Plutonium and other tetravalent metals complexed to a Keggin polyoxometalate

We report the synthesis and characterization of the first plutonium(IV) complex with a Keggin ion chelator. This plutonium-polyoxometalate (POM) coordination compound was crystallized as its caesium salt, Cs20[Pu(PW11O39)2]2·13H2O, using microgram quantities of Pu. Single crystal XRD and solid-state UV-vis absorbance analysis confirm the stabilization of Pu4+ by the Keggin ligand. The unit cell contains two [Pu(PW11O39)2]10- complexes (Pu(PW11)2) bridged by cesium counterions. The Raman spectrum of Pu(PW11)2 reveals features consistent with those observed for the analogous tetravalent complexes with Zr4+, Hf4+, Ce4+, and Th4+. For the first coordination sphere of Pu, the average Pu-O bond distances in the two Pu sites are 2.35(3) and 2.34(3) Å, matching the value extrapolated from the bonding trend based on the other 8-coordinated tetravalent cations coordinated to the Keggin POM. However, the long-range arrangement of the Pu(PW11)2 complexes within the lattice is unique in the series of MIV(PW11)2 compounds, with pairs of Pu(PW11)2 complexes organized perpendicular to each other. The aqueous speciation of Pu4+ in the presence of the POM was probed via UV-visible absorbance, coupled with small angle X-ray scattering (SAXS) analysis of the analogous systems Th(PW11)2, Ce(PW11)2, Hf(PW11)2, and Zr(PW11)2. The solution-state results indicate that, contrary to their trivalent counterparts, tetravalent cations quantitatively form the 1:2 species in solution (i.e., [Pu(PW11O39)2]10-(aq)) and no 1:1 species (e.g., [Pu(PW11O39)(H2O)x]3-(aq)). Finally, in the solid-state, a strong and linear correlation was found between the metal-oxygen bond distances in the MIV(PW11)2 compounds and the corresponding metal dioxides (PuO2, ThO2, CeO2, ZrO2, etc.), allowing for the extrapolation to other tetravalent ions (i.e., Pa4+, Am4+, and Bk4+). The results indicate that our microscale POM approach could be a viable pathway to harness the physicochemical properties of rare actinide ions in discrete molecules, beyond the traditional oxide extended solids.