Synthesis and Characterization of a Uranium(III) Complex Containing a Redox-Active 2,2′-Bipyridine Ligand

Hydrotris(3,5-dimethylpyrazolyl) borate uranium(III) diiodide derivatives have been prepared as an entry into low-valent uranium chemistry with these ligands. The bis(tetrahydrofuran) adduct, Tp*Ul(2)(THF)(2) (1) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), was synthesized by addition of sodium hydrotris (3,5-dimethylpyrazolyl) borate (NaTp*) to an equivalent of Ul(3)(THF)(4). Addition of 2,2'-bipyridine (2,2'-bpy) to 1 displaced the THF molecules producing Tp*Ul(2)(2,2'-bpy) (2). Both derivatives were characterized by H-1 NMR and IR spectroscopies, magnetic measurements, and X-ray crystallography. Reduction of both species was attempted with two equivalents of potassium graphite. The reduction of 1 did not result in a clean product, but rather decomposition and ligand redistribution. Howover, compound 2 was reduced to form Tp*U-2(2,2'-bpy), 3, which is composed of a uranium(Ill) ion with a radical monoanionic bipyridine ligand. This was confirmed by X-ray crystallography, which revealed distortions in the bond lengths of the bipyridine consistent with reduction. Further support was obtained by H-1 NMR spectroscopy, which showed resonances shifted far upfield, consistent with radical character on the 22-bipyridine ligand. Future studies will explore the reactivity of this compound as well as the consequences for redox-activity in the bipyridine ligand.