Decatungstate Anion for Photocatalyzed âWindow Ledgeâ Reactions

Article pubs.acs.org/accounts

Decatungstate Anion for Photocatalyzed “Window Ledge” Reactions Davide Ravelli, Stefano Protti, and Maurizio Fagnoni* PhotoGreen Lab, Department of Chemistry, University of Pavia, Viale Taramelli 12, 27100 Pavia, Italy CONSPECTUS: The majority of organic reactions are commonly carried out inside a lab, under a fume hood. A particular case is that of photochemical reactions, a field where the pioneering experiments by Giacomo Ciamician demonstrated more than one century ago that different processes can be carried out outdoors, for example, on the balcony of his own department, upon exposure of the reacting mixtures to sunlight. The main problem related to this chemistry of the “window ledge” is that most organic compounds are colorless and their absorption in the solar light region is in most cases negligible. Recently, the impressive development in the use of visible light absorbing photocatalysts (e.g., RuII or IrIII complexes, as well as organic dyes) made light-induced processes convenient even for non-photochemistry practitioners. It is thus possible to easily perform the reactions by simply placing the reaction vessel in a sunny place outside the lab. However, most of these processes are based on single electron transfer (SET) reactions (photoredox catalysis). Other photocatalysts able to activate substrates via alternative paths, such as hydrogen atom transfer (HAT), are emerging. In the last years, we were deeply involved in the use of the decatungstate anion ([W10O32]4−, a polyoxometalate) in synthesis. Indeed, such a versatile species is able to promote the photocatalytic C−H activation of organic compounds via either SET or HAT reactions. Interestingly, though the absorption spectrum of [W10O32]4− does not extend into the visible region, it shows an overlap with solar light emission. In this Account, we provide an overview on the application of decatungstate salts as photocatalysts in window ledge chemistry. We initially discuss the nature of the photogenerated species involved in the mechanism of action of the anion, also supported by theoretical simulations. The first-formed excited state of the decatungstate anion decays rapidly to the active species, a dark state tagged wO, featuring the presence of electron-deficient oxygen centers. Next, we describe the main applications of decatungstate chemistry. A significant part of this Account is devoted to photocatalyzed synthesis (C−X bond formation, with X = C, N, O, and oxidations) carried out by adopting sunlight (or simulated solar light). This synthetic approach is versatile, and most of the reactions involved C−H activation in cycloalkanes, alkylaromatics, amides, ethers (1,4-dioxane, oxetane, benzodioxole, and THF), aldehydes, nitriles, and cyclopentanones, and the ensuing addition of the resulting radicals onto electron-deficient olefins. Finally, the increasing role of the decatungstate anion in water depollution and polymerization is briefly discussed.

1. INTRODUCTION In early days of photochemistry, the “window ledge” was the ideal place to carry out experiments. The first synthetic protocol was described in 1812 by John Davy, who exposed to sunlight a vessel containing a gaseous mixture of chlorine and carbon monoxide. 1 The obtained product was called “phosgene”, from the combination of the Greek words phos (light) and genesis. In the 20th century, Ciamician and Paternò2 and, after World War I, Schönberg3 performed pioneering experiments demonstrating the potential of window ledge chemistry (Figure 1). In particular, the Italian chemists Ciamician and Paternò were both fascinated by the chance of combining the peculiar advantages of a photochemical approach with the use of a free and renewable energy source, such as sunlight. Indeed, in a photochemical reaction, the photon alone is responsible for the activation of one of the reactants, and reactive intermediates are thus generated in situ without the intervention of high temperatures or aggressive chemicals.4 Light leaves no traces at the end of the reaction, © 2016 American Chemical Society

thus simplifying the workup and the purification steps, with a minimization of the produced waste.4,5 The main drawback of this approach is the discontinuous nature of solar light, which is dependent on both weather conditions and the geographic position. At the end of the 20th century, innovative solar reactors able to concentrate solar light for either “gram-per-day” or “kilogram-per-year” reactions were devised.6 Compound parabolic collectors (CPCs, such as the PROPHIS loop in Figure 2a), parabolic dish reactors (Figure 2b), or SOLFIN (solar synthesis of fine chemicals) apparatus enabled the concentration of direct sunlight (up to 5000 suns) to perform photochemical reactions on multi-kilogram scale. The SOLFIN reactor (concentration factor = factor up to 4 suns, volume up to 25 L) was exploited by our group for the gram-scale synthesis of the building block terebic acid (83% yield, Figure 2c).7 Received: July 1, 2016 Published: September 20, 2016 2232

