99Tc and Re Incorporated into Metal Oxide Polyoxometalates: Oxidation State Stability Elucidated by Electrochemistry and Theory

The radioactive element technetium-99 (99Tc, half-life = 2.1 × 105 years, β– 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-P2W17O6110– (α1) and α2-P2W17O6110– (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 K7–nHn[TcVO(α1-P2W17O61)] (TcVO-α1), 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 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 MV→ MIV reduction processes for TcVO-α1 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-α1 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 (EXAFS) and X-ray absorption near edge structure (XANES) data that reveal that the TcV is “pulled” into the -α1 framework and that may facilitate the reduction of TcVO-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