Photocatalytic CO Reduction with High Turnover Frequency and Selectivity of Formic Acid Formation Using Ru(II)

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Photocatalytic CO2 reduction with high turnover SPECIAL FEATURE frequency and selectivity of formic acid formation using Ru(II) multinuclear complexes

Yusuke Tamakia, Tatsuki Morimotoa, Kazuhide Koikeb, and Osamu Ishitania,c,1

aDepartment of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama 2-12-1-NE-1, Meguro-ku, Tokyo 152-8551, Japan; bNational Institute of Advanced Industrial Science and Technology, Onogawa 16-1, Tsukuba, 305-8569, Japan; and cAdvanced Low Carbon Technology Research and Development Program (ALCA), Japan Science and Technology Agency, Sanbancho, Chiyoda-ku, Tokyo 102-0075, Japan

Edited by Thomas J. Meyer, University of North Carolina at Chapel Hill, Chapel Hill, NC, and approved July 24, 2012 (received for review February 6, 2012)

Previously undescribed supramolecules constructed with various ratios of two kinds of Ru(II) complexes—a photosensitizer and a catalyst—were synthesized. These complexes can photocatalyze the reduction of CO2 to formic acid with high selectivity and durability using a wide range of wavelengths of visible light and NADH model compounds as electron donors in a mixed solution of dimethylformamide–triethanolamine. Using a higher ratio of the photosensitizer unit to the catalyst unit led to a higher yield of formic acid. In particular, of the reported photocatalysts, a trinuc- lear complex with two photosensitizer units and one catalyst unit Φ photocatalyzed CO2 reduction ( HCOOH ¼ 0.061, TONHCOOH ¼ 671) −1 with the fastest reaction rate (TOFHCOOH ¼ 11.6 min ). On the other hand, photocatalyses of a mixed system containing two CHEMISTRY kinds of model mononuclear Ru(II) complexes, and supramolecules Chart 1. Structures and abbreviations of Ru(II) complexes with a higher ratio of the catalyst unit were much less efficient, and black oligomers and polymers were produced from the Ru complexes during photocatalytic reactions, which reduced the yield catalysts with Ru(II) and Re(I) complexes involves two key of formic acid. The photocatalytic formation of formic acid using principles: First, in the triplet metal-to-ligand-charge-transfer 3 the supramolecules described herein proceeds via two sequential ( MLCT) excited state of the Ru(II) unit, the excited electron processes: the photochemical reduction of the photosensitizer unit should be located at the bridging ligand. Second, a nonconjugated by NADH model compounds and intramolecular electron transfer bridging ligand should be used because a conjugated system in the to the catalyst unit. bridge ligand lowers the reducing power of the catalyst unit (9). The efficiencies and the stability of these supramolecular photo- Φ 0 12–0 15 ∼ 200 ecently, global warming and shortages of fossil fuels and car- catalysts ( CO ¼ . . , TONCO ) are much greater bon resources have become serious issues. The development than those of the other reported supramolecular-type photocata- R – – of technologies to convert CO2 into useful organic compounds lysts for both CO2 reduction (13 16) and H2 evolution (16 19). using sunlight as an energy source would serve as an ideal solution We sought to apply this architecture to construct unique supra- to these problems. molecular photocatalysts for the reduction of CO2 to selectively Formic acid, which is the two-electron reduction product of form formic acid (which has not been reported to date). We suc- CO2, has recently attracted attention as a storage source of H2 cessfully developed such photocatalysts having high efficiency, (1, 2). Formic acid itself is an important chemical. It has been high selectivity for formic acid formation, high stability, and a fast employed as a preservative and an insecticide and is also a useful reaction rate. acid, reducing agent, and source of carbon in synthetic chemical The structures and abbreviations of the synthesized supra- industries. molecular complexes are shown in Chart 1 and include 2þ 4 ′ ′ Only a few photocatalysts for the selective formation of formic ½RuðdmbÞnðBLÞ3−n [dmb ¼ ,4 -dimethyl-2,2 -bipyridine; 1 acid from CO2 have been reported (3–8). Although oligo(p-phe- BL ¼ ,2-bis(4'-methyl-[2,2'-bipyridin]-4-yl)ethane] as the photo- 2þ nylenes) (3) or a mixed system of phenazine and Co cyclam (4) sensitizer unit; and ½RuðdmbÞmðBLÞ2−mðCOÞ2 as the catalyst x y x; y successfully photocatalyzed the reduction of CO2 to formic acid, unit. The abbreviations and in ð Þ indicate the numbers of x y these systems cannot work with visible light. It has been reported each unit in one molecule ( : photosensitizer, : catalyst). Their 2 1 0 that ½RuðbpyÞ ðCOÞ þ (bpy ¼ 2,2′-bipyridine) acted as a cata- model mononuclear complexes are also abbreviated; ( , ) and 2 2 0 1 lyst for reducing CO2 (5–8). Under basic conditions, a mixed sys- ( , ) represent the photosensitizer and catalyst, respectively. 2þ tem of this complex with ½RuðbpyÞ3 as a redox photosensitizer Results and Discussion photocatalyzed the reduction of CO2 to formic acid with high For evaluation of the photocatalysis, we determined both the selectivity (6, 8). However, this photocatalytic system is limited by formation quantum yield (Φ) and the turnover number (TON) instability as evidenced by the fact that the catalyst decomposed following prolonged irradiation and generated black precipitates. We have recently developed a unique architecture for con- Author contributions: O.I. designed research; Y.T. and T.M. performed research; K.K. structing visible-light-driven supramolecular photocatalysts, analyzed data; and Y.T. and O.I. wrote the paper. ∧ 2þ ∧ consisting of a ½RuðN NÞ3 (N N ¼ a diimine ligand)–type The authors declare no conflict of interest. complex as a photosensitizer and a Re(I) diimine complex as This article is a PNAS Direct Submission. a catalyst (9–12). These supramolecules can selectively photo- 1To whom correspondence should be addressed. E-mail: [email protected]. catalyze the reduction of CO2 to CO with high efficiency. The This article contains supporting information online at www.pnas.org/lookup/suppl/ requirements for constructing efficient supramolecular photo- doi:10.1073/pnas.1118336109/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1118336109 PNAS ∣ September 25, 2012 ∣ vol. 109 ∣ no. 39 ∣ 15673–15678 Downloaded by guest on October 1, 2021 Table 1. Photocatalytic properties of the supramolecules and the model system*

