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

Abstract

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.

How the Ancillary Ligand X Drives the Redox Properties of Biscyclopentadienyl Pentavalent Uranium Cp2U(═N–Ar)X Complexes

“…The U­[5d] valence space of the heavy element includes the 5f/6s/6p/6d/7s/7p shells (14 valence electrons). Several studies have shown that the ZORA/BP86/TZP approach reproduces the experimental geometries and ground-state properties of f-element compounds with a satisfying accuracy. − ,, In our case, we carried out first the full geometry optimizations of the species under consideration, in the gas phase, at the spin unrestricted level. Next, the geometries were re-optimized in the THF solvent using the COSMO model.…”

“…For instance, it is likely that the more pronounced covalent character of the actinide−ligand bonds than lanthanide−ligand ones, involving the 5f electrons will play a role on their electrochemical properties. 4,5,17,18,22 In view of the rich diversity of organo−uranium complexes and the importance of the redox chemistry in the reactivity processes, 17,18,27,30,35,61,62 the aim of this work is to investigate the redox properties not only to get access theoretically to the electrochemical potentials but also to corelate these properties to the ligand's nature. The target systems are the series of pentavalent bis(cyclopentadienyl) imido-uranium(V) Cp 2 U( N−Ar)X complexes derived from [(C 5 Me 5 ) 2 U V (N−Ar)(X)] (Ar = 2,6-i Pr 2 -C 6 H 3; X = OTf, SPh, NPh 2 , OPh, Me, Ph, C CPh, NCPh 2 ) complexes (Figure 1), synthetized by the Kiplinger's group, 38,39 for which U V /U IV and U VI /U V redox (E 1/2 ) half-wave potentials have been measured for almost all of them.…”