An Isolable Bis(silylene)-Stabilized Germylone and Its Reactivity

The first zerovalent germanium complex ("germylone") 3, [Si ^II (Xant)Si ^II ]Ge ⁰ , stabilized by a chelating bis(N-heterocyclic silylene)xanthene donor ligand 1 was successfully synthesized via the dechlorination of the corresponding {[Si ^II (Xant)Si ^II ]GeCl} ⁺ Cl ^- complex 2 with KC ₈ ; it was structurally and spectroscopically characterized, and also studied by density functional theory (DFT) calculations. Natural bond orbital (NBO) analysis of 3 unambiguously exhibits two lone pairs of electrons (one σ-type lone-pair and one 3p(Ge) lone-pair) on the zerovalent Ge atom. This is why the Ge atom can form the corresponding mono- and bis-AlBr ₃ Ge → Al Lewis adducts [Si ^II (Xant)Si ^II ]Ge(AlBr ₃ ) 4 and [Si ^II (Xant)Si ^II ]Ge(AlBr ₃ ) ₂ 5, respectively. Due to the electron-rich character of the Ge ⁰ atom, the germylone 3 displayed quite unusual reactivities. Thus, the reaction of 3 with 9-borabicyclo[3.3.1]nonane (9-BBN) as a potential Lewis acid furnished the first boryl(silyl)germylene complex 6, possessing a heteroallylic B···Ge···Si π-conjugation. When 3 was allowed to react with Ni(cod) ₂ (cod = 1,5-cyclooctadiene), the unique {[Si ^II (Xant)Si ^II ]Ge ^I } ₂ Ni ^II complex with a three-membered ring Ge ₂ Ni-metallacycle was obtained via reductive coupling of two Ge ⁰ atoms on the Ni center. Moreover, 3 was suitable to form a frustrated Lewis pair (FLP) with BPh ₃ , which was capable of heterolytic H ₂ cleavage at 1 atm and room temperature, representing, for the first time, that a metallylone could be applied in FLP chemistry.