The littlest aromate

In recent months, two research groups have reported new molecules containing three-membered aromatic boron rings. The chemists behind each of these discoveries made a claim that their molecules have established a new precedent as the smallest or lightest aromatic species now known. Both have a legitimate shot at being named a record holder, but their cases hinge on how chemists define aromaticity.

For the claim of lightest aromatic ring, a team led by Holger Braunschweig of Julius Maximilian University Würzburg synthesized a sandwich molecule containing two triboracyclopropenyl anion rings (B₃^2–) bearing dicyclohexylamino groups and connected by sodium ions, Na₄[B₃(NR₂)₃]₂. The Würzburg team has shown through computational, spectroscopic, and electrochemical studies that the B₃^2– ring structure is consistent with classical π aromatic carbon compounds such as benzene (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201508670).

For the claim of smallest aromatic ring, Gernot Frenking of Philipps University Marburg, Mingfei Zhou of Fudan University, and colleagues used a laser to vaporize a boron target in the presence of nitrogen or carbon monoxide to generate [B₃(N₂)₃]⁺ and [B₃(CO)₃]⁺ complexes. With computational and spectroscopic studies, the team has provided evidence that the B₃⁺ ring is also a π aromatic species (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201509826).

The merits of these two claims notwithstanding, the new boron rings raise broader questions of how to define aromaticity and to what end chemists should go to determine which molecule really is or could be the littlest aromate. C&EN challenged those authors and other chemical bonding experts to come up with definitive answers.

Chemists typically apply aromaticity to five- and six-membered hydrocarbon and heterocyclic ring systems that have 4n + 2 π electrons, where n is any integer. This formula foretells whether a planar ring molecule will have a stabilizing conjugated π electron system involving delocalized p orbitals.

But aromaticity expands well beyond those confines. The largest aromatic molecule with a single ring is a 50-membered expanded porphyrin with 50 π electrons. At the opposite end of the spectrum, the cyclopropenyl cation, C₃H₃⁺, a three-membered ring with two π electrons first isolated as a salt by Ronald Breslow and coworkers in 1967, is generally accepted as the simplest organic aromatic species.

However, there’s a dizzying array of other possibilities. Chemists have been applying the concept of aromaticity more broadly to include noncarbon systems such as boron rings, all-metal cluster systems such as three-membered gallium ring compounds, and spherical compounds such as fullerenes. Although there’s some lingering controversy, many chemists now accept that compounds can exhibit multiple types of aromaticity. Instead of just delocalized π bonds, theorists say σ bonds, normally thought of as being localized between two atoms, can also be delocalized over a molecular framework, such as a three-membered ring.

With those options on the table, trying to decide which aromatic ring is smallest or lightest becomes a quagmire. Does a chemist restrict the candidates to only traditional π aromatic rings or also include σ aromaticity? Should any molecule that is theoretically plausible based on computational analysis be considered, or should the choices be limited only to molecules that can be synthesized? And in that case, does observing the molecule in the gas phase qualify, or should a compound have to be isolated and put into a bottle for it to count?

“Any new compound obtained in a molecular beam, or in matrix isolation, or as a solid-state material is important for understanding the role of aromaticity,” says computational chemist Alexander I. Boldyrev of Utah State University, whose group has studied inorganic aromaticity.

In helping answer the question of how low aromaticity can go, Boldyrev and doctoral student Ivan A. Popov, at C&EN’s request, calculated the parameters for the possible three-membered rings that can be formed from the first nine elements of the periodic table.

Until the new boron compounds came along, the smallest π aromatic hydrocarbon was C₃H₃⁺ and its derivatives (C₃R₃⁺), Boldyrev and Popov note. These compounds have three C–C σ bonds and one delocalized three-centered two-electron π bond holding the ring together.

The more exciting possibilities, Boldyrev suggests, lie with the noncarbon candidates. “Aromaticity in inorganic chemistry is much more important in explaining the structure, stability, and reactivity than in organic chemistry because of the diversity of atoms and the diversity of types of aromaticity,” Boldyrev says.

