Polyoxometalate Photocatalysis for Liquid-Phase ... - ACS Publications

Subscriber access provided by University of Sunderland

Review

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

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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


Page 2 of 49

Page 3 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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


Page 4 of 49

Page 5 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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.


Page 6 of 49

Page 7 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

Page 8 of 49

Page 9 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

Page 10 of 49

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

Page 11 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 49

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


Page 13 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

R–H


R

activation by decatungstate

O

Page 14 of 49

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

Page 15 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 49

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

Page 17 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

Page 20 of 49

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


Page 21 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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.


Page 22 of 49

Page 23 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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.


Page 24 of 49

Page 25 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

Page 26 of 49

Page 27 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

Page 28 of 49

Page 29 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

Page 30 of 49

Page 31 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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


Page 32 of 49

Page 33 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

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)

Pope, M. T. In Heteropoly and Isopoly Oxometalates; Springer: Berlin, 1983. Hill, C. L.; Prosser-McCartha, C. M. Homogeneous Catalysis by Ttransition Metal Oxygen Anion Clusters. Coord. Chem. Rev. 1995, 143, 407−455. 33 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(3)

Kozhevnikov, I. V. Catalysis by Heteropoly Acids and Multicomponent Polyoxometalates in Liquid-Phase Reactions. Chem. Rev. 1998, 98, 171−198.

(4)

Mizuno, N.; Misono, M. Heterogeneous Catalysis. Chem. Rev. 1998, 98, 199−218.

(5)

Sadakane, M.; Steckhan, E. Electrochemical Properties of Polyoxometalates as Electrocatalysts. Chem. Rev. 1998, 98, 219−238.

(6)

Katsoulis, D. E. A Survey of Applications of Polyoxometalates. Chem. Rev. 1998, 98, 359−388.

(7)

Hill, C. L. In Comprehensive Coordination Chemistry I; McCleverty, J. A., Meyer, T. J., Eds.; Elsevier Pergamon: Amsterdam, 2004; Vol. 4, p. 679.

(8)

Long, D.-L.; Burkholder, E.; Cronin, L. Polyoxometalate Clusters, Nanostructures and Materials: From Self Assembly to Designer Materials and Devices. Chem. Soc. Rev. 2007, 36, 105−121.

(9)

Long, D.-L.; Tsunashima, R.; Cronin, L. Polyoxometalates: Building Blocks for Functional Nanoscale Systems. Angew. Chem., Int. Ed. 2010, 49, 1736−1758.

(10)

Song, Y.-F.; Tsunashima, R. Recent Advances on Polyoxometalate-Based Molecular and Composite Materials. Chem. Soc. Rev. 2012, 41, 7384−7402.

(11)

Lv, H.; Geletii, Y. V.; Zhao, C.; Vickers, J. W.; Zhu, G.; Luo, Z.; Song, J.; Lian, T.; Musaev, D. G.; Hill, C. L. Polyoxometalate Water Oxidation Catalysts and the Production of Green Fuel. Chem. Soc. Rev. 2012, 41, 7572−7589.

(12)

Wang, S.-S.; Yang, G.-Y. Recent Advances in Polyoxometalate-Catalyzed Reactions. Chem. Rev. 2015, 115, 4893−4962.

(13)

Vilà-Nadal, L.; Cronin, L. Design and Synthesis of Polyoxometalate-Framework Materials from Cluster Precursors. Nat. Rev. 2017, 2, 17054.

(14)

Gumerova, N. I.; Rompel, A. Synthesis, Structures and Applications of Electron-Rich Polyoxometalates. Nat. Rev. Chem. 2018, 2, 0112.

(15)

Rindl, M. A Reversible Photochemical Reaction. S. African J. Sci. 1916, 11, 362−366.

(16)

Chalkley, L. The Extent of the Photochemical Reduction of Phosphotungstic Acid. J. Phys. Chem. 1952, 56, 1084−1086.

(17)

Papaconstantinou, E; Dimotikali, D.; Politou, A. Photochemistry of Heteropoly Electrolytes. The 18-Molybdodiphosphate. Inorg. Chim. Acta 1980, 43, 155−158.

(18)

Yamase, T.; Sasaki, R.; Ikawa, T. Photochemical Studies of the Alkylammoniun Molybdates. Part 5. Photolysis in Weak Acid Solutions. J. Chem. Soc., Dalton Trans. 1981, 628−634. 34 ACS Paragon Plus Environment

Page 34 of 49

Page 35 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(19)

Photocatalytic Oxidation of Organic Compounds Using Heteropoly Electrolytes of Molybdenum and Tungsten. J. Chem. Soc., Chem. Commun. 1982, 12−13.

(20)

Hill, C. L.; Brown, R. B. Sustained Epoxidation of Olefins by Oxygen Donors Catalyzed by Transition Metal-Substituted Polyoxometalates, Oxidatively Resistant Inorganic Analogs of Metalloporphyrins. J. Am. Chem. Soc. 1986, 108, 536−538.

(21)

Hill, C. L.; Bouchard, D. A. Catalytic Photochemical Dehydrogenation of Organic Substrates by Polyoxometalates. J. Am. Chem. Soc. 1985, 107, 5148−5157.

(22)

Fox, M. A.; Cardona, R.; Gaillard, E. Photoactivation of Metal Oxide Surfaces: Photocatalyzed Oxidation of Alcohols by Heteropolytungstates. J. Am. Chem. Soc. 1987, 109, 6347−6354.

(23)

Ward, M. D.; Brazdil, J. F.; Grasselli, R. K. Photocatalytic Alcohol Dehydrogenation Using Ammonium Heptamolybdate. J. Phys. Chem. 1984, 88, 4210−4213.

(24)

Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis. Chem. Rev. 2013, 113, 5322−5363.

