Dioxygen in Polyoxometalate Mediated Reactions - Chemical

Review Cite This: Chem. Rev. 2018, 118, 2680−2717

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Dioxygen in Polyoxometalate Mediated Reactions Ira A. Weinstock,*,† Roy E. Schreiber,‡ and Ronny Neumann*,‡ †

Department of Chemistry, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel

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‡

ABSTRACT: In this review article, we consider the use of molecular oxygen in reactions mediated by polyoxometalates. Polyoxometalates are anionic metal oxide clusters of a variety of structures that are soluble in liquid phases and therefore amenable to homogeneous catalytic transformations. Often, they are active for electron transfer oxidations of a myriad of substrates and upon reduction can be reoxidized by molecular oxygen. For example, the phosphovanadomolybdate, H5PV2Mo10O40, can oxidize Pd(0) thereby enabling aerobic reactions catalyzed by Pd and H5PV2Mo10O40. In a similar vein, polyoxometalates can stabilize metal nanoparticles, leading to additional transformations. Furthermore, electron transfer oxidation of other substrates such as halides and sulfurcontaining compounds is possible. More uniquely, H5PV2Mo10O40 and its analogues can mediate electron transfer-oxygen transfer reactions where oxygen atoms are transferred from the polyoxometalate to the substrate. This unique property has enabled correspondingly unique transformations involving carbon−carbon, carbon−hydrogen, and carbon−metal bond activation. The pathway for the reoxidation of vanadomolybdates with O2 appears to be an inner-sphere reaction, but the oxidation of one-electron reduced polyoxotungstates has been shown through intensive research to be an outer-sphere reaction. Beyond electron transfer and electron transfer−oxygen transfer aerobic transformations, there a few examples of apparent dioxygenase activity where both oxygen atoms are donated to a substrate.

CONTENTS 1. Introduction 2. Polyoxometalates 2.1. Heteropolyoxometalates 2.2. Structural Types of Polyoxometalates 2.3. Lacunary and Transition Metal Substituted Polyoxometalates 3. Palladium(II) Catalyzed Reactions 3.1. Oxidative Hydration and Amination of Alkenes 3.2. Arene Coupling and Hydroxylation 3.3. Coupling Reactions with Alkenes 3.4. Oxidative Dehydrogenation of Alcohols 3.5. Oxidation of CO to CO 2 and CO/O 2 Oxidation Reactions 4. Other Metal−Polyoxometalate Cocatalytic Systems 4.1. Oxidations by Polyoxometalate Protected Metal(0) Nanoparticles 4.2. Catalysis with H2−O2 Mixtures 4.3. Oxidation of Alkanes: Shilov Type Reactions 5. Phosphovanadomolybdate Catalyzed Reactions 5.1. Electron Transfer Oxidations 5.1.1. Dehydrogenation of Dienes 5.1.2. Oxidation of Alcohols and Amines 5.1.3. Oxidation of Activated Phenols 5.1.4. Other Electron-Transfer Oxidations 5.2. Electron Transfer−Oxygen Transfer Reactions © 2017 American Chemical Society

5.2.1. Carbon−Carbon Bond Cleavage Reactions 5.2.2. Activation of Carbon−Metal Bonds 5.2.3. Reactions in Strong Acids 6. Activation of Dioxygen 6.1. Phosphovanadomolybdates 6.2. Autooxidations and Reactions with Sacrificial Reductants 6.3. Outer-Sphere Electron Transfer from Polyoxotungstates to Dioxygen 6.3.1. Outer-Sphere Electron Transfer 6.3.2. Electron Self-Exchange between Polyoxometalate Anions 6.3.3. Electron Transfer to O2 6.4. Dioxygenase Type Reactions 7. Oxidation of Sulfides 8. Conclusions Author Information Corresponding Authors ORCID Author Contributions Notes Biographies Acknowledgments

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Special Issue: Oxygen Reduction and Activation in Catalysis Received: July 25, 2017 Published: December 1, 2017

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DOI: 10.1021/acs.chemrev.7b00444 Chem. Rev. 2018, 118, 2680−2717

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1. INTRODUCTION Polyoxometalate chemistry was born early in the 19th century with the first synthesis of such a compound by Berzelius in 1824.1 More than 100 years later the first crystal structure of phosphotungstic acid, H3PW12O40, was determined by Keggin.2 From that time onward, polyoxometalate chemistry has developed into a subfield of inorganic chemistry. Although the majority of the research in this area has been centered around synthesis, synthetic strategies, and the development of continuously more complicated structures,3−13 it was realized that H3PW12O40 and analogous so-called heteropoly acids such as phosphomolybdic acid, H3PMo12O40, and silicotungstic acid were strong Brønsted acids. This led to their use in acidcatalyzed reactions,14,15 for laboratory research purposes but also industrial applications.16 Somewhat later on in the mid 1970s the redox properties of phosphovanadomolybdates, H3+xPVxMo12‑xO40 (x = 1−6), were shown to be advantageous for oxidative catalytic transformations, and since then the use of polyoxometalates in oxidation catalysis has become a significant research area.17−19 In this review we will limit ourselves to homogeneous liquid phase catalysis although there is a body of research where polyoxometalates have been used as photocatalysts20,21 and high temperature gas phase reactions.22−25 This latter research has been mostly directed to the formation of methacrylic acid from isobutane and derivatives by oxidative dehydrogenation. Some similar research on propane oxidative dehydrogenation has also been reported. Three research directions have dominated the field of liquid phase oxidative transformations catalyzed or mediated by polyoxometalates. First is the use of peroxides, mostly hydrogen peroxide as oxidant with the formation of peroxo intermediates capable of oxygenation of nucleophiles26−28 and in some cases even of alkanes and arenes.29 Second is the use of polyoxometalates for water oxidation,30−34 and third is the use of dioxygen as terminal oxidant in polyoxometalate mediated thermal transformations which is the subject of this review.

Figure 1. Lindqvist structure [M6O19]n‑ showing addenda and types of oxygen atoms. Left: balls and sticks; right: polyhedral. Gray, addenda; red, oxygen.

However, the vast majority of known polyoxometalate structures have group 5 and 6 transition metals as addenda atoms. In particular, V(V), Mo(VI), and W(VI) are known for forming anionic polyoxometalates in aqueous as well as nonaqueous solutions. For W(VI) for example, these include among others the paratungstate anion [H2W12O42]10−, the metatungstate anion [H2W12O40]6− and the decatungstate anion [W10O32]4−, Figure 2.36 In these cases, the group 5 or 6 addenda atoms have an octahedral geometry with 4−6 bridging oxygens and 0−2 terminal oxygen atoms surrounding them. In the rest of this section we will concentrate on this subclass of polyoxometalates. 2.1. Heteropolyoxometalates

In some cases, the polyoxometalate is built around a structural group that is not the addendum atom or oxygen, for example phosphate or silicate. The central atom in these cases, such as phosphorus or silicon in the example, is called a heteroatom. One of the simplest heteropolyoxometalates is the Keggin anion, Figure 3. Heteroatoms usually take tetrahedral positions within polyoxometalates but can also be octahedral in some cases. They are usually either transition metals such as V, Cr, Mn, Fe, Co, etc. or nonmetals such as Si, P, S, As, I, etc. Other heteroatoms such as Na and Al are also known. The heteroatom is important both in directing the growth of the polyoxometalate as well as tuning its stability and physical properties. The same synthesis conditions with different heteroatoms in solution may cause the formation of a different structure.41 Moreover, it was shown that the heteroatom attaches to the growing polyoxometalate at a very early stage, already after the attachment of two addenda atoms in the case of the formation of a Keggin structure.42 Since heteroatoms are not exposed to the outside chemical environment, changing them is often used in order to fine-tune the overall physical properties of the polyoxometalate.

2. POLYOXOMETALATES Metal-oxides may take the form of small anionic complexes such as WO42‑ and MnO4‑ or of noncharged extended structures such as TiO2, WO3, and Fe2O3. In many cases, however, the energy landscape for the oxolation process, in which small oxide complexes condense to form extended structures, is not monotonic. That is, there exist “magic numbers” for the number of metal atoms in a cluster species. These are specific clusters that are kinetically or thermodynamically stable and are called polyoxometalates.35 A more detailed look into clusters of this type shows that they are well-defined, usually polyanionic, molecular clusters. They are made of metal atoms, termed addenda atoms, which are bridged by oxygen atoms, Figure 1.36 Many metals can function as addenda atoms for polyoxometalates. For example, aluminum was shown to form [AlO4Al12(OH)24(H2O)12]7+.37 Addition of stabilizing agents greatly increases the scope of addenda atoms and clusters that can be stabilized. In such a way, iron may form species such as [FeO4Fe12O12(OH)12(O2C(CCl3)12)]17‑,38 titanium may form species such as [Ti18O27(OH)(OtBu)17],39 and uranium may form species such as [U6O13(1,2,4-tBu3C5H2)4(2,2′-(C5H4N)2)2].40

2.2. Structural Types of Polyoxometalates

Many different structures are known for polyoxometalates. In general terms, these structures are divided into two main types by the number of terminal oxygen atoms per tungsten. This affects the directionality of d orbitals within the addenda atoms and therefore changes physical properties such as redox reversibility.36 Thus, type I polyoxometalates have one terminal oxygen atom per addendum atom, while type II polyoxometalates have two terminal oxygen atoms per addendum atom. Addenda atoms that possess no terminal oxygen atoms are not counted toward this classification. Polyoxometalates with three terminal oxygen atoms per addendum atom are not known and are predicted not to exist. In terms of redox properties, a 2681


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Figure 2. Isopolytungstates. (a) Paratungstate [H2W12O42]10−, (b) metatungstate [H2W12O40]6−, and (c) decatungstate [W10O32]4−. Gray, tungsten; red, oxygen.

solid-state structure. This is therefore a notably stable, or “magic number”, cluster on the path to an extended structure. A second polyoxometalate structure with six addenda atoms contains a hexagonal heteroatom in the center of a hexagonal structure, Figure 4. This is the Anderson structure, an example of a type II polyoxoanion.

Figure 3. Keggin structure [XM12O40]n‑. Left: balls and sticks; right: polyhedral. Gray, addenda; red, oxygen; blue, heteroatom.

distinction should be made between type I polyoxometalates that have one terminal oxygen atom and type II polyoxometalates that have two terminal oxygen atoms. Whereas the reduction of type I polyoxometalates is reversible due to a nonbonding HOMO orbital, type II polyoxometalates have an antibonding HOMO orbital, and their reduction therefore causes structural changes and is not reversible.36 The reduction of type I polyoxometalates forms polyoxo blues (or heteropoly blues), so-called due to their deep blue color that has a broad visible light absorbance band with a maximum around 700 nm.36 It is possible to reduce a polyoxometalate molecule by multiple electrons reversibly without inducing deformation of the molecules. Polyoxometalates may therefore be used to store electrons and as molecular electrodes. The reduction potentials of polyoxometalates are determined by the nature of the addenda atoms, structure, overall charge and the level of protonation. Generally, tungstate polyoxometalates are the most difficult to reduce with typical reduction potentials between +0.1 and −1 V vs NHE. Molybdates have reduction potentials approximately 400 mV more positive. The molecular charges of polyoxometalates also greatly influence their reduction potentials. One study found that the reduction potentials of Keggin heteropolytungstates become 180 mV more negative for every additional charge on the heteroatom.43 The redox potential of the polyoxometalate in many cases determines catalytic activity. The overall charge, along with the nature of the countercation, also affects the solubility of the polyoxometalate and the therefore the choice of solvent. The simplest polyoxometalate molecule is considered to be the Lindqvist structure [M6O19]n‑, where M is the addendum atom, Figure 1. This structure consists of six addenda atoms that are ordered in a close packed structure, forming a type I polyoxoanion. A hypothetical extension of this structure would form either a face-centered cubic or a hexagonal close packed

Figure 4. Anderson structure [XM6O24]n‑. Left: balls and sticks; right: polyhedral. Gray, addenda; red, oxygen; blue, heteroatom.

A large family of polyoxometalates is derived from the αKeggin structure. It is comprised of a tetrahedral heteroatom surrounded by four interconnected triads of addenda atoms, giving the chemical formula of [XM12O40]n‑, Figure 3. Each cluster may be viewed as a neutral Td symmetry MVI 12O36 shell, with a tetrahedral Xn+O4(8−n)− oxoanion at its center. As the heteroatom is varied, the size and charge of the central oxoanion changes, while the MVI 12O36 shell remains unchanged in size or symmetry. Although there are many variants of this structure based on different heteroatoms and addendum atoms, many uses in oxidation catalysis have been connected to the polyoxometalate where X = P and M is a mixture of Mo and V. As the number of vanadium atoms increases the charge on the anion also increases thereby reducing the stability of the polyoxometalate. As will be discussed below, in terms of stability and activity, the [PV2Mo10O40]5− anion is favored for many applications. It should be noted that this polyoxometalate is in fact a mixture of five isomers based on the relative positions of the two vanadium atoms. The isomers are in dynamic equilibrium, and their relative abundance is solvent dependent. The isomers have not, and likely cannot, be separated; they can be divided into isomers where vanadium atoms are vicinal and those where vanadium atoms are distal, Figure 5. In oxidation catalysis, the acid, that is protonated form, of the polyoxometalate is typically used. The vanadiumbound oxygen atoms show higher proton affinity that further increases upon reduction of vanadium(V) to vanadium(IV). This results in the formation of stable isomeric forms with three 2682


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Co, and many others substitutes one or more addenda atoms. The lacunary polyoxometalate acts as a weak-field ligand and induces only a small d-orbital splitting. This leads to a tendency for polyoxometalates to stabilize high spin states in complexed transition metal cations. For example, a rare high spin Mn(V) ion was stabilized by an α 2 lacunary Wells−Dawson polytungstate.47,48 A Keplerate polyoxometalate with 30 Fe(III) ions was also found to be high spin on all iron centers.49

Figure 5. Different isomers of the [PV2Mo10O40]5− polyoxometalate and their free energies in acetonitrile at 298 K from DFT calculations.44 Tungsten, black; vanadium, orange; phosphorus, yellow.

(of the five) protons bound to nucleophilic sites around the same vanadium atom.44 The DFT calculated free energies of the different isomers are slightly different but not significantly so. An example of a structural type derived from the Keggin structure is the Wells−Dawson anion. In this case, starting from the Keggin structure, three adjacent addenda atoms are removed, one from each of three triads, and two of the resulting moieties are connected, giving a cluster of the chemical formula [X2M18O62]n‑, Figure 6. This structure

3. PALLADIUM(II) CATALYZED REACTIONS The original use in the 1970s of polyoxometalates in oxidation catalysis was initiated by the Matveev group at the Institute of Catalysis at Russian Academy of Science in Novosibirsk. One major research direction was the use of polyoxometalates as a cocatalyst in palladium(II) catalyzed reactions according to the following general scheme, Scheme 1. This represents also the Scheme 1. Pd(II) Catalyzed Reactions and the Cocatalysis of H3+xPVxMo12‑xO40

first application of phosphovanadomolybdates, H3+xPVxMo12‑xO40 (x = 1−6) in oxidation catalysis. In essence, as will be shown below, the objective was to expand the use of versatile Pd(II) catalysis in numerous transformations by facilitating the reoxidation of Pd(0). During this period, reactions were mostly carried out in water and other polar solvents. In water, it was observed that the oxidation potential of H3+xPVxMo12‑xO40 was independent of the vanadium content.50 Beyond the reduced stability of the polyoxometalate with increasing amount of vanadium as noted above, H3+xPVxMo12‑xO40 was rather unique in that it was a relatively strong oxidant, but the reduced catalyst could be easily reoxidized by O2. Kinetic studies, without spectroscopic support, led to the suggestion of the formation of radical intermediates during the reoxidation reaction.51 This reactivity is quite different compared to the molybdate only catalyst, H3PMo12O40, which also oxidized Pd(0), but is not easily reoxidized by O2. Similarly, VO2+ is also an efficient oxidant, but the VO2+ formed is not reoxidized by O2.

