Concave–Convex π–π Template Approach Enables the Synthesis of [10]Cycloparaphenylene–Fullerene [2]Rotaxanes

Abstract

The cycloparaphenylenes (CPPs) are a class of strained macrocycles that until 2008 were considered beyond the reach of organic synthesis. With its cyclic array of ten para-substituted phenylene rings, [10]­CPP possesses a concave π-system that is perfectly preorganized for the strong supramolecular association of convex fullerenes such as C60. Although mechanically interlocked CPP architectures have been observed in the gas phase, the rational synthesis of bulk quantities has not been achieved yet, which is likely due to the fact that conventional template strategies are not amenable to CPP rings that lack heteroatoms. Here, we report the synthesis of two [2]­rotaxanes in which a [10]­CPP ring binds to a central fullerene bis-adduct and is prevented from dethreading by the presence of two bulky fullerene hexakis-adduct stoppers. The final step in the rotaxane synthesis is surprisingly efficient (up to ca. 40% yield) and regioselective because the fullerene acts as an efficient convex template, while [10]­CPP acts as a supramolecular directing group, steering the reaction at the central fullerene exclusively toward two trans regioisomers. Comprehensive physicochemical studies confirmed the interlocked structure, shed light on the dynamic nature of the CPP–fullerene interaction, and revealed intriguing consequences of the mechanical bond on charge transfer processes. In light of recent advances in the synthesis of nanohoops and nanobelts, our concave–convex π–π templating strategy may be broadly useful and enable applications in molecular electronics or complex molecular machinery.

The Supramolecular Chemistry of Strained Carbon Nanohoops

“…Our group recently achieved the synthesis of two discrete [2]rotaxanes, in which [10]CPP serves as the macrocyclic component (Figure a) . The strong concave–convex interaction between C 60 and [10]CPP was used to “trap” the macrocycle on a central fullerene bis‐adduct binding site by conducting a carefully optimized Bingel–Hirsch reaction in the final step.…”

Supramolekulare Chemie von gespannten Kohlenstoffnanoreifen

“…Unserer Gruppe gelang kürzlich die Synthese von zwei diskreten [2]Rotaxanen, bei denen [10]CPP als makrozyklische Komponente fungiert (Abbildung a) . Die starke konkav‐konvexe Wechselwirkung zwischen C 60 und [10]CPP wurde genutzt, um den Makrozyklus auf einer zentralen Fullerenbisaddukt‐Bindungsstelle mit einer sorgfältig optimierten Bingel‐Hirsch‐Reaktion letztlich “einzufangen”.…”

“…Eindeutige Beweise für die mechanisch verzahnte Struktur von 24 wurden durch VT‐NMR‐Spektroskopie und CID‐Massenspektrometrie gewonnen. Bei der Untersuchung der beiden Rotaxanisomere durch transiente Absorptionsspektroskopie beobachteten Guldi und Mitarbeiter interessante Unterschiede zwischen der Ladungsseparations‐ (CS) und Rekombinationskinetik (CR; Abbildung a unten; [10]CPP : Elektronendonor; Fullerenbisaddukt: Elektronenakzeptor) eines “gebundenen” und eines “ungebundenen” Zustands des Nanoreifens (der Komplexisierungs‐/Dekomplexierungsprozess verläuft auf der Zeitskala des Anregungs‐Abfrage‐Experiments langsam).…”

“… a) DFT‐optimierte Struktur des Rotaxans 24 ( trans ‐2‐Regioisomer) und Zusammenfassung der Ergebnisse der transienten Absorptionsspektroskopie . b) Struktur sowie Pd II ‐ (oben) sowie F − ‐induzierte (unten) Emission der Rotaxane 25 und 26 , übernommen aus Lit.…”

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.…”