Polyoxometalate Photocatalysis for Liquid-Phase Selective Organic

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Polyoxometalate Photocatalysis for Liquid-Phase Selective Organic Functional Group Transformations Kosuke Suzuki, Noritaka Mizuno, and Kazuya Yamaguchi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03498 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 8, 2018

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ACS Catalysis

Polyoxometalate Photocatalysis for Liquid-Phase Selective Organic Functional Group Transformations Kosuke Suzuki†‡*, Noritaka Mizuno†, and Kazuya Yamaguchi†*

†Department

of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1

Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡Precursory

Research for Embryonic Science and Technology (PRESTO), Japan Science and

Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan

1 ACS Paragon Plus Environment

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ABSTRACT Polyoxometalates (POMs), which are anionic metal oxide clusters, have recently attracted considerable attention as photocatalysts owing to their unique photoinduced charge-transfer properties, redox properties, acid–base properties, and reactivities. In this review, we present a summary of recent developments in POM photocatalysis for organic synthesis. Various organic functional group transformations can be selectively induced by photoirradiation in the presence of catalytic amounts of suitably designed POMs. In particular, many liquid-phase functional group transformations based on the activation of substrates by decatungstate have been reported. However, decatungstate photocatalysis requires irradiation with UV light because of the large energy gaps between the O2−-based highest occupied molecular orbitals (HOMOs) and the W6+based lowest unoccupied molecular orbitals (LUMOs) therein. Various strategies have been developed in efforts to utilize visible light, including hybridization with photosensitizers, metal substitution, and coordination of ligands (substrates) at the vacant sites of lacunary POMs. We also present here an overview of our recent work on the development of visible-light-responsive POM catalysts by HOMO- and LUMO-engineering strategies for both the oxidation and reduction of organic substrates, including amines, alcohols, nitroarenes, sulfides, sulfoxides, and pyridine N-oxides.

KEYWORDS photocatalysis, polyoxometalate, functional group transformation, visible light, UV light


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1. INTRODUCTION Polyoxometalates (POMs) are metal oxide nanoclusters that adopt a wide variety of structures.1– 14

Furthermore, they exhibit unique structure-dependent chemical and physical properties,

including acid–base and redox properties, making them attractive materials for application as catalysts, photocatalysts, sensors, and magnetic materials.1–14 In addition, POMs are thermally and oxidatively stable in comparison with organic- or organometallic-based homogeneous catalysts. Therefore, POMs have received much attention especially in the field of oxidation catalysis. The metal atoms (polyatoms) in fully oxidized POMs typically adopt the d0 electronic configuration, and they exhibit ligand-to-metal charge transfer (LMCT), for example, O2−(2p) to W6+(5d) in polyoxotungstates, upon irradiation with UV and/or near UV light. Photoirradiation of POMs induces intramolecular charge transfer from the O2−-based highest occupied molecular orbital (HOMO) to the W6+-based lowest unoccupied molecular orbital (LUMO), leading to the formation of photoexcited states. The resultant photoexcited POMs are highly reactive, both as oxidizing agents and reducing agents, allowing them to be utilized for various photocatalytic reactions. The first report on the photochemistry of POMs was published over 100 years ago.15 However, a long time passed after the publication of this seminal work before Chalkley reported the photoredox conversion of α-Keggin-type phosphotungstic acid (H3[PW12O40]) into a reduced POM, the so-called heteropoly blue, by photoirradiation with UV light in the presence of 2propanol as a reducing reagent.16 This reduced POM could be reoxidized by O2 in air. Systematic investigation of photoredox catalysis using POMs started in the 1980s, and Yamase, Papaconstantinou, and Hill, among others, contributed greatly to these early studies.17–23 These pioneering authors reported the efficient photocatalytic oxidation of various organic substrates such as alkanes, alcohols, sulfides, and alkenes using UV light and POMs. POM-based photocatalysts possess several advantages over other photoredox catalysts such as organometallic coordination complexes24–26 and organic dyes.27 First, POMs exhibit strong absorption of UV and near UV light and undergo efficient intramolecular charge transfer in response to photoirradiation. In addition, POM frameworks are typically stable during photoredox reactions and can reversibly transfer electrons to and from a range of substrates while maintaining their structures.28 Importantly, the redox potentials of POMs can be controlled at the


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molecular level by tailoring their structures and constituent elements and/or by the introduction of additional metal cations into the POM framework, allowing modification of their oxidation and reduction abilities. Thus, POM photocatalysts may be applied to a range of photocatalytic oxidation and reduction reactions. Furthermore, POMs can accommodate multiple electrons within their frameworks, allowing them to promote multielectron redox reactions, which are especially necessary for processes that convert sunlight into chemical energy. Accordingly, POM photocatalysis has been applied to a wide range of reactions, including H2 evolution, water oxidation (O2 evolution), CO2 reduction, metal reduction, and the degradation of organic pollutants and dyes. Indeed, several excellent review articles on these applications have been published.11,28–33 However, in the present review, we have focused mainly on the application of POM photocatalysis to liquid-phase organic synthesis. As for the photocatalysis of organometallic coordination complexes, organic dyes, and solid-state catalysts, please refer to recent excellent reviews. 24–27,34 Early studies on POM photocatalysis for organic functional group transformations employed Keggin-type ([XM12O40]n−), Dawson-type ([X2M18O62]n−), and decatungstate ([W10O32]4−) (M = Mo, W; X = Si, P, S, Ge, etc.) POMs (Figure 1).17–23,35 In particular, the tetran-butylammonium (TBA) salt of decatungstate [W10O32]4− has recently received considerable attention owing to its unique photocatalytic reactivity. Since the TBA salt of decatungstate is commercially available, it can be utilized easily for organic synthesis chemists. Decatungstate anions present strong O-to-W LMCT absorption bands in the UV light region (λmax = 324 nm, ε = 14100 M−1 cm−1)36 and exhibit UV-light-responsive photocatalysis for a range of functional group transformations.34,37-39 In this review, we first show recent progress in the decatungstate photocatalysis using UV light (including sunlight). When UV light is irradiated to decatungstate, active species that can abstract hydrogen atoms from various organic substrates are generated. Various reactions have been developed skillfully using radical species generated by utilizing such active species; of particular note is aerobic oxidation, including oxidation of alcohols, alkanes, alkenes, and aromatic hydrocarbons, etc., and bond formation reactions, including formation of C–C, C–N, C–Si, and C–F, etc. Generally, these reactions proceed efficiently with relatively small amounts of decatungstate (typically, 0.5–2 mol%). For organic synthesis using decatungstate, excellent reviews are summarized by Orfanopoulos37 and Ryu,40 so please refer to


