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 Th-[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- →Si20 interaction energy (Eint) as a function of X- and Y and found the largest Eint 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, 35Cl NMR spectroscopy proved to be highly diagnostic for an experimental assessment of the Cl- →Si20 interaction as the paramagnetic shielding and, in turn, δ(35Cl) of the endohedral Cl- ion correlate inversely with Eint. Finally, we disclose the synthesis of [PPN][Cl@Si20Y20] (Y = Me, Et, Br) and provide a thorough characterization of these new silafulleranes.
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