Confined Lewis Pairs: Investigation of the X →Si Interaction in Halogen-Encapsulating Silafulleranes.

A joint theoretical and experimental study on 32 endohedral silafullerane derivatives [X@Si Y ] (X=F-I; Y=F-I, H, Me, Et) and -[Cl@Si H Y ] (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 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 Si Y clusters as Lewis acids, we classify our inseparable host-guest complexes [X@Si Y ] as "confined Lewis pairs". Third, Cl NMR spectroscopy proved to be highly diagnostic for an experimental assessment of the Cl →Si interaction as the paramagnetic shielding and, in turn, ( Cl) of the endohedral Cl ion correlate inversely with . Finally, we disclose the synthesis of [PPN][Cl@Si Y ] (Y=Me, Et, Br) and provide a thorough characterization of these new silafulleranes.