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them as well. Recent developments of cooperative decatungstate photocatalysis and metal complex catalysis are also included in this review. Because of the large energy gaps between the O2-based HOMO and the W6+-based LUMO in decatungstate, the reactions should require irradiation with UV light. In order to effectively use energy, development of photocatalysts which can utilize visible light is highly desired. Since POMs can precisely control various properties and store/transfer multi- electrons and protons as described above, they have great potential to develop excellent visible-lightresponsive photocatalysts. Design of visible-light-responsive POM-based photocatalysts is still under development, but several design strategies have been reported so far; for example, hybridization with photosensitizers, metal substitution, and coordination of ligands (substrates) at the vacant sites of lacunary POMs (Figure 2). In this review, we summarize the development of visible-light-responsive POM-based photocatalysts for each design strategy, and also introduce our recent development of HOMO- and LUMO-engineering strategies for both oxidation and reduction reactions of organic substrates, including amines, alcohols, nitroarenes, sulfides, sulfoxides, and pyridine N-oxides.


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Figure 1. Anion structures of (a) fully-occupied and (b) lacunary POMs. The light-green and gray polyhedra represent [MO6] and [XO4], respectively. The red spheres represent oxygen atoms at the vacant sites of lacunary POMs.


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Figure 2. Strategies for visible-light-responsive POM catalysts through hybridization with photosentsitizer, metal substitution, and introduction of ligands (substrates) at the vacant sites of lacunary POMs. 2. DECATUNGSTATE PHOTOCATALYSIS Experimental35,41–50 and theoretical studies51–53 of decatungstate photocatalysis have revealed that it typically proceeds based on the following mechanism: The photoactivation of decatungstate ([W10O32]4−) with UV light irradiation leads to the formation of the excited state ([W10O32]4−*) by intramolecular O-to-W LMCT. The [W10O32]4−* species rapidly decays to the actual reactive state (termed wO), which is a relaxed excited state, probably of triplet multiplicity (Scheme 1). A quantum yield for the generation of the reactive species is around 0.5–0.6. A recent theoretical study demonstrated that the redox potential of the reactive species wO is around +2.44 V vs. a saturated calomel electrode (SCE).53 Owing to its high reactivity, wO can activate various organic substrates through hydrogen-atom transfer (HAT) or single-electron transfer (SET). In 1984, Yamase and co-workers reported that UV irradiation of decatungstate in the presence of heterogeneous catalysts such as RuO2, IrO2, or Pt results in the oxidation of alcohols to aldehydes with accompanying H2 evolution.41 Oxidative transformations of various organic compounds are achieved when the photocatalytic reactions are carried out in the presence of O2. 7 ACS Paragon Plus Environment

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Conversely, when the reactions are carried out under anaerobic conditions, nucleophilic carbon radical species are generated by the homolytic cleavage of nonactivated C–H bonds in the organic compounds, and these radical species readily react with organic substrates, resulting in the formation of C–C, C–N, C–Si, and C–F bonds. Notably, photocatalytic reactions by decatungstate typically proceed site-selectively by polar and steric effects. Examples of siteselective reactions are comprehensively summarized in an excellent review by Ryu and coworkers, and the origin of the selectivity is also mentioned in detail.40 Decatungstate presents absorption bands in the UV region (λ < ca. 400 nm), and a xenon lamp is typically used as the light source for these reactions. Because sunlight comprises both UV and visible light, irradiation with sunlight can also be effective.54 Furthermore, in addition to reactions in batch-type photoreactors such as flasks and test tubes, photocatalytic reactions can also be performed in continuous-flow irradiation systems.55,56 In this chapter, decatungstate photocatalysis is classified by reaction type, and a summary of each is presented.

[W10O32]4–*

R–H

HAT

R• H+[W10O32]5–

UV light

[W10O32]4–

wO [W10O32]5–

+2.44 V (vs SCE)

R–X•+

R–X SET

Scheme 1. Photoactivation of Decatungstate ([W10O32]4−) with UV Light 2.1. Photocatalytic Aerobic Oxidation of Organic Substrates by Decatungstate Photocatalytic oxidation of organic compounds such as alcohols, alkanes, and alkenes proceeds efficiently under aerobic conditions using UV-driven decatungstate photocatalysis (Scheme 2). Tanielian and co-workers examined the mechanism of decatungstate-photocatalyzed oxidation in detail by means of laser flash photolysis, pulse radiolysis, and continuous photolysis studies.49 As discussed above, illumination of decatungstate generates the short-lived excited state that decays to form the reactive species wO, which is responsible for the subsequent transformation of organic substrates. It is considered that the reaction of wO with most organic substrates such as aliphatic alcohols and alkanes typically proceeds through a HAT process. For example, under 8 ACS Paragon Plus Environment

