Structural and Spectroscopic Characterization of Plutonium and Other Tetravalent Metals Complexed to a Keggin Ion

We report the isolation of the first plutonium(IV) complex with a Keggin ion chelator: Cs₂₀[Pu(PW₁₁O₃₉)₂]₂·13H₂O. Single crystal XRD and solid-state UV-vis absorbance analysis demonstrate the stabilization of Pu⁴⁺ by the Keggin ligand. The unit cell contains two [Pu(PW₁₁O₃₉)₂]^10- complexes (Pu(PW₁₁)₂) bridged by Cs⁺. Raman and ³¹P NMR spectra of Pu(PW₁₁)₂ are consistent with the analogous Zr⁴⁺, Hf⁴⁺, Ce⁴⁺, and Th⁴⁺ complexes. The Pu-O bond distances at the two Pu sites are 2.35(3) and 2.34(3) Å, matching the value extrapolated from the bonding trend built with the other 8-coordinated tetravalent cations. However, the long-range arrangement of the Pu(PW₁₁)₂ complexes within the lattice is unique in the series of M^IV(PW₁₁)₂ compounds: pairs of Pu(PW₁₁)₂ are organized perpendicular to each other. Based on solution-state UV-visible absorbance, small-angle X-ray scattering (SAXS), and ³¹P NMR, the tetravalent cations quantitatively form the 1:2 species in solution ([Pu(PW₁₁O₃₉)₂]^10-_(aq)) and no 1:1 species ([Pu(PW₁₁O₃₉)(H₂O)_<i>x</i>]^3-_(aq)). Finally, a linear correlation exists between the metal-oxygen distances in the M^IV(PW₁₁)₂ compounds and the corresponding metal dioxides, allowing for extrapolation for Pa⁴⁺, Am⁴⁺, and Bk⁴⁺. The results indicate that our microscale POM approach represents a viable pathway to probe properties of rare actinide ions in discrete molecules, beyond the traditional oxide extended solids.