Exploring the Oxidation States of Neptunium with Schiff Base Coordination Complexes

A pair of neptunium Schiff base coordination complexes, (NpO2L)-O-VI(MeOH) and (NpL2)-L-VI{H2L = N,N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediaminel, have been synthesized and analyzed by several characterization methods including single-crystal X-ray diffraction, electronic absorption, H-1 NMR, cyclic voltammetry, and theoretical interpretation. Structural analysis reveals that (NpO2L)-O-VI(MeOH) and (NpL2)-L-IV are isomorphous with the previously reported (UO2)-O-IV L(MeOH) and (ML2)-L-IV (M = Pu, Ce, U, Th) complexes, respectively, allowing for a direct comparison across the series. The reduction of (NpO2)-O-VI L-(MeOH) in situ or direct synthesis from a ((NpO2)-O-V)(+) source shows evidence of a pentavalent neptunyl ((NpO2L)-O-V)(x)(n-) species as determined by UV/vis/NIR and H-1 NMR spectroscopy. The synthesis of ((NpO2L)-O-VI)(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 (NpL2)-L-V are similar to those in the (NpO2)-O-VI 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.