Very strong anionic homoaromaticity in (deloc-1,3,4)-1-sila-3,4-diboracyclopentane-1-ides, the importance of the energy of the reference system for homoaromatic stabilization energies

(deloc-1,3,4)-1-Sila-3,4-diboracyclopentane-1-ide (3a) was prepared as solvent separated ion pair [Li(Et2O)(thf)3][3a] and as contact ion pair [Li(Et2O)][3a]. Both were fully characterized by NMR spectroscopy as well as by X-ray structure analyses. Their five-membered rings are strongly distorted as seen from short transannular Si⋯B distances of 206 and 208 pm as compared with 276 pm in the undistorted 1-sila-3,4-diboracyclopentane (4c). This distortion is also found in the unsubstituted prototype 3u by geometry optimizations at the MP2/6-311+G** level of theory. In addition, computations show that a three-center-two-electron (3c2e) bond between the silicon and the two boron atoms is present in the distorted 3u. The planar classical reference molecule 3u* with a 2c2e π bond between the boron atoms is not accessible by computations. Therefore, the energy difference between 3u and 3u* was estimated by isodesmic equations to be about 80 kcal mol−1 (at MP4/6-311+G**), considerably larger than any homoaromatic stabilization energy (HSE) ever discussed. The origin of this huge HSE of 3u is strong electrostatic destabilization of the reference system 3u* due to intramolecular charge separation. The series of bishomoaromatic systems is thus extended by a borderline case of very large HSE. The very small HSE of 1u at the other end of the series is also due to the energy of the reference: 1u* is strongly stabilized by hyperconjugation.