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

Abstract

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.

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

“…The peroxide mode frequency depends on the coordination to uranyl polyhedra and ranges from 904 to 822 cm −1 [137,138]. In the case of LiK 3 [(UO 2 ) 4 (O 2 ) 2 (C 10 H 12 O 8 N 2 ) 2 (H 2 O) 2 ]·18 H 2 O, two partially resolved vibrational features are located near 830 cm −1 [139]. Speciation assignment of these bands was achieved using both isotopic labeling and DFT calculations.…”

Self‐Assembly of Uranyl–Peroxide Nanocapsules in Basic Peroxidic Environments

“…Among these building blocks, a dimer containing two uranyl ions bridged by a bidentate peroxo ligand is central for understanding the initiation of the growth mechanism of the capsules (Figure ). Furthermore, the recent use of organic ligands such as picolinate or pyridine‐2,6‐dicarboxylate allowed for the isolation of several uranyl–peroxide dimers . Density functional theory (DFT) and complete active space self‐consistent field (CASSCF) calculations have been used to study the electronic structure and bonding of uranyl–peroxide dimers .…”

“…Furthermore, the recent use of organic ligands such as picolinate or pyridine‐2,6‐dicarboxylate allowed for the isolation of several uranyl–peroxide dimers . Density functional theory (DFT) and complete active space self‐consistent field (CASSCF) calculations have been used to study the electronic structure and bonding of uranyl–peroxide dimers . While uranyl–peroxide bonding in these species is mainly ionic, some covalent contributions are observed.…”

“…These orbital interactions are responsible for the slight favoring of butterfly bending in the U‐O 2 ‐U moiety, effectively minimizing the repulsion between adjacent peroxo moieties in cyclic facets and enhancing charge delocalization. However, the U‐O 2 ‐U dihedral angle is highly pliable and bending the angle from planar to 130° gains only a few kcal mol −1 . Furthermore, the alkali cations used as counterions during the synthesis of uranyl–peroxide nanocapsules play a critical role in solution.…”

“…During the past few years, several computational studies on uranyl–peroxide nanocapsules have been published; however, the reactivity corresponding to the growth of these species and the role of the alkali counterions is not yet well understood . Moreover, in the growth of transition‐metal‐containing polyoxometalates, mechanistic studies are rare.…”