(25)

Narayanama, J. M. R.; Stephenson, C. R. J. Visible Light Photoredox Catalysis: Applications in Organic Synthesis. Chem. Soc. Rev. 2011, 40, 102−113.

(26)

Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. Photoredox Catalysis in Organic Chemistry. J. Org. Chem. 2016, 81, 6898−6926.

(27)

Romero, N. A.; Nicewicz, D. A. Organic Photoredox Catalysis. Chem. Rev. 2016, 116, 10075−10166.

(28)

Yamase, T. Photo- and Electrochromism of Polyoxometalates and Related Materials. Chem. Rev. 1998, 98, 307−326.

(29)

Streb, C. New trends in Polyoxometalate Photoredox Chemistry: From Photosensitisation to Water Oxidation Catalysis. Dalton Trans. 2012, 41, 1651−1659.

(30)

Cameron, J. M.; Wales, D. J.; Newton, G. N. Shining a Light on the Photo-Sensitisation of Organic–Inorganic Hybrid Polyoxometalates. Dalton Trans. 2018, 47, 5120−5136.

(31)

Marcì, G.; I. Garc í a-L ó pez, E.; Palmisano, L. Heteropolyacid-Based Materials as Heterogeneous Photocatalysts. Eur. J. Inorg. Chem. 2014, 1, 21−35.

(32)

Walsh, J. J.; Bond, A. M.; Forster, R. J.; Keyes, T. E. Hybrid Polyoxometalate Materials for Photo(electro-) Chemical Applications. Coord. Chem. Rev. 2016, 306, 217−234.

(33)

Sivakumar, R.; Thomas, J.; Yoon, M. Polyoxometalate-based Molecular/nano Composites: Advances in Environmental Remediation by Photocatalysis and Biomimetic 35 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Approaches to Solar Energy Conversion. J. Photochem. and Photobiol. C: Photochem. Rev. 2012, 13, 277−298. (34)

Lang, X.; Chen, X.; Zhao, J. Heterogeneous Visible Light Photocatalysis for Selective Organic Transformations. Chem. Soc. Rev. 2014, 43, 473−486.

(35)

Hill, C. L. Introduction of Functionality into Unactivated Carbon-Hydrogen Bonds. Catalytic Generation and Nonconventional Utilization of Organic Radicals. Synlett 1995, 127−132 and references therein.

(36)

Duncan, D. C.; Netzel, T. L.; Hill, C. L. Early-Time Dynamics and Reactivity of Polyoxometalate Excited States. Identification of a Short-Lived LMCT Excited State and a Reactive Long-Lived Charge-Transfer Intermediate following Picosecond Flash Excitation of [W10O32]4– in Acetonitrile. Inorg. Chem. 1995, 34, 4640−4646.

(37)

Tzirakis, M. D.; Lykakis, I. N.; Orfanopoulos, M. Decatungstate as an Efficient Photocatalyst in Organic Chemistry. Chem. Soc. Rev. 2009, 38, 2609−2621.

(38)

Fagnoni, M.; Dondi, D.; Ravelli, D. Albini, A. Photocatalysis for the Formation of the C− C Bond. Chem. Rev. 2007, 107, 2725−2756.

(39)

Ravelli, D.; Protti, S.; Fagnoni, M. Carbon–Carbon Bond Forming Reactions via Photogenerated Intermediates. Chem. Rev. 2016, 116, 9850−9913.

(40)

Ravelli, D.; Fagnoni, M.; Fukuyama, T.; Nishikawa, T.; Ryu, I. Site-Selective C–H Functionalization by Decatungstate Anion Photocatalysis: Synergistic Control by Polar and Steric Effects Expands the Reaction Scope. ACS Catal. 2018, 8, 701−713.

(41)

Yamase, T.; Takabayashi, N.; Kaji, M. Solution Photochemistry of Tetrakis(tetrabutylammonium) Decatungstate(VI) and Catalytic Hydrogen Evolution from Alcohols. J. Chem. Soc., Dalton Trans. 1984, 793−799.

(42)

Yamase, T.; Usami, T. Photocatalytic Dimerization of Olefins by Decatungstate(VI), [W10O32]4–, in Acetonitrile and Magnetic Resonance Studies of Photoreduced Species. J. Chem. Soc., Dalton Trans. 1988, 183−190.

(43)

Ermolenko, L. P.; Giannotti, C.; Delaire, J. A. Laser Flash Photolysis Study of the Mechanism of Photooxidation of Alkanes Catalysed by Decatungstate Anion. J. Chem. Soc., Perkin Trans. 2 1997, 25−30.

(44)

Texier, I.; Delouis, J. F.; Delaire, J. A.; Giannotti, C.; Plaza, P.; Martin, M. M. Dynamics of the First Excited State of the Decatungstate Anion Studied by Subpicosecond Laser Spectroscopy. Chem. Phys. Lett. 1999, 311, 139−145. 36 ACS Paragon Plus Environment

Page 36 of 49

Page 37 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(45)

Tanielian, C. Decatungstate Photocatalysis. Coord. Chem. Rev. 1998, 178−180, 1165 − 1181.

(46)

Tanielian, C.; Cougnon, F.; Seghrouchni, R. Acetone, a Substrate and a New Solvent in Decatungstate Photocatalysis. J. Mol. Catal. A: Chem. 2007, 262, 164−169.

(47)

Duncan, D. C.; Fox, M. A. Early Events in Decatungstate Photocatalyzed Oxidations: A Nanosecond Laser Transient Absorbance Reinvestigation. J. Phys. Chem. A 1998, 102, 4559−4567.

(48)

Texier, I.; Delaire, J. A.; Giannotti, C. Reactivity of the Charge Transfer Excited State of Sodium Decatungstate at the Nanosecond Time Scale. Phys. Chem. Chem. Phys. 2000, 2, 1205−1212.

