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Supramolecular Photocatalysts for Reduction of CO2 Yusuke Tamaki, and Osamu Ishitani ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00440 • Publication Date (Web): 29 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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Supramolecular Photocatalysts for Reduction of CO2 Yusuke Tamaki and Osamu Ishitani* Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-1, O-okayama, Meguro-ku, Tokyo, 152-8550, Japan. e-mail: [email protected]

keywords: supramolecular complex, CO2 reduction, photocatalyst, photosensitizer, sacrificial electron donor

Abstract Photocatalytic reduction of CO2 into energy-rich compounds utilizing solar light as an energy source is expected to provide a solution to serious problems of the shortage of fossil resources and global warming. In this perspective, we summarize advances in supramolecular photocatalysts for the reduction of CO2, of which photosensitizer and catalyst units are connected via a bridging ligand. The first successful Ru(II)-Re(I) supramolecular photocatalysts reported in 2005 indicated molecular architecture for developing efficient supramolecular photocatalysts for CO2 reduction to CO with high selectivity and durability. On the basis of this architecture, both the bridging ligands and Re(I) catalyst unit were optimized to increase the photocatalytic activity. In addition, the compositional units of supramolecular photocatalytic systems were modified: (1) Ir(III) and Os(II) complexes, free- and metallo-porphyrins, and chlorophyll functioned as alternative or better photosensitizer units in comparison to the Ru(II) complexes, (2) Ru(II) carbonyl complexes reduced CO2 giving HCOOH selectively, and (3) dihydrobenzoimidazole derivatives were suitable sacrificial electron donors for evaluating potential of supramolecular photocatalytic systems. These researches have been providing efficient photocatalytic systems for CO2 reduction with high selectivity, durability, and reaction rate under visible-light irradiation.

1. Introduction Worldwide energy consumption has continually increased owing to increases in both population and the amount of energy consumed per person. In 2012 energy consumption (5.8 × 1020 J)1 was higher than that in 2001 (4.3 × 1020 J)2, and in 2050 and 2100 it is projected to be double and triple that in 2001, respectively.3 The energy consumed in 2001 was mainly (86%) obtained from fossil resources, with roughly equal parts derived from oil, coal, natural gas, and, recently, shale gas and oil. However, owing to their limited amounts, it will become impossible for fossil resources by themselves to meet the increasing demand for energy in the future. It should also be pointed out that considerable amounts of fossil resources have been used as chemical starting materials for the manufacture of various products such as polymers. The shortage of “fossil fuels” effectively means a shortage of chemical resources. Another serious problem arising from the mass consumption of fossil resources is the increase in the atmospheric CO2 concentration. When fossil resources are used to generate electricity in thermal generating plants or for transportation by 1

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airplanes, cars, etc., they are burned to form CO2, which is eventually released into the atmosphere. In addition, when fossil resources are utilized as chemical resources, most of the synthetic chemicals and polymers produced are finally burned and converted into CO2 after they have fulfilled their purpose. It has recently been sounded that the increase in the atmospheric CO2 concentration may induce severe global warming. A promising candidate as an energy source, which might be able to supply a huge amount of energy, is solar light. The energy received worldwide from sunlight in 1 hour is 4.3 × 1020 J, which is almost same as the worldwide consumption of energy in 2001 (4.3 × 1020 J).3 In addition, the reduction of CO2 to energy-rich compounds using solar light as an energy source is expected to provide a solution to all of the three serious problems described above, i.e., the shortages of energy and carbon resources and global warming. From this viewpoint, the conversion of CO2 into energy-rich compounds using solar light as an energy source has been one of the main targets for solar-energy conversion to chemical energy, so-called artificial photosynthesis. In this research area, the photocatalytic reduction of CO2 has been extensively investigated using metal complexes that can act as redox photosensitizers (PS) by initiating photochemical electron transfer and as catalysts (Cat) by accepting electrons and reducing CO2.4-6 As the photosensitizers, some metal complexes display strong absorption in the visible region and can be stably converted into one-electron reduced and oxidized species. Other metal complexes can accept two electrons in total to reduce CO2 to stable compounds such as CO and HCOOH, which requires the introduction of two electrons into CO2. These two kinds of the functional metal complexes can be combined with a bridging ligand to give so-called supramolecular photocatalysts (Chart 1).5

Chart 1. Conceptual image of a supramolecular photocatalyst for CO2 reduction. The advantage of the supramolecular photocatalysts is the acceleration of electron transfer between the two components, which improves the performance of the photocatalytic system. This also leads to higher durability of the photosensitizer unit, because its unstable excited and/or reduced states can be consumed more rapidly, and the speed of the photocatalytic reaction might be increased. In particular, these advantages should be heightened on the surface of photofunctional solid materials such as semiconductor photocatalysts and electrodes to which photocatalysts are attached by anchoring groups, because in the case of supramolecular photocatalysts, intramolecular electron transfer should proceed rapidly from the photosensitizer unit to the catalyst unit even on the surface without depending on density of the supramolecules on the surface (Chart 2a), whereas it should strongly depend on distance between a 2


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photosensitizer and a catalyst and become much slower or does not proceed if the photosensitizer and catalyst are attached separately to the surface in low density (Chart 2b). Recently, we reported examples of hybrid systems of supramolecular photocatalysts and photofunctional solid materials: (1) photocatalysts with a light-harvesting function consisting of a supramolecular photocatalysts and periodic mesoporous organosilica;7 (2) hybrid photocatalysts consisting of semiconductor photocatalysts, e.g., TaON and carbon nitride, and supramolecular photocatalysts, which have both strong oxidizing and reducing powers;8-12 and (3) photoelectrochemical systems consisting of a supramolecular photocatalyst on a NiO p-type semiconductor as a photoanode and a CoOx-loaded TaON photocathode, which can photocatalyze the reduction of CO2 with water as a reductant.13-14

Chart 2. Conceptual images of the hybrid systems of (a) a supramolecular photocatalyst and (b) a mixed system of mononuclear complexes on a surface of heterogeneous materials.

There are two types of reaction mechanism in the initial stage of photocatalytic reactions that use supramolecular photocatalysts (PS-Cat) for CO2 reduction, namely, reductive quenching (RQ) and oxidative quenching (OQ) mechanisms. The difference between the two mechanisms is the process of quenching of the excited state of the photosensitizer unit (*PS), which is produced by absorption of a photon (eq. 1). In the OQ mechanism, *PS donates an electron to the catalyst unit giving a charge-separated state with a one-electron oxidized species (OEOS) of the photosensitizer unit (PS+) and a one-electron reduced species (OERS) of the catalyst unit (Cat–) (eq. 2). PS+ extracts an electron from a sacrificial electron donor (D) (eq. 3). In the RQ mechanism, on the other hand, *PS is initially reduced by a sacrificial electron donor giving an OERS of the photosensitizer unit (PS−) (eq. 4), and the unpaired electron is transferred from PS− to Cat (eq. 5). In both mechanisms, the reduction of CO2 proceeds on the reduced form of Cat. (1)

(2) (3)

3


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(4) (5) In the supramolecular photocatalytic systems that proceed via the OQ mechanism, there is a disadvantage, i.e., rapid intramolecular backward electron transfer, in which the electron added to the reduced catalyst unit rapidly returns to the oxidized photosensitizer unit (eq. 6). Therefore, most of the efficient photocatalytic systems that have been reported for the reduction of CO2 in solution proceed via the RQ mechanism. (6) In this perspective, we summarize the reported supramolecular photocatalysts for the reduction of CO2. Their photocatalytic activities are evaluated in terms of the following properties: (1) Selectivity (Γ) for the products, i.e., the ratio of the amount of the target product to the total amount of reduced products. In many cases, H2 is generated as a byproduct during the photocatalytic reaction (eq. 7). Γ = [target product (mol)]/[reduced compounds (mol)]

(7)

(2) Quantum yield (Φ) of the product, which is calculated using eq. 8. It is noteworthy that the formation of CO and HCOOH from CO2 requires two electrons, and the formation of H2 from protons also requires two electrons. Φproduct = [product (mol)]/[absorbed photons (einstein)]

(8)

(3) Turnover number (TON), which indicates the stability of the photocatalyst (eq. 9). TON = [product (mol)]/[photocatalyst (mol)]

(9)

(4) Turnover frequency (TOF), which indicates the speed of the photocatalytic cycle (eq. 10). TOF = TON/[reaction time (min or h)]

2.

(10)

Ru(II)-Ni(II)

and

Ru(II)-Co(III)

systems

4


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Chart 3. Structures and abbreviations of Ru(II)-Ni(II) and Ru(II)-Co(III) complexes.

