Tuning Uranium Redox Chemistry in Asymmetric Polyoxometalate Complexes: Access to U(IV), U(V), and Transient U(VI)

We report the synthesis and characterization of asymmetric sandwich-type complexes [TpU^IVEW₁₁O₃₉]^x- (E = P; x = 4, E = Si; x = 5, Tp = trispyrazolylborate). These complexes represent rare examples of the selective formation of asymmetric actinide-polyoxometalate (An-POM) complexes. Oxidation studies show that the U^V analogues are stable and accessible, with both complexes isolated. Attempts to access U^VI with this framework yield mixed results. Attempts to produce [TpU^VIPW₁₁O₃₉]^2- only led to decomposition, while electrochemical oxidation experiments gave evidence of transient formation of [TpU^VISiW₁₁O₃₉]^3- in solution. Rapid decomposition of [TpU^VISiW₁₁O₃₉]^3- inhibits full characterization. These studies indicate that the redox properties and stability of An-POM complexes containing actinides in higher valencies are dependent on the anionic charge of the assembly.