Observation of all-metal aromatic molecules

Abstract

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 posses a pyramidal structure containing an M+ cation interacting with a square Al42- unit. Ab initio studies indicate that Al42- exhibits characteristics of aromaticity with two delocalized p electrons (thus following the (4n+2) p 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.