Hydrothermal Synthesis, Structure, and Magnetic Properties of the Mixed-Valent Np(IV)/Np(V) Selenite Np(NpO₂)₂(SeO₃)₃

The reaction of NpO2 with SeO2 in the presence of CsCl at 180 °C results in the formation of Np(NpO2)2(SeO3)3 (1). The structure of 1 consists of three crystallographically unique Np centers with three different coordination environments in two different oxidation states. Np(1) is found in a neptunyl(V), ONpO+, unit that is further ligated in the equatorial plane by three chelating SeO32- anions to create a hexagonal bipyramidal NpO8 unit. A second neptunyl(V) cation also occurs for Np(2); it is bound by four bridging selenite anions and by the oxo atom from the Np(1) neptunyl cation to form a pentagonal bipyramidal, NpO7, unit. The third neptunium center, Np(3), which contains Np(IV), is found in a distorted NpO8 dodecahedron. Np(3) is bound by five bridging selenite anions and by three neptunyl units via cation−cation interactions. The NpO7 pentagonal bipyramids and NpO8 hexagonal bipyramids share both corners and edges. Both of these polyhedra share corners via cation−cation interactions with the NpO8 dodecahedra creating a three-dimensional structure with small channels that house the stereochemically active lone pair of electrons on the selenite anions. Magnetic susceptibility data follow Curie−Weiss behavior over the entire temperature range measured (5 ≤ T ≤ 320 K). The effective moment, μeff = 2.28 μB, which represents an average over the three crystallographically inequivalent Np atoms, is within the expected range of values. There is no evidence of long-range ordering of the Np moments at temperatures down to 5 K, consistent with the negligible Weiss constant determined from fitting the susceptibility data. Crystallographic data: 1, orthorhombic, space group Pbca, a = 10.6216(5), b = 11.9695(6), and c = 17.8084(8) Å and Z = 8 (T = 193 K).