Uranium(IV) Nucleophilic Carbene Complexes

The mono-, bis-, and tris-carbene uranium complexes [Li(THF)(2)U(SCS)Cl-3(THF)] (2a), [U(SCS)(2)(THF)(2)] (3a), and [{Li(OEt2)}(2)U(SCS)(3)] (1) were synthesized in good yields by reactions of UCl4 and the stoichiometric amount of Li-2(SCS) [(SCS)(2-) = [Ph2P(=S)](2)C2-. Complex 3a was also obtained by comproportionation reaction of 1 and 0.5 molar equiv of UCl4 and further reacted with 1 molar equiv of UCl4 to give the neutral mono-carbene [U(SCS)Cl-2(THF)(2)] (5a). Treatment of U(NEt2)(4) with H2C(Ph2PS)(2) in THF led to a mixture of 3a and [U(SCS)(NEt2)(2)] (6), while the same reaction in Et2O gave the mixed alkyl-carbene compound [U(SCS)(SCHS)(NEt2)] (7) in 85% yield. The cationic uranium carbene complex [U(SCS)(NEt2)(THF)(3)][BPh4] (9) was isolated in almost quantitative yield from reaction of [U(NEt2)(3)][BPh4] and H2C(Ph2PS)(2). Mono-carbenes 2a, 5a, and 9 were used as precursors for the synthesis of Cp and COT derivatives (Cp = C5H5, COT = C8H8). Treatment of 2a with 1 or 2 molar equiv of TlCp gave [Tl{U(Cp)(SCS)}(2)(mu-Cl)(3)] (10) and [U(Cp)(2)(SCS)] (11) in 90% and 79% yield, respectively, whereas [U(Cp*)(2)(SCS)] (12) (Cp* = C5Me5) was obtained only by reaction of [U(Cp*)(2)Cl-2] and Li-2(SCS). Reactions of Sa or 9 with K2COT gave [U(COT)(SCS)(THE)] (13) in 78% and 99% yield, respectively. 2a, [Li(THF)(Et2O)U(SCS)(mu-Cl)(3)](2) (2b), [U(SCS)(2)(py)(2)] center dot 1.5py center dot 0.5THF (3b center dot 1.5py center dot 0.5THF), [U(SCS) {CS(Ph2PS)(2)} (Py)] (4), 7 center dot toluene, [Li(THF)(2)U(SCS)(NEt2)(mu-O)](2) (8), 10 center dot 2toluene, 11, 12, and 13 center dot 0.5pentane were characterized by X-ray diffraction. The crystal data revealed that, in contrast to transition metal complexes, changes in the coordination sphere of the U(IV) center have little influence on the U=C bond. This feature was explained by DFT analysis of analogous U(IV) and Zr(IV) compounds [M(SCS')Cl-2(py)(2)] and [M(SCS')(Cp)(2)] [M = U, Zr; SCS' = C(H2PS)(2)]. Although the 5f orbitals are more radially contracted than the 6d atomic orbitals, the 5f AOs are lower in energy in uranium and can lead to greater angular overlaps in symmetry-constrained systems. As a result, the seven 5f orbitals play a buffer role by engaging in covalent interactions with the carbon center to stabilize the nucleophilic carbene lone pairs.