Carbon–Carbon Reductive Elimination from Homoleptic Uranium(IV) Alkyls Induced by Redox-Active Ligands

The synthesis, characterization, and reactivity of the
homoleptic
uranium­(IV) alkyls U­(CH2C6H5)4 (1-Ph), U­(CH2-p-CH3C6H4)4 (1-p-Me), and U­(CH2-m-(CH3)2C6H3)4 (1-m-Me2) are reported. The
addition of 4 equiv of K­(CH2Ar) (Ar = Ph, p-CH3C6H4, m-(CH3)2C6H3) to UCl4 at −108 °C produces 1-Ph in good yields
and 1-p-Me and 1-m-Me2 in moderate yields. Further characterization
of 1-Ph by X-ray crystallography confirmed η4-coordination of each benzyl ligand to the uranium center.
Magnetic studies produced an effective magnetic moment of 2.60 μB at 23 °C, which is consistent with a tetravalent uranium
5f 2 electronic configuration. Addition of 1 equiv
of the redox-active α-diimine MesDABMe (MesDABMe = [ArNC­(Me)­C­(Me)NAr];
Ar = 2,4,6-trimethylphenyl (Mes)) to 1-Ph results in
reductive elimination of 1 equiv of bibenzyl (PhCH2CH2Ph), affording (MesDABMe)­U­(CH2C6H5)2 (2-Ph). Treating an equimolar mixture of 1-Ph and 1-Ph-d28 with MesDABMe forms the products from monomolecular
reductive elimination, 2-Ph, 2-Ph-d14, bibenzyl,
and bibenzyl-d14. This is confirmed by 1H NMR spectroscopy and GC/MS analysis of both organometallic
and organic products. Addition of 1 equiv of 1,2-bis­(dimethylphosphino)­ethane
(dmpe) to 1-Ph results in formation of the previously
synthesized (dmpe)­U­(CH2C6H5)4 (3-Ph), indicating the redox-innocent chelating
phosphine stabilizes the uranium center in 3-Ph and prevents
reductive elimination of bibenzyl. Full characterization for 3-Ph, including X-ray crystallography, is reported.