Tetraalkylammonium Uranyl Isothiocyanates

Three tetraalkylammonium uranyl isothiocyanates, [(CH3)4N]3UO2(NCS)5 (1), [(C2H5)4N]3UO2(NCS)5 (2), and [(C3H7)4N]3UO2(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, UO2(NCS)53–, 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, UO2X42–.