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

The first zerovalent
germanium complex (“germylone”) 3, [SiII(Xant)­SiII]­Ge0, stabilized
by a chelating bis­(N-heterocyclic silylene)­xanthene donor ligand 1 was successfully synthesized via the dechlorination of the
corresponding {[SiII(Xant)­SiII]­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 [SiII(Xant)­SiII]­Ge­(AlBr3) 4 and [SiII(Xant)­SiII]­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 {[SiII(Xant)­SiII]­GeI}2NiII 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.