Thiocyanate Complexes of Uranium in Multiple Oxidation States: A Combined Structural, Magnetic, Spectroscopic, Spectroelectrochemical, and Theoretical Study

A comprehensive
study of the complexes A4[U­(NCS)8] (A = Cs,
Et4N, nBu4N) and A3[UO2(NCS)5] (A = Cs, Et4N) is described,
with the crystal structures of [nBu4N]4[U­(NCS)8]·2MeCN and Cs3[UO2(NCS)5]·O0.5 reported. The magnetic properties
of square antiprismatic Cs4[U­(NCS)8] and cubic
[Et4N]4[U­(NCS)8] have been probed
by SQUID magnetometry. The geometry has an important impact on the
low-temperature magnetic moments: at 2 K, μeff =
1.21 μB and 0.53 μB, respectively.
Electronic absorption and photoluminescence spectra of the uranium­(IV)
compounds have been measured. The redox chemistry of [Et4N]4[U­(NCS)8] has been explored using IR and
UV–vis spectroelectrochemical methods. Reversible 1-electron
oxidation of one of the coordinated thiocyanate ligands occurs at
+0.22 V vs Fc/Fc+, followed by an irreversible oxidation
to form dithiocyanogen (NCS)2 which upon back reduction
regenerates thiocyanate anions coordinating to UO22+. NBO calculations agree with the experimental spectra, suggesting
that the initial electron loss of [U­(NCS)8]4– is delocalized over all NCS– ligands. Reduction
of the uranyl­(VI) complex [Et4N]3[UO2(NCS)5] to uranyl­(V) is accompanied by immediate disproportionation
and has only been studied by DFT methods. The bonding in [An­(NCS)8]4– (An = Th, U) and [UO2(NCS)5]3– has been explored by a combination of
DFT and QTAIM analysis, and the U–N bonds are predominantly
ionic, with the uranyl­(V) species more ionic that the uranyl­(VI) ion.
Additionally, the U­(IV)–NCS ion is more ionic than what was
found for U­(IV)–Cl complexes.