Exploring crown-ether functionalization on the stabilization of hexavalent neptunium

Abstract

Crown-ether molecules are used in radiochemical separations due to their high selectivity for a range of metal cations. Previous investigations regarding the interactions of 18-crown-6 (18C6) with ²³⁷Np suggested the formation of a Np(V) inclusion complex, but also reported rapid reduction of Np(VI) to Np(V) in the presence of the ether molecule. Herein, we investigate the impact of crown ether functionalization by exploring the Np(V) and Np(VI) dicyclohexano-18-crown-6 (DCH-18C6) systems. Two [X(DCH-18C6)]₂[Np(VI)O₂Cl₄] compounds (X = K (1) and Na (2)) were crystallized and characterized by single crystal X-ray diffraction and Raman spectroscopy. Additional studies of Np(VI), Np(V), and Np(V)/Np(VI) in solution indicated redox stability in the presence of functionalized crowns and preferential crystallization of Np(VI) DCH-18C6 solids. These results indicate that functionalization of the crown can lead to higher resistance to radiolysis and increased stability of the Np(VI) oxidation state in solution.