Exploring the Oxidation States of Neptunium with Schiff Base Coordination Complexes

A pair of neptunium Schiff base coordination complexes, Np VI O 2 L­(MeOH) and Np IV L 2 {H 2 L = N,N′-bis­[(4,4′-di­ethyl­amino)­salicylidene]-1,2-phenylene­diamine}, have been synthesized and analyzed by several characterization methods including single-crystal X-ray diffraction, electronic absorption, 1H NMR, cyclic voltammetry, and theoretical interpretation. Structural analysis reveals that Np VI O 2 L­(MeOH) and Np IV L 2 are isomorphous with the previously reported U VI O 2 L­(MeOH) and M IV L 2 (M = Pu, Ce, U, Th) complexes, respectively, allowing for a direct comparison across the series. The reduction of Np VI O 2 L­(MeOH) in situ or direct synthesis from a (NpVO2)+ source shows evidence of a pentavalent neptunyl (Np V O 2 L) x n– species as determined by UV/vis/NIR and 1H NMR spectroscopy. The synthesis of (Np V O 2 L) x n– directly from a (NpVO2)+ starting material gives a similar spectrum. Theoretical analysis offers insight into the electronic structure for a better understanding of the bonding patterns and relative stability of the different oxidation states. Computational results show that the Np–L covalent interactions in Np IV L 2 are similar to those in the Np VI O 2 L­(MeOH) complex, indicating that neither the presence of the axial oxo ligands nor the oxidation state significantly modify the nature of the Np–L bonds.