(49)

Tanielian, C.; Duffy, K.; Jones, A. Kinetic and Mechanistic Aspects of Photocatalysis by Polyoxotungstates: A Laser Flash Photolysis, Pulse Radiolysis, and Continuous Photolysis Study. J. Phys. Chem. B. 1997, 101, 4276−4282.

(50)

Giannotti, C.; Richter, C. Aerobic Photolysis of Saturated C–H Bond with Decatungstate Anion, Mechanism and Products. Trends Photochem. Photobiol. 1997, 4, 43−54.

(51)

Awad, M. K.; Anderson, A. B. Photodimerization of Cyclohexene and Methane by Decatungstate Anions: Molecular Orbital Theory. J. Am. Chem. Soc. 1990, 112, 1603 − 1606.

(52)

Jen, S. F.; Anderson, A. B.; Hill, C. L. Alkane Reactions with Photoactivated Decatungstate in Neutral and Acid Solution: Molecular Orbital Theory. J. Phys. Chem. 1992, 96, 5658−5662.

(53)

De Waele, V.; Poizat, O.; Fagnoni, M.; Bagno, A.; Ravelli, D. Unraveling the Key Features of the Reactive State of Decatungstate Anion in Hydrogen Atom Transfer (HAT) Photocatalysis. ACS Catal. 2016, 6, 7174−7181.

(54)

Ravelli, D.; Protti, S.; Fagnoni, M. Decatungstate Anion for Photocatalyzed “Window Ledge” Reactions. Acc. Chem. Res. 2016, 49, 2232−2242 and references therein.

(55)

Davide, F.; Ravelli, D.; Protti, S.; Fagnoni, M. Decatungstate Photocatalyzed Acylations and Alkylations in Flow via Hydrogen Atom Transfer. Adv. Synth. Catal. 2015, 357, 3687−3695.

(56)

Laudadio, G.; Govaerts, S.; Wang, Y.; Ravelli, D.; Koolman, H. F.; Fagnoni, M.; Djuric, S. W.; Noël, T. Selective C(sp3) − H Aerobic Oxidation Enabled by Decatungstate Photocatalysis in Flow. Angew. Chem., Int. Ed. 2018, 57, 4078−4082. 37 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(57) (58)

Zakrzewski, J.; Chauveu, F.; Giannotti, C. C. R. Acad. Sci. Paris 1989, 309, 809. Maldotti, A.; Amadelli, R.; Carassiti, V.; Molinari, A. Catalytic Oxygenation of Cyclohexane by Photoexcited (nBu4N)4W10O32: the Role of Radicals. Inorg. Chim. Acta 1997, 256, 309−312.

(59)

Lykakis, I. N.; Orfanopoulos, M. Deuterium Kinetic Isotope Effects in Homogeneous Decatungstate Catalyzed Photooxygenation of 1,1-Diphenylethane and 9-Methyl-9Hfluorene: Evidence for a Hydrogen Abstraction Mechanism. Tetrahedron Lett. 2005, 46, 7835−7839.

(60)

Lykakis, I. N.; Tanielian, C.; Seghrouchni, R.; Orfanopoulos, M. Mechanism of Decatungstate Photocatalyzed Oxygenation of Aromatic Alcohols: Part II. Kinetic Isotope Effects Studies. J. Mol. Catal. A: Chem. 2007, 262, 176−184.

(61)

Jaynes, B. S.; Hill, C. L. Selective Ethylation and Vinylation of Alkanes via Polyoxotungstate Photocatalyzed Radical Addition Reactions. J. Am. Chem. Soc. 1993, 115, 12212−12213.

(62)

Jaynes, B. S.; Hill, C. L. Radical Carbonylation of Alkanes via Polyoxotungstate Photocatalysis. J. Am. Chem. Soc. 1995, 117, 4704−4705.

(63)

Zheng, Z.; Hill, C. L. Alkanes to Nitriles and α-Iminoesters. Polyoxotungstate Photocatalytic Radical Chain Initiation. Chem. Commun. 1998, 2467−2468.

(64)

Dondi, D.; Fagnoni, M.; Molinari, A.; Maldotti, A.; Albini, A. Polyoxotungstate Photoinduced Alkylation of Electrophilic Alkenes by Cycloalkanes. Chem. Eur. J. 2004, 10, 142−148.

(65)

Dondi, D.; Fagnoni, M.; Albini, A. Photomediated Synthesis of β-Alkylketones from Cycloalkanes. Tetrahedron 2006, 62, 5527−5535.

(66)

Dondi, D.; Fagnoni, M.; Albini, A. Tetrabutylammonium Decatungstate-Photosensitized Alkylation of Electrophilic Alkenes: Convenient Functionalization of Aliphatic C–H Bonds. Chem. Eur. J. 2006, 12, 4153−4163.

(67)

Ryu, I.; Tani, A.; Fukuyama, T.; Ravelli, D.; Fagnoni, M.; Albini, A. Atom-Economical Synthesis of Unsymmetrical Ketones through Photocatalyzed C–H Activation of Alkanes and Coupling with CO and Electrophilic Alkenes. Angew. Chem., Int. Ed. 2011, 50, 1869 −1872.


Page 38 of 49

Page 39 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(68)

Esposti, S.; Dondi, D.; Fagnoni, M.; Albini, A. Acylation of Electrophilic Olefins through Decatungstate-Photocatalyzed Activation of Aldehydes. Angew. Chem., Int. Ed. 2007, 46, 2531−2534.

(69)

Tzirakis, M. D.; Orfanopoulos, M. Acyl Radical Reactions in Fullerene Chemistry: Direct Acylation of [60]Fullerene through an Efficient Decatungstate-Photomediated Approach. J. Am. Chem. Soc. 2009, 131, 4063−4069.