DOI: 10.1021/acs.accounts.6b00339 Acc. Chem. Res. 2016, 49, 2232−2242

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Accounts of Chemical Research

undesirable side reactions.8 A valuable approach is photocatalysis, where the photocatalyst alone is responsible for light absorption and for the activation of a reactant through a chemical reaction.9 The past decade has seen an impressive development of synthetic protocols involving catalysts able to absorb into the visible region, such as iridium and ruthenium complexes or organic dyes.10 This approach is mainly based on the peculiar behavior of the excited catalyst, able to act either as a stronger oxidant or as a stronger reductant than its ground state.10 The key advantage of using visible light is obviously the large availability of glassware and light sources that can be employed for performing the reaction. Vials, tubes, or flasks made of Pyrex can be employed for carrying out batch reactions by using inexpensive compact fluorescent lamps (CFLs) or almost monochromatic light emitting diodes (LEDs) as the light sources. Visible light represents ca. 44% of the overall solar energy, and sunlight may become the elective light source in the future, making the window ledge the ideal place to carry out a photochemical reaction.8 Figure 3 collects some representative cases of “window ledge” chemistry. The first example is related to the use of RuII(bpy)32+, able to photocatalyze the oxidation of benzyl halides to aromatic ketones (Figure 3a)11 or the [2 + 2] cycloaddition of acyclic enones (Figure 3b).12 The second is the gold-photocatalyzed α-alkynylation of tertiary aliphatic amines by iodoalkynes, where a compact disc was used to “maximize” the amount of sunlight reaching the reaction

Figure 1. Early example of window ledge chemistry made by G. Ciamician (digitalization of an original gelatin silver print earlier than 1920). By courtesy of the Photochemical Nanosciences Laboratory of the Chemistry Department "Giacomo Ciamician".

The development of protocols for the solar synthesis of complex organic molecules, however, is not a trivial issue.8 Only a few organic molecules exhibit a good absorption within the sun emission spectrum. In most cases, they absorb in the UV region (that represents only 3% of the emission energy reaching the Earth), where the high energy of photons may induce

Figure 2. (a) PROPHIS reactor (DLR, Cologne, Germany). Reproduced with permission from ref 6c. Copyright 2016 American Chemical Society. (b) Laboratory scale parabolic dish solar concentrator (Ege University, Turkey). Reproduced with permission from ref 6c. Copyright 2016 American Chemical Society. (c) SOLFIN apparatus (Plataforma Solar de Almeria, Spain). Reproduced with permission from ref 7. Copyright 2009 the Royal Society of Chemistry. 2233


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Figure 3. Some examples of window ledge chemistry. Panel a reproduced with permission from ref 11. Copyright 2011 American Chemical Society. Panel b reproduced with permission from ref 12. Copyright 2009 American Chemical Society. Panel c reproduced with permission from ref 13. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA. Panel d by courtesy of Prof. P. Melchiorre (University of Tarragona, Spain).

mixture (Figure 3c).13 The electron transfer may also occur within a colored electron donor−acceptor (EDA) complex upon irradiation with sunlight, as demonstrated in the stereoselective α-alkylation of aldehydes with phenacyl bromides (in the presence of a chiral organocatalyst, Figure 3d).14 In all cases, the desired product was prepared by simply exposing the (stirred) reaction mixture in a sunny place outside the laboratory. However, in recent years, there has been increasing attention to the development of photocatalysts for the direct activation of substrates through a hydrogen atom transfer (HAT) process.15 Only few classes of photocatalysts have been successfully tested, namely, aromatic ketones and, more recently, polyoxometalates (POMs).9,15 The latter compounds are early transition metal (V, Nb, Ta, Mo, W) oxygen-anion clusters (a variety of heteroatoms such as P, As, and Si can be also included) that found wide application in synthesis or for water oxidation.16 Only some POMs, and in particular tungsten-based POMs, however, are photoactive. In the last years, we focused our attention on the use of decatungstate ([W10O32]4−; Figure 4) salts. Early studies demonstrated the capability of this cluster to activate C−H bonds in a variety of organic derivatives, including alcohols17 and alkanes,18 resulting in the formation of radical species.

