Achiral <i>C</i>_<i>s</i>-Symmetric Half-Sandwich Scandium(III) Complexes with Imine–Cyclopentadienyl Ligands Catalyze Isotactic Polymerization of 1-Hexene

Abstract

Unchelated scandium(III) trichloride complexes, 2-(ArN═CH)C 6 H 4 Me 4 CpScCl 3 Li(THF) 4 (Ar = 2,6- i Pr 2 C 6 H 3 ( 1a ), 2,6-Et 2 C 6 H 3 ( 1b ), 2,6-Me 2 C 6 H 3 ( 1c )), were obtained from the reaction of ScCl 3 (THF) 3 with the lithium salt of the corresponding ligand, 2-(ArC 6 H 3 N═CH)C 6 H 4 Me 4 CpLi, 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)C 6 H 4 Me 4 CpScCl 2 ([Ar = 2,6- i Pr 2 C 6 H 3 ( 2a ), 2,6-Et 2 C 6 H 3 ( 2b ), 2,6-Me 2 C 6 H 3 ( 2c )). Attempts to synthesize dialkyl scandium(III) complexes by the reaction of Sc(CH 2 SiMe 3 ) 3 (THF) 2 with the corresponding free ligands were not successful. The scandium(III) trialkyl complex 2-[Li(THF) 3 (2,6- i Pr 2 C 6 H 3 )N═CH]C 6 H 4 Me 4 CpSc(CH 2 SiMe 3 ) 3 ( 3 ) was synthesized by a one-pot reaction of ScCl 3 (THF) 3 with 2-(2,6- i Pr 2 C 6 H 3 N═CH)C 6 H 4 Me 4 CpLi and 3 equiv of Me 3 SiCH 2 Li in THF sequentially. The scandium(III) dialkyl complex 2-(2,6- i Pr 2 C 6 H 3 N═CH)C 6 H 4 Me 4 CpSc(CH 2 SiMe 3 ) 2 ( 4 ) was obtained from the reaction of the dichloride complex 2a with 2 equiv of Me 3 SiCH 2 Li in hexane. Complexes 1b, c were directly converted to complexes 2b, c without purification and characterization. All other scandium(III) complexes were characterized by 1 H and 13 C 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 AlR 3 /Ph 3 CB(C 6 F 5 ) 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.