Redox-Inactive Metal Cations Modulate the Reduction Potential of the Uranyl Ion in Macrocyclic Complexes

Capture and activation of the water-soluble uranyl dication (UO₂²⁺) remains a challenging problem, as few rational approaches are available for modulating the reactivity of this species. Here, we report the divergent synthesis of heterobimetallic complexes in which UO₂²⁺ is held in close proximity to a range of redox-inactive metals by a tailored macrocyclic ligand. Crystallographic and spectroscopic studies confirm assembly of homologous U^VI(μ-O_Ar)₂M cores with a range of mono-, di-, and trivalent Lewis acids (M). Cyclic voltammetry data demonstrate that the U^VI/U^V reduction potential in these complexes is modulated over a span of 600 mV, depending linearly on the Lewis acidity of the redox-inactive metal with a sensitivity of 61 ± 9 mV/pK_a unit. These findings suggest that interactions with Lewis acids could be effectively leveraged for rational tuning of the electronic and thermochemical properties of the 5f elements, reminiscent of strategies more commonly employed with 3d transition metals.