(70)

Tzirakis, M. D.; Orfanopoulos, M. Photochemical Addition of Ethers to C60: Synthesis of the Simplest [60]Fullerene/Crown Ether Conjugates. Angew. Chem., Int. Ed. 2010, 34, 5891−5893.

(71)

Tzirakis, M. D.; Alberti, M. N.; Orfanopoulos, M. Hydroxyalkylation of [60]Fullerene: Free Radical Addition of Alcohols to C60. Chem. Commun. 2010, 46, 8228−8230.

(72)

Okada, M.; Fukuyama, T.; Yamada, K.; Ryu, I.; Ravelli, D.; Fagnoni, M. Sunlight Photocatalyzed Regioselective β-Alkylation and Acylation of Cyclopentanones. Chem. Sci. 2014, 5, 2893−2898.

(73)

Montanaro, S.; Ravelli, D.; Merli, D.; Fagnoni, M.; Albini, A. Decatungstate As Photoredox Catalyst: Benzylation of Electron-Poor Olefins. Org. Lett. 2012, 14, 4218 − 4221.

(74)

Orareya, H.; Ravelli, D.; Fagnoni, M.; Albini, A. Decatungstate Photocatalyzed Benzylation of Alkenes with Alkylaromatics. Adv. Synth. Catal. 2013, 355, 2891−2899.

(75)

Quattrini, M. C.; Fujii, S.; Yamada, K.; Fukuyama, T.; Ravelli, D.; Fagnonia, M.; Ryu, I. Versatile Cross-Dehydrogenative Coupling of Heteroaromatics and Hydrogen Donors via Decatungstate Photocatalysis. Chem. Commun. 2017, 53, 2335−2338.

(76)

Murphy, J. J.; Bastida, D.; Paria, S.; Fagnoni, M.; Melchiorre, P. Asymmetric Catalytic Formation of Quaternary Carbons by Iminium Ion Trapping of Radicals. Nature 2016, 532, 218−222.

(77)

Perry, I. B.; Brewer, T. F.; Sarver, P. J.; Schultz, D. M.; DiRocco, D. A.; MacMillan, D. W. C. Direct Arylation of Strong Aliphatic C–H Bonds. Nature 2018, 560, 70−75.

(78)

Ryu, I.; Tani, A.; Fukuyama, T.; Ravelli, D.; Montanaro S.; Fagnoni, M. Efficient C– H/C–N and C–H/C–CO–N Conversion via Decatungstate-Photoinduced Alkylation of Diisopropyl Azodicarboxylate. Org. Lett. 2013, 15, 2554−2557.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(79)

Page 40 of 49

Orareya, H.; Dondi, D.; Raveli, D.; Fagnoni, M. Decatungstate-Photocatalyzed Si−H/C− H

Activation

in

Silyl

Hydrides:

Hydrosilylation

of

Electron-Poor

Alkenes.

ChemCatChem 2015, 7, 3350−3357. (80)

Liang, T.; Neumann, C. N.; Ritter, T. Introduction of Fluorine and Fluorine-Containing Functional Groups. Angew Chem., Int. Ed. 2013, 52, 8214−8264.

(81)

Halperin, S. D.; Fan, H.; Chang, S.; Martin, R. E.; Britton, R. A Convenient Photocatalytic Fluorination of Unactivated C-H Bonds. Angew. Chem., Int. Ed. 2014, 53, 4690−4693.

(82)

Benzylic: Nodwell, M. B.; Bagai, A.; Halperin, S. D.; Martin, R. E.; Knust, H.; Britton, R. Direct Photocatalytic Fluorination of Benzylic C–H Bonds with NFluorobenzenesulfonimide. Chem. Commun. 2015, 51, 11783−11786.

(83)

Halperin, S. D.; Kwon, D.; Holmes, M.; Regalado, E. L.; Campeau, L.-C.; DiRocco, D. A.; Britton, R. Development of a Direct Photocatalytic C–H Fluorination for the Preparative Synthesis of Odanacatib. Org. Lett. 2015, 17, 5200−5203.

(84)

Nodwell, M. B.; Yang, H.; Čolović, M.; Yuan, Z.; Merkens, H.; Martin, R. E.; Benard, F.; Schaffer, P.; Britton, R. 18F-Fluorination of Unactivated C–H Bonds in Branched Aliphatic Amino Acids: Direct Synthesis of Oncological Positron Emission Tomography Imaging Agents. J. Am. Chem. Soc. 2017, 139, 3595−3598.

(85)

Yuan, Z.; Nodwell, M. B.; Yang, H.; Malik, N.; Merkens, H.; Bénard, F.; Martin, R. E.; Schaffer, P.; Britton, R. Site-Selective, Late-Stage C−H 18F-Fluorination on Unprotected Peptides for Positron Emission Tomography Imaging. Angew. Chem., Int. Ed. 2018, 57, 12733−12736.

(86)

West, J. G.; Huang, D.; Sorensen, E. J. Acceptorless Dehydrogenation of Small Molecules through Cooperative Base Metal Catalysis. Nat. Commun. 2015, 6, 10093.

(87)

Liu, X.; Li, Y.; Peng, S.; Lu, G.; Li, S. Photocatalytic Hydrogen Evolution under Visible Light Irradiation by the Polyoxometalate α-[AlSiW11(H2O)O39]5–-Eosin Y System. Int. J. Hydrogen Energy 2012, 37, 12150−12157.

(88)

Matt, B.; Fize, J.; Moussa, J.; Amouri, H.; Pereira, A.; Artero, V.; Izzet, G.; Proust, A. Charge Photo-Accumulation and Photocatalytic Hydrogen Evolution under Visible Light at an Iridium(III)-Photosensitized Polyoxotungstate. Energy Environ. Sci. 2013, 6, 1504− 1508.


