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

The first zerovalent germanium complex ("germylone") 3, [Si^II(Xant)Si^II]Ge⁰, 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 KC₈; 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-AlBr₃ Ge → Al Lewis adducts [Si^II(Xant)Si^II]Ge(AlBr₃) 4 and [Si^II(Xant)Si^II]Ge(AlBr₃)₂ 5, respectively. Due to the electron-rich character of the Ge⁰ 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}₂Ni^II complex with a three-membered ring Ge₂Ni-metallacycle was obtained via reductive coupling of two Ge⁰ atoms on the Ni center. Moreover, 3 was suitable to form a frustrated Lewis pair (FLP) with BPh₃, which was capable of heterolytic H₂ cleavage at 1 atm and room temperature, representing, for the first time, that a metallylone could be applied in FLP chemistry.