Neutral and Cationic Alkyl Tantalum Imido Complexes: Synthesis and Migratory Insertion Reactions

The synthesis and reactivity of dibenzyl cationic tantalum imido complexes are described. The trialkyl
tantalum imido compounds Bn3TaNCMe3 (1) and Np3TaNCMe3 (2) were synthesized as starting
materials for the study of dialkyl cationic tantalum imido complexes. Compound 1 undergoes insertion
reactions with diisopropylcarbodiimide and 2,6-dimethylphenylisocyanide to give (bisamidinate)imido
complex 5 and (bisiminoacyl)imido complex 6, respectively. Treatment of compound 1 with B(C6F5)3
gives the zwitterionic tantalum complex [Bn2TaNCMe3][BnB(C6F5)3] (7), which is stabilized by η6-coordination of the benzyl triaryl borate anion. The aryl anion can be displaced by 3 equiv of pyridine
to give the Lewis base complex 8. Treatment of compound 1 with [Ph3C][B(C6F5)4] gives the cationic
tantalum imido complex [Bn2TaNCMe3][B(C6F5)4] (3). This salt forms insoluble aggregates unless
trapped by THF coordination or an insertion reaction with an alkyne or an alkene. Cation 3 undergoes
migratory insertion reactions with diphenylacetylene, phenylacetylene, norbornene, and cis-cyclooctene
to give the corresponding alkenyl or modified alkyl imido complexes. The characterization of these products
and the significance of these insertion reactions with respect to Ziegler−Natta polymerizations and
hydroamination reactions are described.