Squeezing more ingredients into the molecule sandwich

Chemists have made a molecule called a metallocene – or a “sandwich compound” – that breaks the conventional limit on how many things you can fit in a sandwich.

The discovery could eventually help chemists make a range of new materials, including solar cells and pharmaceuticals.

The quirky shapes of metallocenes have intrigued chemists for decades. They’re valuable in making polymers, perovskite solar cells, measuring glucose and catalysing thousands of other reactions.

They’re made from metal atoms, surrounded – or sandwiched – by carbon-containing (organic) molecules. Two chemists were awarded the 1973 Nobel Prize in Chemistry for the discovery of metallocenes.

In general, these sandwich molecules follow the “18-electron rule”: they’re most stable when there are 18 valence electrons in the central metal atom. Previously, chemists have been able to fill the sandwich to 20 electrons, but not more than that.

List of different metallocene compounds
Examples of metallocene chemical compounds, their electron counts, and applications. Credit: Takebayashi et al., 2023

A team of Japanese, Russian and German scientists have made a new metallocene molecule, which can stably hold 21 electrons.

“Having more than 18 electrons is known to be rare because if you deviate from 18, the chemical bonds of the metallocenes start to elongate, break, and change structure,” says Dr Satoshi Takebayashi, a researcher at the Okinawa Institute of Science and Technology, Japan, and co-author on a paper describing the research, published in Nature Communications.

“However, we added two more electrons to a 19-electron metallocene and created a 21-electron metallocene.

“I think most people didn’t think this was possible, but our 21-electron metallocene is stable in solution and solid states and can be stored for a long time.”

The molecule has a cobalt atom at its centre. The researchers are now looking to see how they can use this molecule to catalyse reactions and make new materials, which Takebayashi hopes will be useful in medicine and energy.

Picture of molecule
Crystal structure of the newly synthesized 21-electron metallocene compound showing the nitrogen (blue), cobalt (red), hydrogen (green) and carbon (grey) atoms. Credit: Takebayashi et al., 2023

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