By virtue of synthesizing a stable B3 ring, a team of inorganic chemists has prepared the lightest aromatic species that is experimentally possible. In addition to helping researchers better understand chemical structure and bonding, the sandwich molecule the team made containing two B3 rings connected by sodium ions could serve a practical purpose as the first of a family of precursor compounds for preparing semiconducting, superconducting, and magnetic materials.
Thomas Kupfer, Holger Braunschweig, and Krzysztof Radacki of Julius Maximilian University Würzburg made the triboracyclopropenyl dianion by treating cyclohexyl-substituted dichloroaminoborane with sodium metal in dimethoxyethane solvent (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201508670). The Würzburg team has shown through computational, spectroscopic, and electrochemical studies that the B3 ring has an electronic structure consistent with classical aromatic carbon compounds such as the cyclopropenyl cation and benzene.
The announcement “is certainly a great breakthrough and opens a new direction in boron chemistry,” comments Alexander I. Boldyrev of Utah State University, whose group has done much of the computational work on planar all-boron rings during the past 15 years.
Aromaticity as defined by the Hückel Rule has typically been the domain of carbon compounds, with only a few noncarbon analogs made from elements heavier than carbon. Chemists have long predicted that planar boron rings from B3 up to B15 or greater might also be aromatic. Researchers have been hot on the trail of making the boron rings, but the rings are challenging to stabilize in isolable compounds. The greatest success has come in generating the molecules with laser beams and studying them in the gas phase.
“This discovery may help settle the discussion on the validity, or not, of applying the concept of aromaticity to noncarbon-based systems,” says Boniface Fokwa of the University of California, Riverside. In 2012, Fokwa was part of a team that prepared the solid-state material Ti7Rh4Ir2B8, which was the first example of an isolable compound containing a planar all-boron ring. “The discovery of the new planar B3 entity strongly hints at its possible stabilization in a solid-state compound as well,” Fokwa says. “Hopefully, exciting new functional materials based on it will be achieved.”