Controlling the redox chemistry of the uranyl ion (UO22+) is challenging due to a lack of reliable methods that afford influence over reduction potentials. Prior work has shown that secondary metal ions can tune the U(VI)/U(V) reduction potential of the uranyl ion, but relatively few ligand systems capable of supporting appropriate structures for such studies are available. In this work, we have explored the use of metallo-ligands for this purpose, with a focus on avoiding arduous syntheses that could preclude reactivity studies of the final uranyl complexes. We have carried out a synthetic study of a family of salen-based metallo-ligands containing Group 10 metals (Ni, Pd, and Pt) that feature a tetradentate site for binding of the uranyl ion. Four new complexes have been prepared in this work; they have been characterized via 1H nuclear magnetic resonance, cyclic voltammetry, and single-crystal X-ray diffraction (XRD) analysis. Electrochemical data reveal a chemically reversible Ni(II)/Ni(I) couple for the monometallic nickel complex, while the monometallic palladium complex affords an irreversible reduction. Upon introduction of the uranyl ion to form a [Ni,U] complex, two reductions become accessible, corresponding to Ni(II)/Ni(I) and U(VI)/U(V) couples. XRD data for the bimetallic [Ni,U] complex reveal that “end-to-end” dimerization occurs in the solid state, and confirms the preparation of the desired complex. Additionally, to synthesize the monometallic platinum complex, the salen-based ligand was altered to prevent the formation of platinum metal. Taken together, the available data suggest that these tailored metallo-ligands will be useful for the study of tunable uranyl chemistry.

Establishing a modular synthetic route to heterobimetallic uranyl complexes

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