The simplest atom is not easy to tame

Researchers have developed a light-driven method to harness atomic hydrogen for chemical reactions

In an international collaborative effort, researchers at the Max Planck Institute of Colloids and Interfaces, University College London and Imperial College London have now found a solution to this problem. Using light, they generate hydrogen atoms under mild conditions and use them to reduce organic molecules without the use of metals. Their findings have now been published in the Journal of the American Chemical Society.

A hydrogen atom can't wait

In the laboratory, chemists frequently encounter hydrogen as H₂: two hydrogen atoms firmly bonded together. The molecule is stable and its use in synthetic chemistry widespread. A single hydrogen atom, on the other hand, is a completely different story.

"From a chemical point of view, a hydrogen atom is incredibly simple, but it is also extremely reactive," says Nils J. Flodén, first author of the study at the Max Planck Institute of Colloids and Interfaces. "Therefore, we had to find a way to generate hydrogen atoms under conditions where we can channel their reactivity to do what we want."

Light releases the hydrogen atoms

For their new approach, the researchers use light. They combine hydrazine with a thiophenol derivative and then expose the mixture to light. The light provides the necessary energy to trigger an electron transfer between the two molecules.

The investigations suggest that the electron transfer results in the formation of a short-lived intermediate belonging to the broader class of so-called Rydberg radicals. Their lifetime is only around 13 picoseconds, which corresponds to 13 trillionths of a second. While Rydberg atoms are crucial in quantum computing, in this work the decay of a Rydberg molecule is used to release a hydrogen radical.

The idea dates back more than a hundred years

It is no secret that hydrogen atoms are difficult to generate and control. In 1912, the American chemist Irving Langmuir found that hydrogen molecules can break down into individual hydrogen atoms at the heated tungsten wire of a light bulb. This required temperatures of more than 2,000 Kelvin. Later, methods involving electrical discharges or mercury UV radiation were developed and made it possible to investigate the properties and reactivity of atomic hydrogen in greater detail. However, these methods were not particularly suitable for organic synthesis, as the conditions involved were too extreme.

What the hydrogen radical makes possible

Using the hydrogen radical produced, the researchers are able to reduce various organic molecules under mild, metal-free conditions. These included highly functionalised alkenes and halogen compounds.

"For organic chemistry, the discovery of new chemical intermediates opens up new possibilities," says Peter H. Seeberger, Director of the Department of Biomolecular Systems at the Max Planck Institute of Colloids and Interfaces. "With this work, atomic hydrogen becomes a practical tool for synthetic chemists."

The researchers see this primarily as a new starting point. This is because the unusual reactivity of H• could be of interest not only for the reduction reactions studied. Applications are conceivable in other areas of organic chemistry and, in the long term, also in the biological sciences.