Rare earth dialkyl cations and monoalkyl dications supported by a rigid neutral pincer ligand: synthesis and ethylene polymerization

A palladium-catalyzed coupling reaction between 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene and 2 equiv. of 1,3-diisopropylimidazolin-2-imine afforded the rigid neutral 2,7-di-tert-butyl-4,5-bis(1,3-diisopropylimidazolin-2-imino)-9,9-dimethylxanthene (XII₂) pincer ligand. Reaction of XII₂ with YCl₃(THF)_3.5 provided [(XII₂)YCl₃] (1). However, compound 1 failed to react cleanly with 3 equiv. of LiCH₂SiMe₃, and the reaction of XII₂ with [Y(CH₂SiMe₃)₃(THF)₂] afforded a complex mixture of products. To access group 3 alkyl complexes without the intermediacy of [(XII₂)M(CH₂SiMe₃)₃], the XII₂ ligand was protonated using [H(OEt₂)₂][B(C₆F₅)₄] to form [H(XII₂)][B(C₆F₅)₄], and subsequent reaction with [M(CH₂SiMe₃)₃(THF)₂] (M = Y, Sc) directly afforded the cationic scandium and yttrium dialkyl complexes [(XII₂)M(CH₂SiMe₃)₂][B(C₆F₅)₄] {M = Y (2) and Sc (3)}. Reaction of 3 with B(C₆F₅)₃ in C₆D₅Br afforded dicationic [(XII₂)Sc(CH₂SiMe₂CH₂SiMe₃)][MeB(C₆F₅)₃][B(C₆F₅)₄] (4) featuring a CH₂SiMe₂CH₂SiMe₃ ligand, formed as a result of methyl anion abstraction from silicon, with concomitant migration of the neighbouring CH₂SiMe₃ group from scandium to silicon. The MeB(C₆F₅)₃ anion in 4 forms a contact ion pair. By contrast, reaction of 1 with [CPh₃][B(C₆F₅)₃] in C₆D₅Br/toluene or o-C₆H₄F₂/toluene afforded dicationic [(XII₂)Sc(CH₂SiMe₃)(η^x-toluene)_n][B(C₆F₅)₄]₂ (5). Compounds 2-4 showed negligible ethylene polymerization activity, whereas 5 is highly active (up to 870 kg mol^-1 h^-1 atm^-1 in o-C₆H₄F₂/toluene under 1 atm of ethylene at room temperature).