Figure 6. Wells−Dawson structure [X2M18O62]n‑. Left: balls and sticks; right: polyhedral. Common naming of regions within the molecule are shown. Gray, addenda; red, oxygen; blue, heteroatom.

contains addenda atoms in two different chemical environments. The top and bottom triads are commonly called “cap” regions and contain three addenda atoms each. The middle region is commonly called the “belt” region and contains 12 addenda atoms. 2.3. Lacunary and Transition Metal Substituted Polyoxometalates

The removal of an addenda atom from a plenary polyoxometalate forms a monodefect of “lacunary” structure. This is often performed by increasing the pH of a solution containing the plenary polyoxometalate to a specific value, being the first step in the degradation of polyoxometalates to simple metal-oxo anions in basic media.35 The lacunary site is basic and therefore often stabilized by an alkali metal or other cation taking the place of the missing addendum atom. It is sometimes possible to remove more than one addendum atom by carefully controlling the pH to form a multiply lacunary polyoxometalate while still maintaining structural integrity.35,36 Control over the specific addenda atoms that are removed may also be achieved in well researched polyoxometalates such as the phosphotungstate Keggin and Wells−Dawson structures.45,46 It is possible to view lacunary polyoxometalates as rigid chelating sites. In the absence of suitable transition metals, the lacunary site may ion-pair with alkali-metal cations. This, however, is a weak interaction, and upon addition of a transition-metal cation with an appropriate size, a complex forms between the metal and the lacunary polyoxometalate.35 This way, it is possible to prepare transition metal-substituted polyoxometalates in which a transition metal such as Mn, Fe,

3.1. Oxidative Hydration and Amination of Alkenes

One of the more studied reactions was the Wacker oxidation of ethylene to acetaldehyde, Scheme 2 (R = H).52 Scheme 2. Wacker or Oxidative Hydration of Alkenes Catalyzed by Pd(II) and H3+xPVxMo12‑xO40

Although the Wacker reaction had already been industrialized using Cu(II)Cl2 as a cocatalyst for reoxidation of Pd(0), there was still interest in improvement, due to the high concentration of HCl present and the resulting corrosiveness of the industrial process. A halide free system was the stated objective with H3+xPVxMo12‑xO40 as the cocatalyst. Indeed, 2683


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H3+xPVxMo12‑xO40 proved to be effective for this reaction. It was suggested that the rate-determining step was the reoxidation of Pd(0), specifically the oxidation of a proposed Pd(0)CH2CH2 complex. During the subsequent years, this oxidative hydration of alkenes attracted interest from several other groups. Research was directed mostly toward kinetic measurements and improving reaction rates and expansion of the substrate scope to additional acyclic terminal alkenes and also cyclic alkenes. The original research showed that, while the oxidative hydration of ethylene to acetaldehyde was possible, the reaction rates were too slow for process development. Interestingly, while originally the objective was to eliminate chloride from the reaction, Grate and co-workers at Catalytica discovered that 10−2 M concentrations of Cl− were essential to allow sufficiently fast Pd(0) oxidation in order to prevent Pd(0) aggregation and catalyst deactivation.53 Similar oxidative hydration reactions were carried out on terminal acyclic alkenes and also dienes.54 Thus, acetone was prepared from propene, and 2-butanone was prepared from 1butene at pH 1.1−1.3.55 Kinetic measurements were carried out and showed that in reaction carried out using H3+xPVxMo12‑xO40 (x = 1−4), the reactions were first order in alkene and approximately 0.5 order in Pd.56,57 A further study in a semicontinuous reactor, which now included also chloride, showed a relative activity of ethene ≤ 1-butene < propene that differed only by a factor of 3 using 50 mM H6PV3Mo9O40 and 0.05−2 mM PdCl2 as catalyst.58 Notably, under these conditions isomerization of 1-butene to 2-butene was also observed. Earlier, it had also been noticed that the presence of chloride leads to the isomerization of terminal acyclic alkenes. Thus, the oxidative hydration of 1-octene catalyzed by 30−40:1 H9PV6Mo6O40/Pd2+ was 95% selective toward the formation of 2-octanone using PdSO4 but yielded all three isomeric octanone products in the presence of PdCl2.59 Similar research in ethanol as solvent showed 86% 2-octanone with PdSO4 and 75% 2-octanone with a rarely used RhCl3 as catalyst.60 Other efforts were made to stabilize the Pd(II)/ Pd(0) catalyst with polyoxometalate catalyst in the absence of chloride. Notable has been the use of pyromellitc acid (benzene-1,2,4,5-tetracarboxylic acid)61 and dipicolinic acid (pyridine-2,6-dicarboxylic acid).62 It was suggested that stabilization of the catalyst was due to the decrease in the oxidation potential of the Pd(II)/Pd(0) in the presence of the chelating ligands. Oxidative hydration of acyclic alkenes using Pd(II)/CuCl2 invariably yields the Markovnikov ketone addition product, with only slight formation of the anti-Markovnikov aldehyde product. In contrast to this, styrene derivatives have been shown to predominately yield the corresponding aldehydes in stoichiometric reactions that was explained by the possible involvement of a η4-palladium−styrene complex.63 Quite surprisingly, different from CuCl2 as cocatalyst, the use of H4PVMo11O40, also a strong Brønsted acid, as cocatalyst led to the retention, 6.8:1, of the aldehyde versus ketone selectivity in the oxidative hydration of styrene, Scheme 3. Heterogeneous analogues of the Wacker oxidative hydration reaction, mostly of 1-butene, have also been reported. There are two basic variants. In one case palladium salts of H3+xPVxMo12‑xO40 were supported on silica,64,65 and in the other variant silica was covered with a palladium salt to which then H3+xPVxMo12‑xO40 was added. In the latter case the protons can be replaced by other metal cations such as Cu2+, Ni2+, and Cs+. At 75 °C, a steady state formation of 2-butanone

Scheme 3. Oxidative Hydration of Styrene with an AntiMarkovnikov Addition

of ∼0.01 g/g of catalyst/h was reported with high selectivity, 95−98%, and very good catalyst stability. Under these conditions the reoxidation of the reduced polyoxometalate is sluggish and is rate determining.66 In research where Mn2+ was added to a heterogeneous catalyst a synergetic interaction between Mn2+ and Pd2+ was suggested that increased catalytic activity for the oxidation of ethene to acetaldehyde.67 Usually the oxidative hydration using the Wacker catalyst is only effective for terminal alkenes, which is considered a limitation. However, more recently, oxidative hydration of cyclic alkenes has been reported using a polyoxometalate. Thus, the transformation of cyclohexene to cyclohexanone was reported in various solvents in the presence of water with up to 70% yield.68 The use of PdSO4 and the mixed addenda polyoxometalate without vanadium, H3W6Mo6O40, was reported to be the optimal catalyst that was also used for the oxidation of cyclopentene to cyclopentanone. For the series, cyclic CnH2n‑2 the yield decreased as n increased. Methylcyclohexenes were less reactive than cyclohexene.69 There are reports of reactions that are akin to the oxidative hydration reaction noted above. In this way acetoxylation of ethene and propene was reported to yield the corresponding vinylacetates using Pd/H9PV6Mo4O40 as catalyst where the addition of NaOAc increased the yield of the acetoxylation product relative to the oxidative hydration product, Scheme 4a.70 Further, 1,3-cyclohexadiene derivatives were diacetoxyScheme 4. Acetoxylation of Alkenes

lated using a three component catalyst, Pd/quinone/ H5PV2Mo10O40, Scheme 4b.71,72 Intramolecular reactions and ethoxylation reactions were also possible in using this latter triple catalyst system. In a similar fashion, oxidative amination reactions (aza Wacker reactions) were developed by reacting diaryl secondary amines with various substitution patterns and acrylate esters to yield the corresponding diarylamino-acrylates, Scheme 5. In general, high yields (15 reaction combinations) were obtained with 5% Pd and 0.6% H6PV3Mo9O40 after reaction at 60 °C for 6 h in DMF.73 Heterogeneous catalysts were also developed for such reaction where Pd and the polyoxometalate were supported on active carbon.74 Interestingly under slightly different reaction conditions, aerobic allylic amination of simple alkenes was observed, where Scheme 5. Oxidation Amination of Acrylates with Secondary Arylamines

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H4PVMo11O40 as cocatalyst, which proceeds with very high selectivity.85 In a further mechanistic study of this reaction it was indicated that the reaction was first order in both palladium and H4PVMo11O40, while with Ag(OAc) as oxidant the reaction was first order in palladium only. There were also differences between H4PVMo11O40/O2 and Ag(OAc) as oxidants vis a vis the regioselectivity of the reaction and in competition experiments, leading to the authors’ postulation that in this cross-coupling reaction there is a Pd(II)/Pd(IV) manifold operating in the oxidative coupling reaction,86 as opposed to a Pd(0)/Pd(II) manifold indicated in all other reactions involving Pd(II) and H3+xPVxMo12‑xO40. It is rather fascinating to note that the selectivity of the benzene oxidative coupling reaction can be changed by a change in the composition of the solvent. Thus, in acetic acid the reaction of benzene with Pd(OAc)2H5PV2Mo10O40/O2 yields almost exclusively biphenyl, and a minor amount of water has little effect. However, a significant increase in the amount of water, for example, using AcOH:H2O in a 30:70 ratio leads to the preferred formation of phenol as product.87 Later, others have made similar observations and performed a complete analysis of the reaction parameters. High turnover numbers are accessible with small amounts of Pd(OAc)2, unfortunately, however, Pd(0) precipitates and the polyoxometalate decomposed after 4 h of reaction.88 Supported catalyst used in a heterogeneous reaction mode show lower activity but apparently are more stable and can be reused.89,90

the formation of a Pd-allyl intermediate was suggested to lead to the shift of the double bond. Thus, a reaction of diphenylamine with 1-decene in the presence of Pd(OCOCF3)2 and (NH4)5H4PMo6V6O40·23H2O in trifluorotoluene under 10 bar air at 40 °C for 24 h gave (E)-1diphenylamino-2-decene in a 73% yield and 78% conversion of the amine.75 3.2. Arene Coupling and Hydroxylation

The oxidative coupling of arenes catalyzed by Pd(II) salts with H3+xPVxMo12‑xO40 as cocatalyst for the reoxidation of Pd(II) was first reported already in the mid 1970s in reaction that included oligomerization of reactive arenes with electron donating moieties such as durene and diphenyloxide76 and dimerization of benzene, toluene, and ortho-xylene, Scheme 6.77 Scheme 6. Oxidative Coupling of Arenes

The isomers obtained are commensurate with an electrophilic coupling mechanism. The coupling of benzene to diphenyl can also be carried out using a nominally supported palladium heterogeneous catalyst although it is possible that Pd(II) is in solution.78 In parallel, mechanistically oriented research was carried out on the coupling of various arenes. The research showed that the rate-determining step using Pd(OAc)2/H5PV2Mo10O40 as the catalyst combination was the reoxidation of the reduced polyoxometalate with O2, since the reaction was first order in both these components.79,80 This was supported by the observation that the rate was independent of the nature of the substrate, i.e., toluene and chlorobenzene were equally reactive. Yet, the reaction was also ∼0.5 order in Pd, suggesting a more complicated picture, possibly related to aggregation of Pd upon reduction. Interestingly addition of Hg(OAc)2 had a directing effect toward substitution at the para position. Years later these arene coupling reactions were revisited. Thus, it was shown that a nonvanadium containing cocatalyst H3PMo12O40 was also active for the coupling of benzene to diphenyl, although reaction conditions were significantly more extreme. Higher O2 pressures and temperatures were needed, coinciding also with lower yields.81,82 Another advance in this oxidative benzene coupling reaction was made by using a solid phase ionic liquid prepared by quaternization of pyridine with acetonitrile. This ionic liquid with a nitrile ligand was proposed to be paired with H5PV2Mo10O40 and ligated to Pd(II).83 In another interesting report, rather unique selectivity was observed for the oxidative coupling of methyl benzoate where the reaction occurred mostly at both ortho positions yielding 1,1′-diphenyl-2,2′dimethylcarboxylate as the major product (Scheme 7).84 The aerobic oxidative cross-coupling of benzene in excess with benzofuran and various derivatives was also reported with

3.3. Coupling Reactions with Alkenes

The first report in the Pd/H3+xPVxMo12‑xO40 catalyzed arylation of ethene with benzene, thiophene, and furan also appeared in the 1970s with formation of vinylacetate and acetaldehyde as coproducts. Alkali metal acetates increased the selectivity to the arylation reaction which was preferably carried out in DMF although the catalytic efficiency was low.91 This reaction was revisited 15 years later where it was found that, for acrylate esters, improved results and high selectivity could be obtained, Scheme 8.92,93 Scheme 8. Arylation of Acrylate Esters

The oxidative arylation of acrylates was successfully extended using indole as a substrate. In this way, various indoles substituted at the 3-position with acrylate esters were prepared (20 combinations).94 The transformation was catalyzed by Pd(OAc)2/H3PMo12O40 with 4-dimethylaminopyridine as an effective additive for the coupling reaction. Curiously the reaction was apparently unsuccessful with a vanadium containing analogue. On the other hand, further investigation showed that the arylation of ethene was much less successful. Addition of dibenzoyl methane (dbm) or acetylacetonate (accac) led to improved catalyst stability by inhibiting precipitation of palladium black, but using benzene as substrate both styrene and trans-stilbene were formed, in addition to the formation of vinyl carboxylate originating from the use of carboxylic acid solvents.95,96 Benzofuran similarly reacted at the C 2 position, for example, with styrene to yield the corresponding trans alkene; N-acetyl-glycine was used as a promotor.97 Interestingly, although aryl amines were reported

Scheme 7. Oxidative Cross-Coupling of Benzofuran with Benzene

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to react with acrylates by an aza Wacker reaction,73 the addition of trimethylbenzoic acid to as a promoter lead also to the reaction of aniline with acrylate esters with high selectivity to an olefination reaction at the para position.98 A fascinating extension of the aforementioned arylation reactions of acrylates is the reaction of nitrogen heterocycles (10 examples), notably pyridine alkylated at the 2-position bonds. Here, the acrylates react at the beta carbon to the heterocyclic ring (an aliphatic sp3 C−H bond) to yield a cationic product with formation of a five-membered ring, Scheme 9.99 The cationic product can be hydrogenated to yield neutral heterocylic compounds with a nitrogen bridge or ring opened to yield the corresponding acrylates.

of base, preferably with secondary alcohols. Thus, an early report describing the oxidation of 2-propanol to acetone under very acidic conditions, that is, catalyzed by PdCl 2 / H5PV2Mo10O40/O2, is somewhat surprising.104 In fact there are no further reports of oxidative dehydrogenation of alcohols involving Pd(II) and acidic Keggin type polyoxometalates. There are, however, a few reports on the catalytic oxidative dehydrogenation of alcohols with Pd(II) in the presence of neutral nonoxidizing polyoxometalates. It is likely, but not proven, that the function of the polyoxometalates in these reactions is to prevent or inhibit the aggregation of Pd(0) species, perhaps by ligation, and then precipitation of palladium black. In such a catalytic system, homogeneous or well dispersed small Pd(0) clusters are themselves oxidized by O2,105 without the involvement of a polyoxometalate as an electron transfer oxidant. In this way, alcohols have been oxidized using Pd(OAc)2, ((n-butyl)4N+)3PMo12O40 formed in situ from an excess of tetrabutyl ammonium acetate and H3PMo12O40. Under these slightly basic conditions due to the presence of acetate, the highly efficient aerobic oxidation (up to 10 000 turnovers) of mostly of benzylic and secondary alcohols was observed.106 Furthermore, a palladium(II) polyoxometalate complex formulated as (PdCl2)0.5Q12{[WZn3(H2O)2][(ZnW9O34)2]} where Q is hexadecyl-trimethylammonium, was a catalyst for the oxidation of primary alcohols to the corresponding aldehydes in high selectivity.107 The unusual aspects of this reaction is that the oxidation of primary alcohols was preferred to that of secondary alcohols and that no base was needed for the reaction to proceed. In a similar vein, Pd(II)dipyridylamine complexes with polyoxometalate anions were active for the oxidation of benzylic alcohols.108

Scheme 9. Aerobic Acrylation of Unactivated sp3 C−H Bonds

The arylation of alkenes was also extended to reaction of arenes with acrylonitrile and its derivatives, which is also an alkene substituted with a cyano electron withdrawing group. The benzene was oxidized with acrylonitrile catalyzed by Pd(OAc)2/H3+xPVxMo12‑xO40/O2 to yield the corresponding cinnamonitriles.100 A similar reaction at the ipso position was also carried out with benzaldehyde in place of benzene where ethanol was added to form the diacetal and FeCl3 was used as a Lewis acid cocatalyst, Scheme 10.101

3.5. Oxidation of CO to CO2 and CO/O2 Oxidation Reactions

Scheme 10. Reaction of Benzaldehyde Derivatives with Acrylonitrile to Yield α-Cyanocinnamaldehydes

In the context of remediation of carbon monoxide there is an interest in its oxidation to CO2. In early work, it was shown that the combination of Pd(II) salts and H3+xPVxMo12‑xO40 in methanol or ethanol with 0.5−1 M H2O was effective for the oxidation of CO to CO2 with the coformation of dimethylcarbonate and diethylcarbonate, respectively. 109,110 It was proposed that the oxidation occurred upon the coordination of CO to reduced palladium species; the presence of VO2+ inhibited the reaction.111,112 In further research by the Neumann group the oxidation of CO to CO2 catalyzed by Pd(0)/H5PV2Mo10O40 in 9:1 AcOH/ water showed by isotope labeling that under anaerobic conditions the oxidation to CO2 takes place through oxygen transfer from the polyoxometalate framework to CO.113 See section 5.2 below for further discussion for such oxygen transfer reactions. On the other hand, under aerobic conditions, that is, reactions using Pd(0)/H5PV2Mo10O40 in 9:1 AcOH/water with CO/O2 as coreactants, reactions showed a very complex reactivity pattern. Thus, the formation of a nucleophilic intermediate seemed likely using the oxidation of thianthrene oxide as a probe substrate, where it was shown that the sulfoxide was more reactive than the sulfide.114 On the other hand, upon addition of toluene to the reaction mixture, predominately nonselective ring hydroxylation was observed, Scheme 12, perhaps more indicative of a radical intermediate. The lack of ortho/para selectivity in the hydroxylation reaction also rules out an electrophilic oxidant. Finally, in the oxidation of a representative terminal alkene, 1-octene, hydrocarbonylation to yield the linear and branched carboxylic

More generally, using a similar reaction protocol, acrylates can be reacted with aliphatic aldehydes to yield 3-carboxy furans in a ring closing reaction that is thought to proceed through allylic palladation of an in situ formed acetal according to Scheme 11.102 Scheme 11. Synthesis of 3-Carboxyfurans from Aliphatic Aldehydes and Acrylate Esters

Under very similar reaction condition acrylate esters, preferably iso-butyl acrylate, can be trisannelated to yield the 1,3,5-tricarboxy esters of benzene in fair yields. The authors suggest that the reaction proceeds through Wacker oxidation of the acrylate ester catalyzed by Pd(OAc)2/H7PV4Mo8O40/O2, followed by acetal formation, and then a CeCl3 catalyzed annelation reaction.103 3.4. Oxidative Dehydrogenation of Alcohols

The oxidative dehydrogenation of alcohols with Pd(II) is a well-known transformation that occurs typically in the presence 2686


