Radical Reductive Elimination from Tetrabenzyluranium Mediated by an Iminoquinone Ligand

Reductive elimination from U(CH2Ph)4 (1-Ph) mediated by 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]quinone (dippiq) was observed, resulting in the formation of (dippap)2U(CH2Ph)2(THF)2 (2) (dippap = 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)amido]phenolate) and bibenzyl. The crossover experiment with U(CD2C6D5)4 showed formation of bibenzyl-d7, indicating that reductive elimination occurs in a stepwise fashion via benzyl radical extrusion, presumably through an iminosemiquinone tris(benzyl) intermediate, (dippisq)U(CH2Ph)3. Synthesis of this intermediate was attempted by addition of the iminoquinone ligand to UI3(THF)4 to form (dippisq)UI3 (3), followed by alkylation with 3 equiv of benzylpotassium. However, this only resulted in the isolation of 2. Reduction of 3 with KC8 afforded the amidophenolate diiodide species (dippap)UI2(THF)2 (4), maintaining the tetravalent oxidation state of the uranium and reducing the ligand. Attempts at the formation of 2 via addition of 2 equiv of benzylpotassium to 4 resulted in decomposition. The uranium mono(alkyl) (dippap)UI(CH2Ph)(THF)2 (5) was observed upon addition of 1 equiv of benzylpotassium to 4. All products have been characterized by 1H NMR and electronic absorption spectroscopy. X-ray crystallography was employed to ascertain ligand reduction in 2, 3, and 5.