Left: Chemical structure of the cyclo[48]carbon [4]catenane. Right: Space-filling representation. Credit: Harry Anderson
For the first time since 1990, researchers have synthesised a new form of molecular carbon which can be studied on the lab bench under ambient conditions.
It somewhat resembles a charm bracelet.
The molecule, cyclo[48]carbon, is a ring of 48 carbon atoms arranged in alternating single and triple bonds.
All other cyclocarbons made before now have been too reactive to study unless kept at extremely low temperatures (4 to 10 Kelvin, or -269 to -263°C) or in the gas phase.
The new molecule is stable in liquid solution at room temperature (20°C) and has a half-life 92 hours under these conditions. This means that after 92 hours on the bench, half of the initial amount of cyclo[48]carbon will not yet have reacted to form something else.
This was made possible because the chemists who synthesised cyclo[48]carbon also threaded it through 3 other large molecular rings (macrocycles) to form a ‘catenane’.
Imagine the catenane as a bracelet: cyclo[48]carbon’s ring of carbon atoms would be the string, while the 3 macrocycles would be the beads strung onto it.
This ‘supramolecular encapsulation’ increased the molecule’s lifetime in solution by preventing the cyclocarbon molecules from reacting together to form a cross-linked material.
“Naked” cyclo[48]carbon has a half-life of only about 1 hour at 20°C and a concentration of 0.9 micromoles per litre.
It decomposes even more rapidly at higher concentrations.
“Achieving stable cyclocarbons in a vial at ambient conditions is a fundamental step,” says Dr Yueze Gao from the Department of Chemistry at the University of Oxford, UK.
“This will make it easier to study their reactivity and properties under normal laboratory conditions.”
Professor Harry Andersen, also from Oxford, adds: “This achievement marks the culmination of a long endeavour to synthesise cyclocarbon catenanes, based on the hope that they might be stable enough to study at room temperature.
“The original grant proposal was written in 2016, based on preliminary results from 2012–2015. It is satisfying to have reached this point, because there were many times when the goal seemed unrealistic and unachievable.”
The findings have been published in the journal Science.