Silicon–Halogen Bond Activation in Mixed Si/Al Compounds and an Approach to Intramolecular Stabilized Silylium Ions

Abstract

Alkenylchlorosilanes functionalized by coordinatively unsaturated Al atoms, R′′(R′)Si(Cl)C(AltBu2)=C(H)R 5 [R = tBu, cHex, 1‐Ad, Ph, 3,5‐(F3C)2C6H3; R′ = Mes, Ph, 4‐tBuC6H4, 4‐tBuOC6H4, 4‐Et2NC6H4, tBu; R′′ = Mes, CH(SiMe3)2, tBu], were obtained in good yields by hydroalumination of the respective alkynylchlorosilanes R′′(R′)Si(Cl)C≡C‐R 3 with tBu2Al‐H. They feature four‐membered, close to planar SiCAlCl heterocycles by intramolecular Al–Cl interactions. The activation of the Si–Cl bonds results in bond lengthening from about 209 pm in 3 to 220 to 225 pm in compounds 5. The 29Si NMR resonances show an increasing shift to a lower field and range from δ = 24.0 for 5a [R = tBu, R′ = R′′ = Mes] to δ = 68.1 for 5j [R = R′ = R′′ = tBu]. The latter value approaches those of solvent coordinated silyl cations such as Et3Si(C6H5Me)+. The related fluorides R′(tBuC≡C)Si(F)C(AltBu2)C = C(H)tBu 9 (R′ = Ph, Mes) were similarly obtained by hydroalumination of alkynylsilanes R′Si(F)(C≡C‐tBu)2 8. Compounds 9 are dimeric via Si–F–Al bridges and form eight‐membered C2Al2Si2F2 heterocycles. The 29Si NMR shifts of δ = –4.7 and –14.7 and Si–F bond lengths of about 170 pm indicate the presence of four‐coordinate, covalently bound Si atoms. Treatment of 8a with two equivalents of tBu2Al‐H led to H/F exchange and formation of tBu2Al‐F which is coordinated by monomeric 9a via Si–F–Al and Al–F–Al bridges to form a six‐membered SiCAlFAlF heterocycle.