The First Titanacyclic Five‐Membered Cumulene. Synthesis, Structure, and Reactivity

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Abstract
The first five‐membered titanacyclic cumulene (5) was prepared by treatment of the titanocene generator Cp2Ti(Me3SiC2SiMe3) with the di‐tert‐butylbutadiyne in a molecular one‐to‐one ratio. The reaction of 5 with “Cp2Zr” or of the analogous complex with “Cp2Ti” leads to cleavage of the central C‐C bond in the metallacyclocumulene complexes to afford the same product, the heterodimetallic ρ, π,‐alkynyl‐bridged titanocene‐zirconocene complex Cp2Zr(μ‐η1η2‐C≡CtBu)Cp2Ti(μ‐η1η2‐C≡CtBu) (6), in which each alkynyl group is π‐bonded to one and π‐bonded to the other metal as shown by X‐ray diffraction studies. In solution the two alkynyl groups become equivalent. The coupling reaction of “Cp2Ti” with bis(trimethylsilyl)butadiyne in a one‐to‐two molecular ratio leads to the unsymmetrically substituted titanacyclopentadiene (7).