Conformation Control of Oligosilanes by Trimethylsilyl Groups: Dodecamethyl-, Undecamethyl-2-trimethylsilyl- and Decamethyl-2, 4-bis(trimethylsilyl)-n-pentasilane Studied by Raman Spectroscopy and Quantum Chemical Calculations

2,4-Bis(trimethylsilyl) substituted n-pentasilane Me₃SiSiMe(SiMe₃)SiMe₂SiMe(SiMe₃)SiMe₃ (1) has been prepared from Me₂SiCl₂ and (SiMe₃)₂SiMeK, and its solid state molecular structure has been determined by single crystal X-ray diffraction. By reaction of 1 with KO^tBu and subsequent treatment of the resulting silyl anion with (MeO)₂SO₂, the novel hexasilane Me₃SiSiMe(SiMe₃)SiMe₂SiMe₂SiMe₃ (2) has been synthesized. By again using the reaction sequence KO^tBu/(MeO)₂SO₂, known n-pentasilane Me₃SiSiMe₂SiMe₂SiMe₂SiMe₃ (3) has been prepared in a novel and very convenient way from 2. All silanes 1, 2, and 3 as well as the intermediate silyl anions obtained from 2 and 3 have been characterized by ²⁹Si and ¹H NMR spectroscopy, 1 and 2 by elemental analyses also. Raman spectra in the temperature range 210–370 K of the pure liquids 1, 2 and 3 as well as of solutions in toluene (1 and 2) or cyclopentane (3) reveal that all of the silanes exist as mixtures of conformers. The symmetric SiSi stretch observed in the range 341 to 363 cm^-1 and possessing by far the largest Raman intensity of all SiSi stretching vibrations splits into five bands for 3 and two bands for 1 and 3. Quantum chemical DFT B3LYP/6–311G(d) calculations located 15 conformational minima for 2 with relative energies up to 19.9 kJmol^-1, and five minima for 1 with relative energies up to 15.5 kJmol^-1. Calculated SiSiSiSi torsion angles vary between 26.2° (smallest value found) and 179.3°, demonstrating the flexibility of backbones composed of silicon.