Polyoxometalates (POMs) have remarkable properties and a great deal of potential to meet contemporary societal demands regarding health, environment, energy and information technologies. However, implementation of POMs in various functional architectures, devices or materials requires a processing step. Most developments have considered the exchange of POM counterions in an electrostatically driven approach: immobilization of POMs on electrodes and other surfaces including oxides, embedding in polymers, incorporation into Layer-by-Layer assemblies or Langmuir-Blodgett films and hierarchical self-assembly of surfactant-encapsulated POMs have thus been thoroughly investigated. Meanwhile, the field of organic-inorganic POM hybrids has expanded and offers the opportunity to explore the covalent approach for the organization or immobilization of POMs. In this critical review, we focus on the use of POM hybrids in selected fields of applications such as catalysis, energy conversion and molecular nanosciences and we endeavor to discuss the impact of the covalent approach compared to the electrostatic one. The synthesis of organic-inorganic POM hybrids starting from bare POMs, that is the direct functionalization of POMs, is well documented and reliable and efficient synthetic procedures are available. However, as the complexity of the targeted functional system increases a multi-step strategy relying on the post-functionalization of preformed hybrid POM platforms could prove more appealing. In the second part of this review, we thus survey the synthetic methodologies of post-functionalization of POMs and critically discuss the opportunities it offers compared to direct functionalization.

Functionalization via covalent grafting of organic functions allows to tune the redox and acid-base properties, and the solubility of polyoxometalates, to enhance their stability and biological activity and to reduce their toxicity, to facilitate their implementation in extended structures and functional devices. We discuss herein the electronic and binding connections, and the various synthesis methodologies. We emphasize on organonitrogen, organosilyl and organophosphonyl derivatives with special attention to synthesis, characterization and potential applications in catalysis and materials science. We also consider the giant molybdenum oxide-based clusters especially the porous capsule-type clusters (Keplerates) which have high relevance to this context.

ContentsI. Introduction 77 II. Scope and Organization of the Review 78 III. Polyoxometalates Incorporating Halides 79 IV. Polyoxometalates Incorporating Group 16 Element-Centered Ligands 81 A. Peroxopolyoxometalates 81 B. Polyoxoalkoxometalates 82 1. Polyoxoalkoxometalates Involving Unidentate Alcohols 82 2. Polyoxoalkoxometalates Involving Chelating Triols 83 C. Heavier Group 16 Element-Centered Ligands 85 1. Thiopolyoxometalates 85 2. Organosulfur and Organoselenium Ligands 85 V. Polyoxometalates Incorporating Group 15 Element-Centered Ligands 86 A. Singly Bonded Nitrogen-Donating Ligands 86 1. Amine and Related Ligands 86 2. Amide Oximes [RC(NH 2 )NOH] and Oximes 87 B. Multiply Bonded Nitrogen Ligands 89 1. Nitrido Derivatives 89 2. Organoimido Derivatives 89 3. Hydrazido and Diazenido Derivatives of Polyoxometalates 90 4. Nitrosyl Derivatives 92 C. Organophosphorus, Organoarsenic, and Organoantimony Ligands 94 1. Organophosphonate and Organoarsonate Ligands 94 2. Organophosphinate and Organoarsinate Ligands 97 VI. Polyoxometalates Incorporating Group 14 Element-Centered Ligands 97 A. Oxocarbon Ligands 97 1. Carbonate 97 2. Carboxylates 97 3. Oxalate and Squarate 97 4. Carbonyl Derivatives 98 B. Silicon Derivatives 98 C. Germanium Derivatives 99 D. Tin and Lead Derivatives 99 VII. Organometallic Derivatives of Polyoxometalates 100 A. Cyclopentadienyl Derivatives of Polyoxometalates 100 B. Cyclopentadienyl Oxide Clusters of Groups 5 and 6 100 C. Organometallic Polyoxometalates 100 1. Polyoxometalate-incorporated Organometalic Complexes 101 2. Polyoxometalate-Supported Organometallic Complexes 101 3. Integrated Cubane-Type Clusters 105 4. Organometallic Cation Salts of Keggin-Type Anions 105 VIII. Concluding Remarks 106 IX. Acknowledgments 106 X. Abbreviations 106 XI. References 106

