Following Nature's lessons, today chemists can cross the boundary of the small molecule world to construct multifunctional and highly complex molecular nano-objects up to protein size and even cell-like nanosystems showing responsive sensing. Impressive examples emerge from studies of the solutions of some oxoanions of the early transition metals especially under reducing conditions which enable the controlled linking of metal-oxide building blocks. The latter are available from constitutional dynamic libraries, thus providing the option to generate multifunctional unique nanoscale molecular systems with exquisite architectures, which even opens the way towards adaptive and evolutive (Darwinian) chemistry. The present review presents the first comprehensive report of current knowledge (including synthesis aspects not discussed before) regarding the related giant metal-oxide clusters mainly of the type {Mo57M′6} (M′ = FeIII, VIV) (torus structure), {M72M′30} (M = Mo, M′ = VIV, CrIII, FeIII, MoV), {M72Mo60} (M = Mo, W) (Keplerates), {Mo154}, {Mo176}, {Mo248} (“big wheels”), and {Mo368} (“blue lemon”) – all having the important transferable pentagonal {(M)M5} groups in common. These discoveries expanded the frontiers of inorganic chemistry to the mesoscopic world, while there is probably no collection of discrete inorganic compounds which offers such a versatile chemistry and the option to study new phenomena of interdisciplinary interest. The variety of different properties of the sphere- and wheel-type metal-oxide-based clusters can directly be related to their unique architectures: The spherical Keplerate-type capsules having 20 crown-ether-type pores and tunable internal functionalities allow the investigation of confined matter as well as that of sphere-surface-supramolecular and encapsulation chemistry – including related new aspects of the biologically important hydrophobic effects – but also of nanoscale ion transport and separation. The wheel-type molybdenum-oxide clusters exhibiting complex landscapes do not only have well-defined reaction sites but also show unprecedented adaptability regarding the integration of various kinds of matter. Applications in different fields, e.g. in materials science and catalysis including those in small spaces, investigated by several groups, are discussed while possible directions for future work are outlined.

From linking of metal-oxide building blocks in a dynamic library to giant clusters with unique properties and towards adaptive chemistry

This critical review surveys the 3d substituted magnetic polyoxotungstate (POT) molecular compounds reported these last ten years, illustrating the huge variety of topologies that these systems can adopt. Both purely inorganic and organic/inorganic species have been considered, and the cited compounds have been classified according to their nuclearity, i.e., the number of magnetic 3d centres embedded in the POT matrix, which can be as high as 48. This review highlights the synthetic conditions, the solubility and the stability in solution of these species. The magnetic, catalytic and electrocatalytic properties of these complexes are also summarized.

Diversity in structures and properties of 3d-incorporating polyoxotungstates

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

Hybrid organic/inorganic Wells−Dawson polyoxotungstates have been prepared through addition of functionalized tricholorostannanes to lacunary α2- and α1-[P2W17O61]10-. Coupling of amines and alcohols to polyoxotungstate platforms led to new structures in good yields. Coupling of chiral amines to the previously unknown organotin-substituted α1 derivatives allowed the isolation of diastereomers, which feature in some cases split 1H, 13C, and 31P NMR spectra. This is the first example of NMR observation of a single pair of diastereomers in the α1-Wells−Dawson series. It opens the way to potential resolution of those chiral polyoxotungstates.

Efficient Preparation of Functionalized Hybrid Organic/Inorganic Wells−Dawson-type Polyoxotungstates

4.11 – Polyoxometalates: Reactivity

Heteropolytungstates result from the acidic polycondensation of [WO4]\" anions around a heteroelement X. Despite their apparent great diversity, due to the different stereochemical and electronic properties of the heteroatoms, the formation of most heteropolytungstates can be described by analogous successive reactions. In a first step, 9-tungstoheteropolyanions are generally obtained in basic medium and have an open structure. Two types, A and B, and for each type two isomers, α and β, are known. In the next steps which occur at lower pH, closing of the framework or association of the open species directly or through tungsten atoms lead to miscellaneous heteropolytungstates. The reactions are often controlled by the nature of the cationic counter-ions in the solution, for example Na or K. Moreover, in these last steps, the tungsten atoms can be replaced by transition metal cations giving rise to polynuclear complexes. New monoor polyvacant species can be obtained by selective hydrolytic reactions of the heteropolyanions formed in acid solution, the nature of these species depending again on the nature of the heteroelement and of the counter-ions.

R. Contant

Papers

The heteropolyoxotungstates: relationships between routes of formation and structures