Heptamolybdate anions

The heptamolybdate anion and its protonated forms, [H Mo7024](6-"), n = 1-3, predominate in solutions of pH3-ca. 5.5. The structure of the anion, Figure 6, has been determined in sodium, potassium, ammonium and isopropylammonium salts, and has been confirmed in solution by low angle X-ray scattering.34 The anion structure can be viewed as ... 

Several examples of anions with the stoichiometry Ln Mo = 4 29 have been reported. The structures reveal an assembly of four heptamolybdate anions bridged to a central Mo042-group by the lanthanide cations, i.e., [Ln4(H20)16(Mo04)(Mo7024)4]14- (Figure 27).371-374 It is unclear whether this assembly survives in solution. 

The monomeric tetrahedral Mo04 species is favored at high pH and vice versa for the polymeric heptamolybdate anion. 

From a catalytic point of view 0.5 is the optimum value [6] whereas the former one (0.17) is the ratio corresponding to the stoechiometry of the Anderson molybdocobaltate or Anderson molybdonickelate [XMoeHeOaJ heteropolyanion. Figure 1 shows the Raman spectra of these CoMo and NiMo solutions before precipitation. They exhibit the characteristic lines of the heptamolybdate anions in solution with the main lines at 942, 895 and 364 cm the line at 1050 cm" corresponding to nitrate anions. Whereas the Mo and CoMo solutions are relatively stable without any precipitation before several hours, a precipitation is rapidly observed for the NiMo based solutions, whatever the metal loading and the Ni/Mo ratio. 

Figure 39 Structures of (a) the heptamolybdate anion and (b) the dimolybdate anion.

A powder X-ray diffraction pattern of this material, collected on a Rigaku diffractometer operated at 30 kV and 40 mA with CuKa radiation, is shown in Figure 3. This diffraction pattern is significantly different from ZnO, and bares no resemblance to that expected from a LDH pillared with the heptamolybdate anion (20,27). This diffraction pattern is almost identical to that of the Zn-LTM phase (22). A least squares refinement of the peak positions gave a hexagonal unit cell ofa = 6.104(9) A, and c = 21.69(9) A. The crystal structure of the Zn/Cu-LTM is a derivative of the Zn-LTM, whose structure solution is reported elsewhere (22). The structure of the Zn-LTM was solved by Rietveld refinement of an isostructural ammonium nickel molybdate, (NH4)HNi2(OH)2(Mo04)2, prepared by precipitation, whose own structure was determined, ab initio from powder synchrotron data (25). 

Examples Ca3Mo7024, tricalcium 24-oxoheptamolybdate, may be shortened to tricalcium hep-tamolybdate the anion, Mo7C> (, is heptamolybdate(6—) S20 -, disulfate(2—) P20 , diphos-phate(V)(4—). 

Ammonium ion forms isopolymolybdates, such as di-, tri-, or heptamolyb-dates with the molybdate anion. Only the dimolybdate, (NH4)2Mo20 , and ammonium heptamolybdate (NH4)6Mo7024 4H2O [12027—67—7], have commercial apphcations. 

Figure 8 The structure of [Mo O IijO) ]8 anion. A heptamolybdate unit in one of the two Molg moieties is emphasized. Coordinated water molecules are indicated by open circles...

In the various preparations, ammonium heptamolybdate, cobalt nitrate, or nickel nitrate are preferred. The preferred phosphorus-containing precursors are phosphorus pentoxide, phosphoric acid, or their anion derivatives such as ammonium dihydrogen phosphate (NH4H2PO4) be-... 

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. 

Recently, it has been shown that hydrotalcite-type clays interlayered by polyoxometalate anions lead to a new class of pillared materials for selective adsorption and catalysis. The heptamolybdate- and decavanade-pillared hydrotalcite-type clays were synthesized by Drezdzon (1988). An organic-anion-pillared clay precursor was prepared and subsequently exchanged with the appropriate isopolymetalate under mildly acidic conditions (pH = 4.T-4.9). These pillared intercalates were found difficult to obtain in highly crystalline form. This is ascribed to the fact that the Mg and Al hydroxide layers have a basic nature, whereas Mo7024 and anions are stable in acidic conditions. 

As a consequence it can be proposed that, at low concentrations, monomeric molybdenum and phosphate are adsorbed and that, as the concentration increases, pho homolybdate is also adsorbed. Moreover, the presence of heptamolybdate cannot be set aside, since it can occurs as a consequence of the PjMojO j anion adsorption and according to a mechanism previously proposed by Van Veen et al. [6] ... 

The influence of cross-linking and bead size on the resolution of mannitol/sorbitol and arabinitol/xylitol mixtures on seven Ca -form cation exchange resins has been studied. Alditols with at least four vicinal hydroxy-groups are retained more strongly than aldoses and ketoses when applied to anion exchange resins as their complexes with hexaammonium heptamolybdate. While the free sugars can be eluted with water, the alditols require 0.1 M aq. ammonia. Oxalic, citric and a-hydroxycarboxylic acids afford stable complexes with molybdate ions, so that they need to be removed from mixtures, if the separation of aldoses and alditols is to be achieved." ... 

Two more sophisticated applications of the molybdate catalyst are worthy of mention. An anion-exchange resin immobiUzed heptamolybdate has provided an effective catalysis while the same resin activated with mononuclear sodium molybdate was completely ineffective [23]. In contrast, a patent apphcation claims a successful activation with the same sodium molybdate for an effective epimerization of D-glucose to o-mannose [28]. Therefore, attention has to be paid when performing such appUcations as the equilibria between the (poly)molybdate species in solution are very complex and extremely pH-sensitive [29]. 

In the case of ferricenium complexes which are characterized by the presence of only one ionic acido group, tumor growth inhibition was observed with the water-soluble derivatives XLIX-LV containing the quite different anions tetrachloroferrate(III), p-oxo-bis[trichloroferrate(III)], picrate and trichloroacetate. When, however, anions such as heptamolybdate [H5M7O24] are introduced which impair the water solubility of fer-... 

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