Lawrence Berkeley National Laboratory

The chemistry of linear uranyl(V/VI) dioxo cations, [Oyl–U–Oyl]+/2+, is dominated by coordination of uranium in the equatorial plane. Effects of this constraint were evaluated by experiment and theory for gas-phase mixed-valence UV/VI coordination dimers in which uranyl moieties are linked by alkyl dicarboxylates, [(UO2+)(UO22+)(OOC-(CH2)n-2-COO2–)2]– (n = 3–12). Faster O2-addition to dimers with short linkers n = 3 and 4, vs. n ≥ 5, suggests a structural difference. Computed structures with the shortest linkers have bridging dicarboxylates and nearly parallel, non-interacting uranyls. Longer linkers, n = 5–7, accommodate uranyl orientations with distinct UV–UVI end-on cation-cation interactions (CCIs), whereby Lewis base Oyl from UV coordinates to the acid UVI, denoted as UVOyl···UVI. The dimer structure for n = 8 has a UV–UVI side-on diamond-shape CCI, with UVOyl···UVI and UVIOyl···UV interactions. Addition of O2 to the n = 4 and 5 dimers yields [(UO22+)2(OOC-(CH2)n-2-COO2–)2(O2–)]–, with UV oxidized to UVI and O2 reduced to O2–. Whereas O2 can associate to and oxidize the exposed UV center for dimers with n = 3 and 4, the more crowded UV site in the CCI structures inhibits O2 addition. The results demonstrate rational structural control of uranyl-uranyl bonding and reactivity in small coordination complexes.