Light-driven method uses hydrogen atoms to transform chemicals without metals

Researchers have developed a light-driven method to generate highly reactive hydrogen atoms for chemical synthesis.

The technique produces individual hydrogen atoms under mild conditions, addressing a long-standing challenge in preparing and using these reactive particles.

First produced by Nobel laureate Irving Langmuir more than a century ago, individual hydrogen atoms are difficult to harness because they react readily with other substances.

Researchers at the Max Planck Institute of Colloids and Interfaces, University College London, and Imperial College London used the generated atoms to reduce organic molecules without metals. It offers a potential approach to chemical transformations under milder conditions.

Using light for hydrogen

The technique allows researchers to produce highly reactive individual hydrogen atoms under mild conditions and use them to reduce organic molecules. The approach could help chemists investigate the unusual reactivity of atomic hydrogen and explore its potential in other chemical reactions.

Hydrogen is the simplest element on the periodic table, with one proton and one electron. However, its behavior changes depending on whether it exists as a single atom or a molecule.

In laboratories, hydrogen is commonly encountered as H₂, a stable molecule consisting of two bonded hydrogen atoms. A single hydrogen atom, represented as H•, is much more reactive and difficult to prepare and control. Its high reactivity makes it challenging to harness for chemical synthesis.

“From a chemical point of view, a hydrogen atom is incredibly simple, but it is also extremely reactive,” said Nils J. Flodén, the study’s first author at the Max Planck Institute of Colloids and Interfaces, in a statement. The researchers therefore sought a method to generate atomic hydrogen under conditions that would allow them to direct its reactivity toward specific chemical transformations.

The new approach combines hydrazine with a thiophenol derivative, then exposes the mixture to light. The light supplies the energy needed to trigger electron transfer between the two molecules. Notably, the investigations suggest that this process produces a short-lived intermediate belonging to a broader class of compounds known as Rydberg radicals. The intermediate exists for approximately 13 picoseconds, or 13 trillionths of a second. As the Rydberg molecule decays, it releases a hydrogen radical that can participate in chemical reactions.

Metal-free reduction reactions

The team, using hydrogen radicals generated through their light-driven process, reduced several organic molecules under mild conditions without metals. The tested compounds included highly functionalized alkenes and halogen compounds.

According to researchers, reduction reactions are important in organic chemistry because they enable chemists to modify molecules by adding hydrogen or otherwise changing their electronic structure. Producing reactive hydrogen under controlled conditions could provide another tool for these transformations.

“For organic chemistry, the discovery of new chemical intermediates opens up new possibilities. With this work, atomic hydrogen becomes a practical tool for synthetic chemists,” said Peter H. Seeberger, Director of the Department of Biomolecular Systems at the institute.

The researchers view the method as a starting point for further investigations rather than a solution limited to the reduction reactions already demonstrated. Atomic hydrogen’s unusual reactivity could potentially be useful in other areas of organic chemistry and, over the longer term, biological sciences. Further research will be needed to determine how broadly the technique can be applied.

Source: https://interestingengineering.com/science/light-driven-technique-makes-elusive-hydrogen-atoms-useful