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Synthesis of technetium-99 (99Tc; t1/2: 2.1 × 105 years, βmax: 253 keV) materials is of importance in studies of the nuclear fuel cycle where Tc is a major fission product (6 % thermal yield from 235U and 239Pu), in understanding radioactive tank waste composition, and in identifying 99mTc compounds for nuclear medicine imaging. One of the most useful synthetic starting materials, (NBu4)TcOCl4, is susceptible to disproportionation in water to form TcO4- and TcIV species, especially TcO2 ·2H2O. This unwanted reaction is especially problematic when working with ligands bearing 'hard' donor atoms, such as oxygen, where the stability with the 'soft' TcV=O3+ core may be low. Polyoxometalates (POMs) are such ligands. They possess defect sites with four hard oxygen atoms and show low (ca. 108) stability constants with transition metals. Tc complexes of POMs are molecular-level models for Tc metal oxide solid-state materials and can provide information on coordination and redox environments of metal oxides that stabilize low-valent Tc. In order to synthesize pure Tc POM complexes [TcVO(α1-P2W17O61)]7- (TcVO- α 1) and [TcVO(α2-P2W17O61)]7- (TcVO- α 2) from (NBu4)TcOCl4, we have identified strategies that minimize formation of TcIV species and optimize the formation of pure TcV species. The parameters that we consider are the amount of ethylene glycol, which is employed as a 'transfer ligand' to prevent hydrolysis of (NBu4)TcOCl4, and the precipitating agent. The TcIV species that contaminates the non-optimized syntheses is likely a TcIV μ-oxido-bridged dimer [TcIV-(μ-O)2-TcIV]. We also employ a novel procedure where the α 2 ligand is photoactivated and reduced (in the presence of a sacrificial electron donor) to subsequently reduce TcVIIO4- to an isolatable TcVO- α 2 product that is remarkably free of TcIV. [ABSTRACT FROM AUTHOR]

The radioactive element technetium-99 (99Tc, half-life = 2.1 × 105 years, ß- of 253 keV), is a major byproduct of 235U fission in the nuclear fuel cycle. 99Tc 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 99Tc 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, α1-P2W17O61l0- (α1) and α2-P2W17O6110- (α2) to identify features of metal oxide materials that can sta ilize 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 K7-nHn[TcVO(α1P2W17O61)] (TcVO-αl), K7_nHn[TcVO(α2-P2W17O61)] (TcVO-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 αl site as compared to the a2 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 VL The MV?MIV reduction processes for TcVOαl are not pH dependent or only slightly pH dependent suggesting that protonation does not accompany reduction of this species unlike the MVO-α2 (M = 99Tc, Re) and ReVO-αl where MV/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 (EXAES) and X-ray absorption near edge structure (XANES) data that reveal that the TcV is "pulled" into the -αl framework and that may facilitate the reduction of TcVO-αl 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. [ABSTRACT FROM AUTHOR]

Technetium-99 (99Tc) (β-max: 293.7 keV; t1/2: 2.1 × 105 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 (99TcO4-) is stable. 99TcO4- 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 99TcO4- and the chemical incorporation of the reduced 99Tc into a metal oxide material. This strategy employs a single material, a polyoxometalate (POM), α2-[P2W17O61]10-, that can be photoactivated in the presence of 2-propanol to transfer electrons to 99TcO4- and incorporate the reduced 99Tc covalently into the α2-framework to form the 99TcVO species, 99TcVO(α2-P2W17O61)7-. This occurs via the formation of an intermediate species that slowly converts to 99TcVO(α2-P2W17O61)7-. Extended X-ray absorption fine structure and X-ray absorption near-edge spectroscopy analysis suggests that the intermediate consists of a 99Tc(IV) α2- species where the 99Tc is likely bound to two of the four W-O oxygen atoms in the α2-[P2W17O61]10- defect. This intermediate then oxidizes and converts to the 99TcVO(α2-P2W17O61)7- product. The reduction and incorporation of 99TcO4- 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 α2-P2W17O6112- through bulk electrolysis followed by the addition of 99TcO4-. The reduction and incorporation of ReO4-, as a nonradioactive surrogate for 99Tc, does not proceed through the intermediate species, and ReVO is incorporated quickly into the α2-[P2W17O61]10- defect. These observations are consistent with the periodic trends of 99Tc and Re. Specifically, 99Tc 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 99Tc into a material. [ABSTRACT FROM AUTHOR]