products∕μmol † Φ ‡ § Φ ¶ τ ∥ k ** 107 −1 −1 η †† entry photocatalyst reductant HCOOH CO H2 HCOOH TONHCOOH em em ns q M s q % 1(2,1) BNAH 30.4 1.8 1.8 0.041 562 0.085 726 1.97 59 2 MeO–BNAH ‡‡ 36.8 2.4 1.9 0.061 671 (396) §§ 4.72 77 3(1,1) BNAH 26.9 2.8 0.9 0.038 315 0.083 745 1.64 54 4(1,2) BNAH 8.4 7.5 1.0 0.030 353 0.089 755 2.33 64 5(1,3) BNAH 5.3 8.1 0.7 0.017 234 0.082 733 2.82 67 6(1,0)+(0,1) BNAH 5.3 1.9 0.5 − 316 0.087¶¶ 766¶¶ 1.02¶¶ 44¶¶

*A 4-mL CO2-saturated dimethylformamide–triethanolamine (DMF–TEOA) (4∶1 v∕v) solution containing reductant (0.1 M) and complexes was irradiated. “Φ ” “ ” “ ” The concentrations of all the photocatalysts were 0.3 mM for HCOOH and Products and 0.05 mM for TONHCOOH, except for (2,1), of which concentrations were adjusted to half of the other complexes because (2,1) has two photosensitizer units. Therefore, the concentrations of the photosensitizer units were same in all the photocatalytic systems in the series of the experiments. †Irradiated in the Light Irradiation Condition (LIC) 1 for 5 h (light intensity: 4.9 × 10−8 einstein s−1). ‡Irradiated in the LIC1 (light intensity: 4.9 × 10−8 einstein s−1). § Irradiated in the LIC2 for 20 h. TONHCOOHs were calculated as [produced HCOOH]/[added supramolecule]. ¶Emission quantum yield of the complexes in DMF–TEOA (4∶1 v∕v) (Excitation wavelength: 480 nm). ∥Emission lifetime of the complexes in DMF–TEOA (4∶1 v∕v) (Excitation wavelength: 456 nm). **Quenching rate constant of emission from the complexes by the reducing agent. †† 0 1k τ ∕ 1 0 1k τ Quenching fractions of emission from the Ru(II) complexes by 0.1 M of the reducing agent, calculated as . q em ð þ . q emÞ. ‡‡1-(4-methoxybenzyl)-1,4-dihydronicotinamide (25). §§Irradiated in the LIC3 for 20 h. ¶¶Determined by emission from (1,0).

