Accessing five oxidation states of uranium in a retained ligand framework

Abstract

Understanding and exploiting the redox properties of uranium is of great importance because uranium has a wide range of possible oxidation states and holds great potential for small molecule activation and catalysis. However, it remains challenging to stabilise both low and high-valent uranium ions in a preserved ligand environment. Herein we report the synthesis and characterisation of a series of uranium(II-VI) complexes supported by a tripodal tris(amido)arene ligand. In addition, one- or two-electron redox transformations could be achieved with these compounds. Moreover, combined experimental and theoretical studies unveiled that the ambiphilic uranium-arene interactions are the key to balance the stabilisation of low and high-valent uranium, with the anchoring arene acting as a δ acceptor or a π donor. Our results reinforce the design strategy to incorporate metal-arene interactions in stabilising multiple oxidation states, and open up new avenues to explore the redox chemistry of uranium.

Fig. 1. Selected chelating ligands previously reported and synthesis and molecular structures in this work.

a Selected chelating ligands capable of supporting multiple oxidation states of uranium. b Synthesis and molecular structures of uranium(II–VI) complexes 1–5 supported by [^AdTPBN₃]^3–. The single crystal structures are shown in thermal ellipsoids at 50% probability. All hydrogen atoms, counterions, and lattice solvents are omitted for clarity. Atom (colour): U (green), N (blue), O (red), C of the anchoring arene (pink), C of others (grey).

Fig. 2. Structural characterization and electrochemistry.

a Superpositions of molecular structures of 1‒5: C of the anchoring arene (magenta), O (pink), U(II) (red), U(III) (green), U(IV) (yellow), U(V) (sky blue), U(VI) (black). b Cyclic voltammograms of 1 (top) and 3 (bottom) at a scan rate of 200 mV/s in [ⁿBu₄N][PF₆]/THF, with internal standard Fc^*+/Fc^* (Fc^* = decamethylferrocene) labelled with *.

Fig. 3. Redox transformations for 1–7.

Black arrows for one-electron processes and red arrows for two-electron processes.

Fig. 4. Spectroscopic and magnetic studies and theoretical calculations.

a UV–Vis–NIR spectra of 1–7 with the inset showing the NIR region. b Magnetic moments as a function of temperature (2–298 K) for uranium(II–V) complexes. c X-band EPR spectra of 1 in toluene and 2 in THF at 10 K. d Kohn-Sham orbitals (isosurface = 0.05) of the four SOMOs of 2; hydrogens were omitted for clarity.