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late-protected NPs were impregnated onto aluminum- or silicon-oxide supports and used as heterogeneous catalysts for aerobic epoxidation, dehydrogenation, and the selective oxidation of secondary alcohols.131 The selective epoxidation of alkenes using only molecular oxygen and no sacrificial reducing agents is an important, and as yet, largely unsolved objective with considerable commercial implications. With regards to selectivity, a major obstacle is the relative ease with which allylic C−H bonds are cleaved. In this regard, the effectiveness of heterogeneous Ag(0) catalysts is limited to the aerobic epoxidation of ethylene, which has no allylic C−H bonds. To address this, PV2Mo10O405‑-stabilized Ag(0) and Ru(0) nanoparticles were investigated131 as catalysts for the aerobic epoxidation of cyclohexene and 1-methylcyclohexene, both of which are sensitive to autoxidation processes that typically lead to poor selectivity. Both polyoxometalateprotected NP systems gave improved selectivity, for these reactions, as well as for epoxidations of a series of primary and secondary olefins. Selectivity was generally higher at smaller percent conversions and decreased as the reactions proceeded, possibly due to polyoxometalate degradation. When 1-octene was used as a model substrate, for example, the selectivity for the formation of 1-octene oxide early in the reaction (at 17% conversion) was 88%. As the reaction proceeded, however, the selectivity decreased, and more of the substrate was converted to autoxidation products as detected by gas chromatography. While aerobic oxidations of activated alcohols, such as benzylic and allylic ones, are well-known, analogous oxidations of secondary alcohols, which are much less reactive, are rarely reported. In this context, Neumann prepared heterogeneous catalysts for the aerobic oxidation of secondary alcohols to ketones by supporting PV2Mo10O405−-stabilized 2.6 nm diameter Pt(0) NPs on α-alumina.132 Oxidations of secondary alcohols were carried out under relatively mild conditions: 14 h at 125°C under two bar O2. At 5% catalyst loading on αalumina, secondary alcohols, such as 2-decanol, 2-octanol, 2hexanol, 2-pentanol, cyclohexanol, and cyclooctanol, were oxidized to ketones with 100% selectivity. For the acid form of PV2Mo10O405‑ as metal(0)-NP stabilizing ligand, turnover numbers (TONs) were typically ca. 10. Interestingly replacement of the polyoxometalate’s H+ counter-cations by Rh(I) cations led an increase in TONs to between 50 and 90. Conversions of alkanes to alkenes, and of alkylarenes to vinylarenes, can occur via dehydrogenation and oxy-dehydrogenation. A polyoxometalate with pendant thiol-terminated ligands, [(C6H13)4N]4[SiW11O40(SiCH2CH2CH2SH)2], was used to stabilize Pd(0) NPs, giving effective catalysts for the aerobic oxy-dehydrogenation of substrates with allylic tertiary C−H bonds.133 For example, after 14 h at 140 °C in α,α,αtrifluorotoluene (chosen for its high boiling point), 4vinylcyclohexene was transformed (with 80% conversion) to styrene (77%) and ethylbenzene (23%) as the only products (a mass balance of 100%). No aggregation of Pd(0) or formation of Pd(0) black was observed during the reaction. Polyoxometalates have also been explored as acidic supports for catalytically active metal(0) NPs. For this, the acid catalyst, α-H3PW12O40, was rendered relatively water insoluble by partially replacing H+ countercations by Cs+ or Ba2+. Gold(0) NPs loaded onto α-CsxH(3‑x)PW12O40 were used to catalyze the combined acid-hydrolysis and aerobic oxidation of cellobiose and cellulose to gluconic acid.134 Along with radical initiators, Au(0) NPs on α-BaxH(3−1/2x)PW12O40 was used for the selective autoxidations of norbornene and cyclooctene to the

Scheme 12. Oxidation of Toluene Catalyzed by Pd(0)/ H5PV2Mo10O40 with CO/O2

acid was observed. In addition, approximately equal amounts of 1,2-epoxyoctane was also formed as an intermediate that, however, proceeded to react in multiple pathways including acid catalyzed ring opening, along with C−C bond cleavage and subsequent formation of furanones. Cyclic alkenes also gave carboxylation products, for example cyclopentene yielded in the presence of methanol, dimethyl cis-1,2-cyclopentanedicarboxylate and dimethyl cis-1,3-cyclopentanedicarboxylate.115

4. OTHER METAL−POLYOXOMETALATE COCATALYTIC SYSTEMS 4.1. Oxidations by Polyoxometalate Protected Metal(0) Nanoparticles

The first reported polyoxometalate-protected metal(0) nanoparticles (NPs) were prepared in 1952 by reacting photochemically reduced α-H3PW12O40 with AgNO3 in water.116 Since then, numerous polyoxometalates have been shown to serve as protecting ligands for a wide variety of metal(0) NPs in both organic solvents and water. In the early 1990s,117 Finke discovered that H2 reductions of (Bu4N)5Na3[(1,5-COD)M· P2W15Nb3O62] (M = Rh(I)118,119 and Ir(I)120,121), gave Rh(0) and Ir(0) NPs, stabilized by an electrosteric, “combined DLVO-type electrostatic plus steric-stabilization”, mechanism.122 Papaconstantinou123 later used photochemically reduced α-H 3 PW 12 O 4 to the prepare polyoxometalate -protected metal(0) NPs of Au, Pd, and Pt. More generally, polyoxometalate-stabilized metal(0) NPs can be prepared using a variety of polyoxometalates, by (1) reacting prereduced polyoxometalates with metal salts, (2) chemically reducing metal salts in the presence of polyoxometalate ligands, or (3) introducing polyoxometalates via ligand exchange to preformed organic-ligand protected metal(0) NPs.124−126 These methods have led to preparations of heteropoly molybdovanadate and tungstate protected metal(0) NPs of Ru, Rh, Pd, Pt, Ir, Ag, and Au. Moreover, direct binding of electrostatically stabilized127 polyoxometalate-ligand monolayers on the surfaces of Ag(0) and Au(0) NPs has been documented by transmission electron microscopy of cryogenically frozen samples (cryo-TEM).128,129 While polyoxometalate-protected metal(0) NPs are catalysts for a variety of reactions,126,130 some have been used as supported or homogeneous catalysts for aerobic oxidations of organic substrates. H5PV2Mo10O40 has been used to stabilize NPs. In this way H5PV2Mo10O40 was reacted with metallic Zn, to give 2e−reduced ZnH5PVIV 2 Mo10O40, which then served as a reducing agent for preparing H5PV2Mo10O40-protected NPs of Ag(0),131 Ru(0),131 and Pt(0).132 The protecting ligand, H5PV2Mo10O40, is not only an effective catalyst for O2 oxidations but, like many polyoxometalates, can simultaneously act as a radical scavenger to inhibit unselective autoxidation process. The polyoxometa2687


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corresponding epoxides.135 Radical-chain autoxidations of both these substrates give high selectivity to the corresponding epoxides. 1 3 6 Palladium(0) NPs supported on αCsxH(3‑x)PW12O40 were used to catalyze the selective reduction of O2 to H2O2 by MeOH137 or iPrOH.138 After demonstrating that polyoxometalates can bind directly to catalytically active colloidal metal(0) NPs, Finke described the polyoxometalate components as “soluble analogs” of heterogeneous metal-oxide supports.118 While many of these feature high catalytic activity and selectivity, reactivity is generally attributed to the activities of the metal(0) cores, with the polyoxometalates providing a stabilizing function, acidic sites, or reversible redox properties for quenching organic-radical intermediates.126 A direct role of polyoxometalate ligands in modifying the catalytic activities of metal(0) cores themselves was reported in 2017.139 After using cryoTEM imaging and UV−vis spectroscopy to document and quantify coverage of 14 nm diameter Au(0) NPs by the monodefect ion, [α-PW11O39]7−, Wang and Weinstock investigated the effect of polyoxometalate ligands on a model reaction, the aerobic (O2) oxidation of CO to CO2, at the surface of the Au(0) NPs in water.139 Rates of CO oxidation increased linearly with percent-coverage of the gold nanoparticles by from zero to 211 ± 19 surface-bound molecules of [α-PW11O39]7−. X-ray photoelectron spectroscopy (XPS) of Au(0) NPs protected by [α-PW11O39]7−, the Wells−Dawson ion [α-P2W18O62]6−, and the monodefect Keggin anion, [αSiW11O39]8−, revealed that binding energies of electrons in the Au-4f7/2 and 4f5/2 atomic orbitals decreased as a linear function of polyoxometalate charge and percent-coverage of the Au(0) surface, providing a direct correlation between the electronic effects of the polyoxometalates on the Au(0) NPs, and rates of CO oxidation by O2

cleavage of H2O2 is more likely to proceed by a Haber−Weiss type mechanism leading to formation of hydroxyl radical as very reactive oxidizing species.141 Most of the research reported in this area has been directed to the hydroxylation of benzene to phenol. Thus, in an early report, Pd(II) in the presence of a lacunary polyoxometalate, PW11O397− was proposed to form complexes with Pd(II) incorporated into the lacunary position; the proposal came as a result of the observed 31P NMR chemical shifts.142,143 Pd(0) species formed in the formation of H2O2 and also in the Haber−Weiss cycle could be oxidized by O2 or by H2O2. Yields in H2 and or O2 were not given. Replacing Pd(II) by Pt(II) gave phenol yields that were 1 order of magnitude less. Further research, using a combination of Pt(II) species and phosphomolybdic acid showed activity for the hydroxylation of benzene likely due to the high oxidation potential of the molybdates versus tungstates.144−146 Such catalysts were also active for the oxidation of cyclohexane to cyclohexanol and cyclohexanone.147,148 Attempts to prepare heterogeneous catalysts were less successful although the addition of Fe(III) did improve the catalytic efficiency.149 Since a H2−O2 mixture tends to be explosive, the possibility of forming H2O2 by reaction of CO and O2 where it was assumed that H2 is formed in situ by a water−gas shift type reaction has also been explored. Indeed, in this way benzene was hydroxylated to phenol; isotope labeling experiments using H218O pointed to the intermediacy of H2O2.150 Although originally it was reported that this reaction proceeds in the absence of Pd, a further report from the same group a few years later would seem to indicate that indeed some Pd/C was indeed needed.151 4.3. Oxidation of Alkanes: Shilov Type Reactions

Ever since the discovery that alkanes, especially methane, can be oxidized in a reaction catalyzed by Pt(II) using a stoichiometric oxidant there has been an interest in developing an aerobic version of this reaction based on the possible redox activity of Pt(II) in the presence of oxidizing polyoxometalates such as H3+xPVxMo12‑xO40 that can be reoxidized with O2 according to a simplified Scheme 14. In practice, it has been

4.2. Catalysis with H2−O2 Mixtures

Mechanistically distinct reactions from what was described in section 3 involving a metal−polyoxometalate cocatalytic system are those carried out using a combination of H2 and O2. Here the metal, typically a noble metal, catalyzes the in situ formation of H2O2 that then reacts with the polyoxometalate, Scheme 13.

Scheme 14. Shilov Type Oxidation of Methane with a Polyoxometalate and O2

Scheme 13. Oxidation with H2−O2 Mixtures

difficult to achieve this goal to a significant extent. An early attempt using simple Pt salts and Na8HPV6Mo6O40 yielded some catalytic turnover but formed methanol and chloromethane in equal amounts and also lead to further oxidation of methanol.152 Years later a Pt(II)bipyrimidine complex was prepared that was ionically bonded to H5PV2Mo10O40 and then supported on silica. Oxidation of methane in this case also showed catalytic turnover; however, a C−C coupling reaction to yield acetaldehyde was a competing reaction. After some time, formaldehyde formed by the further oxidation of methanol was also observed which more or less coincided

Such an H2O2 polyoxometalate reaction can yield two outcomes. Formation of tungsten or molybdenum-peroxo intermediates competent for oxygenation of nucleophiles such sulfides and alkenes. An example of such a transformation is the oxidation of propylene with an Au(0)−polyoxometalate catalyst that depending on the method of catalyst preparation tended to yield acrolein, acetic acid, and total combustion products.140 However, more typically, especially in the presence of vanadium addenda in the polyoxometalate framework or alternatively another oxidizing transition metal the oxidative 2688


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Scheme 15. Pathway for the Oxidative Dehydrogenation of α-Terpinene

Supported catalysts on fluorapatite,160 and other carbon supports,161,162 have also been reported for this reaction, as has the aromatization of 1-4-dihydropyridines.163 5.1.2. Oxidation of Alcohols and Amines. During the subsequent decade, additional electron transfer or redox oxidation reactions catalyzed by H5PV2Mo10O40 and analogues were described. One transformation that was discovered involved the oxidation of benzylic alcohols to the corresponding benzaldehyde derivatives using a nonacid catalyst, Na5PV2Mo10O40, supported on active carbon.164 It should be noted that others reported the oxidative of aliphatic cyclic alcohols to the corresponding ketones.165 Years later it became apparent that Na5PV2Mo10O40 did not directly catalyze this reaction, but likely its support on active carbon led to formation of active species, such as quinones, on the carbon surface. These active species are apparently responsible for the activity observed. In fact, it was shown that quinones, such as p-cloranil (2,3,5,6-tertrachloro-1,4-benzoquionone), in combination with Na5PV2Mo10O40 oxidized benzylic alcohols.166 Based on kinetic and UV−vis and EPR spectroscopic measurements, it was suggested that the actual oxidizing species responsible for the alcohol oxidation is a semiquinone−polyoxometalate complex. In another extension of this reaction, Cu2PVMo11O40 was supported on MgO and used for the oxidation of primary alcohols such as 1-hexanol to hexanal albeit at 380 °C and long reaction times of 50 h.167 A decavanadate catalyst was also reported to be active for oxidative dehydrogenation of secondary alcohols,168 as were iron and other substituted phosphomolybdates.169−172 It was also observed that the same catalytic system, 164 and also the ammonium salt of [PV6Mo6O40]9−, was also active for the dehydrogenation of benzylic and primary amines,173 as was a phosphovanadotungstate supported on silica albeit in a vapor phase reaction.174 In these reactions the amines, RCH 2NH2, are apparently dehydrogenated to the corresponding imine RCH = NH, which is hydrolyzed in the presence of even traces of water to yield the corresponding aldehydes, RCHO. The latter are not observed, reacting with the initial imines to form Schiff bases as the initial observable product. However, over longer reaction times these Schiff bases are eventually transformed entirely to the aldehyde product via a hydrolysis/dehydrogenation cycle, Scheme 16. This hydrolysis/dehydrogenation strategy has been extended to semicarbazones, phenylhydrozones, and oximes yielding the corresponding aldehydes.175 5.1.3. Oxidation of Activated Phenols. Another electron transfer oxidation reaction that was studied was the oxidation of activated phenols, that is those with a relatively low oxidation

with the deactivation of the catalyst presumably due to the coordination of CO to Pt.153 More recently, it has been reported that such an oxidation reaction can also take place with [PdCl4]2− in trifluoroacetic acid to yield both the C−C coupled product, acetic acid, and methyltrifluoroacetate as the trapped methanol product.154 Another notable effort in this area involved the use of microfluidics in order to implement genetic algorithms for the discovery of homogeneous catalysts for the oxidation of methane with O2. In this way hundreds of reactions were screened. Not surprisingly Pt(II) was identified as the most active catalyst, but a combination of Fe(II) and H5PV2Mo10O40 was found to be the best cocatalysts, rather than H5PV2Mo10O40 alone. Reactivity of the optimal combination led to a maximum turnover number of ∼50 with formation of equal amounts of methanol and formic acid.155

5. PHOSPHOVANADOMOLYBDATE CATALYZED REACTIONS 5.1. Electron Transfer Oxidations

5.1.1. Dehydrogenation of Dienes. Although phosphovanadomolybdates, especially H3+xPVxMo12‑xO40, were used in many transformations as a cocatalyst, especially in palladium catalyzed reactions as described above, a major field of interest involves the use of this compound as a catalyst itself. Thus, in the literature there has been much research related to both synthetic and mechanistic aspects of these H3+xPVxMo12‑xO40 catalyzed reactions. Although the use of molybdenum/ vanadium/tungsten oxides typically as insoluble nonstoichiometric compounds had been known for some time in high temperature gas phase oxidations, the first reports on the use of soluble polyoxometalates in the context of catalytic oxidations that involved molecular oxygen in the liquid phase began only in the late 1980s. One paper in 1989 reported on the aromatization of dienes, such as cyclohexadiene and limonene to the corresponding arenes, benzene, and p-cymene.156 Further mechanistic research on the oxidative dehydrogenation of α-terpinene to p-cymene, Scheme 15, firmly established the paradigm that H5PV2Mo10O40 (the compound most often used) could oxidize substrates by an electron transfer mechanism.157 Use of both UV−vis and EPR spectroscopy showed together the formation of a substrate−H5PV2Mo10O40 complex where one vanadium atom was reduced and a cation radical of the organic substrate was formed. It was also suggested that the reoxidation of the polyoxometalate was through an inner sphere reaction of H7PVIV 2 Mo10O40 with O2 via formation of a μ-peroxo intermediate followed by formation of water. It should be noted that there were several previous reports on the kinetics of oxidation of reduced polyoxometalates of these types in general support of the idea of a four electron oxidation of the reduced vanadium containing polyoxometalate by O2 but no spectroscopic support for exactly which reactions may be occurring in solution.158,159

Scheme 16. Hydrolysis/Dehydrogenation Pathways for Oxidation of Compounds Containing CN Bonds

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potential, to the corresponding benzoquinones,176−179 or via “tail to tail” coupling reactions to the corresponding diphenoquinones, presumably through diphenol intermediates.180 In the latter case it was clearly shown that the reactivity is directly correlated with the oxidation potential of the substrate. Also, it should be noted that 2,3,6-trimethyl phenol also undergoes dimerization, but inexplicably the reaction yields the 2,2′,3,3′,5,5′-hexamethyl-4,4′-biphenol and not the diphenoquinone. An interesting reaction of industrial interest is the oxidation of 2-methyl-1-napthol to 2-methyl-1,4-naphthoquinone (menadione).181 In a similar reaction aniline can also be oxidized to polyaniline or 1,4-benzoquinone depending on the catalyst/aniline ratio.182 5.1.4. Other Electron-Transfer Oxidations. The use of nitroxyl compounds, notably TEMPO (2,2,6,6-terapiperidine oxide), as precursors to nitrosium species that are wellestablished stoichiometric oxidants, was also reported. Such systems were used, for example, for the oxidation of alcohols to the corresponding aldehydes and ketones. The N-hydroxy product of this reaction can be reoxidized using H5PV2Mo10O40, thereby leading to an aerobic catalytic cycle based on TEMPO-H5PV2Mo10O40.183 Supporting the catalysts on magnetic silica has proved to be useful for catalyst recovery.184 The oxidation of a bromide anion under acidic conditions to provide an electrophilic brominating agent in situ, that then reacted with arenes at a ring position, was first described in reactions carried out in aqueous acetic acid; turnover numbers were low.185 This reaction was further developed by use of a less polar solvent, dichloroethane, with tetraglyme as an agent to dissolve H5PV2Mo10O40. Using gaseous HBr, selective para oxybromination of phenol was observed with high turnover,186 but under these conditions H5PV2Mo10O40 was not stable and likely not the actual catalyst.187 Catalytic iodination can also be catalyzed by H5PV2Mo10O40, where the polyoxometalate acts to oxidize I2 to I+ according to Scheme 17, thereby leading to a 100% theoretical yield based on iodine. High yields were typically obtained.188

from water as evidenced by the formation of 18-O labeled amides using H218O.191 5.2. Electron Transfer−Oxygen Transfer Reactions

