Triazolium-Catalyzed Metal-Free Photochemical Reduction of CO2 into Formate

The use of renewable solar energy for the photochemical CO2 reduction reaction (CO2RR) has garnered significant interest owing to its dual benefits: reducing the greenhouse effect and producing high-energy alternatives to fossil fuels. Transition metals play a crucial role in catalyzing the CO2RR; however, their scarcity and high cost pose significant challenges in achieving sustainable chemistry. In this study, we developed a metal-free CO2RR catalytic system using triazolium, a small and structurally simple compound, as the key catalyst. Visible-light irradiation of a solution containing a carbazole photosensitizer and triazolium salt under 1 atm pressure of CO2 afforded formate in high yield at room temperature. No other reduced products of CO2 or by-products such as methanol, formaldehyde, oxalate, CO, or H2 were detected. The turnover number of the catalyst exceeded 2300, indicating high catalytic activity and durability. Mechanistic studies revealed that the triazolium salt serves as a hydrogen atom transfer (HAT) catalyst, with a carbon-centered radical as the catalytically active species. A study on the relationship between the chemical structure and catalytic activity of various triazolium analogs elucidated the essential structural properties required for carbon-centered HAT catalysts. The results of this study provide valuable insights for future research on CO2RR utilizing transition-metal-free organic molecules as catalysts, which could significantly contribute to environmentally friendly and sustainable methods.