Polyoxometalates (POMs) are discrete anionic metaloxygen clusters which can be regarded as soluble oxide fragments. They exhibit a great diversity of sizes, nuclearities, and shapes. They are built from the connection of {MOx} polyhedra, M being a d-block element in high oxidation state, usually VIV,V, MoVI, or WVI.1 While these species have been known for almost two centuries, they still attract much interest partly based on their large domains of applications. They play a great role in various areas ranging from catalysis,2 medicine,3 electrochemistry,4 photochromism,5 to magnetism.6 This palette of applications is intrinsically due to the combination of their added value properties (redox properties, large sizes, high negative charges, nucleophilicity...). Parallel to this domain, the organic-inorganic hybrids area has followed a similar expansion during the last 10 years. The concept of organic-inorganic hybrid materials * To whom correspondence should be addressed. E-mail: dolbecq@ chimie.uvsq.fr. Chem. Rev. 2010, 110, 6009–6048 6009

Hybrid Organic−Inorganic Polyoxometalate Compounds: From Structural Diversity to Applications

Ever since the discovery and development of polyoxometalates (POMs), it has been known that they can exist in electron-rich reduced forms of different archetypes, structural flexibilities and functionalities. There are now reliable synthetic strategies for electron-rich POMs — materials that have unique and potentially useful catalytic, electronic and magnetic properties. This Review covers the synthesis and applications of these reduced species, and also highlights their differences and advantages relative to fully oxidized POMs. More than 200 reduced POM structures are described in this Review, with emphasis placed on how reduction influences POM structure, function and properties. The metals in polyoxometalates need not be in their highest oxidation states. Indeed, polyoxometalates can exist in reduced forms, and several different metals can be incorporated into various structural archetypes. This Review describes the synthesis and characterization of these complexes, along with their topical catalytic, electronic and biological properties.

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Synthesis, structures and applications of electron-rich polyoxometalates

Today, sensing represents one of the key topics in current science and technology. Polyoxometalates (POMs), which are defined as early transition metal clusters, are considered as one of the most growing fields of research and development in sensing. This paper discusses the promising prospects of POMs in sensing. The paper starts with brief definitions about the formation of POMs. The two basic structures of POMs, Keggin and Dawson, as well as some combined structures are discussed. The interesting properties of POMs particularly as acid catalysts, in medicine, in redox chemistry and in magnetism are briefly mentioned. The main methods used for the deposition of POMs on solid supports (substrates) including chemisorption, electrodeposition, encapsulation in polymers and sol–gels, immobilization using the Langmuir–Blodgett process, layer by layer assemblies as well as deposition via formation of hybrid POM–organic moieties are discussed with their advantages and disadvantages. Finally, the potential applications of immobilized POMs on solid substrates as sensors for the detection and determination of analytes both in liquid and in the gas phase are addressed and compared.

Polyoxometalates: formation, structures, principal properties, main deposition methods and application in sensing

Metal-organic framework nanosheets: An emerging family of multifunctional 2D materials

Hybrid organic–inorganic films consisted of molecular layers of a Keggin-structure polyoxometalate (POM: 12-tungstophosphoric acid, H3PW12O40) and 1,12-diaminododecane (DD) on 3-aminopropyl triethoxysilane (APTES)-modified silicon surface, fabricated via the layer-by-layer (LBL) self-assembly method are evaluated as molecular materials for electronic devices. The effect of the fabrication process parameters, including primarily compositions of deposition solutions, on the structural characteristics of the POM-based multilayers was studied extensively with a combination of spectroscopic methods (UV, FTIR, and XPS). Well-characterized POM-based films (both single-layers and multilayers) in a controlled and reproducible way were obtained. The conduction mechanisms in single-layered and multilayered structures were elucidated by the electrical characterization of the produced films supported by the appropriate theoretical analysis. Fowler−Nordheim (FN) tunneling and percolation mechanisms were encountered in ...

Polyoxometalate-based layered structures for charge transport control in molecular devices.