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aerobic conditions, wO reacts with 2-propanol to produce a one-electron-reduced species ([W10O32]5−), and the 2-propanol and O2 are converted into acetone and hydrogen peroxide. When adamantane is employed as a substrate, the corresponding hydroperoxides are generated. These reactions proceed through HAT from the substrates by wO, followed by rapid deprotonation of •wOH and/or trapping by O2. For instance, Giannotti and co-workers have reported the photooxidation of adamantane.50,57 In their system, adamantane hydroperoxide species are formed as primary products and then decompose to 1- and 2-adamantanols upon treatment with trimethylphosphite. Furthermore, Maldotti and co-workers reported the selective oxidation of cyclohexane to cyclohexanol (under low O2 pressure) and cyclohexanone (under high O2 pressure).58 In the case of more easily oxidized substrates such as amines, aromatic hydrocarbons, benzylic alcohols, and alkenes, SET competes with HAT. Thus, the reaction of wO with such substrates may occur by HAT and/or SET (Scheme 1). Orfanopoulos and co-workers explored the mechanism for the decatungstate-catalyzed oxidation of several easily oxidizable substrates, utilizing a combination of time-resolved techniques and kinetic isotope studies.59,60 They reported that the oxidation of alkylarenes, such as 1,1-diphenylethane and 9-methyl-9H-fluorene, and benzylic alcohols proceeds exclusively via HAT (Scheme 3),49,60 much like that proposed for aliphatic alcohols and alkanes, and not via SET, which is predominantly observed in alkene oxidation. In addition, their primary and β-secondary kinetic isotope effect experiments provided strong evidence for a stepwise mechanism in which hydrogen-atom abstraction is involved in the rate-determining step. OH

UV light [W10O32]4–, O2


OH


O

OH

O + O

Scheme 2. Decatungstate Photocatalysis for Oxidation of Aliphatic Alcohols, Alkanes, and Aromatic Alcohols under Aerobic Conditions


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OH H R X

OH +

[W10O32]4–

UV light

R

H

wO

TS OH OO R

HOO R

OH

O2

R + H+ [W10O32]5– O2

X

X

X

HAT

X

R = CH3, tBu X = NO2, CF3, F, H

O

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HOO

net:

2HOO

H 2O 2 + O 2

OH H R

UV light [W10O32]4–

+ O2

X

[W10O32]4– O R + H O 2 2 X

Scheme 3. Proposed Mechanism for the Oxidation of Benzyl Alcohols via HAT by Decatungstate Photocatalysis 2.2. Photocatalytic C–C Bond Formation by Decatungstate An important feature of decatungstate photocatalysis is that it allows carbon radical species to be generated from nonactivated C–H bonds by the aforementioned HAT process, and these radicals can be utilized for C–C bond formation by reacting them with suitable acceptors under anaerobic conditions. In the infancy of this research area, Hill and co-workers developed C–C bond formation reactions involving the addition of carbon radicals to various substrates such as acetylene (vinylation), ethylene (ethylation), electron-deficient alkenes, and CO (Scheme 4).34,61,62 They also reported the selective synthesis of nitriles and α-imino esters, in which nonactivated alkane C–H bonds are cleaved to afford the corresponding radical species that then react with methyl cyanoformate, giving the corresponding nitrile or α-imino ester under high or low temperature, respectively (Scheme 4).63 Following these studies, Albini and co-workers developed methods for the alkylation of electron-deficient alkenes using decatungstate photocatalysis.64–66 In their work, alkyl radicals arising from cycloalkanes are efficiently trapped by electron-deficient alkenes to afford the corresponding alkylated products (Scheme 5).66 By activation of alkanes with decatungstate photocatalysis, three-component reactions of alkyl radicals, CO, and electron-deficient alkenes proceed efficiently under high CO pressure (80 atm) (Scheme 5).67 Notably, decatungstate photocatalysis also efficiently generates various types of acyl radicals by activating aldehydes, 10 ACS Paragon Plus Environment

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and trapping the acyl radicals with electron-deficient alkenes affords the corresponding acylated products (ketones) (Scheme 6).68 Furthermore, Orfanopoulos and co-workers developed the acylation of C60 fullerene by photoactivation of aldehydes with decatungstate photocatalysis (Scheme 6).69 In addition, they also generated radical species from ethers,70 crown ethers,70 thioethers,70 and alcohols,71 and these radical species affords the corresponding monoadducts of C60 fullerene. acetylene

R–H


ethylene EWG

R

CO NCCO2CH3

R R EWG

R R

O

R–CN

Scheme 4. Decatungstate Photocatalysis for C–C Bond Formation Reactions by Generation of Carbon Radicals from Nonactivated Alkane C–H Bonds

+

+ CO + (80 atm)


EWG

EWG

CH3CN

43–65% O


EWG

EWG

CH3CN

58–77%

Scheme 5. Decatungstate Photocatalysis for Alkylation of Electron-Deficient Alkenes

O R

H

O

EWG


O

R

EWG 40–63%

R O C60 fullerene R

35–50%

Scheme 6. Decatungstate Photocatalysis for Acylation of Electron-Deficient Alkenes and C60 Fullerene


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Decatungstate photocatalysis is applicable to the activation of the β-C–H bonds in cyclopentanone, allowing efficient β-alkylation with electron-deficient alkenes (Scheme 7).72 In the presence of CO, the three-component β-alkylation of cyclopentanone occurs.72 This photocatalytic strategy has also been applied to the benzylation of electron-deficient alkenes with alkylaromatics via the activation of substituted benzyl silanes73 and toluenes (Scheme 7).74 In addition to reactions with electron-deficient alkenes, Ravelli, Ryu, and co-workers have recently reported the C–H functionalization of heteroaromatics via cross-dehydrogenative coupling between R–H hydrogen donors (alkanes, ethers, amides, and aldehydes) and heteroaromatics (Scheme 8).75 In their study, C–H functionalization of heteroaromatics such as quinoline, isoquinoline, quinazoline, quinoxaline, phthalazine, and benzothiazole proceeded efficiently in the presence of a stoichiometric amount of the oxidant K2S2O8.

