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

An isolable NHC-coordinated disilenyl germylene R2SiSiR-GePh·NHCiPr2Me2 (5b: R = 2,4,6-triisopropylphenyl, NHCiPr2Me2 = 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·NHCiPr2Me2 with phenyllithium. The disilenyl germylene 5b isomerizes at 40 °C to the NHC-adduct of the corresponding heavier cyclopropylidene analogue 6b. Both 5b and 6b 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 5b even at low temperature while in the case of cyclic 6b the low-valent germanium center requires a considerable thermal activation.