Redox properties of biscyclopentadienyl uranium(V) imido-halide complexes: a relativistic DFT study

International audienceCalculations of ionization energies (IE) and electron affinities (EA) of a series of biscyclopentadienyl imido-halide uranium(V) complexes Cp*2U(=N-2,6-(i)Pr2-C6H3)(X) with X = F, Cl, Br, and I, related to the U(IV)/U(V) and U(V)/U(VI) redox systems, were carried out, for the first time, using density functional theory (DFT) in the framework of the relativistic zeroth order regular approximation (ZORA) coupled with the conductor-like screening model (COSMO) solvation approach. A very good linear correlation (R(2) = 0.993) was obtained, between calculated ionization energies at the ZORA/BP86/TZP level, and the experimental half-wave oxidation potentials E1/2. A similar linear correlation between the computed electron affinities and the electrochemical reduction U(IV)/U(III) potentials (R(2) = 0.996) is obtained. The importance of solvent effects and of spin-orbit coupling is definitively confirmed. The molecular orbital analysis underlines the crucial role played by the 5f orbitals of the central metal whereas the Nalewajski-Mrozek (N-M) bond indices explain well the bond distances variations following the redox processes. The IE variation of the complexes, i.e., IE(F) < IE(Cl) < IE(Br) < IE(I) is also well rationalized considering the frontier MO diagrams of these species. Finally, this work confirms the relevance of the Hirshfeld charges analysis which bring to light an excellent linear correlation (R(2) = 0.999) between the variations of the uranium charges and E1/2 in the reduction process of the U(V) species

DFT investigation of methane metathesis with L2AnCH3 actinide complexes catalysts (L = Cl, Cp, Cp*; An = Ac, Th, Pa, U, Np, Pu)

In order to understand the catalytic activity of the actinide complexes L2AnCH3 (An = Ac, Th, Pa, U, Np and Pu; L = Cl, Cp and Cp∗) towards the activation of the CH bond of methane, relativistic ZORA/DFT investigations have been carried out. The results obtained from Linear Transit (LT) and Intrinsic Reaction Coordinate (IRC) calculations show that the mechanism involved in these reactions starts with a proton transfer from methane to the methyl group of the complex leading to the formation of a four center transition state characteristic of a bond metathesis process. The U(III) and Np(III) complexes exhibit a high ability to activate the methane CH bond, the activation energies being respectively equal to 10.5, 17.1 and 21.0 kcal/mol for Cl2NpCH3, Cp2NpCH3 and Cp∗2UCH3 respectively whereas the Th(III) complexes exhibit the highest activation energy, 34.9 kcal/mol for Cp∗2ThCH3. Since the initial step of the reaction is viewed as a proton transfer, the analysis of the charges evolution and frontier molecular orbitals of the complexes and the transition states, shows that a facile polarization of the bonds involved in the reaction has the effect of reducing the activation energy. The role of the metallic 5f orbitals in the reactivity of the L2AnCH3 compounds towards CH4 is analyzed and discussed. More important the 5f actinide orbital contribution, less important is the activation energy