Page 41 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(89)

Lv, H.; Song, J.; Zhu, H.; Geletii, Y. V.; Bacsa, J.; Zhao, C.; Lian, T.; Musaev, D. G.; Hill, C. L. Visible-Light-Driven Hydrogen Evolution from Water Using a Noble-MetalFree Polyoxometalate Catalyst. J. Catal. 2013, 307, 48−54.

(90)

Lv, H.; Guo, W.; Wu, K.; Chen, Z.; Bacsa, J.; Musaev, D. G.; Geletii, Y. V.; Lauinger, S. M.; Lian, T.; Hill, C. L. A Noble-Metal-Free, Tetra-nickel Polyoxotungstate Catalyst for Efficient Photocatalytic Hydrogen Evolution. J. Am. Chem. Soc. 2014, 136, 14015−14018.

(91)

Zhao, J.; Ding, Y.; Wei, J.; Du, X.; Yu, Y.; Han, R. A Molecular Keggin Polyoxometalate Catalyst with High Efficiency for Visible-Light Driven Hydrogen Evolution. Int. J. Hydrogen Energy 2014, 39, 18908−18918.

(92)

Zhang, Z.-M.; Zhang, T.; Wang, C.; Lin, Z.; Long, L.-S.; Lin, W. Photosensitizing Metal–Organic Framework Enabling Visible-Light-Driven Proton Reduction by a Wells– Dawson-Type Polyoxometalate. J. Am. Chem. Soc. 2015, 137, 3197−3220.

(93)

Schönweiz, S.; Heiland, M.; Anjass, M.; Jacob, T.; Rau S.; Streb, C. Experimental and Theoretical Investigation of the Light-Driven Hydrogen Evolution by Polyoxometalate– Photosensitizer Dyads. Chem. Eur. J. 2017, 23, 15370−15376.

(94)

Geletii, Y. V.; Huang, Z.; Hou, Y.; Musaev, D. G.; Lian, T.; Hill, C. L. Homogeneous Light-Driven Water Oxidation Catalyzed by a Tetraruthenium Complex with All Inorganic Ligands. J. Am. Chem. Soc. 2009, 131, 7522−7253.

(95)

Yin, Q. S.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L. A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals. Science 2010, 328, 342−345.

(96)

Tanaka, S.; Annaka, M.; Sakai, K. Visible Light-Induced Water Oxidation Catalyzed by Molybdenum-based Polyoxometalates with Mono- and Dicobalt(III) Cores as OxygenEvolving Centers. Chem. Commun. 2012, 48, 1653−1655.

(97)

Lv, H.; Song, J.; Geletii, Y. V.; Vickers, J. W.; Sumliner, J. M.; Musaev, D. G.; Kögerler, P.; Zhuk, P. F.; Bacsa, J.; Zhu, G.; Hill, C. L. An Exceptionally Fast Homogeneous Carbon-Free Cobalt-Based Water Oxidation Catalyst. J. Am. Chem. Soc. 2014, 136, 9268 −9271.

(98)

Han, X.-B.; Zhang, Z.-M.; Zhang, T.; Li, Y.-G.; Lin, W.; You, W.; Su, Z.-M.; Wang, E.B. Polyoxometalate-Based Cobalt–Phosphate Molecular Catalysts for Visible LightDriven Water Oxidation. J. Am. Chem. Soc. 2014, 136, 5359−5366.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(99)

Al-Oweini, R.; Sartorel, A.; Bassil, B. S.; Natali, M.; Berardi, S.; Scandola, F.; Kortz, U.; Bonchio, M. Photocatalytic Water Oxidation by a Mixed-Valent MnIII3MnIVO3 Manganese Oxo Core that Mimics the Natural Oxygen-Evolving Center. Angew. Chem., Int. Ed. 2014, 53, 11182−11185.

(100) Han, X.-B.; Li, Y.-G.; Zhang, Z.-M.; Tan, H.-Q.; Lu, Y.; Wang, E.-B. PolyoxometalateBased Nickel Clusters as Visible Light-Driven Water Oxidation Catalysts. J. Am. Chem. Soc. 2015, 137, 5486−5493. (101) Schwarz, B.; Forster, J.; Goetz, M. K.; Yücel, D.; Berger, C.; Jacob, T.; Streb, C. VisibleLight-Driven Water Oxidation by a Molecular Manganese Vanadium Oxide Cluster. Angew. Chem., Int. Ed. 2016, 55, 6329−6333. (102) Szczepankiewicz, S. H.; Ippolito, C. M.; Santora, B. P.; Van de Ven, T. J.; Ippolito, G. A.; Fronckowiak, L.; Wiatrowski, F.; Power, T.; Kozik, M. Interaction of Carbon Dioxide with Transition-Metal-Substituted Heteropolyanions in Nonpolar Solvents. Spectroscopic Evidence for Complex Formation. Inorg. Chem. 1998, 37, 4344−4352. (103) Gao, G.; Li, F.; Xu, Lin; Liu, X.; Yang, Y. CO2 Coordination by Inorganic Polyoxoanion in Water. J. Am. Chem. Soc. 2008, 130, 10838−10839. (104) Khenkin, A. M.; Efremenko, I.; Weiner, L.; Martin, J. M. L.; Neumann, R. Photochemical Reduction of Carbon Dioxide Catalyzed by a Ruthenium-Substituted Polyoxometalate. Chem. Eur. J. 2010, 16, 1356−1364. (105) Ettedgui, J.; Diskin-Posner, Y.; Weiner, L.; Neumann, R. Photoreduction of Carbon Dioxide to Carbon Monoxide with Hydrogen Catalyzed by a Rhenium(I) Phenanthroline− Polyoxometalate Hybrid Complex. J. Am. Chem. Soc. 2011, 133, 188−190. (106) Ci, C.; Carbó, J. J.; Neumann, R.; Graaf, C.; Poblet J. M. Photoreduction Mechanism of CO2 to CO Catalyzed by a Rhenium(I)–Polyoxometalate Hybrid Compound. ACS Catal. 2016, 6, 6422−6428. (107) Haviv, E.; Shimon, L. J. W.; Neumann, R. Photochemical Reduction of CO2 with Visible Light Using a Polyoxometalate as Photoreductant. Chem. Eur. J. 2017, 23, 92−95. (108) Bonchio, M.; Carraro, M.; Scorrano, G.; Bagno, A. Photooxidation in Water by New Hybrid Molecular Photocatalysts Integrating an Organic Sensitizer with a Polyoxometalate Core. Adv. Synth. Catal. 2004, 346, 648−654. (109) Ishizuka, T.; Ohkawa, S.; Ochiai, H.; Hashimoto, M.; Ohkubo, K.; Kotani, H.; Sadakane, M.; Fukuzumi, S.; Kojima, T. A Supramolecular Photocatalyst Composed of a


