Endo- and exohedral complexes of superphane with cations.

Quite recently it has been shown in a previous study that superphane, that is, [2.2.2.2.2.2](1,2,3,4,5,6)cyclophane, is a very convenient molecule in the study of endohedral complexes and especially in the study of the influence of the caged entity (i.e., guest) on the structure of the host molecule. This advantage results from the presence of two parallel benzene rings joined together by six quite flexible ethylene bridges (spacers). This article examines the energetic and structural properties of endo- and exohedral complexes of superphane with the following cations: H+ , Li+ , Na+ , K+ , Be2+ , Mg2+ , Ca2+ , B3+ , Al3+ , Ga3+ . The stability of endohedral complexes has been shown to be strongly dependent on the charge and radius of the caged cation. The inclusion of the cation inside the superphane molecule causes its 'swelling', which is manifested by an increase in the distance between the benzene rings and elongations of the ring and spacer C-C bonds. In the case of exohedral complexes, three forms are investigated: with the cation above the benzene ring, with the cation interacting with the superphane window in the equatorial position, and with the cation interacting with the center of the C-C spacer bond. The first of these forms has been shown to be preferred. The cation⋯acceptor distance depends on the cation radius. Among the cations investigated, H+ and Be2+ are particularly reactive and predisposed to induce significant structural changes in the superphane molecule, forming C-H bond or C-Be-C bridges, respectively.