Photoreduction of 99Tc pertechnetate by nanometer-sized metal oxides: new strategies for formation and sequestration of low-valent technetium

Technetium-99 ((99)Tc) (β(-)(max): 293.7 keV; t(1/2): 2.1 × 10(5) years) is a byproduct of uranium-235 fission and comprises a large component of radioactive waste. Under aerobic conditions and in a neutral-basic environment, the pertechnetate anion ((99)TcO(4)(-)) is stable. (99)TcO(4)(-) is very soluble, migrates easily through the environment and does not sorb well onto mineral surfaces, soils, or sediments. This study moves forward a new strategy for the reduction of (99)TcO(4)(-) and the chemical incorporation of the reduced (99)Tc into a metal oxide material. This strategy employs a single material, a polyoxometalate (POM), α(2)-[P(2)W(17)O(61)](10-), that can be photoactivated in the presence of 2-propanol to transfer electrons to (99)TcO(4)(-) and incorporate the reduced (99)Tc covalently into the α(2)-framework to form the (99)Tc(V)O species, (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-). This occurs via the formation of an intermediate species that slowly converts to (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-). Extended X-ray absorption fine structure and X-ray absorption near-edge spectroscopy analysis suggests that the intermediate consists of a (99)Tc(IV) α(2)- species where the (99)Tc is likely bound to two of the four W-O oxygen atoms in the α(2)-[P(2)W(17)O(61)](10-) defect. This intermediate then oxidizes and converts to the (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-) product. The reduction and incorporation of (99)TcO(4)(-) was accomplished in a "one pot" reaction using both sunlight and UV irradiation and monitored as a function of time using multinuclear nuclear magnetic resonance and radio thin-layer chromatography. The process was further probed by the "step-wise" generation of reduced α(2)-P(2)W(17)O(61)(12-) through bulk electrolysis followed by the addition of (99)TcO(4)(-). The reduction and incorporation of ReO(4)(-), as a nonradioactive surrogate for (99)Tc, does not proceed through the intermediate species, and Re(V)O is incorporated quickly into the α(2)-[P(2)W(17)O(61)](10-) defect. These observations are consistent with the periodic trends of (99)Tc and Re. Specifically, (99)Tc is more easily reduced compared to Re. In addition to serving as models for metal oxides, POMs may also provide a suitable platform to study the molecular level dynamics and the mechanisms of the reduction and incorporation of (99)Tc into a material.     

(99)Tc and re incorporated into metal oxide polyoxometalates: oxidation state stability elucidated by electrochemistry and theory

The radioactive element technetium-99 ((99)Tc, half-life = 2.1 × 10(5) years, β(-) of 253 keV), is a major byproduct of (235)U fission in the nuclear fuel cycle. (99)Tc is also found in radioactive waste tanks and in the environment at National Lab sites and fuel reprocessing centers. Separation and storage of the long-lived (99)Tc in an appropriate and stable waste-form is an important issue that needs to be addressed. Considering metal oxide solid-state materials as potential storage matrixes for Tc, we are examining the redox speciation of Tc on the molecular level using polyoxometalates (POMs) as models. In this study we investigate the electrochemistry of Tc complexes of the monovacant Wells-Dawson isomers, α(1)-P(2)W(17)O(61)(10-) (α1) and α(2)-P(2)W(17)O(61)(10-) (α2) to identify features of metal oxide materials that can stabilize the immobile Tc(IV) oxidation state accessed from the synthesized Tc(V)O species and to interrogate other possible oxidation states available to Tc within these materials. The experimental results are consistent with density functional theory (DFT) calculations. Electrochemistry of K(7-n)H(n)[Tc(V)O(α(1)-P(2)W(17)O(61))] (Tc(V)O-α1), K(7-n)H(n)[Tc(V)O(α(2)-P(2)W(17)O(61))] (Tc(V)O-α2) and their rhenium analogues as a function of pH show that the Tc-containing derivatives are always more readily reduced than their Re analogues. Both Tc and Re are reduced more readily in the lacunary α1 site as compared to the α2 site. The DFT calculations elucidate that the highest oxidation state attainable for Re is VII while, under the same electrochemistry conditions, the highest oxidation state for Tc is VI. The M(V)→ M(IV) reduction processes for Tc(V)O-α1 are not pH dependent or only slightly pH dependent suggesting that protonation does not accompany reduction of this species unlike the M(V)O-α2 (M = (99)Tc, Re) and Re(V)O-α1 where M(V/IV) reduction process must occur hand in hand with protonation of the terminal M═O to make the π*(M═O) orbitals accessible to the addition of electrons. This result is consistent with previous extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) data that reveal that the Tc(V) is "pulled" into the -α1 framework and that may facilitate the reduction of Tc(V)O-α1 and stabilize lower Tc oxidation states. This study highlights the inequivalency of the two sites, and their impact on the chemical properties of the Tc substituted in these positions.     