Figure 4. Structure of the decatungstate anion (W atoms are in blue, O atoms in red).

An interesting example was the photocatalyzed hydroperoxidation of isobutane at room temperature, where the efficiency of decatungstate was found superior with respect to other radical initiators (Scheme 1a).19 The photogenerated radical from cyclohexane was also employed for C−C bond formation in ethylations, vinylations, and carbonylations (Scheme 1b). The corresponding adducts were obtained with high selectivities but at low conversion degrees (2−5%), since undesired side pathways became significant upon prolonged 2234


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Accounts of Chemical Research Scheme 1. Early Experiments on the Photocatalytic Activity of Decatungstate. In Blue the H Cleaved

Scheme 2. Events Occurring upon Decatungstate Excitation

Recently, we employed theoretical simulations to characterize the different species generated upon excitation of the decatungstate anion, showing characteristic absorption bands in the 600−800 nm region.33,34 Computational methods based on time-dependent density functional theory (TD-DFT) predicted successfully the spectrum of different clusters belonging to the polyoxometalate family. Indeed, a benchmark study carried out on the Lindqvist cluster [W6O19]2− demonstrated that the use of a hybrid functional (PBE0), combined with scalar relativistic corrections and a continuum treatment of the solvent, predicted the most important electronic transitions and convoluted the overall electronic spectrum down to 200 nm, in fair agreement with experimental data.33 We later proved that this approach has some generality, since the spectra of all of the species involved in decatungstate-mediated photochemical reactions, [W10O32]4−, [W10O32]5−, H+[W10O32]5−, and [W10O32]6−, have been predicted to a good approximation.34 Another fundamental topic is the mechanism of activation of organic substrates (R−X) by wO. In the presence of R−X, wO may react either via a SET or HAT15 (when X = H) mechanism, leading to a radical cation (R−X•+) or a radical (R•), respectively, with the concomitant formation of the monoreduced form of decatungstate.28c SET reactions were demonstrated to occur mainly with substrates easy to oxidize and lacking a weak R−H bond. This is the typical case of aromatic amines and aromatic hydrocarbons, where the bimolecular rate constants are near the diffusion limit.28c HAT reactions occur in the case of alkanes, phenols, aliphatic alcohols, aliphatic amines, and halogenated hydrocarbons, with typical rate constants in the 107−108 M−1 s−1 range.31 A mixed mechanism, however, could not be excluded for some substrates.28c Interestingly, this diverted behavior can be easily appreciated from the LFP experimental traces following the decay at 780 nm of the wO transient in the presence of increasing concentrations of biphenyl (representative of SET reactions, Figure 5a) or butan-1-ol (representative of HAT reactions, Figure 5b). Only HAT reactions lead to the formation of an appreciable amount of the reduced species (of which the formation quantum yield may equal that of wO), as indicated by the 300−400 ns region of the traces, where the signal increases along with the amount of added substrate (Figure 5b).28c Thus, the pathways occurring for butan-1-ol 28c and biphenyl28c are those described in Scheme 3. The regeneration of the starting decatungstate, required for preparative applications, involves a back hydrogen donation (in the first

