Calculations 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.

International audienc

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In order to understand the catalytic activity of the actinide complexes L 2 AnCH 3 (An = Ac, Th, Pa, U, Np and Pu; L = Cl, Cp and Cp*) towards the activation of the C-H 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 C-H bond, the activation energies being respectively equal to 10.5, 17.1 and 21.0 kcal/mol for Cl 2 NpCH 3 , Cp 2 NpCH 3 and Cp* 2 UCH 3 respectively whereas the Th(III) complexes exhibit the highest activation energy, 34.9 kcal/mol for Cp* 2 ThCH 3. 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 L 2 AnCH 3 compounds towards CH 4 is analyzed and discussed. More important the 5f actinide orbital contribution, less important is the activation energy.

International audienceInvestigations of the CF bond activation by actinide monocation An+ (An = Ac, Th, Pa, U, Np, Pu and Am) are carried out using relativistic density functional theory (DFT) computations. Originally, the aim of the study is to compare the ability of different actinide ions to break strong bonds particularly in the context of accidental radioactive dissemination. The An+ reaction with the fluorinated hydrocarbon CH3F was selected as a representative system in this context. Unexpectedly, the considered An+ were found to react differently. Via linear transit (LT) and intrinsic reaction coordinate (IRC) calculations, three reaction mechanisms for the CF bond activation, leading to the An–F+ formation, were revealed; the first one, i.e. ‘harpoon’ mechanism which was observed in the case of Pu+, Am+, while the second called ‘insertion-elimination’ mechanism concerned the case of Th+, Pa+, U+ and Np+. DFT computations highlight the particular case of the Ac+ system which presents two different mechanisms according to its spin state: a mechanism qualified as ‘harpoon-like’ for the triplet state and an ‘insertion-elimination’ mechanism for the singlet state. The activation barrier for the fluorine elimination from CH3F is weak for all the studied systems, from 0.9 kcal/mol for Th+ to 8.2 kcal/mol for Am+. Th+ is found as the most effective ion to activate the CF bond and a considerable exergonic character (−81.5 kcal/mol) for this reaction is expected. The performed orbital, population and charge analyses permitted to reveal the role of the actinide 7s, 6d and 5f orbitals and of electron transfers during the reaction