of the product along with the turnover frequency (TOF) in part. The photosensitizer unit can be selectively excited using LIC1 Since suitable irradiation conditions for obtaining these values and LIC2, while, in the case of LIC3, the irradiated light is are different, we chose the following reaction conditions (SI Text): absorbed mainly by the photosensitizer unit and partially by the Light Irradiation Condition (LIC) 1 for determining Φ:For catalyst unit where slow photolysis of the photocatalysts was evaluating the number of photons absorbed by the photocatalyst, observed and TON obtained in LIC3 was slightly lower than that monochromic light is required, and the absorbance of the reac- in LIC2 (Table 1). The reductants (BNAH, MeO-BNAH) and the tion solution at the wavelength of irradiation should be suffi- BNA2 scarcely absorbed >420-nm light (Fig. S1A). ciently high, at least 2 where 99% of the irradiated photons is In a typical run for determining TON, a solution of dimethyl- absorbed by the photocatalyst. We used 480-nm monochromic formamide (DMF) and triethanolamine (TEOA) (4∶1 v∕v) con- light obtained using a 500-W Xe lamp with a band-pass filter taining (1,1) (0.05 mM) and 1-benzyl-1,4-dihydronicotinamide (FWHM ¼ 10 nm); the light intensity was 4.9 × 10−8 or (BNAH, 0.1 M) as a sacrificial electron donor was bubbled with −9 −1 4.9 × 10 einstein s , which was controlled using neutral den- CO2 for 20 min and then irradiated (LIC2) to give formic acid sity (ND) filters. The concentration of the photosensitizer unit with high selectivity. The TON of formic acid formation exceeded (not the photocatalyst) was 0.3 mM, and the solution was vigor- 300 after 20-h irradiation with small amounts of CO and H2 ously mixed during the irradiation. The incident light flux was (Eq. 1, Fig. 1). On the other hand, formic acid was not produced completely absorbed by the photosensitizer unit because the at all in the absence of BNAH or without irradiation. absorbance of the reaction solutions was larger than 3 at 480 nm and the light-pass length was 1 cm. In this reaction condition, it is difficult to determine reason- [1] able TON values because too-large amounts of the products— i.e., formic acid and BNA2s—should be accumulated in the reac- 13 tion solution after long irradiation and the products obstruct the In a labeling experiment using CO2, a strong signal attribu- photocatalytic reaction (see below). The photon flux determined ted to H13COO− was observed at 168.2 ppm in the 13C NMR that the rates of the photocatalytic reactions—i.e., the ordinary spectrum of a DMF-d7-TEOA (4∶1 v∕v) solution containing (1,1) — 13 light source even using a 500-W Xe lamp cannot supply enough (0.5 mM), BNAH (0.1 M), and CO2 (532 Torr) after irradiation 59 Upper photon flux for determining maximum TOF values. For obtaining (LIC3) for 14.5 h (TONHCOOH ¼ )(Fig. S2, )*. Also, in 1 J 188 more exact values of TON and TOF, we reduced the concentra- the H NMR of the same solution, a doublet ( CH ¼ Hz) tions of the photocatalysts in the reaction solutions and used dif- attributable to the proton coupled to the 13CofH13COO− was ferent light sources as follows. observed at 8.52 ppm, but no singlet due to the proton of LIC2 for determining TON: >500-nm light was obtained using H12COO− was detected (Fig S2, Lower)†. These results clearly a 500-W high-pressure mercury lamp equipped with a uranyl glass show that the carbon source of formic acid is CO2. and a K2CrO4 (30% w∕w, d ¼ 1 cm) aqueous solution filter, Fig. 2 shows the IR spectra of (0,1) in DMF (a) and in which supplied stronger light flux than that used in LIC1. The DMF-TEOA (b). Two peaks observed in DMF are attributed concentration of the photosensitizer unit was 0.05 mM. The to the symmetric and asymmetric stretching bands of the two CO TONs were determined after 20-h irradiation in all cases, and ligands (2094 and 2040 cm−1, respectively). On the other hand, ν calculated as the produced amount of formic acid divided by only one peak appeared in the CO region in the DMF-TEOA the amount of supramolecule added. Using a merry-go-round

irradiation apparatus, we could irradiate up to eight samples *It is reasonable that the TONHCOOH (59 for 14.5-h irradiation) in the case of the NMR simultaneously with the same light-intensity. measurement was much smaller than that obtained in the LIC2 (TON ¼ 315 for 20-h LIC3 for determining TOF and measuring NMR: >420-nm irradiation) because the concentration of the photocatalyst in the reaction solution light was obtained using a 500-W high-pressure mercury lamp or was 10 times higher. † 13 13 a 500-W Xenon lamp with a cutoff filter. This visible-light source The peak of H COOH in DMF-d7 was observed at 163.2 ppm in C-NMR spectrum and had shifted to 166.2 ppm after addition of TEOA (1.4 M). Lehn et al. also reported that the 13 − could irradiate the reaction solution with the highest intensity in peak of H COO was observed at 167.4 ppm in a DMF-DMF-d7-TEOA (3∶1∶1 v∕v∕v) our laboratory. mixed solution (8).

15674 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1118336109 Tamaki et al. Downloaded by guest on October 1, 2021 SPECIAL FEATURE