irradiation.20,21 Dehydrogenations gave access to non-thermodynamic (less-substituted) alkenes in good selectivity (Scheme 1c), promoted by the preference shown by excited decatungstate toward the activation of highly substituted C−H bonds in the starting alkane (tertiary position > secondary > primary).22,23 Many efforts have been devoted to the mechanistic aspects of the reactions.24 The monitoring of the UV−vis spectra of the solutions was a fundamental tool due to the formation of deeply blue-colored species upon irradiation.25 The excited state of [W10O32]4− was proposed to have an oxygen-to-metal charge-transfer (LMCT) character as highlighted by nano- and microsecond laser flash photolysis (LFP) experiments performed on tetrabutylammonium decatungstate (TBADT) in deaerated acetonitrile. Picosecond flash excitation experiments demonstrated that the primary LMCT excited state, formed upon absorption of a UV photon by decatungstate salts, survived for ca. 30 ps in both aerated and deaerated acetonitrile, and decayed rapidly to generate an intermediate state that persisted for tens of nanoseconds.26 This intermediate triplet state (a dark state tagged wO)27 is the actual chemically active species and shares an oxyradical-like character, due to the presence of an electron-deficient oxygen center.28 In detail, upon excitation (λ = 355 nm) of TBADT in acetonitrile with a 15 ns laser pulse, wO was clearly identified thanks to its absorption around 780 nm. Its decay followed first-order kinetics with a lifetime of τ = 55 ± 20 ns. wO was claimed as a relaxed excited state since it was not quenched by O2 and neither the first formed LMCT state nor wO produced singlet oxygen under illumination in O2-saturated acetonitrile solution (Scheme 2).27 The decay profile recorded for wO further indicated the presence of a long-lived species identified as the one-electron-reduced form of decatungstate (Scheme 2).27 From steady-state irradiation experiments made in the presence of isopropanol, the quantum yield for formation of wO (ΦwO) was determined to be ca. 0.6,27 which is obviously the maximum value measured for the formation of chemical products.22,29−32 2235


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Figure 5. LFP experimental traces of sodium decatungstate (NaDT) solutions containing increasing concentration of (a) biphenyl and (b) butan-1ol. Adapted with permission from ref 28c. Copyright 2000 The Royal Society of Chemistry.

reactions that make use of natural sunlight or simulated solar light to promote decatungstate-photocatalyzed organic transformations.

Scheme 3. Two Modes of Action in Decatungstate Photocatalysisa

a

2. DECATUNGSTATE PHOTOCATALYZED WINDOW-LEDGE CHEMISTRY The two main fields where decatungstate salts (ammonium, potassium, sodium, etc.) were used upon solar light absorption are (i) synthesis (formation of C−C, C−N, or C−O bonds, photo-oxidations)36 and (ii) water depollution.37 As for synthetic applications, a typical case is the addition of photogenerated nucleophilic radicals onto CC or NN bonds, where TBADT is commonly used. Figure 7a shows the proposed photocatalytic cycle that is based on the capability of excited decatungstate to cleave homolytically the C−H bond in suitable hydrogen donors (R−H). The thus formed nucleophilic radical R• gives conjugate addition onto Michael acceptors, and the alkylated olefin along with the regenerated photocatalyst is then obtained by back hydrogen donation.15 The starting solution was poured in a cylindrical Pyrex glass vessel (Figure 7b), deaerated with nitrogen and closed with a rubber septum. The irradiation was carried out by placing the mixture on an external surface (e.g., a window ledge)38 or by using a solar light simulator (Solarbox) equipped with a Xe lamp. During the reaction, the typical blue color of the reduced form H+[W10O32]5− (Figure 7b) developed, a clue of the occurrence of the reaction. The TBADT window ledge chemistry is important from the environmental point of view since no artificial energy is required (for stirring/heating/cooling the solution or for the light source). An added bonus is that the involved process is 100% atom economical. Albeit direct solar light is beneficial for the photocatalyzed transformations, we had evidence that a satisfying conversion of the reagent took place even under a cloudy sky.38 Scheme 4 collects some representative examples of C−C bond formation by using dimethyl maleate 1 as the electron-poor olefin. A variety of hydrogen donors (R−H) was used, and only 2 mol % TBADT was sufficient to promote the photocatalyzed alkylations of 1 (in blue, the cleaved C−H bond). An excess of R−H was required, and the reactions took up to 9 days (8 h per day) for the complete consumption of 1. In detail, cyclohexane, DMF, and 1,4-dioxane were smoothly

See ref 28c for details.