Page 42 of 49

Page 43 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Polyoxometalate and a Photosensitizing Water-Soluble Porphyrin Diacid for the Oxidation of Organic Substrates in Water. Green Chem. 2018, 20, 1975−1980. (110) Zheng, S.-T.; Yang, G.-Y. Recent Advances in Paramagnetic-TM-Substituted Polyoxometalates (TM = Mn, Fe, Co, Ni, Cu). Chem. Soc. Rev. 2012, 41, 7623−7646. (111) Oms, O.; Dolbecq, A.; Mialane, P. Diversity in Structures and Properties of 3dIncorporating Polyoxotungstates. Chem. Soc. Rev. 2012, 41, 7497−7536. (112) Putaj, P.; Lefebvre, F. Polyoxometalates Containing Late Transition and Noble Metal Atoms. Coord. Chem. Rev. 2011, 255, 1642−1685. (113) Kikukawa, Y.; Yamaguchi, K.; Mizuno, N. Zinc(II) Containing γ-Keggin Sandwich-Type Silicotungstate: Synthesis in Organic Media and Oxidation Catalysis. Angew. Chem., Int. Ed. 2010, 49, 6096−6100. (114) Kikukawa, Y.; Suzuki, K.; Sugawa, M.; Hirno, T.; Kamata, K.; Yamaguchi, K.; Mizuno, N. Cyanosilylation of Carbonyl Compounds with Trimethylsilyl Cyanide Catalyzed by an Yttrium‐Pillared Silicotungstate Dimer. Angew. Chem., Int. Ed. 2012, 51, 3686−3690. (115) Suzuki, K.; Sugawa, M.; Kikukawa, Y.; Kamata, K.; Yamaguchi, K.; Mizuno, N. Strategic Design and Refinement of Lewis Acid-Base Catalysis by Rare-Earth-MetalContaining Polyoxometalates. Inorg. Chem. 2012, 51, 6953−6961. (116) Suzuki, K.; Sato, R.; Mizuno, N. Reversible Switching of Single-Molecule Magnet Behaviors by Transformation of Dinuclear Dysprosium Cores in Polyoxometalates. Chem. Sci. 2013, 4, 596−600. (117) Kikukawa, Y.; Kuroda, Y.; Suzuki, K.; Hibino, M.; Yamaguchi, K.; Mizuno, N. A Discrete Octahedrally Shaped [Ag6]4+ Cluster Encapsulated within Silicotungstate Ligands. Chem. Commun. 2013, 49, 376−378. (118) Minato, T.; Suzuki, K.; Yamaguchi, K.; Mizuno, N. Synthesis and Disassembly/Reassembly of Giant Ring-Shaped Polyoxotungstate Oligomers. Angew. Chem., Int. Ed. 2016, 55, 9630−9633. (119) Suzuki, K.; Shinoe, M.; Mizuno, N. Synthesis and Reversible Transformation of CunBridged (n = 1, 2, or 4) Silicodecatungstate Dimers. Inorg. Chem. 2012, 51, 11574 − 11581. (120) Suzuki, K.; Kikukawa, Y.; Uchida, S.; Tokoro, H.; Imoto, K.; Ohkoshi, S.; Mizuno, N. Three-Dimensional Ordered Arrays of 58×58×58 Å3 Hollow Frameworks in Ionic


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 49

Crystals of M2Zn2-Substituted Polyoxometalates. Angew. Chem., Int. Ed. 2012, 51, 1597− 1601. (121) Kikukawa, Y.; Suzuki, K.; Yamaguchi, K.; Mizuno, N. Synthesis, Structure Characterization, and Reversible Transformation of a Cobalt Salt of a Dilacunary γKeggin

