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

Abstract

The increasing CO2 concentration in the atmosphere is an urgent social problem that has to be resolved. Reducing CO2 into compounds useful as energy sources and carbon materials is desirable. For the CO2 reduction reaction (CO2RR) to be operational on a global scale, the catalyst system must: (1) use only renewable energy, (2) be built from abundantly available elements, and (3) not require highenergy reactants. Although light is an alluring energy source, most existing methods utilize electricity. Furthermore, catalyst systems are based on rare heavy metals. Herein, we present a transition-metal-free catalyst system for CO2RR using visible light and containing a carbazole photocatalyst and an organohydride co-catalyst based on benzimidazoline. It produced formate with a turnover number exceeding 8000. No other reduced products such as H2 and CO were generated, confirming the high selectivity of the system. This finding is essential for operating artificial photosynthesis on a useful scale.

Sensitizer-Free Photochemical Regeneration of Benzimidazoline Organohydride

“…Based on the experimental results and knowledge obtained from the literature, 24 a reaction mechanism is proposed (Figure 6). BI + -Ph-NMe 2 is excited with visible light to obtain BI + -Ph-NMe 2 *.…”

“…The mass balances (sum of the yields of BI + -Ar and recovered BIH-Ar ) were 67 and 100% for BIH-1Nap-OH and BIH-Ph-NMe 2 , respectively, indicating that BI + -Ph-NMe 2 / BIH-Ph-NMe 2 can constitute a robust organohydride-recycling photocatalytic system. As suggested in a previous study, the observed moderate yield (54%) of BIH-Ph-NMe 2 in the photochemical regeneration of organohydrides is not problematic for future application as a catalyst . Control experiments suggested that light irradiation, a base, and ascorbic acid were indispensable for this reaction (entries 10–12).…”

“…13 Photochemical Reaction. 24 In a flame-dried Schlenk tube under an argon atmosphere, L-ascorbic acid (88.1 mg, 0.50 mmol, 20 equiv), K 2 CO 3 (76.1 mg, 0.55 mmol, 22 equiv), BI + -Ph-NMe 2 (9.8 mg, 0.025 mmol, 1.0 equiv), 8 mL of CH 3 CN, and 2 mL of distilled water were added. The solution was degassed by three freeze−pump−thaw cycles and backfilled with argon.…”

“…Hasegawa et al reported photochemical hydrodesulfonylation using a catalytic amount of BI + -1Nap-O – in the presence of stoichiometric NaBH 4 (Figure c) . Recently, Glusac and our group , independently reported the photochemical regeneration of BIH (Figure d,e). Using photochemical regeneration, metal-free CO 2 reduction to formate was achieved (Figure e), in which the excited state of the carbazole photosensitizer PS2 with high reducing capability – performed the first one-electron transfer in the two-electron reduction process of BI + -Me to BIH-Me .…”

Triarylamine-Substituted Benzimidazoliums as Electron Donor–Acceptor Dyad-Type Photocatalysts for Reductive Organic Transformations

“…Studies of the redox chemistry of benzimidazolines (BIH–R), which are artificial analogues of the reduced form of nicotinamide adenine dinucleotide (NADH), began more than three decades ago, and the applications of these substances in various chemical fields have been probed in detail. ,,,,, In contrast, investigations of the redox chemistry of their oxidized forms, benzimidazoliums (BI + –R), are extremely limited. , In addition, it is not common for cationic photocatalysts to be utilized to promote reduction reactions. − , Therefore, the investigation described in this manuscript introduces a new aspect of BI + –R chemistry. As described above, among the subjects of future investigation includes more exploration of the influence of the para -substituent of the −PhNAr 2 moiety as well as solvent effects on the catalytic mechanism of 1 .…”