Cationic Scandium Allyl Complexes Bearing Mono(cyclopentadienyl) Ligands: Synthesis, Novel Structural Variety, and Olefin‐Polymerization Catalysis

The one‐pot salt‐metathesis reaction of ScCl 3 , cyclopentadienyl lithium salts, and allylmagnesium chlorides afforded with ease the corresponding base‐free half‐sandwich scandium di(η 3 ‐allyl) complexes [(C 5 Me 4 SiMe 3 )Sc(C 3 H 5 ) 2 ] ( 1 a ), [(C 5 Me 5 )Sc(C 3 H 5 ) 2 ] ( 1 b ), and [(C 5 Me 5 )Sc(2‐MeC 3 H 4 ) 2 ] ( 1 c ) in high yields. Reaction of 1 a with 1 equivalent of [PhNMe 2 H][B(C 6 F 5 ) 4 ] in toluene gave rapidly the N , N ‐dimethylaniline‐coordinated cationic mono(η 3 ‐allyl) complex [(C 5 Me 4 SiMe 3 )Sc(η 3 ‐C 3 H 5 )(η 6 ‐PhNMe 2 )][B(C 6 F 5 ) 4 ] ( 2 ). The similar reaction of 1 a with [Ph 3 C][B(C 6 F 5 ) 4 ] yielded the analogous toluene‐separated ion pair [(C 5 Me 4 SiMe 3 )Sc(η 3 ‐C 3 H 5 )(η 6 ‐PhMe)][B(C 6 F 5 ) 4 ] ( 3 ). When [PhNMe 2 H][BPh 4 ] was treated with 1 a , the contact ion pair [(C 5 Me 4 SiMe 3 )Sc(η 3 ‐C 3 H 5 )( μ,η 6 ‐Ph)BPh 3 ] ( 4 ) was obtained. Recrystallization of 2 , 3 , and 4 in THF yielded the corresponding thf‐separated ion pair complexes [(C 5 Me 4 SiMe 3 )Sc(η 3 ‐C 3 H 5 )(thf) 2 ][B(C 6 F 5 ) 4 ] ( 5 ) and [(C 5 Me 4 SiMe 3 )Sc(η 3 ‐C 3 H 5 )(thf) 2 ][BPh 4 ] ( 6 ). The N , N ‐dimethylaniline‐coordinated cationic scandium allyl complex 2 and the toluene‐coordinated analogue 3 showed high activity (activity: 3 > 2 ) toward the polymerization and copolymerization of isoprene and norbornene to afford random copolymers with a broad range of isoprene content (33–86 mol %). The tight ion pair 4 and the thf‐coordinated complexes 5 and 6 showed no activity under the same conditions. These results offer unprecedented insight into the structure–activity relationship of a cationic metal polymerization‐catalyst system.