Pentamethylcyclopentadienyl-substituted hypersilylsilylene: reversible and irreversible activation of CC double bonds and dihydrogen

Abstract

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*[(Me₃Si)₃Si]Si: as well as its unique reactivity. Silylene Cp*[(Me₃Si)₃Si]Si: activates dihydrogen to give the corresponding dihydrosilane Cp*[(Me₃Si)₃Si]SiH₂ at particularly mild conditions as well as ethylene to afford the three-membered cyclic silirane c-Cp*[(Me₃Si)₃Si]Si(H₂CCH₂). The addition of N-heterocyclic carbene NHC (NHC = 1,3,4,5-tetramethyl-imidazol-2-ylidene) to dihydrosilane Cp*[(Me₃Si)₃Si]SiH₂ induces the reductive elimination of Cp*H, which according to DFT calculations is thermodynamically preferred over H₂ elimination. With NHC, Cp*[(Me₃Si)₃Si]Si: forms a typical donor–acceptor complex with concomitant change in hapticity of the Cp* ligand from η² to η¹ (σ). In contrast, the reaction with the N-heterocyclic silylene c-[(CHCH(^tBuN)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.