For example, the B₃^2– ring in the Braunschweig group’s Na₄[B₃(NR₂)₃]₂ has 11 valence electrons—six electrons in three classical B–B σ bonds, three electrons shared in σ bonds to the amino substituents, and two valence electrons in a delocalized π bond. Although the B₃^2– ring is lighter than the cyclopropenyl cation, the latter is still smaller; moving left to right across the periodic table, the atomic radii of the elements decrease, meaning carbon rings typically will be smaller than boron rings. So Braunschweig’s group can claim that B₃^2– holds the title of lightest π aromatic ring in a molecule that has been isolated.

On the other hand, Frenking claims that the B₃⁺ ring in his team’s [B₃(N₂)₃]⁺ and [B₃(CO)₃]⁺ compounds is the lightest π aromatic system to be experimentally observed. It has eight valence electrons in three B–B σ bonds and one delocalized π bond. “We have beaten the record by three electrons!” Frenking says. But beyond that, he also thinks B₃⁺ is the smallest π aromatic system, beating out even C₃H₃⁺, because it has no substituent atoms protruding from the ring. Frenking adds that he has no problem with that stipulation because the N₂ and CO ligands in his compounds serve only to stabilize the aromatic cation and aren’t required for the ring to be aromatic, unlike C₃H₃⁺ and Braunschweig’s molecule.

Computational chemist Anastassia N. Alexandrova of the University of California, Los Angeles, says chemists should hold their horses before declaring a π champion. In 2003, when she was a graduate student in Boldyrev’s group, she ran calculations on the naked Bring. Alexandrova found that B₃^–, which has been produced in the gas phase and studied spectroscopically, is doubly aromatic with three B–B σ bonds, one delocalized σ bond, and one π bond. Alexandrova, who also ran calculations for C&EN, says B₃^– has the shortest bond lengths of the boron candidates and, like Frenking’s ring, has fewer atoms than C₃H₃⁺, so it should be deemed smallest.

But are the carbon and boron species really the lightest π aromates possible? “Theory says no,” Frenking reveals. A predicted Be₃^2– ring capped by two metal atoms (M₂Be₃, where M = Li, Na, Cu) exhibits π aromaticity. However, these species have larger ring sizes.

Considering σ delocalized bonding changes the aromaticity picture further, Alexandrova says. All the chemists agree that the trihydrogen cation, H₃⁺, with two σ electrons in a single delocalized bond, is the unbeatable smallest and lightest aromate. Many people may never have heard of H₃⁺, but it’s one of the most abundant molecular species in the universe. It’s stable in the low-temperature and low-density conditions of interstellar space but does not enjoy those comforts on Earth. The molecule can be generated from H₂ in a plasma discharge in the lab, but it has not yet been incorporated into an isolable compound.

Boldyrev and Popov’s calculations indicate that the next element in the periodic table—helium—can’t form a stable aromatic ring. That leaves Li₃⁺ as the next candidate. In 2003, Alexandrova and Boldyrev predicted that Li+has one delocalized σ bond, although another study had conflicting results on whether it is aromatic. “I am sure Li₃⁺ was experimentally observed well ahead of our work,” Boldyrev says. “But nobody had discussed its possible aromaticity.” Although it is lighter than the carbon and boron species, its ring size dwarfs them.

Further down the first row of elements, Boldyrev and Popov’s as well as Alexandrova’s calculations suggest it’s not likely anyone will be able to make stable aromatic triangular species out of highly electronegative nitrogen, oxygen, or fluorine. Alexandrova’s calculations suggest that N₃⁺ would have the smallest ring area if it could be made. Mixed-element compounds such as C₂B⁺ and NO₂^– are other possibilities, they note, but they are either nonaromatic or are larger than the C₃ candidates.

“Playing with the rules, or sometimes against the rules, is part of human nature,” says Bernd Wrackmeyer of the University of Bayreuth, who wrote a commentary in Angewandte Chemie on the new B₃ species created by Frenking, Braunschweig, and coworkers and where they fall in the aromaticity pecking order. “The development of modern inorganic chemistry, where the role of organic groups is reduced mainly to providing kinetic stabilization, has opened an almost unlimited playground full of challenging tasks.”