Chemists from the UK have obtained a cycle of 48 carbon atoms in a solution. To do this, they had to wrap three other cycles around the carbon cycle so that the cyclocarbon molecules did not react with each other. The resulting cyclocarbon was stable in solution at low temperatures, but instantly decomposed when the solvent evaporated, the authors of the study write in Science.
In cyclocarbon molecules, carbon atoms are linked into a cycle by multiple bonds. There are no other atoms in these compounds, so they can be considered a separate allotropic modification of carbon. In solution or solid phase at room temperature, these substances are unstable and quickly disintegrate, so they are usually obtained on substrates at temperatures below 10 Kelvin.
Chemists led by Harry L. Anderson from Oxford University decided to obtain cyclocarbon in solution, despite its instability. The scientists' idea was to synthesize a derivative of cyclocarbon in which several cyclic organic molecules would be wrapped around the carbon chain. The chemists assumed that these molecules would protect the carbon chain from collisions with neighbors, and, as a result, from rapid decomposition.
The scientists began by synthesizing the cyclocarbon base. First, they protected the reactive triple bonds in the starting materials by cobalt coordination. Then, from the alkynes obtained, the scientists assembled a catenane in which the cyclocarbon base with protected triple bonds was surrounded by three organic cycles.
The scientists then set about removing the cobalt protection from the triple bonds. To do this, they had to test many reaction conditions, and eventually settled on a mixture of meta-chloroperbenzoic acid and bipyridine. The scientists mixed this with catenane in dichloromethane, and purified the resulting product using gel permeation chromatography. Mass spectra of the solution obtained after chromatography confirmed the formation of a catenane with a free 48-membered carbon cycle at its core. This was also confirmed by NMR spectroscopy on carbon and hydrogen nuclei.
Cyclocarbon protected by additional rings turned out to be relatively stable in dilute solution — at room temperature, the half-life was 92 hours. But when chemists tried to obtain free cyclocarbon in the same way without additional rings and a catenane structure, the decomposition proceeded very quickly — and scientists were unable to obtain reliable evidence of its formation.
Thus, chemists obtained for the first time a cyclocarbon with 48 atoms in solution braided in a catenane. As scientists note, in the future the stabilization method they used may be useful for other unstable substances.
We recently reported how chemists used a scanning tunneling microscope to manipulate atoms to produce an 18-membered cyclocarbon.
