Molecular structure of 1,1,2,2-tetra-tert-butyldisilane: unusual structural motifs in sterically crowded disilanes

Abstract

The molecular structure of 1,1,2,2-tetra-tert-butyldisilane has been determined by gas-phase electron diffraction supported by ab initio calculations, in the solution phase by Raman spectroscopy, and in the solid phase by Raman spectroscopy and X-ray crystallography. The gas-phase structure (C₂ symmetry) was found to be almost anticlinal, a most unusual and unexpected result. In the favoured conformation, contact between tert-butyl groups at each end of the molecule is avoided by a large deviation of the angles around the silicon atoms from the parent tetrahedral angle of 109.5°. In fact, the Si–Si–C angles returned from the gas electron diffraction refinement are 117.0(5) and 110.7(6)°, indicating the large degree of flexibility about the silicon centres. The ab initio methods and gas electron diffraction results indicate that there is only one conformer of Bu^t₂HSiSiHBu^t₂ in the gaseous mixture. Variable temperature Raman studies indicate the possibility of a further higher energy conformer existing in the liquid phase. However, this seems quite improbable from other observations made for the Raman spectra at all temperatures. The X-ray structure is close to that observed in the gas phase, with ϕHSiSiH = 94.2(18)°. There is a large amount of disorder about one of the silicon postions and one of the tert-butyl groups within the crystal structure, which makes detailed direct comparison with the gaseous structure difficult.