Silicotungstate

and

Sandwich-Type

Di-

and

Tetra-Cobalt-Containing

Silicotungstate Dimers. Inorg. Chem. 2013, 52, 8644−8652. (122) Kuriyama, Y.; Kikukawa, Y.; Suzuki, K.; Yamaguchi, K.; Mizuno, N. Water- and Temperature-Triggered Reversible Structural Transformation of Tetranuclear Cobalt(II) Cores Sandwiched by Polyoxometalates. Chem. Eur. J. 2016, 22, 3962−3966. (123) Hanaya, T.; Suzuki, K.; Sato, R.; Yamaguchi, K.; Mizuno, N. Creation of Bismuth– Tungsten Oxide Nanoclusters using Lacunary Polyoxometalates. Dalton Trans. 2017, 46, 7384−7387. (124) Sato, R.; Suzuki, K.; Sugawa, M.; Mizuno, N. Heterodinuclear Lanthanoid-Containing Polyoxometalates: Stepwise Synthesis and Single-Molecule Magnet Behavior. Chem. Eur. J. 2013, 19, 12982−12990. (125) Sato, R.; Suzuki, K.; Minato, T.; Shinoe, M.; Yamaguchi, K.; Mizuno, N. Field-Induced Slow Magnetic Relaxation of Octahedrally Coordinated Mononuclear Fe(III)-, Co(II)-, and Mn(III)-Containing Polyoxometalates. Chem. Commun. 2015, 51, 4081−4084. (126) Suzuki, K.; Sato, R.; Minato, T.; Shinoe, M.; Yamaguchi, K.; Mizuno, N. A Cascade Approach to Hetero-Pentanuclear Manganese-Oxide Clusters in Polyoxometalates and their Single-Molecule Magnet Properties. Dalton Trans. 2015, 44, 14220−14226. (127) Suzuki, K.; Sato, R.; Minato, T.; Yamaguchi, K.; Mizuno, N. Sequential Synthesis of 3d3d’-4f Heterometallic Heptanuclear Clusters in between Lacunary Polyoxometalates. Inorg. Chem. 2016, 55, 2023−2029. (128) Minato, T.; Suzuki, K.; Ohata, Y.; Yamaguchi, K.; Mizuno, N. A Modular Synthesis Approach to Multinuclear Heterometallic Oxo Clusters in Polyoxometalates. Chem. Commun. 2017, 53, 7533−7536. (129) Takashima, T.; Nakamura, R.; Hashimoto, K. Visible Light Sensitive Metal Oxide Nanocluster Photocatalysts: Photo-Induced Charge Transfer from Ce(III) to Keggin-Type Polyoxotungstates. J. Phys. Chem. C 2009, 113, 17247−17253. (130) Takashima, T.; Yamaguchi, A.; Hashimoto, K.; Nakamura, R. Multielectron-Transfer Reactions at Single Cu(II) Centers Embedded in Polyoxotungstates Driven by Photo-


Page 45 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Induced Metal-to-Metal Charge Transfer from Anchored Ce(III) to Framework W(VI). Chem. Commun. 2012, 48, 2964−2966. (131) Zhao, C.; Huang, Z.; Rodríguez-Córdoba, W.; Kambara, C. S.; O’Halloran, K. P.; Hardcastle, K. I.; Musaev, D. G.; Lian, T.; Hill, C. L. Synthesis and Characterization of a Metal-to-Polyoxometalate Charge Transfer Molecular Chromophore. J. Am. Chem. Soc. 2011, 133, 20134−20137. (132) Glass, E. N.; Fielden, J.; Kaledin, A. L.; Musaev, D. G.; Lian, T.; Hill, C. L. Extending Metal-to-Polyoxometalate Charge Transfer Lifetimes: The Effect of Heterometal Location. Chem. Eur. J. 2014, 20, 4297−4307. (133) Zhao, C.; Kambara, C. S.; Yang, Y.; Kaledin, A. L.; Musaev, D. G.; Lian, T.; Hill, C. L. Synthesis, Structures, and Photochemistry of Tricarbonyl Metal Polyoxoanion Complexes, [X2W20O70{M(CO)3}2]12– (X = Sb, Bi and M = Re, Mn). Inorg. Chem, 2013, 52, 671−678. (134) Glass, E. N.; Fielden, J.; Huang, Z.; Xiang, X.; Musaev, D. G.; Lian, T.; Hill, C. L. Transition Metal Substitution Effects on Metal-to-Polyoxometalate Charge Transfer. Inorg. Chem. 2016, 55, 4308−4319. (135) Zhao, C.; Glass, E. N.; Chica, B.; Musaev, D. G.; Sumliner, J. M.; Dyer, R. B.; Lian, T.; Hill, C. L. All-Inorganic Networks and Tetramer Based on Tin(II)-Containing Polyoxometalates: Tuning Structural and Spectral Properties with Lone-Pairs. J. Am. Chem. Soc. 2014, 136, 12085−12091. (136) Zhang, Z.; Lin, Q.; Zheng, S.-T.; Bu, X.; Feng, P. A Novel Sandwich-Type Polyoxometalate Compound with Visible-Light Photocatalytic H2 Evolution Activity. Chem. Commun. 2011, 47, 3918−3920. (137) Suzuki, K.; Hanaya, T.; Sato, R.; Minato, T.; Yamaguchi, K.; Mizuno, N. Hexanuclear Tin(II) and Mixed Valence Tin(II,IV) Oxide Clusters within Polyoxometalates. Chem. Commun. 2016, 52, 10688−10691. (138) Forster, J.; Rösner, B.; Khusnyarov, M. M.; Streb, C. Tuning the Light Absorption of a Molecular Vanadium Oxide System for Enhanced Photooxidation Performance. Chem. Commun. 2011, 47, 3114−3116. (139) Seliverstov, A.; Streb, C. Chirality Meets Visible-Light Photocatalysis in a Molecular Cerium Vanadium Oxide Cluster. Chem. Commun. 2014, 50, 1827−1829.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(140) Tucher, J.; Nye, L. C.; Ivanovic-Burmazovic, I.; Notarnicola, A.; Streb, C. Chemical and Photochemical Functionality of the First Molecular Bismuth Vanadium Oxide. Chem. Eur. J. 2012, 18, 10949−10953. (141) Seliverstov, A.; Streb, C. A New Class of Homogeneous Visible-Light Photocatalysts: Molecular Cerium Vanadium Oxide Clusters. Chem. Eur. J. 2014, 20, 9733−9738. (142) Tucher, J.; Peuntinger, K.; Margraf, J. T.; Clark, T.; Guldi, D. M.; Streb, C. TemplateDependent Photochemical Reactivity of Molecular Metal Oxides. Chem. Eur. J. 2015, 21, 8716−8719. (143) Tucher, J.; Wu, Y.; Nye, L. C.; Ivanovic-Burmazovic, I.; Khusniyarov, M. M.; Streb, C. Metal Substitution in a Lindqvist Polyoxometalate Leads to Improved Photocatalytic Performance. Dalton Trans. 2012, 41, 9938−9943. (144) Tucher, J.; Schlicht, S.; Kollhoff, F.; Streb, C. Photocatalytic Reactivity Tuning by Heterometal and Addenda Metal Variation in Lindqvist Polyoxometalates. Dalton Trans. 2014, 43, 17029−17033. (145) Dave, M.; Streb, C. Oxidative Photoreactivity of Mono-Transition-Metal Functionalized Lacunary Keggin Anions. Dalton Trans. 2015, 44, 18919−18922. (146) Suzuki, K.; Tang, F.; Kikukawa, Y.; Yamaguchi, K.; Mizuno, N. Visible-Light-Induced Photoredox Catalysis with a Tetracerium-Containing Silicotungstate. Angew. Chem., Int. Ed. 2014, 53, 5356−5360. (147) Suzuki, K.; Jeong, J.; Yamaguchi, K.; Mizuno, N. Photoredox Catalysis for Oxygenation/Deoxygenation between Sulfides and Sulfoxides by Visible-LightResponsive Polyoxometalates. New J. Chem. 2016, 40, 1014−1021. (148) Suzuki, K.; Tang, F.; Kikukawa, Y.; Yamaguchi, K.; Mizuno, N. Hydrogen Evolution Using the Visible-light-induced Metal-to-polyoxometalate Multiple Electron Transfer. Chem. Lett. 2014, 43, 1429−1431. (149) Yu, H.; Wang, J.; Zhai, Y.; Zhang, M.; Ru, S.; Han, S.; Wei, Y. Visible-Light-Driven Photocatalytic Oxidation of Organic Chlorides Using Air and an Inorganic-Ligand Supported Nickel-Catalyst Without Photosensitizers. ChemCatChem 2018, DOI: 10.1002/cctc.201800629. (150) Li, C.; Suzuki, K.; Mizuno, N.; Yamaguchi, K. Polyoxometalate LUMO Engineering: a Strategy for Visible-Light-Responsive Aerobic Oxygenation Photocatalysts. Chem. Commun. 2018, 54, 7127−7130.


