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 (XII(2)) pincer ligand. Reaction of XII(2) with YCl(3)(THF)(3.5) provided [(XII(2))YCl(3)] (1). However, compound 1 failed to react cleanly with 3 equiv. of LiCH(2)SiMe(3), and the reaction of XII(2) with [Y(CH(2)SiMe(3))(3)(THF)(2)] afforded a complex mixture of products. To access group 3 alkyl complexes without the intermediacy of [(XII(2))M(CH(2)SiMe(3))(3)], the XII(2) ligand was protonated using [H(OEt(2))(2)][B(C(6)F(5))(4)] to form [H(XII(2))][B(C(6)F(5))(4)], and subsequent reaction with [M(CH(2)SiMe(3))(3)(THF)(2)] (M = Y, Sc) directly afforded the cationic scandium and yttrium dialkyl complexes [(XII(2))M(CH(2)SiMe(3))(2)][B(C(6)F(5))(4)] {M = Y (2) and Sc (3)}. Reaction of 3 with B(C(6)F(5))(3) in C(6)D(5)Br afforded dicationic [(XII(2))Sc(CH(2)SiMe(2)CH(2)SiMe(3))][MeB(C(6)F(5))(3)][B(C(6)F(5))(4)] (4) featuring a CH(2)SiMe(2)CH(2)SiMe(3) ligand, formed as a result of methyl anion abstraction from silicon, with concomitant migration of the neighbouring CH(2)SiMe(3) group from scandium to silicon. The MeB(C(6)F(5))(3) anion in 4 forms a contact ion pair. By contrast, reaction of 1 with [CPh(3)][B(C(6)F(5))(3)] in C(6)D(5)Br/toluene or o-C(6)H(4)F(2)/toluene afforded dicationic [(XII(2))Sc(CH(2)SiMe(3))(η(x)-toluene)(n)][B(C(6)F(5))(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-C(6)H(4)F(2)/toluene under 1 atm of ethylene at room temperature).