Lacunary POMs

Various catalytic systems for H202- and 02-based oxidations catalyzed by POMs have been developed. Typical examples are listed in Table 13.1. The systems can be classified into four groups according to the stmctures of POMs (1) mixed-addenda POMs, (2) transition-metal-substituted POMs, (3) POMs, and (4) lacunary POMs. In this chapter, liquid-phase homogeneous oxidations by POMs with H202 and 02 are described according to the above classification. 

The mono-ruthenium-substituted silicotungstate, [SiWn039Ru3+(H20)]5, synthesized by reaction of the lacunary POM Si Vn039 x with Ru3+ in acetone, was an efficient catalyst for the... 

The lacunary POMs can be obtained by the removal of one or more addenda atoms from fully occupied POMs. The most stable lacunary compounds are obtained with Si4 + as the heteroatom. While the dodecatungstosilicates are stable in acidic solution, hydrolytic cleavages ofW—O bonds occur and well-defined lacunary POMs with eleven, ten and nine tungsten atoms are produced when the pH increases. 

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. 

Inorganic-organic hybrid POMs have attracted much attention because the electron and steric properties of the POMs significantly affect the catalytic properties of a metal center [88-91]. One hybrid catalyst consisted of a metallo-salen complex attached covalently to a lacunary POM [SiWn039]8- via two propyl spacers. In contrast with an usual Mn(III) salen species, the metallosalen complex modified with the POM afforded a more oxidized Mn(IV) salen-POM or Mn(III) salen cation radical-POM compound [89]. 

Polyoxometalates have found applications in analytical and clinical chemistry, catalysis (including photocatalysis), biochemistry (electron transport inhibition), medicine (antitumoral, antiviral, and even anti-HIV activity), and solid-state device [19,20], Recently, three-dimensional ordered materials are modified by the incorporation of POM into wall structures using a direct synthesis approach [21,22], The motivation for the work lies in the novel properties that POM is introduced into silica networks [19,20], The incorporation is that the POM is linked to silica matrix by covalent grafting. A class of POM that are particularly attractive for the preparation of hybrid materials are the lacunary POM. Lacunary POMs include the mono and divacant unsaturated Keggin fragments [X" W,039]< ">- (X"+=B Ge P [22], [ Y -SiWioOgg] [21], Lacunary... 

Figure 4. SEM images of the macroporous hybrid silica materials based on lacunary POMs (A) SiMo SiOj (B) GeMo SiOj, (C) PMo SiOj.CD) PMo SiOj with high magnification. ...

For the preparation of RE-POM complexes, traditional aqueous solution synthesis and hydrothermal synthesis [2] are the two main synthetic methods. In traditional aqueous solution synthesis, reactions are carried out in the temperature range from room temperature to the boiling point of water, and in general start from plenary or lacunary POMs and simple RE salts. Hydrothermal reactions typically proceed in the temperature range 120-200 C under autogenous pressure, and usually use simple metal salts of all the required elements as the starting materials. As different solubility problems are minimized under hydrothermal conditions, the... 

Transition metals substituted polyoxometalates are obtained starting from vacant or lacunary POMs. The latter are derived from the saturated parent POM, through formal loss of one or more MOe octahedral units. Such complexes present several coordinatively unsaturated oxygen atoms, which surround the defect and form a polydentate site, able to coordinate a multitude of transition metals, M . Fundamental modes of coordination of the transition metals are in pocket and out of pocket , the latter including sandwich-like structures and metal oxide cluster stabilization, depending both on the dimensions of M and on the stoichiometic ratio between M and the vacant units. " ... 

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