Polyoxometalates: Very large clusters - Nanoscale magnets

Main-Group Element, Organic, and Organometallic Derivatives of Polyoxometalates.

The earlier tremendous interest in molybdenum blue solutions-an enigma for generations of chemists-became even more pronounced when it turned out that the basic building units correspond to giant wheel-shaped clusters with nanostructured cavities, displaying exceptional aesthetic beauty. This discovery will stimulate new ideas in supramolecular chemistry, colloid chemistry, and materials science. The giant wheels represent nanosensors and nanoreactors, enabling the initiation of chemical processes at different positions, like a "structured landscape", and can even be used as robust synthons for the construction of compounds with typical solid-state structure, a situation comparable to crystal engineering.

Soluble molybdenum blues-"des Pudels Kern".

Giant metal-oxide-based spheres and their topology: from pentagonal building blocks to keplerates and unusual spin systems

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

The compounds (NMe4)5[As2Mo8V4AsO40] . 3 H2O 2a, (NH4)21[H3Mo57V6(NO)6O183(H2O)18] . 65 H2O 3a, (NH2Me2)18(NH4)6[Mo57V6(NO)6O183(H2O)18] . 14 H2O 3b and (NH4)12[Mo36(NO)4O108(H2O)16] . 33 H2O 4a (3a and 4a were not correctly reported in the literature regarding to their composition, structures and the oxidation states of the metal centres) which contain large isolated anionic species, have been prepared (among them 3 a, 3 b, and 4 a in rather high yield) and characterized by complete crystal structure analysis as well as IR/Raman, UV/VIS/NIR, ESR spectroscopy and magnetic susceptibility measurements, redox titrations, bond valence sum calculations, elemental analyses and thermogravimetric studies. Perspectives for polyoxometalate chemistry referring to the synthesis of ''extremely'' large nanoscaled species are discussed, together with the occurrence of a large transferable {Mo17} building block in the compounds 3a, 3b and 4a which also exists in the corresponding iron compound Na3(NH4)12[H15Mo57Fe6(NO)6O183(H2O)18] . 76H2O 7a.

New Perspectives in Polyoxometalate Chemistry by isolation of compounds containing very large moieties as transferable building blocks: (NMe4)5[As2Mo8V4AsO40] · 3H2O, (NH4)21[H3Mo57V6(NO)6O183(H2O)18] · 65 H2O, (NH2Me2)18(NH4)6[Mo57V6(NO)6O183(H2O)18] · 14 H2O, and (NH4)12[Mo36(NO)4O108(H2O)16] · 33 H2O

New Perspectives in Polyoxometalate Chemistry by Isolation of Compounds Containing Very Large Moieties as Transferable Building Blocks: (NMe4)5(As2Mo8V4AsO40)×3 H2O, (NH4)21(H3Mo57V6(NO)6O183( H2O)18)×65 H2O, (NH2Me2)18(NH4)6(Mo57V6(NO)6O183(H2O)18)× 14 H2O, and (NH4)12(Mo36(NO)4O108(H2O)16)×33 H2O.

L. Dloczik

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