More efficient and sustainable energy conversion technologies, among other applications, hinge on lowering the amount of energy needed to trigger specific reactions on the surface of electrodes. Called electrocatalysis, the process conserves energy by transferring electrons and speeding up the reaction time, but the molecules involved typically cannot shuttle other particles or directly activate components of the system. Now, a team led by researchers at YOKOHAMA National University and the University of Tokyo have designed a new class of mediators that more actively and precisely control electrocatalysis reactions.
They published their work on Jan. 8 in the Journal of the American Chemical Society .
The reactions in electrocatalysis are referred to as redox reactions, a shorthand meaning "reduction-oxidation." When one component in a system gains an electron, it's reduced. When another component loses an electron, it's oxidized. According to the researchers, conventional redox mediators are mostly limited to chauffeuring those electrons between components.
"Most redox mediators act only as passive electron carriers and cannot actively control substrate activation or proton transfer," said co-corresponding author Naoki Shida, associate professor, Faculty of Engineering at YOKOHAMA National University. "The key message of this work is that redox mediators can do more than shuttle electrons."
The team set out to understand whether interactions — specifically halogen bonding that appears only after oxidation — could be used to regulate electron transfer coupled with protons and to enable more efficient, selective bond formation between carbon and nitrogen. Selective C-N bonding, as it is called, enables precise bonds between specific carbon and nitrogen atoms without involving other atoms. Such control is highly desirable for developing pharmaceuticals and advanced materials, the researchers said.
"Noncovalent interactions — meaning the weak forces between molecules or molecular components, such as halogen bonding — and their integration into redox mediator design has remained underexplored," said co-corresponding author Dr. Kayo Suda, Graduate School of Arts and Sciences, The University of Tokyo. "Halogen bonding is a directional, tunable interaction involving polarized halogen atoms and has recently emerged as a powerful tool in molecular recognition and catalysis. Yet, until this work, its application in the context of redox catalysis had remained largely unexplored."
The team turned to haloanthracene, a molecule that has at least one hydrogen atom replaced by a halogen atom. The byproducts of haloanthracene, called derivatives, exhibit stronger halogen bonding when oxidized with one electron, according to the researchers.
In experimental work and further analysis, they found that this redox-triggered halogen bonding allowed for dynamic substrate capture and organization that, in turn, promotes more efficient C-N bond formation.
"By designing mediators whose noncovalent interactions are switched on by oxidation, we show that halogen bonding can actively control proton-coupled electron transfer and reaction pathways," said Daisuke Yokogawa, associate professor, Graduate School of Arts and Sciences, The University of Tokyo. "This establishes redox-switchable noncovalent interactions as a new design principle for efficient and selective molecular electrocatalysis."
Next, the researchers said they plan to extend this concept to other bond-forming reactions and catalytic transformations.
"Our ultimate goal is to establish a general platform for molecular electrocatalysts that control reactivity and selectivity through redox-responsive noncovalent interactions, enabling more efficient, predictable and sustainable electrochemical synthesis," said co-corresponding author Mahito Atobe, professor, Faculty of Engineering at YOKOHAMA National University.
Atobe and Shida are also affiliated with YOKOHAMA National University's Institute of Advanced Science. Shida is also affiliated with PRESTO, Japan Science and Technology Agency. Other authors include Atsuki Hirama, Shohei Yoshinaga and Azusa Kikuchi, Department of Chemistry and Life Science, YOKOHAMA National University; Su-Gi Chong, Institute of Advanced Sciences, YOKOHAMA National University; and Moto Kikuchi and Yusuke Ishigaki, Department of Chemistry, Faculty of Science, Hokkaido University.
The Japan Society for the Promotion of Science, Murata Science Foundation and the Japan Science Society's Sasakawa Scientific Research Grant supported this research.
