Pentamethylcyclopentadienyl-substituted hypersilylsilylene: reversible and irreversible activation of C&z.dbd;C double bonds and dihydrogen

Recent studies of low-valent main group species underscore their resemblance to transition metal complexes with regards to the ability to activate small molecules. Here, we report synthesis and full characterisation of the persistent (hypersilyl)(pentamethylcyclopentadienyl)silylene Cp*[(Me 3 Si) 3 Si]Si: as well as its unique reactivity. Silylene Cp*[(Me 3 Si) 3 Si]Si: activates dihydrogen to give the corresponding dihydrosilane Cp*[(Me 3 Si) 3 Si]SiH 2 at particularly mild conditions as well as ethylene to afford the three-membered cyclic silirane c -Cp*[(Me 3 Si) 3 Si]Si(H 2 CCH 2 ). The addition of N-heterocyclic carbene NHC (NHC = 1,3,4,5-tetramethyl-imidazol-2-ylidene) to dihydrosilane Cp*[(Me 3 Si) 3 Si]SiH 2 induces the reductive elimination of Cp*H, which according to DFT calculations is thermodynamically preferred over H 2 elimination. With NHC, Cp*[(Me 3 Si) 3 Si]Si: forms a typical donor-acceptor complex with concomitant change in hapticity of the Cp* ligand from η 2 to η 1 (σ). In contrast, the reaction with the N-heterocyclic silylene c -[(CH&z.dbd;CH( t BuN) 2 ]Si: leads to an unusual "masked" disilene with the former Cp* ligand bridging the two silicon centres. The heterodimer is stable in the solid state, but dissociates reversibly to the constituting silylene fragments in solution. A hypersilyl half-sandwich silicon( ii ) complex in action.