σ,π-Conjugated Bis(germylene) Adducts with NHC and CAACs.

Heavier tetrylenes attract attention for their potential in synthesis, catalysis and small molecule activation. The coordination by N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs) results in substantial structural and electronic differences although typically only one of these yields stable derivatives for one and the same tetrylene. We now report both NHC- and CAAC-coordination to a bridged bis(germylene) motif. The NHC-coordinated bis(germylene) exhibits pyramidal germanium centers with lone pairs of electrons, while with CAAC an unprecedented stable bis(germene) with two Ge=C bonds is isolated. Spectroscopic and crystallographic evidence as well as DFT calculations confirm the effects of σ,π-conjugation between the two germanium centers in both cases. The coordination of NHC is reversible as the reaction with BPh3 liberates the transient bis(germylene) and thus provides an alternative low-temperature route towards polymers with Ge=Ge bonds.

Metathesis of Ge=Ge double bonds.

The metathesis of carbon-carbon double bonds-the 'reshuffling' of their constituting carbene fragments-is a tremendously important preparative tool in industry and academia. Metathesis of heavier alkene homologues is restricted to occasional unproductive examples in phosphorus chemistry and cross-metathesis to mixed heavier alkynes. We now report the thermally induced, transition-metal-free metathesis of purpose-built unsymmetrically substituted digermenes. The A2Ge=GeAB starting materials are thus converted to symmetrically substituted derivatives of the A2Ge=GeA2 and ABGe=GeAB types. The use of tethered auxiliary donors (dimethylaniline groups) in substituents B ensures intramolecular donor-acceptor stabilization of the transient germylene fragments, the intermediacy of which is proven by trapping experiments. Density functional theory calculations shed light on the thermodynamic driving force of the metathesis and validate the crucial role of the tethered donor. With an analogously equipped bridged tetragermadiene precursor (A2Ge=GeB-X-BGe=GeA2), heavier acyclic diene metathesis polymerization occurs, in analogy to the widespread acyclic diene metathesis (ADMET) polymerization in the carbon case, yielding a polydigermene.