Already in 1942 it was reported that mixed addenda acidic polyoxometalates could oxidize naphthalene in the vapor phase to phthalic and maleic anhydride.192 In ensuing years such polyoxometalates were studied as catalysts in various gas phase reactions that proceeded by what was afterward called Mars− van Krevelen type reactions. 193 In this scenario the polyoxometalate acts as an electron acceptor from the substrate on the one hand, leading to a reactive intermediate species and on the other hand acts as an oxygen donor, thereby leading to an oxygenated product. Therefore, under anaerobic conditions, the oxidation reactions take place by oxygen transfer from the polyoxometalate. Separately or in situ, the polyoxometalate can be reoxidized with O2. A generic representation of such socalled electron transfer-oxygen transfer reactivity is presented in Scheme 18. Scheme 18. Generic Electron Transfer−Oxygen Transfer Reactions

Despite this well-known paradigmatic reactivity in the gas phase at high temperatures, there were not any proven examples of such reactions at low temperatures in the liquid phase until the year 2000. Reactions were carried out on two model substrates using H5PV2Mo10O40 as catalyst.194,195 In the case of anthracene, the reaction is thought to proceed by a twoelectron oxidation to anthrone as shown in Scheme 19. This is followed by an additional much faster four-electron oxidation to 9,10-anthraquinone (not shown), which is the product that is finally obtained. The second model reaction deals with the oxidation of xanthene as shown in Scheme 20 where the product first formed is 9-xanthenol through a two-electron oxidation. 9Xanthenol is then oxidized more quickly by an additional twoelectron oxidation to 9-xanthone as the final product. The oxygen transfer from H5PV2Mo10O40 to the two substrates was investigated using various techniques to show that other commonly observed liquid-phase oxidation mechanisms, autoxidation and oxidative nucleophilic substitution, were not involved in these reactions. This was carried out by the use of 18-O labeled molecular oxygen, polyoxometalate, and water; performing reactions under anaerobic conditions; performing reactions with an alternative nucleophile (acetate); and determining the reaction stoichiometry. All evidence supported a Mars−van Krevelen type mechanism and ruled out autoxidation and oxidative nucleophilic mechanisms. Furthermore, the mode of activation of both substrates was determined to be by electron transfer, as opposed to hydrogen atom transfer from the hydrocarbon to the polyoxometalate. An outer-sphere electron transfer coupled with the formation of a donor−acceptor complex was suggested as a key activating reaction. The outer sphere electron transfer, using Marcus theory, also explained the thermodynamic feasibility of the reaction that is fortuitously enabled by the high negative charge of the polyoxometalate. Intermediates were also directly observed by EPR and NMR spectroscopy measurements. For anthracene, the immediate result of electron-transfer, the

Scheme 17. Catalytic Iodination of Arenes

S o m e r e c e n t e x a m p le s d e s cr ib in g t h e us e o f H3+xPVxMo12‑xO40 as catalysts include the oxidative cyanation of quinolone and its derivatives at the 4-position using trimethylsilyl cyanide as the cyanation reagent.189 Pyridine and its derivatives did not react. Another interesting transformation is the trifluoromethylation of arenes that can be carried out using sodium trifluoromethylsulfinate (Langlois’s reagent). Instead of using t-butylhydroperoxide as oxidant, the reaction can be carried out using H6PV3Mo9O40 and O2.190 Secondary and tertiary thioamides can be converted to the corresponding amides in a reaction catalyzed by H6PV3Mo9O40, where the oxygen atom of the amide formed appears to come 2690


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Scheme 19. Pathway for the Oxidation of Anthracene to Anthrone

Scheme 20. Pathway for the Oxidation of Xanthene to 9-Xanthenol

different. Here vanadium(IV) remains imbedded with the coordination sphere of the complex, [PVVVIVMo10O40]6−, and this structure is associated with the distal isomers of H5PV2Mo10O40. From this one can see that all isomers of H5PV2Mo10O40 are similarly reduced by electron transfer, an observation that is also supported by DFT calculations.199 An important question that arose was whether both species were involved in oxygen transfer. Again, using EPR spectroscopy the anaerobic oxidation of 13CO to 13CO2 was studied. Using 2D correlations between 13C and the V(IV) center, it was clear that species I was active in oxygen transfer while species II was not. From these EPR studies one can summarize that the ion paired complex [VIVO]2+[PVVMo10O39]8− is active in oxygen transfer. It is important to note, however, that VO2+ itself does not appear to be an active oxygen transfer agent and furthermore is inactive to reoxidation by O2. The polyoxometalate framework is crucial to the functionality in terms of oxygen transfer and reoxidation with O2. It should also be noted that other vanadium containing polyoxometalates are electron transfer oxidants but H5PV2Mo10O40 and its analogs are quite unique in their activity as electron transfer−oxygen transfer catalysts.200 The aforementioned experimental study was further supplemented by a high level computational DFT study.201 As a basis of the study the energetics of both outer sphere proton and electron transfer as well as proton coupled electron transfer were computed. The calculations showed that protons mediate the formation of coordinatively unsaturated structures by displacement of oxygen atoms preferably around vanadium atoms. Further calculations showed that electron transfer, for example from anthracene or xanthene, further facilitates the formation of coordinatively unsaturated structures; that is, reduced structures tend more toward formation of defects. After electron transfer, coordination of the reductant/substrate leads to formation of reactive ensembles followed by oxygen transfer. In essence, the DFT calculations support the experimental findings pointing to the crucial importance of formation of defects on the polyoxometalate surface in enabling oxygen transfer.

formation of cation radical and reduced polyoxometalate, was observed by EPR spectroscopy. The EPR spectrum, together with kinetics experiments, including kinetic isotope experiments an 1H NMR, supported an electron-transfer oxygen transfer mechanism where the rate-determining step is the oxygentransfer reaction between the polyoxometalate and the intermediate cation radical. For xanthene, the cation radical was not observed. Instead, the initial cation radical undergoes fast additional proton and electron transfer to yield a stable benzylic cation observable by 1H NMR. Here also kinetics experiments supported an oxygen-transfer rate-determining step between the xanthenyl cation and the polyoxometalate. This research on the oxidation of anthracene and xanthene established the concept of the electron-transfer oxygen transfer mechanism catalyzed by H5PV2Mo10O40 and assumingly analogous compounds with different V−Mo ratios but left open questions on the way H5PV2Mo10O40 compound itself reacted at the molecular level. These questions were studied by both EPR experiments and studies based on DFT calculations. Although the structure of the oxidized form of H5PV2Mo10O40, Figure 5, is well-known as is the dependence of the structure of this compound as a function of temperature,196 much less is known about the structure of the one-electron reduced compound [PVVVIVMo10O40]6−, which is the key component in complexes that lead to oxygen transfer from the polyoxometalate to the substrate as shown above. Thus, highfield pulsed EPR spectroscopy at very low temperatures enabled the resolution of hyperfine couplings belonging to 31 P, 51V, 95 Mo, and 17O nuclei with respect to the vanadium(IV) center in the [PVVVIVMo10O40]6− anion.197 Two different species were identified. The first, species I, is similar to the vanadyl (VO2+) dication and is not clearly within the coordination sphere of the polyoxometalate, leading to the conclusion that the vanadyl cation is ion paired to a lacunary (defect) Keggin structure, [VIVO]2+[[PVVMo10O39]8−. The existence of such ion pairs had been suggested in the past.198 We found that this species originates from isomers of [VIVO]2+[[PVVMo10O39]8−, where the vanadium atoms are nearest neighbors, Figure 5. Another compound, species II, is 2691


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5.2.1. Carbon−Carbon Bond Cleavage Reactions. Over the years it has been noticed that especially H5PV2Mo10O40, but also other similar vanadium containing compounds, shows rather unique reactivity in various C−C bond cleavage reactions. In a series of papers it was shown, largely without mechanistic insight, that cycloalkanones such as 2-methylcyclohexanone and 1-phenylalkanones such as methyl-benzylketone can be oxidized to 6-oxoheptanoic acid and benzaldehyde and acetic acid, respectively.202−204 Aliphatic ketones also undergo this C−C bond cleavage reaction as does cyclohexanone which yields adipic acid rather efficiently; this latter transformation may be an interesting substitute for the presently practiced industrial oxidation with nitric acid.205,206 The mechanism of the oxidation of cyclohexanone to adipic acid with air, catalyzed by H3+xPVxMo12‑xO40 (x = 1 and 2), was investigated later on.207 In water, a redox mechanism was proposed where the reoxidation of the reduced polyoxometalate by oxygen is rate limiting. In acetic acid/water and with very low amounts of H3+xPVxMo12‑xO40 a radical chain autoxidation mechanism was proposed, in line with earlier observations that H3+xPVxMo12‑xO40 can act as an antioxidant, that is, prevent autoxidation at significant concentrations.157 The next development in this area of C−C bond cleavage reactions was related to the oxidation of primary aliphatic alcohols, which under typical oxidizing conditions react to yield the corresponding aldehydes and then carboxylic acids. Atypically, H5PV2Mo10O40 catalyzes a different reaction as shown in Scheme 21 for several exemplary substrates where C− C bond cleavage occurs between the α and β carbon atoms to the alcohol substituent.208

importance of coordination of the polyoxometalates to the substrate was obvious from the observation that cis-1,2cyclohexanediol reacted much faster than trans-1,2-cyclohexanediol. Others also reported on high selectivity to adipic acid in this reaction.209 Secondary and tertiary alcohols did not react in this way, instead undergoing acid catalyzed elimination to yield the corresponding alkenes that were not oxidized. The findings that H5PV2Mo10O40 and similar compounds can lead to C−C bond cleavage of diols and primary alcohols as well as the strong Brønsted acidity of these polyoxometalates have led to a flurry of activity in the area of oxidation of biomass and biomass related compounds.210 One area of research has been the oxidation of glucose, other saccharides, oligosaccharides, and polymers such as cellulose, starch, and xylan (hemicellulose) and even algae to formic acid and to CO2 in varying degrees of conversion and selectivity.211−220 This research first reported by the Wasserschied group has yielded many variants, but basically all of the research has in common the ability of H3+xPVxMo12‑xO40 to react with the diol, ketone, and primary alcohol groups that these compounds have in common. It is interesting to note that in one case, using H3PMo12O40 in place of the vanadium containing analogues, the formation of glycolic acid from cellulose as the major product with less formation of formic acid has been reported.221 However, since it is less likely that an electrontransfer oxygen transfer pathway is in place here and the reaction temperature is quite high, 180 °C, it is possible that the reaction mechanism is different than from what has been described above. Another interesting result is the oxidation of fructose to 1,5-furan-dicarbaldehyde as a major product using Cs3HPVMo11O40 as a heterogeneous catalyst.222 The authors suggest that the reaction occurs via acid catalyzed dehydration of fructose to 5-hydroxymethyl furfural and then further oxidation to the dialdehyde. On the other hand, others have reported that the oxidation of 5-hydroxymethyl furfural catalyzed by H5PV2Mo10O40 yields maleic anhydride and maleic acid.223 They suggest a mechanism where there is C− C bond cleavage between the hydroxymethyl groups and the furan ring. Specifically, they note that 1,5-furan-dicarbaldehyde and further oxidized species (2,5-furandicarboxylic acid and 5formyl-2-furancarboxylic acid) are not intermediates in the formation of maleic anhydride. Another biomass related substrate of interest is glycerol, which is formed as a byproduct in the transesterification of triglycerides with methanol in biodiesel production. Thus, the transformation of glycerol to lactic acid was catalyzed by H3PMo12O40 but also by AlPMo12O40 and CrPMo12O40.224,225 Although formally this is an isomerization reaction, the observation that dihydroxyacetone, glyceraldehyde, and pyruvaldehyde were formed as intermediates suggests a more complicated redox process yet to be deciphered. Interestingly, inclusion of VO2+ as a cation leads to formation of acrylic acid as the main product by an additional dehydration of lactic acid.226 5.2.2. Activation of Carbon−Metal Bonds. An interesting goal toward functionalization of alkanes would be a system where alkanes are activated by a coordination compound to yield organometallic intermediate containing alkyl-metal moieties. One of the next possible steps in such an alkane functionalization scheme could be oxygen insertion into the carbon−metal bond to yield the corresponding alcohol or oxidative dimerization of alkyl substituents, Scheme 23. These two types of reactivity have been demonstrated using

Scheme 21. Oxidation of Primary Alcohols by H5PV2Mo10O40

Deuterium labeling and kinetic isotope effect kinetics on the oxidation of 1-butanol showed that C−H bond cleavage was not involved in the rate-determining step. Furthermore, 13C NMR measurements that showed significant shift upon addition of the polyoxometalate to 1-butanol as well 18-O isotope labeling studies lead to the proposition of a reaction pathway as shown in Scheme 22. It should be also noted that diols also undergo C−C bond cleavage reactions. The Scheme 22. Mechanism for Oxidation of Primary Alcohols

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H5PV2Mo10O40 is strongly a function of acidity, whereby the oxidation potential can reach 1.1 V versus SCE in >50% aqueous H2SO4.229 In common organic solvents and in water, the oxidation potential of H5PV2Mo10O40 is much lower, ∼0.45 V versus SCE. Thus, while H5PV2Mo10O40 can catalyze the oxidation of xanthene and other easily oxidizable substrates in common organic substrates, the reaction fails for toluene and its derivatives. However, carrying out the reaction in >50% aqueous H2SO4 allows the transformation of toluene and many of its derivatives to the corresponding benzaldehyde compounds, in high yield. From another point of view, intuitively and as confirmed by DFT calculation, in strong acid aqueous H2SO4, H5PV2Mo10O40 can be protonated to a cationic intermediate, which is more reactive as an electron transfer oxidation. The suggested oxidation pathway for toluene and in contrast the pathway for xanthene under nonacidic conditions is presented in Scheme 24. The benzylic alcohol products formed remain in only trace amounts as they are quickly oxidized to the corresponding carbonyl product. The formation of carboxylic acids was not observed. It is notable also to state that the oxidation of compounds such as 4-nitrotoluene under very acidic conditions, or nitrobenzene in the absence of solvent,230 raised the question whether the cation radical of benzene, a compound relatively inert to electron transfer oxidation could lead to formation of a cation radical in the presence of H5PV2Mo10O40 in very acidic solution.231 Using high field W-band EPR, the presence of C6H6+• was verified. However, this compound was not susceptible to oxygen transfer from the polyoxometalate to yield phenol, because the intermediate C 6 H 6 + • − H5PVIVVVMo10O40 is at a thermodynamic minimum toward the formation of phenol. On the other hand, O2 was found to directly react with C6H6+•− H5PVIVVVMo10O40 although the yields were only 60%. As discussed above, H5PV2Mo10O40, was shown to be active for the deconstruction of glucose and other carbohydrates to formic acid, although the reaction of cellulose, likely due to its low solubility, was typically incomplete and required more forceful conditions. Analysis of these reactions reveals that the

Scheme 23. Oxidation of Alkyl−Metal Bonds

H5PV2Mo10O40 as catalyst. In the first case, stable tetraalkyl tin compounds, SnR4 (R = Me, n-Bu) were reacted with H5PV2Mo10O40 polyoxometalate that mediated the insertion of an oxygen atom from the H5PV2Mo10O40 into the tin− carbon bond of n-Bu4Sn 1-butanol and (n-Bu3Sn)2O.227 The UV−vis and EPR spectra show that the reaction is initiated by electron transfer from n-Bu4Sn to H5PVV2 Mo10O40 to yield an ion pair n-Bu4Sn+•−H5PVIVVVMo10O40 that is observable by 13 C NMR. The H5PVIVVVMo10O40 moiety was identified UV− vis and EPR. The n-Bu4Sn+•−H5PVIVVVMo10O40 intermediate ion pair is relatively unstable and forms more stable Bu+ and Bu3Sn+ cations coordinated to the polyoxometalate, which were also identified by ESI-MS and calculated to be viable by DFT calculations. In the presence of molecular oxygen as the terminal oxidant the reaction is catalytic in H5PV2Mo10O40. In the second example of reactions a series of new dialkyl cobalt(III) complexes [(R)2CoIII(bpy)2]ClO4 (R = Me, Et, nPr, and Bz) were prepared.228 Upon addition of H5PV2Mo10O40 as is typical, an intermediate, [(R) 2 Co IV (bpy) 2 ] 2+ − H5PVIVVVMo10O40, was formed where both the cobalt(IV) and the vanadium(V) moieties were identified and quantified by X-ray photoelectron spectroscopy. At 60 °C a catalytic reaction can be sustained. For aliphatic substituents, ethane, nbutane, and n-hexane are formed selectively when R = Me, Et, and n-Pr, respectively. When R = Bz, there was mostly oxygen insertion into the cobalt−carbon bond and benzaldehyde was the major product. 5.2.3. Reactions in Strong Acids. The H5PV2Mo10O40 polyoxometalate is typically synthesized by reacting phosphoric acid, vanadium(V) oxide, and molybdenum oxide under strongly acidic conditions through a “self-assembly” reactions. Therefore, H5PV2Mo10O40 is also stable under strongly acidic conditions. Furthermore, the oxidation potential of

Scheme 24. Reaction Pathways for Hydroxylation of Xanthene in Acetonitrile (Top) and Toluene (Bottom) in Aqueous H2SO4

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Scheme 25. Formation of Synthesis Gas from Cellulose

product formation as shown in Scheme 26, followed by catalyst reoxidation and (ii) direct aerobic oxidation of the intermediate as recently demonstrated in the oxidation of benzene.230 Thus, although much is known on the mechanisms of substrate oxidation much less is known about the reactions of such reduced polyoxometalates with oxygen. It has been shown that a Fe(II)-substituted polyoxotungstate binds O2,233 but more generally the reoxidation of reduced polyoxotungstates with O2 in water has been shown to proceed by an outer-sphere electron transfer (ET; see section 6.3 below). The mechanism is less clear in the case of phosphovanadomolybdates. Moreover, although the redox potentials for different α-Keggin polyoxometalates show the following general trend polyoxotungstates < polyoxomolybdates < polyoxovanadomolybdates,234 the rates of their aerobic reoxidation are not correlated with the redox potential: polyoxotungstates (seconds) ≫ polyoxovanadomolybdates (minutes) ≫ polyoxomolybdates (days). It has been suggested that aerobic reoxidation of H6PVVVIVMo10O40 after reaction with α-terpinene occurs through inner sphere electron transfer to oxygen,157 but other publications suggest different scenarios.235−237 Despite the mechanistic uncertainty, electrochemical reduction of H5PV2Mo10O40 to form both two-electron and one-electron reduced species shows the following reoxidation stoichiometry in each case (eqs 1 and 2). By extension, this stoichiometry and associated first order kinetics could support many possible scenarios yet to be deciphered.

formation of formic acid is coupled with the formation of H7PVIV 2 Mo10O40, which is oxidized to water with O2 (see for example Scheme 15). By carrying out the reaction in more concentrated H2SO4, two objectives can be attained.232 First the formic acid formed is dehydrated to CO. Second, under such strongly acidic conditions H7PVIV 2 Mo10O40 cannot be reoxidized to H5PV2Mo10O40 and water, but by electrolysis, H2 can be released, Scheme 25. In this way, instead of forming formic acid and water from carbohydrates, they, including cellulose, can be quantitatively deconstructed to form synthesis gas (CO and H2). H5PV2Mo10O40 also easily catalyzes the oxidation of CO with H2O to CO2 (water gas shift reaction) under these conditions with the coformation of H7PVIV 2 Mo10O40 that again can be electrochemically oxidized to H5PV2Mo10O40 and H2. Thus, the ratio CO/H2 can be controlled.

