Polyoxometalate reviews

The applications of polyoxometalates in catalytic dehalogenation of halocar-bons have been succinctly reviewed by Hill and coworkers [188]. This reaction involves the photocatalytic transformation of organic halides coupled with the oxidation of sacrificial organic reductants (secondary alcohols or tertiary amides) (Eq. (9)) [189, 190] ... 

Neumann and Fish have studied the novel polyoxometalate salt 25, which features 12 fluorous ammonium cations [12]. This material was insoluble in EtOAc (and toluene) at room temperature, but dissolved at 80 °C to give an effective catalyst system for the oxidation of alkenes and alcohols by 30% aqueous H2O2. CooHng precipitated the catalyst, which was reused. Additional examples of thermomorphic fluorous catalysts have been briefly described in meeting abstracts [60,61] and will Hkely soon appear in the peer-reviewed literature. 

PossibiKties of electrocatalysis of reactions at electrodes are among the powerful incentives for the electrochemical study of POMs. Interesting results were obtained both in electrocatalytic reductions and oxidations, provided the appropriate form of the POM is used. Two recent reviews devoted to the electrochemical properties of polyoxometalates as electrocatalysts are available [8, 9]. The second one focuses more specifically on electrocatalysis on modified electrodes. In the present text, attention will be drawn specially to the basic principles that could be considered to govern most of solution processes. The principles will be illustrated by several recent experimental results, even though earlier achievements will also be described briefly. 

Lack of space has forced exclusion of some clusters, and topics described in reviews elsewhere are afforded reduced coverage. Literature citation for established classes of aggregates such as the polyoxometallates is limited to contemporary leading references. Dimetallic and linear polymetallic complexes are excluded. 

The present chapter reviews the structural chemistry (Section 2) and properties and applications (Section 3) of polyoxometalates that incorporate one or more rare-earth elements. In most cases these are discrete anionic entities within the crystal and in solution, but there are also extended lattices in which POM groups are linked by rare-earth cations. Solids which can best be described as mixed oxides, or which appear to be salts of common polyoxometalate architectures such as the... 

TABLE 1 Reviews of aspects of polyoxometalate rare-earth chemistry... 

Interest in polyoxometalate complexes of the rare earths has been driven to a large extent by their photophysical and photochemical properties. Table 1 lists several reviews. In general, photoexcitation into LMCT (O -> W, O -> Mo) bands results in intramolecular energy transfer to the rare earth with subsequent emission and luminescence. 

Molybdenum coordination has recently been comprehensively reviewed in Comprehensive Coordination Chemistry, particularly in the areas of mononuclear complexes, polyoxometaUate synthesis, structure, and reactivity see also Polyoxometalates), and in chalcogen-containing cluster complexes. ... 

PolyoxometaUate ions containing molybdenum form an enormous range of compounds which have been reviewed extensively2 (see also Polyoxometalates). They may be subdivided into (i) isopolyanions containing only Mo and (ii) heteropolyanions where other metal and/or nonmetal atoms may be present in the core structure. 

A large fraction of polytungstates that have been tested prove to have potent antitumoral and antiviral activity (rabies, influenza, scrapie, and the retroviruses responsible for AIDS and SAIDS). Toxicity varies widely, bnt some species are less toxic than azidothymidine (AZT) and are effective against AZT-resistant cell lines. The polyoxoanions bind to the gpl20 envelope glycoprotein of the retrovirus, and in some cases inhibit the function of the rus reverse transcriptase (cf AZT). Reviews of Structure and Reactivity of Polyoxometalates have recently appeared. ... 

Figure 5.29 A schematic energy diagram of relaxation processes of the O— -M LMCT excitation energy in polyoxometaloeuropates lattices [80b]. (Reprinted with permission from T. Yamase, Photo- and electrochromism of polyoxometalates and related materials, Chemical Reviews, 98, no. 1, 307-326 (Figure 12), 1998. 1998 American Chemical Society.)...

Since the application of POMs is so widespread across areas such as homogeneous and heterogeneous catalysis, as well as acid and redox catalysis, it is not possible to exhaustively review all the applications. Thus the rest of this chapter focuses only on the catalytic properties of the polyoxometallates in heterogeneous gas- or liquid-phase oxidation reactions, and reviews the most recent progress in the knowledge of their properties and working process, underlining both their potential and their limitations. 

A general introduction to the chemistry of POMs [1] and more recent reviews [2,3,7] illustrate the broad range and potential scope of this field. The present article concentrates on the structural chemistry of polyoxometalates which incorporate actinide (An) elements as heteroatoms. A review of actinide... 

The present review focuses first on actinide complexes of polymolybdates (section 2) and pol5durrgstates (section 3) that have beetr isolated and structurally characterized. There are currently no reported actinide polyvanadates, -niobates, or -tantalates. A final section (4) summarises solution studies of equilibria between actinide cations and polyoxometalate anions. 

By far the largest number of actinide POMs are polytungstates, and exploration of this field has been closely linked and often inspired by the development of lanthanide polyoxometalate chemistry. Before plunging into this area it is appropriate to review the main structural features that are often observed for actinide (and lanthanide) POMs. Such features have been termed lacunary... 

Since the publication of CCC (1987), various aspects of the chemistry of polyoxometalates have been reviewed or assembled in symposia proceedings see Table 1. Within the same time period, three volumes of Inorganic Syntheses have included sections on polyoxometalates (1985, 25,186-191 1990, 27, 71-135 1997, 31, 167-201). 

Polyoxometalates (POMs) are discrete molecular structures composed of metal cations bridged by oxide anions. They are not multi-metal species with metal-metal bonding, one conventional definition of clusters, but they are clusters in the generic sense of the word. A number of general reviews on POMs have appeared in recent years.1-13 Since a great majority of these structures are anionic, and thus salts with obligatory cations, and constituted primarily of early-transition-metal (d-block) elements in their highest oxidation states, the... 

Sadakane, M., and Steckhan, E. 1998. Electrochemical properties of polyoxometalates as electrocatalysis. Chemical Reviews 98, 219-237. 

The vast majority of homogeneous catalysts are transition metal complexes and many systems have been reported, for example, Ru(III) [129], W(VI) [130], polyoxometallates [131], Re(V) [132], Fe(III) [133], and Pt(II) [134] with hydrogen peroxide, Mn(II) [135-137] with peracetic acid, and Ti-tartrate with alkyl hydroperoxides [75]. The subject of epoxidation by H2O2 has been reviewed [138-140]. 

Polyoxometallates (ROMs) have attracted interest because they are fully inorganic mimics of biological-type catalysts such as porphyrins or salen compounds. POMs are oxygen anion multimetallic clusters containing early transition metals such as V, Nb, Ta, W, and Mo [151-153]. Recent reviews are provided by Bregeault... 

---