Other Heteropoly Anions

Mild acidification of molybdate-phosphate solutions yielded the colorless [P2Mos023] 6 anion, isolated as the sodium salt48,49 The heteropoly acid containing pentavalent antimony, H3[SbMo12O40] 48 H20, was reported to have been prepared by refluxing M/20 molybdic acid with M/75 potassium pyroantimonates°), however, more work is necessary to elucidate to exact nature of molybdenum heteropoly compounds containing antimony. 

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At present count, some 68 elements (other than Mo and W) have been observed as heteroatoms in polyanions (see Table 5). Since each element may form more than one heteropoly anion (e.g. at least 18 tungstophosphates) and there are polyanions with combinations of several heteroatoms, the total number of known heteropolyanions is extremely large. It is therefore out of the question to attempt a complete accounting of every type of heteropolyanion on the following pages. We shall restrict discussion to the main structural types. More complete reviews are available.1-3... 

Between 35 and 40 heteroatoms are known to form heteropoly anions and their corresponding acids. Large heteroaloms such as Ceoctahedra share faces to form MojGg groups which are corner connected to each other.39... 

Heteropolyanions and isopolyanions are polymeric oxoanions (polyoxometalates) (2, 3, 5, 6). The structure of a heteropolyanion or polyoxoanion molecule itself is called a primary structure (5, 6, 77). There are various kinds of polyoxoanion structure (Section II.A. 1). In solution, heteropoly anions are present in the unit of the primary structure, being coordinated with solvent molecules and/ or protonated. Most heteropolyanions tend to hydrolyze readily at high pH (Section 1I.C). Protonation and hydrolysis of the primary structure may be major structural concerns in solution catalysis. Heteropoly compounds in the solid state are ionic crystals (sometimes amorphous) consisting of large polyanions, cations, water of crystallization, and other molecules. This three-dimensional arrangement is called the secondary structure. For understanding catalysis by solid heteropoly compounds, it is important to distinguish between the primary structure and the secondary structure (5, 6, 17). Recently, it has been realized that, in addition... 

Throughout specific ranges of pH and other conditions, most solutions of heteropolymolybdates and heteropolytungstates appear to contain predominantly one distinct species of anion. It is generally reasonable to assume that this predominant species is identical with the anion existing in the solid state, in equilibrium with the solution, or is closely related to it some heteropoly anions are remarkably stable. 

Molybdenum is of intermediate stability compared with heteropoly anions of other elements. With 12-heteropolytungstates and 12-heteropolyvanadates, the stabilities follow the order W > Mo > V. 

Other studies on heteropoly anions include electron paramagnetic resonance on the reduced forms of heteropoly compounds196, 198 and thermal and in situ X-ray studies of some heteropoly compounds200. No thermodynamic data are available on heteropoly compounds. 

In addition to the traditional iso- and heteropoly anions just discussed, recent work has revealed that others, many of them very large, with crown-type structures can be obtained. Examples of these are the [NaPsW Ono]14- ion, which has a double crown structure with five-fold symmetry, [H7P8W4gOi84]33 , which has a double crown structure with four-fold symmetry and two structurally similar crown species of molybdenum, [Mo36On2]8 , and Mo34Oiio(NO)4(H20)14. ... 

Heteropoly anion polyoxoanion with at least one kind of positive-valent element (other than hydrogen) in addition to the addenda... 

Association of molybdates (tungstates) with phosphate-like structures leads to a class of compounds called heteropoly compounds or heteropoly acids of Mo or W, hereafter schematically written as Mo —P or W—P heteropoly compounds. The heteropoly anions, which may contain Mo, W, P, and other elements, are paired, both in the solid state and in solution, with cations such as H+, NH4, and Na+. Here, only some Mo —P heteropoly compounds playing an important role in hydrotreating catalysts during the catalyst preparation (impregnation) or as supported oxidic phases are discussed. The Mo — P heteropoly compounds have the following properties 18,19) ... 

In crystals, the relatively large heteropoly anions cause interstices between each other which are occupied by cations or by water molecules. Therefore, there exist no strong bondings between the individual heteropoly tungstate ions. Instead, they are joined by hydrogen bondings through water of hydration molecules. 

This chapter will present recent progress made in reaction-controlled phase-transfer catalysis for the epoxidation of olefins, focusing on work with hetero-pol)q5hosphotungstates and quaternary ammonium ions from our group. We have systemically investigated the influence of composition of the heteropoly anion and various quaternary ammonium ions on the catalyst activity. The epoxidation of propylene, allyl chloride, and others olefins and the stability of the catalyst in recycle will be summarized and discussed in detail. 

At this time we do not attempt to assign all of the P NMR peaks observed at low water concentration. Most likely the peaks that disappear first as water is added (at 175 and 440 ppm) originate from the PWnCo ion with the empty sixth coordination site on cobalt. One of these species could have K ion paired to the heteropoly anion. We continue to investigate this phenomenon for PWnCo and other heteropoly mngstates. 

The structures of both the heptamolybdate and octamolybdate ions in crystals are discussed on page 954 in relation to other heteropoly and isopoly anion structures. 

Spectra of Reduced Anions. IR spectra of the PV2W10O40" anions are very similar to those reported for other heteropoly tungstates with the Keggin structure. Of particular interest however is the T2 mode of the central PO4 group which occurs as an intense sharp absorption at 1080... 

Selinites, on the other hand, have attracted recent interest compounds MMn(Se03)2- H20 (M = H, NH4, K, Rb, Cs) have been described and X-ray analyses of Mn2(Se03)3-3H20 and MnH(SeOj)(Se2 05) reveal distorted [MnO ] octahedra. lodates of Mn are perhaps represented in the yellow to brown-yellow solids M2Mn(I03)5-There is also a range of isopoly- and heteropoly-anion compounds which we have summarized in Table 26. 

Amorphous zirconium phosphate a-Zr(HP04)2 (Chapter 5.7) is an excellent sorbant for use in renal dialysis [6], Radioactive Cr P04 is a neoplastic suppressant and is much used in cancer treatment. The heteropoly anion PaWigOg is a potent inhibitor of viral DNA but other more complex anions of this type may prove to be of greater use (Chapter 5.7). Radioactive P has uses in medicine (Chapter 13.5). 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

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