Bimetallic Arylamide-Ligated Rare-Earth Metal Complexes: Synthesis, Characterization, and Stereo-Selectively...

Bimetallic complexes are expected to offer unique catalytic property, by facilitating cooperative effects between proximate functional groups or adjacent active metal centers, and thus have attracted increasing attention in the chemical community. Treatment of Ln(CH₂SiMe₃)₃(THF)₂ or Ln(CH₂C₆H₄NMe₂-o)₃ with 1,4-(C₆H₅NH)₂C₆H₄ in a 2:1 molar ratio in tetrahydrofuran (THF) generated a series of bimetallic arylamide-ligated rare-earth metal alkyl complexes [1,4-(C₆H₅N)₂C₆H₄][Ln(CH₂SiMe₃)₂(THF)₂]₂ (Ln = Sc (1), Lu (2), Y (3)), and aminobenzyl complexes [1,4-(C₆H₅N)₂C₆H₄][Ln(CH₂C₆H₄NMe₂-o)₂(THF)]₂ (Ln = Sc (4), x = 0; Lu (5), Y (6), x = 1) in 65-73% isolated yields. To reveal the polymerization difference between bimetallic and monometallic rare-earth metal complexes, the monoarylamide-ligated scandium bis(aminobenzyl) complex [(C₆H₅)₂N]Sc(CH₂C₆H₄NMe₂-o)₂ (7) was prepared by the reaction of Sc(CH₂C₆H₄NMe₂-o)₃ with 1 equiv of diphenylamine (C₆H₅)₂NH. All these rare-earth metal complexes were characterized by elemental analysis and NMR spectroscopy. The molecular structures of complexes 4 and 6 were authenticated by single-crystal X-ray diffraction. Complexes 2, 3, 5, and 6 alone were highly active for 2-vinylpyridine (2VP) polymerization at room temperature, giving poly-2VP with good iso-selectivity (mm). After activation with 2 equiv of [Ph₃C][B(C₆F₅)₄] or [PhNHMe₂][B(C₆F₅)₄], these complexes demonstrated an improved iso-selectivity (mm up to 96%) toward 2VP polymerization compared to their neutral analogues. In comparison, the bimetallic scandium complexes 1 and 4 showed relatively poor activity toward 2VP polymerization under the same conditions. However, the stereoselectivity of the polymerization could be switched from iso-tacticity to syndio-rich selectivity solely by tuning active species from only one scandium precatalyst. The catalyst system of complex 4/[PhNMe₂H][B(C₆F₅)₄] was able to promote a controlled syndio-specific polymerization of 2VP. The polymerization was experimentally verified to proceed via group transfer mechanism. Preliminary results indicated that the bimetallic rare-earth metal complexes showed a higher polymerization activity than the corresponding monometallic species mostly resulting from the cooperative effect.