Table 1. Photocatalytic properties of Ru(II)-Ni(II) and Ru(II)-Co(III) systems.

entry Photocatalyst Donora Product Γb / % TON Ref. 1 RuNi1 AscH CO 72 < 1 15 2 [Ru(phen)3]2+ + [Ni(cyclam)]2+ AscH CO 36 < 1 15 3 RuNi2 AscH CO 11 < 1 16 4 [Ru(bpy)3]2+ + Ni2 AscH CO 9 2 16 5 RuNi3 TEOA CO 50 2 17 6 [Ru(bpy)3]2+ + [Ni(bpy)3]2+ TEOA CO 79 2 17 7 RuCo1 TEOA CO 73 3 17 8 RuCo2 TEOA CO 79 5 17 9 [Ru(bpy)3]2+ + [Co(bpy)3]3+ TEOA CO 35 9 17 a b AscH: ascorbic acid; TEOA: triethanolamine. The byproduct was H2 in these systems.

The structures and abbreviations of Ru(II)-Ni(II) and Ru(II)-Co(III) complexes are shown in Chart 3 and their photocatalytic activities are summarized in Table 1. The first bimetallic system used for the photochemical reduction of CO2 was reported by Kimura et al. in 1992.15 RuNi1, which was composed of a [Ru(phen)3]2+ (phen = 1,10-phenanthroline) derivative as the photosensitizer unit and [Ni(cyclam)]2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) as the catalyst unit (Chart 3), was irradiated in the presence of ascorbic acid (AscH) as a sacrificial electron donor using light with a wavelength of >350 nm to reduce CO2 to CO. The selectivity for CO formation over H2 formation using RuNi1 (entry 1: ΓCO = 72%) was higher than that using the mixed system (entry 2: ΓCO = 36%). However, the TON for the CO formation using RuNi1 was less than 1, which indicated that RuNi1 did not work as a photocatalyst. Although a pyridinium cation was introduced as an electron-mediating unit between the photosensitizer and catalyst units (RuNi2), the photocatalytic activity was not improved (entry 3: TONCO < 1), and the TON was lower than that of a mixed system comprising the corresponding model complexes, i.e., [Ru(bpy)3]2+ (bpy = 2,2’-bipyridine) and [Ni(6-((N-benzylpyridin-4-yl)methyl)-1,4,8,1l-tetraazacyclotetradecane)]3+ (Ni2) (entry 4: TONCO = 2).16 In 1999, Komatsuzaki et al. reported other bimetallic complexes, which consisted of a [Ru(bpy)2(phen)]2+-type complex as the photosensitizer unit and a Ni(II) or Co(III) polypyridyl complex as the catalyst unit.17 Solutions containing the bimetallic complex were irradiated in the presence of triethanolamine (TEOA) as a sacrificial electron donor under a CO2 atmosphere using light at 400–750 nm to give both CO and H2. RuNi3 produced almost equivalent amounts of CO and H2 (entry 5: TONCO = TONH2 = 2), whereas the mixed system ([Ru(bpy)3]2+ + [Ni(bpy)3]2+) mainly produced CO (entry 6: TONCO = 2, TONH2 < 1). The photocatalytic performance of the Ru(II)-Co(III) bimetallic complexes (RuCo1, entry 7: TONCO = 3, TONH2 = 1; RuCo2, entry 8: TONCO = 5, TONH2 = 1) was lower in comparison to that of a mixed system of the corresponding model complexes, namely, [Ru(bpy)3]2+ and [Co(bpy)3]3+ (entry 9: TONCO = 9, TONH2 = 16). There are three important requirements for a supramolecular photocatalyst: (1) it should have at least two functions in one molecule, such as photosensitizer and catalyst units; (2) it should be catalytically 5


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active, i.e., TON > 1; and (3) its photocatalytic activity should be better than that of a mixed system of the corresponding mononuclear model complexes. From these standpoints, no supramolecular photocatalyst for CO2 reduction was reported before 2005.

6


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3. Ru(II)-Re(I) systems

7


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Chart 4. Structures and abbreviations of Ru(II)-Re(I) complexes.

Table 2. Photocatalytic properties of Ru(II)-Re(I) systems. Photocatalyst RuRe1 [Ru(4dmb)3]2+ + fac-Re(4dmb)(CO)3Cl

Donora BNAH

12 13 14 15 16 17 18 19

RuRe2 RuRe3 RuRe4 RuRe5 RuRe6 RuRe7 RuRe8 RuRe9

BNAH BNAH BNAH BNAH BNAH BNAH BNAH BNAH

CO CO CO CO CO CO CO CO

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

RuRe10 RuRe11

BNAH BNAH BNAH BIH BNAH BNAH BNAH BIH BNAH BNAH BIH BNAH BNAH AscNa BI(CO2H)H BNAH BNAH BNAH BNAH BNAH BNAH

CO CO CO CO CO CO CO CO CO CO CO CO CO HCOOH CO CO CO CO CO CO CO

entry 10 11

a

BNAH:

RuRe12 RuRe13 RuRe14 RuRe15 RuRe16 RuRe17 RuRe18 RuRe19 RuRe20 RuRe21 RuRe22 RuRe23 RuRe24 RuRe25 RuRe26 RuRe27

BNAH

Product CO CO HCOOH

Γ / % 96

Φproduct 0.12

TON 170

TOF / -

Ref. 18

changeb

0.062

101

-

18,37

97 96 97c 76d 94 >99 91 73 98 >99 95 >99

0.093 0.16 0.13

50 3 14 28 240 232 97 180

-

18 18 18 18 18 23 23 26,37

0.11 0.11 0.15 0.45 0.10 0.10 0.13 0.54 0.18 0.12 0.50 0.09 0.16 0.002 0.13 -

120 120 207 3029 144 27 233 2915 253 >1000 123 204 25 130 315 50 110 190 283 313

4.7 35.7 4.2 2.3 2.8

26 26 30 33 30 30 This This 31 31 24 27 27 38 39 34 22 35 35 32 32

81 81 69 75

1-benzyl-1,4-dihydronicotinamide;

1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole;

AscNa:

sodium

BIH: ascorbate;

BI(CO2H)H:

b

2-(1,3-dimethyl-2,3-dihydro-1H-benzimidazol-2-yl)benzoic acid. The main product was changed from CO in the initial stage to HCOOH after several-hours irradiation.37 cIrradtiation for 1 h. dIrradiation for 15 h.

The structures and abbreviations of Ru(II)-Re(I) complexes are shown in Chart 4 and their photocatalytic activities are summarized in Table 2. We reported RuRe1 as the first successful example of a supramolecular photocatalyst for CO2 reduction.18 The complex RuRe1 consisted of a [Ru(N^N)3]2+ photosensitizer unit and a fac-Re(N^N)(CO)3Cl catalyst19-20 unit connected via two 4-methyl-bpy moieties 8


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bridged by a -CH2CH(OH)CH2- chain. RuRe1 photocatalyzed the reduction of CO2 using 1-benzyl-1,4-dihydronicotinamide (BNAH) as a sacrificial electron donor under visible-light irradiation (λex > 500 nm). CO was produced with high selectivity, efficiency, and durability (entry 10: ΦCO = 0.12, TONCO = 170), which were much higher than those of a 1:1 mixture of the corresponding mononuclear model

complexes,

namely,

[Ru(4dmb)3]2+

(4dmb

=

4,4’-dimethyl-2,2’-bipyridine)

and

fac-Re(4dmb)(CO)3Cl (entry 11: ΦCO = 0.062, TONCO = 101). The following reaction mechanism was clarified: [Process I] the selective light absorption by the Ru photosensitizer unit gave the triplet metal-to-ligand-charge-transfer (3MLCT) excited state via intersystem crossing from the corresponding singlet (1MLCT) excited state (eq. 11); [Process II] the excited Ru unit was reductively quenched by BNAH giving the OERS of the Ru unit (eq. 12); [Process III] intramolecular electron transfer proceeded from the OERS of the Ru unit to the Re unit (eq. 13); and [Process IV] CO2 was reduced on the Re unit (eq. 14).

