Uranium Going the Soft Way: Low-Valent Uranium(III) Coordinated to an Arene-Anchored Tris-Thiophenolate Ligand.

The synthesis of a tripodal, S-based ligand, namely the mesitylene-anchored, tris-thiophenolate-functionalized (mes(Me,AdArS)3)3- (1)3-, and its coordination chemistry with low-valent uranium to form [UIII((SArAd,Me)3mes)] (1-U) are reported. Single-crystal X-ray diffraction analysis reveals a C3-symmetric molecular structure. Full characterization of 1-U was performed using nuclear magnetic resonance, UV-vis-NIR electronic absorption, and electron paramagnetic resonance spectroscopies as well as SQUID magnetometry, thus confirming the U(III) oxidation state. Alternating current magnetic studies show that 1-U exhibits single-molecule magnet behavior at low temperatures in a non-zero external field. Comparison of these results to those of the previously reported mesitylene-anchored complexes, [UIII((OArAd,Me)3mes)] and [UIII((OArtBu,tBu)3mes)], indicates a drastic change in the electronic structure when moving from phenolate-based ligands to thiophenolate-based 1, which is further discussed by means of computational analysis (NBO, DFT, and QTAIM). Despite the U-O bonds being stronger, a much higher covalency was found for the U-S analogue.