Uranium-Mediated Thiourea/Urea Conversion on Chelating Ligands

Abstract

2,6-Dipicolinoylbis(N,N-dialkylthioureas) and H2LR2 react with uranyl salts and a supporting base (e.g., NEt3) under formation of monomeric or oligomeric complexes of the compositions [UO2(LR2)(solv)] (solv = donor solvents) or [{UO2(LR2)(µ2-OMe)}2]2–. In such complexes, the uranyl ions are commonly coordinated by the “hard” O,N,O or N,N,N donor atom sets of the central ligand unit and the lateral sulfur donor atoms remain uncoordinated. Their individual structures, however, depend on the reaction conditions, particularly on the equivalents of NEt3 used. An unprecedented, selective hydrolysis of the uranium-coordinating bis(thioureato) ligands results in an S/O donor atom exchange at exclusively one thiourea side-arm, when an excess of NEt3 is used. The resulting trimeric uranyl complexes are isolated in fair yields and have a composition of [(UO2)3(L2Et2)2(µ2–OR)(µ3-O)]–. H2L2Et2 represents the newly formed 2,6-dipicolinoyl(N,N-diethylthiourea)(N,N-diethylurea) and R = H, Me, or Et. {L2Et2}2– binds to the uranyl units via the pyridine ring, the dialkylurea arm, and the central carbonyl groups, while the thiourea unit remains uncoordinated. The central cores of the products consist of oxido-centered triangular {(UO2)3O}4+ units. The observed reactivity is metal-driven and corresponds mechanistically most probably to a classical metal-catalyzed hydrodesulfurization. The hydrolytic thiourea/urea conversion is only observed in the presence of uranyl ions. The products were isolated in crystalline form and studied spectroscopically and by X-ray diffraction. The experimental findings are accompanied by DFT calculations, which help to understand the energetic implications in such systems.