Abstract
A cyclometalated iridium(III) complex having 2-(pyren-1-yl)-4-methylquinoline ligands [Ir(pyr)] has a strong absorption band in the visible region (ε444nm = 67,000 M−1 cm−1) but does not act as a photosensitizer for photochemical reduction reactions in the presence of triethylamine as an electron donor. Here, 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (BI(OH)H) was used instead of the amine, demonstrating that BI(OH)H efficiently quenched the excited state of Ir(pyr) and can undergo the photochemical carbon dioxide (CO2) reduction catalyzed by trans(Cl)-Ru(dmb)(CO)2Cl2 (dmb = 4,4′-dimethyl-2,2′-bipyridine, Ru) to produce formate as the main product. We also synthesized a binuclear complex combining Ir(pyr) and Ru via an ethylene bridge and investigated its photochemical CO2 reduction activity in the presence of BI(OH)H.
Highlights
Today, the consumption of fossil resources releases a tremendous amount of carbon dioxide (CO2), which has had a serious impact on global climate change
We reported that the Ru(II)–Ru(II) supramolecular photocatalyst can selectively reduce CO2 to formic acid (HCOOH) by using 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3dihydro-1H-benzo[d]imidazole (BI(OH)H) as an electron donor (ED) with a high turnover number (TONHCOOH) and a high quantum yield ( HCOOH; Tamaki et al, 2015)
Ir(pyr)–Ru was synthesized according to Scheme 1
Summary
The consumption of fossil resources releases a tremendous amount of carbon dioxide (CO2), which has had a serious impact on global climate change. Some metal complex photocatalytic systems that consist of a photosensitizer (PS) and a catalyst (CAT) can selectively induce CO2 reduction and suppress hydrogen (H2) evolution. These systems require a sacrificial electron donor due to the relatively low oxidation power of the PS unit in the excited state (Yamazaki et al, 2015; Tamaki and Ishitani, 2017; Kuramochi et al, 2018a) Step-by-step excitation of both the semiconductor and the metal complex produces an electron with high reducing power and a hole with high oxidizing power, allowing for CO2 reduction by weaker electron donors such as methanol (Figure 1; Sekizawa et al, 2013)
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