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

Functionalization of polyoxometalates: towards advanced applications in catalysis and materials science

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 and post-functionalization: a step towards polyoxometalate-based materials

Heterogeneous selective oxidation catalysts based on coordination polymer MIL-101 and transition metal-substituted polyoxometalates

Covalent Immobilization of a TiW5 Polyoxometalate on Derivatized Silicon Surfaces

The methoxo-bridged, dimeric, ZrIV-substituted Lindqvist-type polyoxometalate (POM) (nBu4N)6[{(μ-MeO)ZrW5O18}2], (TBA)61, has been synthesized by stoichiometric hydrolysis of Zr(OnPr)4, [{Zr(OiPr)3(μ-OnPr)(iPrOH)}2], or [{Zr(OiPr)4(iPrOH)}2] and [{WO(OMe)4}2] in the presence of (nBu4N)2WO4, providing access to the systematic nonaqueous chemistry of ZrW5 POMs for the first time and an efficient route to 17O-enriched samples for 17O NMR studies. 1H NMR provided no evidence for dissociation of 1 in solution, although exchange with MeOH was shown to be slow by an EXSY study. Reactions with HX at elevated temperatures gave a range of anions [{XZrW5O18}n]3n- (X = OH, 3; OPh, 4; OC6H4Me-4, 5; OC6H4(CHO)-2, 6; acac, 7; OAc, 8), where n = 2 for 3 and n = 1 for 4−8, while 1H and 17O NMR studies of hydrolysis of 1 revealed the formation of an intermediate [(μ-MeO)(μ-HO)(ZrW5O18)2]6-. Electrospray ionization mass spectrometry of 1 and 3 illustrated the robust nature of the ZrW5O18 framework, and X-ray crystal structu...

Synthesis and reactivity of the methoxozirconium pentatungstate (nBu4N )6 [{(μ-MeO )ZrW5O18}2] : Insights into proton-transfer reactions, solution dynamics, and assembly of {ZrW5O18}2- building blocks

The tetra-n-butylammonium (TBA) salt of [(MeO)TiW5O18]3−1 was reacted with alcohols ROH to give primary, secondary and tertiary alkoxide derivatives [(RO)TiW5O18]3− (R = Et 2, iPr 3 and tBu 4), whilst hydrolysis afforded [(µ-O)(TiW5O18)2]6−5 rather than the hydroxo derivative (R = H). In reactions with iPrOH and tBuOH, impurity peaks observed at 1015 and 1020 ppm in the 17O NMR spectra indicate alkoxide degradation and TiO bond formation via reactions analogous to those occurring at the surfaces of solid heteropolyacids. Aryloxides [(ArO)TiW5O18]3− were prepared by reacting 1 with phenols ArOH (Ar = C6H56, C6H4Me-4 7, C6H4tBu-4 8, C6H4OH-4 9, C6H4OH-3 10, C6H3(OH)2-3,5 11 and C6H4CHO-2 13). TiW5O18 units were linked by reacting 1 with 9 to give [(µ-1,4-OC6H4O)(TiW5O18)2]6−12. 17O and 183W NMR spectra are reported and X-ray crystal structures were obtained for TBA salts of anions 3–10 and 13, which showed that the titanium is six-coordinate in all cases. Reactions were monitored by 1H NMR, including a 2D-EXSY study of methoxo exchange, and the slow rates observed are probably associated with the reluctance of titanium in these anions to achieve seven-coordination.

Sagar S. Petkar

Papers

Covalent Immobilization of a TiW5 Polyoxometalate on Derivatized Silicon Surfaces

Synthesis and reactivity of the methoxozirconium pentatungstate (nBu4N )6 [{(μ-MeO )ZrW5O18}2] : Insights into proton-transfer reactions, solution dynamics, and assembly of {ZrW5O18}2- building blocks

Non-aqueous synthetic methodology for TiW5 polyoxometalates: protonolysis of [(MeO)TiW5O18]3− with alcohols, water and phenols