Lewis Base Activation by Uranium(III) Complexes

The combination of a bulky hypersilyl potassium [(Me3Si)3SiK] reagent with Tp*2UI (Tp* = hydrotris­(3,5-dimethylpyrazolyl)­borate) in the presence of ethereal Lewis donors resulted in the formation of base-activated products Tp*2U­[O­(CH2)4Si­(SiMe3)3] (1-THF) and Tp*2U­[O­(CH2)2OMe] (2-DME). The reactivity with another Lewis base, pyridine, was explored by treating Tp*2UI and hypersilyl potassium or benzyl potassium in the presence of pyridine, which resulted in formation of Tp*2U­[NC5H5-4-Si­(SiMe3)3] (3-py-Si) and Tp*2U­(NC5H5-4-Bn) (4-py-Bn, Bn = benzyl), respectively. Multinuclear paramagnetic NMR spectroscopy (1H, 11B­{1H}, 29Si­{1H}) supported the formation of the Lewis base activated uranium compounds as corroborated by electronic absorption spectroscopy and X-ray crystallography. To recognize the mechanistic possibilities, radical trap experiments were performed and [K­(18-crown-6)]­[4-benzylpyridinide] (4-K), Tp*U­(IV)­[(NC­(Me)­C­(H)­C­(Me)­N)-B­(H)­(3,5-dimethylpyrazole)2] (6-Tp*UTp′), and [Tp*2U­(NC5H5)]2 (5-py-py) were observed.