Synthesis and structure of dawson polyoxometalate-based, multifunctional, inorganic-organic hybrid compounds: organogermyl complexes with one terminal functional group and organosilyl analogues with two terminal functional groups

Four novel multifunctional polyoxometalate (POM)-based inorganic-organic hybrid compounds, [α(2)-P(2)W(17)O(61){(RGe)}](7-) (Ge-1, R(1) = HOOC(CH(2))(2(-)) and Ge-2, R(2) = H(2)C═CHCH(2(-))) and [α(2)-P(2)W(17)O(61){(RSi)(2)O}](6-) (Si-1, R(1) and Si-2, R(2)), were prepared by incorporating organic chains having terminal functional groups (carboxylic acid and allyl groups) into monolacunary site of Dawson polyoxoanion [α(2)-P(2)W(17)O(61)](10-). In these POMs, new modification of the terminal functional groups was attained by introducing organogermyl and organosilyl groups. Dimethylammonium salts of the organogermyl complexes, (Me(2)NH(2))(7)[α(2)-P(2)W(17)O(61)(R(1)Ge)]·H(2)O MeN-Ge-1 and (Me(2)NH(2))(7)[α(2)-P(2)W(17)O(61)(R(2)Ge)]·4H(2)O MeN-Ge-2, were obtained as analytically pure crystals, in 22.8% and 55.3% yields, respectively, by stoichiometric reactions of [α(2)-P(2)W(17)O(61)](10-) with separately prepared Cl(3)GeC(2)H(4)COOH in water, and H(2)C═CHCH(2)GeCl(3) in a solvent mixture of water/acetonitrile. Synthesis and X-ray structure analysis of the Dawson POM-based organogermyl complexes were first successful. Dimethylammonium salts of the corresponding organosilyl complexes, (Me(2)NH(2))(6)[α(2)-P(2)W(17)O(61){(R(1)Si)(2)O}]·4H(2)O MeN-Si-1 and (Me(2)NH(2))(6)[α(2)-P(2)W(17)O(61){(R(2)Si)(2)O}]·6H(2)O MeN-Si-2, were also obtained as analytically pure crystalline crystals, in 17.1% and 63.5% yields, respectively, by stoichiometric reactions of [α(2)-P(2)W(17)O(61)](10-) with NaOOC(CH(2))(2)Si(OH)(2)(ONa) and H(2)C═CHCH(2)Si(OEt)(3). These complexes were characterized by elemental analysis, thermogravimetric and differential thermal analyses (TG/DTA), FTIR, solid-state ((31)P) and solution ((31)P, (1)H, and (13)C) NMR, and X-ray crystallography.     

Peptide Bond Hydrolysis Catalyzed by the Wells-Dawson Zr(α(2)-P(2)W(17)O(61))(2) Polyoxometalate

In this paper we report the first example of peptide hydrolysis catalyzed by a polyoxometalate complex. A series of metal-substituted Wells-Dawson polyoxometalates were synthesized, and their hydrolytic activity toward the peptide bond in glycylglycine (GG) was examined. Among these, the Zr(IV)- and Hf(IV)-substituted ones were the most reactive. Detailed kinetic studies were performed with the Zr(IV)-substituted Wells-Dawson type polyoxometalate K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O which was shown to act as a catalyst for the hydrolysis of the peptide bond in GG. The speciation of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O which is highly dependent on the pD, concentration, and temperature of the solution, was fully determined with the help of (31)P NMR spectroscopy and its influence on the GG hydrolysis rate was examined. The highest reaction rate (k(obs) = 9.2 (±0.2) × 10(-5) min(-1)) was observed at pD 5.0 and 60 °C. A 10-fold excess of GG was hydrolyzed in the presence of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O proving the principles of catalysis. (13)C NMR data suggested the coordination of GG to the Zr(IV) center in K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O via its N-terminal amine group and amide carbonyl oxygen. These findings were confirmed by the inactivity of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O toward the N-blocked analogue acetamidoglycylglycinate and the inhibitory effect of oxalic, malic, and citric acid. Triglycine, tetraglycine, and pentaglycine were also fully hydrolyzed in the presence of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O yielding glycine as the final product of hydrolysis. K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O also exhibited hydrolytic activity toward a series of other dipeptides.     

