Polyoxometalates represent a diverse range of molecular clusters with an almost unmatched range of physical properties and the ability to form structures that can bridge several length scales. The new building block principles that have been discovered are beginning to allow the design of complex clusters with desired properties and structures and several structural types and novel physical properties are examined. In this critical review the synthetic and design approaches to the many polyoxometalate cluster types are presented encompassing all the sub-types of polyoxometalates including, isopolyoxometalates, heteropolyoxometalates, and reduced molybdenum blue systems. As well as the fundamental structure and bonding aspects, the final section is devoted to discussing these clusters in the context of contemporary and emerging interdisciplinary interests from areas as diverse as anti-viral agents, biological ion transport models, and materials science.

Polyoxometalate clusters, nanostructures and materials: From self assembly to designer materials and devices

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

Recent Developments in the Preparation and Chemistry of Metallacycles and Metallacages via Coordination

Oxidative stress is known to play an important role in the pathogenesis of a number of diseases. In particular, it is linked to the etiology of Alzheimer's disease (AD), an age-related neurodegenerative disease and the most common cause of dementia in the elderly. Histopathological hallmarks of AD are intracellular neurofibrillary tangles and extracellular formation of senile plaques composed of the amyloid-beta peptide (Aβ) in aggregated form along with metal-ions such as copper, iron or zinc. Redox active metal ions, as for example copper, can catalyze the production of Reactive Oxygen Species (ROS) when bound to the amyloid-β (Aβ). The ROS thus produced, in particular the hydroxyl radical which is the most reactive one, may contribute to oxidative damage on both the Aβ peptide itself and on surrounding molecule (proteins, lipids, …). This review highlights the existing link between oxidative stress and AD, and the consequences towards the Aβ peptide and surrounding molecules in terms of oxidative damage. In addition, the implication of metal ions in AD, their interaction with the Aβ peptide and redox properties leading to ROS production are discussed, along with both in vitro and in vivo oxidation of the Aβ peptide, at the molecular level.

https://hal.archives-ouvertes.fr/hal-01635507/document

Oxidative stress and the amyloid beta peptide in Alzheimer's disease.

A series of remarkable crystalline compounds [Cu2(BTC)4/3(H2O)2]6[HnXM12O40]·(C4H12N)2 (X = Si, Ge, P, As; M = W, Mo) were obtained from the simple one-step hydrothermal reaction of copper nitrate, benzentricaboxylate (BTC), and different Keggin polyoxometalates (POMs). In these compounds, the catalytically active Keggin polyanions were alternately arrayed as noncoordinating guests in the cuboctahedral cages of a Cu-BTC-based metal−organic framework (MOF) host matrix. X-ray crystallographic analyses, TG, FT-IR, UV−vis, N2 adsorption studies, and acid−base titration demonstrated their high stability and toleration for thermal and acid−base conditions. No POM leaching or framework decomposition was observed in our study. The representative acid catalytic performance of a compound containing PW12 species was assessed through the hydrolysis of esters in excess water, which showed high catalytic activity and can be used repeatedly without activity loss. Moreover, catalytic selectivity, which is dependent on th...

Highly stable crystalline catalysts based on a microporous metal-organic framework and polyoxometalates.

Organic-inorganic hybrid materials based on polyoxometalate building blocks with capping La 3 + ions and bidentate oxygenated ligands have been obtained by reaction at room temperature of the [e-PMo 1 2 O 3 6 -(OH 4 {La(H 2 O) 4 )] 5 + polyoxocation with glutarate (C 5 H 6 O 2 2-) and squarate (C 4 O 4 2-) organic ligands. [e-PMo 1 2 O 3 7 -(OH) 3 {La(H 2 O) 4 (C 5 H 6 O 4 ) 0 . 5 } 4 ].21 H 2 O (1) and [e-PMo 1 2 O 3 9 (OH){La(H 2 O) 6 } 2 -(La(H 2 O) 5 (C 4 O 4 ) 0 . 5 } 2 ].17H 2 O (2) form unprecedented 1D chains built from alternating polyoxocations and organic ligands connected through La-O links. The structures of these materials are compared to the 2D hybrid organic-inorganic framework [NC 4 H 1 2 ] 2 -[Mo 2 2 O 5 2 (OH) 1 8 {La(H 2 O) 4 } 2 {La(CH 3 CO 2 ) 2 } 4 ].8H 2 O (3) isolated from the hydrothermal reaction of elemental precursors (MoO 4 2-, Mo, La 3 + ) in acetate buffer. Compound 3 is built from previously undescribed polyoxometalate units with twenty-two MO v centers capped by six La 3 + ions, four of which are bridged by acetate ligands.

