Aluminum Effects in the Syndiospecific Copolymerization of Styrene with Ethylene by Cationic Fluorenyl Scandium Alkyl Catalysts

A series of half-sandwich fluorenyl (Flu′) scandium dialkyl complexes Flu′Sc­(CH₂SiMe₃)₂(THF)_n (1, Flu′ = C₁₃H₉, n = 1; 2, Flu′ = 2,7-^tBu₂C₁₃H₇, n = 1; 3, Flu′ = 9-SiMe₃C₁₃H₈, n = 1; 4, Flu′ = 2,7-^tBu₂-9-SiMe₃C₁₃H₆, n = 1; 5, Flu′ = 9-CH₂CH₂NMe₂C₁₃H₈, n = 0; 6, Flu′ = 2,7-^tBu₂-9-CH₂CH₂NMe₂C₁₃H₆, n = 0) have been synthesized and structurally characterized. In comparison with the well-known cyclopentadienyl-ligated scandium catalyst system [(C₅Me₄SiMe₃)­Sc­(CH₂SiMe₃)₂(THF)]/[Ph₃C]­[B­(C₆F₅)₄], the analogous combinations of the fluorenyl-ligated, THF-containing complexes 1–4 with [Ph₃C]­[B­(C₆F₅)₄] show relatively low activities, albeit with similar syndioselectivities for styrene polymerization and styrene–ethylene copolymerization. However, on treatment with 15 equiv of AlⁱBu₃, the 1–4/[Ph₃C]­[B­(C₆F₅)₄] combinations show a dramatic increase in catalytic activity without changes in the stereoselectivity. In contrast, the combinations of complexes 5 and 6, which have an amino group attached to the fluorenyl ring and intramolecularly bonded to the metal center, exhibit very low activity, no matter whether or not AlⁱBu₃ is present, affording syndiotactic polystyrenes with broad molecular weight distributions. The DFT calculations of the activation mechanism by using the representative catalysts suggest that AlⁱBu₃ can capture the THF molecule from the catalyst precursors 1–4 at first,and then the new, THF-free cationic half-sandwich scandium active species [Flu′Sc­(CH₂SiMe₃)]­[B­(C₆F₅)₄] with less steric hindrance around the metal center is generated in the presence of an activator such as [Ph₃C]­[B­(C₆F₅)₄]. The DFT calculations on the syndioselectivity of styrene (co)­polymerization catalyzed by [Flu′Sc­(CH₂SiMe₃)]­[B­(C₆F₅)₄] have also been carried out, thus shedding new light on the mechanistic aspects of the (co)­polymerization processes.