O


O

O EWG

CH3CN

R TMS


EWG 41–73% R

EWG

R

EWG

CH3CN

EWG EWG 54–99%

EWG UV light [W10O32]4–

EWG

0.5 M LiClO4 CH3CN/H2O

EWG EWG 29–75%

Scheme 7. Decatungstate Photocatalysis for β-Alkylation of Cyclopentanone and Benzylation of Electron-Deficient Alkenes


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UV light [W10O32]4–, K2S2O8

+ N

CH3CN/CH2Cl2

+ N H

N 77%

Scheme 8. Decatungstate Photocatalysis for C–H Functionalization of Heteroaromatics via Cross-Dehydrogenative Coupling Cooperative photocatalysis using decatungstate and metal complexes is a promising and novel catalytic strategy for organic synthesis. Melchiorre and co-workers reported a combination of decatungstate photocatalysis and asymmetric amine catalysis for the enantioselective synthesis of quaternary carbons (Scheme 9).76 They utilized iminium ions, which are generated upon the condensation of chiral amine catalysts and β,β-substituted cyclic enones, as radical traps for C–C bond formation. Enantioselective radical conjugate addition to β,β-substituted cyclic enones afforded the corresponding quaternary carbon stereocenters with high fidelity.76 Very recently, MacMillan and co-workers have successfully developed an efficient protocol for the direct C(sp3) arylation of a diverse set of aliphatic cyclic compounds through a combination of decatungstate photocatalysis and nickel catalysis (Ni(dtbbpy)Br2; dtbbpy = 4,4′di-tert-butyl-2,2′-bipyridine) (Scheme 10).77 In this reaction, the active species derived from the photoactivation of decatungstate promotes HAT from an alkyl nucleophile (e.g., norbornane) to afford

singly

reduced

decatungstate

(H+[W10O32]5−)

and

a

carbon-centered

radical.

Disproportionation of the singly reduced decatungstate (re)generates decatungstate ([W10O32]4−) and doubly reduced decatungstate (2H+[W10O32]6−). Reduction of Ni(dtbbpy)Br2 (Ep(Ni2+/Ni0) = −1.47 V vs. Ag/Ag+) by 2H+[W10O32]6− (E1/2([W10O32]5−/[W10O32]6−) = −1.52 V vs. Ag/Ag+) most likely affords a Ni0 species, which captures the carbon-centered radical species to generate a Ni+-alkyl species. Then, subsequent oxidative addition into the alkyl bromide bond by the Ni+alkyl species affords a Ni3+(aryl)(alkyl) species, followed by reductive elimination to afford the C(sp3)–C(sp2) cross-coupled product.77


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R–H


R

activation by decatungstate

O

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O

R

R2

H

NH2

R2

33–75% (77–93%ee)

N

activation by iminium ion

Scheme 9. Cooperative Decatungstate Photocatalysis and Asymmetric Catalysis for Enantioselective Synthesis of Quaternary Carbon Species

R H

+

Br

390 nm LED light [W10O32]4–, Ni(dtbby)Br2 K3PO4 (1.1 eq)

Z

R Z

CH3CN 61% (Z = N, R = CF3)

proposed catalytic cycle H

HAT H+[W10O32]5–

H+[W10O32]5–

LnNi0

LnNi+

Alk

–HBr

wO decatungstate catalysis

SET electron relay

SET

R Br

Z

nickel catalysis Br

LED light

[W10O32]4–

LnNi+ Br

2H+[W10O32]6–

LnNi3+ Alk

Ar

R Z

Scheme 10. Cooperative Decatungstate Photocatalysis and a Nickel Catalysis for Direct C(sp3)–H Arylation 2.3. Photocatalytic C–X (X = N, Si) Bond Formation by Decatungstate Decatungstate photocatalysis has been applied to C–N bond formation using azodicarboxylates as radical traps (Scheme 11).78 Alkanes, ethers, and aldehydes have been utilized for the 14 ACS Paragon Plus Environment

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generation of carbon radical species for the construction of C–N bonds by reacting them with the N–N bonds of azodicarboxylates. Under CO atmosphere (80 atm), the three-component reaction of cycloalkanes, CO, and azodicarboxylates proceeded to afford C–CO–N bonds.

Scheme 11. Decatungstate Photocatalysis for C–N Bond Formation Reactions The activation of the Si–H bond in trisubstituted silanes is possible using decatungstate photocatalysis, and this has been applied to the hydrosilylation of electron-deficient alkenes (Scheme 12).79 The homolytic cleavage of the Si–H bond generates a silyl radical, which can be trapped by electron-deficient alkenes. Aromatic tertiary silanes afford the corresponding products in satisfactory to good yields, whereas a competition between C–H and Si–H cleavage is observed when aliphatic silanes are used.