Hybrid Organic–Inorganic 1D and 2D Frameworks with ε-Keggin Polyoxomolybdates as Building Blocks

The reactivity of the [α-SiW 1 1 O 3 9 ] 8 - monovacant polyoxometalate with lanthanide has been investigated for four different trivalent rare-earth cations (Ln = Nd I I I , Eu I I I , Gd I I I , Yb I I I ). The crystal structures of KCs 4 [Yb(α-SiW 1 1 O 3 9 )-(H 2 O) 2 ].24H 2 O (1), K 0 . 5 Nd 0 . 5 [Nd 2 (α-SiW 1 1 O 3 9 )(H 2 O) 1 1 ].17H 2 O (2a), and Na 0 . 5 Cs 4 . 5 [Eu(α-SiW 1 1 O 3 9 )(H 2 O) 2 ].23H 2 O (3a) are reported. The solid-state structure of compound 1 consists of linear wires built up of [α-SiW 1 1 O 3 9 ] 8 - anions connected by Yb 3 + cations, while the linkage of the building blocks by Eu 3 + centers in 3a leads to the formation of zigzag chains. In 2a, dimeric [Nd 2 (α-SiW 1 1 O 3 9 ) 2 (H 2 O) 8 ] 1 0 - entities are linked by four Nd 3 + cations. The resulting chains are connected by lanthanide ions, leading to a bidimensional arrangement. Thus, the dimensionality, the organization of the polyoxometalate building units, and the Ln/[α-SiW 1 1 O 3 9 ] 8 - ratio in the solid state can be tuned by choosing the appropriate lanthanide. The luminescent properties of compound 3a have been studied, showing that, in solution, the polymer decomposes to give the monomeric complex [Eu(α-SiW 1 1 O 3 9 )(H H O) 4 ] 5 - . The lability of the four exogenous ligands connected to the rare earth must allow the functionalization of this lanthanide polyanion.

Solid-State and solution studies of [Ln(n)(SiW11O39)] polyoxoanions: an example of building block condensation dependent on the nature of the rare earth.

Five Co(II) silicotungstate complexes are reported. The centrosymmetric heptanuclear compound K(20)[{(B-beta-SiW(9)O(33)(OH))(beta-SiW(8)O(29)(OH)(2))Co(3)(H(2)O)}(2)Co(H(2)O)(2)]47 H(2)O (1) consists of two {(B-beta-SiW(9)O(33)(OH))(beta-SiW(8)O(29)(OH)(2))Co(3)(H(2)O)} units connected by a {CoO(4)(H(2)O)(2)} group. In the chiral species K(7)[Co(1.5)(H(2)O)(7))][(gamma-SiW(10)O(36))(beta-SiW(8)O(30)(OH))Co(4)(OH)(H(2)O)(7)]36 H(2)O (2), a {gamma-SiW(10)O(36)} and a {beta-SiW(8)O(30)(OH)} unit enclose a mononuclear {CoO(4)(H(2)O)(2)} group and a {Co(3)O(7)(OH)(H(2)O)(5)} fragment. The two trinuclear Co(II) clusters present in 1 enclose a mu(4)-O atom, while in 2 a mu(3)-OH bridging group connects the three paramagnetic centers of the trinuclear unit, inducing significantly larger Co-L-Co (L=mu(4)-O (1), mu(3)-OH (2)) bridging angles in 2 (theta(av(Co-L-Co))=99.1 degrees ) than in 1 (theta(av(Co-L-Co))=92.8 degrees ). Weaker ferromagnetic interactions were found in 2 than in 1, in agreement with larger Co-L-Co angles in 2. The electrochemistry of 1 was studied in detail. The two chemically reversible redox couples observed in the positive potential domain were attributed to the redox processes of Co(II) centers, and indicated that two types of Co(II) centers in the structure were oxidized in separate waves. Redox activity of the seventh Co(II) center was not detected. Preliminary experiments indicated that 1 catalyzes the reduction of nitrite and NO. Remarkably, a reversible interaction exists with NO or related species. The hybrid tetranuclear complexes K(5)Na(3)[(A-alpha-SiW(9)O(34))Co(4)(OH)(3)(CH(3)COO)(3)]18 H(2)O (3) and K(5)Na(3)[(A-alpha-SiW(9)O(34))Co(4)(OH)(N(3))(2)(CH(3)COO)(3)]18 H(2)O (4) were characterized: in both, a tetrahedral {Co(4)(L(1))(L(2))(2)(CH(3)COO)(3)} (3: L(1)=L(2)=OH; 4: L(1)=OH, L(2)=N(3)) unit capped the [A-alpha-SiW(9)O(34)](10-) trivacant polyanion. The octanuclear complex K(8)Na(8)[(A-alpha-SiW(9)O(34))(2)Co(8)(OH)(6)(H(2)O)(2)(CO(3))(3)]52 H(2)O (5), containing two {Co(4)O(9)(OH)(3)(H(2)O)} units, was also obtained. Compounds 2, 3, 4, and 5 were less stable than 1, but their partial electrochemical characterization was possible; the electronic effect expected for 3 and 4 was observed.

