Thiocyanate complexes of uranium in multiple oxidation states: A combined structural, magnetic, spectroscopic, spectroelectrochemical, and theoretical study

A comprehensive study of the complexes A[U(NCS)] (A = Cs, EtN, BuN) and A[UO (NCS)] (A = Cs, EtN) is described, with the crystal structures of [BuN][U(NCS) ]·2MeCN and Cs[UO(NCS) ]·O reported. The magnetic properties of square antiprismatic Cs[U(NCS)] and cubic [Et N][U(NCS)] have been probed by SQUID magnetometry. The geometry has an important impact on the low-temperature magnetic moments: at 2 K, ? = 1.21 ? and 0.53 ?, respectively. Electronic absorption and photoluminescence spectra of the uranium(IV) compounds have been measured. The redox chemistry of [EtN][U(NCS)] 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) which upon back reduction regenerates thiocyanate anions coordinating to UO . NBO calculations agree with the experimental spectra, suggesting that the initial electron loss of [U(NCS)] is delocalized over all NCS ligands. Reduction of the uranyl(VI) complex [EtN][UO(NCS)] to uranyl(V) is accompanied by immediate disproportionation and has only been studied by DFT methods. The bonding in [An(NCS)] (An = Th, U) and [UO(NCS)] 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. © 2014 American Chemical Society.