Fig. 3. UV-vis absorption spectra of (1,0) (black line) and (0,1) (red line), Fig. 1. Photocatalytic formation of formic acid (red filled circle), CO (green which was converted to (0,1)′ (see the main text), in DMF-TEOA (4∶1 v∕v) filled square), and H2 (orange filled triangle) as a function of irradiation time. and (0,1) in DMF (blue line). The (red dotted line) shows the amount of formic acid generated by photocatalysis, under the assumption that the side reaction effect by BNA2s and the The UV–visible light (UV-vis) absorption spectra of (1,0), effect of the decrease of BNAH can be neglected: A CO2 saturated DMF-TEOA 0 1 0 1 ′ 4∶1 ∕ ( , ), and ( , ) are shown in Fig. 3. The metal-to-ligand- ( v v, 4 mL) solution containing BNAH (0.1 M) and (1,1) (0.05 mM) was 0 1 ′ irradiated by >500-nm light. charge-transfer (MLCT) band of ( , ) was red-shifted compared with that of (0,1) because of the weak ligand field of the − 4 2 4 −1 COOC2H NðC H OHÞ2 ligand compared with that of the CO (4∶1 v∕v) solution (1958 cm ). This clearly indicates that (0,1) ligand. However, because the MLCTabsorption band of (1,0) was was converted to a complex with only one carbonyl ligand in the observed at a much longer wavelength than that of (0,1)′, it can be þ presence of TEOA. A candidate is ½RuðdmbÞ2ðCOÞðCOOHÞ , concluded that the irradiated lights absorbed only by the photo- − which could be produced by addition of OH from contaminated sensitizer unit(s) in LIC1 (480 nm) and LIC2 (>500 nm). CHEMISTRY water. However, this should not be the main complex in the re- The emission quantum yields of (1,0) and the multinuclear ν þ action solution because the CO of ½RuðdmbÞ2ðCOÞðCOOHÞ in complexes observed in DMF-TEOA are summarized in Table 1. −1 DMF-TEOA (1;955 cm ) was similar to but clearly different They were very similar to the emission quantum yield of (1,0) 0 1 Φ 0 088 from that of ( , ) in the same solvent system. The structural observed in a DMF solution ( em ¼ . ). Therefore, in the transition of (0,1) in DMF-TEOA is attributable to the addition multinuclear complexes, the intramolecular quenching of the of deprotonated TEOA to the CO ligand giving excited state of the photosensitizer unit by the catalyst unit þ 0 1 ′ − ½RuðdmbÞ2ðCOÞfCOOC2H4NðC2H4OHÞ2g ,(, ) , as shown with the COOC2H4NðC2H4OHÞ2 ligand(s) does not occur in in Eq. 2. The formation of (0,1)′ was also confirmed by an DMF-TEOA (Scheme 1)‡. ESI-MS spectrum of the reaction solution in which the parent Under the photocatalytic reaction conditions, BNAH should peak of (0,1)′ (m∕z ¼ 674) was observed as one of the main function as a reductant, but TEOA cannot, as suggested by the signals (Fig. S3). A similar conversion of one of the CO ligands observation that the emission from the 3MLCT excited state of − to COOC2H4NðC2H4OHÞ2 should occur in all multinuclear the photosensitizer unit was mostly quenched by BNAH (Table1), complexes because similar IR spectral changes were observed while quenching by TEOA was negligible (Scheme 1). However, in DMF-TEOA in all cases. TEOA should have other important roles in the photocatalytic reaction. It has been reported that TEOA can act as a base that captures a proton from the one-electron oxidation product of BNAH (i.e., BNAH ·þ) and suppresses the back-electron transfer from the reduced photosensitizer unit to BNAH ·þ (process 1 in Scheme 2) (9). Another potential role for TEOA as a base involves the control of product distribution in CO2 reduction. Tanakaet al. have reported that in the case of the electrochemical 2þ [2] reduction of CO2 using ½RuðbpyÞ2ðCOÞ2 as a catalyst, the product distribution changed depending on the pH of the reaction solution: Formic acid was the main product under basic conditions, but CO and H2 were produced with higher yields under acidic conditions (5–7). Actually, in the case of the photocatalytic reaction using (1,1), the formation rate of formic acid was 4.7 times lower in the absence of TEOA compared with that in the presence of TEOA, and the formation selectivity of formic acid was also lower in the absence of TEOA (Fig. S5A). The reducing agent BNAH can potentially act either as a two-electron or one-electron donor (20–22). The two-electron process gives BNA þ via an electrochemical, chemical, and electrochemical (ECE) sequence (process 2 in Scheme 2), while the one-electron process gives 4,4′- and 4,6′-BNA2 via the coupling

‡The intramolecular quenching of emission from (1,1) was observed in a DMF solution without TEOA as shown in Fig. S4A. This is probably due to electron transfer from the excited state of the photosensitizer unit to the catalyst unit because the reduction 4∶1 ∕ Fig. 2. IR spectra of (0,1) in DMF (blue line) and in DMF-TEOA ( v v) potential of (0,1)(−1.52 Vvs.Ag∕AgNO3) was more positive than that of (0,1)′ (red line). (−1.92 V) (Fig. S4B).

Tamaki et al. PNAS ∣ September 25, 2012 ∣ vol. 109 ∣ no. 39 ∣ 15675 Downloaded by guest on October 1, 2021 Scheme 1. Initial process of the photocatalytic reaction using (1,1).

