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-(Pr2C6H3)-Pr-i (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 degrees 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-(Pr2C6H3)-Pr-i (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-(Pr2C6H3)-Pr-i)N=CH]C6H4Me4CpSc(CH2SiMe3)(3) (3) was synthesized by a one-pot reaction of ScCl3(THF)3 with 2-(2,6-(Pr2C6H3N)-Pr-i=CH)C6H4Me4CpLi and 3 equiv of Me3SiCH2Li in THF sequentially. The scandium(III) dialkyl complex 2-(2,6-(Pr2C6H3N)-Pr-i=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 H-1 and C-13 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.