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--butyl-9,9-dimethylxanthene and 2 equiv. of 1,3-diisopropylimidazolin-2-imine afforded the rigid neutral 2,7-di--butyl-4,5-bis(1,3-diisopropylimidazolin-2-imino)-9,9-dimethylxanthene (XII) pincer ligand. Reaction of XII with YCl(THF) provided [(XII)YCl] (1). However, compound 1 failed to react cleanly with 3 equiv. of LiCHSiMe, and the reaction of XII with [Y(CHSiMe)(THF)] afforded a complex mixture of products. To access group 3 alkyl complexes without the intermediacy of [(XII)M(CHSiMe)], the XII ligand was protonated using [H(OEt)][B(CF)] to form [H(XII)][B(CF)], and subsequent reaction with [M(CHSiMe)(THF)] (M = Y, Sc) directly afforded the cationic scandium and yttrium dialkyl complexes [(XII)M(CHSiMe)][B(CF)] {M = Y (2) and Sc (3)}. Reaction of 3 with B(CF) in CDBr afforded dicationic [(XII)Sc(CHSiMeCHSiMe)][MeB(CF)][B(CF)] (4) featuring a CHSiMeCHSiMe ligand, formed as a result of methyl anion abstraction from silicon, with concomitant migration of the neighbouring CHSiMe group from scandium to silicon. The MeB(CF) anion in 4 forms a contact ion pair. By contrast, reaction of 1 with [CPh][B(CF)] in CDBr/toluene or -CHF/toluene afforded dicationic [(XII)Sc(CHSiMe)(η-toluene)][B(CF)] (5). Compounds 2-4 showed negligible ethylene polymerization activity, whereas 5 is highly active (up to 870 kg mol h atm in -CHF/toluene under 1 atm of ethylene at room temperature).