Confined Lewis Pairs: Investigation of the X^- →Si₂₀ Interaction in Halogen-Encapsulating Silafulleranes

A joint theoretical and experimental study on 32 endohedral silafullerane derivatives [X@Si₂₀ Y₂₀ ]^- (X=F-I; Y=F-I, H, Me, Et) and $$ -[Cl@Si₂₀ H₁₂ Y₈ ]^- (Y=F-I) is presented. First, we evaluated the structure-determining template effect of Cl^- in a systematic series of concave silapolyquinane model systems. Second, we investigated the X^- →Si₂₀ interaction energy ( $$ ) as a function of X^- and Y and found the largest $$ values for electron-withdrawing exohedral substituents Y. Given that X^- ions can be considered as Lewis bases and empty Si₂₀ Y₂₀ clusters as Lewis acids, we classify our inseparable host-guest complexes [X@Si₂₀ Y₂₀ ]^- as "confined Lewis pairs". Third, ³⁵ Cl NMR spectroscopy proved to be highly diagnostic for an experimental assessment of the Cl^- →Si₂₀ interaction as the paramagnetic shielding and, in turn, $$ (³⁵ Cl) of the endohedral Cl^- ion correlate inversely with $$ . Finally, we disclose the synthesis of [PPN][Cl@Si₂₀ Y₂₀ ] (Y=Me, Et, Br) and provide a thorough characterization of these new silafulleranes.