Three-Dimensional Network of Cation–Cation-Bound Neptunyl(V) Squares: Synthesis and in Situ Raman Spectroscopy Studies

Cation–cation
interactions (CCIs) are an essential feature of actinyl chemistry,
particularly neptunyl­(V). To better understand the formation mechanisms
of CCIs, the crystallization process of Np­(V) CCI compounds has been
explored during the evaporation of acidic Np­(V) stock solutions using
X-ray diffraction and both ex situ and in situ Raman spectroscopy.
At least four Np solid products have been isolated from evaporation
of the same Np­(V) acidic solution. In situ evaporation using a continuous
wave laser (532 nm) as a local heat source produced similar solid
products to ex situ experiments with matching Raman signatures. The
formation of these products is highly dependent on the evaporation
conditions. Slower evaporation appears to favor the formation of a
new neptunyl­(V) compound, (NpO2)­Cl­(H2O)2 (1), over other solid products. The structure
of 1 features a three-dimensional network of NpO2+ cations, where neighboring Np­(V) ions are only
connected to each other through CCIs in a square arrangement. The
ONpO stretching region shows similar Raman bands in
both the solids and solution suggesting that CCIs between Np­(V) cations
exist prior to crystallization. These results provide new insight
into the formation mechanism of Np­(V) CCI compounds from solutions.