Synthesis and Characterization of a Series of U(IV) trans-bis(Wittig) Adducts Across the UX4 Halide Series

Addition of 2 equiv. of the Wittig reagents CH2PAr3 (Ar = Ph; 3,5-di-tert-butylphenyl (tBuAr)) to the tetravalent uranium halides, UX4(solvent)n (X = Cl, n = 0; X = Br, solvent = THF, n = 2; X = I, solvent = 1,4-dioxane, n = 2), generates the trans-bis(Wittig) adducts UX4(CH2PAr3)2 (Ar = Ph, X = Cl (1Ph-Cl), I (1Ph-I); Ar = tBuAr, X = Cl (2tBu-Cl), Br (2tBu-Br), I (2tBu-I)) in low to good yields. Complexes 1Ph-X exhibit poor solubility in aromatic solvents but are partially soluble in 1,2-difluorobenzene, while 2tBu-X possesses greatly improved solubility in these solvents. In all cases, 1Ph-X and 2tBu-X are sensitive to polar coordinating solvents, such as THF or DME, decomposing in these solutions to intractable products. The solid-state molecular structures of 1Ph-XꞏC7H8 and 2tBu-Xꞏn(o-DFB), reveal U-CWittig bonds that range from = 2.506(3) – 2.58(1) Å, generally shorter than those found in other uranium-Wittig (2.60(1) – 2.71(1) Å) and untethered, monodentate U-CNHC (2.62(1) – 2.79(1) Å) complexes. Notably, a general contraction of the U-CWittig bond is observed in the UX4(CH2PAr3)2 as the halide series is descended, which may be attributable to the poorer π-donation of the heavier halides that gives rise to increased Lewis acidity at the uranium center that results in contraction of the U-CWittig bond. Attempts to oxidize 2tBu-Cl with Ag+ or Fc+ salts leads to complicated product mixtures from which a few crystals of {[(tBuAr)3PCH2]UCl3(μ-Cl)}2·C7H8 can be isolated, whereas addition of the reductant Cp*2Co to 2tBu-I, in the presence of an extra equiv. of CH2P(tBuAr)3, leads to the formation of the highly encumbered U(III) tris(Wittig) adduct UI3[CH2P(tBuAr)3]3 (3tBu-I). Preliminary experiments show these complexes are amenable to substitution reactions as treatment of 2tBu-Cl with 2 equiv. of LiCH2SiMe3 generates the thermally sensitive bis(alkyl) bis(Wittig) complex trans-UCl2[CH2P(tBuAr)3]2(CH2SiMe3)2 (4tBu-TMS), a mixed ylide-alkyl system featuring distinct U-CWittig and U-Calkyl σ-bonds. This chemistry demonstrates that untethered, neutral Wittig ligands, when coordinated to uranium, are compatible with redox transformations and metathesis reactions. The report of these UX4(CH2PAr3)2 complexes significantly expands the library of known uranium-Wittig compounds and contributes to the relatively small collection of uranium complexes featuring neutral C-donor ligands.