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

Abstract

A comprehensive study of the complexes A4[U(NCS)8] (A = Cs, Et4N, (n)Bu4N) and A3[UO2(NCS)5] (A = Cs, Et4N) is described, with the crystal structures of [(n)Bu4N]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 UO2(2+). 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.

A Structural and Spectroscopic Study of the First Uranyl Selenocyanate, [Et4N]3[UO2(NCSe)5]

“…The U=O bond length is 1.771(2) Å and average U-N, N=C and C=Se bond lengths of 2.459 Å, 1.149 Å and 1.794 Å respectively can be compared to the uncoordinated [N=C=Se]´ion (N=C: 1.081(14) Å and C=Se: 1.846(7) Å) in 2. Upon coordination to the uranyl ion the N=C bond lengthens slightly and the C=Se bond shortens slightly, suggesting a reorganization in the π-framework of the ligand; this effect has also been observed in uranyl thiocyanates experimentally and theoretically [11,14] 8 ] (An = Th, U, Pu) [16], the lack of perturbation of the π-system in the [NCS(e)]´ligands suggests no π-overlap in the U-N bond.…”

“…A further interesting photophysical property of certain uranyl compounds are thermochromic effects. Thus the compound [C 4 mim] 3 [UO 2 (NCS) 5 ] (C 4 mim = 1-butyl-3-methylimidazolium) is thermochromic in ionic liquids [10] but in organic solvents [Et 4 N] 3 [UO 2 (NCS) 5 ] is not [11]. We have reported on the latter compound recently and now extend our study to the selenocyanate [ …”

“…The uranyl group has characteristic vibrations in both the infrared and Raman spectra ( Figures S2 and S3). [11], whilst the C=Se stretch of 1 is visible in the Raman spectrum at 635 and 672 cm´1. 1 H NMR spectroscopy was uninformative ( Figure S4).…”

“…Therefore, on the basis of the metric parameters from the X-ray structure, vibrational and NMR spectroscopic data we suggest that the bonding in these compounds are ionic with little perturbation of the [NCSe]´anionic fragment upon coordination. Our recent theoretical study of the [NCS]´compound suggested a predominantly ionic interaction [11].…”

“…Given the ionicity of the U-N bond, ligand exchange processes may be faster than the lifetime of the uranyl excited state and so contributes to the shorter lifetime [25]. [11]. The photophysics of this compound has also been investigated (Figure 3).…”