(11)

(12)

(13)

(14)

other

hand,

RuRe2

(entry

12)

and

RuRe3

(entry

13),

which

contained

On

the

bpy

or

4,4’-bis(trifluoromethyl)-2,2’-bipyridine ((CF3)2bpy) instead of 4dmb as peripheral ligands on the Ru units, displayed much lower photocatalytic abilities compared with RuRe1 and even the mixed system ([Ru(4dmb)3]2+ + fac-Re(4dmb)(CO)3Cl). This difference was caused by the unfavorable intramolecular electron transfer from the OERS of the Ru unit to the Re unit, because the unpaired electron should be mainly localized on the peripheral diimine ligands with a lower energy level of the LUMO and the intramolecular electron transfer becomes endergonic (in the case of RuRe3, for example, E1/2red(Ru) = – 1.23 V; E1/2red(Re) = –1.76 V vs. Ag/AgNO3). In the case of RuRe1, on the other hand, the Ru and Re units were reduced by one electron at an almost equal potential (E1/2red = –1.77 V). These results clearly indicate that the reduction potential of the photosensitizer unit should be equal to or more negative than that of the catalyst unit to construct an effective supramolecular photocatalyst for CO2 reduction. 9


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The selection of the bridging ligand also strongly affected the performance of the supramolecular photocatalysts. Neither RuRe4 (entry 14) nor RuRe5 (entry 15), which contained a conjugated bridging ligand, could not act as effective photocatalysts. In the case of RuRe4, the photochemically added electron should be mainly localized on the bridging ligand in the Ru site (inefficient Process III), because the energy level of the π* orbital on the phenanthroline-imidazolyl motif of the bridging ligand is lower than that on the bpy motif in the Re site. In the case of RuRe5, on the other hand, the intramolecular electron transfer should proceed rapidly from the reduced Ru unit to the Re unit; however, the photocatalytic ability was still low. This was because the extended conjugation of the bridging ligand lowered the reducing power of the OERS of the Re unit (E1/2red = –1.10 V), which should inhibit the reductive catalytic activity of the Re unit. Table 3 shows the relationship between the photocatalyses of the mononuclear Re complexes, i.e., fac-[Re(N^N)(CO)3(PR3)]+, and their first reduction potentials.21 This indicates that a threshold for photocatalysis exists at E1/2red = –1.4 V. The photocatalytic activity of RuRe23 (entry 36), in which the two diimine moieties in the bridging ligand were connected by a carbon-carbon double bond, was also much lower than that of the corresponding non-conjugated complex (RuRe9, entry 19).22 This difference should occur for the same reason (E1/2red of RuRe23 was – 1.34 V, whereas that of RuRe9 was –1.77 V). Table 3. Relationship between photocatalyses of fac-[Re(N^N)(CO)3(PR3)]+ and their first reduction potentials.a fac-[Re(N^N)(CO)3(PR3)]+ N^N PR3 4dmb P(OEt)3 bpy P(O-i-Pr)3 bpy P(OEt)3 bpy P(OMe)3 bpy PEt3 bpy P(n-Bu)3 (CF3)2bpy P(OEt)3 a

ΦCO

TONCO

E1/2red

0.18 0.20 0.16 0.17 0.024 0.013 0.005

4.1 6.2 5.9 5.5 0.83 0.65 0.10

–1.55 –1.44 –1.43 –1.41 –1.39 –1.39 –1.03

A 4 mL solution in dimethylformamide (DMF) containing the complex (2.6 mM) and TEOA (1.26 M) as

a sacrificial electron donor was irradiated at 365 nm under a CO2 atmosphere. The light intensity was 1.27 × 10-8 einstein·s-1.

From these results and investigations, the following architecture for the construction of efficient supramolecular photocatalysts comprising photosensitizer and catalyst units for CO2 reduction can be proposed: (1) the electron that is photochemically captured by the photosensitizer unit should be mainly localized on the bridging ligand to enable efficient intramolecular electron transfer to the catalyst unit (Process III); and (2) a non-conjugated linker should be used between the two diimine moieties in the bridging ligand, because the introduction of conjugation between the diimine moieties drastically lowers the reducing power of the catalyst unit (Process IV). On the basis of this architecture, we have successfully 10


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developed various efficient and durable supramolecular photocatalysts for CO2 reduction, as is described later. The peripheral ligand of the Re unit also affected the photocatalytic ability. The introduction of triethylphosphite as a peripheral ligand (RuRe7) instead of Cl− improved the photocatalytic activity (entry 17: ΦCO = 0.16, TONCO = 232), whereas the introduction of a pyridine ligand (RuRe8) lowered the durability of the photocatalyst (entry 18: TONCO = 97).23 A mechanistic study clearly showed that, in the first stage of the photocatalytic reaction, RuRe7 was rapidly converted into a complex containing an

–

OC(O)OC2H4N(C2H4OH)2 ligand, which was produced by the reaction of CO2 and deprotonated TEOA at the Re center, instead of the triethylphosphite ligand (eq. 15).23-25 The formation of this CO2 adduct is described in details below. This adduct was an actual photocatalyst in the photocatalytic reaction of CO2 reduction. Although similar rapid substitution of the ligand occurred in the case of RuRe8, the free pyridine in the solution accelerated the decomposition of the photocatalyst.

(15 )

The length of the alkyl chain between the diimine moieties in the bridging ligand affected the abilities of the supramolecular photocatalysts. RuRe9, which contained a -C2H4- chain, exhibited higher photocatalytic activity (entry 19: ΦCO = 0.13, TONCO = 180) than the supramolecular photocatalysts containing a -C4H8- or -C6H12- chain (RuRe10, RuRe11), while the photocatalytic abilities of RuRe10 (entry 20) and RuRe11 (entry 21) were similar to each other.26 A weak but definite electronic interaction between the Ru and Re units was only observed via the -C2H4- chain in the excited state of RuRe9. This should be the main reason for the difference in the photocatalysis, because the 3MLCT excited state of

RuRe9 was more efficiently quenched by the sacrificial electron donor BNAH than those of the others (Process II: the quenching rate constants kq = 1.53 × 107 M-1s-1 for RuRe9, 1.09 × 107 M-1s-1 for RuRe10, and 1.10 × 107 M-1s-1 for RuRe11). An increase in the interaction between the Ru and Re units was achieved by the introduction of two -C2H4- chains (RuRe19), which caused a further improvement in photocatalytic ability (entry 32: ΦCO = 0.16, TONCO = 204, kq = 3.07 × 107 M-1s-1).27 Re dicarbonyl complexes, cis,trans-[Re(N^N)(CO)2(PR’3)2]+ (R’ = p-F-C6H4, Ph, OEt),28-29 were also employed as the catalyst unit instead of the Re tricarbonyl complexes. These Ru(II)-Re(I) binuclear complexes (RuRe12 ~ RuRe14) also photocatalyzed the reduction of CO2 to CO.30 The Ru(II)-Re(I) binuclear complex with P(p-F-C6H4)3 ligands in the Re unit, RuRe12, exhibited the highest photocatalytic activity (entry 22: ΦCO = 0.15, TONCO = 207, TOFCO = 4.7 min−1). The introduction of a -CH2OCH2- chain between the two diimine moieties in the bridging ligand also increased the photocatalytic ability (RuRe16, entry 28: ΦCO = 0.18, TONCO = 253).31 This was caused by an increase in the oxidizing power of the excited photosensitizer unit because of the electron-withdrawing property of the 11


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ether group, which caused Process II to be more efficient (kq = 5.73 × 107 M-1s-1). The reaction mechanism after the intramolecular electron transfer from the OERS of the Ru unit to the Re unit was partially clarified in the photocatalytic reaction with RuRe16, as illustrated in Scheme 1. In the initial stage of the photocatalytic reaction, the Re dicarbonyl unit in some of the Ru-Re complexes was converted into the corresponding

Re

tricarbonyl

species

containing

an

aminoethylcarbonate

ligand

2+

[Re(N^N)(CO)3{OC(O)OC2H4N(C2H4OH)2}] , i.e., RuRe16 was partially converted into RuRe17 in the reaction solution, in which a CO2 molecule was captured by the Re complex with the aid of deprotonated TEOA. In the system of RuRe16, a photostationary state consisting of a 3:1 mixture of RuRe16 and

RuRe17 was observed upon further irradiation. Although RuRe17 itself photocatalyzed the reduction of CO2 to CO with ΦCO = 0.12 (entry 29), photocatalysis using the mixture of RuRe16 and RuRe17 was more efficient (ΦCO = 0.19). In this system, RuRe17 acted as the photocatalyst and RuRe16 assisted the photocatalytic reaction as an external photosensitizer. It is noteworthy that RuRe16 had stronger oxidizing power in the excited state than RuRe17, which induced a larger quenching fraction (ηq) by BNAH of 0.82 (RuRe17: ηq = 0.58).