Several key properties of the water oxidation catalyst Rb(8)K(2)[{Ru(IV)(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)] and its mechanism of water oxidation are given. The one-electron oxidized analogue [{Ru(V)Ru(IV)(3)O(6)(OH(2))(4)}(gamma-SiW(10)O(36))(2)](11-) has been prepared and thoroughly characterized. The voltammetric rest potentials, X-ray structures, elemental analysis, magnetism, and requirement of an oxidant (O(2)) indicate these two complexes contain [Ru(IV)(4)O(6)] and [Ru(V)Ru(IV)(3)O(6)] cores, respectively. Voltammetry and potentiometric titrations establish the potentials of several couples of the catalyst in aqueous solution, and a speciation diagram (versus electrochemical potential) is calculated. The potentials depend on the nature and concentration of counterions. The catalyst exhibits four reversible couples spanning only ca. 0.5 V in the H(2)O/O(2) potential region, keys to efficient water oxidation at low overpotential and consistent with DFT calculations showing very small energy differences between all adjacent frontier orbitals. The voltammetric potentials of the catalyst are evenly spaced (a Coulomb staircase), more consistent with bulk-like properties than molecular ones. Catalysis of water oxidation by [Ru(bpy)(3)](3+) has been examined in detail. There is a hyperbolic dependence of O(2) yield on catalyst concentration in accord with competing water and ligand (bpy) oxidations. O(2) yields, turnover numbers, and extensive kinetics data reveal several features and lead to a mechanism involving rapid oxidation of the catalyst in four one-electron steps followed by rate-limiting H(2)O oxidation/O(2) evolution. Six spectroscopic, scattering, and chemical experiments indicate that the catalyst is stable in solution and under catalytic turnover conditions. However, it decomposes slowly in acidic aqueous solutions (pH < 1.5).

We propose a simple experimental procedure based on the cloud point measurement of a non-ionic surfactant as a tool for (i) estimating the super-chaotropic behaviour of polyoxometalates (POMs) and for (ii) establishing a classification of POMs according to their affinity towards polar surfaces.

Polyoxometalates (POMs) are attractive candidates for the rational design of multi-level charge-storage materials because they display reversible multi-step reduction processes in a narrow range of potentials. The functionalization of POMs allows for their integration in hybrid complementary metal oxide semiconductor (CMOS)/molecular devices, provided that fine control of their immobilisation on various substrates can be achieved. Owing to the wide applicability of the diazonium route to surface modification, a functionalized Keggin-type POM [PW11 O39 {Ge(p-C6 H4 -CC-C6 H4 -${{\rm N}{{+\hfill \atop 2\hfill}}}$)}](3-) bearing a pending diazonium group was prepared and subsequently covalently anchored onto a glassy carbon electrode. Electron transfer with the immobilised POM was thoroughly investigated and compared to that of the free POM in solution.

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Steady-state irradiation under visible light of a covalent Ir(III)-photosensitized polyoxotungstate is reported. In the presence of a sacrificial electron donor, the photolysis leads to the very efficient photoreduction of the polyoxometalate. Successive formation of the one-electron and two-electron reduced species, which are unambiguously identified by comparison with spectroelectrochemical measurements, is observed with a significantly faster rate reaction for the formation of the oneelectron reduced species. The kinetics of the photoreduction, which are correlated to the reduction potentials of the polyoxometalate (POM), can be finely tuned by the presence of an acid. Indeed light-driven formation of the two-electron reduced POM is considerably facilitated in the presence of acetic acid. The system is also able to perform photocatalytic hydrogen production under visible light without significant loss of performance over more than 1 week of continuous photolysis and displays higher photocatalytic efficiency than the related multi-component system, outlining the decisive effect of the covalent bonding between the POM and the photosensitizer. This functional and modular system constitutes a promising step for the development of charge photoaccumulation devices and subsequent photoelectrocatalysts for artificial photosynthesis.Photoconversion of light into chemical fuels is emerging as a major scientific challenge. [1][2][3][4] In the past decades, molecular approaches have mostly focused on one hand on the design of photosensitive systems displaying long-lived photo-induced charge separation states to permit further electron transfers 5-10 and, on the other hand, on catalysts able to use these photogenerated charges for achieving either oxygen [11][12][13][14][15][16][17][18] or hydrogen evolution. [19][20][21][22][23][24] As these two reactions are multi-electronic processes while photosensitizers deliver electrons and holes sequentially, the charges need to be directed to a charge accumulation site. 25 However, only a few molecular photoactive systems with a designed charge accumulation site have been described so far. [26][27][28][29][30] Another requirement is crucial for efficient charge accumulation in such systems: when partially filled, the reservoir should not interfere with the photoactive moiety. Indeed, in classical donor-acceptor (D-A) systems, the electron acceptor, once reduced, potentially becomes an electron donor and often displays lightabsorbing properties. Thus it may act, in a subsequent light-driven process, as a deleterious 47,48 In the previously mentioned study, transient absorptions measurements only allowed for the characterization of the first photo-induced electron transfer. Charge photo-accumulation studies can be achieved in the presence of an additional electron donor in the solution that can irreversibly quench the charge separation state, regenerate the initial state of the photosensitizer and make a second photo-induced process possible. We herein provide unprecedente...