Electrochromic multilayer films consisting of polyoxometalate (POM) cluster α-K10[P2W17O61]·17H2O (P2W17), copper(II) complex [CuII(phen)2](NO3)2 (phen = 1,10-phenithroline), and iron complex [FeII(phen)3](ClO4)2 were fabricated on silicon, quartz and ITO substrates by layer-by-layer self-assembly method. The multilayer films, PSS/CuII(phen)2/[(P2W17/CuII(phen)2)]n and PSS/FeII(phen)3/[(P2W17/FeII (phen)3)]n were characterized by UV−vis spectra, X-ray photoelectron spectra, cyclic voltammetry (CV), chronoamperometric (CA) and in-situ spectral electrochemical measurements. The interesting feature of the electrochromic film is its adjustable color by reduction of both transition metal complex and polyoxometalate at different potentials. The multilayer films also exhibit high optical contrast, suitable response time and low operation potential due to the presence of mono-lacunary-substituted polyoxometalate and transition metal complex. This is the first example that the color of electrochromic film can be a...

Electrochromic multilayer films of tunable color by combination of copper or iron complex and monolacunary dawson-type polyoxometalate.

Electrochromic multilayer films based on trilacunary Dawson-type polyoxometalate

An ultrathin multilayer film consisting of polyoxometalate cluster a-K 8 [P 2 W 1 7 O 6 1 Ni 1 1 (H 2 O)].18H 2 O(P 2 W 1 7 Ni) and polyallylamine hydrochloride (PAH) was fabricated on quartz and indium tin oxide substrates by layer-by-layer self-assembly. The multilayer films, (P 2 W 1 7 Ni/PAH) n , were characterized by UV-visible spectra, cyclic voltammetry, chronoamperometry, atomic force microscopy, and in situ spectroelectrochemical measurements. The multilayer film exhibits both suitable response time and low operation potential due to the presence of nickel-substituted polyoxometalate, providing valuable information for exploring the possible application of polyoxometalate-based electrochromic materials. Further, P 2 W 1 7 Ni embedded in a self-assembled polyelectrolyte matrix can also be used as a pH sensor because of the pH dependence of its electrochemical response.

Electrochromic and pH-Sensitive Multilayer Films Based on Nickel-Substituted Dawson-Type Polyoxometalate

A novel selenium–molybdenum oxide chain inserted with copper (II) complex: Synthesis, structure, and magnetic property

A new bicapped Keggin heteropolyoxoanion derivative [Co(bpy)3]2[ VIVO40(VIVO)2] [{Co(bpy)2(H2O)}2{ VIVO40(VIVO)2}] · 8H2O (1) (bpy = 2,2′-bipyridine), has been hydrothermally synthesized and structurally characterized by elemental analysis, single crystal X-ray diffraction, IR and X-ray photoelectron spectra. Complex 1 consists of two [Co(bpy)3]2+ cations, an anion of [ VIVO40(VIVO)2]4− and a neutral unit [{Co(bpy)2(H2O)}2{ VIVO40(VIVO)2}] in which { VIVO40(VIVO)2}4− is coordinated to two Co atoms through its terminal oxygen atoms of the two capping V atoms at opposite sites together with bipyridine and water ligands. The most interesting feature of 1 is that a polyoxoanion and a neutral component coexist in the crystal structure. Hydrogen-bonding contacts are also formed between different polyanions through the lattice water molecules.

Zhenqing Wang

Papers

Electrochromic multilayer films of tunable color by combination of copper or iron complex and monolacunary dawson-type polyoxometalate.

Electrochromic multilayer films based on trilacunary Dawson-type polyoxometalate

Electrochromic and pH-Sensitive Multilayer Films Based on Nickel-Substituted Dawson-Type Polyoxometalate

A novel selenium–molybdenum oxide chain inserted with copper (II) complex: Synthesis, structure, and magnetic property

A novel cobalt(II) complex with polyoxometalate-based ligand by virtue of coexistence of both a capped-Keggin anion and a neutral unit