ηq = kqτ[donor] / (1 + kqτ[donor]) (16) where kq is the rate constant of reductive quenching reaction of the excited photosensitizer unit by the sacrificial electron donor and τ is the emission lifetime of the photosensitizer unit in the absence of the sacrificial electron donor. In the cases of trinuclear complexes, of which two photosensitizer units were connected to the diimine ligand of Re(I) dicarbonyl catalyst units (RuRe26, RuRe27), the supramolecular photocatalyst with -CH2CH2- bridging chains (RuRe27, entry 40: TONCO = 313) also showed better photocatalytic ability than that with -CH=CH- chains (RuRe26, entry 39: TONCO = 283).32

12


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Scheme 1. Reaction mechanisms of CO2 reduction using RuRe16. The efficiency and durability of the Ru-Re supramolecular photocatalysts can be drastically improved by using 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor (RuRe12, entry 23: ΦCO = 0.45, TONCO = 3029, TOFCO = 35.7 min−1) instead of BNAH.33 The reasons for this fascinating effect are described in detail in section 7-2.

(17)

We recently successfully applied high CO2 capturing ability of Re(N^N)(CO)3{OC2H4N(C2H4OH)2} (eq. 17),25 in which deprotonated TEOA coordinates to the Re(I) center, to a supramolecular photocatalyst.24 When [Re(N^N)(CO)3(DMF)]+ was dissolved in a DMF–TEOA (5:1 v/v) mixed solution, Re(N^N)(CO)3{OC2H4N(C2H4OH)2} and [Re(N^N)(CO)3(DMF)]+ were produced as an equilibrium mixture. Bubbling CO2 into the solution rapidly induced the insertion of CO2 into the Re-O bond in the form

of

Re(N^N)(CO)3{OC2H4N(C2H4OH)2}

giving

the

CO2

adduct

Re(N^N)(CO)3{OC(O)OC2H4N(C2H4OH)2}. Because this CO2 insertion into the Re complex (N^N = bpy) is an equilibrium reaction with a very large equilibrium constant (KCO2 = 1.5 × 103 M–1), the Re complex can efficiently capture a CO2 molecule even from gases containing low concentrations of CO2.25 This strategy can be applied to the supramolecular photocatalytic systems. The Ru(II)-Re(I) binuclear complex with a DMF ligand in the Re unit was dissolved in a DMF–TEOA (5:1 v/v) mixed solution giving a mixture of RuRe(DMF) and RuRe(TEOA) in a ratio of 1:7.3. Bubbling CO2 into this solution 13


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quantitatively gave the corresponding CO2 adduct RuRe17, for which the equilibrium constant was KCO2 = 1.7 × 103 M–1. Visible-light irradiation of a solution of RuRe17 in the presence of BIH as a sacrificial electron donor under CO2 selectively produced CO with high efficiency and durability (entry 30: ΦCO = 0.50, TONCO > 1000). Since the Re catalyst unit has two functions, i.e., capturing and reducing CO2,

RuRe17 could photocatalyze CO2 reduction even under Ar gas containing dilute CO2.24 Under a gas mixture with a ratio of CO2:Ar = 10:90 (10% CO2), the rate of CO evolution was almost equivalent to that under 100% CO2 (Figure 1a). Although, under 1% and 0.5% CO2, the rates of CO production became slower, the initial TOFCO was maintained at 74% and 53% of that under pure CO2, respectively. These values were proportional to the concentration of RuRe17 in the reaction mixture (Figure 1b). On the other hand, irradiation of a solution without TEOA under 1% CO2 did not produce any CO. These results clearly indicate that the Re catalyst unit containing the deprotonated TEOA ligand captured CO2 from mixtures with low concentrations of CO2 and also reduced CO2 to CO using an electron provided by the Ru photosensitizer unit.

Figure 1. (a) Photocatalytic reactions under 100%, 10%, 1%, and 0.5% CO2 atmosphere. (b) Linear relationship between the initial rate of CO formation and the ratio of the CO2-capturing complex RuRe17. Supramolecular photocatalysts containing multiple photosensitizer or catalyst units have been reported by our group (RuRe6)18, Rieger’s group (RuRe22)34, and Furue’s group (RuRe24, RuRe25)35. The supramolecular photocatalysts with multiple catalyst units (RuRe6, entry 16: TONCO = 240; RuRe22, entry 35: TONCO = 315; and RuRe25, entry 38: TONCO = 190) exhibited higher photocatalytic durability compared with RuRe1 (entry 10: TONCO = 170). On the other hand, RuRe24, which contained two photosensitizer units and one catalyst unit, showed lower durability (entry 37: TONCO = 110) than RuRe1. In the photocatalytic CO2 reduction using mixed systems of the [Ru(4dmb)3]2+ photosensitizer and the Re(I) tricarbonyl complex catalyst, we found that one of the deactivation processes during the decomposition of [Ru(4dmb)3]2+ photosensitizer is the photochemical decomposition of the OERS of [Ru(4dmb)3]2+ (eq. 18), although the Re(I) catalyst retained its tricarbonyl structure during the photocatalytic reactions.36 It should be noted that the decomposition product [Ru(4dmb)2(solvent)2]2+ worked as a catalyst for CO2 reduction with the [Ru(4dmb)3]2+ photosensitizer, which produced HCOOH 14


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as a main products. Therefore, in the mixed photocatalytic system of [Ru(4dmb)3]2+ and fac-Re(4dmb)(CO)3Br, the product distribution was drastically changed during the irradiation and HCOOH became as a main product after long irradiation.37 Although the change of the product distribution was also observed in the cases of Ru(II)-Re(I) supramolecular photocatalytic systems, the effect was lower owing to the higher stability of the photosensitizer unit as described below.

(18) Taking into account the future aim of utilizing water as an electron donor, photocatalysis in an aqueous solution is highly important. The photocatalyses of RuRe20 (entry 33)38 and RuRe21 (entry 34)39 using sacrificial electron donors that can be used in an aqueous solution are described in detail in section 7-3.

4. Ir(III)-Re(I) and Os(II)-Re(I) systems Chart 5. Structures and abbreviations of Ir(III)-Re(I) and Os(II)-Re(I) complexes.

Table 4. Photocatalytic properties of Ir(III)-Re(I) and Os(II)-Re(I) systems entry Photocatalyst 41 IrRe1 42 43 OsRe1 44 OsRe2 45 OsRe3 a BNAH:

Donora BNAH BIH BIH BIH BIH

Product(s) Γ / % Φproduct TON CO >99 0.21 130 CO >99 0.41 1700 CO >99 0.10 762 CO >99 0.12 1138 CO >99 973 1-benzyl-1,4-dihydronicotinamide;

TOF / min-1 1.6 3.3 -

Ref. 37 37 41 41 This work BIH:

1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole.

The structures and abbreviations of Ir(III)-Re(I) and Os(II)-Re(I) complexes are shown in Chart 5 and their photocatalytic activities are summarized in Table 4. As described above, decomposition of the [Ru(N^N)3]2+-type photosensitizer unit represented one of the deactivation processes of the Ru-Re supramolecular photocatalysts. This reaction gave the [Ru(N^N)2(solvent)2]2+ species (eq. 18), which can act as a catalyst for the reduction of CO2 to give HCOOH as the main product in basic conditions.40 This may cause another effect that is problematic for the evaluation of photocatalysis, in particular the product 15


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distribution.37 Therefore, for the evaluation of the catalysis by various metal complexes of the reduction of CO2 it should be useful to develop alternative photosensitizers with similar photophysical and photochemical properties to [Ru(N^N)3]2+-type complexes, of which the photochemical decomposition products do not catalyze CO2 reduction. For this purpose, Ir(III) complexes containing two cyclometalated ligands (C^N) and one diimine ligand are very suitable because they did not supply catalytic species for CO2 reduction even after long irradiation.37 The Ir(III) complex was also investigated as photosensitizer units for supramolecular photocatalysts. An Ir(III) complex with two 1-phenylisoquinoline (piq) ligands connected to the Re catalyst unit (IrRe1) exhibited favorable properties such as visible-light absorption (λabs < 560 nm) and a long emission lifetime (τem = 3.0 µs), and IrRe1 photocatalyzed the reduction of CO2 to CO with high selectivity (ΓCO >99%) using BNAH as a sacrificial electron donor.37 The efficiency and durability were both high (entry 41: ΦCO = 0.21, TONCO = 130). Formic acid was not detected even after 15-h irradiation at λex < 500 nm, whereas the corresponding Ru-Re supramolecular photocatalysts

