Novel Examples of Three-Dimensional Aromaticity:  1,3-Dehydro-silaadamantane Dications. A Theoretical (DFT, GIAO NMR, NICS) Study^†

A DFT study of the hitherto elusive 1,3-dehydro-silaadamantane dications 2²⁺−5²⁺ has been carried out. Computed nucleus independent chemical shift (NICS) values are strongly indicative of three-dimensional heteroaromaticity in the resulting caged dications (via 2-electron, 4-center homoconjugation). In the optimized structures, silicon is pyramidalized. Although charge calculations (NPA and MKS) indicate significant positive charge build-up at silicon(s), the ²⁹Si GIAO NMR chemical shifts are unusually shielded. The latter finding agrees with the recent DFT calculations on 7-silanobornadien-7-ylium monocation 10⁺, suggesting that silicon shielding is a consequence of unusual bonding and homoconjugation in the dications. Both NICS values and silicon shielding decrease in going from 2²⁺ to 5²⁺. Natural bond orbital (NBO) analysis was utilized to shed light on the origin of the three-dimensional heteroaromaticity in these dications.