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

We report the formation of a Np(IV) complex from the complexation of Np(VI)O22+ with the redox-active ligand tBu-pdiop2-=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)O22+ to Np(IV). In contrast, the complexation of U(VI)O22+ with tBu-pdiop2- did not induce the reduction of U(VI)O22+, 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 5f orbitals in Np compared to those in U. The present findings indicate that the redox chemistry between U(VI)O22+ and Np(VI)O22+ should be clearly differentiated in redox-active ligand systems.