Observation of All-Metal Aromatic Molecules

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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 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.