Solution and Solid-State Studies on the Halide Binding Affinity of Perfluorophenyl-Armed Uranyl-Salophen Receptors Enhanced by Anion-π Interactions

Interaction with Simple Monopyridinecarboxylic Ligands Revealing Unexpected Structural Types of Uranyl Halides

The inclusion of monopyridinecarboxylic acids into the structures of uranyl halides results in the formation of unexpected structural units. Molecules of picolinic and nicotinic acids acting as bridging ligands favor the formation of unprecedented dinuclear units in two uranyl bromide complexes, which comprise two metal centers in different, tetragonal- and pentagonal-bipyramidal, coordination geometries. Moreover, the different positions of the nitrogen atom in the molecule of nicotinic acid induce significant bending of the heterodimer. The uranyl chloride complex with isonicotinic acid also exhibits a structure containing metal atoms in two unique geometries. The structure consists of cationic and anionic isolated fragments. The anionic part is unprecedented and represents the first example of a 1:3 uranyl halide unit with a tetragonal bipyramid surrounding the central atom.

Diversity and uniformity in anion-π complexes of thiocyanate with aromatic, olefinic and quinoidal π-acceptors

Despite the progress in the study of anion-π interactions, there are still inconsistencies in the use of this term and the experimental data about factors affecting the strength of such bonding are limited. To shed light on these issues, we explored supramolecular associations between NCS^- anions and a series of aromatic, olefinic or quinoidal π-acceptors. Combined experimental and computational studies revealed that all these complexes were formed by an attraction of the anion to the face of the π-system, and the arrangements of thiocyanate followed the areas of the most positive potentials on the surfaces of the π-acceptors. The stabilities of the complexes increased with the π-acceptor strength (reflected by their reduction potentials), and were essentially independent of the magnitudes of the maximum electrostatic potentials on their surfaces. The complexes showed intense absorption bands in the UV-Vis range, and the energies of these bands were correlated with the difference of the redox potentials of the anions and π-acceptors. Such features, as well as results of atoms-in-molecules and non-covalent index analyses suggested that besides electrostatics, molecular orbital interactions play a substantial role in the formation of these complexes. The unified trends in variations of the characteristics of the complexes between thiocyanate and a variety of π-acceptors point to their common nature. To embrace diversity and uniformity of the anion-π associates, we suggest (following the halogen bond's definition) that anion-π bonding occurs when there is evidence of a net attraction between the anions and the face of the electrophilic π-system.

Anion-π Complexes of Halides with p-Benzoquinones: Structures, Thermodynamics, and Criteria of Charge Transfer to Electron Transfer Transition

Interchange of complex formation and electron-transfer reactions between halide anions and p-benzoquinones were established via UV-vis spectral and X-ray structural measurements and computational analysis. Solution-phase interaction of the p-benzoquinone acceptors with Cl^-, Br^-, or I^- donors led to the formation of anion-π complexes showing strong absorption bands in the UV-vis range. Formation constants and calculated interaction energies of these complexes increased, and donor/acceptor separations decreased with increasing reduction potentials of p-benzoquinones. Mulliken correlation and NBO analysis indicated a charge-transfer nature of these anion-π associates. Most notably, the increase of the acceptor strength led to a transition between the formation of the persistent anion-π complexes and electron-transfer reactions. Thermodynamic analysis accounted for the experimental observations of anion radicals and trihalide anions in solutions of p-benzoquinones with iodide or (for the strongest acceptor) bromide donors. Kinetics of these processes indicated that anion-π complexes represent critical intermediates of the redox reactions. In contrast to Cl^-, Br^-, or I^- anions, interaction of p-benzoquinones with F^- anions led to the formation of σ-complexes, and the appearance of anion radicals in such systems was related to the follow-up reactions of these complexes.

X-Ray crystallographic and computational study on uranyl-salophen complexes bearing nitro groups

In the solid state, salophen-UO₂ complexes bearing one, two, or three NO₂ groups lack the pronounced ligand curvature that represents a structural hallmark for this class of compounds. A detailed structural study based on single-crystal X-ray crystallography and computational methods, comprising molecular dynamics, gas-phase Hartree Fock, and DFT calculations, was carried out to investigate the coordination properties of the uranyl cation.