reaction of BNA · as oxidation products (process 3 in Scheme 2). To clarify the role of BNAH, we used high performance liquid chromatography to quantify both the decrease of BNAH and the formation of its oxidation product(s) in the photocatalytic Scheme 2. Oxidation processes of BNAH. reaction solutions (LIC2: Fig. 4). For example, 20-h irradiation induced the consumption of 141 μmol of BNAH, and the produc- irradiation of a solution containing (1,0) and (0,1) (0.3 mM each) tion of 63 μmol of formic acid, 4 μmol of H2, and 3 μmol of CO. § instead of (1,1) gave a much smaller amount of formic acid with This reaction solution also contained 29 μmol of 4,4′-BNA2 (23), an induction period in the initial stage, but formation of formic and 25 μmol and 14 μmol of the diastereomers of 4,6′-BNA2.On the other hand, BNA þ was not detected in the reaction solution. acid ceased after irradiation for only 1 h. Furthermore, the irra- The formation of all reduction products (HCOOH, H2, and CO) diation of the mixed system with lower light intensity (LIC1: 4 9 × 10−9 −1 requires two electrons per molecule, and the formation of one . einstein s ) produced no formic acid at all, whereas 1 1 Φ 0 045 BNA2 molecule from two BNAH molecules donates two elec- ( , ) still functioned as a photocatalyst with HCOOH ¼ . Φ trons. Thus, we conclude that BNAH acted only as a one-electron (Fig. S6). It should be emphasized that HCOOH was similar even donor. The electron balance in the photocatalytic reduction of when using a one-order difference in light intensity. During the 1 0 0 1 CO2 to formic acid using (1,1) is shown in Eq. 3, below. photocatalytic reaction of the mixed system of ( , ) and ( , ), the solution changed color from red to black: This color change was [3] more significant at lower light intensity after the same quantity of photons was introduced into the reaction solution (Fig. 6A). Although BNA2s are recognized as good electron donors (24), However, no such color change was observed when (1,1) was used they did not act as reducing agents in this photocatalytic reaction as a photocatalyst (Fig. 6B). and were the final oxidation products of BNAH. Back electron The ratio between the photosensitizer unit(s) and the catalyst transfer is expected to occur too fast for the one-electron oxidized unit(s) in the supramolecular system strongly influenced the out- þ products of BNA2s (BNA2 · ) to be irreversibly decomposed come of photocatalysis. A higher ratio of the photosensitizer unit 4 [Eq. ]. Since the oxidation potentials of BNA2s [for example, to the catalyst unit generated a higher yield of formic acid (LIC1: E∘ 4; 4 0 0 26 oxð -BNA2Þ¼ . V vs. SCE] (24) are more negative than Fig. 5). In the cases in which the reaction solutions had the same E∘ 0 57 that of BNAH [ oxðBNAHÞ¼ . V vs. SCE] (25), it is possi- absorbance at 480 nm [this light can be absorbed only by the ble that the accumulation of BNA2s obstructs the photochemical photosensitizer unit(s)], the results of the photocatalytic reac- 1 1 electron transfer from BNAH to the excited state of ( , ) tions are summarized in Table 1. As an example, (2,1) has two k 1 64 × 107 −1 −1 [ qðBNAHÞ¼ . M s ] in view of the much faster photosensitizer units and one catalyst unit and generated formic 1 1 quenching rate constant of the emission from the excited ( , ) acid with the highest selectivity and yield. In contrast, using (1,3) k 4; 4 0 3 09 × 108 −1 −1 by BNA2s: qð -BNA2Þ¼ . M s . For example, with one photosensitizer unit and three catalyst units, CO was the after 8-h irradiation, the reaction solution contained 291 μmol of BNAH and 51 μmol of BNA2s where only 18% of the excited state of (1,1) was quenched by BNAH, whereas 54% could be quenched in the first stage of the photocatalytic reaction. We believe that this is the main reason why the photocatalytic efficiency decreased after the 8-h irradiation. If the effects of the side reaction and the decrease of BNAH were prevented, the formation of HCOOH would increase by 1.85-fold after 8-h irradiation (red dotted line in Fig. 1). This increased rate should be approximately the same as that in the first stage of the photocatalytic reaction.

[4]

The photocatalytic ability of (1,1) was much greater than that of a mixed system of the corresponding mononuclear model complexes (ð1; 0Þþð0; 1Þ; 1∶1) (Fig. 5). The irradiation of the reaction solution containing 0.3 mM of (1,1) by 480-nm mono- 4 9 × 10−8 −1 Fig. 4. Photocatalytic production of the reduction and oxidation products— chromatic light (LIC1: light intensity: . einstein s ) — Φ 0 038 i.e., HCOOH þ CO þ H2 (red filled circle) and BNA2s (blue filled diamond) gave formic acid with HCOOH ¼ . . On the other hand, the and consumption of BNAH (black filled circle): a CO2 saturated DMF-TEOA (4∶1 v∕v, 4 mL) solutions containing BNAH (0.1 M) and (1,1) (0.05 mM) § It has been known that the yield of another isomer of 4,4′-BNA2 is very low. was irradiated at >500-nm light.

15676 ∣ www.pnas.org/cgi/doi/10.1073/pnas.1118336109 Tamaki et al. Downloaded by guest on October 1, 2021 SPECIAL FEATURE

Fig. 6. UV-vis absorption spectral changes of solutions containing the mix- ture of (1,0) and (0,1) (0.3 mM each: A)or(1,1) (0.3 mM: B) during irradiation (0–10 h at 1-h intervals): CO2 saturated DMF-TEOA (4∶1 v∕v, 4 mL) solutions containing BNAH (0.1 M) and the complex(es) were irradiated by 480-nm light of intensity 4.9 × 10−9 einstein s−1.

