Observation of All-Metal Aromatic Molecules

Aromaticity is a concept invented to account for the unusual stability of an important class of organic molecules: the aromatic compounds. Here we report experimental and theoretical evidence of aromaticity in all-metal systems. A series of bimetallic clusters with chemical composition MAl4 -(M = Li, Na, or Cu), was created and studied with photoelectron spectroscopy and ab initio calculations. All the MAl4 -species possess a pyramidal structure containing an M+cation interacting with a square Al4 2-unit. Ab initio studies indicate that Al4 2-exhibits characteristics of aromaticity with two delocalized π electrons (thus following the 4n + 2 electron counting rule) and a square planar structure and maintains its structural and electronic features in all the MAl4 -complexes. These findings expand the aromaticity concept into the arena of all-metal species.