Noble metals in polyoxometalate chemistry: palladium-containing derivatives of the monolacunary Keggin and Wells-Dawson tungstophosphates

We have prepared the three novel Pd(II)-containing tungstophosphates [Pd(2)(α-PW(11)O(39)H(0.5))(2)](9-) and two structural isomers of [Pd(2)(α(2)-P(2)W(17)O(61)H(n))(2)]((16-2n)-) via simple synthetic procedures and characterized their potassium salts by single-crystal X-ray diffraction, elemental analysis, and IR and multinuclear ((31)P and (183)W) NMR spectroscopy. This study sheds light on the long-standing question about the nature and structure of the actual products formed in the reaction of Pd(II) ions with monolacunary Keggin-type [α-XW(11)O(39)](n-) and Wells-Dawson-type [α(2)-P(2)W(17)O(61)](10-) heteropolytungstates.     

Coordination of rare-earth elements in complexes with monovacant Wells-Dawson polyoxoanions

The alpha-1 and alpha-2 isomers of the monovacant Wells-Dawson heteropolyoxoanion [P(2)W(17)O(61)](10-) are complexants of trivalent rare-earth (RE) ions and serve to stabilize otherwise reactive tetravalent lanthanide (Ln) and actinide (An) ions in aqueous solution. Aspects of the bonding of Ln ions with alpha-1-[P(2)W(17)O(61)](10-) and alpha-2-[P(2)W(17)O(61)](10-) were investigated to address issues of complex formation and stability. We present structural insights about the Ln(III) coordination environment and hydration in two types of stoichiometric complexes, [Ln(alpha-1-P(2)W(17)O(61))](7-) and [Ln(alpha-2-X(2)W(17)O(61))(2)](17-) (for Ln identical with Sm, Eu, Lu; X identical with P, As). The crystal and molecular structures of [(H(2)O)(4)Lu(alpha-1-P(2)W(17)O(61))](7-) (1) and [Lu(alpha-2-P(2)W(17)O(61))(2)](17-) (2) were solved and refined through use of single-crystal X-ray diffraction. The crystallographic results are supported with corresponding insights from XAFS (X-ray absorption fine structure) for a series of nine solid-state complexes as well as from optical luminescence spectroscopy of the Eu(III) analogues in aqueous solution. All the Ln ions are eight-coordinate with oxygen atoms in a square antiprism arrangement. For the 1:1 stoichiometric Ln/alpha-1-[P(2)W(17)O(61)](10-) complexes, the Ln ions are bound to four O atoms of the lacunary polyoxometalate framework in addition to four O atoms from solvent (water) molecules as [(H(2)O)(4)Ln(alpha-1-P(2)W(17)O(61))](7-). This structure (1) is the first of its kind for any metal complex of alpha-1-[P(2)W(17)O(61)](10-), and the data indicate that the general stoichiometry [(H(2)O)(4)Ln(alpha-1-P(2)W(17)O(61))](7-) is maintained throughout the lanthanide series. For the 1:2 stoichiometric Ln/alpha-2-[X(2)W(17)O(61)](10-) complexes, no water molecules are in the Ln-O(8) coordination sphere. The Ln ions are bound to eight O atoms-four from each of two heteropolyanions-as [Ln(alpha-2-X(2)W(17)O(61))(2)](17-). The average Ln-O interatomic distances decrease across the lanthanide series, consistent with the decreasing Ln ionic radius.     

Functionalization of heteropolyanions-osmium and rhenium nitrido derivatives of Keggin- and Dawson-type polyoxotungstates: synthesis, characterization and multinuclear ((183)W, (15)N) NMR, EPR, IR, and UV/Vis fingerprints

Reaction of K(10)[alpha(2)-P(2)W(17)O(61)] or K(10)[alpha(1)-P(2)W(17)O(61)] or [Bu(4)N][OsCl(4)N] in a water/methanol mixture, and subsequent precipitation with (Bu(4)N)Br provided [alpha(2)-P(2)W(17)O(61){Os(VI)N}](7-) and [alpha(1)-P(2)W(17)O(61){Os(VI)N}](7-) Dawson structures as tetrabutylammonium salts. Reactions of [(Bu(4)N)(4)][alpha-H(3)PW(11)O(39)] with either [ReCl(3)(N(2)Ph(2))(PPh(3))(2)] or [Bu(4)N][ReCl(4)N] are alternatives to the synthesis of [(Bu(4)N)(4)][alpha-PW(11)O(39){Re(VI)N}]. (183)W and (15)N NMR, EPR, IR, and UV-visible spectroscopies and cyclic voltammetry have been used to characterize these compounds and the corresponding [(Bu(4)N)(4)][alpha-PW(11)O(39){Os(VI)N}] Keggin derivative.     

