Main group elements in low oxidation states have the potential to carry out chemistry that’s usually reserved for transition metals. For example, at low oxidation states, these atoms might be able to carry out oxidative addition and reductive elimination. But coming up with stable forms of such low-valent compounds is challenging.
A team led by Robert J. Gilliard Jr. of the University of Virginia and Gernot Frenking of Philipps University of Marburg have synthesized, isolated, and characterized the first known stable beryllium radical cation (J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.9b13777). Beryllium in compounds is usually in the +2 oxidation state. The beryllium radical cation is in the +1 oxidation state.
To make the radical cation, the researchers started with a Be(0) compound that Holger Braunschweig and coworkers at Julius Maximilian University of Würzburg had previously reported (Nat. Chem. 2016, DOI: 10.1038/nchem.2542). In that previous study, electrochemical measurements suggested that a Be(I) radical cation likely exists.
“Neither we nor—to the best of my knowledge—anybody else ever succeeded in preparing, isolating, and fully characterizing such a species,” Braunschweig says. The new compounds represent “a breakthrough in the chemistry of low-valent, radical species of the s-block elements,” he adds.
Gilliard’s team tried many different oxidizing agents to make the Be(I) cation. Most of those reagents decomposed the resulting compounds and left the researchers unable to isolate the product. They finally achieved success using TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl). They isolated two compounds, each containing the same beryllium radical cation but different anions.
On paper, the radical cation looks similar to the neutral compound Braunschweig reported in 2016. “But the electronics are completely different because it’s a radical cation,” Gilliard says. The C–Be–C angle in the radical cation is twisted and slightly bent whereas it’s linear in the parent compound.
The work represents “a remarkable achievement in beryllium chemistry,” says Paul G. Plieger, an expert on the element at Massey University. “Beryllium radicals are extremely rare, with the most well-known being beryllium hydride, a semistable radical that has only been observed in the gas phase on earth and on the sun’s surface. The authors have isolated two salts of the same Be(I) radical cation, which is unprecedented. To achieve isolation and structural characterization of the first examples is an exciting and very noteworthy achievement.”
Gilliard doesn’t see an immediate commercial application for the radical cation, but he doesn’t rule out the possibility that chemists will find one. “The motivation is not all about copying transition metal chemistry. It’s also about discovering new fundamental chemistry that we haven’t seen before,” he says. “Every time we do that, if you wait a year or two or three, you always discover something significant that you couldn’t have predicted.”