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Synlett 2022; 33(12): 1137-1141
DOI: 10.1055/a-1709-0280
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Organic Photoredox Catalysis in Synthesis – Honoring Prof. Shunichi Fukuzumi’s 70th Birthday

A Highly Durable, Self-Photosensitized Mononuclear Ruthenium Catalyst for CO2 Reduction

Kenji Kamada
a   Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
,
Hiroko Okuwa
a   Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
,
Taku Wakabayashi
a   Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
,
Keita Sekizawa
b   Toyota Central R&D Laboratories, Inc., Nagakute 480-1192, Japan
,
Shunsuke Sato
b   Toyota Central R&D Laboratories, Inc., Nagakute 480-1192, Japan
,
Takeshi Morikawa
b   Toyota Central R&D Laboratories, Inc., Nagakute 480-1192, Japan
,
Jieun Jung∗
a   Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
,
Susumu Saito  ∗
a   Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
c   Research Center for Materials Science (RCMS), Nagoya University, Chikusa, Nagoya 464-8602, Japan
› Author Affiliations

This work was supported by the Asahi Glass Foundation (Step-up-grant to S.S.), the Japan Society for the Promotion of Science (Scientific Research (B) 19H02713 to S.S. and Early-Career Scientists 21K14642 to J. J.), and partially by the Ministry of the Environment of the Government of Japan.
› Further Information
 
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Abstract

A novel mononuclear ruthenium (Ru) complex bearing a PNNP-type tetradentate ligand is introduced here as a self-photosensitized catalyst for the reduction of carbon dioxide (CO2). When the pre-activation of the Ru complex by reaction with a base was carried out, an induction period of catalyst almost disappeared and the catalyst turnover numbers (TONs) over a reaction time of 144 h reached 307 and 489 for carbon monoxide (CO) and for formic acid (HCO2H), respectively. The complex has a long lifespan as a dual photosensitizer and reduction catalyst, due to the sterically bulky and structurally robust (PNNP)Ru framework. Isotope-labeling experiments under 13CO2 atmosphere indicate that CO and HCO2H were both produced from CO2.


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Key words

CO2 reduction - photocatalyst - ruthenium complex - carbon monoxide - formic acid

The recent depletion of fossil fuels has impelled scientists to search for alternative carbon and energy sources.[1] Carbon dioxide (CO2) is an attractive feedstock for producing valuable chemicals and fuels as it is a nontoxic, low-cost, and renewable resource;[2] thus, there remains considerable interest in the photocatalytic reduction of CO2.[3] [4] Since the one-electron reduction of CO2 to CO2 •– is highly endergonic,[5] few successful results featured the photoredox one-electron activation of CO2 over homogeneous catalysis.[6] A more favorable pathway is multielectron reduction of CO2 to more reduced products, such as carbon monoxide (CO), formic acid (HCO2H), methanol, methane, and C2–C3 compounds. For the two-electron reduction of CO2, transition-metal complexes have been recruited as photocatalysts in the common strategy.[7] [8] In this area, most photocatalytic reduction of CO2 have been realized by two-component systems, in which a CO2 reduction catalyst is used separately with a photosensitizer in the presence of an electron donor.[9] An ideal alternative is the self-photosensitized, single-active-site catalyst, which functions as a dual photosensitizer and reduction catalyst, since it could mitigate the number of reaction partners/sites and intermolecular electron/energy transfer that may cause non-negligible energy loss.

We have demonstrated that (PNNP)iridium (Ir) and (PNNP)ruthenium (Ru) complexes (phosphine–bipyridine (bpy)–phosphine, PNNP), with their central metals coordinatively saturated, serve as efficient molecular pre-catalysts for the hydrogenation of C4-dicarboxylic acids[10] or unactivated amides.[11] These complexes retain a high catalytic activity with unsurpassed durability even under harsh conditions such as high temperature/H2 pressure, owing to their inherent structural robustness. Furthermore, the PNNP ligands have the (bpy)CH2P unit, which could potentially act as proton donor(s) for potent activation of carbonyl compounds as well as CO2.[12]