6. ACTIVATION OF DIOXYGEN 6.1. Phosphovanadomolybdates

It is notable that, similar to iron and copper enzyme based systems and their biomimetic analogues, O2 is activated by the reduced phosphovanadomolybdate species to yield a higher valent oxo species. However, different from these iron or copper catalysts, the higher valent species is not the reactive oxygen donor, rather the reduced vanadium(IV)-oxo species is the oxygen donor. Above we have described various alternative approaches for use of dioxygen as a terminal oxidant in reactions catalyzed by phosphovanadomolybdates, notably the H5PV2Mo10O40. Both electron transfer and electron transfer− oxygen transfer type reactions have been described, Scheme 26.

2H 7PV2Mo10O40 + O2 → 2H5PV2Mo10O40 + 2H 2O

(1)

4H6PV2Mo10O40 + O2 → 4H5PV2Mo10O40 + 2H 2O

(2)

Scheme 26. Oxidation of Generic Substrates (S) by H5PV2Mo10O40

6.2. Autooxidations and Reactions with Sacrificial Reductants

As stated the reduced catalyst, whose structure has been studied by EPR experiments and DFT calculations,113,197,201 can be reoxidized by molecular oxygen. This spatiotemporal separation between the substrate oxidation and the catalyst regeneration is responsible for high selectivity of such reactions. It should be stressed, as indicated in Scheme 26, that the reaction of 1 equiv of H5PV2Mo10O40 with 1 equiv of substrate or reducing agent typically leads first to an observable intermediate consisting of an oxidized substrate and reduced H5PV2Mo10O40.157,195,208,227 In principle, in the presence of O2 two scenarios can be considered for a catalytic reaction including reoxidation of the reduced H5PV2Mo10O40 (i)

Another type of reactivity observed using polyoxometalate as catalysts falls into the general category of autoxidation. In these types of reactions, a polyoxometalate may react with a substrate via a proton coupled electron transfer or by a hydrogen atom transfer mechanism. The resulting radical, R•, can react with O2 very quickly to yield a peroxo radical intermediate, R−O− O•. Radical chain reactions (autoxidation) stemming from this peroxo radical species may be elicited especially at low catalyst concentrations. However, some polyoxometalates, especially more oxidizing molybdenum and vanadium containing compounds can react efficiently with such species to form peroxo intermediates, R−O−O−POM, that may significantly affect the overall reactivity observed. An early example of such a situation was where the presence of H3PMo12O40 changed the reactivity in the oxidation of alkanes from oxygenation to oxidative dehydrogenation to yield alkenes rather than oxygenates.238 Using tert-butylhydroperoxide (TBHP) to initiate radical chain oxidations, the capture of an intermediate led instead to dehydrogenation wherein an alkyl radical formed, R•, did not react with O2 but rather underwent further dehydrogenation resulting in the formation of alkenes. These 2694


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polyoxometalates in water are below 1, and the partial negative charges residing on the outermost (more accessible) MO ligands, are smaller than those on the bridging oxide anions embedded within the clusters). (3) The V, Mo, or W addendum atoms of most polyoxometalates are coordinatively saturated by bridging or terminal oxide ligands, severely limiting their direct coordination to potential electron donors or acceptors. (4) Numerous polyoxometalates can be reversibly reduced by one or more electrons, and possess well-defined redox potentials. (5) Many polyoxometalates possess compact, approximately spherical structures. While formally tetrahedral (Td), for example, the van der Waals volumes of the 12 terminal oxide ligands at the surfaces of plenary Keggin ions give them an approximately spherical shape,282 a geometric consideration inherent to the electrostatic laws upon which much of Marcus’s development is predicated. Moreover, as soluble molecular structures with rationally tunable properties, polyoxometalates can be deployed as physicochemical probes of inorganic reaction mechanisms. The first step, however, is to obtain fundamental parameters related to the outer-sphere election-transfer properties of the polyoxometalates themselves. Information of this kind is often derived from investigation of their electron self-exchange reactions. The first detailed analysis of electron self-exchange between polyoxometalates was reported by Rasmussen and Brubaker in 1964.283 They studied the reaction between [CoIIW12O40]6− and its one-electron oxidized derivative, [CoIIIW12O40]5−, both of which are substitutionally inert with respect to the redox-active cobalt ions.284 An additional rationale for using this system in conjunction with Marcus’ model was the expectation that reduction of the (central) tetrahedrally coordinated Co(III) ions (d6, with a 5E ground state) to tetrahedral Co(II) (d7, with a 4A ground state), would occur rapidly by a thermal process, i.e., with no need for electronic excitation. Since that time, Marcus, Sutin, and others continued to improve upon a number of initial approximations and assumptions in the theory.277,280,285−288 By now, after decades of interaction between theoreticians and experimentalists, classical and semiclassical approaches have been refined, and empirical observations have led to important conclusions regarding the scope and limitations of the relatively simple, yet readily accessible and mechanistically informative models.289−296 In parallel with those advances, more accurate and versatile spectroscopic techniques for quantifying rates of electron self-exchange between polyoxometalates were developed. Those methods have made it possible to more thoroughly verify297−299 the accuracy of key theoretical parameters. The new techniques were a natural outgrowth of spectroscopic investigations of reduced polyoxometalates (heteropoly “blues”) themselves.300−306 In particular, however, the use of NMR spectroscopy followed the observation by Kozik and Baker that one-electron-reduced phosphotungstate cluster-anions, although paramagnetic, gave sharp 31P NMR signals.297−299 In 1987 stopped-flow techniques were used to study the kinetics of cross reactions between one-electron reduced polyoxometalates both with oxidized polyoxometalates and O2. Plots of the rate constants as a function of driving force showed a good fit to the Marcus model.307 More recently, an iso-structural series of α-Keggin heteropolytungstate anions308 was used to systematically investigate polyoxometalate electronself-exchange reactions309,310 and electron-transfer from

reactions are very fast as can be appreciated by the formation of the kinetic rather than the formation of the thermodynamically more stable product. Similarly, a VIVO containing polyfluorooxometalate, [H2F6NaVVW17O56]8−, partially redirected the oxidation of alkylbenzenes by autoxidation to oxidative dehydrogenation.239 Such observations showing inhibition of autoxidation and others presented below point to a high probability that reduced polyoxomolybdates and mixed addenda vanadomolbydates react with O2 by an inner sphere reaction, presumably through a cascade of reactions involving formation of superoxide and peroxide species. The aerobic oxidation of aldehydes to the corresponding carboxylic acid is a reaction that can occur quite easily over long periods of time, due to the relatively weak C−H bond in the aldehyde functional group. Thus, C−H bond cleavage can be thermally initiated or metal catalyzed, leading to radical chain reactions and formation of the corresponding carboxylic acids. Such reactions have been carried out with a variety of polyoxometalates and in a variety of contexts, such as decontamination of air containing formaldehyde to industrial synthesis of methacrylic acid from methacrolein.240−245 The easy oxidation of aldehydes and the possibility of “capturing” peroxo intermediates has led to the use of aldehyde/O2 combinations for the epoxidation of alkenes.246−250 Similarly the combination of cumene/O2 can also be used for alkene epoxidation.251 Another strategy used to aerobically oxidize substrates with polyoxometalates is to reduce the polyoxometalate with a sacrificial donor that is not an aldehyde. In this case the reduced polyoxometalate presumably reacts with O2 to yield an active intermediate capable of oxygenation. Most of the efforts in this area have been to use ascorbic acid as reducing agent, with vanadium containing polyoxometalates, toward to hydroxylation of benzene to phenol.252−257 Zinc metal has also been used as a reducing agent in this context.258 There is also a report that H5PV2Mo10O40 supported on carbon nitride can oxidize benzene with an additional reducing agent,259 but this reaction should be checked to see if the support acts as an electron donor. Finally, sodium sulfite has also been reported as a convenient reducing agent toward the epoxidation and then acid catalyzed hydrolysis of alkenes to diols catalyzed by H5PV2Mo10O40 as a bifunctional catalyst.260 There is also a considerable body of literature on the autoxidation of various hydrocarbon substrates such as cyclohexene, cyclohexane, ethylbenzene, p-xylene, and long chain alkanes.261−271 There was also a report on the aerobic epoxidation of linear alkenes, using an iron-substituted polyoxometalate. Initial reports suggested that selectivity was very high,272 but it would appear that the results obtained are commensurate with autoxidation.273,274 As a word of caution, in our experience even slight/trace contamination of linear alkene with an aldehyde can considerably influence reaction selectivity. 6.3. Outer-Sphere Electron Transfer from Polyoxotungstates to Dioxygen

Shortly after publication of Marcus’s seminal works on electron transfer,275−277 it was recognized that many redox processes involving polyoxometalates were likely to occur via outersphere278−281 mechanisms. This is because of the following: (1) Many polyoxometalates possess charge densities considerably smaller than those of traditional noncoordinating anions, such as ClO4− and NO3−. (2) The partial negative charges on the terminal oxide−anion ligands, MO, of many polyoxometalate structures are small (e.g., the pKa values of protonated forms of 2695


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reduced polyoxometalates to dioxygen.311,312 The iso-structural anions were obtained by varying the heteroatom, Xn+, in the plenary, Td symmetry α-Keggin ion, Xn+W12O40(8‑n)‑, from Al3+ to Si4+ to P5+. This resulted in stepwise changes in ion charge and reduction potential, whose combined effects on reaction rates were used to elucidate fundamental aspects of the electron-transfer processes. Based on the above developments, it is now possible to provide a detailed account of outer-sphere electron transfer reactions between polyoxometalate clusteranions and dioxygen. 6.3.1. Outer-Sphere Electron Transfer. Some introductory comments regarding quantitative and theoretical models of outer-sphere electron transfer are in order. In 1942 Debye extended Smoluchowski’s method for evaluating fundamental frequency factors pertaining to collision rates between neutral particles D and A randomly diffusing in solution, to include charged reacting species in dielectric media containing dissolved electrolytes.313−316 Debye’s colliding-sphere model assumed that collisions between Dn (electron donors with charge n) and the Am (electron acceptors with charge m) led to the transient formation of short-lived complexes, [Dn, Am]. Predicted rate constants for these reactions vary in a nonlinear fashion as a function of ionic strength, and the models are intimately tied to electrolyte theory. Marcus278,280,317 and others279,281 extended this model to include reactions in which electron transfer occurred during collisions between the “donor” and “acceptor” species, i.e., between the short-lived “precursor” complexes, [Dn, Am], Scheme 27.

between the reactants, often leading to ligand exchange or atom transfer. The relationship between the Gibbs free energy and activation energies of electron transfer reactions is nicely depicted by the two-dimensional intersection of potential energy surfaces in Figure 7.280 The curves represent the

Figure 7. Potential energy surfaces for outer-sphere electron transfer between reactants, R, giving products, P. The positions of the precursor and successor complexes are labeled A and B, respectively, and electron transfer occurs at position S. Reproduced from ref 296 with permission from John Wiley & Sons, Inc.

energies and spatial locations of reactants and products, and the x-axis corresponds to the motions of all atomic nuclei. The profile of the reactants and surrounding medium is represented by curve R, and the products plus surrounding medium by curve P. The minima in each curve, i.e., positions A and B, represent the equilibrium nuclear configurations, and associated energies, of the precursor and successor complexes shown in Scheme 27. The difference in energy between A and B is not the Gibbs free energy for the overall reaction, ΔG°, but rather, the “corrected” Gibbs free energy, ΔG°′. For reactions of charged species, the difference between ΔG° and ΔG°′ can be substantial. Weak electronic interaction between the reactants results in the indicated splitting of the potential surfaces (position S in Figure 7). This gives rise to electronic coupling between electronic states of the reactants, described by the electronic matrix element, HAB. This is equal to one-half the separation of the upper and lower curves at the intersection of the R and P surfaces. (The dashed lines represent the approach of two reactants in the absence of electronic interaction.) The reorganization energy, λ, is the energy associated with the difference in nuclear coordinates between the R and P surfaces. Electronic transitions are orders of magnitude more rapid than nuclear motion, such that nuclear coordinates remain effectively unchanged during electron transfer (the Franck− Condon principle). To obey the Franck−Condon principle, while also complying with the first law of thermodynamics (i.e., conservation of energy), electron transfer that does not involve quantum-mechanical “tunneling” can occur only at nuclear coordinates for which the potential energy of the reactants (and surrounding medium) equals that of the products (and surrounding medium). The only position at which both these conditions are satisfied is at the intersection of the two surfaces, S, in Figure 7. The simplest form of the Marcus model requires that electron-transfer be adiabatic. This means that changes in

Scheme 27. Electron Transfer between Short-Lived Precursor and Successor Complexes

Electron transfer within these transient complexes resulted in the formation of short-lived “successor” complexes [(D(n+1), A(m‑1)]. The Debye−Smoluchowski description of the diffusioncontrolled collision frequency between Dn and Am was retained. This has important implications for application of the Marcus model, particularly to electron-transfer reactions of polyoxometalates in which charged donors or acceptors are involved. In these cases, use of the Marcus model is only defensible if collision rates between the reactants vary with ionic strength as required by the Debye−Smoluchowski model. After electron transfer (the transition from [Dn, Am] to [(D(n+1), A(m‑1)] in Scheme 27), the successor complex dissociates to give the products, D(n+1) and A(m‑1). The distinction between the successor complex and products is important because the Marcus model describes rate constants as a function of the difference in energy between precursor and successor complexes, rather than between reactants and products. Outer-sphere electron-transfer reactions are characterized by the absence of strong electronic interactions (e.g., bond formation) between atomic or molecular orbitals populated by the electron transferred from reactants to products. Nevertheless, outer-sphere reactions do require some electronic communication, usually less than 1 kcal mol−1, between donor and acceptor orbitals. Inner-sphere electron-transfer reactions, by contrast, frequently involve the formation of covalent bonds 2696


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associated with this is the outer-sphere (solvent) reorganization energy, λout. In practice, ΔG‡ values, eq 3, cannot be measured directly. However, bimolecular rate constants, k, are related to ΔG‡ by the Eyring equation, eq 8, where κ is the transmission coefficient and Z is the collision frequency in units of M−1 s−1.

nuclear coordinates are slow enough for the system to (effectively) remain at equilibrium along the reaction coordinate. It also requires that the transmission coefficient, κ (i.e., the probability of electron transfer each time the reactants reach position S) be close to unity. Marcus related the free energy of activation, ΔG‡ to the corrected Gibbs free energy, ΔG°′, via a quadratic equation, eq 3).280,317,318 zz e λ⎛ ΔG° ′ ⎞ = 1 2 exp( −χr12) + ⎜1 + ⎟ Dr12 4⎝ λ ⎠

k = kZ exp( −ΔG‡/RT )

2

ΔG

++

The transmission coefficient is often set equal to unity, which in many cases gives reasonable results. With this assumption, expansion of eq 3 gives eq 9, in which the Coulombic work term (i.e., the first term on the right side of eq 3) is abbreviated as W(r).

(3)

The energies ΔG°′ and λ are indicated in Figure 7, and χ is the reciprocal Debye radius, eq 4.289,316 ⎛ 4πe 2 ⎞1/2 2 χ = ⎜⎜ ∑ nizi ⎟⎟ ⎝ DkT i ⎠

ΔG++ = W (r ) +

(4)

e2 exp( −cr12) Dr12

(ΔG°′)2 λ ΔG°′ + + 4 2 4λ

(9)

Substituting eq 9 into eq 8, and taking the natural logarithm, one obtains eq 10.

In eq 4, D is the dielectric constant of the medium, e is electron charge, k is the Boltzmann constant, and ∑nizi2 = 2μ, in which μ is the total ionic strength of electrolytes in the reaction solution containing molar concentrations, n, of species, i, of charge z. In the first term in eq 3 retained from Debye’s colliding sphere model, the electron-donor and electronacceptor species are viewed as spheres of radii r1 and r2, possessing charges of z1 and z2, respectively. This term describes the electrostatic energy (Coulombic work) required to bring the two spheres from an infinite distance apart, to the center-to-center separation distance, r12 = r1 + r2. This is the distance of closest approach, representing formation of the precursor complex, [Dn, Am] at position A in Figure 7). The magnitude of the Coulombic term is attenuated by a factor, exp(−χr12), which accounts for the effects of the medium (of dielectric constant D and ionic strength, μ). The corrected free energy, ΔG°′, is modified by the charges of the reactants and products, as shown in eq 5, where z1 and z2 are the charges respectively of the electron donor and acceptor. ΔG°′ = ΔG° + (z1 − z 2 − 1)

(8)

RT ln Z − RT ln k = W (r ) +

(ΔG°′)2 λ ΔG°′ + + 4 2 4λ (10)

Values of ln k vs ΔG°′ for a series or reactions can be fitted to eq 10 by nonlinear regression using λ as an adjustable parameter. If |ΔG°′| ≪ λ, the last term in eq 10 can be ignored, and λ obtained from the slope of the approximately linear dependence of ln k on ΔG°′. In many cases, ΔG° can be readily obtained from electrochemical data, while λ is often more difficult to determine. One frequently used direct method for determining λ values is to derive them from rate constants for related electron self-exchange reactions, eq 11. *Am + Am − 1 ⇌ *Am − 1 + Am

(11)

In self-exchange reactions, ΔG°′ = 0 and eq 3 is reduced to eq 12.

