Undergraduates learn that aromaticity means a specific number of electrons in conjugated π orbitals above and below a ring. But like many concepts taught in first-year organic chemistry, the truth is more complex. Chemists are actively debating the concept of aromaticity. Some argue that σ and δ orbitals can form aromatic systems and that multiple types of aromaticity can exist in a single molecule. Masaichi Saito of Saitama University and colleagues have added another log to that fire by confirming double aromaticity in a bench-stable compound (Commun. Chem. 2018, DOI: 10.1038/s42004-018-0057-4).
Chemists have previously predicted a number of double aromatic compounds, such as those involving boron. In 1988, Vanderbilt University’s James C. Martin reported a hexaiodobenzene dication that had σ aromaticity (J. Am. Chem. Soc. 1988, DOI: 10.1021/ja00225a038), although later research questioned if the molecule was truly a dication (Chem.—Eur. J. 2012, DOI: 10.1002/chem.201102960). Inspired by Martin’s work, Saito synthesized a hexakis(phenylselenyl)benzene dication, which has selenium atoms attached to each of the six carbon atoms in the central benzene ring. Martin’s group had investigated that molecule’s σ aromaticity as early as 1990 but did not report the research in the scientific literature and did not examine the compound’s π aromaticity.
Saito explains that selenium’s large size allows overlapping σ orbitals from the six atoms to create aromaticity. Phenyl substituents have two functions: They keep the selenium atoms from getting too close to each other and donate electron density to the molecule to make oxidation easier.
A two-electron oxidation leaves the inner ring of carbon π orbitals with six electrons and the outer ring of selenium σ orbitals with 10, both satisfying the 4n+2 electron rule for aromaticity, according to the researchers. X-ray crystallography data showed that C–C bond lengths were nearly identical, and Se–Se bond lengths fell within a 0.1-Å range—suggesting the symmetry needed for aromaticity. Also, the group found that the C–Se distances were too long to be double bonds.
Saito’s work “appears to be the first σ and π doubly aromatic compound that has been crystallized and structurally characterized using X-ray diffraction,” says Brown University physical chemist Lai-Sheng Wang.
Alexander I. Boldyrev, a theoretical chemist at Utah State University who has predicted boron-containing double aromatic molecules, says Saito has indeed found a real example of π and σ aromaticity. “What is most important [is] this is a bottled compound,” he says. Boldyrev thinks the molecule could help chemists understand bonding, structure, and stability in other molecules, especially inorganic compounds.