Exploring the Oxidation States of Neptunium with Schiff Base Coordination Complexes.

A pair of neptunium Schiff base coordination complexes, NpVIO2L(MeOH) and NpIVL2 {H2L = 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 NpVIO2L(MeOH) and NpIVL2 are isomorphous with the previously reported UVIO2L(MeOH) and MIVL2 (M = Pu, Ce, U, Th) complexes, respectively, allowing for a direct comparison across the series. The reduction of NpVIO2L(MeOH) in situ or direct synthesis from a (NpVO2)+ source shows evidence of a pentavalent neptunyl (NpVO2L)xn- species as determined by UV/vis/NIR and 1H NMR spectroscopy. The synthesis of (NpVO2L)xn- 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 NpIVL2 are similar to those in the NpVIO2L(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.