The isotope 99Tc (βmax, 293.7; half-life, 2.1 × 105 years) is an abundant product of uranium-235 fission in nuclear reactors and is present throughout the radioactive waste stored in underground tanks at the Hanford and Savannah River sites. Understanding and controlling the extensive redox chemistry of 99Tc is important in identifying tunable strategies to separate 99Tc from spent fuel and from waste tanks and, once separated, to identify and develop an appropriately stable waste form for 99Tc. Polyoxometalates (POMs), nanometer-sized models for metal oxide solid-state materials, are used in this study to provide a molecular level understanding of the speciation and redox chemistry of incorporated 99Tc. In this study, 99Tc complexes of the (α2-P2W17O61)10- and (α1-P2W17O61)10- isomers were prepared. Ethylene glycol was used as a "transfer ligand" to minimize the formation of TcO2∙xH2O. The solution structures, formulations, and purity of TcVO(α1/α2-P2W17O61)7- were determined by multinuclear NMR. X-ray absorption spectroscopy of the complexes is in agreement with the formulation and structures determined from 31P and 183W NMR. Preliminary electrochemistry results are consistent with the EXAFS results, showing a facile reduction of the TcVO(α1-P2W17O61)7- species compared to the TcVO(α2-P2W17O61)7- analog. The α1 defect is unique in that a basic oxygen atom is positioned toward the α1 site, and the TcVO center appears to form a dative metal-metal bond with a framework W site. These attributes may lead to the assistance of protonation events that facilitate reduction. Electrochemistry comparison shows that the ReV analogs are about 200 mV more difficult to reduce in accordance with periodic trends. [ABSTRACT FROM AUTHOR]

The isotope (99)Tc (β(max), 293.7; half-life, 2.1 × 10(5) years) is an abundant product of uranium-235 fission in nuclear reactors and is present throughout the radioactive waste stored in underground tanks at the Hanford and Savannah River sites. Understanding and controlling the extensive redox chemistry of (99)Tc is important in identifying tunable strategies to separate (99)Tc from spent fuel and from waste tanks and, once separated, to identify and develop an appropriately stable waste form for (99)Tc. Polyoxometalates (POMs), nanometer-sized models for metal oxide solid-state materials, are used in this study to provide a molecular level understanding of the speciation and redox chemistry of incorporated (99)Tc. In this study, (99)Tc complexes of the (α(2)-P(2)W(17)O(61))(10-) and (α(1)-P(2)W(17)O(61))(10-) isomers were prepared. Ethylene glycol was used as a "transfer ligand" to minimize the formation of TcO(2)·xH(2)O. The solution structures, formulations, and purity of Tc(V)O(α(1)/α(2)-P(2)W(17)O(61))(7-) were determined by multinuclear NMR. X-ray absorption spectroscopy of the complexes is in agreement with the formulation and structures determined from (31)P and (183)W NMR. Preliminary electrochemistry results are consistent with the EXAFS results, showing a facile reduction of the Tc(V)O(α(1)-P(2)W(17)O(61))(7-) species compared to the Tc(V)O(α(2)-P(2)W(17)O(61))(7-) analog. The α(1) defect is unique in that a basic oxygen atom is positioned toward the α(1) site, and the Tc(V)O center appears to form a dative metal-metal bond with a framework W site. These attributes may lead to the assistance of protonation events that facilitate reduction. Electrochemistry comparison shows that the Re(V) analogs are about 200 mV more difficult to reduce in accordance with periodic trends., (© 2011 American Chemical Society)

