Thiocyanate Complexes of Uranium in Multiple Oxidation States: A Combined Structural, Magnetic, Spectroscopic, Spectroelectrochemical, and Theoretical Study.
A comprehensive study of the complexes A(4)[U(NCS)(8)] (A = Cs, Et4N, (Bu4N)-Bu-n) and A(3)[UO2(NCS)(5)] (A = Cs, Et4N) is described, with the crystal structures of [(Bu4N)-Bu-n](4)[U(NCS)(8)]center dot 2MeCN and Cs-3[UO2(NCS)(5)]center dot O-0.5 reported. The magnetic properties of square antiprismatic Cs-4[U(NCS)(8)] and cubic [Et4N](4)[MU(NCS)(8)] have been probed by SQUID magnetometry. The geometry has an important impact on the low-temperature magnetic moments: at 2 K, mu(eff) = 1.21 mu(B) and 0.53 mu(B), 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.