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). A rigid and robust neutral pincer ligand has been developed, and used for the synthesis of a pair of dicationic scandium alkyl complexes, one of which is a highly active catalyst for ethylene polymerization.