Interaction of the five-membered zirconacyclocumulene complex Cp2Zr(η4-tBuC4tBu) with acetylenes. Synthesis of zirconacyclopentadienes and seven-membered zirconacyclocumulenes

In the interaction of the five-membered zirconacyclocumulene Cp2Zr(η4-tBuC4tBu) (6) with 1,4-bis(trimethylsilyl)butadiyne Me3SiCtriple bond; length of mdashC–Ctriple bond; length of mdashCSiMe3 in toluene at 60–80 °C, two isomeric zirconacyclopentadiene complexes (7) and (8) are formed in 4.4–4.7 : 1 ratio. Complex 7 has been isolated from the solution in the individual state and its structure Cp2Zr[η2-tBuCdouble bond; length as m-dashC(C2tBu)–C(SiMe3)double bond; length as m-dashC(C2SiMe3)] has been determined by X-ray crystallography. Prolonged heating of a mixture of 6 and Me3SiCtriple bond; length of mdashC–Ctriple bond; length of mdashCSiMe3 at 100 °C leads to a gradual disappearance of the resulting zirconacyclopentadienes and to the formation of the seven-membered zirconacyclocumulene complex Cp2Zr[η4-tBuC4(tBu)–C(C2SiMe3)double bond; length as m-dashCSiMe3] (9) as the main product and two its analogues, viz., Cp2Zr[η4-Me3SiC4(SiMe3)–C(C2SiMe3)double bond; length as m-dashCSiMe3] (1) and Cp2Zr[η4-tBuC4(tBu)–C(C2tBu)double bond; length as m-dashCtBu] (2) as the admixtures. The same products are produced from individual compound 7 at 100 °C. Heating of complex 6 with tolane and phenylacetylene in toluene at 100 °C also gives rise to the corresponding zirconacyclopentadiene metallacycles, viz. Cp2Zr[η2-tBuCdouble bond; length as m-dashC(C2tBu)–C(Ph)double bond; length as m-dashCPh] (10) and Cp2Zr[η2-tBuCdouble bond; length as m-dashC(C2tBu)–CHdouble bond; length as m-dashCPh] (11). However these metallacycles do not form seven-membered zirconacyclocumulene complexes in any significant amounts even under prolonged keeping of the mixture at 100 °C. The structure of complexes 9–11 has also been established by X-ray crystallography. Mechanisms of the reactions found are discussed.