Synthesis, structure determination, and infrared spectroscopy of (NpO2)2(SO4)(H2O)4: Prevalence of cation–cation interactions and cationic nets in neptunyl sulfate compounds

The compound (NpO2)2(SO4)(H2O)4 was synthesized by evaporation of a Np5+ sulfate solution. The crystal structure was determined using single crystal X-ray diffraction and refined to an R1=0.0310. (NpO2)2(SO4)(H2O)4 crystallizes in triclinic space group P-1, a=8.1102(7) Å, b=8.7506(7) Å, c=16.234(1) Å, α=90.242(2)°, β=92.855(2)°, γ=113.067(2)°, V=1058.3(2) Å3, and Z=2. The structure contains neptunyl pentagonal bipyramids that share vertices through cation–cation interactions to form a sheet or cationic net. The sheet is decorated on each side by vertex sharing with sulfate tetrahedra, and adjacent sheets are linked together through hydrogen bonding. A graphical representation of (NpO2)2(SO4)(H2O)4 was constructed to facilitate the structural comparison to similar Np5+ compounds. The prevalence of the cationic nets in neptunyl sulfate compounds related to the overall stability of the structure is also discussed.