Fig. 5. Photocatalytic formation of formic acid as a function of irradiation scribed above, the formation of the oligomers and/or polymers time: CO2 saturated DMF-TEOA (4∶1 v∕v, 4 mL) solutions containing BNAH was enhanced with the lower light intensity. Therefore, this de- (0.1 M) and (2,1) (0.15 mM: purple filled circle), (1,1) (0.3 mM: red filled circle), activation process should be accelerated with either higher con- (1,2) (0.3 mM: blue filled circle), (1,3) (0.3 mM: orange filled circle), or a mix- centration of the complex(es) in solution or lower irradiated light ture of (1,0) and (0,1) (0.3 mM each: black filled circle), were irradiated by 2 1 480-nm light of intensity 4.9 × 10−8 einstein s−1. intensity. It should be emphasized that ( , ) showed the highest photocatalytic ability in any photocatalytic reaction condition. major product, and the photocatalytic activity was much lower with this photocatalyst than with (2,1), even though the reaction solution of (2,1) contained only 1/6 the number of catalyst units present in the (1,3) solution. All complexes produced only a small CHEMISTRY amount of H2. The photocatalytic activities of (1,2) and (1,3) decreased rapidly (Fig. 5), and the color of the solution changed [5] from red to black. For CO formation with (1,2) and (1,3), there was an induction period, and the production of CO accelerated In the cases of (1,2) and (1,3), an additional absorption band after the color change (Fig. S5B). However, no such phenomenon was observed with a maximum at 600 nm during the irradiation was observed with (1,1) and (2,1). These results suggest that the (LIC1: Fig. S8B). This can be attributed to the reduced catalyst actual photocatalyst for CO formation with (1,2) and (1,3)is unit, because this absorption alone disappeared rapidly on intro- the black product(s) produced during the photocatalytic reaction. ducing air into the solution. The remaining spectra after the in- Table 1 summarizes the emission quantum yields of (1,0) and troduction of air were very similar to that of the black polymer the supramolecules using 480-nm light for excitation. Although (Fig. 6A). The formation of formic acid involves a two-electron the emission yields from the supramolecules were slightly smaller reduction, which could occur by the twofold reductive quenching than that from (1,0), there was no correlation between the emis- of the excited photosensitizer unit by BNAH. Probably, the larger sion yield and the photocatalytic ability. Table 1 also shows the number of catalyst units in the supramolecule gives rise to electron η fractions of the emission quenched by 0.1 M BNAH ( q). Clearly donation from the one-electron reduced species of the photosen- (1,1) and (2,1) are better photocatalysts, although emission from sitizer unit to the different catalyst units. This should increase the (1,2) and (1,3) was more efficiently quenched by BNAH than that lifetime of the reduced catalyst unit and trigger the oligomerization from (1,1) and (2,1). of the catalyst units. This is a major reason why (1,2)and(1,3)have The photocatalytic activities of (1,2), (1,3), and the mixed low photocatalytic ability. Solutions of (1,1) and (2,1)didnotun- system of (1,0) and (0,1) deteriorated rapidly and the reaction dergo any similar color change during irradiation (Fig. 6B and solution turned black even in the early stages of irradiation, as Fig. S8A). An increase in the number of photosensitizer units in described above. As a typical example, Fig. 6A shows the changes the supramolecule should increase the likelihood of injection of in the UV-vis absorption spectrum of the mixed system during a second electron into the catalyst unit, thereby suppressing the irradiation (LIC1). Tanaka et al. observed the precipitation of process of deactivation of the catalyst unit. We checked the black solids during the photocatalytic reaction of the mixed sys- dependence of the photocatalytic efficiency on the concentration 2þ 2þ of (2,1)(0.02–0.15 mM) in the LIC1 (light intensity: 4.9 × tem of ½RuðbpyÞ3 and ½RuðbpyÞ2ðCOÞ2 ; they suggested that −8 −1 the precipitation causes the deactivation of the photocatalyst (6). 10 einstein s )asshowninFig. S9A. This clearly shows that Deronzier and Ziessel et al. reported that the electrochemical the efficiency is not affected by the concentration of (2,1)atall. 2þ 2 1 reduction of ½RuðbpyÞ2ðCOÞ2 caused the dissociation of the Only 59% of the emission from the photosensitizer unit of ( , ) bpy ligand to give a black polymer with ruthenium-ruthenium was quenched by BNAH even in the first stage of the photocata- bonds (i.e., ½RuðbpyÞðCOÞ2n, Eq. 5) (26). As the absorption lytic reaction (Table 1, entry 1). If this process of reductive quench- spectrum of this polymer was similar to that in Fig. 6A, the de- ing of the excited state of (2,1) can be improved, the efficiency and activation of the photocatalysts ð1; 0Þþð0; 1Þ,(1,2), and (1,3)is stability of the photocatalyst should also be improved. For this pur- pose, the stronger reducing agent 1-(4-methoxybenzyl)-1,4-dihy- presumably due to the formation of similar oligomers and/or ∥ polymers from the reduced catalyst or catalyst unit produced dur- dronicotinamide (MeO-BNAH) , having the redox potential ing irradiation¶. Lower photocatalytic abilities of ð1; 0Þþð0; 1Þ, 0.50 V vs. SCE (25), was used as a sacrificial electron donor instead 0 57 η 59% (1,2), and (1,3) in the LIC1 were observed compared with those of BNAH [redox potential ¼ . V(25)and q ¼ ]. As a η B in the LIC2 (“products” vs. “TON ” in Table 1). As de- result, q increased to 77% (Table 1, entry 2: Fig. S1 ), the photo- HCOOH 2 1 Φ catalytic ability of ( , ) was substantially improved (i.e., HCOOH ¼ ¶ ν A unique IR absorption band attributable to CO of the oligomers was observed at −1 ∥ 2;029 cm in the case of (1,3)(Fig. S7). The UV-vis absorption spectra of MeO-BNAH, BNAH, and 4,4′-BNA2 are shown in Fig. S1A.