Influence of steric and electronic properties of the defect site, lanthanide ionic radii, and solution conditions on the composition of lanthanide(III) alpha1-P2W17O6110- polyoxometalates

This study identifies the principles that govern the formation and stability of Ln complexes of the (alpha(1)-P(2)W(17)O(61))(10-) isomer. The conditional stability constants for the stepwise formation equilibria, K(1cond) and K(2cond), determined by (31)P NMR spectroscopy, show that the high log K(1cond)/log K(2cond) ratio predicts the stabilization of the 1:1 Ln/ (alpha(1)-P(2)W(17)O(61))(10-) species. The value of log K(1cond) increases as the Ln series is traversed, consistent with the high charge/size requirement of the basic alpha(1) defect site. The conditional stability constants, K(2), are very low and are highly dependent on the countercations in the buffer. The source of the instability is understood from the crystal structures of the early-mid lanthanide analogues, where the close contact of the (alpha(1)-P(2)W(17)O(61))(10-) units result in severe steric encumbrance. The electronic properties of the alpha(1) defect along with the lanthanide ionic radii and countercation composition are important parameters that need to be considered for a rational synthesis of lanthanide polyoxometalates.     

Lanthanide complexes of [alpha-2-P2W17O61]10-: solid state and solution studies

We have isolated the 1:1 Ln:[alpha-2-P2W17O61]10- complexes for a series of lanthanides. The single-crystal X-ray structure of the Eu3+ analogue reveals two identical [Eu(H2O)3(alpha-2-P2W17O61)]7- moieties connected through two Eu-O-W bonds, one from each polyoxometalate unit. An inversion center relates the two polyoxometalate units. The Eu(III) ion is substituted for a [WO]4+ unit in the "cap" region of the tungsten-oxygen framework of the parent Wells-Dawson ion. The point group of the dimeric molecule is Ci. The extended structure is composed of the [Eu(H2O)3(alpha-2-P2W17O61)]214- anions linked together by surface-bound potassium cations. The space group is P, a = 12.7214(5) A, b = 14.7402(7) A, c = 22.6724(9) A, alpha = 71.550(3), beta = 84.019(3)degrees, gamma = 74.383(3), V = 3883.2(3) A3, Z = 1. The solution studies, including 183W NMR spectroscopy and luminescence lifetime measurements, show that the molecules dissociate in solution to form monomeric [Ln(H2O)4(alpha-2-P2W17O61)]7- species.     

New polyoxometalate compounds built up of lacunary Wells-Dawson anions and trivalent lanthanide cations

Five new polyoxometalate compounds built on lacunary Wells-Dawson anions and trivalent lanthanide cations, KNa3[Nd2(H2O)10(alpha2-P2W17O61)].11H2O (1), (H3O)[Nd3(H2O)17(alpha2-P2W17O61)].6.75H2O (2), (H2bpy)2[Nd2(H2O)9 (alpha2-P2W17O61)].4.5H2O (3), (H2bpy)2[La2(H2O)9(alpha2-P2W17O61)].4.5H2O (4), and (H2bpy)2[Eu2(H2O)9(alpha2-P2W17O61)].5H2O (5), have been synthesized and characterized by elemental analysis, IR, TG, and single-crystal X-ray diffraction. Compound 1 shows a bisupporting polyoxometalate cluster structure where two {Nd(H2O)7}3+ fragments are supported on the polyoxometalate dimer [{Nd(H2O)3(alpha2-P2W17O61)}2]14-; this represents the first bisupporting polyoxometalate compound based on a polyoxometalate dimer. Compound 2 displays a 1D chain structure built up of bisupporting polyoxoanions [{Nd(H2O)7}2{Nd(H2O)3(alpha2-P2W17O61)}2]8- and Nd3+ ions. Compounds 3-5 are isostructural and show a 2D structure constructed of 1D polyoxometalate chains of [Ln(H2O)2(alpha2-P2W17O61)]n(7n-) linked by Ln3+ ions. Compounds 2-5 represent the first extended structures formed by lacunary Wells-Dawson anions and trivalent lanthanide ions. The influence of the Ln3+/[alpha2-P2W17O61]10- ratio on the syntheses of these five compounds has been studied. Furthermore, the fluorescent activity of compound 5 is reported.     

