Knot your usual hydrocarbons

The nanocarbon family just got bigger and more complex. Chemists in Japan have created intertwined molecules, including a trefoil knot and interlocked rings known as catenanes, made up entirely of benzene rings (Science 2019, DOI: 10.1126/science.aav5021). This adds mechanically bonded molecules to the menagerie of carbon nanostructures, which includes fullerenes and nanotubes. Mechanical bonds are key to the movement of many types of molecular machines, the construction of which garnered their creators the 2016 Nobel Prize in Chemistry.

“Occasionally a molecule is made that just looks impossible. This paper has three,” says David A. Leigh, an expert in topologically complex molecules at the University of Manchester who was not involved in the research.

Nagoya University’s Kenichiro Itami and Yasutomo Segawa led the team that created the nanocarbon structures. Previous examples of catenanes and molecular knots typically contain heteroatoms like nitrogen. These heteroatoms are necessary for stitching together the complex topology via interactions with metals or templating molecules. Itami says the chemists were inspired by these topologically complex structures but that they wanted to use only benzene, Itami’s favorite molecule, to make them.

Chemists, including Itami and Segawa, have previously synthesized all-benzene nanorings. But to interlock them, they needed a new strategy. They struck on the idea of using a silicon template to adjoin nanoring fragments. Once they’d cyclized the fragments, they could cleave off the silicon, leaving behind interlocked molecules. Creating the trefoil knot was particularly challenging. Itami points out that there’s only a 0.3% overall yield for the last three steps.

There were some surprises when they analyzed the nanocarbons. Although the chemists’ calculations and crystallographic data indicated that the trefoil knot would be rigid, nuclear magnetic resonance experiments revealed that the molecule’s interlaced chains move quickly in solution, even at temperatures as low as –95 °C.

“This fantastic work brings together the fields of molecular machines and carbon nanoscience, enabled beautifully by organic synthesis,” says Ramesh Jasti, a nanocarbon expert at the University of Oregon. “There is a whole topological world of carbon nanoscience outside of fullerenes, carbon nanotubes, and graphene that we are only beginning to discover.” Jasti calls the new structures “an inspiring beginning.”

“Being able to control topology has the potential to vary properties and characteristics in new ways,” Leigh adds. “Perhaps these molecules, or their relatives, will have useful properties different to other nanocarbons.” But even if they don’t, Leigh says Itami and Segawa’s all-benzene structures “showcase modern organic chemistry’s ability to make the beautiful and extraordinary.”