Uranyl Peroxide Oxalate Cage and Core–Shell Clusters Containing 50 and 120 Uranyl Ions

Cage clusters built from uranyl hexagonal bipyramids
and oxalate
ligands crystallize from slightly acidic aqueous solution under ambient
conditions, facilitating structure analysis. Each cluster contains
uranyl ions coordinated by peroxo ligands in a bidentate configuration.
Uranyl ions are bridged by shared peroxo ligands, oxalate ligands,
or through hydroxyl groups. U50Ox20 contains
50 uranyl ions and 20 oxalate groups and is a topological derivative
of the U50 cage cluster that has a fullerene topology.
U120Ox90 contains 120 uranyl ions and 90 oxalate
groups and is the largest and highest mass cluster containing uranyl
ions that has been reported. It has a core–shell structure,
in which the inner shell (core) consists of a cluster of 60 uranyl
ions and 30 oxalate groups, identical to U60Ox30, with a fullerene topology. The outer shell contains 12 identical
units that each consist of five uranyl hexagonal bipyramids that are
linked to form a ring (topological pentagon), with each uranyl ion
also coordinated by a side-on nonbridging oxalate group. The five-membered
rings of the inner and outer shells (the topological pentagons) are
in correspondence and are linked through K cations. The inner shell
topology has therefore templated the location of the outer shell rings,
and the K counterions assume a structure-directing role. Small-angle
X-ray scattering data demonstrated U50Ox20 remains
intact in aqueous solution upon dissolution. In the case of clusters
of U120Ox90, the scattering data for dissolved
crystals indicates the U60Ox30 core persists
in solution, although the outer rings of uranyl bipyramids contained
in the U120Ox90 core–shell cluster appear
to detach from the cluster when crystals are dissolved in water.