Chemists create smaller all-carbon rings

Two new carbon allotropes have been zapped into being and imaged using atomic force microscopy (AFM). Researchers at Tongji University led by Wei Xu used on-surface synthesis to make rings composed of 10 and 14 carbon atoms, known as cyclo[10]carbon (C₁₀) and cyclo[14]carbon (C₁₄) . Although C₁₀ and C₁₄ have been studied in the gas phase, this is the first time that chemists have managed to make and characterize them on a surface, an accomplishment that gives deeper insight into their bonding.

To make C₁₀ and C₁₄, Xu’s team started with fully chlorinated naphthalene (C₁₀ Cl₈) and anthracene (C₁₄ Cl₁₀) and used an AFM tip to gradually pluck off chlorines. Removing the chlorines prompted the precursor molecules to undergo ring-opening reactions, which ultimately produced the 10- and 14-carbon rings (Nature 2023, DOI: 10.1038/s41586-023-06741-x).

The synthesis differs from that of cyclo[16]carbon( C₁₆) and cyclo[18]carbon( C₁₈) , which used more complex precursors. The University of Oxford’s Harry L. Anderson, who co-led the syntheses of those larger rings, says in an email that starting with readily available polycyclic aromatic chlorocarbons “will open up the field by making cyclocarbons easier to synthesize.”

AFM imaging reveals that C₁₀’s and C₁₄’s bonding characters are distinct from their larger all-carbon siblings, C₁₆ and C₁₈. Bonds in the larger rings are polyynic, meaning that they alternate between single and triple bonds. C₁₀’s bonds are cumulenic—essentially consecutive double bonds. C₁₄ also has only double bonds, but those bonds are not all the same, which indicates that the molecule is somewhere in between cumulenic and polyynic.

IBM Research-Zurich’s Leo Gross, who collaborated with Anderson on the syntheses of C₁₆ and C₁₈, says it’s great to see that C₁₀ has the cumulenic structure predicted by theory and that C₁₄ is a transition structure between cumulenic and polyynic. “For us experimentalists it is nice that we see cyclocarbons in all these different forms,” he says in an email.