Attempted Synthesis of a Homocyclic Bis(silyl)silylene Leads to the Formation of a Tricyclo[3,1,1,1^2,4]octasilane

The reduction of 1,1-dibromo-cyclopentasilane with a mildly reducing magnesium(I) dimer [{(MesNacnac)Mg-}₂] was examined, which gave rise to the formation of endocyclic disilene 1. The formation of 1 was further confirmed by a trapping experiment with MeOH. Additionally, 1 was found to dimerize slowly to 1-dimer. Furthermore, the formation of the homocyclic silylene was demonstrated by performing the reduction in the presence of Et₃SiH as a trapping agent. To prevent the 1,2-trimethylsilyl shift, which causes a rapid degradation of the silylene, a second synthetic strategy was established. Therefore, two different tetrasilanes 4 and 5 were synthesized. Compounds 4 and 5 were subsequently reacted with 2.1 equiv of trifluoromethanesulfonic acid to corresponding bis(trifluoromethanesulfonates) 6 and 7. 1,4-Dihalo-tetrasilanes 8a,b and 9 as well as hexasilanes 10 and 11 were obtained by a subsequent nucleophilic substitution of the triflate substituent with X^- (X = Cl^-, Br^-, or R₃Si^-). Dianionic species 12 and 13 were synthesized by the reaction of corresponding hexasilanes 10 and 11 with 2.1 equiv of KOt-Bu. Surprisingly, the salt metathesis reaction of dianionic compound 13 does not lead to the formation of the expected 1,1-halocyclopentasilanes. Instead the formation of cyclobutasilane 14 was observed. The reaction of 9 with lithium led again to the formation of 14 alongside with the formation of cyclohexasilane 15. Interestingly, 14 underwent a ring-opening formation by the reaction with gaseous HCl in the presence of AlCl₃ and compound 16 was formed. The reduction of 16 allowed straightforward access to 17 as a structurally complex hitherto unknown tricyclic polysilane.