RuRe9 produced a significant amount of HCOOH (TONCO = 190, TONHCOOH = 55), which lowered the selectivity for the formation of CO (ΓCO = 76%) in the same reaction conditions. Note that in the initial stage of the photocatalytic reaction using RuRe9, the selectivity for CO was much higher (97% after 1-h irradiation). When BIH was used instead of BNAH, the efficiency and durability were greatly increased (entry 42: ΦCO = 0.41, TONCO = 1700). Although a longer irradiation period caused a gradual decline in the photocatalysis of IrRe1, a ligand-substituted Ir(III) species was not detected but the isoquinoline moieties in the Ir(III) unit were hydrogenated. In many supramolecular photocatalytic systems used for the reduction of CO2, [Ru(N^N)3]2+-type complexes have been used as photosensitizers. However, the visible-light absorption of the Ru(II) units in efficient supramolecular photocatalysts is limited to λabs < 560 nm. Although the extension of the conjugation in the N^N ligand18, 22 and/or the introduction of electron-withdrawing groups into the N^N ligand18 can give rise to a red shift in the absorption of the Ru complex, these structural changes drastically lower the photocatalytic activity owing to slower intramolecular electron transfer from the reduced photosensitizer unit to the catalyst unit, as described above. To use visible light with longer wavelengths, we employed an Os(II) complex as the photosensitizer unit because [Os(N^N)3]2+-type complexes have a relatively strong singlet-to-triplet (S-T) “forbidden” absorption at much longer wavelengths (λabs < 730 nm) than those of the singlet-to-singlet absorption (λabs < 500 nm), and their OERS have a similar reducing power to the corresponding [Ru(N^N)3]2+-type complexes. A [(5dmb)2Os(4dmb)]2+-type (5dmb = 5,5'-dimethyl-2,2'-bipyridine) complex was connected to a cis,trans-[Re(N^N)(CO)2{P(p-X-C6H4)3}2]+ (X = F, Cl) catalyst unit via an ethylene chain. This complex absorbed visible light over a much wider range of wavelengths (λabs < 730 nm) in comparison to the corresponding Ru analogue (RuRe12), and worked as photocatalysts for CO2 reduction to CO in the presence of BIH, even under irradiation at λex > 620 nm.41 RuRe12 did not act as a photocatalyst in the same reaction conditions because it did not absorb light at λabs > 560 nm. The reaction mechanism was similar to that of the Ru(II)-Re(I) photocatalysts, i.e., (1) the selective light-absorption by the Os photosensitizer unit giving its 3MLCT excited state; (2) reductive quenching of the excited Os unit by BIH 16


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giving the OERS of the Os unit; (3) intramolecular electron transfer to the Re catalyst unit; and (4) CO2 reduction on the reduced Re unit. The photocatalytic abilities were affected by the phosphine ligands in the Re unit, and OsRe2 with Cl substituents in the phenyl groups exhibited better photocatalytic activity (entry44: ΦCO = 0.12, TONCO = 1138, TOFCO = 3.3 min− 1) compared with that of OsRe1 with F substituents (entry 43: ΦCO = 0.10, TONCO = 762, TOFCO = 1.6 min−1). OsRe3, in which the catalyst unit was fac-[Re(N^N)(CO)3(PPh3)]+, also showed relatively high photocatalytic ability (entry 45: TONCO = 973). As described above, we now have three options for the photosensitizer unit, namely, Ru(II), Ir(III), and Os(II) complexes, for the construction of efficient supramolecular photocatalysts for the reduction of CO2. [Ru(N^N)3]2+-type complexes combine several characteristics that are appropriate for a redox photosensitizer, i.e., (1) relatively strong visible-light absorption (λabs < 560 nm); (2) a long lifetime of the excited state (τem ~ 1 µs); (3) a reducing power of the OERS that is strong enough to donate an electron to the catalyst unit for CO2 reduction; and (4) the relatively high stability of the OERS. However, this series of complexes also suffers from a drawback, i.e., decomposition to give the [Ru(N^N)2(solvent)2]2+ species40, 42, which makes the evaluation of the products distribution difficult. On the other hand, both Ir(III)37 and Os(II)41 complexes are more stable than the Ru(II) photosensitizers, and the decomposed species do not catalyze CO2 reduction. Another advantage of [Ir(C^N)2(N^N)]+-type photosensitizers compared with the [Ru(N^N)3]2+-type is their anisotropy, which might induce a rectification effect when they are immobilized on a semiconducting surface. The Os(II) photosensitizer has the specific characteristics of S-T absorption, which allows the use of a wider range of visible-light wavelengths (λabs < 730 nm) even compared to the Ru complexes, although its high toxicity (actually that of OsVIIIO4) possibly inhibits its wider application.

5.

17


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Porphyrin-Re(I) systems

Chart 6. Structures and abbreviations of porphyrin-Re(I) complexes.

Table 5. Photocatalytic properties of porphyrin-Re(I) systems. entry 46 47 48 49 50 51 52 53 54 55 56 57 58 59 a TEA:

Photocatalyst PorRe1 PorRe2 Pd(Por) + fac-[Re(bpy)(CO)3(3-picoline)]+ PorRe3 PorRe4 PorRe5 Zn(Por) + fac-[Re(bpy)(CO)3(3-picoline)]+ PorRe6 PorRe7 PorRe8 PorRe9 PorRe10 PorRe11 ChlRe1 triethylamine; TEOA:

Donora Product Φproduct TEA CO 0.0064 TEA CO TEA CO TEOA CO TEOA CO TEOA CO TEOA CO TEA CO TEA CO TEA CO TEA CO TEA CO TEA CO BIH CO triethanolamine;

TON 2 3 14 32 332 103 13 < 1 < 1 < 1 < 1 < 1 18

Ref. 43 44 44 45 45 46 45 47 47 47 47 47 47 48 BIH:

1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole.

The structures and abbreviations of porphyrin-Re(I) complexes are shown in Chart 6 and their photocatalytic activities are summarized in Table 5. The use of supramolecular photocatalysts based on metalloporphyrines43-47 or chlorophylls48, which have strong absorption bands in the visible region (Soret band and Q band), as photosensitizer units with the Re catalyst for CO2 reduction has been reported by 18


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several groups. Inoue et al. used a Zn(II) porphyrin photosensitizer (PorRe1), which photochemically reduced CO2 to CO in the presence of triethylamine (TEA) under irradiation at λex = 428 nm (entry 46:

ΦCO = 0.0064).43 The photochemical reduction of CO2 using PorRe1 uniquely proceeded via the OQ mechanism of the S2 excited state of the photosensitizer unit by the catalyst unit. The short lifetime of the charge-separated state (90 ps) should lower the efficiency of the subsequent process, i.e., the reduction of the OEOS of the photosensitizer unit. Perutz et al. reported the photocatalysis using PorRe2 with a Pd(II) tetraphenylporphyrin derivative as the photosensitizer and a fac-[Re(N^N)(CO)3(3-picoline)]+ derivative as the catalyst, which were connected to each other via an amide linkage.44 This complex photochemically reduced CO2 to CO in the presence of TEA (entry 47: TONCO = 2), although its photocatalytic ability was lower in comparison to that of a mixed system of the corresponding mononuclear complexes, Pd(II) tetraphenylporphyrin (Pd(Por)) and fac-[Re(bpy)(CO)3(3-picoline)]+ (entry 48: TONCO = 3). The bimetallic complexes with a Zn(II) tetraphenylporphyrin derivative instead of the Pd one as the photosensitizer unit also photocatalyzed the reduction of CO2 to CO by irradiation of light at λex > 520 nm. However, their photocatalytic abilities (PorRe3, entry 49: TONCO = 14; PorRe4, entry 50: TONCO = 32) were also lower than that of a mixed system of Zn(II) tetraphenylporphyrin (Zn(Por)) and fac-[Re(bpy)(CO)3(3-picoline)]+ (entry 52: TONCO = 103).45 The insertion of a methylene group between the -NHCO- linkage and the Re unit improved the photocatalysis compared to the mixed system (PorRe5, entry 51: TONCO = 332).46 This difference should be caused by the stronger reducing power of OERS of the Re unit of PorRe5 than those of PorRe3 and PorRe4 (PorRe5: E1/2red = –1.68 V vs. Fc/Fc+; PorRe3: E1/2red = –1.44 V; PorRe4: E1/2red = –1.42 V) in the similar manner described in section 3. Tschierlei, Schwalbe, and their co-workers reported binuclear complexes, of which a diimine moiety of Re(I) unit was connected to free- and metallo-porphyrins via a linker with π conjugation. The binuclear complex with a Zn center (PorRe6) photochemically reduced CO2 to CO in the presence of TEA under UV-visible light irradiation at λex > 375 nm (entry 53: TONCO = 13), whereas the other binuclear complexes without metal center or with Fe, Co, Cu, and Pd centers (PorRe7 ~ PorRe11) did not work as photocatalysts (entry54-58). Tamiaki and co-workers reported a supramolecular photocatalyst that employed a chlorophyll as the photosensitizer unit (ChlRe1), which photocatalyzed CO2 reduction to CO using BIH as a sacrificial electron donor (entry 59: TONCO = 18).48

6. Ru(II)-Ru(II) systems

19


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Chart 7. Structures and abbreviations of Ru(II)-Ru(II) complexes.

