Boranyl-Functionalized [Ge₉ ] Clusters: Providing the Idea of Intramolecular Ge/B Frustrated Lewis Pairs

The unique three-dimensional structure of spherical, homoatomic nine-atom germanium clusters opens various possibilities for the spatial arrangement of functional groups. Ligands comprising lone pairs have recently been introduced in the cluster sphere, and we now report the addition of a boranyl group to the cluster featuring a Ge-B exo-cluster bond. The reaction of the twofold-silylated cluster [Ge₉ {Si(TMS)₃ }₂ ]^2- (TMS=trimethylsilyl) with 2-chloro-1,3,2-diazaborolidines DAB^R -Cl leads to the first boranyl-functionalized [Ge₉ ] clusters [Ge₉ {Si(TMS)₃ }₂ DAB^R ]^- (R=methyl (1 a), iso-propyl (2 a), ortho-tolyl (3 a)). The anions 2 a and 3 a were structurally characterized as [NHC^Dipp Cu]⁺ complexes (NHC^Dipp =1,3-di(2,6-diisopropylphenyl)imidazolylidine) through single crystal X-ray structure determination. Quantum-chemical calculations manifest the frustrated Lewis pair (FLP) character of the boranyl-functionalized cluster [Ge₉ {Si(TMS)₃ }₂ BCy₂ ]^- (4 a).