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

Stable disilenes having two different trialkylsilyl substituents [Si^ASi^BSi=SiSi^ASi^B ((E)-2 and (Z)-2) and Si^A₂Si=SiSi^B₂ (3), where Si^A = t-BuMe₂Si and Si^B = i-Pr₂MeSi] were prepared and characterized. Disilene (E)-2 was found by X-ray crystallography to have the highest planarity around the Si=Si bond among isolated tetrasilyldisilenes. Temperature-dependent ²⁹Si NMR spectra of a mixture of (E)- and (Z)-2 revealed the facile intramolecular isomerization between (E)-2 and (Z)-2 with a free energy of activation less than 15 kcal/mol at 300 K; the value is ca. 10 kcal/mol smaller than those of E,Z-isomerization of other known stable disilenes with aryl or alkyl substituents. The origin of the low barrier for the silyl-substituted disilenes is ascribed to the effective σ-π interaction at the transition state of the isomerization. In addition to the rotational isomerization, the isomerization of 2 to 3 was also observed at higher temperatures.