Synthesis, Structure Elucidation, and Redox Properties of⁹⁹Tc Complexes of Lacunary Wells−Dawson Polyoxometalates: Insights into Molecular⁹⁹Tc−Metal Oxide Interactions

Abstract

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.

Strategies for the Photoreduction of Tc‐99 Pertechnetate to Low‐Valent Tc by Keggin Polyoxometalates

“…The Tc–W distance is the same as that found in the Tc V =O alpha 2‐Lacunary Wells–Dawson ion , . Since this coordination environment is basically identical to that of a lacunary Keggin ion, this suggests that at least some of the Tc has been incorporated into the Keggin ion most likely as Tc V =O, consistent with the XANES data.…”

Strategies for the Photoreduction of Tc‐99 Pertechnetate to Low‐Valent Tc by Keggin Polyoxometalates

“…The Tc–W distance is the same as that found in the Tc V =O alpha 2‐Lacunary Wells–Dawson ion , . Since this coordination environment is basically identical to that of a lacunary Keggin ion, this suggests that at least some of the Tc has been incorporated into the Keggin ion most likely as Tc V =O, consistent with the XANES data.…”

Incorporation of Technetium into Spinel Ferrites

“…The shorter TcO distance is ascribed to a fitting artifact. In some cases, the shorter TcO distance is consistent with the distance of a terminal oxo group of Tc­(V), 1.64 Å . However, the shorter TcO distance varies widely from 1.51 to 1.69 Å.…”

Photoreduction of99Tc Pertechnetate by Nanometer-Sized Metal Oxides: New Strategies for Formation and Sequestration of Low-Valent Technetium

“…260 mV, which is due to an incorporated Re species into the POM framework. The CV is identical to the chemically synthesized K-Re V O-α2 complex 40 This experiment supports a rapid reduction of both rhenium (VII) and technetium (VII) as evidenced by the rapid rise in the rest potential upon addition of MO 4…”

“…The species formed in this series of reactions as well as in those with electrolyte (vide infra) are: (1) 99 Tc V O-α2 (−11.84 and −13.29 ppm); (2) an “intermediate” that subsequent studies suggest contains 99 Tc and phosphorus (−14.50 ppm); (3) unreacted α2 (−8.51 and −12.58 ppm); and (4) the decomposition product, α-[P 2 W 18 O 62 ] 6– (−12.65 ppm). These chemical shifts (except for the 99 Tc/P containing intermediate) have been assigned previously in independent studies where 99 Tc V O-α2 has been chemically synthesized and characterized …”

Strategies for the Photoreduction of Tc‐99 Pertechnetate to Low‐Valent Tc by Keggin Polyoxometalates

“…The Tc–W distance is the same as that found in the Tc V =O alpha 2‐Lacunary Wells–Dawson ion , . Since this coordination environment is basically identical to that of a lacunary Keggin ion, this suggests that at least some of the Tc has been incorporated into the Keggin ion most likely as Tc V =O, consistent with the XANES data.…”

Incorporation of Technetium into Spinel Ferrites

“…The shorter TcO distance is ascribed to a fitting artifact. In some cases, the shorter TcO distance is consistent with the distance of a terminal oxo group of Tc­(V), 1.64 Å . However, the shorter TcO distance varies widely from 1.51 to 1.69 Å.…”

Photoreduction of99Tc Pertechnetate by Nanometer-Sized Metal Oxides: New Strategies for Formation and Sequestration of Low-Valent Technetium

“…260 mV, which is due to an incorporated Re species into the POM framework. The CV is identical to the chemically synthesized K-Re V O-α2 complex 40 This experiment supports a rapid reduction of both rhenium (VII) and technetium (VII) as evidenced by the rapid rise in the rest potential upon addition of MO 4…”

“…The species formed in this series of reactions as well as in those with electrolyte (vide infra) are: (1) 99 Tc V O-α2 (−11.84 and −13.29 ppm); (2) an “intermediate” that subsequent studies suggest contains 99 Tc and phosphorus (−14.50 ppm); (3) unreacted α2 (−8.51 and −12.58 ppm); and (4) the decomposition product, α-[P 2 W 18 O 62 ] 6– (−12.65 ppm). These chemical shifts (except for the 99 Tc/P containing intermediate) have been assigned previously in independent studies where 99 Tc V O-α2 has been chemically synthesized and characterized …”

Strategies for the Photoreduction of Tc‐99 Pertechnetate to Low‐Valent Tc by Keggin Polyoxometalates

“…The Tc–W distance is the same as that found in the Tc V =O alpha 2‐Lacunary Wells–Dawson ion , . Since this coordination environment is basically identical to that of a lacunary Keggin ion, this suggests that at least some of the Tc has been incorporated into the Keggin ion most likely as Tc V =O, consistent with the XANES data.…”

Incorporation of Technetium into Spinel Ferrites

“…The shorter TcO distance is ascribed to a fitting artifact. In some cases, the shorter TcO distance is consistent with the distance of a terminal oxo group of Tc­(V), 1.64 Å . However, the shorter TcO distance varies widely from 1.51 to 1.69 Å.…”

Photoreduction of99Tc Pertechnetate by Nanometer-Sized Metal Oxides: New Strategies for Formation and Sequestration of Low-Valent Technetium

“…260 mV, which is due to an incorporated Re species into the POM framework. The CV is identical to the chemically synthesized K-Re V O-α2 complex 40 This experiment supports a rapid reduction of both rhenium (VII) and technetium (VII) as evidenced by the rapid rise in the rest potential upon addition of MO 4…”

“…The species formed in this series of reactions as well as in those with electrolyte (vide infra) are: (1) 99 Tc V O-α2 (−11.84 and −13.29 ppm); (2) an “intermediate” that subsequent studies suggest contains 99 Tc and phosphorus (−14.50 ppm); (3) unreacted α2 (−8.51 and −12.58 ppm); and (4) the decomposition product, α-[P 2 W 18 O 62 ] 6– (−12.65 ppm). These chemical shifts (except for the 99 Tc/P containing intermediate) have been assigned previously in independent studies where 99 Tc V O-α2 has been chemically synthesized and characterized …”