Chemists roll out rings made from stacked sandwich complexes

A new type of sandwich complex has got chemists going in circles. Researchers led by Peter W. Roesky at Karlsruhe Institute of Technology found they could coax 18 metallocene units to curve into a nanometer-scale ring. The chemists call this new type of supersize, circular sandwich complex a cyclocene.

Cyclocenes might one day function as tiny electrical coils in molecular electronics, although such applications are not their primary goal, according to Roesky. “The synthetic challenge was the main driving force,” he says. Roesky’s team made the cyclocenes by alternating layers of metal ions—either strontium, samarium, or europium—with layers of cyclooctatetraenes. Each cyclooctatetraene carried two bulky triisopropylsilyl groups that forced the metallocenes to bend as they stacked (Nature 2023, DOI: 10.1038/s41586-023-06192-4).

“The discovery of the bending was accidental,” Roesky says. When his group previously made a multi-decker sandwich complex with three metal ions and four bulked-up cyclooctatetraene groups, the chemists noticed that the layers didn’t stack straight-up but instead curved. “We realized this might have the potential to form a ring,” he says—in other words, they could add layers to the sandwich until the top of the sandwich complex curved so far back that it wrapped around and touched the bottom.

The chemists construct the cyclocenes by taking monomers of the metal coordinated to the bulky cyclooctatetraene on one side and tetrahydrofuran solvent molecules on the other. They then remove the solvent, which makes the cyclocenes form. Computational work revealed that there’s an energetic benefit to forming the cyclocene rather than a zigzag metallocene wire.

Tetsuro Murahashi, who studies sandwich complexes at the Tokyo Institute of Technology and was not involved in the work, says the finding is an important advance. “The chemistry of sandwich compounds, the subject of the 1973 Nobel Prize in chemistry, has recently been examined for dimensional extensions involving multiple metal atoms,” he says in an email. “The present discovery is a remarkable achievement in this direction, showing that multiple-decker-type sandwich complexes give a circular closed-ring structure.”

Rebecca Musgrave, who studies metallopolymers at King’s College London and was also not involved in the research, agrees that the cyclocenes are a major development in metallocene chemistry. Chemists have only observed cyclocenes in the solid state, but if they can be maintained in solution, “fascinating electrochemical studies could probe the relationship between metal centers in these rings,” she says in an email.

Next, Roesky and colleagues are looking into other aromatic rings and metals to see if they can make cyclocenes of different sizes.