Unusual conformational properties of 1,3-dimethyl-1,1,3,3-tetrakis(trimethylsilyl)trisilane: A preparative, X-ray, Raman spectroscopic and ab initio study

The heptasilane Me(SiMe₃)₂SiSiH₂SiMe(SiMe₃)₂ was synthesized from Me(SiMe₃)₂SiK and H₂Si(OSO₂CF₃)₂. Crystals suitable for a X-ray single crystal analysis could be grown, with the somewhat surprising result that the two dihedral angles (H₃)CSiSi(H₂)Si are different in the crystal (24.58(10)° and 31.67(11)°). SiSiSi-bonds angles are widened, with values up to 117°. Ab initio calculations at the density functional B3LYP level employing 6-311G(d) basis sets predict minima for five conformers 1-5 with relative energies 0.0, 3.1, 8.2, 10.8 and 18.1 kJ/mol, respectively. Moreover, SiSiSiSi dihedral angles spanning the range 43.5-172.3° are predicted, reflecting the small forces which are required for distorting these angles.In the Raman spectrum of a solution in toluene, three lines at 350, 340 and 330 cm⁻¹ are observed in a wavenumber range which is typical for the SiSi-pulsation of methylated oligosilanes. The relative intensity ratio of the bands is temperature dependent, reflecting the changes in conformer concentrations that occur according to Boltzman’s law. Supported by the ab initio calculations, the Raman band at 350 cm⁻¹ is assigned to an ‘averaged’ conformer 1 and 2, because a rapid interconversion between 1 and 2 has to be assumed due to a small barrier separating them. The bands with wavenumbers 340 and 330 cm⁻¹ originate from conformers 3 and 4. From the Raman spectra, relative energies 0.0 (1 + 2), 2.2 (3) and 6.3 (4) kJ/mol are deduced, the presence of 5 is not observed. Caused by solvent effects, these values differ somewhat from the ab initio results.