Speciation of organic-soluble europium(III) α1-Wells-Dawson complexes

In this contribution, we provide a comprehensive understanding of the speciation of the Eu(III) complex of the lacunary Wells-Dawson isomer, α1-[P(2)W(17)O(61)](10-) in organic media. The Wells-Dawson polyoxometalate, α1-[P(2)W(17)O(61)](10-) (abbreviated as α1) forms well-defined complexes with europium(III) (and other lanthanide(III)) ions in aqueous solution of predominantly 1 : 1 stoichiometries. The 8-coordinate Eu(III) ion is bound to 4 basic terminal oxygens (O(α1)) and four water molecules (O(H(2)O)) that complete the coordination sphere. Tetra-n-butylammonium (TBA) cations are employed to render the [(H(2)O)(4)Eu(α1-P(2)W(17)O(61))](7-) (Eu-α1) complex soluble in acetonitrile. Europium(III) provides the unique opportunity to employ luminescence spectroscopy and multinuclear NMR to probe the coordination environment. We interrogate the innermost coordination sphere of the Eu(III) ion in acetonitrile solution and in MeCN/H(2)O mixtures. We provide evidence toward the fractional displacement and coordination of acetonitrile within the TBA salts, that is consistent with recent EXAFS data. (31)P NMR and Stern-Volmer quenching studies suggest that dimerization to the 2 : 2 species is negligible in acetonitrile and MeCN-H(2)O mixtures. The decreasing transition energy in the excitation spectroscopy of the TBA-Eu-α1 analog upon dilution is consistent with a nephelauxetic effect, which is attributed to a slight increase in covalency upon replacement of water with acetonitrile. Determination of the number of bound waters (q) is also consistent with acetonitrile-water exchange. The reactivity of the 1 : 1 TBA-Eu-α1 with heterocyclic aromatic amines (1,10-phenanthroline, phen, and 2,2' bipyridine, bipy) in MeCN was probed by titrations monitoring the Eu(III) emission upon sensitization by the "antenna ligands". Binding constants for the products 1 : 1 TBA(x-y)H(y)[(Phen)(H(2)O)(2)Eu(α1-P(2)W(17)O(61))] and 1 : 2 TBA(x-y)H(y)[(Phen)(2)Eu(α1-P(2)W(17)O(61))] (denoted 1 : 1 TBA-Eu-α1:phen and 1 : 2 TBA-Eu-α1:phen, respectively), were determined: logK(1): 7.05 ± 0.04 and logK(2): 4.63 ± 0.10. These are reasonably strong formation constants for Ln phenanthroline complexes. In comparison the bipyridine complexes are much weaker. Excitation spectroscopy reveals that the coordination environment about the Eu(III) center is consistent with the ternary 1 : 1 TBA-Ln-α1:phen or 1 : 2 TBA-Ln-α1:phen complexes. Multinuclear NMR spectroscopy shows significant chemical shift changes at 1 : 1 and 1 : 2 stoichiometries and the chemical shift of bound water tracks with the titration to validate expulsion of the H(2)O upon coordination of phenanthroline.     

Confirmation of the semivacant Wells-Dawson polyoxotungstate skeleton. The structures of [Ce{X(H4)W17O61}2]19- (X = P, As) indicate the probable location of internal protons

Reaction of Ce(III) with lacunary versions of [H(4)XW(18)O(62)](7-) (X = P, As) yields the 1:2 complexes [Ce(H(4)XW(17)O(61)](19-) (X = As, 1; P, 2) in good yield, characterized in solution and the solid state by NMR spectroscopy and X-ray crystallographic analysis, respectively. The structures confirm a syn C(2) conformation that is analogous to that observed for [Ln(alpha(2)-P(2)W(17)O(61))(2)](17-) but with "empty" O(4) tetrahedra that are in positions remote from the cerium atom. Bond valence sum calculations for these structures show that the four protons that are required for charge balance in all salts of the XW(18) anions and their lacunary derivatives are almost certainly bound to the oxygen atoms of the empty tetrahedra.     

Preparation and Tungsten-183 NMR Characterization of [alpha-1-P(2)W(17)O(61)](10)(-), [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-), and [alpha-2-Zn(H(2)O)P(2)W(17)O(61)](8)(-)

The preparation of the alpha-1 and alpha-2 isomers of the Wells-Dawson 17 tungsto derivatives by standard methods is accompanied by a significant proportion of the other isomer present as an impurity. In this study, the alpha-1 and alpha-2 isomers of [Zn(H(2)O)P(2)W(17)O(61)](8)(-) have been prepared in >98% purity by reacting isomerically pure K(9)Li[alpha-1-P(2)W(17)O(61)] and K(10)[alpha-2-P(2)W(17)O(61)], respectively, with ZnCl(2), while rigorously controlling the pH at 4.7. The molecules were isolated as potassium salts. For (183)W NMR and (31)P NMR characterization, both molecules were ion exchanged by cation-exchange chromatography, maintaining the pH at 4.7, to obtain the lithium salts. Removal of water and isolation of a solid sample of [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-) was achieved by lyophilization at -40 degrees C. The chemical shift data from (31)P and (183)W NMR spectroscopy of the isolated [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-) and [alpha-2-Zn(H(2)O)P(2)W(17)O(61)](8)(-) isomers are consistent with a mixture of the alpha-1 and alpha-2 isomers reported previously;(1) the molecules have the expected C(1) and C(s)() symmetry, respectively. The [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-) isomer is stable in the pH range of 4.6-6 at temperatures <35 degrees C. Using the same ion exchange and lyophilization techniques, the lacunary [alpha-1-P(2)W(17)O(61)](10)(-) isomer was isolated as the lithium salt; characterization by (183)W NMR spectroscopy confirms the C(1) symmetry.     

