Structural and Spectroscopic Trends in Actinyl Iodates of Uranium, Neptunium, and Plutonium

Two neptunyl(VI) iodates, NpO2(IO3)2(H2O) (1) and NpO2(IO3)2·H2O (2), have been prepared from the aqueous reactions of Np(V) in HCl with KIO4 or H5IO6 at 180 °C and have been characterized by single crystal X-ray diffraction and Raman spectroscopy. Both compounds consist of two-dimensional arrangements of pentagonal bipyramidal [NpO7] polyhedra with axial neptunyl, NpO2 2+, dioxocations. In 1, the neptunium centers are bound in the equatorial plane by four bridging iodate anions and one terminal water molecule. The iodate anions link the [NpO7] units into corrugated sheets that interact with one another through intermolecular IO3 -···IO3 - interactions as also observed in UO2(IO3)2(H2O). Compound 2 is isostructural with the recently reported PuO2(IO3)2·H2O, where oxygen atoms from bridging iodate anions occupy the five equatorial sites around the neptunyl moieties. The iodate anions occur as both μ2- and μ3-units and link the neptunyl polyhedra into sheets. Both types of iodate anions have their stereochemically active lone-pair of electrons aligned on one side of each layer creating a polar structure. Raman spectra of 1, UO2(IO3)2(H2O), and PuO2(IO3)2·H2O show a sequential shift of the ν1(AnO2 2+) stretch to lower wavenumber as the atomic number of the actinide is increased. Crystallographic data: 1, orthorhombic, space group Pcan, a = 7.684(2) Å, b = 8.450(2) Å, c = 12.493(3) Å, Z = 4; 2, orthorhombic, space group Pna21, a = 7.314(1) Å, b = 11.631(2) Å, c = 9.449(2) Å, Z = 4.