride complexes, 2-(ArNCH)­C6H4Me4CpScCl3Li­(THF)4 (Ar = 2,6-iPr2C6H3 (1a), 2,6-Et2C6H3 (1b), 2,6-Me2C6H3 (1c)), were obtained from the reaction of ScCl3(THF)3 with the lithium salt of the corresponding ligand, 2-(ArC6H3NCH)­C6H4Me4CpLi, in THF. After heating at 120 °C under vacuum for 30 min, the attached LiCl and THF were removed from complexes 1 to give the chelated scandium­(III) dichloride complexes 2-(ArNCH)­C6H4Me4CpScCl2 ([Ar = 2,6-iPr2C6H3 (2a), 2,6-Et2C6H3 (2b), 2,6-Me2C6H3 (2c)). Attempts to synthesize dialkyl scandium­(III) complexes by the reaction of Sc­(CH2SiMe3)3(THF)2 with the corresponding free ligands were not successful. The scandium­(III) trialkyl complex 2-[Li­(THF)3(2,6-iPr2C6H3)­NCH]­C6H4Me4CpSc­(CH2SiMe3)3 (3) was synthesized by a one-pot reaction of ScCl3(THF)3 with 2-(2,6-iPr2C6H3NCH)­C6H4Me4CpLi and 3 equiv of Me3SiCH2Li in THF sequentially. The scandium­(III) dialkyl complex 2-(2,6-iPr2C6H3NCH)­C6H4Me4CpSc­(CH2SiMe3)2 (4) was obtained from the reaction of the dichloride complex 2a with 2 equiv of Me3SiCH2Li in hexane. Complexes 1b,c were directly converted to complexes 2b,c without purification and characterization. All other scandium­(III) complexes were characterized by 1H and 13C NMR spectroscopy and elemental analyses. The structures of complexes 1a, 2c, 3, and 4 were determined by single-crystal X-ray crystallography, which indicates that the imine N atoms in complexes 1a and 3 do not coordinate to the central scandium atoms. Complexes 2a–c and 4 were found to exhibit moderate catalytic activity for propylene and 1-hexene polymerization upon activation with AlR3/Ph3CB­(C6F5)4 or methylaluminoxane (MAO) and produce atactic polypropylene and isotactic poly­(1-hexene). The effects of molecular structures and reaction conditions on the catalytic behavior of these complexes were examined and the possible catalytic mechanism was discussed.