Table 6. Photocatalytic properties of Ru(II)-Ru(II) systems. entry Photocatalyst Donora Product Γ / % Φproduct TON TOF / min-1 Ref. 60 RuRu1 BNAH HCOOH 90 0.038 315 52 61 BNAH HCOOH 91 0.041 562 7.8 52 62 MeO-BNAH HCOOH 89 0.061 671 11.6 52 RuRu2 63 BI(OH)H HCOOH 87 0.46 2766 44.9 60 64 BIH HCOOH 72 0.18 641 10.2 60 65 RuRu3 BNAH HCOOH 77 0.030 353 52 66 RuRu4 BNAH HCOOH 70 0.017 234 52 67 RuRu5 BNAH HCOOH 91 337 This work 68 RuRu6 BNAH CO 98 40 This work a BNAH: 1-benzyl-1,4-dihydronicotinamide; MeO-BNAH: 1-(4-methoxybenzyl)-1,4-dihydronicotinamide; BI(OH)H:

1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole;

BIH:

1,3-dimethyl-2-phenyl -2,3-dihydro-1H-benzo[d]imidazole.

The structures and abbreviations of Ru(II)-Ru(II) complexes are shown in Chart 7 and their photocatalytic activities are summarized in Table 6. The molecular architecture of the Ru(II)-Re(I) supramolecular photocatalysts can be applied to other photocatalytic systems with a different catalyst instead of the Re unit. As was originally reported by Tanaka and his coworkers, [Ru(N^N)2(CO)2]2+-type 20


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complexes are well-known electrochemical catalysts for CO2 reduction, which catalytically produce HCOOH with high selectivity in basic conditions.49-51 The [Ru(4dmb)3-m(BL)m]2+-type (m = 1, 2, 3; BL = 1,2-bis(4'-methyl-[2,2']bipyridin-4-yl)-ethane) 2+

cis-[Ru(4dmb)2-n(BL)n(CO)2]

photosensitizer(s)

were

connected

to

(n = 1, 2) catalyst(s) using a non-conjugated bridging ligand or ligands,

where the ratio between the number of photosensitizer and catalyst units was tunable in the range from 1:3 to 2:1 (RuRu1 - RuRu4 in Chart 7). All these complexes could photocatalytically reduce CO2 to HCOOH in the presence of BNAH under irradiation at λex > 500 nm.52 The ratio between the number of photosensitizer and catalyst units strongly affected the photocatalytic activity. Using a higher ratio of the photosensitizer to the catalyst led to a higher yield of HCOOH: RuRu2 (entry 61: ΓHCOOH = 91%) and

RuRu1 (entry 60: ΓHCOOH = 90%) displayed high selectivity in the photocatalytic reaction, while RuRu3 (entry 65: ΓHCOOH = 77%) and RuRu4 (entry 66: ΓHCOOH = 70%) were rapidly deactivated during the photocatalytic reaction. The deactivation of the photocatalytic activity was caused by the change in the catalyst unit(s). In the cases of RuRu3 and RuRu4, the color of the reaction solutions rapidly changed to black during the photocatalytic reaction. On the other hand, the solutions of RuRu1 and RuRu2 remained a similar color (orange or red) for a long period. [Ru(bpy)2(CO)2]2+ has been reported to release a bpy ligand by electrochemical reduction to give a black polymer with Ru-Ru bonds, i.e., [Ru(bpy)(CO)2]n (eq. 19).53-54

(19) The conversion of CO2 into HCOOH requires a two-electron reduction. In the cases of RuRu3 and RuRu4, the larger number of catalyst units in one photocatalyst should lowered the opportunity for each catalyst unit to accept the second electron from the OERS of the photosensitizer unit. This should facilitate the decomposition of the catalyst unit, i.e., the loss of the diimine ligand and formation of the Ru-Ru bond. On the other hand, in the cases of RuRu1 and RuRu2, the oligomerization/polymerization of the catalyst unit did not proceed, because the higher ratio of the photosensitizer unit to the catalyst unit increased the opportunity for the transfer of the second electron to the catalyst unit and favored the reduction of CO2 over the deactivation process. The effects of the ratio between the photosensitizer unit(s) and the catalyst unit(s) on the photocatalytic durability in Ru(II)-Ru(II) systems were the opposite of those in Ru(II)-Re(I) systems. The Ru(II)-Re(I) supramolecular complexes with multiple catalyst units exhibited higher durability compared to the corresponding Ru-Re binuclear complexes.18, 34-35 In the cases of Ru(II)-Ru(II) systems, on the other hand, RuRu2 with multiple photosensitizer units was the most effective photocatalyst.52 These differences should be induced by the lower stability of the OERS of cis-[Ru(4dmb)2-n(BL)n(CO)2]2+. Ru

multinuclear

cis,trans-Ru(BL)(CO)2Cl240, 55,

complexes

containing

the

other

Ru(II)

catalyst

unit,

i.e.,

(RuRu5 and RuRu6) also photocatalyzed the reduction of CO2 in the

presence of BNAH under irradiation at λex > 500 nm. The ratios between the photosensitizer unit and the catalyst unit(s) were 1:1 and 1:3. RuRu5 with the ratio of 1:1 mainly produced HCOOH (entry 67: 21


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TONHCOOH = 337, TONCO = 22, TONH2 = 12), whereas RuRu6 with three catalyst units produced CO selectively (entry 68: TONHCOOH < 1, TONCO = 40, TONH2 < 1). The reason why RuRu6 did not produce HCOOH should be due to polymerization of the catalyst units. cis,trans-Ru(bpy)(CO)2Cl2 has also been reported to release chloride anions by electrochemical reduction giving [Ru(bpy)(CO)2]n (eq. 20).56-59 The production of CO was probably caused by photocatalysis by the Ru oligomer that contained the Ru photosensitizer unit formed from RuRu6.

(20) In

photocatalysis

using

RuRu2,

the

utilization

of

1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (BI(OH)H) instead of BNAH (entry 61: ΦHCOOH = 0.041, TONCO = 562, TOFCO = 7.8 min−1) as a sacrificial electron donor substantially increased the efficiency, durability, and rate of the photocatalytic production of HCOOH (entry 63:

ΦHCOOH = 0.46, TONCO = 2766, TOFCO = 44.9 min−1), although the use of BIH did not have this effect (entry 64: ΦHCOOH = 0.18, TONCO = 641, TOFCO = 10.2 min−1).60 The reasons for this fascinating effect and the differences between BNAH, BI(OH)H, and BIH are described in detail in section 7-2.

7. Sacrificial electron donors In this section, the characteristics and reactivity of frequently used sacrificial electron donors are summarized, because they strongly affect the parameters that have been used for the evaluation of photocatalysis, i.e., quantum yield, turnover number, and turnover frequency. One of the final targets in the photocatalytic reduction of CO2 is the use of water as a reductant. However, supramolecular photocatalysts require a sacrificial electron donor, because the oxidizing power of the photosensitizer unit is insufficient to capture electrons from water. Therefore, the investigations described in this paper mostly only described the reduction site as a model of “a half reaction” in the photocatalytic reduction of CO2 with water. Because we wish to know how photocatalysts function if an electron is supplied to them without difficulty, i.e., the electron donating process to the photocatalyst is not a rate-limiting process for the evaluation of its photocatalysis, sacrificial electron donors for supramolecular photocatalytic systems, in which the excited photosensitizer unit is reductively quenched by the sacrificial electron donor in the initial stage of the photocatalytic reaction, should have the following properties: (1) Strong reducing power. In the cases of the supramolecular photocatalysts with the [Ru(4dmb)3]2+-type photosensitizer

(Charts

4

and

7),

TEOA,

TEA,

2-({2-[bis(carboxymethyl)amino]ethyl}(carboxymethyl)amino)acetic acid (EDTA), and AscH, which have frequently been used as sacrificial electron donors in various photochemical reactions, are inappropriate, because they have insufficient reducing power to reduce the excited [Ru(4dmb)3]2+-type photosensitizer unit. (2) Short lifetime of the OEOS of the sacrificial electron donor. If the OEOS is stable, reverse electron 22


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transfer from the reduced photosensitizer unit to the OEOS of the electron donor should proceed efficiently, which should lower the efficiency of the photocatalytic reaction. A typical example is 1,4-diazabicyclo[2.2.2]octane (DABCO), of which the reducing power is strong enough to quench all the photosensitizer units reported in this paper. However, photocatalytic reactions do not proceed efficiently with this reductant, because the OEOS of DABCO (DABCO·+) is stable. (3) No inhibitory effect of the fully oxidized form of the reductant. During the photocatalytic reaction, the oxidized product of the electron donor may hinder the progress of the photocatalytic reaction. From these viewpoints, we have employed a series of sacrificial electron donors for supramolecular photocatalysts, i.e., model compounds of the redox coenzyme NAD(P)H and dihydrobenzoimidazole derivatives, which exhibited very different degrees of reactivity during the photocatalytic reactions as described below.

