Synthesis and characterization of a uranyl(VI) complex with 2,6-pyridine-bis(methylaminophenolato) and its ligand-centred aerobic oxidation mechanism to a diimino derivative

Design of Promising Uranyl(VI) Complexes Thin Films with Potential Applications in Molecular Electronics

In this work, it is proposed the development of organic semiconductors (OS) based on uranyl(VI) complexes. The above by means of the synthesis and the characterization of the complexes by Infrared spectroscopy, Nuclear magnetic resonance spectroscopy, mass spectrometry, and X-ray diffraction. Films of these complexes were deposited and subsequently, topographic and structural characterization was carried out by Scanning Electron Microscopy, X-ray diffraction, and Atomic Force Microscopy. Additionally, the nanomechanical evaluation was performed to know the stiffness of uranyl films using their modulus of elasticity. Also, the optical characterization took place in the devices and their bandgap value ranges between 2.40 and 2.93 eV being the minor for the film of the uranyl complex with the N on pyridine in position 4 (2 c). Finally, the electrical behavior of the uranyl(VI) films was evaluated, and important differences were obtained: the uranyl complex with the N on pyridine in position 2 (2 a) film is not influenced by changes in lighting and its current density is in the order of 10-3 A/cm2. The film with uranyl complex with the N on pyridine in position 3 (2 b) and 2 c presents a greater current flow under lighting conditions and two orders of magnitude larger than in film 2 a. In these films 2 b and 2 c, ohmic behavior occurs at low voltages, while at high voltages the charge transport changes to space-charge limited current behavior.

Fate of Oxidation States at Actinide Centers in Redox-Active Ligand Systems Governed by Energy Levels of 5 f Orbitals

We report the formation of a NpIV complex from the complexation of NpVI O2 2+ with the redox-active ligand tBu-pdiop2- =2,6-bis[N-(3,5-di-tert-butyl-2-hydroxyphenyl)iminomethyl]pyridine. To the best of our knowledge, this is the first example of the direct complexation-induced chemical reduction of NpVI O2 2+ to NpIV . In contrast, the complexation of UVI O2 2+ with tBu-pdiop2- did not induce the reduction of UVI O2 2+ , not even after the two-electron electrochemical reduction of [UVI O2 (tBu-pdiop)]. This contrast between the Np and U systems may be ascribed to the decrease of the energy of the 5 f orbitals in Np compared to those in U. The present findings indicate that the redox chemistry between UVI O2 2+ and NpVI O2 2+ should be clearly differentiated in redox-active ligand systems.

Effects of coordinating heteroatoms on molecular structure, thermodynamic stability and redox behavior of uranyl(vi) complexes with pentadentate Schiff-base ligands

Uranyl(vi) complexes with pentadentate N₃O₂-, N₂O₃- and N₂O₂S₁-donating Schiff base ligands, tBu,MeO–saldien–X²⁻ (X = NH, O and S), were synthesized and thoroughly characterized by ¹H NMR, IR, elemental analysis, and single crystal X-ray diffraction. The crystal structures of UO₂(tBu,MeO–saldien–X) showed that the U–X bond strength follows U–O ≈ U–NH > U–S. Conditional stability constants (β_X) of UO₂(tBu,MeO–saldien–X) in ethanol were investigated to understand the effect of X on thermodynamic stability. The log β_X decrease in the order of UO₂(tBu,MeO–saldien–NH) (log β_NH = 10) > UO₂(tBu,MeO–saldien–O) (log β_O = 7.24) > UO₂(tBu,MeO–saldien–S) (log β_S = 5.2). This trend cannot be explained only by Pearson's Hard and Soft Acids and Bases (HSAB) principle, but rather follows the order of basicity of X. Theoretical calculations of UO₂(tBu,MeO–saldien–X) suggested that the ionic character of U–X bonds decreases in the order of U–NH > U–O > U–S, while the covalency increases in the order U–O < U–NH < U–S. Redox potentials of all UO₂(tBu,MeO–saldien–X) in DMSO were similar to each other regardless of the difference in X. Spectroelectrochemical measurements and DFT calculations revealed that the center U⁶⁺ of each UO₂(tBu,MeO–saldien–X) undergoes one-electron reduction to afford the corresponding uranyl(v) complex. Consequently, the difference in X of UO₂(tBu,MeO–saldien–X) affects the coordination of tBu,MeO–saldien–X²⁻ with UO₂²⁺. However, the HSAB principle is not always prominent, but the Lewis basicity and balance between ionic and covalent characters of the U–X interactions are more relevant to determine the bond strengths.

The U–X bond strength and thermodynamic stability of uranyl(vi) complexes with pentadentate N₂O₂X₁-donating ligands (X = NH, O, S) are affected by the difference in X. In contrast, the X atom does not largely affect the redox behavior of the complexes.