Metal-free reduction of CO2 to formate using a photochemical organohydride-catalyst recycling strategy

Increasing levels of CO 2 in the atmosphere is a problem that must be urgently resolved if the rise in current global temperatures is to be slowed. Chemically reducing CO 2 into compounds that are useful as energy sources and carbon-based materials could be helpful in this regard. However, for the CO 2 reduction reaction (CO 2 RR) to be operational on a global scale, the catalyst system must: use only renewable energy, be built from abundantly available elements and not require high-energy reactants. Although light is an attractive renewable energy source, most existing CO 2 RR methods use electricity and many of the catalysts used are based on rare heavy metals. Here we present a transition-metal-free catalyst system that uses an organohydride catalyst based on benzimidazoline for the CO 2 RR that can be regenerated using a carbazole photosensitizer and visible light. The system is capable of producing formate with a turnover number exceeding 8,000 and generates no other reduced products (such as H 2 and CO). Photochemical reduction of CO 2 is a significant challenge and many existing methods use catalysts containing rare metals. Now a metal-free version of this reaction—with high selectivity for formate generation over H 2 or CO—has been achieved that features a combination of carbazole photosensitizer and organohydride catalyst.