Cation-cation interactions and antiferromagnetism in Na[Np(V)O2(OH)2]:: Synthesis, structure, and magnetic properties

Na[NpO2(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 degrees 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 P2(1)2(1)2(1), a = 5.8904(3) angstrom, b = 7.6297(4) angstrom, c = 8.1540(4) angstrom, 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, [PbUO2(TeO3)(2)], and (UO2)Cl-2(H2O). Each chain extends along the b-axis and is rotated approximately 60 degrees from neighboring chains, thus permitting the bonding of a dioxo ligand on a neptunyl in one chain as an equatorial ligand of a neighboring NpO2+ 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) mu(B) per formula unit.