case) or an oxidation (in the latter) of the deactivated photocatalyst. Thus, the versatility of the decatungstate anion as photocatalyst in promoting SET or HAT reactions with organic substrates is apparent. Despite the absorption spectrum of [W10O32]4− not extending into the VIS region, there is an overlap between solar light emission and decatungstate absorption (Figure 6).35 A decatungstate photocatalyzed “window ledge” chemistry is thus possible. Accordingly, this Account collects only those

Figure 6. Overlap of the solar UV emission spectrum (full line) and NaDT absorption spectrum (dashed line). Reprinted with permission from ref 35. Copyright 1999 Elsevier. 2236


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Figure 7. (a) Solar light decatungstate photocatalyzed functionalization of electron-poor olefins. (b) Glass vessel (after the irradiation) used for this window ledge chemistry. Courtesy of Mr. L. Capaldo (University of Pavia, Italy).

Scheme 4. TBADT Photocatalyzed Synthesis of Succinates 2−6

Scheme 5. Solar Light Induced Photocatalyzed Functionalization of 7

We further investigated the TBADT-photocatalyzed reactions of unsaturated sulfones under similar conditions (Scheme 6).41 Solar light irradiation of phenyl vinyl sulfone 11 in the presence of either heptanal or cyclopentanone gave ketone 1241 and sulfone 1340 in 77% and 70% yield, respectively. A Solarbox (equipped with a 1.5 kW xenon lamp; 500 W/m2) was used instead for the mild and effective generation of α-oxy radicals from oxetanes.42 By using our approach, vinyl sulfone was

a

Reactions performed at the University of Pavia (Italy) during the July−September 2008 period. b1 (0.45 M), heptanal (0.5 M); ca. 10 g of compound 5 isolated. cTBADT (4 mol %). dReaction performed at the University of Osaka (Japan) during November 2014.

Scheme 6. TBADT Photocatalyzed Conjugate Additions onto Vinyl Sulfone 11

photoadded to 1 in 50−60% isolated yield (compounds 2− 4).38 In the acylation by aliphatic aldehydes the reagents could be used in ca. 0.5 M amount, thus reducing the amount of solvent and improving the ecosustainability of the process.38 Compound 5 was thus prepared in 90% yield in up to 10 g scale.38 Selective C−H cleavage of the γ-methine hydrogen in isocapronitrile likewise allowed the exclusive formation of 6.39 In the case of cyclopentanone (7, Scheme 5), we were able to perform the regioselective activation of the less labile βhydrogens promoted by polar effects. The C−H cleavage of the more labile α-hydrogens would have led to an unfavorable polar radical transition state and to an unstabilized electron-deficient α-carbon, making β-selective C−H bond cleavage feasible.40 Cyclopentanones 8−10 were thus obtained by β-regioselective alkylation of 7 with selected Michael acceptors. The endo adduct (10) was exclusively obtained in the reaction with methylene norbornanone. 2237


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Accounts of Chemical Research converted to 14 (66% yield) after 8 h irradiation.42 This is an alternative strategy for the synthesis of oxetanes (surrogates of a carbonyl group) usually carried out via building of the fourmembered ring. Window ledge chemistry was used for the smooth conversion of C−H to C−N bonds by radical addition onto azodicarboxylates.43 Diisopropyl azodicarboxylate (DIAD, 15) was employed as radical trap in an atom-economical amination process (Scheme 7). The reactions worked to some extent even

Our photocatalytic approach was also applied to the PEGylation of single-walled carbon nanotubes (SWCNTs).45 The method is based on the formation of α-oxy radicals directly from poly(ethylene glycol)s (PEGs). The surface functionalization was improved when using low-molecular-weight PEGs (e.g., PEG 200) with respect to PEG 600.45 These radicals attack the nanotubes surface modifying their physicochemical properties while preserving their structure, as witnessed by TEM analysis (Figure 8). The image of pristine SWCNTs (Figure 8a) shows entanglements of nanotube bundles connected by amorphous carbon particles with the presence of metal particles as impurities. In contrast, single bundles were observed in the samples after the photocatalyzed PEGylation (Figure 8b). A particular case is the TBADT photocatalyzed benzylation of electron-poor reducible olefins, such as fumaronitrile.46 Laser flash photolysis experiments supported a mechanism involving the initial formation of a polar exciplex between substituted toluenes 20a−d and the photocatalyst (PC) in the excited state (Scheme 8).

