Chiral C₁-Symmetric Group 4 Metallocenes as Catalysts for Stereoregular α-Olefin Polymerization. Metal, Ancillary Ligand, and Counteranion Effects

This contribution reports the synthesis and activity as precatalysts for stereoselective propylene polymerization of several chiral non-C₂ symmetric zirconocene and hafnocene complexes, (R)- and (S)-Me₂Si(Me₄C₅)-(C₅H₃R*)MR₂ (R = Cl(2) or Me(3)) where R* = (1R,2S,5R)-trans-5-methyl-cis-2-(2-propyl)cyclohexyl ((-)-menthyl; M = Zr, (a) and M = Hf, (b)) and (1S,2S,5R)trans-5-methyl-cis-2-(2-propyl)cyclohexyl ((+)-neomenthyl; M = Zr (c)). Metallocene dichlorides were prepared from MCl₄ and Li₂Me₂Si(Me₄C₅)(C₅H₃R*) and converted to the corresponding dimethyl complexes with MeLi·LiBr. All complexes were characterized by standard techniques, with absolute configuration established by circular dichroism and X-ray diffraction. For the (R) — R* = (—)-menthyldichloro complex (2a): space group = F2₁2₁2₁; a = 9.404(2), b = 9.817(3), c = 28.684(7) Å(-120 °C), Z = 4; R(F) — 0.056, R_w(F) = 0.061 for 2025 reflections having I > 3σ(I). For the (R) — R* = (—)-menthyldimethyl complex (3a): space group = P2₁; a = 9.501(3), b = 9.394(3), c = 15.565(3) Å, β = 103.76(2)°(-120 °C), Z = 2; R(F) = 0.037, R_W(F) = 0.039 for 1528 reflections having I > 3σ(I). Reaction of either (R)-3a or (R)-3b with B(C₆F₅)₃ in toluene yields two spectroscopically discernible methyl cations. The temperature dependence of the ion-pair equilibrium constant in toluene yields ΔH = -0.7(1) kcal/mol and ΔS = —3.1(1) eu for (R)-3a and ΔH = 0.14(3) kcal/mol and AS = -3.1(1) for (R)-3b. “Cationic” propylene polymerization catalysts were generated from 2 + methylalumoxane or 3 + methylalumoxane, B(C₆F₅)₃, Ph₃C⁺B(C₆F₅)₄^_, or HN(ⁿBu₃)⁺B(C₆F₅)₄^_. Polymerization activities, stereoregularities, and polymer molecular weights are strongly dependent on cocatalyst identities and concentrations, suggesting strong, structure-sensitive ion-pairing effects. Polypropylene isotacticities as high as 95% mmmm pentad content are observed, with stereoregularity increasing and polymerization activity falling with decreasing reaction temperature.