Very recently, we disclosed that a (PNNP)Ir complex was able to reduce CO2 mainly to HCO2H with 87% selectivity together with CO to achieve a catalyst turnover number (TON) of 2560, upon photoirradiation in the presence of the sacrificial electron donor 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH).[13] The TON is the highest among CO2 reduction photocatalysts without an additional photosensitizer (self-photosensitized mononuclear metal complexes), given remarkable stability of the Ir photocatalyst that effectively prevents exogenous molecules from attacking to outer- and inner spheres of the Ir complex and mitigates its decomposition. Given our continuous interest in developing new photocatalysis for the conversion of CO2 into valuable molecules, we report here a photocatalytic CO2 reduction (Scheme [1a]) using a new (PNNP)Ru complex Ru-1 (Scheme [1b]), which showed a substantial increase of catalytic activity after its pre-activation. Although some self-photosensitized catalysts based on rhenium,[14] iridium,[15] osmium,[16] and other metal complexes[17] for CO2 reduction have been reported to date, Ru complexes are nearly out of scope.[18]

Zoom Image
Scheme 1 (a) CO2 reduction with a Ru complex under photoirradiation (λ ≥ 400 nm). (b) X-ray crystallographic and schematic structures of Ru-1 and pre-activation of Ru-1 to Ru-1·2KOt-Bu; Mes = 2,4,6-Me3(C6H2). Hydrogen atoms and solvents were omitted. Displacement ellipsoids are shown at the 50% probability.

Besides, many molecular photocatalysts for CO2 reduction are deactivated within a few hours under visible-light illumination and the quantum efficiency of CO2 photoreduction has been unsatisfactory. Our results represent a rare example of a mononuclear Ru-based self-photosensitized catalyst that has a long lifespan, owing to the sterically bulky and structurally robust (PNNP)Ru framework. The Mes-PNNP ligand of Ru-1 was synthesized based on the previous procedure.[13] A toluene mixture of Mes-PNNP and [RuCl2(PPh3)3] was simply heated, and the resulting stable intermediate complex was purified and isolated by the column chromatography on silica gel. The reaction of the intermediate Ru complex with NaBPh4 in CHCl3/CH3CN (1:1, v/v) by heating gave the desired Ru-1. Detailed synthetic procedures for Ru-1 are described in the Supporting Information. The complex was unambiguously characterized by elemental analysis, NMR, high-resolution mass spectrometry (HRMS), and single-crystal X-ray diffraction analysis (Figure S1, Table S1).

We first examined the photocatalytic activity of Ru-1 (0.1 mM) under photoirradiation (λ ≥ 400 nm) in a CO2 (1 atm) saturated, mixed solution of dimethylacetamide (DMA) and triethanolamine (TEOA) (5:1, v/v) in the presence of BIH (0.1 M) as an electron donor (Scheme [1a]).[19] To our delight, photoreduction of CO2 proceeded through a long induction period of catalyst (Figure [1a]). The amounts of products continuously increased as the irradiation time was prolonged; after the longest irradiation time of 58 h, CO, HCO2H, and H2 were produced with TONs of 30, 21, and 0.2, respectively (Table [1], entry 1).

Table 1 Optimization and Control Experiments for Photocatalytic CO2 Reduction by the Ru Complex in a Mixed DMA/TEOA (5:1, v/v) Solution

Entry

[Ru] (mM)

BIH (M)

λ (nm)

Gas

TONs in 58 h

CO

HCO2H

H2

 1

0.1a

0.1

≥400

CO2

30

21

0.2

 2

0.1b

0.1

≥400

CO2

67

62

0.7

 3

0.1c

0.1

≥400

CO2

146

185

3.7

 4

0.1d

0.1

≥400

CO2

93

164

27

 5

0.1e

0.1

≥400

CO2

82

145

24

 6

0.1c

0.1

≥400

Ar

0

0

2.3

 7

0.1c

0.1

dark

CO2

0

0.5

0.5

 8

0.1c

–

≥400

CO2

0

1.0

0

 9

–

0.1

≥400

CO2

0

1.3

0

10

0.1c

0.1

≥400

CO2

307f

489f

28f

a Without pre-activation.

b Pre-activated by KOt-Bu (1 equiv).

c Pre-activated by KOt-Bu (2 equiv).

d Pre-activated by KOt-Bu (4 equiv).

e Pre-activated by KOt-Bu (8 equiv).

f TON in 144 h.

Zoom Image
Figure 1 CO (red), HCO2H (blue), and H2 (black) formation upon visible-light irradiation (λ ≥ 400 nm) of a CO2-saturated mixture of DMA/TEOA (5:1, v/v) containing a catalytic amount of (a) Ru-1 or (b), (c) Ru-1·2KOt-Bu (0.1 mM) and BIH (0.1 M) at 298 K.