ΔG++ = W (r ) +

(5)

λ 4

(12)

In eq 5 the electrostatic correction to ΔG° vanishes when z1 − z2 = 1.318 In these cases, the difference in Gibbs free energy between the successor and precursor complexes is not dramatically increased or decreased by electrostatic interactions resulting from the transfer of one unit charge. ΔG° is often calculated from electrochemical data, eq 6, where E° is the difference between the standard reduction potentials of the electron donor and acceptor, n is the number of electrons transferred, and F is the Faraday constant.

In this case, λ can be calculated from the self-exchange rate constant, k, by using eq 13, obtained by substituting eq 12 into eq 8, or after linearization to eq 14.

(6)

For reactions in solution, Z is often ca. 1011 M−1 s−1 (values of Z = 6 × 1011 M−1 s−1 are also used).281 The Coulombic term, W(r), is calculated from the charges and radii of the reactants, the dielectric constant of the solvent, and the ionic strength of the solution, after which, λ is obtained from k. The rate constant, k12, for electron transfer between species, Am and Bn (eq 15), not related to one another by oxidation or reduction, can be calculated using the Marcus cross relation.

ΔG° = −nFE°

⎡ ⎛ ⎤ λ⎞ k = Z exp⎢ −⎜W (r ) + ⎟ /RT⎥ ⎝ ⎠ ⎣ ⎦ 4

RT ln k = RT ln Z − W (r ) −

The λ term in eq 3 is the reorganization energy associated with the transition from precursor to successor complexes. It is comprised of “inner-sphere” and “outer-sphere” reorganization energies, λin and λout, eq 7. λ = λ in + λout

(7)

The inner-sphere energy term, λin, arises from changes in nuclear coordinates (associated with changes in bond lengths and angles) of the donor and acceptor molecules or complexes. In addition, the electronic properties and charge distribution of the successor complex is different from that of the precursor complex, resulting in reorientation or other subtle changes involving solvent molecules near the reacting pair. The energy

Am + Bn − 1 ⇌ Am − 1 + Bn

λ 4

(13)

(14)

(15)

The cross relation is derived from kinetic expressions for the two related self-exchange reactions in eqs 16 and 17, characterized by rate constants, k11 and k22, and reorganization energies, λ11 and λ22. 2697


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*Am + An − 1 ⇌ *Am − 1 + An

(16)

*Bm + Bn − 1 ⇌ *Bm − 1 + Bn

(17)

(eq 23) is rigorously correct for solutions of mixed electrolytes. In this model, specific-interaction parameters are moved from the denominator to a second term, in which b is an adjustable parameter.

It furthermore requires that the reorganization energy for the cross reaction, λ12, be equal to the mean of the reorganization energies, λ11 and λ22, associated with the two related selfexchange reactions (eq 18) 1 λ12 ≅ (λ11 + λ 22) 2

log k = log k 0 + 2z1z 2αμ1/2 /(1 + μ1/2 ) + br12

Ignoring the second term in eq 23 gives the “truncated” Guggenheim equation, eq 24, which is identical to the Davies equation but with βr equal to unity.

(18)

log k = log k 0 + 2z1z 2αμ1/2 /(1 + μ1/2 )

This averaging over the outer-sphere components of λ11 and λ22, i.e., λ11out and λ22out, is only valid if Am and Bn+1 (eq 15) are both spherical and identical in size, with r1 = r2. These assumptions make it possible to describe the rate constant for the cross reaction, k12, as shown in eqs 19−21. k12 = (k11k 22k12f12 )1/2 C12

More elaborate models can give improved fits to high ionic strength values but do not generally provide additional insight. In practice and as espoused by Espenson,325 the truncated Guggenheim equation, eq 24, is often used for reactions in solutions with ionic strength values up to 0.1 M. If this fails, the Guggenheim equation, eq 23, is sometimes used. With more adjustable parameters, it is more likely to produce a linear plot. However, the slopes of these plots often deviate from theoretical values, calculated using the charge product, z1z2, of the reacting species. If a linear fit is obtained (even though the slope is incorrect), this might still argue against the presence of significant ion pairing or other medium effects.326 6.3.2. Electron Self-Exchange between Polyoxometalate Anions. The electron self-exchange reaction between airstable [CoIIW12O40]6‑327 and its one electron-oxidized derivative, [CoIIIW12O40]5−,283 and use of the latter ion in numerous studies of outer-sphere electron transfer have been reviewed.323 This section concerns the acquisition of fundamental kinetic parameters from the self-exchange reactions of polyoxometalates with more negative reduction potentials, whose reduced forms readily transfer electrons to O2. The first examples deal with Keggin and Wells−Dawson phosphotungstate anions, Figures 3 and 6. Based on line broadening observed in 31P NMR spectra, rate constants were determined for self-exchange reactions between the Keggin anions α-[PW12O40]3− (PW123‑) and one-electron reduced α-[PW12O40]4− (α-PW124‑), between the Wells−Dawson anions α-[P2W18O62]6− (α-P2W186‑) and α[P2W18O62]7− (α-P2W187‑) and between the trimolybdenumcapped derivatives α-[P2Mo3W15O62]6− and α[P2Mo3W15O62]7−.298 In the one electron reduced phosphotungstate anion, αPW124‑, the exchanged electron (whose d−d transitions account for the blue color of the reduced anion), is located in a predominantly nonbonding b2 orbital of the C4v symmetry WO6 polyhedra, with rates of intramolecular electron transfer between W atoms of ca. 10 11 to 10 12 s −1 at room temperature.299,305,328 More recently, computational data show the “blue” electron in the plenary Keggin ion is delocalized by population of a LUMO comprised of d orbitals from all 12 W329 or Mo atoms.44,201 In the one electron reduced Wells−Dawson anion α-P2W187‑, the blue electron is rapidly exchanged (1.7 × 1011 s−1) between the 12 “belt” tungsten atoms of the complex, 2 9 8 , 3 0 5 and in α[P2Mo3W15O62]7− the added electron is localized within the trimolybdenum cap, with exchange between Mo atoms 25 times slower than between W atoms in α-P2W187‑.299 In a preliminary investigation, 31P NMR spectra were acquired from solutions prepared from the free acids (proton forms) of α-PW123‑ and α-PW124‑ (pH 1), from the Li+ salts of α-P2W186‑ and α-P2W187‑ (pH 1 and pH 4), and from the K+ salts of α-[P2Mo3W15O62]6− and α-[P2Mo3W15O62]7− (pH 2.5).

where ln f12

W −W

(

)

( ) Z

(20)

and C12 = exp[−(W12 + W21 − W11 − W22)/2RT]

(21)

As seen above in eq 3, Z is the pre-exponential factor. The Coulombic work terms, wij, are associated with all four combinations of the reacting species. If the self-exchange rate constant for one reacting pair, k22, is available, k12 and eqs 19−21 can be used to calculate the rate constant for the second pair, k11. From this value, the associated reorganization energy, λ11, can be obtained from eq 14. The C12 and f12 terms often approach unity for molecules that possess small charges280 but can be quite important for reactions of highly charged inorganic complexes and certainly for polyoxometalate cluster anions. As the Marcus equation (eq 3) is based on collision rate theory, compliance with collision-rate models for reactions of charged species such as polyoxometalates is a prerequisite to its use. The key question in these cases is whether electrontransfer rate constants vary with ionic strength as predicted by electrolyte theory. In 1923, Debye and Hückel315,319 described the behavior of electrolyte solutions at the limit of very low concentration. Discussions in the literature subsequently lasted into the 1980s, by which time a number of complex approaches had been promulgated. The latter can give excellent fits up to large ionic strength values but are generally not used by most kineticists. One convenient option is to use the Debye−Hückel equation, also referred to as the Davies equation:320,321 log k = log k 0 + 2z1z 2αμ1/2 /(1 + βr12μ1/2 )

(24)

324

(19)

12 21 1 ln K12 + RT = 4 ln k11k222 + W11 − W22 RT

(23)

(22)

In eq 22, α (dimensionless) and β (in units of units of Å1/2 mol−1/2) are the Debye−Hückel constants, equal to 0.509 and 0.329, respectively, at 25 °C in water. k0 is the rate constant for the reaction at infinite dilution (μ = 0 M). Kineticists often work with mixed-electrolyte solutions. In such cases, the Davies equation is not rigorously correct based on principles from thermodynamics. In addition, some workers argue that setting r equal to the distance between reacting species is not justified.322 Nonetheless, as demonstrated below, it can give excellent results, even in reactions of highly charged ions such as polyoxometalates.323 Alternatively, the Guggenheim equation 2698


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1 2 λ in = ( )∑ fi (d10 − d 2 0)i 2

No ionic-strength effects were observed when the total polyoxometalate concentrations of equimolar mixtures of αPW123‑ and α-PW124‑ were increased from 0.002 to 0.02 M. Based on reported Keggin-ion diffusion coefficients,303,330,331 the rate constant for electron transfer between the anions appeared to be diffusion controlled (i.e., k > 107 M−1 s−1, based on a hydrodynamic radius of 5.6 Å).330 Electron transfer between α-P2W186‑ and α-P2W187‑ was also fast (k > 104 M−1 s−1, based on a hydrodynamic radius of 10 Å).331 The line broadening observed in 31P NMR spectra of mixtures of α[P2Mo3W15O62]6− and α-[P2Mo3W15O62]7− gave a rate constant of 2 × 103 M−1 s−1. The slower rate was attributed to localization of the blue electron in the Mo3O13 triad of the 7− ion. In 1990, Kozik and Baker297 reported a highly detailed study of electron transfer between α-PW123‑ and α-PW124‑ and between α-PW124‑ and the two-electron reduced anion αPW125−. In α-PW125‑ the two d electrons are paired by a “multiroute superexchange” mechanism297,299,303 or are delocalized,44,201,329 in either case giving a diamagnetic reduced ion. A more limited study of self-exchange between α-P2W186‑ and α-P2W187‑ was also reported.297 Experimental rate constants were compared with theoretical values calculated using Sutin’s semiclassical model for outer-sphere electron transfer.279,280,332 Rate constants for self-exchange in 1:1 mixtures of α-PW123‑ and α-PW124‑ (free-acid forms), the most thoroughly studied system, were determined by 31P NMR over a range of ionic strength values (μ = 0.026−0.616 M). Observed rate constants, kobs, were fitted to the Debye−Hückel equation (eq 22), using an internuclear distance of r = 11.2 Å or twice the ionic radius of the Keggin ions. A linear relationship was obtained, and the slope gave a charge product of z1z2 = 12.5, close to the theoretical value of 12.0. The linearity and close-to-theoretical slope was surprising in that the Debye−Hückel expression was derived for monovalent ions at low ionic strength values (up to 0.01 M). Agreement at quite high ionic strength values (>0.5 M) was attributed to the fact that polyoxometalates,333−335 owing to the very pronounced inward polarization of their exterior oxygen atoms, have extremely low solvation energies and very low van der Waals attractions for one another.297 For the α-PW124‑/α-PW125‑ couple, reasonably good agreement with eq 22 was observed up to μ = 0.125 M. Agreement with eq 22, and slopes giving predicted z1z2 values, indicated that the precursor complexes that formed prior to electron transfer likely involved “free” α-PW123‑, αPW124‑, and α-PW125‑ ions rather than alkali-metal-cation paired species such as those shown in eq 25. (This general conclusion was later questioned by Swaddle.)322

Bond lengths within both Keggin anions were obtained from X-ray crystallographic data. It was assumed that λin would result primarily from changes in W−O bond lengths and slight changes in bond angles. Force constants of individual W−O bonds in oxidized and reduced Keggin anions were estimated from IR data, in combination with normal coordinate analyses reported for two hexametalate anions.337 A value of λin = 6.1 kcal mol−1 was obtained, and the authors noted that, if small changes in bond angles were included, a slightly larger λin value would be obtained, giving even better agreement between predicted and experimentally determined rate constants. Calculation of the outer-sphere reorganization energy (λout) was done using eq 27 and is straightforward, provided that accurate values of the hydrodynamic radii, r1 and r2, of the reacting species are available (ne is the transferred charge and η and DS, are the refractive index and dielectric constant of the reaction medium). ⎛1 1 1 ⎞⎛ 1 1 ⎞ λ12(out) = (ne 2)⎜ + − ⎟ ⎟⎜ 2 − r2 r1 + r2 ⎠⎝ η DS ⎠ ⎝ r1

(27)

The internuclear distance, r12, at which the probability of electron transfer is at a maximum was set equal to r1 + r2. The outer-sphere reorganization energy λout is neither an explicit function of the ionic strength nor of the charges of the reacting species. Thus, unlike the Coulombic work term, which is highly sensitive to charge- and ionic-strength effects, the contribution of λout to ΔG‡ is often regarded as a constant property of the reactant pair. For reaction between α-PW124‑ and α-PW123‑ at 298 K in water, a value of λout = 16.4 kcal mol−1 and a total reorganization energy of λ = λin + λout = 22.3 kcal mol−1 were calculated. Excellent agreement between theory and experiment provided support for the assumptions made by Kozik and Baker. In light of the unexpectedly high degree to which their rate data obeyed the Debye−Hückel equation at large ionic strength values, their assignment of λ = 22.3 kcal mol−1 is probably correct or very close. In addition, their use of Sutin’s semiclassical model for outer-sphere electron transfer279 set a standard for work in this area. They also concluded that “electron delocalization of itself does not significantly contribute to the activation energy” of electron transfer.296 This finding has general implications for electron-transfer reactions of polyoxometalate anions. Predicted rate constants for exchange between the one- and two-electron reduced anions α-PW124‑ and α-PW125‑ were k0 = 2.2 × 103 M−1 s−1 and k = 3.1 × 106 M−1 s−1 (μ = 0.125 M), while experimental values were k0 = (1.6 ± 0.3) × 102 M−1 s−1 (by extrapolation) and k = 2.5 × 105 M−1 s−1, respectively. These smaller than expected values were attributed to the additional energy needed to decouple the paired electrons in diamagnetic, two-electron reduced α-PW125‑. For the Wells−Dawson anions, α-P2W186‑ and α-P2W187‑, observed rate constants, k0 = 1.1 × 103 and k = 1.4 × 104 M−1 s−1 (at μ = 0.59 and 1.49 M, respectively), were several orders of magnitude smaller than predicted values of k = 9.9 × 106 and 5.5 × 107 M−1 s−1. This was attributed to nonuniformity in the distribution of electron density over the reduced Wells−Dawson anion, α-[P2W18O62]7−, with the “blue” electron localized over the 12 belt W atoms at the midsection of the anion.

α‐[PW12O40 ]4 − + [Na(H 2O)x ]+ ⇌ [(Na(H 2O)x + )(α‐[PW12O40 ]4 − )]3 −

(26)

(25)

Predicted self-exchange rate constants for the α-PW123‑/αPW124‑ system were k0 = 3.6 × 105 M−1 s−1 after extrapolation to infinite dilution (μ = 0 M) and k = 1.0 × 108 M−1 s−1 at μ = 0.616 M. Experimental values were k0 = 1.1 × 105 M−1 s−1 (by extrapolation using 22) and k = 8.2 × 107 M−1 s−1 at μ = 0.616 M. Reorganization energies for the α-PW123‑/α-PW124‑ couple were calculated using literature methods.336 The inner-sphere reorganization energy, λin, was calculated using eq 26, where d1 and d2 are atomic coordinates and f is a force constant. 2699


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Weinstock subsequently309,310 used 27Al NMR spectroscopy to determine rate constants as a function of ionic strength in water for self-exchange between α- and β-Keggin isomers of fully oxidized, one- and two-electron reduced AlW12O40n− ions. The reactions occurred at the slow-exchange limit of the NMR time scale. As done by Kozik and Baker, a hard-sphere collision distance of 1.12 nm was used, corresponding to twice the hydrodynamic radius of a fully oxidized Keggin-type Xn+W12O(8‑n)− anion.338,339 Values of z1z2 obtained from slopes 40 of linear fits to eq 23 (up to ca. 0.2 M ionic strength values) were within experimental error of the actual charge products. Aqueous solution chemistries were most extensively defined for the reaction in eq 28 for which an experimentally determined charge product, z1z2, of 29 ± 2 was statistically identical to the actual value of 30.

Variation of Xn+ in the central oxoanion, from Al3+ to P5+, results in a linear increase in reduction potentials from −130 to +255 mV versus NHE (corresponding to nearly 9 kcal mol−1).308,340−343 Because these molecular ions possess approximately spherical shapes, they are ideal mechanistic probes for investigating outer-sphere electron transfer to O2. To evaluate electron transfer from reduced polyoxometalates to O2, it is first necessary to understand electron exchange between dioxygen (O2) and the superoxide radical anion (O2•−). In 1989, Lind and Merényi reported the first experimentally determined rate constant, k22, for the electron self-exchange between O2 and O2•− in water.344 This was done by reacting 32O2 with isotopically labeled O2•−, generated by γirradiation of 36O2, eq 30.