The tetra-n-butylammonium (TBA(+)) salts for a series of lanthanoid(III) (Ln = Nd, Sm, Eu, Tb, Dy, Yb, and Y) complexes with the alpha-1-isomer of the Wells-Dawson heteropolyoxoanion, alpha-1-[P(2)W(17)O(61)](10-), were prepared and characterized by voltammetry, controlled-potential bulk electrolysis, Eu L(3)-edge XANES spectroelectrochemistry, and Ln L(3)-edge X-ray absorption spectroscopy. Aspects of the series behavior across the 4f period for the complex anions with nominal 1 : 1 Ln(III):alpha-1-[P(2)W(17)O(61)](10-) stoichiometries are detailed. The voltammetric response of the alpha-1 ligand alone in dry acetonitrile (with 0.1 M (TBA)PF(6) electrolyte) is concentration independent and remarkably well-defined with five waves attributable to W-based redox processes. The formation of heteropoly blue solutions upon electrochemical reduction results in chemical instabilities and isomerization. The deliberate addition of water turns an otherwise ideal response into a broad and poorly resolved one, wherein the first reduction process is shifted 150 mV to more positive electrode potentials. Upon its coordination with Ln(III) ions, the voltammetric response develops notable complexities with as many as ten concentration-dependent couples attributable to W-based redox processes of the Ln:alpha-1 complexes. The results from in situ Eu L(3)-edge XANES of the Eu(III):alpha-1 complex provide no evidence for the one-electron reduction of Eu(III) at controlled electrode potentials comparable to those that were previously found to form Eu(II) in the potassium salt of the corresponding Eu(III) complex upon reduction in an aqueous electrolyte. To explain the contrasting system behaviors, the Ln(III) coordination environments in the TBA(5)H(2)[(H(2)O)(n)Ln(alpha-1-P(2)W(17)O(61))] solid salts, including the extent of Ln hydration (n) upon their dissolution in acetonitrile, were determined through use of EXAFS, which demonstrates a structural uniformity among the salts and their acetonitrile solutions, wherein the average Ln-O interatomic distances and O coordination numbers reveal variations that are consistent with the effects of the lanthanoid contraction. The side-by-side comparison of the solid and solution data provides evidence that is consistent with a partial solvent (H(2)O-CH(3)CN) exchange upon dissolution in MeCN. Details of the Ln(III) coordination chemistry, wherein the decrease in the ionic radius from the large, light Nd(III) to the small, heavy Yb(III), are presented in the context of known structural and physical phenomena of acetonitrile solvates, aqua ions, and the corresponding water-soluble K(7)[(H(2)O)(4)Eu(alpha-1-P(2)W(17)O(61))] complex.

We report a facile, high yield synthesis and characterization of discrete, ternary porphyrin-metal-polyoxometalate (Por-M-POM) complexes where a group (IV) transition metal ion is bound both to the porphyrin core and to the lacunary site of a Keggin POM, PW(11)O(39) (-7). The remarkably robust complexes exploit the fact that Hf(IV) and Zr(IV) are 7-8 coordinate and reside outside the plane of the porphyrin macrocycle, thus enabling the simultaneous coordination to meso-tetraphenylporphyrin (TPP) or meso-tetra(4-pyridyl)porphyrin (TPyP) and to the defect site in the Keggin framework. The physical properties of the (TPP)Hf(PW(11)O(39))[TBA](5), (TPyP)Hf(PW(11)O(39))[TBA](5), and (TPP)Zr(PW(11)O(39))[TBA](5) complexes are similar because the metal ions have similar oxidation states, and coordination chemistry.This architecture couples the photonic properties of the porphyrin to the POM because the metal ion is incorporated into both frameworks. Thus the ternary complexes can serve as a basis for the characterization of Hf(IV) and Zr(IV) porphyrins bound to oxide surfaces via the group (IV) metal ions. The Hf(Por) and Zr(Por) bind strongly to TiO(2) nanoparticles and indium tin oxide (ITO) surfaces, but significantly less binds to crystalline SiO(2) or TiO(2) surfaces. Together, the strong binding of the metalloporphyrins to the POM, nanoparticles, and the ITO surfaces, and paucity of binding to crystalline surfaces, suggests that the 3-4 open coordination sites on the Hf(Por) and Zr(Por) are predominantly bound at surface defect sites.

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.