Tamaki et al. PNAS ∣ September 25, 2012 ∣ vol. 109 ∣ no. 39 ∣ 15677 Downloaded by guest on October 1, 2021 0 061 671 . in the LIC1 and TONHCOOH ¼ in the LIC2) compared not require participation of two molecules of the photocatalyst. Φ 0 041 with that using BNAH (i.e., HCOOH ¼ . and TONHCOOH ¼ The disproportionation of one-electron reduced supramolecules 562 ). The TOF of formic acid formation (TOFHCOOH) reached is not the main process for the formation of formic acid. 11.6 min−1 in the LIC3 (7.8 min−1 in the case using BNAH)**. Tothe best of our knowledge, this system affords the fastest photo- Materials and Methods catalytic reduction of CO2 reported to date. Details of synthesis of ruthenium(II) complexes, general experimental condi- It is noteworthy that the formation rate of formic acid using tions, and photocatalytic reaction conditions, and analysis of formic acid are (2,1) as a photocatalyst was strongly dependent on the irradiated provided in SI Text. B light flux as shown in Fig. S9 . Even in the reaction condition with Conclusion the highest light flux (LIC3), TOF was still increasing, and, there- We successfully developed unique supramolecular photocatalysts fore, the rate-limiting process of the photocatalytic CO2 reduction constructed with various ratios of two kinds of ruthenium(II) using (2,1) was the excitation of the photosensitizer unit; i.e., higher TOF should be achievable by using a stronger light source. complexes for reduction of CO2 to formic acid with high selec- HCOOH tivity and durability. The ratio between photosensitizer units and Finally, the reaction mechanism of formation of formic acid — using the good photocatalysts; i.e., (1,1) and (2,1), is discussed. catalyst units strongly affected the photocatalytic activities i.e., Formation of formic acid requires two-electron reduction of the higher ratio of the photosensitizer unit caused a higher — 2 1 CO2. It is clear that the initial one-electron transfer to the catalyst yield of formic acid and ( , ) exhibited the highest photocata- Φ 0 061 671 lytic ability ( HCOOH ¼ . , TONHCOOH ¼ , TOFHCOOH ¼ unit proceeds from the one-electron-reduced species (OER) of −1 the photosensitizer unit, which is produced via the reductive 11.6 min ). The following sequence takes place twice for the for- quenching process of the excited photosensitizer unit by BNAH. mation of one formic acid molecule in the photocatalytic reaction: The second-electron reduction of the catalyst unit, which is prob- First, the excitation of the photosensitizer unit; second, the reduc- ably converted to the CO2-adduct form via chemical processes tive quenching of the excited state of the photosensitizer unit by after the first reduction, should also proceed through the same BNAH; and third, the intramolecular electron transfer from the re- sequence—i.e., photochemical formation of the OER of the duced photosensitizer unit to the catalyst unit. The addition of CO2 photosensitizer unit and then its donation of another electron to into the catalytic site probably occurs between the two sequences. the catalyst unit—because the reductant BNAH works only as a one-electron donor. Therefore, the maximum quantum yield of ACKNOWLEDGMENTS. This work was partially supported by Japan Science Φ formic acid formation ( HCOOH) should be 0.5 using the photo- and Technology Agency (Research Seeds Quest Program). Y.T. thanks the catalytic systems (27). Since the quantum yield of formation of Japan Society for the Promotion of Science (JSPS) for a Research Fellowship formic acid was not dependent on both the light intensity and for Young Scientists. the concentration of the photocatalyst, the sequential two-step photochemical electron transfer processes from the OER of the **Since the corresponding one-electron-oxidized and dimerized products—i.e., — photosensitizer unit to the catalyst unit should proceed intramo- ðMeO-BNAÞ2 was detected by ESI-Mass spectrum of the irradiated reaction solution lecularly; namely, the photocatalytic formation of formic acid (m∕z ¼ 488), the photocatalytic reaction using MeO-BNAH probably proceeded via a similar mechanism to that using BNAH. proceeds with participation of only one supramolecule and does

