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)(2)C6H3)(4) (1-m-Me-2) are reported. The addition of 4 equiv of K(CH2Ar) (Ar = Ph, p-CH3C6H4, m-(CH3)(2)C6H3) to UCl4 at -108 degrees C produces 1-Ph in good yields and 1-p-Me and 1-m-Me-2 in moderate yields. Further characterization of 1-Ph by X-ray crystallography confirmed eta(4)-coordination of each benzyl ligand to the uranium center. Magnetic studies produced an effective magnetic moment of 2.60 mu(B) at 23 degrees C, which is consistent with a tetravalent uranium 5f(2) electronic configuration. Addition of 1 equiv of the redox-active alpha-diimine (Mes)DAB(Me) ((Mes)DAB(Me) = [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 ((Mes)DAB(Me))U(CH2C5H5)(2) (2-Ph). Treating an equimolar mixture of 1-Ph and 1-Ph-d(28) with (Mes)DAB(Me) forms the products from monomolecular reductive elimination, 2-Ph, 2-Ph-d(14), bibenzyl, and bibenzyl-d(14). This is confirmed by H-1 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.