Properties of Polyoxometalates

Early transition metal oxygen anion clusters, or polyoxometalates for short, are a large and rapidly growing class of inorganic complexes [1-15]. These compounds attracted us as it was apparent they simultaneously exhibited a unique set of properties we felt could be utilized to address new catalytic transformations, photoredox chemistry, and, ultimately construction of sophisticated single-molecule multifunctional devices capable of several temporally linked functions. These prognostications have already been borne out to a considerable extent. This article reviews the title subject, an enterprise still in its infancy. Presented sequentially in this chapter are the basic properties of polyoxometalates, general features of the photochemistry, the mechanisms elucidated in polyoxometalate photochemistry thus far, and an overview of the photochemistry of two representative complexes in tabular form (Table I). 

Photosensitization and Photocatalysis Using Inorganic and Organometallic Compounds, 307-330. 

Wio032 (right), are shown in polyhedral notation. In polyhedral notation, each component octahedra represents one WO0 unit with each vertex being an oxygen nucleus. The heteroatom in the first structure resides in a central internal cavity of Td symmetry in the molecule. 

Reports of the photosensitivity of polyoxometalates date to at least 1916 [24]. Early work on the photochromism and photochemistry of alkylammonium polymolybdates by Yamase and co-workers [25-30] and the photooxidation of alcohols by heteropoly acids by Papaconstantinou and coworkers [31] gave some indication of the potential of such chemistry. Papaconstantinou reported that alcohols and carboxylic acids are photooxidized under aerobic conditions by heteropoly acids in water with clean transformation of the primary and secondary alcohols to the corresponding carbonyl compounds, equation (1) [32]. The products were not identified for the other substrates. The rate law was reported for a couple 

SubH2 = hydrocarbons, alcohols, alkyl halides, carboxylic acids, ketones, ethers, nitriles, thioethers 

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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. 

Hill, C. L. Prosser-McCartha, C. M. Photocatalytic and Photoredox Properties of Polyoxometalate Systems. In Photosensitization and Photocatalysis Using Inorganic and Organometallic Compounds, Kalyanasundaram, K., Gratzel, M., Eds. Kluwer Academic Publishers Dordrecht, 1993, pp 307-330. 

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

Renneke, R., Kadkhodayan, M., Pasquah, M., etal. (1991). Roles of Surface Protonation on the Photodynamic, Catalytic, and Other Properties of Polyoxometalates Probed by the Photochemical Functionalization of Alkanes. Implications for Irradiated Semiconductor Metal Oxides, J. Am. Chem. Soc., 113, pp. 8357-8367. 

Hill, C. and Prosser-McCartha, C. (1993). Photocatalytic and Photoredox Properties of Polyoxometalates Systems, in M. Gratzel and K. Kalyanasundaram (eds). Photosensitization and Photocatalysis Using Inorganic andOrganometallic Compounds, Kluwer Academic Publishers, Dordrecht, pp. 307-330. 

Some of the other properties of polyoxometalates that make them of interest are (i) accessibility, (ii) low cost, (iii) low toxicity, and (iv) high versatility. Many polyoxometalates, including the complexes in Figure 1, are readily prepared in quantity from inexpensive and commercially available... 

Lopez X, Bo C, Poblet IM (2002) Electronic properties of polyoxometalates electron and proton affinity of mixed-Addenda Keggin and Wells-Dawson anions. J Am Chem Soc 124 12574-12582... 

Poblet JM, Lopez X, Bo C (2003) Ab iiritio and DPT modelling of complex materials towards the understanding of electronic and magnetic properties of polyoxometalates. Chem Soc Rev 32 297-308... 

Hiskia A, Mylraias A, Papaconstantinou E (2001) Comparison of the photoredox properties of polyoxometalates and semictuiducting particles. Chem Soc Rev 30 62-69... 

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