Keggin heteropolyanions

This technique has been applied to the study of the reduction of the Keggin heteropolyanion [PM012O40]3- immobilized at the surface of a BDD electrode in aqueous media with two electrolytes HCIO4 1.0 M and LiC104 0.1 M. In the case of carbon electrodes like Glassy Carbon or Pyrolytic Graphite electrodes, in aqueous acidic media, three reversible waves are observed in the potential range (0.6, 0.2)... 

The number and strength of the acid centers of tungstic heteropolyacids have been determined by ammonia adsorption calorimetry. Ammonia is irreversibly absorbed, with the formation of the corresponding ammonium salts. An increase in the number of protons in Keggin heteropolyanions decreases the acidic strength. 

The role of transition metals as counter-cations in polyoxometaUates used as oxidation catalysts has been reviewed [75]. Transition metals have important and complex effects on textural, acid-base and redox properties of the heteropolyanions, as described in a number of studies. The interaction of the molybdophos-phoric Keggin heteropolyanion with the iron counter-ion has been studied and... 

This brief overview of the rich chemistry of the complexes of monolacunary Keggin heteropolyanions was aimed at collecting relevant information for those interested in further studies on these types of complexes and on their potential applications. To keep this review short, only the most common characterisation techniques used in the studies of these systems were described. Other potentially interesting techniques and properties that have recently been used in their study in our laboratory, such as fast atom bombardment mass spectrometry, were not considered. 

Other examples of Keggin heteropolyanion-supported transition metal complexes that are structurally characterized, include [Cu (phen)2]4PMo V ... 

Lactones in rhodium catalysis, 32 365-366 Lacunary Keggin anion, heteropolyanions, 41 ... 

The possibilities of POMs compositions and structures are virtually enormous, and the number of these molecules actually prepared and characterized continues to grow unabated. However, examination of their short electrochemical studies, which are becoming a usual part of their characterization, shows that a limited selection of representative groups of POMs is sufficient for a description of the main electrochemical behaviors of this class of chemicals. Therefore, a-Keggin- and Dawson-type heteropolyanions of phosphotungstate, silicotungstate, phosphomolybdate, and silicomolybdate mixed addenda heteropolyanions, and transition-metal substituted heteropolyanions including sandwich-type derivatives were selected for the present article. 

Several deficit or lacunary derivatives of the Keggin structure exist as individual species or as fragments of other heteropolyanion structures. Removal of one M06 octahedron (stoichiometric loss of MO"+) from a- or /3-Keggin anions leads to isomers of [XM11O39]"-. Removal of three adjacent octahedra leads to A- and B-type XM9034 anions. The structures of these are shown in Figure 11. 

Infrared (IR) spectroscopy is a convenient and widely used method for the characterization of heteropolyanions. Keggin, Dawson, and lacunary heteropolyanions can be distinguished by their characteristic bands. 

Phase-transfer catalysis has been developed by the combination of Keggin-type heteropolyanions and quaternary countercations such as tetrahexyl-ammonium or cetylpyridinium ion. The oxidations of alcohols (306), allyl alcohols (307), olefins (308), alkynes (309), /J-unsaturated acids (310), v/ c-diols (311), phenol (312), and amines (313) are the examples. 

Izumi et al. (369) found that Keggin-type heteropolyanions containing Mo or V show a promoting effect in the reductive carbonylation of nitrobenzene by PdCl2 to form methyl N-phenylcarbamate in the presence of methanol (Eq. 44) ... 

It has been pointed out that these pillared intercalates are intrinsically difficult to synthesize in highly crystalline form because the layered hosts are basic, whereas most heteropolyacids are acidic and tend to decompose. Narita et al. (392) tried direct synthesis of a heteropolyanion-pillared layered double hydroxide by a coprecipitation reaction of Zn2+ and A1J+ ions in the presence of a moderately acidic lacunary Keggin anion, a-SiWn039 XRD of the product showed a basal spacing of 14.6 A, which corresponds to a gallery height of 9.9 A. The surface area was found to be 97 m2 g, which is three times that of the layered double hydroxide. 

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