Synthesis, Isolation, and Study of Heterobimetallic Uranyl Crown Ether Complexes

Although crown ethers can selectively bind many metal cations, little is known regarding the solution properties of crown ether complexes of the uranyl dication, UO₂²⁺. Here, the synthesis and characterization of isolable complexes in which the uranyl dication is bound in an 18-crown-6-like moiety are reported. A tailored macrocyclic ligand, templated with a Pt(II) center, captures UO₂²⁺ in the crown moiety, as demonstrated by results from single-crystal X-ray diffraction analysis. The U(V) oxidation state becomes accessible at a quite positive potential (E_1/2) of -0.18 V vs Fc^+/0 upon complexation, representing the most positive U^VI/U^V potential yet reported for the UO₂ core. Isolation and characterization of the U(V) form of the crown complex are also reported here; there are no prior reports of reduced uranyl crown ether complexes, but U(V) is clearly stabilized by crown chelation. Joint computational studies show that the electronic structure of the U(V) form results in significant weakening of U-O_oxo bonding despite the quite positive reduction potential at which this species can be accessed, underscoring that crown-ligated uranyl species could demonstrate unique reactivity under only modestly reducing conditions.