With an expectation that a change of axial ligand(s) of Ru complex could shorten the induction period, the chloride (Cl–) was removed from Ru-1 by heating at 140 °C in the presence of different amounts of KOt-Bu. The pre-activation of Ru-1 indeed diminished the induction period, accompanied by the enhancement of photocatalytic activity (Figure [1a] vs. 1b). Concomitant process of the hydrogen abstraction and the Cl– elimination from Ru-1 was observed by electron-spray ionization (ESI) HRMS measurements, by which the identical species (m/z found: 954.4567 ± 0.0019) was consistently detected as the major product (Figure S2), irrespective of the amounts of KOt-Bu used (1–8 equiv relative to Ru-1).[20] The removal of Cl– and H+ was similarly observed when inducing hydrogenation catalysts, which were formed by deprotonation at the (2-pyridyl)CH2P unit ((py)CH2P) of (PNP)RuCl pincer complexes or (PNNP)RuCl2 complexes.[11] [21] The UV/Vis absorption spectra of pre-activated complexes cover a broader range of absorption wavelengths (350–800 nm), presumably due to a quinoid structure (Figures S3 and S12).[22,23] When 1 equiv of KOt-Bu was used for the pre-activation, the TONs reached 67, 62, and 0.7 for CO, HCO2H, and H2, respectively (Figure S4a, Table [1], entry 2). Increasing the amount of KOt-Bu to 2 equiv substantially improved the TONs to 146, 185, and 3.7 for CO, HCO2H, and H2, respectively (Table [1], entry 3). The photocatalyst derived from Ru-1·2KOt-Bu showed high durability, as the apparent reduction rate was almost kept constant even after 144 h irradiation with TONs of 307, 489, and 28 for CO, HCO2H, and H2, respectively (Table [1], entry 10; Figure [1c]). UV/Vis absorption measurement of the reaction mixture also supports the remarkable durability of the photocatalyst, where the absorption spectra remained almost unchanged over 6 h through 144 h photoirradiation (Figure S5). In contrast, use of a 4 or 8 equiv of KOt-Bu decreased the lifetime of the Ru complex for photoreduction of CO2 (Figures S4b and S4c), lowering overall reduction efficiency (Table [1], entries 4 and 5). Control experiments (Table [1], entries 6–9) confirmed that no CO2-reduction products were formed in the absence of either one of CO2, light, electron donor, or Ru complex. The quantum yields (QYs) at λex = 450 nm, estimated on the basis of the number of photons absorbed by Ru-1·2KOt-Bu, were determined to be 0.7%, 1.2%, and 0.4% for CO, HCO2H, and H2 production, respectively, by referring to a ferrioxalate actinometer (see the experimental section of the Supporting Information and Figure S6).[24]

Cyclic voltammograms (CVs) of Ru-1 and Ru-1·2KOt-Bu were measured to elucidate their electrochemical behaviors. When Ru-1 (1 mM) was dissolved in a DMA solution containing tetrabutylammonium hexafluorophosphate (NBu4PF6, 0.1 M) under argon (Ar) atmosphere, the CV of the Ru complex exhibited an irreversible wave (Figure [2a]). Replacement of the Ar with CO2 bubbling for 10 min enhanced the catalytic current. In the CVs of Ru-1·2KOt-Bu, the enhancement of the catalytic current was also observed under CO2 atmosphere (Figure [2b]).

Zoom Image
Figure 2 CVs of (a) Ru-1 and (b) Ru-1·2KOt-Bu in the presence of Ar (black) and CO2 (blue). Spectra taken in a DMA solution containing NBu4PF6 (0.1 M) using Ag/AgNO3 reference, Pt counter, and glassy carbon working electrodes. Scan rate: 100 mV/s.

Isotope-labeling experiments using 13C-labeled carbon dioxide (13CO2) were conducted to verify the carbon source of CO and HCO2H. Gas chromatography–mass spectroscopy (GC–MS) analysis of the gas phase after the photoreduction identified 13CO (m/z = 29) as the reduction product (Figure [3]), whereas 12CO (m/z = 28) was mainly observed in an experiment using a 12CO2-saturated solution (Figure S7). 1H NMR measurement of a photoirradiated DMF-d 7/TEOA (5:1, v/v) solution containing Ru-1·2KOt-Bu (1 mM) and BIH (0.1 M) under 13CO2 atmosphere showed a doublet (J = 182 Hz) attributable to the hydrogen atom bound to the 13C atom of H13COOH (Figure [4a]), whereas a singlet was observed when an unlabeled CO2-saturated solution was used (Figure [4b]).

