Chiral and achiral (imino)phenoxy-based cationic group 4 non-metallocene complexes as catalysts for polymerization of renewable α-methylene-γ-butyrolactones

Abstract

Protonolysis of M(Bn)₄ (M = Zr, Ti; Bn = benzyl) with equimolar 2,4-di-tert-butyl-6-[(2,6-diisopropylphenylimino)methyl]phenol [(2,6-ⁱPr₂C₆H₃)NC(3,5-^tBu₂C₆H₂)OH] in toluene at −30 °C to 25 °C cleanly affords the corresponding achiral (imino)phenoxy-tribenzyl complexes, [(2,6-ⁱPr₂C₆H₃)NC(3,5-^tBu₂C₆H₂)O]Zr(Bn)₃ (1) and [(2,6-ⁱPr₂C₆H₃)NC(3,5-^tBu₂C₆H₂)O]Ti(Bn)₃ (2). A chiral dibenzyl complex 3 incorporating the unsymmetric, tetradentate amino(imino)bis(phenoxy) ligand, [2,4-Br₂C₆H₂(O)(6-CH₂(NC₅H₉))CH₂NCH(2-adamantyl-4-MeC₆H₂O)]Zr(Bn)₂ (3), has also been prepared using the same protonolysis protocol. Abstractive activation of 1 with B(C₆F₅)₃·THF in CD₂Cl₂ at room temperature (RT) affords clean, quantitative formation of the corresponding zirconium cation [((2,6-ⁱPr₂C₆H₃)NC(3,5-^tBu₂C₆H₂)O)Zr(Bn)₂(THF)]⁺[BnB(C₆F₅)₃]⁻ (4). Likewise, benzyl abstraction of 2 with B(C₆F₅)₃·THF in CD₂Cl₂ at RT generates the cationic titanium complex [((2,6-ⁱPr₂C₆H₃)NC(3,5-^tBu₂C₆H₂)O)Ti(Bn)₂(THF)]⁺[BnB(C₆F₅)₃]⁻ (5), accompanied by a small amount of decomposed species as a result of C₆F₅ transfer. The dibenzyl cations 4 and 5 have been characterized spectroscopically, and their structures have been confirmed by single crystal X-ray diffraction analysis. Characteristics of the coordination polymerization of renewable α-methylene-γ-butyrolactone monomers by the cationic catalysts derived from achiral complexes 1 and 2 as well as chiral complex 3 have been investigated, representing the first study of such polymerization by non-metallocene catalysts.