1. Enthaler S (2008) Carbon dioxide—The hydrogen-storage material of the future? 15. Komatsuzaki N, Himeda Y, Hirose T, Sugihara H, Kasuga K (1999) Synthesis and photo- ChemSusChem 1:801–804. chemical properties of ruthenium-cobalt and ruthenium-nickel dinuclear complexes. 2. Joó F (2008) Breakthroughs in hydrogen storage—Formic acid as a sustainable storage Bull. Chem. Soc Jpn 72:725–731. material for hydrogen. ChemSusChem 1:805–808. 16. Rau S, Walther D, Vos JG (2007) Inspired by nature: Light-driven organometallic – 3. Matsuoka S, et al. (1992) Photocatalysis of ollgo(p-phenylenes): Photochemical reduc- catalysis by heterooligonuclear Ru(ii) complexes. Dalton Trans 915 919. tion of carbon dloxlde with trlethylamine. J Phys Chem 96:4437–4442. 17. Ozawa H, Haga M-A, Sakai K (2006) A photo-hydrogen-evolving molecular device driving visible-light-induced EDTA-reduction of water into molecular hydrogen. J 4. Ogata T, et al. (1995) Phenazine-photosensitized reduction of CO2 mediated by a cobalt- Am Chem Soc 128:4926–4927. cyclam complex through electron and hydrogen transfer. J Phys Chem 99:11916–11922. 18. Rau S, et al. (2006) A supramolecular photocatalyst for the production of hydrogen 5. Ishida H, Tanaka K, Tanaka T (1987) Electrochemical CO2 reduction catalyzed by – 2þ þ and the selective hydrogenation of tolane. Angew Chem Int Ed 45:6215 6218. ½RuðbpyÞ2ðCOÞ2 and ½RuðbpyÞ2ðCOÞCI : The effect of pH on the formation of 19. Arachchige SM, et al. (2009) Design considerations for a system for photocatalytic CO and HCOO−. Organometallics 6:181–186. hydrogen production from water employing mixed-metal photochemical molecular 6. Ishida H, Terada T, Tanaka K, Tanaka T (1990) Photochemical CO2 reduction catalyzed devices for photoinitiated electron collection. Inorg Chem 48:1989–2000. 2þ by ½RuðbpyÞ2ðCOÞ2 using triethanolamine and 1-benzyl-1,4-dihydronicotinamide as 20. Martens FM, Verhoeven JM (1981) Photo-induced electron transfer from NADH and – an electron donor. Inorg Chem 29:905 911. other 1,4-dihydronicotinamides to methyl viologen. Recl Trav Chim Pays Bas 100:228–236. 7. Ishida H, et al. (1990) Ligand effects of ruthenium 2,2′-bipyridine and 1 ,10-phenan- 2þ 21. Pac C, et al. (1984) Redox-photosensitized reactions. 11: RuðbpyÞ3 -photosensitized throline complexes on the electrochemical reduction of CO2. J Chem Soc Dalton Trans reactions of 1-benzyl-1,4-dihydronicotinamide with aryl-substituted enones, deriva- 2155–2160. tives of methyl cinnamate, and substituted cinnamonitriles—electron-transfer me- 8. Lehn J-M, Ziessel R (1990) Photochemical reduction of carbon dioxide to formate cat- chanism and structure reactivity relationships. J Org Chem 49:26–34. alyzed by 2,2′-bipyridine- or 1,10-phenanthroline-ruthenium(II) complexes. J Organo- 22. Ishitani O, Yanagida S, Takamuku S, Pac C (1987) Redox-photosensitized reactions. 13. 2þ met Chem 382:157–173. RuðbpyÞ3 -Photosensitized reactions of an NADH model, 1-benzyl-1,4-dihydronicoti- 9. Gholamkhass B, et al. (2005) Architecture of supramolecular metal complexes for namide, with aromatic carbonyl-compounds and comparison with thermal-reactions. – photocatalytic CO2 reduction: ruthenium-rhenium bi- and tetranuclear complexes. J Org Chem 52:2790 2796. Inorg Chem 44:2326–2336. 23. Kano K, Matsuo T (1976) Photoinduced one-electron reduction of 1-benzyl-3-carba- 10. Sato S, Koike K, Inoue H, Ishitani O (2007) Highly efficient supramolecular photoca- moylpyridinium chloride and 3,5-bis(ethoxycarbonyl)-2,6-dimethylpyridine. Bull Chem Soc Jpn 49:3269–3273. talysts for CO2 reduction using visible light. Photochem Photobiol Sci 6:454–461. 24. Patz M, Kuwahara Y, Suenobu T, Fukuzumi S (1997) Oxidation mechanism of NAD 11. Koike K, Naito S, Sato S, Tamaki Y, Ishitani O (2009) Architecture of supramolecular dimer model compounds. Chem Lett 6:567–568. metal complexes for photocatalytic CO2 reductionIII: Effects of length of alkyl chain 25. Fukuzumi S, Koumitsu S, Hironaka K, Tanaka T (1987) Energetic comparison between connecting photosensitizer to catalyst. J Photochem Photobiol A 207:109–114. photoinduced electron-transfer reactions from NADH model compounds to organic 12. Tamaki Y, et al. (2012) Development of highly efficient supramolecular CO2 reduction and inorganic oxidants and hydride-transfer reactions from NADH model compounds photocatalysts with high turnover frequency and durability. Faraday Discuss 155:115–127. to para-benzoquinone derivatives. J Am Chem Soc 109:305–316. 13. Kimura E, Bu X, Shionoya M, Wada S, Maruyama S (1992) A new nickel(II) cyclam 26. Chardon-Noblat S, Collomb-Dunand-Sauthier M-N, Deronzier A, Ziessel R, Zsoldos D (cyclam ¼ 1,4,8,11-tetraazacyclotetradecane) complex covalently attached to 0 (1994) Formation of polymeric [fRu ðbpyÞðCOÞ2gn] films by electrochemical reduction 2þ 1 RuðphenÞ3 (phen ¼ ,10-phenanthroline): A new candidate for the catalytic photo- of ½RuðbpyÞ2ðCOÞ2ðPF6Þ2: Its implication in CO2 electrocatalytic reduction. Inorg Chem reduction of carbon dioxide. Inorg Chem 31:4542–4546. 33:4410–4412. 14. Kimura E, Wada S, Shionoya M, Okazaki Y (1994) New series of multifunctionalized 27. Yui T, Tamaki Y, Sekizawa K, Ishitani O (2011) Photocatalytic reduction of CO2: From nickel(II)-cyclam (cyclam ¼ 1,4,8,11-tetraazacyclotetradecane) complexes: Application molecules to semiconductors. Photocatalysis, Topics in Current Chemistry. Vol 303,ed to the photoreduction of carbon dioxide. Inorg Chem 33:770–778. CA Bignozzi (Springer Berlin/Heidelberg), pp 151–184.

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