Confined Lewis Pairs: Investigation of the X⁻→Si₂₀ Interaction in Halogen‐Encapsulating Silafulleranes

Abstract

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.