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

The convergent assembly of metal-organic frameworks has enabled the design of porous materials using a structural building unit approach, but functional systems incorporating pre-assembled structural building unit 'pore' openings are rare. Here, we show that the face-directed assembly of a ring-shaped macrocyclic polyoxometalate structural building unit, {P(8)W(48)O(184)}(40-) with an integrated 1-nm pore as an 'aperture synthon', with manganese linkers yields a vast three-dimensional extended framework architecture based on a truncated cuboctahedron. The 1-nm-diameter entrance pores of the {P(8)W(48)O(184)}(40-) structural building unit lead to approximately spherical 7.24-nm(3) cavities containing exchangeable alkali-metal cations that can be replaced by transition-metal ions through a cation exchange process. Control over this process can be exerted by either electrochemically switching the overall framework charge by manipulating the oxidation state of the manganese linker ions, or by physically gating the pores with large organic cations, thus demonstrating how metal-organic framework-like structures with integrated pores and new physical properties can be assembled.

Face-directed self-assembly of an electronically active Archimedean polyoxometalate architecture

The molybdenum blue reaction for the determination of orthophosphate revisited: Opening the black box.

Mo (W,V)Cu (Ag)S(Se) cluster compounds

Fe-M-S complexes derived from MS42- anions (M = molybdenum, tungsten) and their possible relevance as analogs for structural features in the molybdenum site of nitrogenase

Crystal and molecular structure of [N(CH3)4]2[Mo2O2S2(S2)2]: a compound with two S22- ligands

(MozS8Ag4)(PPh3)., has been prepared by the extraction of an aqueous solution of (NH4)2M~S4 with a solution of (C6HS)3P and AgN03 in CH2C12. The crystal and molecular structure of ( M O ~ S ~ A ~ ~ ) ( P P ~ ~ ) ~ was determined from single-crystal X-ray diffractometer data. The compound crystallizes in the triclinic space group PI with one molecule in the unit cell: a = 12.246 (2), 6 = 12.905 (2), c = 13.211 (2) A; 01 = 97.81 ( I ) , /3 = 113.31 (l) , y = 91.52 (1)'; V = 1892.1 A3; dcalcd = 1.69 g ~ m ~ . Anisotropic (except for the C atoms) least-squares refinement resulted in a conventional final R factor of 0.058 for 5183 independent data. The structure can be described as a cage (with a center of inversion), fused by two six-membered SAg2S2Mo rings which are connected by metalsulfur bonds. Mo (106.42 (10)-112.98 (9)') and Ag (distorted, angles varying between 92.62 (8) and 129.08 (8)') are tetrahedrally coordinated. M o S ~ ~ acts as a terdentate ligand.

Preparation and x-ray crystal and molecular structure of {Mo2S8Ag4}(PPh3)4, a compound with a metal-sulfur cage

The mixed-metal cluster compound (Me2NH2)6[(SCN)9W3S4SnCl3].0.5H2O (1) with the cubane-type [W3(mu3-S)4Sn] core has been prepared by reacting W3S44+ in aqueous HCI solution with SnCl2.2H2O in the presence of Me2NH2Cl and NaSCN. 1 was characterized by X-ray crystallography and by Sn-119 Mossbauer, photoelectron, and electronic absorption spectroscopy. Crystals of 1 are monoclinic, of space group C2/c, with a = 42.792(10) angstrom, b = 11.988(2) angstrom, c = 21.725(3) angstrom, beta = 95.24(2)-degrees, and Z = 8. The structure was refined by full-matrix least-squares techniques to R = 0.069 and R(w) = 0.071 for 7690 reflections with F > 4sigma(F). The formation of 1 is due to a synergetic effect: The affinity of the W3S4 ligand for cations like Sn2+ is increased as its electron-donor strength is increased by the reducing cation itself. An alternative description would be to insert a plug formed mainly from the Sn 5s level into a socket formed from acceptor levels localized on the W3 triangle, which is equivalent to a novel type of host pest interactions.

Reductive Addition at the W3S44+ Core by Sn2+ or an Unusual Supramolecular System: A Synergetic Reaction Leading to the Host Guest Compound (Me2NH2)6[(SCN)9W3S4SnCl3].cntdot.0.5H2O

Achim Mueller

Papers

Crystal and molecular structure of [N(CH3)4]2[Mo2O2S2(S2)2]: a compound with two S22- ligands

Preparation and x-ray crystal and molecular structure of {Mo2S8Ag4}(PPh3)4, a compound with a metal-sulfur cage

Reductive Addition at the W3S44+ Core by Sn2+ or an Unusual Supramolecular System: A Synergetic Reaction Leading to the Host Guest Compound (Me2NH2)6[(SCN)9W3S4SnCl3].cntdot.0.5H2O

Structure and vibrational-spectrum of "bis(tetrathiotungstato)zincate(ii), [zn(ws4)2]2-

Self‐Assembly in Aqueous Solution of Wheel‐Shaped Mo154 Oxide Clusters into Vesicles.