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

Relativistic zero order regular approximation (ZORA) density functional theory computations, coupled with the conductor-like screening model for solvation effects, are used to investigate the redox properties of a series of biscyclopentadienyl pentavalent uranium(V) complexes Cp2U(═N-Ar)X (Ar = 2,6-Me2-C6H3; X = OTf, C6F5, SPh, C═CPh, NPh2, Ph, Me, OPh, N(TMS)2, N═CPh2). Regarding the UV/UIV and UVI/UV couple systems, a linear correlation (R2 ∼ 0.99) is obtained at the ZORA/BP86/TZP level, between the calculated ionization energies and the measured experimental E1/2 half-wave oxidation potentials (UVI/UV) and between the electron affinities and the reduction potentials E1/2 (UV/UIV). The study brings to light the importance of solvation effects that are needed in order to achieve a good agreement between the theory and experiment. Introducing spin-orbit coupling corrections slightly improves this agreement. Both the singly occupied molecular orbital and the lowest unoccupied molecular orbital of the neutral UV complexes exhibit a majority 5f orbital character. The frontier molecular orbitals show a substantial ancillary ligand X σ and/or π character that drives the redox properties. Moreover, our investigations allow estimating the redox potentials of the X = Ph, X = C6F5, and N(TMS)2 UV complexes for which no experimental electrochemical data exist.