Technetium and Rhenium Auto-reduction, Polymerization and Lability towards Group VII Polyoxometalate Chemistry

Group VII Tc and Re have long been studied to develop both radiopharmaceuticals and technologies for nuclear materials management. Fundamental research has targeted understanding this periodic table crossroads where polyoxometalates meets metal-metal bonded complexes. Here we have isolated green hygroscopic and metastable crystals of (Re^VI,oct)₂(Re^VII,tet)₂(OH)₂(O)₁₂⋅H₂O (Re^VI,VII-green, tet=tetrahedral, oct=octahedral), determined by single-crystal x-ray diffraction. In addition to color, Re-L₁ X-ray absorption near-edge spectroscopy confirms the reduced oxidation state. Re^VI,VII-green provides the first demonstration of Re autoreduction, long-observed for Mn and Tc. We also isolated and structurally characterized [Tc₄O₄(H₂O)₂(ReO₄)₁₄]^2- (Tc₄Re₁₄) polyanion crystals that contain Tc(V) and Re(VII), consistent with greater stability of reduced Tc compared to reduced Re. Small angle X-ray scattering of both compounds and prior-reported polyanion [Tc₄O₄(H₂O)₂(TcO₄)₁₄]^4- (Tc₂₀) dissolved in acetonitrile indicated a qualitative lability order of oxo-linkages of Re-O-Re > Re-O-Tc >Tc-O-Tc, and lability of Tc₂₀ was also probed by ⁹⁹Tc nuclear magnetic resonance spectroscopy. Computation provided insight into ⁹⁹Tc chemical shifts as well as lability. Based on both reducibility and solution phase dynamics of polynuclear compounds investigated here, Re is an imperfect surrogate for Tc, and further expansion of group VII polyoxometalate chemistry seems promising.