Exploration of Nano‐Saturns: A Spectacular Sphere–Ring Supramolecular System

Abstract

Saturn‐like systems consisting of nanoscale rings and spheres are fascinating motifs in supramolecular chemistry. Several ring molecules are known to include spherical molecules at the center of the cavity via noncovalent attractive interactions. In this Minireview, we generalize the molecular design, the structural features, and the supramolecular chemistry of such “nano‐Saturns”, which consist of monocyclic rings and fullerene spheres (mainly C60), on the basis of previous experimental and theoretical studies. Ring molecules are classified into three types (loop, belt, and disk) according to their shapes and possible interactions. Whereas typical belt‐shaped rings tend to form tight complexes due to the wide contact area via π–π interactions, flat disk‐shaped rings generally form weak complexes due to the narrow contact area mainly via CH–π interactions. In spite of the small association energies, disk‐shaped rings are attractive because such rings can mimic the planet Saturn precisely as exemplified by an anthracene cyclic hexamer–C60 complex.

The Supramolecular Chemistry of Strained Carbon Nanohoops

“…To provide a suitable context for the association constants presented above, recent work on “saturn complexes” based on unstrained, disc‐shaped macrocycles deserves a mention, despite technically being beyond the scope of this Minireview. As a consequence of the smaller surfaces involved in noncovalent interactions, the association constants for these complexes with C 60 ( 11 ⊃C 60 K A =2300 m −1 , toluene; 12 ⊃C 60 K A =580 m −1 , toluene) are three to four orders of magnitude lower than those for the [ n ]CPPs .…”

“…The key proposal of this study, which included several [ n ]CPP and [ n ]CPPA fullerene complexes as examples, is that collective charge fluctuations are responsible for the interactions in nanohoop/fullerene complexes and that only computational approaches which take this into account are suited to provide accurate predictions (Figure b). Finally, two recent articles cover theoretical aspects of the interaction of fullerenes with belt‐shaped compounds such as the “belt”‐shaped [ n ]CPPs and “disc”‐shaped compounds that give rise to “saturn” complexes …”

Supramolekulare Chemie von gespannten Kohlenstoffnanoreifen

“…Um einen geeigneten Kontext für die oben vorgestellten Assoziationskonstanten zu geben, sind jüngere Arbeit an “Saturn‐Komplexen” auf der Grundlage von ungespannten, scheibenförmigen Makrozyklen zu erwähnen, obwohl sie technisch gesehen nicht mehr in den Rahmen dieses Kurzaufsatzes passen. Aufgrund der kleineren Oberfläche, die an nicht‐kovalenten Wechselwirkungen beteiligt ist, sind die Assoziationskonstanten für diese Komplexe mit C 60 ( 11 ⊃C 60 K A =2300 m −1 , Toluol; 12 ⊃C 60 K A =580 m −1 , Toluol) drei bis vier Größenordnungen niedriger als bei den [ n ]CPPs .…”

“…Die zentrale Behauptung dieser Studie, die mehrere [ n ]CPP ‐ und [ n ]CPPA ‐Fullerenkomplexe als Beispiele umfasste, ist, dass kollektive Ladungsschwankungen für die Wechselwirkungen in Nanoreifen/Fulleren‐Komplexen verantwortlich sind und dass sich nur solche Rechenansätze für genaue Vorhersagen eignen, die dies berücksichtigen (Abbildung b). Schließlich behandeln zwei neuere Artikel theoretische Aspekte der Wechselwirkung von Fullerenen mit Verbindungen wie den “gürtelförmigen” [ n ]CPPs und “scheibenförmigen” Verbindungen, die in “Saturn‐Komplexen” vorliegen …”

A Long‐Lived Azafullerenyl Radical Stabilized by Supramolecular Shielding with a [10]Cycloparaphenylene

“…Highly reactive C 59 NC radicals can be shielded by nesting them in carbon nanobelts consisting of single phenyl units connected in para position-cycloparaphenylenes (CPPs). [25][26][27][28][29][30][31] Our approach takes advantage of 1) the 1.4 nm cavity of [10]CPP, resembling the inner space of carbon nanotubes favoring p-p host-guest interactions,t oa ccommodate aC 59 NC radical (Figure 1a), 2) the diminished environmental exposure of the radical resulting from the favorable orientation of the CPPs close to the exposed radical, and 3) the well-established chemistry of C 59 NC,w hich prevents chemical addition to 1,4substituted phenylenes because of steric hindrance. [25][26][27][28][29][30][31] Our approach takes advantage of 1) the 1.4 nm cavity of [10]CPP, resembling the inner space of carbon nanotubes favoring p-p host-guest interactions,t oa ccommodate aC 59 NC radical (Figure 1a), 2) the diminished environmental exposure of the radical resulting from the favorable orientation of the CPPs close to the exposed radical, and 3) the well-established chemistry of C 59 NC,w hich prevents chemical addition to 1,4substituted phenylenes because of steric hindrance.…”