Regiodiscriminating Reactivity of Isolable NHC-Coordinated Disilenyl Germylene and Its Cyclic Isomer

An isolable NHC-coordinated disilenyl germylene R₂SiSiR-GePh·NHC^{<i>i</i>Pr₂}^Me₂ (<b>5b</b>: R = 2,4,6-triisopropylphenyl, NHC^{<i>i</i>Pr₂}^Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) is obtained from the reaction of the α-chloro silyl functionalized heavier vinylidene analogue R₂(Cl)­Si-RSiGe·NHC^{<i>i</i>Pr₂}^Me₂ with phenyllithium. The disilenyl germylene <b>5b</b> isomerizes at 40 °C to the NHC-adduct of the corresponding heavier cyclopropylidene analogue <b>6b</b>. Both <b>5b</b> and <b>6b</b> display near perfect regiodiscrimination in their reactions with phenylacetylene and xylyl isocyanide, affording in the case of each reagent two different regioisomers of NHC-coordinated cyclic germylenes. DFT calculations reveal that the SiSi bond accounts for the high reactivity of <b>5b</b> even at low temperature while in the case of cyclic <b>6b</b> the low-valent germanium center requires a considerable thermal activation