Zoom Image
Figure 3 Mass spectrum of CO generated under 13CO2 using a mixed DMA/TEOA (5:1, v/v) solution containing Ru-1·2KOt-Bu (0.5 mM) and BIH (0.1 M) at 298 K upon irradiation with a Xe lamp (λ ≥ 400 nm) for 24 h.
Zoom Image
Figure 4 1H NMR spectra of a mixture of DMF-d 7/TEOA (5:1, v/v) containing Ru-1·2KOt-Bu (1 mM) and BIH (0.1 M) after 24 h photoirradiation (λ ≥ 400 nm) under (a) 13CO2 and (b) unlabeled CO2.

Almost the same J value with the 13C–H coupling (J = 181 Hz) was observed in the 13C NMR spectrum that was measured without 1H decoupling (Figure S8). Similarly, the 13C incorporation was confirmed when Ru-1 was used instead of Ru-1·2KOt-Bu (Figures S9 and S10). The results clearly substantiate that CO and HCO2H were originated from CO2 rather than from carbon contaminants in the reaction mixture.

In summary, a self-photosensitized, PNNP-ligated Ru photocatalyst has been developed to effectively photoreduce CO2. The Ru complex has a fairly long lifespan as a photoreduction catalyst, owing to the sterically bulky and structurally robust (PNNP)M structure. The pre-activation of Ru-1 upon treatment with a base gave an activated Ru complex, which harvests light more effectively over a broad range of wavelengths (350–800 nm) and represents a significant improvement of catalytic activities. The present results will open a new avenue not only for simple and straightforward photocatalytic reduction of CO2 but also for the development of photoredox catalysis for organic synthesis using self-photosensitized Ru complex catalysts.


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Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

We acknowledge K. Oyama, and Y. Maeda at the chemical instrument room (RCMS, NU), as well as H. Okamoto and H. Natsume at the glass workshop (NU) for their technical support.

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/a-1709-0280.
  • Supporting Information

Corresponding Authors

Jieun Jung
Department of Chemistry, Graduate School of Science, Nagoya University
Chikusa, Nagoya 464-8602   
Japan   

Susumu Saito
Department of Chemistry, Graduate School of Science, Nagoya University
Chikusa, Nagoya 464-8602
Japan   

Publication History

Received: 30 August 2021

Accepted after revision: 29 November 2021

Accepted Manuscript online:
29 November 2021

Article published online:
05 January 2022

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   Permissions and Reprints All articles of this category
Zoom Image
Scheme 1 (a) CO2 reduction with a Ru complex under photoirradiation (λ ≥ 400 nm). (b) X-ray crystallographic and schematic structures of Ru-1 and pre-activation of Ru-1 to Ru-1·2KOt-Bu; Mes = 2,4,6-Me3(C6H2). Hydrogen atoms and solvents were omitted. Displacement ellipsoids are shown at the 50% probability.
Zoom Image
Figure 1 CO (red), HCO2H (blue), and H2 (black) formation upon visible-light irradiation (λ ≥ 400 nm) of a CO2-saturated mixture of DMA/TEOA (5:1, v/v) containing a catalytic amount of (a) Ru-1 or (b), (c) Ru-1·2KOt-Bu (0.1 mM) and BIH (0.1 M) at 298 K.
Zoom Image
Figure 2 CVs of (a) Ru-1 and (b) Ru-1·2KOt-Bu in the presence of Ar (black) and CO2 (blue). Spectra taken in a DMA solution containing NBu4PF6 (0.1 M) using Ag/AgNO3 reference, Pt counter, and glassy carbon working electrodes. Scan rate: 100 mV/s.
Zoom Image
Figure 3 Mass spectrum of CO generated under 13CO2 using a mixed DMA/TEOA (5:1, v/v) solution containing Ru-1·2KOt-Bu (0.5 mM) and BIH (0.1 M) at 298 K upon irradiation with a Xe lamp (λ ≥ 400 nm) for 24 h.
Zoom Image
Figure 4 1H NMR spectra of a mixture of DMF-d 7/TEOA (5:1, v/v) containing Ru-1·2KOt-Bu (1 mM) and BIH (0.1 M) after 24 h photoirradiation (λ ≥ 400 nm) under (a) 13CO2 and (b) unlabeled CO2.