Rare-Earth Metal Complexes Supported by A Tridentate Amidinate Ligand: Synthesis, Characterization, and Catalytic Comparison in Isoprene Polymerization

To systematically investigate the dependence of the initiating group and metal size on polymerization performance, a family of rare-earth metal bis(alkyl)/bis(benzyl)/bis(amide) complexes supported by a monoanionic tridentate amidinate ligand [(2,6-(Pr2C6H3)-Pr-i)NC(Ph)N(C6H4-2-OMe](-) (<bold>HL</bold>) were synthesized and well-characterized. Treatment of rare-earth metal tris(alkyl)/tris(benzyl)/tris(amide) complexes Y(CH2C6H4NMe2-o)(3) or Y(CH2SiMe3)(3)(THF)(2) or Ln[N(SiHMe2)(2)](3)(THF)(x) (Ln = Sc, x = 1; Ln = Y, La, Sm, Lu, x = 2) with 1 equiv of <bold>HL</bold> gave the corresponding mono(amidinate) rare-earth metal bis(alkyl)/bis(benzyl)/bis(amide) complexes [(2,6-(Pr2C6H4)-Pr-i)NC(Ph)N(C6H4-2-OMe)]Y(CH2C6H4NMe2-o)(2) (<bold>1</bold>), [(2,6-(Pr2C6H4)-Pr-i)NC(Ph)N(C6H4-2-OMe)]Y(CH2SiMe3)(2)(THF) (<bold>2</bold>), and [(2,6-(Pr2C6H4)-Pr-i)NC(Ph)N(C6H4-2-OMe)]Ln[N(SiHMe2)(2)](2)(THF)(n) (Ln = Y, n = 1 (<bold>3</bold>); Ln = La, n = 1 (<bold>4</bold>); Ln = Sc, n = 0 (<bold>5</bold>); Ln = Lu, n = 0 (<bold>6</bold>); Ln = Sm, n = 0 (<bold>7</bold>)) in good isolated yields. These complexes were characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray diffraction. In the presence of excess AlMe3 and on treatment with 1 equiv of [Ph3C][B(C6F5)(4)], these complexes could serve as precatalysts for cationic polymerization of isoprene, in which the dependence of the polymerization activity and regioselectivity on the initiating group and metal size was observed.