Neptunyl Peroxide Chemistry: Synthesis and Spectroscopic Characterization of a Neptunyl Triperoxide Compound, Ca₂[NpO₂(O₂)₃]·9H₂O

Little is known about the crystal chemistry of neptunyl peroxide compounds compared to uranyl peroxide compounds, for which dozens of structures have been described. Uranyl peroxides are formed over a broad range of pH and solution conditions, but neptunyl peroxide chemistry is complicated by the ability of H₂O₂ to act as an oxidizing or reducing agent for Np, depending on the conditions present. The combination of Np­(V) in 1 M HCl, H₂O₂, and CaCl₂ under alkaline conditions leads to the immediate crystallization of a neptunyl triperoxide monomer, Ca₂[NpO₂(O₂)₃]·9H₂O, which is the first Np­(VI)-based peroxide compound to be characterized in the solid state and is isostructural to Ca₂[UO₂(O₂)₃]·9H₂O. The crystal structure reveals bond distances of 1.842(7) Å that are the longest reported to date for nonbridging Np­(VI)-O_yl bonds. Computational studies probe the oxidation state and bond distances of the monomer unit and differences in Raman spectra of the neptunyl and uranyl triperoxide compounds.