Stabilising pentavalent actinides--visible-near infrared and X-ray absorption spectroscopic studies of the utility of the [(Np3W4O15)(H2O)3(MW9O33)3]18- (M = Sb, Bi) structural type

We report the interaction between B-type tri-lacunary heteropolyoxotungstate anions and actinyl(V) cations in aqueous solution, yielding a greater understanding of the stability of the O≡An≡O(1+) linear dioxo actinide moiety. Previously we reported that B-α-[BiW(9)O(33)](9-) and B-α-[SbW(9)O(33)](9-) will react with NpO(2)(1+) to yield [(Np(3)W(4)O(15))(H(2)O)(3)(MW(9)O(33))(3)](18-) (M = Bi, or Sb). Single crystal structural characterisation of salts of these complexes revealed a core in which three Np(V) atoms interact with a central W(VI) atom through bridging oxo groups. These bridging oxygen atoms come from one of the two axial oxygens in O≡Np≡O(1+) and represent a highly unusual interaction for a discrete molecular species. In this study visible/near infra-red spectroscopy indicates that [(Np(3)W(4)O(15))(H(2)O)(3)(BiW(9)O(33))(3)](18-) could be readily stabilized in solution at near neutral pH for several months, with (NH(4))(14)Na(4)[(Np(3)W(4)O(15))(H(2)O)(39)BiW(9)O(33))(3)]·62H(2)O crystallising from solution in high yield. At lower pH and [BiW(9)O(33)](9-) : NpO(2)(1+) ratios additional Np(V) species could be observed in solution. Stabilization of [(Np(3)W(4)O(15))(H(2)O)(3)(SbW(9)O(33))(3)](18-) in solution proved more challenging, with several distinctive Np(V) near infra-red transitions observed in solution. Slow complexation kinetics and reduction to Np(IV) was also observed. High [SbW(9)O(33)](9-) : NpO(2)(1+) molar ratios and careful control of solution pH was required to prepare solutions in which [(Np(3)W(4)O(15))(H(2)O)(3)(SbW(9)O(33))(3)](18-) was the only neptunium containing species. In stark contrast to the NpO(2)(1+) chemistry, [BiW(9)O(33)](9-) readily oxidizes PuO(2)(1+) to PuO(2)(2+) yielding further evidence of the decreased stability of Pu(V)vs. Np(V). Np L(II)-edge XAFS measurement revealed very good agreement with single crystal diffraction data for the Np structural environment for [(Np(3)W(4)O(15))(H(2)O)(3)(MW(9)O(33))(3)](18-) (M = Bi, or Sb) in the solid state. There was also good agreement between coordination shells for [(Np(3)W(4)O(15))(H(2)O)(3)(BiW(9)O(33))(3)](18-) in the solid state and in solution, yielding further confirmation of the high stability of this particular cluster.     

Europium(III) reduction and speciation within a Wells-Dawson heteropolytungstate

The redox speciation of Eu(III) in the 1:1 stoichiometric complex with the alpha-1 isomer of the Wells-Dawson anion, [alpha-1-P 2W 17O 61] (10-), was studied by electrochemical techniques (cyclic voltammetry and bulk electrolysis), in situ XAFS (X-ray absorption fine structure) spectroelectrochemistry, NMR spectroscopy ( (31)P), and optical luminescence. Solutions of K 7[(H 2O) 4Eu(alpha-1-P 2W 17O 61)] in a 0.2 M Li 2SO 4 aqueous electrolyte (pH 3.0) show a pronounced concentration dependence to the voltammetric response. The fully oxidized anion and its reduced forms were probed by Eu L 3-edge XANES (X-ray absorption near edge structure) measurements in simultaneous combination with controlled potential electrolysis, demonstrating that Eu(III) in the original complex is reduced to Eu(II) in conjunction with the reduction of polyoxometalate (POM) ligand. After exhaustive reduction, the heteropoly blue species with Eu(II) is unstable with respect to cluster isomerization, fragmentation, and recombination to form three other Eu-POMs as well as the parent Wells-Dawson anion, alpha-[P 2W 18O 62] (6-). EXAFS data obtained for the reduced, metastable Eu(II)-POM before the onset of Eu(II) autoxidation provides an average Eu-O bond length of 2.55(4) A, which is 0.17 A longer than that for the oxidized anion, and consistent with the 0.184 A difference between the Eu(II) and Eu(III) ionic radii. The reduction of Eu(III) is unusual among POM complexes with Lindqvist and alpha-2 isomers of Wells-Dawson anions, that is, [Eu(W 5O 18) 2] (9-) and [Eu(alpha-2-As 2W 17O 61) 2] (17-), but not to the Preyssler complex anion, [EuP 5W 30O 110] (12-), and fundamental studies of materials based on coupling Eu and POM redox properties are still needed to address new avenues of research in europium hydrometallurgy, separations, and catalysis sciences.     

