Activation of Tantalocene(V) Alkyl and Alkylidene Complexes with Strong Organo Lewis Acids and Application to Polymerization Catalysis

This work reexamines the reaction of the tantalocene trialkyl complex Cp2TaMe3 with the strong organo
Lewis acids (LAs) E(C6F5)3 (E = B, Al) under various reaction conditions, investigates the activation of
Schrock's alkylidene complex Cp2Ta(CH2)Me with Al(C6F5)3, and employs the alkylidene complex
for the polymerization of functionalized alkenes such as methyl methacrylate (MMA) and N,N-dimethylacrylamide (DMAA). Three cationic and zwitterionic tantalocene complexes have been isolated
and structurally characterized, the cation [Cp2TaMe2]+[MeB(C6F5)3]- (1), the zwitterion Cp2Ta+[CH2Al(C6F5)3-]Me (2), and the cation [Cp2Ta(CH2)MeAl(C6F5)2]+[Al(C6F5)4]- (3). Complex 3, which is formed
by the unusual nucleophilic attack of a C6F5 group within the aluminate moiety in 2 on Al(C6F5)3 present
in excess, consists of a cationic portion that can be viewed as a hybrid of two extreme structures:  Cp2Ta+[CH2Al(C6F5)2]Me and the cationic tantalocene alkylidene−LA adduct [Cp2Ta(CH2)MeAl(C6F5)2]+.
The investigation into the scope of such a unique reaction type shows it to be specific to the aluminate/alane (Al-/Al) pair. All three isolated complexes are inactive for polymerization of MMA and DMAA;
however, the combination of Cp2Ta(CH2)Me with 2 equiv of Al(C6F5)3 is highly active, producing
high-molecular-weight polymers presumably via an Al-/Al bimolecular propagation process.