Rare‐Earth‐Metal–Hydrocarbyl Complexes Bearing Linked Cyclopentadienyl or Fluorenyl Ligands: Synthesis, Catalyzed Styrene Polymerization, and Structure–Reactivity Relationship

A series of rare‐earth‐metal–hydrocarbyl complexes bearing N‐type functionalized cyclopentadienyl (Cp) and fluorenyl (Flu) ligands were facilely synthesized. Treatment of [Y(CH 2 SiMe 3 ) 3 (thf) 2 ] with equimolar amount of the electron‐donating aminophenyl‐Cp ligand C 5 Me 4 H‐C 6 H 4 ‐ o ‐NMe 2 afforded the corresponding binuclear monoalkyl complex [({C 5 Me 4 ‐C 6 H 4 ‐ o ‐NMe(μ‐CH 2 )}Y{CH 2 SiMe 3 }) 2 ] ( 1 a ) via alkyl abstraction and CH activation of the NMe 2 group. The lutetium bis(allyl) complex [(C 5 Me 4 ‐C 6 H 4 ‐ o ‐NMe 2 )Lu(η 3 ‐C 3 H 5 ) 2 ] ( 2 b ), which contained an electron‐donating aminophenyl‐Cp ligand, was isolated from the sequential metathesis reactions of LuCl 3 with (C 5 Me 4 ‐C 6 H 4 ‐ o ‐NMe 2 )Li (1 equiv) and C 3 H 5 MgCl (2 equiv). Following a similar procedure, the yttrium‐ and scandium–bis(allyl) complexes, [(C 5 Me 4 ‐C 5 H 4 N)Ln(η 3 ‐C 3 H 5 ) 2 ] (Ln=Y ( 3 a ), Sc ( 3 b )), which also contained electron‐withdrawing pyridyl‐Cp ligands, were also obtained selectively. Deprotonation of the bulky pyridyl‐Flu ligand (C 13 H 9 ‐C 5 H 4 N) by [Ln(CH 2 SiMe 3 ) 3 (thf) 2 ] generated the rare‐earth‐metal–dialkyl complexes, [(η 3 ‐C 13 H 8 ‐C 5 H 4 N)Ln(CH 2 SiMe 3 ) 2 (thf)] (Ln=Y ( 4 a ), Sc ( 4 b ), Lu ( 4 c )), in which an unusual asymmetric η 3 ‐allyl bonding mode of Flu moiety was observed. Switching to the bidentate yttrium–trisalkyl complex [Y(CH 2 C 6 H 4 ‐ o ‐NMe 2 ) 3 ], the same reaction conditions afforded the corresponding yttrium bis(aminobenzyl) complex [(η 3 ‐C 13 H 8 ‐C 5 H 4 N)Y(CH 2 C 6 H 4 ‐ o ‐NMe 2 ) 2 ] ( 5 ). Complexes 1 – 5 were fully characterized by 1 H and 13 C NMR and X‐ray spectroscopy, and by elemental analysis. In the presence of both [Ph 3 C][B(C 6 F 5 ) 4 ] and Al i Bu 3 , the electron‐donating aminophenyl‐Cp‐based complexes 1 and 2 did not show any activity towards styrene polymerization. In striking contrast, upon activation with [Ph 3 C][B(C 6 F 5 ) 4 ] only, the electron‐withdrawing pyridyl‐Cp‐based complexes 3 , in particular scandium complex 3 b , exhibited outstanding activitiy to give perfectly syndiotactic ( rrrr >99 %) polystyrene, whereas their bulky pyridyl‐Flu analogues ( 4 and 5 ) in combination with [Ph 3 C][B(C 6 F 5 ) 4 ] and Al i Bu 3 displayed much‐lower activity to afford syndiotactic‐enriched polystyrene.