Gallium Additions To Aromaticity

Aromaticity, a phenomenon in which delocalized bonding enhances the stability of cyclic molecular frameworks, has traditionally been the province of organic chemistry. But no more. Enterprising chemists have been showing with greater frequency that carbon isn't always necessary for aromaticity; metal-containing compounds of all sorts can mimic the behavior of classical aromatic systems.

A pair of chemists reported the latest manifestations of metalloaromaticity in the form of two novel gallium compounds during presentations before the Division of Inorganic Chemistry at the ACS national meeting taking place this week in Salt Lake City. Creation of these types of compounds is helping chemists gain an ever-better understanding of chemical bonding.

Gregory H. Robinson of the University of Georgia described his group's synthesis of a Ga₆ cage compound, the first example of a neutral aromatic gallium species with an octahedral structure. And Philip P. Power of the University of California, Davis, discussed his group's preparation of the first aromatic metallabenzene containing two gallium atoms. Both researchers have previously dabbled with making gallium-containing ring compounds.

Alexander I. Boldyrev of Utah State University, whose research includes computational and gas-phase studies on metalloaromatic species, notes that the Robinson and Power compounds "represent remarkable advancements of delocalized bonding in main-group inorganic chemistry." These researchers are helping to move the field forward by making stable, isolable solids that "show us the great potential for aromaticity and antiaromaticity concepts beyond organic chemistry," Boldyrev says.

Robinson's group made its compound, Ga₆R₄L₂, where R is mesityl and L is an N-heterocyclic carbene, by reducing the precursor compound LGaRCl₂ with potassium metal (J. Am. Chem. Soc. 2009, 131, 3168). The compound has 14 delocalized valence electrons in its skeleton and exhibits spherical aromaticity, similar to closed-cage boranes such as B₆H₆^2???.

The synthesis was part of a larger effort Robinson discussed to use N-heterocyclic carbenes as versatile ligands for main-group compounds, including diatomic boron, silicon, and phosphorus species. "The metalloaromaticity of the Ga₆ compound is traced to the stabilizing electron-donor capabilities of the N-heterocyclic carbene," Robinson told C&EN.

Power's group prepared a digallatabenzene, K₂[C₄H₂Ph₂Ga₂R₂], where Ph is phenyl and R is a bulky terphenyl, by inserting phenylacetylene into the digallene RGa=GaR, then reducing the cyclic product with potassium metal. The dianion ring has six π electrons, the same as benzene, with the negative charges balanced by potassium cations.

Power discussed the digallatabenzene as part of a presentation on his group's exploration of adding hydrogen, ammonia, and small unsaturated hydrocarbons to digallium and other group 13 and 14 metal compounds (Angew. Chem. Int. Ed. 2009, 48, 2027 and 2031). Looking beyond digallatabenzene, Power believes that getting three or four gallium atoms into a stable six-membered aromatic ring is feasible.