Twisted Carbon-Carbon Double Bonds. Crystal and Molecular Structure of 4,5-Di-tert-butyl-1,1,2,2-tetrafluoro-1,2-disilacyclohexa-3,5-diene

4,5-Di-tert-butyl-1,1,2,2-tetrafluoro-1,2-disilacyclohexa-3,5-diene, C₁₂H₂₀Si₂F₄, crystallizes in a monoclinic system with space group P2₁/c, a = 12.584 (5) Å, b = 16.843 (7) Å, c = 15.293 (4) Å, and β = 69.70 (3)°. The density calculated is 1.295 g cm⁻³ with Z = 8. Intensity data were obtained on a single-crystal diffractometer with Cu Kα radiation. The crystal structure was solved by direct methods and refined by full-matrix least squares to R = 0.083 on F for 1923 independent reflections. The two crystallographic independent molecules are related approximately by a pseudotwofold rotation symmetry. Both molecules have similar dimensions and adopt a twisted-boat conformation which deviates somewhat from C₂ symmetry. The bonds connecting the two tert-butyl groups are long, averaging 1.54 (2) Å, as compared to the normal C(sp²)-C(sp²) single bond length of 1.48 Å. The averaged Si-Si bond length and Si-Si-C bond angles are 2.331 (7) Å and 95.5 (5)°, respectively. The carbon-carbon double bonds are all twisted and can be understood on the basis of the polarized zwitterionic transition state with the ring strain acting as the steric source and tert-butyl and SiF₂ parts as the donor and acceptor groups. The averaged CC bond lengths and twisted angles are 1.345 Å for a twist of 24° and 1.388 Å for a 27° twist. Both ¹³C NMR and UV spectra are consistent with this picture.