A novel technology facilitates hydride transfer to carbon dioxide, converting it into formic acid at a high turnover number

Excessive use of fossil fuels leads to undesired carbon dioxide (CO₂)generation, accelerating climate change. One way to tackle this is by converting CO₂ into value-added chemicals. On this front, researchers have recently utilized a novel redox couple, [Ru(bpy)₂(pbn)]²⁺/[Ru(bpy)₂(pbnHH)]²⁺, for the purpose. It catalyzes hydride transfer to CO₂, reducing it to formic acid in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole. This novel method can help us develop novel renewable materials.

Climate change is a global environmental concern. A major contribution to climate change comes from excessive burning of fossil fuels. They produce carbon dioxide (CO₂), a greenhouse gas responsible for global warming. In this light, governments globally are framing policies to curb such carbon emissions. However, merely curbing carbon emissions may not be enough. Managing the generated carbon dioxide is also necessary.

On this front, scientists have suggested chemically converting CO₂ into value-added compounds, such as methanol and formic acid (HCOOH). Producing the latter requires a source of hydride ion (H^-), which is equivalent to one proton and two electrons. For instance, the nicotinamide adenine dinucleotide (NAD⁺/NADH) reduction-oxidation couple is a hydride (H^-) generator and reservoir in biological systems.

Against this backdrop, a group of researchers led by Professor Hitoshi Tamiaki from Ritsumeikan University, Japan, have now developed a novel chemical method that reduces CO₂ to HCOOH using NAD⁺/NADH-like ruthenium complexes. Their work was published in the journal ChemSusChem on 13 January 2023.

Prof. Tamiaki explains the motivation behind their research. “Recently, a ruthenium complex with an NAD⁺ model – [Ru(bpy)₂(pbn)](PF₆)₂ – was shown to undergo photochemical two-electron reduction. It produced the corresponding NADH-type complex [Ru(bpy)₂(pbnHH)](PF₆)₂ under visible light irradiation in the presence of triethanolamine in acetonitrile (CH₃CN),” he elaborates. “Further, the bubbling of CO₂ into the [Ru(bpy)₂(pbnHH)]²⁺ solution regenerated [Ru(bpy)₂(pbn)]²⁺ and produced formate ion (HCOO^-). However, its yield was quite low. Therefore, transferring H^- to CO₂ required an improved catalytic system.”

Consequently, the researchers explored various reagents and reaction conditions to facilitate CO₂ reduction. Based on those experiments, they proposed a photoinduced two-electron reduction of the [Ru(bpy)₂(pbn)]²⁺/[Ru(bpy)₂(pbnHH)]²⁺ redox couple in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH). Moreover, water (H₂O), instead of triethanolamine, in CH₃CN further improved the yield.

In addition, the researchers explored the underlying reaction mechanism using techniques like nuclear magnetic resonance, cyclic voltammetry, and UV-Vis spectrophotometry. Based on this, they proposed the following: First, the photo-excitation of [Ru(bpy)₂(pbn)]²⁺ produces [Ru^III(bpy)₂(pbn•⁻)]²⁺* radical, which undergoes reduction by BIH to give [Ru^II(bpy)₂(pbn•⁻)]²⁺ and BIH•⁺. Following this, H₂O protonates the ruthenium complex, generating [Ru(bpy)₂(pbnH•)]²⁺ and BI•. The obtained product undergoes disproportionation to generate [Ru(bpy)₂(pbnHH)]²⁺ and gives back [Ru(bpy)₂(pbn)]²⁺. Then, the former is reduced by BI• to produce [Ru(bpy)(bpy•⁻)(pbnHH)]⁺. This complex is an active catalyst and transfers H^- to CO₂, producing HCOO^- and formic acid.

The researchers showed that the proposed reaction demonstrated a high turnover number – moles of CO₂ converted by a mole of catalyst – of 63.

Excited by these findings, the researchers hope to develop a new methodology of energy conversion (sunlight to chemical energy) for the production of novel renewable materials.

“Our method would also decrease the total amount of CO₂ gas on Earth and help maintain the carbon cycle. Thus, it could reduce global warming in the future,” adds Prof. Tamiaki. “Further, the novel organic hydride transfer technology will provide us with invaluable chemical compounds.”

We sure hope his vision is realized soon!

Title of original paper: Photoinduced Catalytic Organic-Hydride Transfer to CO₂ Mediated with Ruthenium Complexes as NAD⁺/NADH Redox Couple Models
Journal: ChemSusChem
DOI: 0.1002/cssc.202300032

About Professor Hitoshi Tamiaki from Ritsumeikan University, Japan

Hitoshi Tamiaki is a Full Professor at Ritsumeikan University, Japan. He received a PhD in Science from Kyoto University in 1986 and joined Ritsumeikan University in 1993. He now leads the Bioorganic Chemistry Lab, where he does pioneering research on natural and artificial photosynthesis as well as chlorophyll science. He has published well over 500 research papers, which together have garnered more than 11,000 citations. He was also named PRESTO researcher of JST (1998–2001), and received the JPA Award in 2006 and the 31st CSJ Award for Creative Work in 2014.

About Specially Appointed Assistant Professor Yusuke Kinoshita from Hokkaido University, Japan

Yusuke Kinoshita is a Specially Appointed Assistant Professor at the Institute for Chemical Reaction Design and Discovery, Hokkaido University. He received a PhD from the Graduate School of Life Sciences, Ritsumeikan University, and was also a postdoctoral fellow and an Assistant Professor at the institution from 2014 to 2022. In March 2022, he was also the recipient of the “Young Award” at the College of Life Sciences at Ritsumeikan University. While his principal research areas range from nanotechnology to life science and biochemistry, his research interests also span organic chemistry and photochemistry. He has published several peer-reviewed papers in these fields in many international journals of repute.

About Specially Appointed Professor Koji Tanaka from Kyoto University, Japan

Koji Tanaka is a Specially Appointed Professor at the Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University. He is a member of the Kitagawa Lab, which carries out important research in the fields of inorganic chemistry and the chemistry of coordination space. A prolific researcher in his own right, with several years of experience, his areas of expertise include nanotechnology, coordination compounds, and materials and inorganic chemistry, and he has published numerous papers in multiple reputed journals in these fields. He was also the recipient of the “Chemical Society of Japan Academic Award” in 1998 for his work.

Funding information

This study was partially supported by Japan Society for the Promotion of Science (JSPS) for Scientific Research on Innovative Areas “Innovation for Light-Energy Conversion (ILEC)”.