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)(?(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).

Automated summary

A palladium-catalyzed coupling reaction synthesized a stable neutral [XII2] ligand from 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene. The reaction of XII2 with YCl3(THF)(3.5) provided [(XII2)YCl3] (1), while the reaction with [Y(CH2SiMe3)(3)(THF)(2)] produced a complex mixture of products. Protonation of XII2 using [H(OEt2)(2)][B(C6F5)(4)] and subsequent reaction with [M(CH2SiMe3)(3)(THF)(2)] directly afforded cationic scandium and yttrium dialkyl complexes.