EWG PhMe2Si–H + EWG

UV light [W10O32]4– CH3CN

SiPhMe2 EWG EWG 36–90%

Scheme 12. Decatungstate Photocatalysis for C–Si Bond Formation Reactions 2.4. Photocatalytic C–F Bond Formation by Decatungstate The introduction of one or more fluorine atoms into organic molecules is becoming of increasing importance in pharmaceutical, agrochemical, and polymer chemistry because fluorine atoms can dramatically modify the chemical and physical properties of a compound, such as its metabolic stability, membrane permeability, and bioactivity.80 Consequently, the development of efficient methods for the introduction of fluorine atoms has attracted considerable attention, with the 15 ACS Paragon Plus Environment

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direct fluorination of nonactivated C–H bonds being particularly important. Britton and coworkers reported the direct fluorination of nonactivated aliphatic C–H bonds using decatungstate photocatalysis and the fluorine-atom-transfer reagent N-fluorobenzenesulfonimide (NFSI) (Scheme 13).81 This system is highly effective for the fluorination of the nonactivated aliphatic C–H bonds in sclareolide and amino-acid derivatives71 as well as benzylic C–H bonds.82 In addition, late-stage fluorination using decatungstate and NFSI allowed the direct synthesis of γfluoroleucine, which is a crucial intermediate for the synthesis of odanacatib, a selective cathepsin K inhibitor.83 Britton and co-workers developed a method for

18F-fluorination

of

unprotected aliphatic amino acids, allowing the direct synthesis of oncological positron emission tomography imaging agents.84 They also reported site-selective, late-stage

18F-fluorination

unprotected peptides for positron tomography imaging.85

Scheme 13. Decatungstate Photocatalysis for C–F Bond Formation Reactions 16 ACS Paragon Plus Environment

of

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2.5. Photocatalytic Acceptorless Dehydrogenation of Alkanes by Cooperative Decatungstate and Metal-Complex Photocatalysis A C=C double bond formation by selective dehydrogenation of ubiquitous C‒C saturated bonds is a very important transformation in both bulk and fine chemical production. In industrial alkene production, dehydrogenation is frequently carried out using heterogeneous catalysts, such as supported precious metals and metal oxides. For laboratory scale precision chemical synthesis, precious metal-based complexes have been used for dehydrogenation. However, most of previously developed catalyst systems have several disadvantages; that is, operation under very harsh reaction conditions, limited substrate scopes, use of expensive precious metals, and/or needs for stoichiometric hydrogen acceptors (terminal oxidants). Sorensen and co-workers succeeded in developing an efficient precious-metal-free acceptorless dehydrogenation system by skillfully combining two different HAT catalysts.86 This system follows a nature-inspired pathway of sequential high- and low-energy hydrogen atom abstractions. In the first HAT, decatungstate photocatalysis can activate a strong C–H bond (~100 kcal mol–1) to form an alkyl radical species. Then, in the second HAT, cobaloxime pyridine chloride (COPC) is utilized for the activation of a weakened C–H bond of the alkyl radical species ( 3.5 eV), the photocatalytic systems using decatungstate typically require irradiation with UV light. Thus, the development of visible-light-responsive photocatalysts is highly demanded if the efficient use of sunlight is to be realized. Recently, several strategies have been developed to utilize visible light, for example, hybridization with visible-light-active chromophores (or “photosensitizers”), metal substitution, and coordination of ligands (substrates) at the vacant sites of lacunary POMs (Figure 2). In this chapter, visible-light-driven POM photocatalysis is classified by the strategy used as a means to utilize visible light. 3.1. Hybridization with Photosensitizers One of the strategies for the development of visible-light-responsive photocatalysts is combining POMs and photosensitizers, such as organometallic complexes, organic dyes, and C60 fullerene, either through electrostatic interactions or covalent bonds (Figures 3 and 4). These systems have mainly been utilized for the photocatalytic conversion of small molecules in processes such as H2 evolution,87–93 water oxidation,94–101 and CO2 reduction.102–107 For H2 evolution, the following combinations of POMs and photosensitizers have been reported: [α-AlSiW11(H2O)O39]5− and eosin Y,87 [P2W17O61]10− and Ir-complexes,88 [Mn4(H2O)2(VW9O34)2]10– and [Ru(bpy)3]2+,89 [Ni4(H2O)2(PW9O34)2]10– and [Ir(ppy)2(dtbbpy)][PF6],90 [Co3+Co2+(H2O)W11O39]7– and eosin Y,91 [P2W18O62]6– embedded in [Ru(bpy)3]2+-based metal–organic framework (MOF).92 Water oxidation to O2 is an important reaction in combination with the use of sunlight because both water and solar energy are abundant and clean resource. Several systems for visible-light-responsive photocatalytic water oxidation have been reported by using the combination of metal-containing POMs and photosensitizers where multinulear metal cores can efficiently oxidized water to O2; for example, [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2]10− and [Ru(bpy)3]3+,94

[Co4(H2O)2(α-PW9O34)2]

[Co2Mo10O38H4]6–)

and

[Ru(bpy)3]2+,96

and

[Ru(bpy)3]3+,95

[[Co4(H2O)2(VW9O34)2]10−

[CoMo6O24H6]3– and

(or

[Ru(bpy)3]2+,97

{[Co4(OH)3(PO4)]4(XW9O34)4}n− (X = Si, Ge, P, As; n = 32, 28) and [Ru(bpy)3]2+,98 19 ACS Paragon Plus Environment

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[MnIII3MnIVO3(CH3COO)3(A-α-SiW9O34)]6–

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and

[Ru(bpy)3]2+,99

[Ni25(H2O)2OH]18(CO3)2(PO4)6(SiW9O34)6]50– and [Ru(bpy)3]2+,100 Recently, Streb et al. reported visible-light-responsive photocatalytic water oxidation by tetra-manganese on vanadium oxide cluster [Mn4V4O17(OAc)3]3− and [Ru(bpy)3]2+.101

Figure 3. Development of visible-light-responsive POM photocatalysts by combining POMs and photosensitizers. (a) Photoactivation of a POM by UV light. (b) Photoactivation of a POMphotosensitizer hybrid by visible light.

