Facile rotation around Si = Si double bonds in tetrakis(trialkylsilyl)disilenes

Stable disilenes having two different trialkylsilyl substituents [(SiSiSi)- Si-A-Si-B=(SiSiSiB)-Si-A ((E)-2 and (Z)-2) and Si(A)2Si=SiSi(B)2 (3), where Si-A = t-BuMe2Si and Si-B = i-Pr2MeSi] were prepared and characterized. Di silene (E)-2 was found by X-ray crystallography to have the highest planari ty around the Si=Si bond among isolated tetrasilyldisilenes. Temperature-de pendent Si-29 NMR spectra of a mixture of (E)- and (Z)-2 revealed the facil e intramolecular isomerization between (E)-2 and (Z)-2 with a free energy o f activation less than 15 kcal/mol at 300 K; the value is ca. 10 kcal/mol s maller than, those of E,Z-isomerization of other known stable disilenes wit h aryl or alkyl substituents. The origin of the low barrier for the silyl-s ubstituted disilenes is ascribed to the effective sigma-pi interaction at t he transition state of the isomerization. In addition to the rotational iso merization, the isomerization of 2 to 3 was also observed at higher tempera tures.