Venturing Past Uranium: Synthesis of a Np(IV) Polyoxomolybdate–Alkoxide Sandwich Complex

The synthesis of a Np(IV) polyoxomolybdate-alkoxide sandwich complex, (TBA)₂[Np{Mo₅O₁₃(OMe)₄NO}₂] (TBA = tetrabutylammonium), is reported. This compound represents a rare example of a neptunium polyoxometalate cluster isolated outside of water, allowing for characterization of its electrochemical properties in nonaqueous solvents. Complexation of An(IV) cations fine-tunes the redox properties of the cluster, with the observed four reversible reductive events varying slightly both in potential and peak separation depending on the actinide present. The new Np(IV) complex also shows an irreversible event assigned to oxidation of Np(IV) to Np(V). New methodology for facile ¹⁷O enrichment of (TBA)₂[Mo₅O₁₃(OMe)₄NO][Na(MeOH)] is presented, which provides a simple pathway to ¹⁷O enriched analogues of the sandwich complexes discussed (Zr(IV), Hf(IV), Th(IV), U(IV), U(V), Np(IV)). ¹⁷O NMR spectroscopy subsequently provides insights into both the nature of metal-oxygen bonding, as well as the influence of unpaired f-electrons on the local environment of the oxygen nuclei.