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. The phenylselenyl groups donate electron density to the benzene to make oxidation easier, and the phenyls' bulk improves the solubility of the dications by keeping them away from each other in solution.
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.
CORRECTION: This story was updated on Oct. 17, 2018, to correct the explanation of how the phenylselenyl groups affect the molecule's behavior.