On the Unusual Global Aromaticity of Two Cyclopenta-Ring-Fused Oligo(m-Phenylenes)

Some years ago, Jishan Wu reported the synthesis of 8MC and 10MC, two homologues of the cyclopenta-ring-fused oligo(m-phenylene) macrocycles mMC, each behaving as an annulene-within-an-annulene (AWA). This was a surprising result as the AWA behavior is rare. Both molecules have a partial polyradical character, enforced by the quest for restoring some aromatic character of benzene rings. However, that restoration brings back some coupling between the two annulenes. Indeed, we found that the geometry and the magnetically induced currents indicate that, while 8MC does have an AWA character, this is not the case of the larger 10MC. Limitations of the design strategy of AWA molecules should be taken into account in future attempts to prepare novel large coronenes.

Simple and efficient visualization of aromaticity: bond currents calculated from NICS values

Aromaticity is a fundamental concept in chemistry, underpinning the properties and reactivity of many organic compounds and materials. The ability to easily and accurately discern aromatic behavior is key to leveraging it as a design element, yet most aromaticity metrics struggle to combine accurate quantitative evaluation, intuitive interpretability, and user-friendliness. We introduce a new method, NICS2BC, which uses simple and inexpensive NICS calculations to generate information-rich and easily-interpreted bond-current graphs. We test the quantitative and qualitative characterizations afforded by NICS2BC for a selection of molecules of varying structural and electronic complexity, to demonstrate its accuracy and ease of analysis. Moreover, we show that NICS2BC successfully identifies ring-current patterns in molecules known to be difficult cases to interpret with NICS and enables deeper understanding of local aromaticity trends, demonstrating that our method adds additional insight.

Dianion and Dication of Tetracyclopentatetraphenylene as Decoupled Annulene-within-an-Annulene Models

The dianion and dication of tetramesityl-substituted tetracyclopentatetraphenylene, a circulene consisting of alternating five- and six-membered rings, have been generated by reduction with alkali metals and oxidation with antimony(V) halides, respectively. They are theoretically predicted to adopt double annulenoid structures called annulene-within-an-annulene models in which the outer and inner conjugation circuits are significantly decoupled. The theoretical structures were experimentally proven by X-ray crystallographic analyses and the electronic configurations were supported by MCD spectra. Based on the ¹³ C NMR chemical shifts, negative and positive charges are shown to be located mainly at the outer periphery, indicating that the dianion and dication have delocalized 22-π and 18-π electron outer perimeters, respectively, and 8-π electron structure at the inner ring. Notably, the dianion has an open-shell character, whereas the dication has a closed-shell ground state.

Design of annulene-within-an-annulene systems by the altanisation approach. A study of altan-[n]annulenes

Computations on neutral and charged [n]annulenes confirm the general effectiveness of altanisation to design paratropic perimeter circulations. An extension of the design strategy is required for open-shell singlet species.

Global Aromaticity in Macrocyclic Polyradicaloids: Hückel's Rule or Baird's Rule?

Aromaticity is one of the most important concepts in organic chemistry to understand the electronic properties of cyclic π-conjugated molecules. Over a century, different aromaticity rules have been developed and validated. For planar monocyclic conjugated polyenes (also known as [n]annulenes), they will be aromatic if they contain [4N + 2] π electrons according to Hückel's rule, or antiaromatic if they have [4N] π electrons. Topological change from a planar to a half-twisted Möbius strip will lead to [4N] ([4N + 2]) aromaticity (antiaromaticity), which is just inverse to Hückel's rule. When the molecules are excited into the first triplet excited state, the Hückel (anti)aromaticity observed in the ground state will become reversed according to Baird's rule. Strictly speaking, these basic rules are only applicable for monocyclic conjugated systems, but some polycyclic systems such as porphyrinoids may also follow these rules if there is a dominant [n]annulene-like conjugation pathway. On the other hand, all-benzenoid polycyclic aromatic hydrocarbons usually display local aromaticity with π electrons predominantly localized at certain benzene rings according to Clar's aromatic sextet rule. In recent years, some proaromatic and antiaromatic molecules with even number of paired electrons have been found to exhibit open-shell diradical character and unique optical, electronic, and magnetic activities. One of the major driving forces is their intrinsic tendency to become aromatic in the open-shell diradical/polyradical forms. A number of stable diradicaloids and linear polyradicaloids have been successfully synthesized by using thermodynamic and kinetic stabilizing strategies. Herein, our particular interest is a type of macrocyclic polyradicaloid in which multiple frontier π-electrons are antiferromagnetically coupled with each other in a cyclic mode. Formally, these free electrons may behave like normal π-electrons in the [n]annulenes, and thus, it raises questions about their possible global aromaticity and which rule they will follow. In the past 5 years, our group has synthesized a series of macrocyclic polyradicaloids and systematically investigated their global aromaticity and electronic properties. Some important findings include: (1) global (anti)aromaticity is generally observed, but there is a balance between local aromaticity and global aromaticity; (2) most of these molecules follow Hückel's rule in the singlet state and display respective (anti)aromatic characteristics; (3) in some special cases, both Hückel's rule and Baird's rule can be applicable, and a unique annulene-within-an-annulene super-ring structure was demonstrated for the first time; (4) global antiaromaticity in the transition state is also important and a slow valence tautomerization process was observed in a supercyclobutadiene tetraradicaloid. These studies demonstrate how these open-shell macrocyclic polyradicaloids adapt their geometry and spin state to reach the lowest-energy state (aromatic). In this Account, we will mainly discuss their synthesis, global aromaticity, and the fundamental structure-radical character-aromaticity-properties relationships. Various experimental methods (e.g., NMR, X-ray crystallographic analysis, and electronic absorption spectroscopy) and theoretical calculations (e.g., anisotropy of the induced current density, nucleus independent chemical shift, and isochemical shielding surface) have been used to elaborate their (anti)aromatic character. At the end, a perspective on the possible three-dimensional global aromaticity in fully conjugated cagelike diradicaloids or polyradicaloids will be also discussed.

Topological ring-currents and bond-currents in the altan-[r,s]-coronenes

π-Electron ring-currents for the altans of the four regular [r,s]-coronenes are calculated by the topological HLPM approach.

Non-alternant non-benzenoid kekulenes: the birth of a new kekulene family

Theoretical and experimental aspects of kekulene and its benzenoid, non-benzenoid and non-alternant type congeners are reviewed.

On the diatropic perimeter of iterated altan-molecules

Ab initio calculations confirm that the design of large paratropic circuits through altanisation fails in altan-[10,5]coronene. This molecule is predicted to be an open-shell singlet biradical, like the cycloacene that it contains.

Aromaticity of the planar hetero[8]circulenes and their doubly charged ions: NICS and GIMIC characterization

All hetero[8]circulenes represent nonaromatic species because the paratropic internal currents (red colour) substantially cancel out the diatropic contribution on the outside edge (blue colour).