On-surface synthesis of a doubly anti-aromatic carbon allotrope

• A collaboration between Oxford Chemistry and IBM Research Europe – Zurich has achieved the synthesis and characterisation of an anti-aromatic carbon allotrope, C₁₆, as reported today in an article published in Nature.
• Cyclo[16]carbon was synthesised on a NaCl bilayer, deposited on a copper substrate, from a molecular precursor C₁₆(CO)₄Br₂, by elimination of bromine and carbon monoxide, at 5 K. The reaction was promoted by applying voltage pulses from the scanning probe tip.
• Atomic force microscopy (AFM) was used to image molecules of C₁₆ revealing a polyynic structure, with eight short C≡C triple bonds and eight long C–C single bonds. In combination with theoretical analysis, this shows that C₁₆ has a doubly anti-aromatic ground state.

Left: Rendered AFM image of cyclo[16]carbon. The eight red protrusions in the ring appear above the eight shorter bonds within the molecule. Upper row: Reaction pathway from the precursor to the final product cyclo[16]carbon. The dissociation reactions are performed by atom manipulation. Lower row: AFM images of the molecules adsorbed on NaCl. (From Nature article.)

C₁₆ is the first structurally characterized anti-aromatic carbon allotrope. It is doubly anti-aromatic, with two circuits of 16-electrons. Anti-aromatic compounds are highly unstable and reactive, so before this work, it was not clear whether any anti-aromatic carbon allotropes could be studied or structurally characterised.

The anion C₁₆^– was also characterised, as well as the neutral molecule. The anion adopts an oval conformation due to strong interactions with the NaCl surface, whereas the neutral molecule is circular. The C₁₆ molecule can be reversibly switched between these two charge states and orbital densities were mapped for the anion. The ability to study different charge states by Kelvin probe force spectroscopy and scanning tunnelling microscopy provides additional evidence to confirm the charge state in the images of neutral C₁₆ molecules. This work builds on previous studies of C₁₈ (Science 2019, 365, 1299) and opens the way to create other elusive carbon-rich molecules by atom manipulation.

You can read more about this study in Nature. https://www.nature.com/articles/s41586-023-06566-8