The concept of aromaticity is a pivotal one in chemistry used to rationalize the notable stability of unsaturated organic molecules, with the benzene ring being a paramount example. Typical π-conjugated molecules obey the aromaticity rule introduced by Hückel, although other types of aromaticity have been also described, that is Möbius aromaticity in annulenes with Möbius conformation and the Baird’s aromaticity for the lowest spin-triplet state, i.e. with two spin-up unpaired electrons.
When benzenoid rings are fused in a polycyclic structure, π-electrons tend to localize forming pseudo-independent aromatic benzene rings (local aromaticity) and very rarely delocalize over the entire molecular framework (global aromaticity).
In this context, conjugated molecules with an annulene-within-an-annulene (AWA) super-ring structure are of special interest. The AWA model consists of inner and outer carbon rings linked through transverse bonds. Many attempts have tried to produce an AWA super-ring in which both rings behave as independent annulenes. But, despite a large amount of theoretical design and many experimental efforts, up to now the synthesis of a decoupled AWA super-ring structure has not been achieved. In the present study, we design, synthesize and characterized experimentally and computationally two novel fully π-conjugated polycyclic hydrocarbon stable macrocycles exhibiting AWA structures. The two compounds, named 8MC and 10MC (Figure 1), are built as alternatively fused five and six membered rings. The tendency to form benzenoid rings with local aromaticity (Clar’s sextet rule) of the two compounds serves as a driving force for the generation of unpaired electrons, resulting in important polyradical character in 8MC and 10MC.
The 1H NMR spectrum of 8MC exhibits a singlet peak at very low field and another weaker broad signal at very high field (Figure 2). These two singlets could be respectively assigned to the outer and inner-ring protons, in good agreement with global aromaticity. The broadening and weakening of all proton resonances upon increasing the temperature is typical of open-shell structures and is related to the population of low-lying states with higher spin multiplicities.
Superconducting quantum interference device (SQUID) measurements of 8MC in powder shows an increase of the product of molar magnetic susceptibility and temperature in agreement with a ground state spin-singlet state with a gap of 3.08 kcal/mol to the lowest spin-triplet. SQUID measurements on 10MC suggest a spin-triplet ground state with the lowest singlet lying at 0.03 kcal/mol.
Theoretical and computational analysis allow us to confirm and rationalize the global aromaticity of the two synthesized super-rings in terms of polyradicaloid character and by means of the Hückel’s and Baird’s aromaticity rules applied to the inner and outer annulene rings. 8MC molecule holds a mild octaradicaloid character (tendency towards the eight unpaired electron configuration) and his structure can be described as a annulene-within-annulene, and despite containing 4n π-electrons, its ground state spin-singlet exhibits global aromaticity as the inner and outer rings follow Baird’s rule. The larger 10MC molecule possesses some decaradicaloid character (tendency towards the ten unpaired electron configuration) with a lower state being a spin-triplet. The super-aromatic AWA structure of 10MC corresponds to a annulene-within-annulene, with the inner and outer rings formally being a local spin-singlet and a spin-triplet, respectively. The global aromaticity emerges as the result of an inner ring satisfying the Hückel’s rule for spin-singlet states, while the outer ring follows the Baird’s rule for triplet states (Figure 3).
Our study exemplifies how rational design can work in conjunction with advanced synthesis procedures and a variety of characterization techniques to achieve fully decoupled AWA super-ring systems with unique properties. Moreover, the work provides a well-defined strategy to obtain novel organic AWA super-ring molecules with tunable global aromaticity.
These results were presented in the article entitled Macrocyclic Polyradicaloids with Unusual Super-ring Structure and Global Aromaticity, recently published in the journal Chem. The work was the result of the collaborative efforts of N. Aratani and H. Yamada from the Institute of Science and Technology (Japan); Y. Hong and D. Kim from Yonsei University (Korea); M. E. Sandoval-Salinas and D. Casanova from Donostia International Physics Center (Spain); and C. Liu, T. Y. Gopalakrishna, T. S. Herng, J. Ding, H. Phan and J. Wu from the National University of Singapore.