Modeling reactive metal oxides. Kinetics,thermodynamics, and mechanism of M(3) cap isomerization in polyoxometalates

An investigation of M(3)O(13) unit ("M(3) cap") isomerization in the classical polytungstodiphosphates alpha- and beta-P(2)W(18)O(62)(6)(-) has been undertaken because cap isomerism is an important and structurally well-studied phenomenon in many polyoxometalate families. The relative thermodynamic stabilities of the alpha (more stable) versus beta isomers were established both in the solid state by differential scanning calorimetry (4.36 +/- 0.64 kcal/mol) and in solution by (31)P NMR (3.80 +/- 0.57 kcal/mol). The isomerization of beta-P(2)W(18)O(62)(6)(-) to alpha-P(2)W(18)O(62)(6)(-), followed by (31)P NMR, has a bimolecular rate constant k(2) of 9.3 x 10(-)(1) M(-)(1) s(-)(1) at 343 K in pH 4.24 acetate buffer. Several lines of evidence establish the validity of suggestions in the literature that isomerization goes through a lacunary (defect) intermediate. First, the rate is proportional to [OH(-)]. Second, isomerization increases at higher ionic strengths, and a Debye-Hückel plot is consistent with a rate-limiting reaction between beta-P(2)W(18)O(62)(6)(-) and OH(-) (two species with a charge product of 6). Third, alkali-metal cations stabilize the bimolecular transition state (K(+) > Na(+) > Li(+)), consistent with recent ion-pairing studies in polyoxometalate systems. Fourth, the monovanadium-substituted products alpha(1)- and alpha(2)-P(2)VW(17)O(62)(7)(-) ((51)V NMR delta -554 ppm) form during isomerization in the presence of VO(2+). The known lacunary compounds (alpha(1)- and alpha(2)-P(2)W(17)O(61)(10)(-)) also react rapidly with the same vanadium precursor. Fifth, solvent studies establish that isomerization does not occur when OH(-) is absent. A mechanism is proposed involving attack of OH(-) on beta-P(2)W(18)O(62)(6)(-), loss of monomeric W(VI) from the M(3) (M(3)O(13)) terminal cap, isomerization of the resulting lacunary compound to alpha-P(2)W(17)O(61)(10)(-), and finally reaction of this species with monomeric W(VI) to form the thermodynamic and observed product, alpha-P(2)W(18)O(62)(6)(-).     

Spectroscopic properties of neodymium(III)-containing polyoxometalates in aqueous solution

The spectroscopic properties of the neodymium(III)-containing polyoxometalates (POMs) [Nd(PW(11)O(39))(2)](11-), [Nd(PMo(2)W(9)O(39))(2)](11-), [Nd(PMo(4)W(7)O(39))(2)](11-), [Nd(PMo(6)W(5)O(39))(2)](11-), [Nd(SiMo(2)W(9)O(39))(2)](13-), [Nd(P(2)W(17)O(61))(2)](17-), [NdW(10)O(36)](9-), [NdP(5)W(30)O(110)](12-) and [NdAs(4)W(40)O(140)](25-) are described. Absorption spectra of aqueous solutions of the complexes have been recorded and the transition intensities are parameterised in terms of the Judd-Ofelt intensity parameters Omega(lambda) (lambda=2, 4, 6). Marked differences were found between the luminescence lifetimes of the complexes of the type Nd(POM) and those of the type Nd(POM)(2), due to a better shielding of the neodymium(III) ions from the bulk water molecules in the latter type of complexes.     

Syntheses and X-ray crystal structures of zirconium(IV) and hafnium(IV) complexes containing monovacant wells-Dawson and Keggin polyoxotungstates