Scheme 7. TBADT Photocatalyzed C−N Bond Formation

Scheme 8. Mechanism of Fumaronitrile Benzylation

in the absence of the photocatalyst, but a dramatic acceleration took place in the presence of 2 mol % TBADT.43 Typical cases are the photoaddition of cycloalkanes (e.g., cC6H12) and THF to give derivatives 16 and 17. The functionalization of 1,3benzodioxole (compound 18) is another valuable reaction, since the methylenedioxy moiety is known to impart a marked biological activity to organic compounds, and actually this scaffold is present in many substances having antitumor, antibacterial, antioxidant, and insecticidal activity.44 A smooth conversion of aldehydes to acyl hydrazides was likewise performed, and the high reactivity of DIAD as a radical trap made the reaction selective even when using secondary aldehydes (compound 19) that generate acyl radicals usually prone to lose CO.43

The presence in solution of LiClO4 and water imparted a stabilization to the exciplex, thus extending its lifetime. A hydrogen transfer within the exciplex then occurred.46 Benzyl radicals 21•a−d were trapped by fumaronitrile radical anion (in turn formed by electron transfer with PC−H•) to give benzylated derivatives 22a−d in a satisfying yield.

Figure 8. TEM images of pristine SWCNTs (a) and after pegylation with PEG-400 (b). Reprinted with permission from ref 45. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA. 2238


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Figure 9. (a) Sunlight TBADT photocatalyzed hydrosilylation of dimethyl maleate. (b) IR spectra recorded during the photopolymerization of EPOX in the presence of TBADT/TTMSS/Ph2I+(1/3/2% w/w) under sunlight at increasing irradiation times. Adapted from ref 48. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA.

TBADT has been likewise used for the photocatalytic generation of silicon-based radicals.47 The homolytic cleavage of the Si−H bond in trisubstituted silanes was applied for the hydrosilylation of electron-deficient alkenes. Dimethylphenylsilane 23 was one of the best silanes in the series (Figure 9a), showing no competitive C−H cleavage from the methyl groups.47 Photogenerated silyl radicals were likewise used to initiate the ring opening photopolymerization of epoxides.48 The initiating system was a mixture comprised of TBADT, tris(trimethylsilyl)silane (TTMSS), and diphenyl iodonium hexafluorophosphate (Ph2I+PF6−) and was effective for the sunlight cationic polymerization of EPOX (3,4epoxycyclohexane)methyl 3,4-epoxycyclohexylcarboxylate, Figure 9b). The reaction took place in October 2010 in Mulhouse (France) under cloudy weather in the presence of air. The reaction was monitored via real time FTIR spectroscopy by following the disappearance of the epoxy groups at about 790 cm−1 (Figure 9b). In the mechanism proposed, the photocatalytically obtained silyl radicals were oxidized by the iodonium salt, and the resulting silyl cations were involved in polymerization event.48 Later, decatungstate was used as counteranion of the diphenyl iodonium cation in the photoinduced polymerization of an epoxy/acrylate blend.49 In other instances, the carbon radical formed (from a hydrocarbon) was trapped by oxygen to form the corresponding oxidized products (alcohol or ketone). Many experiments were carried out on cyclohexane with the aim to perform an ecosustainable process starting from a cheap hydrocarbon by using sunlight and dioxygen. Large scale experiments (4 L) were performed at the Plataforma Solar de Almeria (Spain).50,51 In detail, an oxygen-saturated solution of TBADT (2.4 mM) in acetonitrile (3717 mL), water (70 mL), and cyclohexane (213 mL, ca. 5% volume) was irradiated in a CPC solar reactor.50 After exposure for 1 week (1857 Wh/m2, 150 mL of cyclohexane was further added), the solution was treated with a reducing agent and afforded 7.98 g of a mixture of cyclohexanol and cyclohexanone, along with 6.31 g of polyoxidation products.50 A tentative mechanism is depicted in Scheme 9. Recently, a 35 W tungsten−bromine lamp equipped with an UV light filter was adopted for the visible-light oxidation of cyclohexane in acetonitrile.52 The addition of acids, such as benzenesulfonic acid or HCl, H2SO4, or H3PO4 aqueous solutions, had a beneficial effect on cyclohexane conversion

