Metalloaromaticity

The concept of aromaticity has comfortably resided within the realm of organic chemistry since Michael Faraday first isolated benzene, C6H6, in 1825. Both iconic and compelling, benzene remains the quintessential aromatic molecule.  From an inorganic perspective, the concept of aromaticity, in contrast, has fallen to borazine—the intriguing compound containing a B3N3 six-membered ring—historically, if inaccurately, referred to as “inorganic benzene”. Even though benzene and borazine share similar physical properties, the chemical behavior of these two compounds is quite disparate: benzene readily undergoes electrophilic aromatic substitution reactions while borazine exclusively yields addition products. This profound difference is largely due to the polar B-N bonds in borazine compared to the decidedly nonpolar C-C bonds in benzene.

Benzene

Borazine

The smallest aromatic ring remains the 2π-electron triphenylcyclopropenium cation, [(C6H5)C]3, first isolated by Breslow in 1957. More than four decades later, we made a striking discovery: rings composed of metals may also display traditional aromatic behavior! We prepared the 2π-electron cyclogallene dianion containing a gallium three membered ring, [RGa]3. Consequently, the concept of metalloaromaticity describes traditional aromatic behavior that is derived from a metallic ring system, instead of a carbon ring system.

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2-

Triphenylcyclopropenium Cation

Aromatic

Cyclogallene Dianion

Metalloaromatic

This fertile area of synthetic organometallic chemistry remains largely unexplored with many seminal findings to be discovered.