Pnictogen Ring Boasts Aromaticity

Chemists at MIT have created a stable aromatic anion composed solely of pnictogens—the elements that make up the periodic table column to the right of carbon (Science 2015, DOI: 10.1126/science.aab0204). Christopher C. Cummins and Alexandra Velian made the inorganic aromatic ring, which has the formula P₂N₃^–, by transferring diphosphorus to the azide anion using a sodium cryptand cation. They crystallized a complex of the resulting salt and determined that the anion is planar and has an aromatic π-electron system, as indicated by electron structure calculations. Until now, the chemists note, to isolate compounds with multiple bonds between pairs of phosphorus atoms, chemists had to use bulky substituents to block the reactive π bond. But no such bulky groups were needed to isolate P₂N₃^–. “This stabilization is best construed as aromaticity,” Cummins and Velian point out, “an effect traditionally reserved for the domain of organic chemistry.”