Issue Date: November 19, 2012
Triangular Gold Cluster Exhibits Delocalized Bonding
A multi-institution research team has synthesized and structurally characterized an unusual gold complex held together by delocalized three-center, two-electron bonding, allowing the complex to be classified as aromatic (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201206712). Aromaticity and antiaromaticity were once thought to be limited to hydrocarbon and heterocyclic ring systems. But several research groups have predicted these phenomena for some metal cluster systems. In a few cases, such as aluminum, they have been observed in gas-phase experiments. A team led by Joseph P. Sadighi at Georgia Tech and Thomas G. Gray at Case Western Reserve University has now prepared and characterized a triangular gold cluster—an Au3+ core bearing N-heterocyclic carbene ligands. They found via crystal structure data that the gold atoms form a nearly equilateral triangle, with the gold-bound carbon atoms coplanar with the ring. Density functional theory calculations predict that the Au–Au bonding arises from a pair of electrons in a highly stabilized molecular orbital lying over all three gold atoms. These features meet the criteria defined by computational chemist Alexander I. Boldyrev of Utah State University for σ-aromaticity. Despite its stability, the gold cluster is readily activated toward two-electron oxidation, a requirement for potential catalysis, Sadighi points out.
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