7-1. NAD(P)H model compounds BNAH, a typical NAD(P)H model compound, has been used as a sacrificial electron donor for various redox-photosensitized reactions, which has stronger reducing power than both TEA (Eoxp = 0.96 V vs. SCE) and TEOA (Eoxp = 0.80 V)61. The redox potential of BNAH has been reported to be E°(BNAH/BNAH·+) = 0.57 V62, which is sufficient to reduce the excited [Ru(4dmb)3]2+-type photosensitizers (E1/2red([Ru(4dmb)2(4dmb·–)]+/*[Ru(4dmb)3]2+) = 0.63 V). In fact, BNAH can reductively quench the excited photosensitizer unit of supramolecular photocatalysts. For example, the quenching fractions (ηq) calculated using eq. 16 was 59% for RuRu2 when [BNAH] = 0.1 M in DMF–TEOA (4:1 v/v).52 A smaller value of ηq causes the value of ΦHCOOH to be lower, because the radiative and non-radiative deactivation processes of the excited photosensitizer unit compete with the photochemical reduction of the photosensitizer unit by the sacrificial electron donor (Process II). Actually, the use of a stronger electron donor, namely, 1-(4-methoxybenzyl)-1,4-dihydronicotinamide (MeO-BNAH, E°ox = 0.50 V)62 improved the efficiency of Process II (kq = 4.7 × 107 M-1s-1, ηq = 77% with MeO-BNAH (0.1 M)) and also the photocatalytic ability of RuRu2 (entry 62: ΦHCOOH = 0.061, TONHCOOH = 671, TOFHCOOH = 11.6 min−1) in comparison to that when BNAH was used (entry 61: kq = 1.9 × 107 M-1s-1, ΦHCOOH = 0.041, TONHCOOH = 562, TOFHCOOH = 7.8 min−1).52 It has been reported that the deprotonation of the OEOS of BNAH (BNAH·+) proceeds more rapidly in the presence of a base giving BNA· (eq. 21).52 Although TEOA does not quench the excited state of the [Ru(4dmb)3]2+-type photosensitizer unit as described above, TEOA functions efficiently as a base to capture a proton from BNAH·+ in the reaction solution. There are two possible fates of BNA· produced in the photocatalytic reactions. One involves a dimerization process that produces BNA dimers (4,4’-BNA2 and 4,6’-BNA2) (eq. 22). In the other process, BNA· donates an electron to other molecules and is thereby converted to BNA+ (eq. 23). In other words, there are two possible roles of BNAH as an electron donor, namely, as a one-electron donor (eqs. 21 and 22) or a two-electron donor (eqs. 21 and 23).

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(21)

(22)

(23) During photocatalytic reactions using RuRe12 and RuRu1, BNAH and its oxidized compounds (both forms of BNA2s, as well as BNA+) were quantitatively analyzed by HPLC with an ODS column.30, 52 Only BNA2s were detected as the oxidized compounds of BNAH, whereas BNA+ was not detected. The decrease in the amount of BNAH was twice the total amounts of produced BNA2s. The amounts of produced CO and HCOOH were almost equivalent to the total amount of BNA2s produced in the photocatalytic reactions using both RuRe12 and RuRu1 (see Figure 2 for the case of RuRu1 as an example). Because the formation of the reduction products (CO and HCOOH) requires two electrons per molecule and the formation of one BNA2 molecule from two BNAH molecules donates two electrons, the electron balances in the photocatalytic reduction of CO2 using RuRe12 and RuRu1 could be determined as shown in eqs. 2430 and 2552, respectively. In other words, BNAH acted only as a one-electron donor in these photocatalytic reactions.

Figure 2. Photocatalytic production of the reduction and oxidation products i.e., HCOOH + CO + H2 (red filled circle) and BNA2s (blue filled diamond) and consumption of BNAH (black filled circle): a CO2 saturated DMF-TEOA (4:1 v/v, 4 mL) solution containing BNAH (0.1 M) and RuRu1 (0.05 mM) was irradiated with light at λex >500 nm. 24


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(24)

(25) The accumulation of BNA2s in the reaction solution is problematic for photocatalytic reactions. Since BNA2s are stronger electron donors (E°ox(4,4'-BNA2) = 0.26 V vs. SCE)63 than BNAH, the quenching reaction of the excited photosensitizer by BNA2s competed with that by BNAH (in the case of

RuRe12, kq(BNAH) = 1.9 × 107 M-1s-1, kq(4,4'-BNA2) = 3.1 × 108 M-1s-1; in the case of RuRu1, kq(BNAH) = 1.6 × 107 M-1s-1, kq(4,4'-BNA2) = 3.1 × 108 M-1s-1). The OEOS of BNA2s (BNA2s·+) are so stable that back electron transfer proceeded preferentially from the reduced photosensitizer to BNA2s·+ (eq. 26).30, 52

(26) These reductive quenching and backward electron transfer processes wasted the energy of the photon absorbed by the photosensitizer unit and lowered the efficiencies of the photocatalytic reactions.

7-2. Dihydrobenzoimidazole derivatives BIH and BI(OH)H have much stronger reducing power (BIH: E1/2ox = 0.33 V; BI(OH)H: E1/2ox = 0.31 V vs. SCE) compared to the NADH model compounds.64-69 Owing to this property, both compounds efficiently quenched the excited state of the Ru photosensitizer unit in the supramolecular photocatalysts (for example, kq = 9.7 × 108 M-1s-1, ηq = 99% for RuRe12 with BIH (0.1 M)) (Process II).33, 60

Irradiation of a mixed solution of DMF–TEOA containing RuRe12 and BIH (0.1 M) induced the

selective catalytic formation of CO with much greater efficiency, durability, and rate of CO formation (entry 23: ΦCO = 0.45, TONCO = 3029, TOFCO = 35.7 min−1) compared with those using BNAH (entry 22:

ΦCO = 0.15, TONCO = 207, TOFCO = 4.7 min−1).33 The efficiency and durability of the photocatalysis using RuRe15 were also much improved by using BIH (entry 27: ΦCO = 0.54, TONCO = 2915) instead of BNAH (entry 26: ΦCO = 0.13, TONCO = 233). The processes involved in the oxidation of BIH in the photocatalytic reactions are very different from those of BNAH as follows. Figure 3 shows the decrease of BIH, increase of BI+, that is two-electron oxidized product of BIH (eq. 26), and the total amounts of the reduction products (mostly CO and a tiny amount of H2) produced during a photocatalytic reaction using RuRe12. The production of HCO3– was also observed by 13C NMR measurements of the reaction solution, and the amount produced was almost equivalent to that of CO. These results clarified the electron balance and material balance of the photocatalytic reaction as shown in eq. 27.33

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Figure 3. Photocatalytic production of the reduction and oxidation products i.e., CO + H2 (red filled circle) and BI+ (blue filled diamond) and consumption of BIH (black filled circle): a CO2 saturated DMF-TEOA (5∶1 v/v, 2 mL) solution containing BIH (0.1 M) and RuRe12 (0.05 mM) was irradiated with light at λex >500 nm. (27) BIH donates two electrons by one-photon excitation of the photocatalyst via a sequence comprising electron transfer, deprotonation, and a further electron-transfer (ECE) (eq. 28), because the one-electron oxidized and deprotonated form of BIH (BI·) has strong reducing power (Epox = –2.06 V vs. Fc+/Fc)64, which was sufficient to donate an electron to RuRe12 even in the ground state (E1/2red = –1.76 V vs. Fc+/Fc) and possibly also to the reaction intermediate(s).

