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@Si20Y20](-) (X=F-I Y=F-I, H, Me, Et) and T-h -[Cl@Si20H12Y8](-) (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--> Si-20 interaction energy (E-int) as a function of X- and Y and found the largest E(int )values for electron-withdrawing exohedral substituents Y. Given that X- ions can be considered as Lewis bases and empty Si20Y20 clusters as Lewis acids, we classify our inseparable host-guest complexes [X@Si20Y20](-) as "confined Lewis pairs". Third, Cl-35 NMR spectroscopy proved to be highly diagnostic for an experimental assessment of the Cl--> Si-20 interaction as the paramagnetic shielding and, in turn, delta(Cl-35) of the endohedral Cl- ion correlate inversely with E-int . Finally, we disclose the synthesis of [PPN][Cl@Si20Y20] (Y=Me, Et, Br) and provide a thorough characterization of these new silafulleranes.