Uranyl Tris(benzoate) Photocatalysts for Site-Selective Hydrocarbon Functionalization

The uranyl dication (UO22+) is a highly active photocatalyst for the functionalization of inert Csp3–H bonds by direct hydrogen atom abstraction (HAA). However, photocatalysis by the uranyl ion remains underexplored. Most reports are limited to reactions catalyzed by simple uranyl salts, such as uranyl nitrate UO2(NO3)2·6H2O (UNO3). We report a set of uranyl tris(benzoate) complexes 1-R containing strongly coordinating and tunable equatorial ligands that resist photodamage and control access to the oxo groups. These catalyst variants with appropriate aryl substituents undergo catalytic reactions at C–H bonds by HAA. The selectivity and reactivity of this step depend on the ligand framework and are distinct from that of UNO3 or other photoactive oxo complexes, such as decatungstate, that lack ancillary ligands. Finally, consistent with the strong, stable axial U–O bond, reaction with exogenous radical acceptors outcompetes radical rebound, enabling C–C and C–N bond formation from the alkyl radical intermediate. Regioselective alkylation and functionalization of a broad range of substrates results, and this photocatalysis shows that modulation of equatorial ligands on UO22+ can influence the reactivity and selectivity of photocatalytic C–H bond functionalization.