ContentsI. Introduction 113 II. Outer-Sphere Electron-Transfer Reactions 115 A. Debye−Smoluchowski and Marcus 115 B. Marcus Equation 116 1. Applications 116 2. Theoretical Calculation of Outer-Sphere Electron-Transfer Rate Constants 117 3. Outer-Sphere Electron Self-Exchange 117 4. The Marcus Cross-Relation 117 C. Ion Pairing between Electrolyte Ions and Electron Donors or Acceptors 118 D. Outer-Sphere Electron Transfer within Stable Donor−Acceptor Complexes 119 III. Electron Self-Exchange between POM Anions 121 A. Keggin Tungstocobaltate Anions 121 B. Keggin and Wells−Dawson Metallophosphate Anions 122 C. Reevaluation of the Reorganization Energy of the [Co III W 12 O 40 ] 5-/[Co II W 12 O 40 ] 6-Couple 125 D. One-Electron Oxidation of [Cu I W 12 O 40 ] 7to [Cu II W 12 O 40 ] 6-129 IV. Oxidation of Organic Electron Donors by POM Anions 129 A. Alkylaromatic Compounds 129 B. Cyclohexadienes 136 C. Organic Acids, Esters, Aldehydes, and Ketones 138 D. Aromatic and Aliphatic Alcohols 142 E. Carbon-Centered Organic Radicals 143 F. Thiols and Organic Sulfides 145 V. Oxidation of Inorganic Electron Donors by POM Anions 148 A. Introduction 148 B. Inorganic Electron Donors 150 C. Hydrogen Sulfide 150 D. Luminescence Quenching of Electronically Excited [Ru(bpy) 3 ] 2+ Cations 152 VI. Reduction of Halogenated Alkanes by Reduced POM Anions 153 A. Carbon Tetrabromide 153 B. Halomethanes 154 C. Carbon Tetrachloride, Halomethanes, and Haloalkanes 155 VII. Reduction of Inorganic Oxidants by Reduced POM Anions 156 157 D. Nitronium Ion 158 E. Peroxodisulfate 159 F. Chlorate and Periodate 159 G. Permanganate 160 VIII. Reduction of Dioxygen by Reduced POM Anions 161 A. Introduction 161 B. Single-Electron Reduction of O 2 161 C. The O 2 /O 2 •-Self-Exchange Reaction 161 D. Reduction of O 2 by Reduced Isopoly-and Heteropolytungstates 162 E. Reduction of O 2 by d-Electron-Containing, Transition-Metal-Substituted Heteropolytungstates 164 F. Reduction of O 2 by Reduced Vanadomolybdophosphates 166 IX. Acknowledgments 167 X. References 167

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, HPVMoO, can oxidize Pd(0) thereby enabling aerobic reactions catalyzed by Pd and HPVMoO. In a similar vein, polyoxometalates can stabilize metal nanoparticles, leading to additional transformations. Furthermore, electron transfer oxidation of other substrates such as halides and sulfur-containing compounds is possible. More uniquely, HPVMoO 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 O 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.

The effect of cation size on the rate and energy of electron transfer to [(M(+))(acceptor)] ion pairs is addressed by assigning key physicochemical properties (reactivity, relative energy, structure, and size) to an isoelectronic series of well-defined M(+)-acceptor pairs, M(+) = Li(+), Na(+), K(+). A 1e(-) acceptor anion, alpha-SiV(V)W(11)O(40)(5-) (1, a polyoxometalate of the Keggin structural class), was used in the 2e(-) oxidation of an organic electron donor, 3,3',5,5'-tetra-tert-butylbiphenyl-4,4'-diol (BPH(2)), to 3,3',5,5'-tetra-tert-butyldiphenoquinone (DPQ) in acetate-buffered 2:3 (v/v) H(2)O/t-BuOH at 60 degrees C (2 equiv of 1 are reduced by 1e(-) each to 1(red), alpha-SiV(IV)W(11)O(40)(6-)). Before an attempt was made to address the role of cation size, the mechanism and conditions necessary for kinetically well behaved electron transfer from BPH(2) to 1 were rigorously established by using GC-MS, (1)H, (7)Li, and (51)V NMR, and UV-vis spectroscopy. At constant [Li(+)] and [H(+)], the reaction rate is first order in [BPH(2)] and in [1] and zeroth order in [1(red)] and in [acetate] (base) and is independent of ionic strength, mu. The dependence of the reaction rate on [H(+)] is a function of the constant, K(a)1, for acid dissociation of BPH(2) to BPH(-) and H(+). Temperature dependence data provided activation parameters of DeltaH = 8.5 +/- 1.4 kcal mol(-1) and DeltaS = -39 +/- 5 cal mol(-1) K(-1). No evidence of preassociation between BPH(2) and 1 was observed by combined (1)H and (51)V NMR studies, while pH (pD)-dependent deuterium kinetic isotope data indicated that the O-H bond in BPH(2) remains intact during rate-limiting electron transfer from BPH(2) and 1. The formation of 1:1 ion pairs [(M(+))(SiVW(11)O(40)(5-))](4-) (M(+)1, M(+) = Li(+), Na(+), K(+)) was demonstrated, and the thermodynamic constants, K(M)(1), and rate constants, k(M)(1), associated with the formation and reactivity of each M(+)1 ion pair with BPH(2) were calculated by simultaneous nonlinear fitting of kinetic data (obtained by using all three cations) to an equation describing the rectangular hyperbolic functional dependence of k(obs) values on [M(+)]. Constants, K(M)(1)red, associated with the formation of 1:1 ion pairs between M(+) and 1(red) were obtained by using K(M)(1) values (from k(obs) data) to simultaneously fit reduction potential (E(1/2)) values (from cyclic voltammetry) of solutions of 1 containing varying concentrations of all three cations to a Nernstian equation describing the dependence of E(1/2) values on the ratio of thermodynamic constants K(M)(1) and K(M)(1)red. Formation constants, K(M)(1), and K(M)(1)red, and rate constants, k(M)(1), all increase with the size of M(+) in the order K(Li)(1) = 21 < K(Na)(1) = 54 < K(K)(1) = 65 M(-1), K(Li)(1)red = 130 < K(Na)(1)red = 570 < K(K)(1)red = 2000 M(-1), and k(Li)(1) = 0.065 < k(Na)(1) = 0.137 < k(K)(1) = 0.225 M(-1) s(-1). Changes in the chemical shifts of (7)Li NMR signals as functions of [Li(5)1] and [Li(6)1(red)] were used to establish that the complexes M(+...

Polyoxometalate cluster anions (POMs) control formation and morphology, and serve as protecting ligands, for structurally and compositionally diverse nanostructures. While numerous examples of POM-protected metal(0) nanoparticle syntheses and reactions can now be found in the literature, the use of POMs to prepare nano-scale analogs of binary inorganic materials, such as metal-oxides, sulfides and halides, is a relatively recent development. The first part of this critical review summarizes the use of POMs as protecting ligands for metal(0) nanoparticles, as well as their use as templates for the preparation of new inorganic materials. Here, key findings that reveal general trends are given additional emphasis. In the second part of the review, new information concerning the structure of POM-protected metal(0) nanoparticles is systematically developed. This information, obtained by the combined use of cryogenic transmission microscopy (cryo-TEM) and UV-vis spectroscopy, provides a new perspective on the formation and structure of POM-decorated nanoparticles, and on the rational design of catalytic and other functional POM-based nano-assemblies.