⇌* α‐AIW12O40 5 − + α‐AIW12O40 6 −

(30)

They obtained a rate constant of k22 = 450 ± 150 M−1 s−1. Because the charge product, z1z2, is equal to zero, the total reorganization energy, λtotal, was obtained from eq 13, which gave a value of 45.5 kcal mol−1. Next, λin was estimated computationally, giving λin = 15.9 kcal mol−1; eq 7 then gave λout = 29.6 kcal mol−1. The “effective” radius of O2 (i.e., r1/2) was then obtained using eq 27, which gave a value of 3 Å. Later work suggested this seemingly large value might reflect orientational restrictions imposed on collisions between the nonspherical reactants, O2 and O2•−.345 The above values, and in particular, k22, were then compared to those previously obtained using the Marcus cross relation, eq 19. When applied to kinetic data for presumptively outer-sphere electron transfer from metallo-organic complexes to O2,346 the Marcus cross relation gave “effective” rate constants, k22, for self-exchange between O2 and O2•− from 1 and 10 M−1 s−1. When organic electron donors with radii of from 5 to 7 Å, similar to the size of the Keggin ion, were reacted with O2, the Marcus cross relation gave k22 ≈ 2 M−1 s−1.347,348 These k22 values are 1−3 orders of magnitude smaller than Lind and Merényi’s directly determined value of 450 M−1 s−1.310,346,349 The discrepancy was reasonably attributed to the small size of O2 (bond length = 1.21 Å),350 relative to that of the electron donors (ca. 7−13 Å).348 Derivation of the Marcus cross relation assumes that the reactants are ideal spheres of identical size. Hence, the size differences between donors and O2 could account for the discrepancies between experimental and calculated values. Given the relative consistency in calculated k22 values, and because the apparent systematic error could be explained, the outer-sphere nature of the electron transfer from the metalloorganic complexes and organic compounds, to O2, was not questioned. For the reverse reaction, however, in which the Marcus cross relation was used to calculate k22 from rate constants for O2•− reductions of presumably outer-sphere electron-accepting complexes, calculated k22 values spanned 13 orders of magnitude.349 As a result, O2•− reductions of metal complexes were believed to be inherently complex. To quantitatively address this problem, a modified form of the Marcus cross relation was derived.345 Critically, donors and acceptors were no longer assumed to be identical in size. In the familiar form of the Marcus cross relation,280 the total reorganization energies for cross reactions, λ12, are approximated as the mean of the reorganization energies of the component self-exchange reactions, i.e., λ12 = (1/2)(λ11 + λ22). When r1 ≠ r2, the inner-sphere reorganization energy, λin, of the donor and

(28)

The observed self-exchange rate constant, k11 = (6.5 ± 1.5) × 10−3 M−1 s−1 (extrapolated to μ = 0), was more than 7 orders of magnitude smaller than that measured by Kozik and Baker for reaction between α-PW125‑ and α-PW124‑ (i.e., 1.1 × 105 M−1 s−1). Use of Sutin’s semiclassical model279 showed this to be a consequence of (primarily) the large negative charges of αAlW125− and α-AlW126−. It should be noted that Swaddle322 subsequently studied the effects of cations on the self-exchange reaction between the α-AlW125‑/6− couple over a range of (mostly) larger ionic-strength values (i.e., from 0.1 to 1 M), at which rate accelerations were found to depend specifically on the identity and concentrations of the cations. Weinstock and co-workers provided additional support for the self-exchange rate constant associated with the α-AlW125‑/6− couple by reacting α-AlW12O406‑ (1e−-reduced) and α-PW12O404‑ (oneelectron reduced), to give α-PW12O405‑ (the 2e−-reduced ion), eq 29.309 α‐AIW12O40 5 − + α‐PW12O40 5 − ⇌ α‐AIW12O40 6 − + α‐PW12O40 4 −

K 22

32 ·− 36 O2 + 36O·− 2 HooI O2 + O2

32

*α‐AIW O 6 − + α‐AIW O 5 − 12 40 12 40

(29)

The experimentally determined rate constant (17 ± 2 M−1 s ) was within experimental uncertainty of that predicted by the Marcus cross relation, eq 19, i.e., 13 ± 6 M−1 s−1. The results provided independent verification of the rate constant determined by 27Al NMR for the α-AlW125‑/6− couple, eq 28. They also estimated the zero ionic strength rate constant for self-exchange between α-SiW12O405‑ and α-SiW12O404‑.311 This was done by using an inner-sphere reorganization energy, λin,280,312 of 12.2 kcal mol−1, the mean of λin for the α-PW124‑/5− and α-AlW125‑/6− couples (15.5 and 8.85 kcal mol −1 , respectively). Sutin’s semiclassical method279 then gave a k0 value of 1.6 × 102 M−1 s−1 (i.e., at μ = 0), between the experimental values for self-exchange between the α-PW124‑/5− and α-AlW125‑/6− pairs. The iso-structural series of one-electron reduced Keggin ions discussed here (i.e., with central heteroatoms, Al(III), Si(IV), and P(V)) was then used to evaluate electron transfer to O2 by this class of polyoxometalate anions. 6.3.3. Electron Transfer to O2. Building on the above studies, electron transfer to O2 was investigated using an isostructural series of one-electron reduced α-Keggin ions, αXn+W12O40(8‑n)− (Xn+ = Al3+ to Si4+ to P5+), whose charges vary with the nature of Xn+,309 from 5− to 4− to 3−, respectively. −1

2700


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acceptor (eq 26) can nevertheless be accurately estimated as the average of contributions from the component reactions: λ12(in) = (λ11(in) + λ22(in))/2. However, the outer-sphere reorganization energy of the cross reaction, λout, is a function of the radii r1 and r2 of the donor and acceptor, respectively (eq 27), such that λ12(out) ≠ (λ11(out) + λ22(out))/2. To handle this, a modified form of the Marcus cross relation was derived for cases in which r1 ≠ r2. This was done by defining the reorganization energy of the cross-reaction as shown in eq 31, with Δ defined as shown in eq 32, in which λ11(out) and λ22(out) have been replaced by eq 27. λ12 =

1 (λ11 + λ 22 + 2Δ) 2

⎛ 1 ⎞⎛ 1 1 1 1 ⎞ ⎟⎟⎜ 2 − Δ = (ne 2)⎜⎜ + − ⎟ 2r2 (r1 + r2) ⎠⎝ η DS ⎠ ⎝ 2r1 1 − (λ11(out) + λ 22(out)) 2

The agreement between experimental and calculated values show that, once size differences are rigorously taken into account, experimental rate constants comply with the Marcus model for outer-sphere electron transfer. This argues that the fundamental parameters determined by Lind and Merényi for electron self-exchange between O2 and O2•− in water can be relied upon when using the Marcus model to evaluate electron transfer to O2. The one-electron reduced heteropolytungstates, αXn+W12O40(9‑n)−, Xn+ = P5+, Si4+, and Al3+ (POM1e) were reacted with O2 at pH values from 2 to 7.2 in water.311 In all three cases, the reaction is first order in both [POM1e] and [O2], and O2 is reduced by two-electrons to give hydrogen peroxide (eq 33; POMox is the fully oxidized anion). The elementary steps include the 1e− reduction of O2 to O2•−, which is then reduced to H2O2 by a second equivalent of POM1e. At pH values below 4.7, the pKa of HO2•, the disproportionation of O2•− and HO2• was orders of magnitude slower than reductions of these species by a second equivalent of POM1e.

(31)

(32)

2POM1e + O2 + 2H+ → 2POM OX + H 2O2

Equation 32 removes λ11(out) and λ22(out) and, in their place, inserts eq 27. Use of eq 31 to derive the Marcus cross relation gives eq 19, but the expressions for ln f12 and W12 (see eqs 20 and 21) now include Δ.342 Equation 19, along with the modified forms of eqs 20 and 21, was then used together with Lind and Merényi’s experimentally determined value of k22 = 450 M−1 s−1, to calculate rate constants, k12, for the 14 electron-transfer reactions from metal complexes to O2.346,349 Notably, the list included one-electron reduced forms of the three iso-structural α-KegginXn+W12O40(8‑n)− anions (Xn+ = Al3+, Si4+, and P5+), discussed above.311 The calculation was done using a single value of r2, the effective radius of O2. Calculated k12 values were fit to experimentally observed rate constants, k12, using r2 as the only adjustable parameter. A value of r2 = 2.5 Å gave excellent agreement. The effective radius of 2.5 Å was then used in the modified Marcus cross relation to calculate rate constants, k21, for the reverse reaction: oxidations of O2•− by metal complexes that typically react by outer-sphere mechanisms. Apart from r2, all parameters used were literature values, including k22 = 450 M−1 s−1. Calculated rate constants are compared with experimental values in Figure 8.346,349

(33)

The one-electron reduced anion, α-AlW12O406− is stable from pH 2 to 7.2, and was used to study the [H+] dependence of electron transfer to O2 over this range of pH values. Cyclic voltammetric data showed that neither α-AlW12O405− (fully oxidized) nor α-AlW12O406− are protonated between pH 2 and 7. Hence, any involvement of H+ in the reaction would not be due to protonation of the polyoxometalate itself but, rather, to protonation of incipient O2•−. Bimolecular rate constants, k12, for the rate-limiting electron transfer to O2 were then determined as a function of pH at a constant ionic strength (μ = 175 mM) and were effectively identical at all pH values studied. Also for the one-electron reduced ions, α-PW12O404− and α-SiW12O405−, no significant change in rate constants was observed as the pH was decreased from 2 to 1. Solvent kinetic-isotope effect experiments were carried out by reacting α-AlW12O406− with O2 in D2O at D+ concentrations corresponding to pH values of 2 and 7.2. Rate constants were identical at both [D+] values, indicating that even at pH 2, well below the pKa (=4.7) of protonated superoxide (HO2•), proton transfer (PT) occurs after rate-limiting electron transfer to O2 (i.e., sequential electron and proton transfer, an ETPT mechanism), rather than via concerted proton−electron transfer (CPET),352−356 in which an electron and proton are transferred simultaneously in a single elementary step. The Marcus cross relation was used to compare measured and calculated rate constants, k12. At pH 2, experimental values of k12 for reactions of α-Xn+W12O40(9‑n)−, Xn+ = P5+, Si4+, and Al3+ with O2, were 1.4 ± 0.2, 8.5 ± 1, and 24 ± 2 M−1 s−1, respectively. Calculated constants, k12(calcd), obtained using the Marcus cross relation and the experimentally determined rate constant,344 k22 = 450 M−1 s−1, for self-exchange between O2 and O2•− gave values consistently larger than actual k12 values by 1−1.5 orders of magnitude. Reasonable agreement was obtained, however, when an “effective” value of k22 = 2 M−1 s−1 was used, as first done by Lind and Merényi347,348 for reactions of organic electron donors similar in size to the 1.12 Å diameter Keggin anion. This provided strong evidence that electron transfer from the Keggin anions to O2 occurred via an outer-sphere mechanism. The ΔG0 for electron transfer to O2 decreases by ca. 9 kcal mol−1 as the heteroatom, Xn+, in POM1e is varied from P5+, Si4+,

Figure 8. Observed (red diamonds) and calculated (circles) rate constants for reactions of O 2•−(aq) with electron accepting coordination complexes. The electron acceptors are as follows: CoIII(sep)3+ (7’), FeIII(edta)(H2O)− (a),351 RuIII(NH3)63+ (8’), FeIIICp+2 (b), RuIII(NH3)5isn3+ (2’), and RuIII(NH3)5phen3+ (1’). Reprinted from ref 345. Copyright 1980 American Chemical Society. 2701


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and Al3+. Nevertheless, the increase in rates, from 1.4 to 8.5 to 24 M−1 s−1, respectively, is significantly smaller than would be expected based on the change in Gibbs free energy. This reveals the effect of donor-anion charge on ET to O2. Namely, as Xn+ is varied from P5+ to Si4+ to Al3+, the corresponding successorcomplex ion pairs [(α-PW12O403−)(O2•−)]4−, [(α-SiW12O404−)(O2•−)]5−, and [(α-AlW12O405−)(O2•−)]6− are subjected to incrementally larger anion−anion repulsions, Figure 9. For Figure 11. Rate constants, k, for ET to O2 as a function of [H+] at constant ionic strength (2.0 M, by additions of LiCl). Black diamonds at 0.1, 0.9, and 1.7 M H+ are k values from control experiments carried out in the absence of added LiCl. Reprinted from ref 312. Copyright 2010 American Chemical Society.

Quantitative evaluation of rate expressions for possible mechanisms showed CPET to be the only plausible one to give a linear dependence of rates on [H+]. The rate of CPET was first-order with respect to the concentrations of each species, [H+] [O2] and [α-PW12O404−]), with the proton involved as a reactant, eq 34. The functional dependence of k values on [H+] gave rate constants of kET = 1.2 M−1 s−1 and kCPET = 0.8 M−2 s−1, respectively, for the ET and CPET pathways, eq 35.

Figure 9. Anion−anion repulsion within successor−complex ion pairs after outer-sphere electron transfer from one-electron reduced polyoxometalates to O2. Reproduced from ref 311. Copyright 2006 American Chemical Society.

example, in the absence of anion repulsion in the successor− complex ion pairs, the rate constant for ET from α-AlW12O406− to O2 was estimated to be ca. 200 M−1 s−1.311 Hence, although more negatively charged polyoxometalates possess more negative reduction potentials, and are thermodynamically more favorable electron donors, this is systematically attenuated by the concomitant increase in anion charge, Figure 9. Quantitative observation of the kinetic effects of anion−anion repulsion within successor−complex ion pairs provided a second line of evidence in support of an outer-sphere mechanism for ET to O2. More generally, the findings revealed the extent to which polyoxometalate anion charge can influence rates of outer-sphere electron transfer to O2. At larger H+ concentrations ([H+]) in water,312 the ETPT mechanism discussed above is accompanied by a parallel pathway: concerted proton−electron transfer (CPET; Figure 10) CPET here refers to the simultaneous transfer of both

α‐[PW12O40 ]4 − + O2 + H+ → α‐[PW12O40 ]3 − + HO2• (34)

1 rate = − d[11e ]/dt = (kET + k CPET[H+])[11e ][O2 ] 2 (35)

Using D2O as solvent, a kinetic-isotope effect (KIE) of kH/kD = 1.7 was observed, pointing to a role for proton transfer in the rate determining step. The stoichiometry of the overall reaction was identical to that previously established for ETPT reactions of α-PW12O404− with O2,311 in that two equivalents of POM1e react with O2 to give H2O2. The relationship between the Gibbs free energy of the reaction, ΔG°, and the emergence of CPET, was explored using the more negatively charged, Keggin-anions, α-SiW12O404− and α-AlW12O406−. In line with theoretical predictions,361−364 CPET became more competitive with ETPT as the ET step became more endergonic. Temperature dependence data were analyzed using a form of the Marcus model357,359,360 that included the temperature dependence of the Gibbs free energy of reaction, ΔG°. This gave reorganization energies for the ET and CPET pathways, λET and λCPET, of 41.5 and 52.4 kcal mol−1, respectively. The CPET reaction is formally termolecular, eq 35. Although simultaneous “three-body” collisions are observed in the gas phase, they do not typically occur in solution. Rather, the firstorder dependence on α-PW12O404−, O2, and H+ was understood as a consequence of the unique nature of protons, and proton mobility, in water. Literature values for “reaction distances” associated with proton diffusion in water are typically 2−3 hydrogen bonds, or 6 ± 1 Å.365−375 At the relatively large [H+] values at which CPET occurs, encounter pairs (αPW12O404−, O2) resulting from bimolecular collisions between α-PW12O404− and O2 form in close proximity (i.e., within a reaction distance) to “excess” protons. The lifetimes of these encounter pairs, τ, were calculated as the inverse of ke (τ = 1/ke), the rate constant for diffusional escape from the solvent cage. Using two different models,376−378 lifetimes of τ = 70 and 200 ps were estimated, both

Figure 10. Parallel ET and CPET pathways in reaction of αPW12O404− with O2 at large [H+] values in water. Reprinted from ref 312. Copyright 2010 American Chemical Society.

species, as distinguished from transfer of a hydrogen-atom (H•).357−360 CPET to dioxygen was documented312 by reacting members from the iso-structural series of 1e−-reduced anions, α-Xn+W12O40(9‑n)−, Xn+ = P5+, Si4+, and Al3+, with O2 at [H+] values of 0.1 to 1.9 M in water. Of the three donor-anions, α-PW12O404− was the only one to display a linear (first-order) dependence of observed rate constants, kobs, on [H+], Figure 11. LiCl was used to maintain constant ionic strength, and control experiments ruled out ion pairing between Li+ and αPW12O404−, while electrochemical measurements ruled out protonation of 11e by H+ over the range of [H+] values used. 2702


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the rate expression for CPET is given as shown in eq 36, which accounts for the formally termolecular, first-order dependence of rate on the concentrations of α-PW12O404−, O2, and H+.

of which are at least 2 orders of magnitude longer than that required for proton dislocation over a single water molecule (i.e., ca. 1.5 ps,379,380 to travel a distance of ca. 2.5 Å). This increases the probability that protons initially at distances larger than 6 Å from (α-PW12O404−, O2) encounter pairs can diffuse to O2 prior to cage escape. It also allows time for protons to repeatedly approach to within 1 or 2 Å from O2, increasing the probability of achieving positions and energies associated with the transition state for CPET. This situation is shown in Figure 12, in which the excess proton is shown on the right-hand side

1 − d[α ‐ PW12O40 4 −]/dt 2 ⎛k ⎞ = ⎜ d ⎟k CPET[α ‐ PW12O40 4 −][O2 ][H+] ⎝ ke ⎠

(36)

The authors suggested that “the emergence of multisite CPET, with hydronium ion as the proton donor, may prove a general feature of sufficiently endergonic reductions of dioxygen by otherwise “outer-sphere” complexes (or electrode reactions) at sufficiently low pH values in water”.312 6.4. Dioxygenase Type Reactions

Dioxygenase type reactions are rather rare and involve the splitting of the O−O without coformation of H2O through use of reducing agents. Mechanistically speaking at some point during the reaction cycle an inactive form of the catalyst (POM-X) reacts with O2 to form a reactive intermediate (POM-X-O), which then reacts with substrate to complete the reaction cycle, Scheme 28.

Figure 12. Formation of a (α-PW12O404−, O2) encounter pair (lifetime of 70−200 ps) close to an 11-Å-diameter fluxional hydrated-proton complex (far right; ball-and-stick structures are drawn to scale). The dioxygen molecule is drawn 6 Å from the positively charged “defect” initially located at the center of the hydrated-proton complex. There is a non-negligible probability for this defect becoming “quantum mechanically delocalized” to within 1 or 2 Å of O2, an event that occurs within a small fraction of the 70−200 ps lifetime of the (αPW12O404−, O2) encounter pairs. The oxygen atoms over which the protonic defect is delocalized in a small fraction of the 70−200 ps lifetime of the encounter pairs are indicated by a partial covering of blue. Reprinted from ref 312. Copyright 2010 American Chemical Society.