(28) In the photocatalytic reactions using BIH as the sacrificial electron donor, TEOA could act as a base to improve the photocatalytic activity of supramolecular systems. Although BIH itself is also expected to function as a base in addition to its role as an electron donor, protonation should reduce the concentration of BIH and the protonated form of BIH (BIHH+) should not quench the excited supramolecular photocatalyst. Therefore, the photocatalytic formation of CO was decreased in the absence of TEOA. The photocatalysis of RuRu2 for the reduction of CO2 to HCOOH was considerably improved by using BI(OH)H (entry 63: ΦHCOOH = 0.46, TONCO = 2766, TOFCO = 44.9 min−1) compared with that using BNAH (entry 61: ΦHCOOH = 0.041, TONCO = 562, TOFCO = 7.8 min−1).60 BI(OH)H also acted as a two-electron donor. The electron balance and material balance of the photocatalysis using RuRu2 and BI(OH)H were also established as given in eq. 29. One molecule of BI(OH)H donated two electrons and 26


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two protons, and one molecule of CO2 accepted two electrons and two protons to give one molecule of the two-electron oxidized and doubly deprotonated form of BI(OH)H (BI(O–)+) and HCOOH. (29)

(30) It is noteworthy that the amount of HCOOH produced in the photocatalytic reaction of RuRu2 using BIH as a sacrificial electron donor was much smaller (entry 64: TONHCOOH = 641, TONCO = 237, TONH2 = 13) than that using BI(OH)H although the reducing powers of BIH and BI(OH)H are very similar to each other. The selectivity for the HCOOH production was also decreased to 72% using BIH compared with that using BI(OH)H (87%). Photocatalysis using BNAH also exhibited high selectivity for the HCOOH formation of 91% (entry 61: TONHCOOH = 562, TONCO = 29, TONH2 = 29). One of the important differences between BI(OH)H, BNAH, and BIH regards the numbers of donated electrons and released protons. One-photon excitation of RuRu2 allowed BI(OH)H to donate two electrons and two protons (eq. 30) in a step-by-step process. On the other hand, BIH supplied two electrons and one proton (eq. 28). In the case of BNAH, one electron and one proton were donated (eqs. 21 and 22), in which the ratio between the numbers of the donated electrons and protons was the same as for BI(OH)H. In other words, BIH released only half the number of protons compared to that of electrons; however, both BI(OH)H and BNAH donated equal numbers of protons and electrons. The production of HCOOH on the [Ru(N^N)2(CO)2]2+ catalyst unit might involve processes that are accelerated by the concentration of protons in the reaction solution. This is also supported by the following results; addition of phenol (0.1 M) as an external proton source to the reaction solution using BIH improved the photocatalysis, which produced a 2.3-times larger amount of formic acid in the initial 1-h irradiation.60 This was similar to the photocatalytic system using BI(OH)H.

7-3. Sacrificial electron donors employable in the photocatalytic reactions in an aqueous solution As described in the Introduction section, systems for the photocatalytic CO2 reduction have to utilize water as an electron donor in the future. From this viewpoint, the photocatalyses of supramolecular photocatalysts in an aqueous solution instead of an organic solvent is important. In order to make supramolecular photocatalysts soluble in an aqueous solution, Cl– was employed as a counter ion instead of PF6– in the cases of RuRe20 and RuRe21. In a first trial, ascorbate (Asc–) was employed as a sacrificial electron donor in an aqueous solution saturated with CO2 (pH 5.5) using RuRe20.38 Although the removal of methyl substituents from the peripheral ligands should make intramolecular electron transfer (Process III) deficient, it was necessary to increase the oxidizing power of the excited Ru photosensitizer unit to utilize Asc– as a sacrificial electron donor. Visible-light irradiation of a CO2-saturated aqueous solution containing RuRe20 and Asc– produced HCOOH with high selectivity (entry 33: ΦHCOOH = 0.002, 27


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TONHCOOH = 25). The product distribution was significantly different from that obtained in photocatalysis using the Ru(II)-Re(I) supramolecular photocatalysts in organic solvents such as a DMF–TEOA mixture, which gave CO selectively. The low photocatalytic activity was caused by a much lower efficiency of the photochemical production of the OERS of the Ru photosensitizer unit (Process II) for the following reasons: (1) lower escape yield of the OERS from the solvent cage after reductive quenching by Asc–; and (2) much faster back electron transfer from the OERS of RuRe20 to the oxidized form of Asc–. The strong oxidizing power of the oxidation product of Asc–, namely, dehydroascorbic acid, might be another problem because it may oxidize the reduced photosensitizer unit. Recently, a water-soluble derivative of BIH containing a carboxylic acid group, i.e., 2-(1,3-dimethyl-2,3-dihydro-1H-benzimidazol-2-yl)benzoic acid (BI(CO2H)H) was developed as a sacrificial electron donor soluble in basic aqueous solutions.39 Visible-light irradiation (λex > 500 nm) of an aqueous solution (pH 9.8) containing RuRe21, BI(CO2H)H (0.1 M), and NaOH (0.1 M) under a CO2 atmosphere produced CO with relatively high efficiency and durability (entry 34: ΦCO = 0.13, TONCO = 130). This remarkable improvement in photocatalytic activity compared with those using Asc– could be attributed to a significant increase in the efficiency of the photochemical production of the OERS of the photosensitizer unit (Process II). The quenching rate constant of the excited Ru photosensitizer unit by BI(CO2H)H was 7.4 × 109 M−1s−1, and BI(CO2–)H also worked as a two-electron donor (eq. 31).

(31)

8. Conclusions In this perspective, we have summarized the supramolecular photocatalysts for the reduction of CO2. In 2005, we reported the first successful Ru(II)-Re(I) supramolecular photocatalysts for CO2 reduction and devised an architecture for developing efficient supramolecular photocatalysts. On the basis of this study, the photocatalytic abilities were increased by optimization of both the bridging ligands and the Re(I) catalyst units. In addition, the photocatalytic reduction of low concentrations of CO2 was achieved by utilizing the insertion of CO2 into the Re(I) unit coordinated by deprotonated TEOA. Ir(III) and Os(II) complexes can also function as alternative photosensitizer units to [Ru(4dmb)3]2+-type complexes and exhibit several improved characteristics. For example, the Ir(III) complexes were more appropriate for the precise evaluation of the product distribution and Os(II) complexes could utilize visible light over a much wider range of wavelengths. Metalloporphyrines and chlorophylls were also used as photosensitizer units with the Re catalyst. In these cases, photocatalytic reduction was initiated by intramolecular oxidative quenching of the excited photosensitizer unit by the catalyst unit. We succeeded in controlling and changing the product distribution from CO to HCOOH by utilizing Ru(II) carbonyl complexes as catalyst units. For the first time in the region of metal complex photocatalysts not only for the CO2 reduction but also for the H2 evolution, we have quantitatively analyzed the oxidized compounds 28


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of a sacrificial electron donor and determined the electron balance and material balance of the photocatalysis. A sacrificial electron donor was also important for evaluating the potential of the supramolecular photocatalysts. We have found several very suitable sacrificial electron donors. Thus, over the last decade, we have achieved the photocatalytic reduction of CO2 to both CO and HCOOH with high selectivity, efficiency, durability, and rates even under visible-light irradiation. However, for the development of practical systems for artificial photosynthesis, many functions must be added to the present systems such as a light-harvesting antenna for utilizing sunlight that contains a very low photon flux and a photocatalytic system for the oxidation of water that utilizes water as an electron source for CO2 reduction. To address these issues, in recent years, we have been developed hybrid systems that comprise supramolecular photocatalysts immobilized on heterogeneous materials such as semiconductor photocatalysts8-12, semiconducting electrodes13-14, and periodic mesoporous organosilica7. In these systems, Ru(II)-Re(I) and Ru(II)-Ru(II) supramolecular photocatalysts were employed such as derivatives of RuRe9 and RuRu5 containing methylphosphonic acid anchoring groups on one of peripheral diimine ligands in the photosensitizer unit. For example, a hybrid system of a Ru(II)-Ru(II) supramolecular photocatalyst and an Ag-loaded TaON semiconductor photocatalyst photocatalyzed the reduction of CO2 to HCOOH by utilizing methanol, which is a much weaker reductant even compared to TEOA.8 A hybrid photocathode consisting of an NiO electrode and an Ru(II)-Re(I) supramolecular photocatalyst photoelectrochemically reduced CO2 to CO.13 A photoelectrochemical cell comprising this hybrid photocathode and a CoOx/TaON photoanode achieved the photochemical reduction of CO2 to CO using water as an electron source with the assistance of external electrical (0.3 V) and chemical (0.1 V) biases.14 These two examples prove that the supramolecular photocatalysts occupy a dominant position with respect to mixed systems containing separated photosensitizer and catalyst, in particular on the surface, because they allow efficient electron transfer between the photosensitizer and catalyst units. Although the development of these hybrid systems should represent progress in the production of systems for artificial photosynthesis, it is insufficient. For example, hybrid systems containing semiconductor photocatalysts cannot yet utilize water as an electron source, and the photoelectrochemical cells still require external biases. Further researches into supramolecular complex photocatalysts, semiconducting materials, and, in particular, interfacial reactions between supramolecular complexes and semiconductors is still needed.

Acknowledgements We thank Dr. Shunsuke Sato for analyzing some data. This work was supported by CREST (Molecular Technology project, JST), Strategic International Collaborative Research Program (PhotoCAT project, JST), and a Grant-in-Aids for Young Scientists (JP16K17891, JSPS).

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ACS Catalysis

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