Controlling Cation‐Cation Interactions in Uranyl Coordination Dimers by Varying the Length of the Dicarboxylate Linker

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. Control of cation‐cation interactions (CCIs) in dinuclear uranyl coordination complexes is achieved by adjusting the length of dicarboxylate linkers. Whereas short linkers impose nearly‐parallel uranyl orientation to minimize repulsive interactions, longer linkers allow for U–O dative bonding in a CCI oriented either end‐on or side‐on. The CCI configurations render the U(V) center inaccessible, thereby inhibiting oxidative addition of O2.