Fate of Oxidation States at Actinide Centers in Redox-Active Ligand Systems Governed by Energy Levels of 5 f Orbitals

Abstract

We report the formation of a Np^IV complex from the complexation of Np^VI O₂ ²⁺ with the redox-active ligand tBu-pdiop^2- =2,6-bis[N-(3,5-di-tert-butyl-2-hydroxyphenyl)iminomethyl]pyridine. To the best of our knowledge, this is the first example of the direct complexation-induced chemical reduction of Np^VI O₂ ²⁺ to Np^IV . In contrast, the complexation of U^VI O₂ ²⁺ with tBu-pdiop^2- did not induce the reduction of U^VI O₂ ²⁺ , not even after the two-electron electrochemical reduction of [U^VI O₂ (tBu-pdiop)]. This contrast between the Np and U systems may be ascribed to the decrease of the energy of the 5 f orbitals in Np compared to those in U. The present findings indicate that the redox chemistry between U^VI O₂ ²⁺ and Np^VI O₂ ²⁺ should be clearly differentiated in redox-active ligand systems.

Keywords: actinide; crystal structures; ligands; neptunium; reduction.