Scheme 28. Generic Dioxygenase Cycle Based on a Polyoxometalate Compound

The first report of a polyoxometalate catalyst hinting that such dioxygenase activation of O2 was possible was using a ruthenium substituted sandwich type polyoxometalate, 11− [WZnRuIII , where the ruthenium 2 (OH)(H2O)(ZnW9O34)2] center was at a sterically hindered terminal position available for O2 coordination.381 It is very important to note that there are very similar procedures for the preparation of such a sandwich compound which yields [Ru2IIIZn2(H2O)2(ZnW9O34)2]14−, where the ruthenium centers are not at the terminal position.382 In this case O2 coordination will not be possible and dioxygenase type activation of O2 could not and was not observed. 383,384 Therefore, in further research with [WZnRu2III(OH)(H2O)(ZnW9O34)2]11− coordination of O2 and formation of possible reactive intermediates was studied by using UV−vis and IR spectroscopy. The former measurement showed the appearance of a new peak at 290 nm while the latter measurement showed a new absorption with an expected isotope shift upon use of 18O2. The reactivity profile 11− of [WZnRuIII after treatment with 2 (OH)(H2O)(ZnW9O34)2] molecular oxygen showed the following characteristics: A substrate/dioxygen reaction stoichiometry of 2:1 was observed using adamantane as substrate and the product selectivity observed in the catalytic oxidation of trans-cyclooctene showed the formation of the trans-cyclooctene oxide and not the cis isomer. trans-Cyclooctene acted as a free radical clock (kisomerization = 109 1/s) where a free radical mechanism would be expected to lead to a planar intermediate and formation of the much more stable cis-cyclooctene oxide. Also, free radical scavengers did not inhibit catalytic hydroxylation of adamantane, and the kinetic isotope effect observed in 1,3-adamantaned2 hydroxylation was also not commensurate with a free radical mechanism. These experiments combined with the fact that the formation of H218O was not observed in a reaction with 18O2 all supported a dioxygenase type reaction that was proposed.

as a proton “defect” within a proton complex in water. In theoretical models based on quantum nuclear path-integral simulations, these positively charged “defects” can be delocalized over ∼4−5 Å in roughly 10% of the configurations sampled, due mainly to zero-point effects on the protontransfer free-energy profile.365,371,372 The 70−200 ps lifetime of (α-PW12O404−, O2) encounter pairs is more than sufficient for its diffusion to locations even closer to O2. The CPET rate constant, kCPET, is orders of magnitude smaller than the diffusional cage escape rate constant, ke, Figure 13. Using a steady-state approximation, with kCPET[H+] ≪ ke,

Figure 13. Elementary steps for ET and CPET. Both reactions proceed via the formation of encounter pairs (α-PW12O404−, O2). These form at a rate equal to the bimolecular rate constant, kd, and dissociate with a “cage-escape” rate constant, ke. At large [H+], kinetically significant fractions of the encounter pairs form in close proximity to rapidly diffusing H+ ions, and rates for CPET (kCPET[H+]) become competitive with those for ET (kET). Reprinted from ref 312. Copyright 2010 American Chemical Society. 2703


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Finally, the isolation of a reactive ruthenium species, its identification by UV−vis and IR spectroscopy, and its use in the stoichiometric epoxidation of alkenes also support a dioxygenase activation of O2. Unfortunately, the reactions are not generally catalytic and the presence of alkenes, except for highly hindered alkenes, inhibits the binding of O2 to the active site. It should be noted that this body of research has been criticized by others. In the first critical paper, the authors were unable to repeat the previously reported reactivity of adamantane, receiving instead a mixture of products clearly indicative of a free radical oxidation process.385 The catalyst used in this research was not characterized. In a second paper, it was further stated that in their hands the synthesis of [WZnRuIII 2 (OH)(H2O)(ZnW9O34)2]11− was not successful, and as a result the initial reports could not be corraborated.386 A different approach toward a dioxygenase type mechanism was to use a first-row transition substituted polyoxometalate where the transition metal (TM is preferably Co(II) or Cu(II)) was ligated with NO2 ligands at the terminal position to yield POM-TM-NO2 compounds. The POM-TM-NO2 complex was shown to be an oxygen donor to alkenes yielding epoxides or C−C bond cleaved products and the nitrosyl complex, the POM-TM-NO, Scheme 29.387,388 Notably, the aerobic

Scheme 30. Possible Mechanistic Pathways for the Oxidation of Alkenes Catalyzed by POM-TM-NO2

reaction was second order in Q8{α2-Cu(NO)P2W17O61} and zero order in O2 suggesting that while O2 was needed it was not involved in the rate-determining step. It was concluded that two Q8{α2-Cu(NO)P2W17O61} molecules reacted in a ratedetermining step to form a “Cu(NO)2Cu” dimer, which further yielded a “Cu-NO-O2−ON-Cu” intermediate and then Q8{α2Cu(NO2)P2W17O61}. Another interesting report on dioxygenase type chemistry involves the use of trivanadium substituted polyoxometalate complexes [SiW9V3O40]7− and [P2W15V3O62]9− that can contain also Fe(II) cations. Using 3,5-t-butyl catechol as a substrate very high dioxygenase type activity mimicking those of catechol dioxygenase enzymes was reported yielding mostly intradiol cleavage products. Very high turnover numbers were reported, and the dioxygenase type activity was rationalized by observation of a reaction stoichiometry of 3,5-t-butyl catechol:O2 = ∼1.390 A further study into these vanadiumbased catechol dioxygenases suggests that the vanadium atom(s) are likely leached from the polyoxometalate structure during the initial reaction stages leading to formation of vanadyl semiquinone catechol dimer complex, previously characterized by Pierpoint, which appears to be the active catalytic species.391 These results indicate that the dioxygenase catalyst is formed by the undesired initial autoxidation of 3,5-t-butyl catechol to the corresponding quinone and hydrogen peroxide by the vanadium substituted polyoxometalate, which then leads to a leaching process and the formation of the active catalyst. In essence, this has been described as an autoxidation product initiated dioxygenase. 392 The possibility of using iron substituted polyoxometalates as dioxygenase type catalysts for the intradiol cleavage of catechols has been investigated where four iron(III) oxalate centers have been positioned between two β-XW9O33q− lacunary species. Coordination of 3,5-t-butyl catechol was verified. Depending on X = As(III), Sb(III), Se(IV), and Te(IV) and the solvent used some intradiol cleavage could be observed, that was best for Te(IV), although autoxidation products were typically the major ones.393

Scheme 29. Oxidation of Alkenes by Polyoxometalates Ligated with NO2 and Dioxygen Activation

oxidation of methyloleate gave methyl-8-formyl-octanoate and nonanal in high yields. On the other hand, cyclododecene and other cycloalkenes were epoxidized; reaction selectivity depended on the substrate and catalyst used. Although NO gas can easily be oxidized by O2 to NO2 and oxidation can also be catalyzed by H3+xPVxMo12‑xO40 to yield nitrous acid and nitrate,389 metal-nitrosyl complexes often are stable to oxidation, in this case aerobic oxidation by a dioxygenase type pathway did appear to occur. For example, [α2-Cu(NO2)P2W17O61]8−, {[(Cu(NO2)]2WZn(ZnW9O34)2}12−, or [Cu(H2O)H2W17F6O55]9− catalysts were prepared by pretreatment of the corresponding aqua coordinated polyoxometalates precursors by treatment with NO2 or are formed when the reactions are carried in nitroalkanes, as solvents or cosolvents. A 31P NMR measurement showed that the Cu-NO2 substituted polyoxometalate acted as an oxygen donor to the C−C double bond yielding a Cu-NO product that is reoxidized to Cu-NO2 under reaction conditions to complete a catalytic cycle. Stoichiometric reactions and kinetic measurements using {α2-Co(NO2)P2W17O61}8− as oxidant and trans-stilbene derivatives as substrates point toward a reaction mechanism for C−C bond cleavage involving two molecules of {α2-Co(NO2)P2W17O61}8− and one molecule of trans-stilbene that is sufficiently stable at room temperature to be observed by 31P NMR, Scheme 30. Further, measurement of the reoxidation reaction kinetics, for example the reaction between O2 and Q8{α2-Cu(NO)P2W17O61} to give Q8{α2-Cu(NO2)P2W17O61} showed that the

7. OXIDATION OF SULFIDES The oxidation of sulfur containing compounds is of interest related to several different research areas ranging from valorization of hydrogen sulfide produced in hydrodesulfurization of fuels, to synthesis of sulfuric acid from sulfur dioxide, to the preparation of sulfoxides and sulfones as synthetic targets but also in the context of decomposition of mustard gas (bis-2chloroethyl)sulfane. The ability to oxidize a sulfur containing 2704


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combination of various cocatalysts.405−410 Another research direction in this area relates to the possibility of easily forming cation radicals from aromatic sulfides such as dibenzothiophene and alkylated analogs due to their extended π-systems. Although these compounds are not typically oxygenated, the formation of the electrophilic intermediates led to the possibility of the oxidative polymerization of such compounds. This could be useful for desulfurization of fuel components typically refractory to classic hydrogenation.411,412 Interestingly, using a similar catalyst, H8PV5Mo7O40 that may be less stable, benzothiophene could be oxidized in the presence of an excess of water and high temperature (120 °C) and high O2 pressures (20 bar) to sulfate and various water-soluble carboxylic acid derivatives.413

compound with O2 is very dependent on the oxidation potential of the substrates. Thus, many thiols can be oxidized to the corresponding disulfides even with a weak tungstenbased oxidant such as the Preyssler polyoxometalate, [NaP5W3oO110]14−.394 Similarly, hydrogen sulfide can be oxidized to elemental sulfur with such weakly oxidizing polyoxometalate catalysts.395 The correlation between reactivity and the oxidation potential of the polyoxometalate catalyst suggests that the reaction proceeds via electron transfer between H2S and the polyoxometalate coupled with reoxidation of the catalyst with O2.396,397 The oxidation of sulfur dioxide to sulfuric acid in water requires a more oxidizing polyoxometalate, such as the phosphovanadomolybdates. The authors suggested that the reaction proceeds through electron transfer from SO2 to the polyoxometalate followed by nucleophilic attack of water onto the activated SO2, followed by reoxidation of the catalyst by O2.398 Although the oxidation of sulfides with hydrogen peroxide and organic peroxides often does not require a catalyst, the oxidation of sulfides with molecular oxygen does require a catalyst. Early research showed that dialkylsulfides, R2S where R = Me, Et, and Bu could be oxidized in quantitative yields to a mixture of the corresponding sulfoxides and sulfones in a reaction preferably catalyzed by H3+xPVxMo12‑xO40 (x = 2 or 3) under high O2 pressures and elevated temperatures.399 A copper substituted polyoxometalate within a MOF framework also proved to be very reactive for this reaction and can act to catalytically remove odors.400 Interestingly, the polyoxometalate alone was inactive. A similar observation was reported for an iron substituted polyoxometalate supported on cationic silica nanoparticles.401 A more detailed study on the oxidation of sulfides to sulfoxides with H5PV2Mo10O40 was carried out more recently.402 First, in order to understand how the oxidation reactions occurred, stoichiometric amounts of arylmethyl sulfides (thioanisoles, ArSMe) were reacted with H5PV2Mo10O40. A green precipitate is formed in AcOH at 70 °C. Gas chromatographic analysis revealed the selective formation of the corresponding sulfoxides, and use of 18-O labeled H5PV2Mo10O40 demonstrated that an oxygen atom was transferred from the polyoxometalate to the sulfide. Under milder reaction condition, UV−vis analysis showed the formation of a charge transfer complex: λmax = 650 and 887 nm that were identified with a thioanisole cation radical and a one electron reduced polyoxometalate, respectively. An EPR spectrum carried out using either thianthrene or diphenylsulfide as substrate showed both the characteristic spectrum of the organic cation radical and spectrum of the one-electron reduced polyoxometalate. Hammett plots correlating reactivity with substitution at the phenyl ring yielded ρ = −3.6, also in support of a cation radical intermediate. An aerobic oxidation of thioanisole in nitromethane yielded the sulfoxide as the major product but also some biphenyl-disulfide as minor product due to the cleavage of the S-CH3 bond of the intermediate cation radical. It would appear that aliphatic sulfides, such as tetrahydrothiophene are significantly more reactive than their aryl substituted counterparts.403 It is not clear that the mechanism of the reaction is the same. For example, one study labeling showed that the sulfoxidation could occur by transfer of an oxygen atom from O2.404 However, in any case this more facile oxidation has led to several studies on the aerobic oxidation/ hydrolysis of 2-choroethyl−ethyl sulfide as a surrogate substrate for sulfur muster using various polyoxometalates, also in

8. CONCLUSIONS In this review, we have presented synthetic applications where polyoxometalates are used for useful oxidative transformations with molecular oxygen as terminal oxidant. Thus, polyoxometalates, notably vanadomolybdates, have significantly added to the “tool-box” of sustainable aerobic transformations especially in the context of organic synthesis but also in other fields such as the valorization of biomass and remediation of sulfur containing compounds. These advances have been made possible through careful mechanistic studies that revealed details of electron transfer to a polyoxometalate and further possible oxygen transfers from a polyoxometalate to an activated substrate. In this way, unique carbon−carbon bond and carbon−metal bond activation processes have been discovered. In parallel, the mode of the interaction of molecular oxygen with reduced polyoxotungstates has been studied by precise inorganic, physical methods revealing an interesting outer sphere reaction mechanism. One can notice that in catalysis in general and for dioxygen activation in particular only a relatively few number of polyoxometalate compounds have been used as catalysts among the multitude of polyoxometalate structures that have been discovered and functionalized in various ways. With so many possibilities and through careful design one may expect many new reactions and further mechanistic understanding in the future. AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ira A. Weinstock: 0000-0002-6701-2001 Roy E. Schreiber: 0000-0003-3705-8073 Ronny Neumann: 0000-0002-5530-1287 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest. Biographies Ira A. Weinstock obtained his Ph.D. in 1990 from the Massachusetts Institute of Technology (MIT), where he worked on alkyne metathesis with Richard R. Schrock. After one year at the Sandia National Laboratory, Albuquerque, New Mexico, he served as Team Leader at the U.S. Department of Agriculture, Madison, Wisconsin, where he 2705


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initiated the use of polyoxometalates as green catalysts for aerobic oxidations of biomass in water. In 2006, he joined the Ben-Gurion University of the Negev, where his research concerns the use of polyoxometalates in molecular and supramolecular chemistry and nanoscience and more recently as redox-active ligands for metal-oxide nanocrystals. Roy E. Schreiber received his Ph.D. from the Weizmann Institute of Science in 2017 under the direction of Prof. Ronny Neumann. His thesis focused on researching the importance of the dense phase reaction mechanism for polyoxometalate chemistry. He is primarily interested in fundamental research for the understanding of mechanistic inorganic chemistry. Ronny Neumann obtained his Ph.D. in 1986 at the Hebrew University of Jerusalem in the area of phase transfer catalysis and then was a postdoc at Princeton University working on metalloporphyrins in synthetic membranes. In 1988 he returned to the Hebrew University where he started a research group concentrating on the use polyoxometalates in oxidation catalysis. In 1999 he moved to the Weizmann Institute of Science. His interests are in the areas of sustainable catalytic oxidations, organic chemistry in water, and more recently in photocatalysis and electrocatalysis in the context of alternative and renewable energy.

ACKNOWLEDGMENTS This research was supported by the Israel Science Foundation Grant Numbers 763/14 (R.N.) and 170/17 (I.A.W.). I.A.W. holds the Irene Evens Chair in Inorganic Chemistry. R.N. is the Rebecca and Israel Sieff Professor of Organic Chemistry. REFERENCES (1) Berzelius, J. J. Beitrag zur Näheren Kenntniss des Molybdäns. Ann. Phys. 1826, 82, 369−392. (2) Keggin, J. F. The Structure and Formula of 12-Phosphotungstic Acid. Proc. R. Soc. London, Ser. A 1934, 144 (851), 75−100. (3) Li, B.; Li, W.; Li, H.; Wu, L. Ionic Complexes of Metal Oxide Clusters for Versatile Self-Assemblies. Acc. Chem. Res. 2017, 50, 1391− 1399. (4) Zhao, J.-W.; Li, Y.-Z.; Chen, L.-J.; Yang, G.-Y. Research Progress on Polyoxometalate-Based Transition-Metal-Rare-Earth Heterometallic Derived Materials: Synthetic Strategies, Structural Overview and Functional Applications. Chem. Commun. 2016, 52, 4418−4445. (5) Miras, H. N.; Vila-Nadal, L.; Cronin, L. Polyoxometalate Based Open-Frameworks (POM-OFs). Chem. Soc. Rev. 2014, 43, 5679− 5699. (6) Du, D.-Y.; Qin, J.-S.; Li, S.-L.; Su, Z.-M.; Lan, Y.-Q. Recent Advances in Porous Polyoxometalate-Based Metal-Organic ∼ Framework Materials. Chem. Soc. Rev. 2014, 43, 4615−4632. (7) Omwoma, S.; Chen, W.; Tsunashima, R.; Song, Y.-F. Recent Advances on Polyoxometalates Intercalated Layered Double Hydroxides: From Synthetic Approaches to Functional Material Applications. Coord. Chem. Rev. 2014, 258−259, 58−71. (8) Wang, Y.; Weinstock, I. A. Polyoxometalate-Decorated Nanoparticles. Chem. Soc. Rev. 2012, 41, 7479−7496. (9) Proust, A.; Matt, B.; Villanneau, R.; Guillemot, G.; Gouzerh, P.; Izzet, G. Functionalization and Post-Functionalization: A Step Towards Polyoxometalate-Based Materials. Chem. Soc. Rev. 2012, 41, 7605−762. (10) Oms, O.; Dolbecq, A.; Mialane, P. Diversity in Structures and Properties of 3d-Incorporating Polyoxotungstates. Chem. Soc. Rev. 2012, 41, 7497−7536. (11) Lopez, X.; Carbo, J. J.; Bo, C.; Poblet, J. M. Structure, Properties and Reactivity of Polyoxometalates: A Theoretical Perspective. Chem. Soc. Rev. 2012, 41, 7537−7571. 2706


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