Photocatalytic CO₂ Reduction Using an Iron–Bipyridyl Complex Supported by Two Phosphines for Improving Catalyst Durability

Abstract

We herein report that an iron (Fe) complex bearing a tetradentate PNNP ligand catalyzes photochemical carbon dioxide (CO2) reduction to produce mainly carbon monoxide (CO) together with formic acid (HCO2H) combined with a photosensitizer. The structurally bulky phosphine moieties stabilized the catalyst and improved its durability over a few days (∼72 h) to give the turnover number (TON) of 397. Operando subnanosecond laser-induced transient absorption measurements allowed us to observe the direct electron transfer from a reduced photosensitizer to the Fe catalyst.

Arylamines as More Strongly Reducing Organic Photoredox Catalysts than fac-[Ir(ppy)3]

“…OPC 3 , which was critical to solving this issue, has an ESOP of −2.1 V vs SCE, which is higher than that of fac -[Ir­(ppy) 3 ] ( E * ox = −1.7 V vs SCE). In contrast, the excited-state lifetime of 3 was <3 ns, which was much shorter than that of fac -[Ir­(ppy) 3 ] (τ = 1.3–1.9 μs). Since it is important to determine whether the ESOP or excited-state lifetime is more essential to organic reactions, Hawker and Alaniz et al compared 3 and fac -[Ir­(ppy) 3 ] and demonstrated that 3 is more effective in the hydrodehalogenation of aryl iodides and bromides.…”

“…68−70 Complexes of earth-abundant Fe(II) were also used as effective CO 2 reduction sites, which were difficult to be realized without photosensitization using fac-[Ir(ppy) 3 ]. 71 Our impression is that the replacement of PMPCs including fac-[Ir(ppy) 3 ] with OPCs would satisfy both the requirements in which the usage of ubiquitous/common elements and the development of sufficiently strong reducing agents should be more significantly vitalized. Moreover, while the photocatalytic transformation of "inorganic" CO 2 has reached a rather advanced stage, 72−77 various longstanding challenges remain in the conversion of highly oxidized/oxygenated "organic" compounds, which are highly abundant in renewable biomass feedstocks, 78−81 into more reduced forms for use as energy-rich platform chemicals using photoexcited electrons with high reducing ability.…”

“…In contrast, the development of strongly reducing OPCs and their application is less advanced than that of strongly oxidizing ones. ,,− Highly reducing photocatalytic systems have great potential to pave new avenues for organic syntheses based on dehalogenation, − carbanion formation from organic radicals, − and the reduction of highly oxidized stable compounds (e.g., carboxylic acids/esters, − amides, − and phosphine oxides), among other mechanisms. Meanwhile, our research group has recently developed several precious-metal complexes for the reductive transformation of various carboxylic acid derivatives − and CO 2 under thermal conditions or visible light irradiation. − Complexes of earth-abundant Fe­(II) were also used as effective CO 2 reduction sites, which were difficult to be realized without photosensitization using fac -[Ir­(ppy) 3 ] . Our impression is that the replacement of PMPCs including fac -[Ir­(ppy) 3 ] with OPCs would satisfy both the requirements in which the usage of ubiquitous/common elements and the development of sufficiently strong reducing agents should be more significantly vitalized.…”

“…Additionally, we introduce recent progress in efforts to observe the photoexcited states of strongly reducing OPCs. In contrast to PMPCs, which are easily transformed into the triplet excited-states (T 1 ) with long lifetimes ( fac -[Ir­(ppy) 3 ]: τ = 1.3–1.9 μs) upon photoirradiation due to the heavy-atom effects, the photophysical properties of photoexcited OPCs, such as their spin multiplicity and excited-state lifetime, have sometimes been controversial. , …”

Mechanism of photocatalytic CO2 reduction to HCO2H by a robust multifunctional iridium complex

“…However, these homogeneous photochemical CO 2 reduction systems generally focus on CO formation, whereas the selective production of HCO 2 H, a widely used energy-enriched compound in both laboratory and industry, is rarely reported. [35][36][37][38][39][40] A major challenge for the CO 2 reduction to generate HCO 2 H results from its disfavored thermodynamics compared to the production of CO since the redox potential for CO 2 /CO (−0.53 V in water, pH = 7) is less negative than that of CO 2 /HCO 2 H (−0.61 V in water, pH = 7). 41,42 Plus, the undesired hydrogen evolution reaction (H + / H 2 , with a redox potential of −0.41 V in water, pH = 7) seems unavoidable during the CO 2 reduction in aqueous medium, as it is thermodynamically more favorable than CO 2 reduction.…”