Figure 4. Structures of visible-light-responsive POMs designed by combining POMs and photosensitizers through covalent bonds.88,93,108


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For the photocatalytic reduction of CO2, sites for CO2 interaction in the catalysts are required. In 1998, Kozik and co-workers reported that mononuclear transition metal-substituted POMs (Co2+, Ni2+, and Mn2+) showed an interaction with CO2 at the substituted metal cations in organic media, which is formed according to spectroscopic analyses using UV–Vis, IR, and 13C NMR spectroscopy.102 Furthermore, Xu and co-workers reported that mononuclear Cosubstituted Keggin-type POMs interact with CO2 in aqueous solution to form the chain-like complexes

(C3H5N2)3(C3H4N2)[PMo11CoO38(CO2)]·4H2O

and

(C3H5N2)4[SiMo11CoO38(CO2)]·4H2O, where CO2 molecules are most likely activated by the μη1,η1-OCO coordination between Co-substituted POMs.103 Moreover, Neumann and co-workers reported that CO2 coordinated to the Ru3+ active site on [Ru(H2O)SiW11O39]5− and that CO2 could be reduced to CO in a toluene solution using trialkylamine as the sacrificial electron donor. However, they used a Xe-lamp, which emits UV light (Scheme 15).104 Furthermore, they developed a visible-light-responsive photocatalytic CO2 reduction system by combining [PW12O40]3−

and

Re+-complex

(Re(L)(CO)3CH3CN–MH[PW12O40];

L

=

15-crown-5-

phenanthroline, M = Na+, H3O+) (Scheme 15).105–107 Only a few photocatalytic systems for functional group transformations that exploit a combination of POMs and photosensitizers have been reported so far. As rare examples, Bonchio and co-workers reported hybrids of divacant γ-Keggin-type lacunary POM and fulleropyrrolidine linked through covalent RSi–O–W bonds (Figure 4). These compounds have been utilized as heterogeneous catalysts for the oxidation of phenol by photoirradiation (λ > 375 nm).108 Furthermore, Kojima and co-workers have recently reported that a hybrid of a Ru-containing POM and a porphyrin linked by electrostatic interactions acted as a supramolecular photocatalysis for the oxidation of alcohols using Na2S2O8 as the oxidant by photoirradiation at 710 nm.109


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Scheme 15. Photocatalytic Reduction of CO2 to CO by Polyoxometalates 3.2. Metal Substitution into POMs For the design of functional inorganic materials, lacunary POMs possessing vacant sites (i.e., coordination sites) are useful as multidentate inorganic ligands that can accommodate various metal cations with controlled numbers and positions. To date, various metal-containing POMs with unique properties, such as catalysis, photocatalysis, and magnetism, have been synthesized by the reactions of lacunary POMs and metal cations in aqueous110–112 or organic media.113–128 In particular, in organic media, the unfavorable isomerization of lacunary POMs can be suppressed, and the reactivity of the lacunary POMs can be controlled. As a result, we have successfully developed the novel synthetic method for various metal-substituted POMs, including lanthanidesubstituted POMs,114–116 silver nanoclusters,117 and giant ring shaped structures (Figure 5a).118 Furthermore, heterometallic clusters can be selectively synthesized by sequential introduction of metal cations into lacunary POMs in organic media (Figure 5b).124–128 Based on these synthetic methods, our group and others have developed visible-light-responsive POMs by engineering the HOMO and/or LUMO of POMs through the introduction of metal cations into the vacant sites (Figure 6). These POMs showed prominent metal-to-POM charge transfer.


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Figure 5. (a) Structures of metal-substituted POMs synthesized in organic media. (b) Sequential synthesis of heterometallic clusters within lacunary POMs in organic media.

Figure 6. Development of visible-light-responsive POM photocatalysts via HOMO and LUMO strategies by introducing metal cations into POMs.


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Hashimoto, Nakamura, and co-workers reported that, by interacting the fully occupied POM [-PW12O40]3− with Ce3+ cations on the solid surface of mesoporous silica, visible-lightresponsive metal-to-POM charge transfer occurred (Figure 7).129,130 This supported catalyst is active for photocatalytic degradation of 2-propanol to CO2, whereby in situ generated Ce4+ and W5+ species work as specific 2-propanol oxidation and O2 reduction sites, respectively. Furthermore, Hill and co-workers reported a pioneering work in the design of visible-lightresponsive POMs by utilizing lacunary POMs. In their work, the dark-red Re+-containing POM [P4W35O124{Re(CO)3}2]16− was synthesized by the reaction of Re+(CO)3(CH3CN)3(BF4) and Dawson-type lacunary POM K10[2-P2W17O61] in aqueous media (Figure 8a).131 DFT calculations revealed that a new HOMO was formed on the Re+ cation, and visible-lightresponsive charge transfer from Re+ to the POM framework occurred.131 Co2+- and Mn+containing POMs have also been shown to exhibit intramolecular charge transfer in response to visible light (Figure 8b).132–134 However, the Mn+-containing POM gradually decomposed upon photoinduced charge transfer. Visible-light-responsive charge transfer in Sn2+-containing POMs, for example, [{Na(μOH2)(OH2)2}6{Sn6(B-SbW9O33)2}2]6–,135 [Sn4(SiW9O34)2]12–,136 and [Sn6(A-α-SiW9O34)2]8–,137 has also been reported (Figure 8c). Feng and co-workers reported photocatalysis by Sn2+containing POMs for hydrogen evolution reactions using Pt nanoparticles as a co-catalyst and methanol as a sacrificial agent.136 Furthermore, Streb and co-workers reported that vanadium oxide clusters,138–142 such as [(Bi(dmso)3)4V13O40]3– and [(Ce(dmso)4)2V11O30Cl], as well as metal-substituted POMs,143–145 such as [VMo5O19]3− and [Mn(H2O)SiW11O39]6−, show visiblelight-responsive photocatalysis for the oxidative degradation of organic dye molecules.

