Thermal Isomerization of the Buchwald Seven-Membered Zirconacyclocumulene and Its Interaction with Acetylenes. Synthesis and Structures of Novel Seven-Membered Zirconacyclocumulene Complexes

On
prolonged heating of the Buchwald seven-membered zirconacyclocumulene Cp2Zr­[η4-Me3SiC4(SiMe3)–C­(C2SiMe3)CSiMe3] (1) in toluene
at 100 °C, the isomerization of 1 takes
place with the formation of a seven-membered zirconacyclocumulene
complex, Cp2Zr­[η4-Me3SiC4(SiMe3)–C­(SiMe3)C­(C2SiMe3)] (8), containing the Me3SiCC group in α-position and the SiMe3 group in β-position with respect to the Zr atom. A remarkable
peculiarity of complex 8 is its ability to undergo a
rapid degenerate rearrangement at room temperature in toluene solution,
as a result of which only two rather than four singlets of the Me3Si groups are observed in the 1H NMR spectrum of 8 under such conditions. At temperatures below 200 K, the
rate of this rearrangement decreases and, as a consequence, the 1H NMR spectrum of 8 acquires the normal view.
If 1 is heated at 100 °C in the presence of acetylenes
such as 1,4-bis­(tert-butyl)­butadiyne (tBuCC–CCtBu) and tolane, the corresponding seven-membered zirconacyclocumulenes,
viz., Cp2Zr­[η4-tBuC4(tBu)–C­(C2SiMe3)CSiMe3] (7) and Cp2Zr­[η4-Me3SiC4(SiMe3)–C­(Ph)CPh] (14), are formed along with free Me3SiCC–CCSiMe3. The interaction of 1 with 1,4-diphenylbutadiyne
(PhCC–CCPh) at 100 °C leads to the formation
of a mixture of products, from which a zirconacyclopentadiene metallacycle,
Cp2Zr­[η2-PhCC­(C2Ph)–C­(C2Ph)CPh] (10), as well as seven-membered
zirconacyclocumulenes Cp2Zr­[η4-PhC4(Ph)–C­(C2SiMe3)CSiMe3] (11), Cp2Zr­[η4-Me3SiC4(SiMe3)–C­(C2Ph)CPh]
(12), and Cp2Zr­[η4-PhC4(Ph)–C­(C2Ph)CPh] (13) were isolated. The structures of 8, 9, 10, 12, and 14 have been
established by X-ray crystallography. The mechanism of the reactions
found is discussed.