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 (E1/2) of
−0.18 V vs Fc+/0 upon complexation, representing
the most positive UVI/UV potential yet reported
for the UO2n+ 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–Ooxo 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.