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.