Page 46 of 49

Page 47 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

(151) Li, S.; Liu, S.; Liu, S.; Liu, Y.; Tang, Q.; Shi, Z.; Ouyang, S.; Ye, J. {Ta12}/{Ta16} Cluster-Containing Polytantalotungstates with Remarkable Photocatalytic H2 Evolution Activity. J. Am. Chem. Soc. 2012, 134, 19716−19721. (152) Gouzerh, P; Proust, A. Main-Group Element, Organic, and Organometallic Derivatives of Polyoxometalates. Chem. Rev. 1998, 98, 77−112. (153) Proust, A.; Thouvenot, R.; Gouzerh, P. Functionalization of Polyoxometalates: Towards Advanced Applications in Catalysis and Materials Science. Chem. Commun. 2008, 1837− 1852. (154) Dolbecq, A.; Dumas, E.; Mayer, C. R.; Mialane, P. Hybrid Organic−Inorganic Polyoxometalate Compounds: From Structural Diversity to Applications. Chem. Rev. 2010, 110, 6009−6048. (155) Proust, A.; Matt, B.; Villanncau, R.; Guillemot, G.; Gouzerh, P.; Izzet, G. Functionalization and Post-Functionalization: A Step Towards Polyoxometalate-Based Materials. Chem. Soc. Rev. 2012, 41, 7605−7622. (156) Cameron, J. M.; Fujimoto, S.; Kastner, K.; Wei, R.-J.; Robinson, D.; Sans, V.; Newton, G. N.; Oshio, H. Orbital Engineering: Photoactivation of an Organofunctionalized Polyoxotungstate. Chem. Eur. J. 2017, 23, 47−50. (157) Fujimoto, S.; Cameron, J. M.; Wei, R.-J.; Kastner, K.; Robinson, D.; Sans, V.; Newton, G. N.; Oshio, H. A Simple Approach to the Visible-Light Photoactivation of Molecular Metal Oxides. Inorg. Chem. 2017, 56, 12169−12177. (158) Cameron, J. M.; Fujimoto, S.; Wei, R.-J.; Newton, G. N.; Oshio, H. PostFunctionalization of a Photoactive Hybrid Polyoxotungstate. Dalton Trans. 2018, 47, 10590−10594. (159) Minato, T.; Suzuki, K.; Yamaguchi, K.; Mizuno, N. Alkoxides of Trivacant Lacunary Polyoxometalates. Chem. Eur. J. 2017, 23, 14213−14220. (160) Suzuki, K.; Jeong, J.; Yamaguchi, K.; Mizuno, N. Visible-Light-Responsive Multielectron Redox Catalysis of Lacunary Polyoxometalates Induced by Substrate Coordination to Their Lacuna. Chem. Asian J. 2015, 10, 144−148. (161) Jeong, J.; Suzuki, K.; Yamaguchi, K.; Mizuno, N. Visible-Light-Responsive Catalysis of a Zinc-Introduced Lacunary Disilicoicosatungstate for the Deoxygenation of Pyridine NOxides. New J. Chem. 2017, 41, 13226−13229.


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(162) Jeong, J.; Suzuki, K.; Hibino, M.; Yamaguchi, K.; Mizuno, N. Selective Deoxygenation of Pyridine N-Oxides through Photoredox Catalysis of a Dilacunary Silicotungstate. ChemistrySelect 2016, 1, 5042−5048. (163) Suzuki, K.; Yamaguchi, K.; Mizuno, N. Photoredox Catalysis of Visible-lightResponsive Divacant Lacunary Silicotungstate for Selective Reduction of Aldehydes. Chem. Lett. 2017, 46, 1379−1382.


Page 48 of 49

Page 49 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Table of Contents


Polyoxometalate Photocatalysis for Liquid-Phase ... - ACS Publications

Recommend Documents