Cation Templating and Electronic Structure Effects in Uranyl Cage Clusters Probed by the Isolation of Peroxide-Bridged Uranyl Dimers

The self-assembly of uranyl peroxide
polyhedra into a rich family of nanoscale cage clusters is thought
to be favored by cation templating effects and the pliability of the
intrinsically bent U–O2–U dihedral angle.
Herein, the importance of ligand and cationic effects on the U–O2–U dihedral angle were explored by studying a family
of peroxide-bridged dimers of uranyl polyhedra. Four chemically distinct
peroxide-bridged uranyl dimers were isolated that contain combinations
of pyridine-2,6-dicarboxylate, picolinate, acetate, and oxalate as
coordinating ligands. These dimers were synthesized with a variety
of counterions, resulting in the crystallographic characterization
of 15 different uranyl dimer compounds containing 17 symmetrically
distinct dimers. Eleven of the dimers have U–O2–U
dihedral angles in the expected range from 134.0 to 156.3°; however,
six have 180° U–O2–U dihedral angles,
the first time this has been observed for peroxide-bridged uranyl
dimers. The influence of crystal packing, countercation linkages,
and π–π stacking impact the dihedral angle. Density
functional theory calculations indicate that the ligand does not alter
the electronic structure of these systems and that the U–O2–U bridge is highly pliable. Less than 3 kcal·mol–1 is required to bend the U–O2–U
bridge from its minimum energy configuration to a dihedral angle of
180°. These results suggest that the energetic advantage of bending
the U–O2–U dihedral angle of a peroxide-bridged
uranyl dimer is at most a modest factor in favor of cage cluster formation.
The role of counterions in stabilizing the formation of rings of uranyl
ions, and ultimately their assembly into clusters, is at least as
important as the energetic advantage of a bent U–O2–U interaction.