A six-carbon 10π-electron aromatic system supported by group 3 metals.

Aromaticity is a fundamental concept with implications spanning all the chemical sciences. Hückel's (4n+2)π-electron rule is the standard criterion to determine aromaticity and it applies well to neutral arenes as well as to charged species such as the cyclopentadienyl anion, the cyclooctatetraene dianion and the cycloheptatrienyl cation (tropylium). In the series of all-carbon aromatic compounds, no example of a benzene tetraanion, which is theoretically a 6C, 10π-electron aromatic system, has been reported although heteroatom analogues of such a system, known as 'electron-rich aromatics', have been studied in detail for a long time. Here we present the isolation of the first tetraanionic-substituted benzene as a ligand coordinated to group 3 metals. The nature of the substituted benzene tetraanion and the aromaticity of the 6C, 10π-electron system were established by X-ray crystallographic studies, multi-nuclei nuclear magnetic resonance spectroscopy, X-ray absorption spectroscopy and density functional theory calculations.