The syntheses and crystal structures of a series of zirconium(IV) and hafnium(IV) complexes with Dawson monovacant phosphotungstate [alpha2-P2W17O61](10-) and in situ-generated Keggin monovacant phosphotungstate [alpha-PW11O39](7-), which was obtained by a reaction of [alpha-PW12O40](3-) with Na2CO3, are described. K15H[Zr(alpha2-P2W17O61)2].25H2O (K-1), K16[Hf(alpha2-P2W17O61)2].19H2O (K-2), (Et2NH2)10[Zr(alpha-PW11O39)2].7H2O (Et2NH2-3), and (Et2NH2)10[Hf(alpha-PW11O39)2].2H2O (Et2NH2-4), being afforded by reactions in aqueous solutions of monolacunary Dawson and Keggin polyoxotungstates with ZrCl2O.8H2O and HfCl2O.8H2O followed by exchanging countercations, were obtained as analytically pure, homogeneous colorless crystals. Single-crystal X-ray structure analyses revealed that the Zr(IV) and Hf(IV) ions are in a square antiprismatic coordination environment with eight oxygen atoms, four of them being provided from each of the two monovacant polyanion ligands. Although the total molecular shapes and the 8-coordinate zirconium and hafnium centers of complexes 1-4 are identical, the bonding modes (bond lengths and bond angles) around the zirconium(IV) and hafnium(IV) centers were dependent on the monovacant structures of the polyanion ligands. Additionally, the characterization of complexes 1-4 was accomplished by elemental analysis, TG/DTA, FTIR, and solution (31P and 183W) NMR spectroscopy.     

Production and reactions of organic-soluble lanthanide complexes of the monolacunary Dawson [alpha1-P2W17O61]10- polyoxotungstate

The incorporation of lanthanides into polyoxometalates provides entry to new classes of potentially useful materials that combine the intrinsic properties of both constituents. To utilize the [alpha1-Ln(H2O)4P2W17O61]7- species in applications of catalysis and development of luminescent materials, the chemistry of this family of lanthanide polyoxometalates in organic solvents has been developed. Organic-soluble polyoxometalate-lanthanide complexes TBA5H2[alpha1-Ln(H2O)4P2W17O61] (Ln = La(III), Sm(III), Eu(III), Yb(III)) were prepared and characterized by elemental analysis, acid-base titration, IR, 31P NMR, and mass spectrometry. The synthetic procedure involves a cation metathesis reaction in aqueous solution under strict pH control. A solid-liquid-phase transfer protocol yielded a unique species (TBA)8K3[Yb(alpha1-YbP2W17O61)2] with three ytterbium ions and two [alpha1-P2W17O61]10- polyoxotungstates. A centrosymmetric dimeric complex [{alpha1-La(H2O)4P2W17O61}2]14- was crystallized from aqueous solution and characterized by X-ray diffraction. ESI mass spectral analysis of the complexes TBA5H2[alpha1-Ln(H2O)4P2W17O61] shows that similar dimers exist in organic solution, in particular for the early lanthanides. Fragmentation in the mass spectrometer of the complexes from dry acetonitrile solution involves double protonation of an oxo ligand and loss of one water molecule. Low mass tungstate fragments combine into [(WO3)n]2- (n = 1-5) ions and their condensation products with phosphate. Reaction of TBA5H2[alpha1-Eu(H2O)4P2W17O61] with 1,10-phenanthroline or 2,2'-bipyridine showed an increase of the europium luminescence. This result is explained by the formation of a ternary complex of [alpha1-Eu(H2O)4P2W17O61]7- and two sensitizing ligands.     

The antimony(III)-bridged heteropolyanion sandwich dimers [Sb(II3(A-α-XW9O34)2]11- (X = SiIV, GeIV and C-shaped double-sandwich [SbIII6O2(PW6O26)(A-α-PW9O34)2]15-

Interaction of potassium antimony(iii) tartrate hydrate K(2)(SbC(4)H(2)O(6))(2)·3H(2)O with the trilacunary Keggin derivatives [A-α-XW(9)O(34)](10-) (X = Si(IV), Ge(IV)) and [A-α-PW(9)O(34)](9-) in aqueous acidic medium (pH 4.8) resulted in three novel polyanions, [Sb(3)(A-α-XW(9)O(34))(2)](11-) (X = Si(IV) (1), Ge(IV) (2)) and [Sb(6)O(2)(A-PW(6)O(26))(A-α-PW(9)O(34))(2)](15-) (3), which were isolated as the hydrated potassium salts K(11)[Sb(3)(A-α-XW(9)O(34))(2)]·31H(2)O (X = Si(IV) (K-1), Ge(IV) (K-2)) and the mixed potassium-sodium salt K(14)Na[Sb(6)O(2)(A-PW(6)O(26))(A-α-PW(9)O(34))(2)]·61H(2)O (KNa-3) salts, respectively, and characterized by single-crystal X-ray diffraction, IR spectroscopy, as well as elemental and thermogravimetric analyses. The Sb(III)-containing polyanions 1-3 possess unique structural features, as they represent the first examples of sandwich-type POMs with trigonal-pyramidal Sb(III)O(3) linkers. The stability of 1-3 in aqueous media was investigated by multinuclear ((183)W, (31)P) NMR and UV-Vis spectroscopy.