Figure 7. Visible-light-responsive metal-to-POM charge transfer on the solid surface of mesoporous silica.


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Figure 8. Structures of visible-light-responsive POMs that exhibit metal-to-POM charge transfer designed by the HOMO engineering strategy. (a) [P4W35O124{Re(CO)3}2]16−,131 (b) [CoW12O40]6−,132

(c)

[Sn6(SiW9O34)2]8−,137

(d)

[{Ce(H2O)}2{Ce(CH3CN)}2(4-O)(-

SiW10O36)2]6−,146 and (e) [NiMo6O18(OH)6]4−.149 In terms of organic functional group transformations, we have reported visible-lightdriven

photocatalysis

by

Ce3+-containing

the

-Keggin-type

POM

TBA6[{Ce(H2O)}2{Ce(CH3CN)}2(4-O)(-SiW10O36)2] (CePOM), which utilizes intramolecular Ce3+-to-POM charge transfer (Figure 8d).146 CePOM exhibits absorption bands in the visiblelight region (λmax = ca. 460 nm). DFT calculations revealed that a new Ce3+-based HOMO appears above the energy level of the O2−-based occupied orbitals. As a result, the HOMO– LUMO energy gap becomes significantly smaller (2.4 eV) in comparison with that of the divacant lacunary silicotungstate TBA4H4[-SiW10O36] (4.6 eV), which is a precursor for the Ce3+-containing POM (Figure 9).146 These results indicated that the new absorption band in the visible-light region is due to Ce3+-to-POM(W6+) charge transfer. By irradiation with visible light, charge transfer from Ce3+-to-POM(W6+) occurs, and the resulting Ce4+-to-POM(W5+) state accepts electron(s) from the substrates. Thereafter, the reduced POM is readily reoxidized by O2 (Scheme 16). The Ce3+-containing POM exhibits efficient photocatalysis for the oxidative 25 ACS Paragon Plus Environment

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dehydrogenation of primary and secondary amines as well as for the regioselective -cyanation of tertiary amines using O2 (1 atm) as the sole oxidant (Scheme 16).146 In addition, selective oxygenation of various sulfides to sulfoxides also proceeds by irradiation with visible light in the presence of CePOM (Scheme 16).147 This system also exhibits visible-light-responsive photocatalysis of H2 evolution reactions with oxidation of alcohols to aldehydes using Pt as the co-catalyst and alcohols as the hydrogen sources under anaerobic conditions.148

Figure 9. Energy diagrams and frontier orbitals of [-SiW10O36]8− (left) and CePOM (right).

Scheme 16. Photocatalytic Aerobic Oxidations of Organic Substrates by CePOM 26 ACS Paragon Plus Environment

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Very recently, Yu, Han, Wei, and co-workers have also reported that the Ni2+-containing Anderson-type POM [NiMo6O18(OH)6]4− exhibits photocatalysis for oxidative cross-coupling of chlorides with amines, as well as for the oxidation of chlorides using O2 under irradiation with white LED light, affording various imines, aldehydes, and ketones (Scheme 17, Figure 8e).149

Scheme 17. Photocatalytic Oxidative Cross-Coupling of Chlorides with Amines and Oxidation of Chlorides by [NiMo6O18(OH)6]4− However, the above-mentioned HOMO engineering strategy for visible-light-responsive photocatalysis lowers the oxidation potentials of the POM photocatalysts, which is not desirable for the design of oxidation catalysis. An alternative strategy for the design of visible-lightresponsive POM catalysts is lowering the energy levels of the LUMOs (Figure 6). By introducing vanadium atoms into a divacant lacunary POM, the visible-light-responsive divanadium-containing -Keggin-type POM TBA4H[-PV2W10O40] was prepared (Figure 10a).150 DFT calculations indicated that, upon incorporating vanadium atoms into the POM, a new W6+/V5+-based mixed LUMO appears and the HOMO–LUMO energy gap becomes smaller in comparison with that of the parent lacunary POM (Figure 10b). In contrast to the abovementioned metal-to-POM charge transfer system realized by HOMO engineering, this vanadium27 ACS Paragon Plus Environment

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containing POM exhibited visible-light-responsive charge transfer by LUMO engineering, resulting in the higher oxidation potential of the photoexcited states compared with HOMO engineered POMs. This divanadium-containing -Keggin-type POM exhibits high photocatalytic performance for the selective oxygenation of sulfides to sulfoxides using visible light (Figure 10c). It should be noted that the photocatalytic performance for this transformation was significantly superior to that for the Ce3+-containing POM as well as decatungstate. Controlling the LUMO energy levels of POMs has also been achieved by preparing a Ta5d/W5d hybrid LUMO in [P8W60Ta12(H2O)4(OH)8O236]20– and [Ta4O6(SiW9Ta3O40)4]20–, which was utilized for H2 evolution reactions under UV-light irradiation.151

Figure 10. (a) Structure of the visible-light-responsive POM [-PV2W10O40]5− realized by the LUMO engineering strategy. (b) LUMO of [-PV2W10O40]5−. (c) Photocatalytic aerobic oxygenation of sulfides into sulfoxides under visible-light-responsive photocatalysis by [PV2W10O40]5−. 3.3. Coordination of Ligands (Substrates) Attaching photosensitizers at the vacant sites of lacunary POMs using phosphonate and siloxane groups is an important strategy for the design of visible-light-responsive POMs. Several research groups have reviewed these methods for the attachment of functional groups to POMs.152–155 Newton, Oshio, and co-workers developed visible-light-responsive POMs by hybridization of Dawson-type lacunary POMs with electron-withdrawing organophosphonate moieties (Figure 11).156–158 For example, by introducing 4-carboxyophenyl phosphorous moiety into the vacant 28 ACS Paragon Plus Environment

