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.