Tridecacyclene: Synthesis and Structural Properties of Non-Planar Polycyclic Aromatic Hydrocarbons and Studies Towards a Fragment of the Fullerene C240
While investigating strategies to prepare precursors to highly strained buckybowls, we focused our attention on the Lewis acid aldol cyclization of 1-acenaphthenone derivatives which has been shown to produce a cyclic tetramer as a byproduct. Surprisingly, despite the interesting structural and electronic properties that have recently been observed as a result of the incorporation of eight-membered rings into polycyclic aromatic hydrocarbons, this cyclic tetramer has largely been ignored. As a result of this, we set our sights on the isolation and characterization of this cyclic tetramer.
The initial approach employed subjecting 1-acenaphthenone to the most common conditions used in trimerization—TiCl4 in boiling o-dichlorobenzene. Surprisingly, this resulted in exclusive formation of the cyclic tetramer, which we have named tridecacyclene. The results of these studies were promising, establishing structural characterization and supramolecular assemblies with C60 in the solid-state. The optoelectronic properties revealed a significantly lower reduction potential (~0.4 eV) than the trimeric species of 1-acenaphthenone. This is attributed to the central eight-membered ring of tridecacyclene. Reduction proceeded through two single-electron processes.
Further examining the electrochemical properties, we were able to gain new insight into the relation of structure and aromaticity. Reduction of tridecacyclene with potassium metal allowed us to characterize the radical anion and dianion through NMR and UV-Vis spectroscopy. Solid-state analysis of the dipotassium adduct revealed that despite the propensity of the reduced form of cyclooctatetraene derivatives to flatten as the molecule adopts a Hückel aromatic core, tridecacyclene maintains its tub shape. Significant bond equalization was observed in the center eight-membered ring—a strong indication of a delocalized π-system. This was supported by harmonic oscillator model of aromaticity calculations of the central ring with the value increasing from 0.09 to 0.48 where a value of 1 indicates a fully aromatic ring.
Tridecacyclene represents the precursor to a fragment of the fullerene C240. A broad variety of reactions to facilitate the necessary strain inducing C–C bonds to formthe fragment have been attempted. To date, we have not been able to synthesize the fragment. However, the parent molecule tridecacyclene shows great promise in the development of non-aqueous redox flow batteries and is currently being explored for this purpose in our laboratory.