Supersilylsilanes R*SiX3: Syntheses, characterization and structures; steric and van-der-Waals effects of substituents X [1]

Supersilylsilanes R*SiX3 (R* = supersilyl = SitBu3; X = H, Me, tBu, Ph, SiMe3, F, Cl, Br, I, OMe, OSO2CF3) are prepared (i) by reactions of supersilylhalosilanes with supersilyl sodium NaR* (Hal/R* exchange), (ii) by reactions of supersilylhalosilanes with hydride H- (Hal/H exchange), (iii) by reactions of supersilylsilanes with halogens Hal2 (H/Hal exchange, R*/Hal exchange), (iv) by reactions of supersilylhalosilanes with nucleophiles like F-, MeO- (Hal/F or Hal/OMe exchange) and (v) by reactions of supersilylsilanes with strong acids (H/OSO2CF3 exchange). NMR chemical shifts δ(29Si) of the SiX3 groups of R*SiX3 strongly depends on the nature of X. The supersilylsilanes R*SiX3 are in part moisture sensitive (especially compounds with SiX3≡SiHHal2 and SiH2Hal), in part sensitive against oxygen (compounds with SiX≡SiBr or SiI), and some of them react with supersilyl sodium by supersilanidation (for example R*SiF3, R*SiH2Cl, R*SiMeHCl) or by reduction (for example R*SiCl3, R*SiMeBr2, R*SiPhBrCl). X-ray structure analyses of disilanes tBu3Si-SiX3 with SiX3≡SitBu3, SiPh3, SiI3, SiPhCl2 show a staggered conformation. Due to steric repulsion of the tBu3Si and SiX3 groups as well as van-der-Waals attraction of the substituents tBu and X in tBu3Si-SiX2 the SiSi bonds are longer than 2.34 Å (the normal SiSi single bond length) and the torsion angles are smaller/larger than 60° (the ideal staggered conformation). From the extent of SiSi bond elongation and CSiSiX angle compression it is concluded that the bulkiness of X increases in direction Cl 33 (tBu3Si-SitBu3 has to date the longest SiSi bond of all disilanes) and the van-der-Waals forces between tBu/X increase in direction tBu/I