Conversion of Uranium(III) Anilido Complexes to Uranium(IV) Imido Complexes via Hydrogen Atom Transfer

Abstract

A family of low-valent uranium(III) anilido complexes supported by the bulky hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) ligand was synthesized by combining Tp*2UBn (Bn = benzyl) (1-Bn) with anilines of varying steric bulk and electronic profiles, including 4-fluoroaniline, 3,5-difluoroaniline, 3,5-bis(trifluoromethyl)aniline, 3,5-dimethylaniline, 2,4,6-trimethylaniline, and 2,4,6-tri-tBu-aniline. The corresponding uranium(III) anilido species, Tp*2UNH(4-fluorophenyl) (2-pF), Tp*2UNH(3,5-difluorophenyl) (2-bisF), Tp*2UNH(3,5-bis(trifluoromethyl)phenyl) (2-bisCF3), Tp*2UNH(3,5-dimethylphenyl) (2-bisCH3), Tp*2UNH(2,4,6-trimethylphenyl) (2-Mes), and Tp*2UNH(2,4,6-tri-tBu-phenyl) (2-Mes*), were isolated, and reactivity was explored. Conversion to their respective uranium(IV) imido species (3-pF, 3-bisF, 3-bisCF3, 3-bisCH3, 3-Mes, and 3-Mes*) was achieved by hydrogen atom transfer using either Gomberg’s dimer or the 2,4,6-tri-tBu-phenoxy radical (·OMes*) which eliminates the need to use potentially explosive organic azides, a reagent that has been commonly used for the synthesis of uranium(IV) imido complexes. For comparison of yields and purity of all methods, the uranium imido complexes were also prepared using the corresponding organic azides. Where applicable, compounds were characterized by multinuclear NMR spectroscopy (1H, 11B, 19F), infrared spectroscopy, electronic absorption spectroscopy, and single crystal X-ray crystallography.