Tetraalkylammonium uranyl isothiocyanates

Three tetraalkylammonium uranyl isothiocyanates, [(CH ₃) ₄N] ₃UO ₂(NCS) ₅ (1), [(C ₂H ₅) ₄N] ₃UO ₂(NCS) ₅ (2), and [(C ₃H ₇) ₄N] ₃UO ₂(NCS) ₅ (3), have been synthesized from aqueous solution and their structures determined by single-crystal X-ray diffraction. All of the compounds consist of the uranyl cation equatorially coordinated to five N-bound thiocyanate ligands, UO ₂(NCS) ₅ ^3-, and charge-balanced by three tetraalkylammonium cations. Raman spectroscopy data have been collected on compounds 1-3, as well as on solutions of uranyl nitrate with increasing levels of sodium thiocyanate. By tracking the Raman signatures of thiocyanate, the presence of both free and bound thiocyanate is confirmed in solution. The shift in the Raman signal of the uranyl symmetric stretching mode suggests the formation of higher-order uranyl thiocyanate complexes in solution, while the solid-state Raman data support homoleptic isothiocyanate coordination about the uranyl cation. Presented here are the syntheses and crystal structures of 1-3, pertinent Raman spectra, and a discussion regarding the relationship of these isothiocyanates to previously described uranyl halide phases, UO ₂X ₄ ^2-.