Redox Properties of Monocyclooctatetraenyl Uranium(IV) and (V) Complexes: Experimental and Relativistic DFT Studies

The redox properties of a series of mono­(cyclooctatetraenyl) uranium­(IV) and (V) complexes [(Cot)­(Cp)­U­(NEt₂)₂] (<b>1</b>) (Cot = η-C₈H₈, Cp = η-C₅H₅) [(Cot)­U­(OⁱPr)₃] (<b>2</b>), [(Cot)­U­(NMe₂)₃] (<b>3</b>), [(Cot)­U­(N­{SiMe₃}₂)₂] (<b>4</b>), [(Cot)­U­(NEt₂)₃]^<i>–</i> (<b>5</b>), and the cyclopentadienyl compound [(C₅Me₅)­U­(NEt₂)₃] (<b>6</b>) have been investigated using cyclic voltammetry and relativistic density functional theory (DFT). Electrochemical measurements of half-wave potentials in tetrahydrofuran were carried out under strictly anaerobic conditions. The calculations of ionization energies (IE) and electron affinities (EA) of these compounds, related to the U^III/U^IV, U^IV/U^V, and U^V/U^VI redox systems, revealed a very good linear correlation (<i>r</i>² = 0.99) between calculated ionization energies at the ZORA/BP86/TZP level and the measured <i>E</i>_1/2 half-wave oxidation potentials. A similar good linear correlation between the computed electron affinities and the electrochemical reduction potentials (<i>r</i>² = 0.98) was obtained. It was found to be crucial to take into account the solvent effect as well as the spin–orbit coupling. The DFT computations permitted the estimation of the oxidation potential of <b>3</b> as well as the reduction potential of <b>5</b> for which the electrochemical measurement failed. An explanation of the different redox behaviors of the complexes has been given, considering the donating ability of the ligands and the nature of their frontier molecular orbitals (MOs). The molecular orbital analysis underlines the determining role of the metal 5f orbitals, whereas a good correlation is observed between the Nalewajski–Mrozek bond indices and the structural variations related to the redox processes