Polyoxometalates (POMs) are large inorganic anions formed via condensation-addition reactions of simple oxoanions of early transition elements such as V, Mo and W.
 
        e.g.               7 MoO₄^2- + 8 H⁺   =     [Mo₇O₂₄]^6- + 4 H₂O                                an isopolymolybdate

Generally, such polyanions have symmetric close-packed structures, they have high thermal and hydrolytic stabilities (over a characteristic range of pH) and are formed rapidly in solution by "self-assembly" processes.
 
 
 

 1.     Selective separation and binding of Ln³⁺ and An⁴⁺
 

The tungstophosphate anion [Na(H₂O)P₅W₃₀O₁₁₀]^14- is a doughnut-shaped anion of C_5v symmetry, with a central cylindrical cavity which encloses a sodium cation and axially coordinated water molecule, see Figure 1.
 

                    Figure 1(a)                                                                 Figure 1(b)
 

Figure 1(a). Polyhedral Representation of [Na(H₂O)P₅W₃₀O₁₁₀]^14- viewed approximately down the molecular C₅ axis
 
Figure 1(b). Ball-and-stick view of  [M(H₂O)P₅W₃₀O₁₁₀]^n- perpendicular to the C₅ axis showing position of the internal cation (green) and its associated water molecule (blue). Pink circles are the phosphorus atoms
 

The anion is stable in aqueous solution between pH < 0 (6 M HCl) and pH 10.
 

Under vigorous hydrothermal conditions (e.g. 170^oC, 24h) the central cation can be replaced with cations of similar size i.e. Ca²⁺, Y³⁺ and most Ln³⁺, Th⁴⁺ and U⁴⁺.
 
                       Eu³⁺ + [Na(H₂O)P₅W₃₀O₁₁₀]^14-  -------->  [Eu(H₂O)P₅W₃₀O₁₁₀]^12- + Na⁺

Cations that are significantly smaller or larger than Na⁺ (e.g. Al³⁺, transition metals) are excluded.
 
 

The following results (Figure 2) show that the replacement reactions are kinetically controlled and that the more highly charged cations compete effectively with excess Na⁺, a major component of high level waste solutions.

Reaction:     M³⁺  +  [Na(H₂O)P₅W₃₀O₁₁₀]^14- ------->  [M(H₂O)P₅W₃₀O₁₁₀]^12-  +  Na⁺  at  170^oC
 
   Figure 2
                (a)  pH effect (t = 24 h;  M=Tm)
                (b)  [Na⁺]/[M] = 100, pH 4 (t = 24 h, M=Nd) 
                (c)  [Na⁺]/[M] = 1 -- 500, pH 4 (t = 24 h, M=Nd)
 

                                                                                                                        Ln^III = La, Ce, Nd, Sm

                                                                                                     (30% isolated yield as ammonium salt)
 
 
 

Polyhedral  representation of {W148} anion

 4. Complexation of Technetium by Niobates and Niobotungstates

Initial experiments have been carried out using rhenium as a nonradioactive surrogate for technetium. Although this investigation is still in an early stage, encouraging results have been obtained with the hexaniobate anion which stabilizes Re(IV) in highly alkaline solutions (pH 12 or greater). The complexes are believed to be analogous to those of Mn(IV) and Ni(IV) that we had characterized several years ago. Isolation of these and analogous complexes with niobotungstates is in progress, and conversion of these salts to inert mixed oxide materials as possible waste forms is planned. Parallel experiments with the corresponding technetium species will be carried out collaboratively at Oak Ridge National Laboratory.

References

"Heteropoly and Isopoly Oxometalates",  Pope, M.T. ; Springer-Verlag, New York, 1983. (pp. 1-180).
"Isopolyanions and Heteropolyanions",  Pope, M.T. in Comprehensive Coordination Chemistry, Wilkinson,      G.; Gillard, R.D.;  McCleverty, J.A. eds., Volume 3, pp. 1023-1058, Pergamon Press, 1987.
"Polyoxometalates.  From Platonic Solids to Anti-Retroviral Activity", Pope, M.T.; Müller, A., eds. Kluwer Academic Publishers, Dordrecht, The Netherlands, 1994, pp 1-411
"Polyoxoanions", Pope, M.T. in Encyclopedia of Inorganic Chemistry, King R.B. ed., John Wiley &Sons, Chichester, England, 1994, 3361-71
“Polyoxometalates: Very Large Structures - Nanoscale Magnets”, Müller, A.; Peters, F.; Pope, M.T.; Gatteschi, D., Chem.Rev. 1998, 98, 239-271
 

"A Heteropolyanion with Fivefold Molecular Symmetry that Contains a Nonlabile Encapsulated Sodium Ion.  The Structure and Chemistry of [NaP₁₅W₃₀O₁₁₀]^14-"  Alizadeh, M.H.; Harmalker, S.P.; Jeannin, Y.; Martin-Frere, J.; Pope, M.T. J. Am. Chem. Soc. 1985, 107, 2662-2669.
"Rigid Non-Labile Polyoxometalate Cryptates [ZP₅W₃₀O₁₁₀]^(15-n)- that Exhibit Unprecedented Selectivity for Certain Lanthanide and Other Multivalent Cations" Creaser, I.; Heckel, M.; Neitz, R.J.; Pope M.T., Inorg. Chem., 1993, 32, 1573-1578.
"The Structures of Europium(III)- and Uranium(IV) Derivatives of  [P₅W₃₀O₁₁₀]^15-. Evidence for Cryptohydration", Dickman, M.H.; Gama, G.J.; Kim, K.-C.; Pope, M.T.,  J.Cluster Sci., 1996, 7, 567-583
 

"Self-Assembly of Supramolecular Polyoxometalates. The Compact, Water-Soluble Heteropolytungstate Anion [As^III₁₂Ce^III₁₆(H₂O)₃₆W₁₄₈O₅₂₄]^76-, Wassermann, K.; Dickman, M.H.; Pope, M.T., Angew.Chem., 1997, 109, 1513-1516; Angew.Chem.Internat.Ed.Engl.,1997, 36, 1445-1448
 "Formation of Cerium(III)- and Uranium(IV)-Tungsten Bronzes Through Thermal Degradation of Polyoxometalates", Wassermann, K.; Pope, M.T.; Salmen, M.; Lunk, H.J. Abstracts of Papers presented at the Annual Meeting of the German Chemical Society (Gesellschaft Deutscher Chemiker), Saarbrücken, Germany, 9/23-25/98

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