Scheme 9. Decatungstate Photocatalyzed Oxidation of Cyclohexane

(from ca. 8% to 12−14%) and on the selectivity of cyclohexanone formation (up to 70%). The acid was claimed to improve the photoredox cycling of [W10O32]4−, thus preserving its stability. The presence of water likewise slightly improved the process. Photocatalytic oxidation of toluene, ethylbenzene, and butanone under the same conditions gave poor results in terms of conversion and selectivity.52 A quite unexplored area involves the use of decatungstate salts for the solar light photodegradation of organic molecules.37 NaDT was exploited for the mineralization of pesticides (bromoxynil, atrazine, imidachloprid, and oxamyl), again adopting a CPC reactor.35 The performance of decatungstate was compared to that of TiO2. Titania proved to be more efficient in the photodegradation of pure pesticides, but NaDT performed better when these contaminants were present in formulated solutions. Atrazine and imidachloprid ((1−6.5) × 10−4 M) were completely destroyed by adopting both photocatalysts (incident energy = ca. 6 × 105 J/m2).35 The decatungstate anion can be conveniently incorporated in a metal−organic framework, as was demonstrated in the synthesis of {[Co2(bpdo)5(H2O)4−W10O32]}n·nbpdo·2nH2O (bpdo = 4,4′-bis(pyridine-N-oxide).53 This compound has the advantage to shift the absorption of the photocatalyst to the visible region (in the 510−620 nm range). The different absorption spectrum has a beneficial effect on the photo2239



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degradation performance, as was demonstrated for the case of rhodamine B (RhB). In fact, the organic dye (2 × 10−5 M aqueous solution) completely disappeared after 9 h sunlight exposition when using the MOF catalyst, whereas circa half of the initial amount of RhB was still present after the same time when using TBADT as the photocatalyst.

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3. CONCLUSIONS Among the available alternatives, decatungstate salts have emerged as the preferred photocatalysts for sunlight induced C−H functionalization in organic compounds through a HAT mechanism. The efficiency and the regioselectivity of such photocatalyst in hydrogen atom abstraction has allowed for the use of a wide range of hydrogen donors, including aldehydes, alcohols, ethers, amides, ketones, and even alkanes. This cluster likewise found application in the science of materials, for polymerization reactions and for decontamination purposes. Despite the impressive range of procedures developed in diverse applications in recent years, several issues remain. First, an unequivocal assignment of the reactive state in photocatalyzed processes is still lacking. Computational studies can help here and were actually found to model satisfactorily several properties of decatungstate and its derivatives, particularly in terms of electronic excitation spectra. As for practical applications, the development of visible light absorbing systems is the main challenge, and some promising efforts have been recently reported, through the preparation of heterogeneous catalysts absorbing in the visible light range.54 Other promising hot topics for future applications in the field of decatungstate chemistry are the development of dual catalytic systems55 and of flow processes56 under solar light irradiation, so far promoted only by UV light.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +39 0382 987323. Tel: +39 0382 987198. E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Davide Ravelli obtained his Ph.D. in 2012 from the University of Pavia (Prof. A. Albini as the supervisor). Since November 2015, he is a fixed term researcher at the same University, and his main research interests focus on the generation of radical intermediates through photocatalyzed hydrogen atom transfer reactions. Stefano Protti completed his Ph.D. (supervisor Prof. M. Fagnoni) from the University of Pavia in 2006. He was postdoctoral fellow at the LASIR laboratory (Lille, France), and at the iBitTec-S Laboratory (CEA Saclay). He is currently researcher at the University of Pavia. His research work is focused on photochemical arylations under metal-free conditions. Maurizio Fagnoni is currently an Associate Professor in the Department of Chemistry at the University of Pavia. His research interests are mainly focused on the photoinduced or photocatalytic generation of reactive intermediates such as (bi)radicals, (phenyl) cations, and radical ions and their application to ecosustainable synthesis. 2240


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