Synthesis of (s-trans-η⁴-Butadiene)tantalocene Cation and Its CC-Coupling Reactions with Ketones, Nitriles, and Alkynes

(η 4 -Butadiene)(η 5 -cyclopentadienyl)tantalum dichloride ( 6 ), synthesized by treatment of CpTaCl 4 ( 5 ) with (butadiene)magnesium ( 7 ), was treated with 2 molar equiv of sodium cyclopentadienide to yield Cp 3 Ta(butadiene) ( 9 ). The spectroscopic analysis revealed the (η 5 -Cp) 2 (η 1 -Cp)(η 2 -C 4 H 6 )Ta structure of 9 . Treatment of 9 with 1 molar equiv of the salt [(Cp 2 Zr−CH 3 ) + (CH 3 B(C 6 F 5 ) 3 ) - ] ( 11 ) resulted in the clean transfer of a cyclopentadienide ligand from tantalum to zirconium with formation of the neutral zirconium compound Cp 3 Zr−CH 3 ( 12 ) and the cation Cp 2 Ta(butadiene) + ( 10; with CH 3 B(C 6 F 5 ) 3 - counteranion). The spectroscopic characterization and an X-ray crystal structure analysis has revealed that 10 is a cationic ( s - trans -η 4 -butadiene)bis(η 5 -cyclopentadienyl)tantalum complex. It reacts cleanly with a variety of ketones at 60 °C to form the respective 7,7-disubstituted tetrahydrotantalaoxepines 13 . These cationic seven-membered metallacyclic σ-allyl complexes undergo a rapid topomerization process on the NMR time scale (Δ G ⧧ top ≈ 12 kcal/mol - 1 ). Organic nitriles add similarly at 60 °C to the ( s - trans -η 4 -butadiene)tantalocene cation ( 10 ) to yield the respective 7-substituted 3,4-dihydrotantala-6 H -azepines ( 14 ) that exhibit a similar dynamic NMR behavior due to conformational equilibration. Two examples of the seven-membered σ-allyl metallacyclic cation complexes 14 were characterized by X-ray crystal structure analyses ( 14a, R = CH 3, and 14d, R = CH 2 - p -C 6 H 4 CH 3 ). In contrast, 2-butyne adds to the ( s - trans -η 4 -butadiene)tantalocene cation in a 1:1 ratio to yield the corresponding metallacyclic (π-allyl)metallocene cation complex 15a (characterized by X-ray diffraction). The addition of 1-pentyne gave a 60:40 mixture of the respective regioisomers 15b and 15c, exhibiting the n -C 3 H 7 substituent at the ring position C5 and C6, respectively.