Effect of Cyanato, Azido, Carboxylato, and Carbonato Ligands on the Formation of Cobalt(II) Polyoxometalates: Characterization, Magnetic, and Electrochemical Studies of Multinuclear Cobalt Clusters

[ε‐PMo12O36(OH)4{La(H2O)4}4]5+: The First ε‐PMo12O40 Keggin Ion and Its Association with the Two‐Electron‐Reduced α‐PMo12O40 Isomer

Five new materials built from polyoxotungstates and Cu(II) ions as linkers have been synthesized by hydrothermal reactions from a mixture of sodium tungstate, copper chloride and bipyridine. The value of the initial pH, the nature of the heteroelement (P or Si) and of the ligand (2,2′- and/or 4,4′-bipyridine) permit the control of the nature of the polyoxotungstate clusters and their connectivity via the copper ions, and hence the dimensionality of the framework. A single phase has been obtained with silicon as heteroelement at an initial pH of 5, namely the 2D material [SiW12O40][Cu(2,2′-bpy)2]2·10H2O (1) with saturated Keggin polyoxotungstates linked by {Cu(2,2′-bpy)2}2+ groups. With phosphorous as heteroelement and at the same initial pH, three different structures have been isolated according to the nature of the ligand. Indeed, the two 1D materials [{Cu5(2,2′-bpy)5(H2O)(HPO4)(PO4)}PW11CuO39]·6H2O (2) with 2,2′-bpy and [4,4′-Hbpy][{Cu2(2,2′-bpy)2(4,4′-bpy)2.5}PW11CuO39]·16H2O (3) with a mixture of 2,2′- and 4,4′-bpy have been characterized, and a coordination polymer with polyoxometalate guests Na3[4,4′-Hbpy]{Cu4(4,4′-bpy)8(H2O)8}[PW11CuO39(H2O)][PW10Cu2O38(H2O)2]·38H2O (4) with 4,4′-bpy has been obtained. Finally, in basic medium (pH = 10) the unprecedented molecular cluster Na2[{Cu8(2,2′-bpy)8}(PW8O31)2]·15H2O (5) has been evidenced. Magnetic studies of compound 2 revealed that the predominant interactions involve only 4 paramagnetic centers, which are interacting within pairs, among the 6 Cu(II) centers. The χMT
				=
				f(T) curve can be fitted using the dinuclear expression appropriate to the HDVV isotropic exchange Hamiltonian H
				=
				−JS1×S2, with S1
				=
				S2
				=
				½ and J
				=
				−105.4 cm−1, showing strong antiferromagnetic interactions within the two Cu(II) pairs.

Laurent Lisnard

Papers

Hybrid Organic–Inorganic 1D and 2D Frameworks with ε-Keggin Polyoxomolybdates as Building Blocks

Solid-State and solution studies of [Ln(n)(SiW11O39)] polyoxoanions: an example of building block condensation dependent on the nature of the rare earth.

Effect of Cyanato, Azido, Carboxylato, and Carbonato Ligands on the Formation of Cobalt(II) Polyoxometalates: Characterization, Magnetic, and Electrochemical Studies of Multinuclear Cobalt Clusters

[ε‐PMo12O36(OH)4{La(H2O)4}4]5+: The First ε‐PMo12O40 Keggin Ion and Its Association with the Two‐Electron‐Reduced α‐PMo12O40 Isomer

Molecular and multidimensional polyoxotungstates functionalized by {Cu(bpy)}2+ groups