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sites of the lacunary POM, the first redox couple of W6+/W5+ was significantly shifted to positive value (a shift of ca. +0.5 V compared with the parent [P2W17O61]10−), most likely owing to lowering of the LUMO energy level. The UV–Vis absorption of this hybrid presented a bathochromic shift with an absorption band tailing into the visible region of the spectrum. DFT calculations indicated that the HOMO–LUMO gap and LUMO state energies of the hybrid are considerably lower than those of the parent [P2W17O61]10− and [P2W18O62]6−.156 In addition, this hybrid shows the high catalytic performance for photocatalytic oxidative degradation of dye molecules by irradiation with visible light. Furthermore, the electronic structure and photoactivity of the hybrid can be controlled by modification of the grafted organic moieties (Figure 11).157

Figure 11. Development of visible-light-responsive POMs by hybridization of Dawson-type lacunary POMs with electron-withdrawing organophosphonate moieties.

By the reaction of lacunary POMs with alcohols in organic media, alcohols could be incorporated in the vacant sites as alkoxides (W–OR).159,160 For example, by reacting Keggintype trivacant lacunary POM [-SiW9O34]10− and methanol six methoxy groups were introduced in the vacant sites (Figure 12).159 Because the introduced methoxy groups can be easily removed by hydrolysis, the methoxy groups at the vacant sites could act as protecting groups and the methoxide can be utilized as a precursor for the synthesis of metal-substituted POMs. In addition, multidentate alkoxy groups can be introduce by using pentaerythritol (C(CH2OH)4). These multidentate alkoxy groups possessed high resistance against hydrolysis, and the alkoxides could 29 ACS Paragon Plus Environment

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be utilized for the synthesis of dimeric inorganic-organic-inorganic hybrid structures by reacting with [-SiW9O34]10− (Figure 12).159 Notably, incorporating colorless 4-methoxybenzyl alcohol in the vacant sites of lacunary POM [-SiW10O36]8− can allow visible-light-responsive charge transfer from the alcohols to the POM (Figure 13a).160 The incorporated alcohols formed a new HOMO, and the HOMO–LUMO energy gaps became significantly smaller (Eg = 2.8 eV) in comparison with those of the parent lacunary POM (Eg = 4.6 eV) (Figure 13b).160 Upon irradiation of the mixture of lacunary POM [-SiW10O36]8− and alcohols, such as benzyl alcohol and 4-methoxybenzyl alcohol, with visible light in organic media under anaerobic conditions, the pale yellow solution immediately changed into a dark-blue solution, showing a new strong absorption band at around 650 nm in the UV– Vis absorption spectra (Figure 13c).160 This blue color is due to the W6+/W5+ intervalence charge transfer of the two-electron-reduced POM. In the blue solution, formation of benzaldehyde, which is the oxidation product of benzyl alcohol, was observed. These results showed that, upon irradiation with visible light, multi-electron transfer from the coordinating alcohols to the POM occurs at the vacant sites, and the two electrons and two protons can be stored in the POM framework. This visible-light-responsive charge transfer can be utilized for the photocatalytic multielectron reduction of various organic substrates, such as nitroarenes, pyridine N-oxides, aldehydes, and sulfoxides using alcohols, including benzylic alcohols and ethanol, as reducing reagents (Scheme18).147,161–163 It should be noted that other reducible functional groups, such as C=C bonds, C≡C bonds, Cl, ketone groups, and amide groups, remain completely intact in this photocatalytic system.

Figure 12. Schematic of the synthesis of alkoxides of lacunary POMs by reacting with alcohols. 30 ACS Paragon Plus Environment

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Figure 13. (a) Development of visible-light-responsive POM catalysis by in situ coordination of alcohols to the vacant sites of the lacunary POM [-SiW10O36]8−. (b) Energy diagrams and frontier orbitals of the complex formed between [-SiW10O36]8− and 4-methoxybenzyl alcohol. (c) Changes in color and UV–Vis absorption spectra of a solution containing the lacunary POM [-SiW10O36]8− and 4-methoxybenzyl alcohol upon irradiation with visible light. 31 ACS Paragon Plus Environment

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Scheme 18. Photocatalytic Selective Reduction of Organic Substrates by [-SiW10O36]8− using Alcohols as Reducing Agents


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CONCLUSION This review presented a thorough overview of the recent developments in the field of POM photocatalysis as it mainly relates to organic synthesis. On the basis of their unique physical properties, POM photocatalysts present new possibilities for various bond formation reactions in organic synthesis as well as oxidation and reduction reactions. In addition, cooperative catalysis employing POMs combined with metal complexes or organocatalysts has been recently reported, further expanding the scope of POM photocatalysis. The study of cooperative POM photocatalysis is still in its infancy, and it will be further investigated. Furthermore, recent frontier orbital engineering studies have realized visible-light-responsive photocatalysis by POMs. Thus, the precise control of charge transfer properties, redox properties, and photoabsorption properties of POMs by tailoring their structures, constituent elements, and frontier orbitals will present exciting new avenues for POM photocatalysis.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported in part by JSPS KAKENHI Grant Numbers 18H04500, 17H03037, 15H05797 and JST PRESTO. REFERENCES (1) (2)

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Silicotungstate

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Sandwich-Type

Di-

and

Tetra-Cobalt-Containing

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Polyoxometalate Photocatalysis for Liquid-Phase Selective Organic

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