Disclosure of Ground-State Zimmerman-Möbius Aromaticity in the Radical Anion of [6]Helicene and Evidence for 4π Periodic Aromatic Ring Currents in a Molecular "Metallic" Möbius Strip

Experimental and Theoretical Characterization of 4π-Electron Möbius Aromatic System of a 1,2-Digermacyclobutadiene†

We present the visualization of the experimental valence electron-density distribution (EDD) in the isolated 1,2-digermacyclobutadiene ring system, revealing the unique 4π electron-delocalization on the four-membered Ge2C2 ring. A remarkably high Möbius 4π-electron aromatic character in the Ge2C2 ring can be suggested from theoretical calculations, in sharp contrast to the significant antiaromaticity of the all-carbon cyclobutadiene ring.

Helical polycyclic hydrocarbons with open-shell singlet ground states and ambipolar redox behaviors

Organic π-conjugated polycyclic hydrocarbons (PHs) with an open-shell diradical character are attracting increasing interest due to their promising applications in organic electronics and spintronics. However, most of the open-shell PHs synthesized thus far are based on planar π-conjugated molecules. Herein, we report the synthesis and characterization of two new quinodimethane-embedded expanded helicenes H1 and H2. The helical structures of both molecules were revealed using X-ray crystallographic analysis. It was elucidated in detailed experimental and theoretical studies that they possess an open-shell singlet biradical structure in the ground state and show a small energy gap and amphoteric redox behavior. Both compounds can also be easily oxidized or reduced into relatively stable charged species. The dianions of H1 and H2 exhibit similar electronic structures to the respective isoelectronic structures of their all-benzenoid helical analogues according to NMR measurements and theoretical calculations. Moreover, the structures of the dication and dianion of H2 were identified by X-ray crystallographic analysis, revealing the effect of electron transfer on their backbones and aromaticity. This study thus opens up new avenues for both helical polycyclic π-systems and diradicaloids.

Unraveling the Aromatic Rule of Cyclic Superatomic Molecules in π-Conjugated Compounds

The aromaticity of π-conjugated compounds has long been a confusing issue. Based on a recently emerged two-dimensional (2D) superatomic-molecule theory, a unified rule was built to decipher the aromaticity of cyclic superatomic molecules of π-conjugated compounds from the chemical bonding perspective. Herein, a series of planar [n]helicenes and [n]circulenes, composed of benzene, thiophene, or furfuran, are systemically studied and seen as superatomic molecules ◊On-2◊F2 or ◊On, where superatoms ◊F and ◊O denote π-conjugated units with 5 and 4 π electrons, respectively. The ascertained superatomic Lewis structures intuitively display aromaticity with each basic unit meeting the superatomic sextet rule of benzene, similar to classical valence bond theory, which is favored by the synthesized complex π-conjugated compounds comprising different numbers and kinds of subrings. The evolutionary trend of ring currents and chemical bonding suggests a local ribbon-like aromaticity in these π-conjugated compounds. Moreover, nonplanar helical π-conjugated compounds have the potential to evolve into spring-like periodic materials with excellent physical properties.

Aromaticity: Quo Vadis

Aromaticity is one of the most deeply rooted concepts in chemistry. But why, if two-thirds of existing compounds can be classified as aromatic, is there no consensus on what aromaticity is? σ-, π-, δ-, spherical, Möbius, or all-metal aromaticity… why are so many attributes needed to specify a property? Is aromaticity a dubious concept? This perspective aims to reflect where the aromaticity community is and where it is going.

Antiaromaticity-Promoted Radical Anion stability in α-vinyl Heterocyclics

As an electron-rich species, radical anions have a wide range of applications in organic synthesis. In addition, aromaticity is an essential concept in chemistry that has attracted considerable attention from...

Disclosure of Ground-State Zimmerman-Möbius Aromaticity in the Radical Anion of [6]Helicene and Evidence for 4π Periodic Aromatic Ring Currents in a Molecular "Metallic" Möbius Strip

In 1966, Zimmerman proposed a type of Möbius aromaticity that involves through-space electron delocalization; it has since been widely applied to explain reactivity in pericyclic reactions, but is considered to be limited to transition-state structures. Although the easily accessible hexahelicene radical anion has been known for more than half a century, it was overlooked that it exhibits a ground-state minimum and robust Zimmerman-Möbius aromaticity in its central noose-like opening, becoming, hence, the oldest existing Möbius aromatic system and with smallest Möbius cycle known. Despite its overall aromatic stabilization energy of 13.6 kcal mol-1 (at B3LYP/6-311+G**), the radical also features a strong, globally induced paramagnetic ring current along its outer edge. Exclusive global paramagnetic currents can also be found in other fully delocalized radical anions of 4N+2 π-electron aromatic polycyclic benzenoid hydrocarbons (PAH), thus questioning the established magnetic criterion of antiaromaticity. As an example of a PAH with nontrivial topology, we studied a novel Möbius[16]cyclacene that has a non-orientable surface manifold and a stable closed-shell singlet ground state at several density functional theory levels. Its metallic monoanion radical (0.0095 eV band gap at HSE06/6-31G* level) is also wave-function stable and displays an unusual 4π-periodic, magnetically induced ring current (reminiscent of the transformation behaviour of spinors under spatial rotation), thus indicating the existence of a new, Hückel-rule-evading type of aromaticity.