Cation−Cation Interactions and Antiferromagnetism in Na[Np(V)O₂(OH)₂]: Synthesis, Structure, and Magnetic Properties

Na[NpO 2 (OH) 2 ] ( 1 ) crystallizes from the hydrothermal reaction of Np(V) and NaOH, in an aqueous solution with a pH of 14, over 3 days at 120 °C. The single-crystal X-ray structure was solved by direct methods and refined via full-matrix least-squares on F 2 . The compound crystallizes in the orthorhombic space group P 2 1 2 1 2 1, a = 5.8904(3) Å, b = 7.6297(4) Å, c = 8.1540(4) Å, Z = 4, with R ( F ) = 1.52%, wR2( F 2 ) = 3.51% obtained using 63 refined parameters and 1400 reflections. The three-dimensional structure is built from (Np(V)O 2 )O(OH) 4 pentagonal bipyramids that are linked through bridging hydroxyl groups to form a one-dimensional chain substructure, analogous to chains observed in the U(VI) phases of moctezumite, [PbUO 2 (TeO 3 ) 2 ], and (UO 2 )Cl 2 (H 2 O). Each chain extends along the b -axis and is rotated approximately 60° from neighboring chains, thus permitting the bonding of a dioxo ligand on a neptunyl in one chain as an equatorial ligand of a neighboring NpO 2 + ion in an adjacent chain. This unusual bonding configuration, known as a cation−cation interaction, fuses the neptunyl chains into a three-dimensional framework that contains small orthogonally intersecting channels, which contain the Na + cations. The magnetic response of 1 as a function of applied field and temperature is consistent with an antiferromagnetic ordering of Np moments below 19.5(5) K. The effective moment determined over the temperature range of 35−300 K is 2.80(15) μ B per formula unit.