Single-electron carbon-carbon bond identified

Chemists at Hokkaido University have experimentally confirmed the existence of a single-electron carbon-carbon bond for the first time (Nature 2024, DOI: 10.1038/s41586-024-07965-1). Linus Pauling proposed the concept of one-electron covalent bonds in 1931, but direct evidence of their existence in carbon systems had proved elusive.

Single-electron C–C bonds have particularly low bond strengths and are consequently highly reactive, which makes them difficult to isolate and study. But electron-rich alkanes such as hexaphenylethane (HPE) derivatives are promising substrates to probe this phenomenon, as their adjacent π systems should stabilise the electron-deficient radical cation product. But actually accessing the transient one-electron state is itself a challenge, because HPEs typically undergo two-electron oxidation, breaking the C–C bond to form a pair of cations.

Takuya Shimajiri, who is now at the University of Tokyo, and colleagues hoped to influence this redox chemistry using structural modifications. The team started with a highly strained HPE substrate containing a rigid bridging acenaphthylene unit between the carbon centers to produce a long C–C single bond—1.8 Å compared with a typical bond length of 1.54 Å. The researchers could then perform a single-electron oxidation of their substrate using iodine, producing isolable crystals of the elusive product. The one-electron nature of the bond was rigorously confirmed through a combination of crystallographic, spectroscopic, and theoretical analyses.

“The authors present a clever design with a not-too-short and not-too-long separation of the two sites,” Max Hansmann, an organic chemist at the Technical University of Dortmund, says in an email. “As an indirect result of this ultra-long C–C bond, the system features a step-wise (one-by-one electron) electrochemistry.”

The work dives into a fascinating fundamental question, Hansmann says, and it will be exciting to see the reactivity and applications for these systems going forward. “But, the rather specific arrangement of the structure may make it difficult to find more such examples. A major goal would be to find systems in which one-electron bonds could be formed intermolecularly, without the need for strict pre-organisation,” he says.