Stability of polyoxometalates

Additional features of polyoxometalates impact some of the six limitations. First, polyoxometalates are oxidatively resistant as a great majority of them are composed of d° transition metal ions (most commonly W(VI), Mo(V), and/or V(V)) and oxide ions. Second, the unusually high stability of polyoxometalates coupled with the tunability of their catalytically relevant properties defines a considerable ability to avoid problems associated with intermediate oxidation states, p-oxo dimers, and product inhibition. Illustrations of two representative polyoxometalates, Wio0324 , an isopolyoxometalate (or isopolyanion) and (TM)XWii039 , a transition metal (TM) substituted heteropolyoxometalate (or heteropolyanion) in polyhedral notation are given in Figure 1. 

An alternative strategy for the functionalization of polyoxometalates relies on self-assembly processes. Up to now we failed to introduced a Schrock-type alkylidyne in the lacunar undodecaphosphotungstate. As the current trend in olefin metathesis reaction now favours ruthenium catalysts, such as the Grubbs s ones, we turn to ruthenium precursors and to more stable Lappert-type carbene fragments, stabilized in the a-position by nitrogen atoms. The reaction of [PWii039p- with the carbene precursor... 

C. HiU, M. S. Weeks, R. F. Schinzai Anti-HIV-1 Activity, Toxicity, and Stability Studies of Representative Stmctural Families of Polyoxometalates J. Med. Chem. 33, 2767 (1990). 

An example of a system for photocatalytic water oxidation with a molecular catalyst based on abundant metals is shown in Fig. Id. The Cobalt polyoxometalate catalyst is oxidized with Ru —trisbipyridine that is generated by quenching of the photo-excited Ru —trisbipyridine sensitizer with peroxodisulfate as sacrificial electron acceptor. The system operates at pH 8 and exhibits a high (30 %) photon-to-02 yield while the stability of the catalyst allowed for turnover numbers >220 that were limited by depletion of electron acceptor only [7]. This performance of the abundant-metal-based catalyst is superior to that of an analogue ruthenium polyoxometalate water oxidation catalyst. 

Systems based on Keggin-type polyoxometalates (POMs) (phosphomolybdic acid derivatives) have been the most effective and widely used catalysts in gas-phase isobutane selective oxidation research. This Keggin POM catalyst application was hrst claimed in 1981 by Rohm and Haas, and after several catalyst mod-ihcations has been patented by other companies such as Asahi Kasei, Sumitomo, Mitshubishi, etc. However, in addition to a Keggin phosphomolybdate, vanadium must also be present in the anion structure in order to obtain a catalyst effective in the selective oxidation of isobutane. Moreover, the addition of - -3 metal cations to the phosphomolybdate anion increases the thermal stability of the catalyst, significantly minimizing its deactivation. " ... 

We have also smdied how the stability of the isomer can be modulated by the presence of different heteroatoms. As a first step, we have evaluated the relative stability of the isomers considering only the [Wig054] cage without the internal anions [(X04)2] . As shown in Table 10.4, there are two groups with different stability order isomers a, y, P and around 3 kcal mol less stable we have isomers P, a and y. The relative stability of the polyoxometalate isomers is significantly altered after encapsulating the two [XO4] anions. 

Abstract The catalytic oxidation is an area of the key technologies for converting petroleum-based feedstocks to useful chemicals such as diols, epoxides, alcohols, and carbonyl compounds. Many efficient homogeneous and heterogeneous oxidation systems based on polyoxometalates (POMs) with green oxidants such as H2O2 and O2 have been developed. This chapter summarizes the remarkable oxidation catalyses by POMs with multimetallic active sites. The multifunctionality of multimetallic active sites in POMs such as cooperative activation of oxidants, simultaneous activation of oxidants and substrates, stabilization of reaction intermediates, and multielectron transfer leads to their remarkable activities and selec-tivities in comparison with the conventional monometallic complexes. Finally, the future opportunities for the development of shape- and stereoselective oxidation by POM-based catalysts are described. 

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