Endo‐ and exohedral complexes of superphane with cations

Abstract

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.

Determining Repulsion in Cyclophane Cages

“…This is another argument that the Ng• • • π interactions in endohedral complexes are destabilizing [18]. It is worth adding here that negative MBO values have also been found more recently for some cation• • • C interactions in endohedral cation@superphane complexes [19] and for X• • • π (X = H, F, Cl) interactions in in forms of some "iron maiden" systems [20].…”

“…Although, as I have shown recently [18,19], the superphane molecule, i.e., [2 6 ](1,2,3,4,5,6) cyclophane, is excellent at studying the nature of the guest• • • host interactions in endohedral complexes, its fully closed structure, due to the presence of up to six ethylene bridges linking the two benzene rings, makes it practically impossible for the trapped entity to escape from the superphane cage. This escape, however, is considerably facilitated in superphane derivatives with n < 6, i.e., a reduced number of ethylene bridges, which leads to the presence of at least one single-carbon window.…”

“…The presence of BP and BCP in such spatially crowded circumstances gave rise to the term "counterintuitive" bond path [15][16][17][18][19][20], which is a bond path that occurs between a pair of atoms for which, based on a variety of data (especially energetic or structural), a stabilizing interaction is not expected. Importantly, such counterintuitive BPs have also been found between many pairs of various atoms, especially strongly electronegative [6,9,[15][16][17][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and having a large radius [15,16,23], although the H• • • H interaction is also a good example [6][7][8][35][36][37][38][39].…”

“…Unfortunately, these changes are often small or even negligible due to the high stiffness of the guest molecule [40]. I have recently shown [18,19] that the superphane molecule, i.e., [2.2.2.2.2.2](1,2,3,4,5,6)cyclophane [41][42][43][44], is very suitable for studying steric effects in endohedral complexes. This is due to high flexibility of this molecule.…”

“…Hence the idea to use superphane derivatives with correspondingly reduced number of ethylene linkers, leading to well-known [2 n ] cyclophanes [57][58][59][60][61][62], where n < 6. Thus, the aim of this article is to show the repulsive effect of the Ng• • • host interaction by using various types of cyclophanes, which should allow the initially trapped Ng atom to escape out of the cyclophane cage much more easily than the superphane molecule possessing as many as six ethylene linkers [18,19]. Such a possible spontaneous, i.e., during geometry optimization, escape of the initially trapped atom out of the cage of a host molecule would be a good evidence that the Ng• • • host interaction inside the Ng@host endohedral complex is, indeed, nonbonding (i.e., repulsive), and not stabilizing (attractive).…”