Lacunary structures

Figure 11 Lacunary structures, [XMll039]("+4) and [XM,034]("+6), derived from a- and anions by...

FIGURE 14 Some multivacant lacunary structures derived from Keggin and Dawson anions, (a) A-a-[PW9 034]9- (b) B-a-[PW9034]9-, (c) a-[P2Wi5 0 56]12- (d) y-[SiW10O36]8-. 

The purpose of selecting XMou-SiOj as the inorganic precursor is that they are chemically active toward siloxanes. At first the hybrid XMo,-Si02 sols were prepared via hydrolysis of TEOS in the presence of XMo,under acidic conditions. A little amount of nitric acid was added to the solution in order to increase the solubility of XMo, and still remained the mono lacunary structures. Second, the sols of the hybrid materials were introduced into the spaces between the templating PS sphere arrays (with suction applied) subsequently, a solid hybrid material skeleton was constructed via in situ sol-gel transformation. The PS templates were removed by extraction with toluene solution. 

The preparation of the lacunary sodium nonatungstosilicate is dependent on adding an acid solution to the mixture of tungstate and silicate ions in a manner such that the aggregation process favors formation of the lacunary structure. Although the preparation described here (based on the previously reported synthesis) produces impure material (by elemental analysis), this... 

Fig. 16.18 Examples of the polyoxomoialatc -[XMi20j " ion losing both one melal frugmenl (XMhOv,) and Ihree melal fragments (isomers A-XMvO,i.i and B-XM.jOij), The smicuircs formed (called lacunary structures) have vacant sites which can be filled by other metal fragments. (From Pope. (VI. T. Muller. A. Cliem. Ini. Ed. Engl. 1991. 30,...

This lacunary structure is anticipated to be metastable on the basis of the so-called Lipscomb criterion [W.N.Lipscomb, Paratungstate ion, Inorg.Chem. 4 132 (1965) ]... 

Figure 11 Lacunary structures, and, derived from ce- and /J-p M,20jo]" anions by...

Heteropolyanion structures in which the local site symmetry of each metal atom is approximately C41, (i.e. each metal has one terminal oxygen) frequently undergo partial hydrolysis to lose an group and generate a so-called lacunary structure, see for example... 

The ability to generate structural defects in reaction intermediates (e.g., leading to lacunary structures) for example by removing building blocks from (large) intermediates due to the presence of appropriate reactants. 

Coming to the comparison of the and 0-2 series, the possibilities of isomerization and/or decomposition of the precursor lacunary complexes and, presumably, of their metal ion-substituted derivatives, make it necessary to insure first that no fast conversion occurs, in particular from the a structure to the 2 one. Figure 13(a) compares the cyclic voltammograms of the two lacunary complexes in the pH 3 medium. The main difference appears on the third redox system a single two-electron, reversible wave is obtained for the complex in contrast, the corresponding system for the U2 isomer is clearly constituted by two. 

The synthesis and characterization of isolated, isomerically pure a2-[P2W17061(Mn+ L)]y- (Mn+ = Mn3 +, Fe3+, Co2+, Ni2+, and Cu2+) as aqueous-soluble potassium salts (L = HzO) and as the more novel, organic-solvent-soluble tetrabutylammonium salts (L = Br) are described herein. These preparations yield mono-metal-substituted complexes of the Dawson 2 isomer, 2-[P2Wi7O6i]10, that are typically >98% free of the (Xi isomer.1,2 The X-ray crystallographic structures of the a2-[P2W17O61]10 lacunary precursor and the a2-[P2W17061(Co" OH2)]8- complex are known.3... 

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. 

The anion has almost D%h symmetry with two A-a-PWgC units (see Figure 11) that sandwich two W05(H20) octahedra and a WOs square pyramid, i.e. the equatorial plane contains two radial H20—W=0 groups and one W=0 group. The structure of [F W O ]14- is currently unknown, but it appears to be a lacunary derivative of P2W21 for it reacts with several divalent metal ions to give P2W19M2 species (see Section 38.4.2.2)., ... 

Several lacunary derivatives of the Dawson structure can be synthesized by controlled alkalinization of aqueous solutions, as shown in Scheme 4. 

Within the last few years there has been considerable activity directed towards the synthesis of organic and organometallic derivatives of heteropolyanions. Reactions of lacunary polyanions with RMC13, where R is an organic or organometallic radical, provide a simple route to such derivatives.97 Some examples of these reactions are presented in Scheme 5. For the most part the reactions proceed smoothly and the products are remarkably stable towards hydrolysis. Apart from their structural characterization via crystallography and NMR spectroscopy the chemistry of these complexes has not been completely described. 

The introduction of organic and organometallic moieties into lacunary polyanion structures has been described in Sections 38.4.2.1 and 38.4.2.2. Other organic derivatives of polyanions are discussed in the following sections. 

In the case of PW 2O40, the lacunary PWn03 is formed at pH = 2. The degradation of PW 039 to give PW9034 occurs at pH > 8 (2). These lacunary or defect derivatives of the Keggin structure are illustrated in Fig. 3. 

Lacunary spedes can act as structural motifs with numerous metal cations, leading to mono-, di- and trinuclear transition-metal-substituted POMs according to the number of vacant sites. The metal incorporation involves the reaction of aquated first-row and occasionally second-row d-block metal ions, M(H20)6z +, with a lacunary POM derived from the Keggin, Wells-Dawson and other POM families. In addition, lacunary spedes assemble into large POM structures, either directly or with incorporation of metal ion linkers. 

---